ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus GmbHGöttingen, Germany10.5194/acp-15-6283-2015A large and ubiquitous source of atmospheric formic acidMilletD. B.dbm@umn.eduhttps://orcid.org/0000-0003-3076-125XBaasandorjM.FarmerD. K.ThorntonJ. A.BaumannK.https://orcid.org/0000-0003-4045-5539BrophyP.ChaliyakunnelS.de GouwJ. A.https://orcid.org/0000-0002-0385-1826GrausM.HuL.KossA.LeeB. H.Lopez-HilfikerF. D.NeumanJ. A.https://orcid.org/0000-0002-3986-1727PaulotF.PeischlJ.https://orcid.org/0000-0002-9320-7101PollackI. B.https://orcid.org/0000-0001-7151-9756RyersonT. B.WarnekeC.WilliamsB. J.XuJ.Department of Soil, Water, and Climate, University of Minnesota,
Minneapolis–Saint Paul, MN 55108, USADepartment of Chemistry, Colorado State University, Fort Collins, CO
80523, USADepartment of Atmospheric Sciences, University of Washington,
Seattle, WA 98195, USAAtmospheric Research & Analysis Inc., Cary, NC 27513, USAChemical Sciences Division, NOAA Earth System Research Laboratory,
Boulder, CO 80305, USACooperative Institute for Research in Environmental Sciences,
University of Colorado, Boulder, CO 80309, USANOAA Geophysical Fluid Dynamics Laboratory, Princeton, NJ 08540, USADepartment of Energy, Environmental, and Chemical Engineering,
Washington University in St. Louis, St. Louis, MO 63130, USADepartment of Physics and Atmospheric Science, Dalhousie University,
Halifax, NS B3H 4R2, Canadanow at: Institute of Meteorology and Geophysics, University of
Innsbruck, 6020 Innsbruck, Austrianow at: School of Engineering and Applied Sciences, Harvard
University, Cambridge, MA 02138, USAnow at: Department of Atmospheric Science, Colorado
State University, Fort Collins, CO 80523, USAD. B. Millet (dbm@umn.edu)9June20151511628363041January201518February201530April201515May2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://acp.copernicus.org/articles/15/6283/2015/acp-15-6283-2015.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/15/6283/2015/acp-15-6283-2015.pdf
Formic acid (HCOOH) is one of the most abundant acids in the atmosphere,
with an important influence on precipitation chemistry and acidity. Here we
employ a chemical transport model (GEOS-Chem CTM) to interpret recent airborne
and ground-based measurements over the US Southeast in terms of the
constraints they provide on HCOOH sources and sinks. Summertime boundary
layer concentrations average several parts-per-billion, 2–3× larger
than can be explained based on known production and loss pathways. This
indicates one or more large missing HCOOH sources, and suggests either a key
gap in current understanding of hydrocarbon oxidation or a large,
unidentified, direct flux of HCOOH. Model-measurement comparisons implicate
biogenic sources (e.g., isoprene oxidation) as the predominant HCOOH source.
Resolving the unexplained boundary layer concentrations based (i) solely on
isoprene oxidation would require a 3× increase in the model HCOOH
yield, or (ii) solely on direct HCOOH emissions would require approximately a
25× increase in its biogenic flux. However, neither of these can
explain the high HCOOH amounts seen in anthropogenic air masses and in the
free troposphere. The overall indication is of a large biogenic source
combined with ubiquitous chemical production of HCOOH across a range of
precursors. Laboratory work is needed to better quantify the rates and
mechanisms of carboxylic acid production from isoprene and other prevalent
organics. Stabilized Criegee intermediates (SCIs) provide a large model
source of HCOOH, while acetaldehyde tautomerization accounts for
∼ 15 % of the simulated global burden. Because carboxylic
acids also react with SCIs and catalyze the reverse tautomerization
reaction, HCOOH buffers against its own production by both of these
pathways. Based on recent laboratory results, reaction between
CH3O2 and OH could provide a major source of atmospheric HCOOH;
however, including this chemistry degrades the model simulation of
CH3OOH and NOx: CH3OOH. Developing better constraints on SCI
and RO2+ OH chemistry is a high priority for future work. The model
neither captures the large diurnal amplitude in HCOOH seen in surface air,
nor its inverted vertical gradient at night. This implies a substantial bias
in our current representation of deposition as modulated by boundary layer
dynamics, and may indicate an HCOOH sink underestimate and thus an even
larger missing source. A more robust treatment of surface deposition is a
key need for improving simulations of HCOOH and related trace gases, and our
understanding of their budgets.
Introduction
Formic acid (HCOOH) is, along with acetic acid (CH3COOH), the dominant
carboxylic acid in the troposphere. Both are major sources of atmospheric
acidity, and together they can contribute > 60 % of the free
acidity in precipitation in remote areas and > 30 % in more
polluted regions (Andreae et al., 1988; Galloway et al., 1982; Keene et
al., 1983; Keene and Galloway, 1984). HCOOH can also be a significant sink
for in-cloud OH radical concentrations (Jacob, 1986), and is
therefore key to atmospheric aqueous-phase chemistry through its effects on
oxidant levels, pH-dependent reaction rates, and solubilities. Recent work
has shown that the atmospheric abundance of HCOOH is substantially larger
than can be explained based on current understanding of its budget
(Cady-Pereira et al., 2014; Le Breton et al., 2012; Paulot et al., 2011;
Stavrakou et al., 2012), implying the existence of a large missing source,
or a dramatic sink overestimate. Here, we employ a chemical transport model (GEOS-Chem CTM) to interpret a combination of recent airborne and
ground-based measurements in terms of the constraints they provide on the
atmospheric biogeochemistry of HCOOH.
HCOOH is produced in the atmosphere during the photochemical oxidation of
biogenic and anthropogenic volatile organic compounds (VOCs), and is emitted
directly through a variety of processes. Photochemical production is thought
to be the largest global source of HCOOH, but the magnitude is highly
uncertain. For instance, Paulot et al. (2009a) recently
estimated the HCOOH yield from isoprene at 10 % under NOx-dominated
conditions, 5–10× higher than standard chemical mechanisms had
implied. HCOOH is also emitted directly from vegetation in a light- and
temperature-dependent manner (Kesselmeier et al., 1998; Kesselmeier and
Staudt, 1999), although the flux is bi-directional, so that the net effect
can be emission or uptake depending on the ambient concentration
(Kesselmeier, 2001; Kuhn et al., 2002). Other emission sources include
biomass and biofuel burning (e.g., Goode et al., 2000), soils
(e.g., Sanhueza and Andreae, 1991), agriculture
(e.g., Ngwabie et al., 2008), and fossil fuel
combustion (e.g., Kawamura et al., 1985; Talbot et al., 1988).
Radiocarbon studies in Europe have shown that atmospheric HCOOH is mainly
composed of modern carbon, even in winter, which would suggest that the
fossil fuel contribution (via emission of precursors or of HCOOH itself) is
minor (Glasius et al., 2000, 2001).
Heterogeneous sources have also been proposed. For example, HCOOH can be
rapidly produced in cloud water from the reaction of hydrated formaldehyde
with OH(aq) (Jacob, 1986; Lelieveld and Crutzen, 1991). However,
formate itself is also rapidly oxidized by OH(aq), and as a result
evasion of HCOOH to the gas phase would only be expected for moderately
acidic clouds (pH < 5) (Jacob, 1986). In addition, HCOOH
production has been observed during organic aerosol aging in the laboratory
(Eliason et al., 2003; Molina et al., 2004; Pan et al., 2009; Park et
al., 2006; Vlasenko et al., 2008; Walser et al., 2007), raising the question
of whether this is also important in the ambient atmosphere
(Paulot et al., 2011). With a
continental organic aerosol source of approximately 150 TgC yr-1 globally
(Heald et al., 2010), a large HCOOH yield from aerosol
oxidation would be needed to have a major impact on its overall budget
(given a recent top-down HCOOH source estimate of ∼ 30 TgC yr-1;
Stavrakou et al., 2012).
HCOOH is soluble in water, with an effective Henry's law constant of
∼ 107 M atm-1 at pH 7 (Sander, 2015),
and is efficiently removed from the atmospheric boundary layer through wet
and dry deposition. On the other hand, photochemical oxidation of HCOOH
proceeds relatively slowly (τ∼ 25 days), so its
effective lifetime in the free troposphere is considerably longer than it is
in the boundary layer. Irreversible uptake on dust is another minor sink
(e.g., Hatch et al., 2007; Paulot et al., 2011), and the overall
atmospheric lifetime of HCOOH has been estimated at approximately 2–4 days
(Chebbi and Carlier, 1996; Paulot et al., 2011; Stavrakou et al., 2012).
Recent advances in remote sensing (Cady-Pereira et al., 2014; Stavrakou
et al., 2012; Zander et al., 2010) and in situ (Baasandorj et al., 2015;
Le Breton et al., 2012; Liu et al., 2012; Veres et al., 2011; Yuan et al.,
2015) measurement capabilities have led to the realization that atmospheric
HCOOH concentrations are much too high to be consistent with present
estimates of the source and sink magnitudes. In turn, this points to a key
gap in present understanding of the atmospheric reactive carbon budget. A
number of missing sources have been proposed to explain this discrepancy,
related to vegetation (Le Breton et al., 2012; Paulot et al., 2011;
Stavrakou et al., 2012), fires (Paulot et al., 2011; R'Honi et al.,
2013), anthropogenic VOCs (Le Breton et al., 2012),
and photooxidation of organic aerosols
(Paulot et al., 2011). In this
paper, we employ measurements from a suite of recent airborne and
ground-based studies to shed light on this issue and derive a better
understanding of atmospheric HCOOH. These studies were carried out over the
US Southeast during summer 2013 as part of the Southeast Nexus study (SENEX;
http://www.esrl.noaa.gov/csd/projects/senex/), the Southern
Oxidant and Aerosol Study (SOAS; http://soas2013.rutgers.edu/),
and the St. Louis Air Quality Regional Study (SLAQRS;
Baasandorj et al., 2015). Both SENEX and SOAS were part of
the larger Southeast Atmosphere Study (SAS; http://www.eol.ucar.edu/field_projects/sas). As we will
show, the ensemble of observational constraints imply that (i) biogenic HCOOH
sources are currently underestimated and predominate the HCOOH budget, and
(ii) there is an undefined and widespread chemical source of HCOOH from a range
of different precursor species.
Simulation of atmospheric HCOOHModel overview
We use the GEOS-Chem global 3-D CTM (http://www.geos-chem.org) to
simulate HCOOH and related chemical species for 2013. The model runs employ meteorological data from the GEOS-5 Forward Processing (GEOS-FP) Atmospheric Data Assimilation System, which have a native resolution of 0.25∘
latitude × 0.3125∘ longitude and 72 vertical levels. For
the present analysis we degrade the resolution to 2∘× 2.5∘ with 47 vertical levels and use a 15 min transport
time step.
GEOS-Chem uses the TPCORE advection algorithm of Lin and Rood (1996), convective transport computed as described by Wu et al. (2007), and the non-local boundary layer mixing scheme of
Lin and McElroy (2010). Wet deposition of HCOOH and other
gases proceeds as described by Amos et al. (2012), and dry deposition is
based on a standard resistance-in-series parameterization (Wang et al.,
1998; Wesely, 1989). The chemical mechanism used here is as described
elsewhere (Fischer et al., 2012; Mao et al., 2010, 2013a, b; Paulot et al., 2011), with a number of updates and
modifications detailed below. Emissions relevant to the simulation of HCOOH
are also described below. Further details on the GEOS-Chem model can be
found at http://www.geos-chem.org.
Global distribution of HCOOH sources in the GEOS-Chem
base-case simulation. Note nonlinear color scale.
Figure 1 shows the global distribution of HCOOH sources in our base-case
simulation. These include 51.0 Tg yr-1 from the photochemical oxidation of VOCs
and 10.5 Tg yr-1 from direct emissions, and are computed as described next.
Sinks include wet + dry deposition (29.8 Tg yr-1 wet; 20.8 Tg yr-1 dry),
photochemical loss (9.5 Tg yr-1), and dust uptake (1.2 Tg yr-1).
Emissions
Global anthropogenic emissions of CO, NOx, and SOx in GEOS-Chem
use the Emissions Database for Global Atmospheric Research (EDGAR) 3.2-FT2000 inventory (Olivier et al.,
2005), while anthropogenic VOC emissions are from the Reanalysis of the Tropospheric Chemical Composition (RETRO) inventory
(Schultz et al., 2007) implemented as described by Hu et al. (2015a). Over North America, these
inventories are overwritten by the US EPA's National Emissions Inventory (NEI) 2005 (www.epa.gov/ttnchie1/net/2005inventory.html) and by Environment Canada's
National Pollutant Release Inventory (NPRI)
(www.ec.gc.ca/inrp-npri/). Emissions from open and
domestic biomass burning are based on the version 3 of the Global Fire Emissions Database (GFED3) for 2011
(van der Werf et al., 2010) and on Yevich and
Logan (2003), respectively. In all cases, HCOOH emissions are
estimated by scaling those of CO to observed HCOOH : CO emission ratios,
following Paulot et al. (2011).
Because GFED3 is not available for 2013, individual fire plumes are removed
from the model-measurement comparisons as described later.
Recent measurements in London, UK (Bannan et al., 2014),
imply an anthropogenic HCOOH : CO emission ratio of 1.22 × 10-3 mol mol-1, 6× higher than the value used here based on Talbot
et al. (1988). However, given a direct CO source from fossil
fuels of ∼ 400 Tg yr-1 globally
(Duncan et al., 2007), even this newly
reported emission ratio would imply a corresponding HCOOH source of
< 1 Tg yr-1. The direct anthropogenic HCOOH source is thus very small
compared to the other sources shown in Fig. 1.
Direct emissions of HCOOH and other VOCs (including isoprene and
monoterpenes) from terrestrial vegetation are estimated using version 2.1 of the Model of Emissions of Gases and Aerosols from Nature (MEGANv2.1) (Guenther et al., 2012), implemented in
GEOS-Chem as described by Hu et al. (2015b).
Bottom-up biogenic VOC flux estimates can vary significantly depending on
the land cover, meteorology, and forest canopy parameterization used to
compute the emissions. Here we employ monthly mean leaf area indices derived
from MODIS observations (Myneni et al., 2007; leaf area index (LAI) of year 2008 for all
ensuing years), vegetation coverage for 15 plant functional types from
version 4 of the Community Land Model (CLM4;
Oleson et al., 2010), and the
Parameterized Canopy Environment Emission Activity (PCEEA) algorithm described by Guenther et al. (2006).
The version 2.1 of the Model of Emissions of Gases and Aerosols from Nature (MEGANv2.1) emissions are then derived using the same GEOS-FP
meteorological fields that drive GEOS-Chem. HCOOH and other compounds
undergoing bi-directional exchange are treated as described by Millet et al. (2010) and Guenther et al. (2012).
Marine VOC emissions, along with direct HCOOH
emissions from agriculture and soils, are treated as described previously
(Paulot et al., 2011).
Figure 1 shows the resulting global distribution of direct HCOOH emissions
from terrestrial vegetation (2.7 Tg yr-1), soils + agriculture (5.9 Tg yr-1),
anthropogenic sources (0.4 Tg yr-1; includes domestic biofuel), and open fires
(1.5 Tg yr-1).
Photochemical production of HCOOHOzonolysis of terminal alkenes
A number of prevalent atmospheric VOCs contain a terminal alkene moiety,
including ethene, propene, isoprene, MACR, MVK, and many of the monoterpenes
(e.g., β-pinene, d-limonene, camphene, sabinene, β-ocimene,
β-phellandrene, and myrcene). Ozonolysis of such compounds leads to
an energy-rich [CH2OO]* Criegee intermediate, along with a carbonyl
compound. In the case of ethene (Atkinson et al.,
2006):
.
The nascent energy-rich Criegee intermediate can then undergo prompt
unimolecular decomposition, or collisional stabilization to yield a
stabilized Criegee intermediate (SCI):
.
The lifetime of stabilized CH2OO is long enough that it can undergo
bimolecular reactions with a range of atmospheric compounds (Hasson et
al., 2001; Hatakeyama and Akimoto, 1994; Neeb et al., 1996,
1997; Newland et al., 2015; Stone et al., 2014; Su et al., 2014; Vereecken
et al., 2012; Welz et al., 2012, 2014). In particular, the
reaction with water vapor leads to hydroxymethyl hydroperoxide (HMHP)
(Hasson et al., 2001; Neeb et al., 1996, 1997):
.
which is known to decompose heterogeneously to HCOOH + H2O
(Neeb et al., 1997; Orzechowska and Paulson, 2005).
While it is uncertain how readily this occurs in the atmosphere (Lee et
al., 1993; Sauer et al., 2001; Valverde-Canossa et al., 2006;
Weinstein-Lloyd et al., 1998), photooxidation of HMHP is likely to produce
HCOOH in high yield as well (Neeb et al., 1997; Paulot et al., 2011;
Stavrakou et al., 2012). For this work we assume prompt conversion of HMHP
to HCOOH. This simplification may slightly overestimate HCOOH production
from the CH2OO SCI, but an offsetting factor is that there are a number
of atmospheric compounds containing terminal alkene groups that are not
explicitly represented in the GEOS-Chem chemical scheme.
Estimates of the CH2OO + H2O rate coefficient
(kCH2OO+H2O) vary widely. Stone et al. (2014) inferred an upper limit
of 9 × 10-17 cm3 molec-1 s-1, with a best
estimate of 5.4 × 10-18 cm3 molec-1 s-1 based on
relative rate considerations. On the other hand, Newland et al. (2015) derived a value for kCH2OO+H2O of
(1.3 ± 0.4) × 10-15 cm3 molec-1 s-1. For the base-case
simulations in this work, we employ kCH2OO+H2O=1×10-17 cm3 molec-1 s-1 following version 3.2 of the Master Chemical Mechanism (MCMv3.2) (Jenkin et al., 1997;
Saunders et al., 2003). We also carry out separate sensitivity analyses
(kCH2OO+H2O=5.4×10-18 and 1.3×10-15 cm3 molec-1 s-1) to test how current uncertainty in this
parameter affects our results. Reaction of CH2OO with the water dimer
may also be significant in the atmosphere (Chao et al., 2015),
and likewise leads to HMHP production (Ryzhkov and Ariya, 2003).
However, we do not include such chemistry here, since (as will be seen) the
above sensitivity runs already span conditions where CH2OO removal is
dominated by reaction with water to form HMHP.
In addition to the sinks above, recent work has found that the CH2OO
SCI can be lost by reaction with HCOOH and other carboxylic acids, and by
self-reaction. Reactions between SCIs and carboxylic acids are treated as
described later (Sect. 2.4). In the case of the CH2OO self-reaction,
the kCH2OO+CH2OO rate coefficient was estimated at (4 ± 2) × 10-10 cm3 molec-1 s-1 by Su et al. (2014),
which is close to the gas kinetic collision value.
However, subsequent work by the same group gives a lower value of (8 ± 4) × 10-11 cm3 molec-1 s-1 (Ting et al.,
2014), which agrees with two other recent estimates of (6 ± 2) × 10-11 cm3 molec-1 s-1 (Buras
et al., 2014) and (7.4 ± 0.6) × 10-11 cm3 molec-1 s-1 (Chhantyal-Pun et al., 2015). The
base-case simulations presented here do not include the CH2OO
self-reaction; rather, we include a separate sensitivity run based on the
Ting et al. (2014) rate to assess the potential impact of this chemistry
on atmospheric HCOOH.
We employ here a stabilized CH2OO yield of 0.37 from ethene ozonolysis
(Atkinson et al., 2006). Alam et al. (2011) reported a higher value of 0.54,
but more recent work by the same group gives a revised estimate of 0.37
(Newland et al., 2015), in agreement with the IUPAC
recommendation. For propene, which is grouped with higher alkenes in the
GEOS-Chem mechanism, we use an SCI yield of 12 % for each of the two
possible Criegee intermediates (CH2OO and CH3CHOO) based on MCMv3.2 (Jenkin
et al., 1997; Saunders et al., 2003). Ozonolysis of isoprene, its oxidation
products, and 2-methyl-3-buten-2-ol (MBO), and the associated SCI chemistry,
is also implemented following MCMv3.2. Treatment of monoterpenes is
described below.
Alkyne oxidation
Oxidation of acetylene (or other terminal alkyne) by OH leads to formic acid
through formation of a peroxy radical that can then decompose to HCOOH plus
an acyl radical, or to a dicarbonyl plus OH (Bohn
et al., 1996; Hatakeyama et al., 1986):
.
Paulot et al. (2011) found
acetylene (C2H2) to be the dominant non-biogenic precursor of
formic acid in their simulations. We use in GEOS-Chem branching ratios of
0.364 and 0.636 for the acid and dicarbonyl channels, respectively, in
acetylene oxidation (Bohn et al., 1996; Hatakeyama et al., 1986; Jenkin
et al., 1997; Saunders et al., 2003).
Isoprene oxidation by OH
The HCOOH yield from OH-initiated isoprene oxidation is uncertain. The
OH-isoprene oxidation scheme employed here is based on the work of Paulot et
al. (2009a, b; 2011), updated to account for peroxy radical
isomerization reactions (Crounse et al., 2011, 2012).
Pathways leading to HCOOH in this mechanism (aside from the ozonolysis
reactions) include photooxidation of glycoaldehyde and hydroxyacetone
(Butkovskaya et al., 2006a, b), degradation of isoprene nitrates (Paulot et al.,
2009a), and oxidation of isoprene epoxides (Paulot et
al., 2009b).
Figure S1 in the Supplement shows the resulting production rate of HCOOH over North America in
GEOS-Chem. The total source from isoprene (including ozonolysis plus OH
chemistry) over this domain corresponds to an average molar yield of 13 %
(2.6 % on a per-carbon basis), and accounts for nearly half of the overall
photochemical HCOOH source in the model. Globally, isoprene oxidation
accounts for ∼ 45 % of the total HCOOH burden in the
GEOS-Chem base-case simulation.
Evidence for direct HCOOH production from glycoaldehyde and hydroxyacetone
as reported by Butkovskaya et al. (2006a, b)
(and implemented here) is mixed. Orlando et al. (2012) did not find
evidence for any significant HCOOH production from these compounds within
the range of atmospherically relevant NOx concentrations. Earlier work
by Jenkin et al. (1993) on the Cl-atom-initiated oxidation
of hydroxyacetone attributed the observed HCOOH production to secondary
chemistry that would only be relevant to chamber conditions. Excluding the
HCOOH source from glycoaldehyde and hydroxyacetone in the model reduces the
global photochemical HCOOH source by over one-third, and (as will be seen)
substantially increases the magnitude of the implied missing source.
Monoterpene oxidation
HCOOH can be produced during OH-initiated oxidation and ozonolysis of
monoterpenes (Larsen et al., 2001; Lee et al., 2006a, b;
Orlando et al., 2000). The corresponding mechanisms and yields are not well
quantified, and the overall effect on atmospheric HCOOH will vary with the
mixture of monoterpenes at hand. For the simulations here we employ a single
lumped monoterpenes species, with molar HCOOH yields of 15.5 % (OH
chemistry) and 7.5 % (ozonolysis)
(Paulot et al., 2011). This
results in an HCOOH source of 9.8 Gmol over the domain of Fig. S1, 18 %
of the model source from isoprene. Globally, monoterpenes account for
∼ 4 % of the HCOOH burden in the base-case simulation.
Keto-enol tautomerization
Andrews et al. (2012) studied the photolysis of C-1
deuterated acetaldehyde (CH3CDO), and observed formation of its isomer
CH2DCHO. Modeling of the photo-isomerization implied that upon
absorbing a photon, the initially excited acetaldehyde undergoes keto-enol
tautomerization (as shown here for the non-deuterated molecule):
.
This in turn suggested that a fraction of the enol could be collisionally
deactivated to form stable vinyl alcohol. Their best estimate for the vinyl
alcohol quantum yield (ϕenol) was 21 ± 4 %. Subsequent
work (Clubb et al., 2012) observationally confirmed the
formation of vinyl alcohol during acetaldehyde irradiation.
As the photooxidation of vinyl alcohol can lead to formic acid (Archibald
et al., 2007; So et al., 2014), we include acetaldehyde tautomerization in
GEOS-Chem to gauge its potential importance for atmospheric HCOOH. There are
likely to be two offsetting pressure effects on ϕenol: (i) competition between the photo-tautomerization reaction and collisional
deactivation of the initially excited acetaldehyde molecule, and (ii) competition between collisional stabilization of the enol and dissociation.
Since we lack quantitative constraints on either, we assume here a 21 %
quantum yield for enol production, with no pressure (or temperature)
dependence. The subsequent photochemical oxidation of vinyl alcohol is then
described according to the theoretical rate coefficients derived by So et
al. (2014).
However, theory also suggests that HCOOH and other carboxylic acids (as well
as other species) can effectively catalyze the reverse tautomerization of
vinyl alcohol back to acetaldehyde (da Silva, 2010; Karton, 2014).
Including the photo-tautomerization of acetaldehyde as well as the
acid-catalyzed reverse reaction (applying the rate derived by da Silva (2010) for both formic and acetic acids), we find that
the vinyl alcohol pathway accounts for 15 % of the global HCOOH burden in
the model.
As shown in Fig. S2, a consequence of this chemistry is that the
efficiency of acetaldehyde tautomerization as a source of atmospheric HCOOH
is inversely related to the abundance of HCOOH itself. HCOOH therefore
buffers its own production by this mechanism; keto-enol tautomerization can
provide a major fraction of the secondary HCOOH source when HCOOH
concentrations are low, but it becomes negligible at higher levels of HCOOH.
CH3O2+ OH
The rate coefficient for the CH3O2+ OH reaction was recently
measured at (2.8 ± 1.4) × 10-10 cm3 molec-1 s-1 (Bossolasco et al., 2014), based on the
CH3O2 absorption cross section reported by Faragó et al. (2013). While the 2–3× uncertainty in that
cross section (Atkinson and Spillman, 2002; Faragó et al., 2013;
Pushkarsky et al., 2000) propagates onto the derived CH3O2+ OH
rate, the value derived by Bossolasco et al. (2014) is large
enough that OH would represent an important CH3O2 sink in low-NO
environments (Fittschen et al., 2014).
The potential implications of this chemistry for HCOOH and other species
hinge on the reaction products, which are not known. Archibald et al. (2009) proposed three possible reaction paths – H-atom
abstraction to yield a Criegee intermediate, O-atom transfer to yield an
alkoxy radical, and nucleophilic substitution to yield an alcohol:
.
In the first case, CH2OO can go on to produce HCOOH as discussed
earlier.
To test the possible importance of this chemistry for atmospheric HCOOH, we
carried out a sensitivity simulation using the reported CH3O2+
OH rate coefficient (Bossolasco et al., 2014) and assuming that
the reaction takes place exclusively via H-atom abstraction. The resultant
CI is then treated in the same way as in the case of ethene ozonolysis.
Sinks of HCOOH
Wet and dry deposition are the predominant sinks for HCOOH in the model, and
these are computed as described earlier. Photochemical oxidation of HCOOH by
OH is treated using the current IUPAC recommendation of kOH+HCOOH= 4.5 × 10-13 cm3 molec-1 s-1
(Atkinson et al., 2006).
While the CH2OO SCI can be a source of HCOOH, recent work by Welz et
al. (2014) suggests that SCIs in
general can also provide a sink of HCOOH and other carboxylic acids, with
SCI + RCOOH rate coefficients in excess of 10-10 cm3 molec-1 s-1 (i.e., approaching the collision limit). The reaction
of CH2OO with HCOOH occurs in competition with the CH2OO +
H2O reaction that leads to HCOOH production. As with the
acetaldehyde–vinyl alcohol system, HCOOH can thus be seen as buffering
against its own production from CH2OO. We performed a separate
sensitivity simulation to evaluate the potential role of this chemistry for
the atmospheric HCOOH budget, with results described in Sect. 4.
Atmospheric observations of HCOOH and related species
Here, we compare model results with recent airborne and ground-based
measurements over North America to derive a better understanding of the
HCOOH budget and potential missing sources. Measurements include the SENEX
aircraft campaign (http://www.esrl.noaa.gov/csd/projects/senex/) over the US Southeast, the
SOAS study (http://soas2013.rutgers.edu/) at the Southeastern Aerosol Research and Characterization (SEARCH)
(http://www.atmospheric-research.com/studies/SEARCH/index.html)
Centreville site near Brent, Alabama, and the SLAQRS study
(Baasandorj et al., 2015) in Greater Saint Louis, MO–IL. Flight
tracks and site locations are shown in Fig. 2.
HCOOH mixing ratios in the summertime boundary layer as
simulated by GEOS-Chem. Plotted is the June–September mean for P> 800 hPa. Also shown are the SENEX flight tracks (in black) and the SOAS and
SLAQRS ground site locations (grey square and circle, respectively).
HCOOH was measured by chemical ionization mass spectrometry during each of
the above campaigns, with analytical details as described by Brophy and
Farmer (2015) for SOAS and Baasandorj et al. (2015) for SLAQRS. Two groups measured HCOOH on-board the WP-3D
aircraft during SENEX: the University of Washington, with details described
by Lee et al. (2014), and NOAA CSD as described by Neuman
et al. (2002, 2010). For 1 min average data over the campaign, the
two data sets agree with R=0.90, a major axis slope of 0.89 (NOAA vs. UW),
and a mean relative error of 11 %. We use here the University of
Washington data set, but the conclusions are not significantly altered if the
NOAA CSD data set is used instead.
Additional chemical measurements shown below for SENEX include VOCs by
proton transfer reaction-mass spectrometry (de Gouw and Warneke,
2007), CO by vacuum ultraviolet resonance fluorescence
(Holloway et al., 2000), HCHO by laser-induced
fluorescence (Cazorla et al., 2015; DiGangi et al., 2011; Hottle et al.,
2009; Kaiser et al., 2014) as well as NO and NO2 by chemiluminescence
(Pollack et al., 2010; Ryerson et al., 2000). Additional measurements
shown for SLAQRS include VOCs by proton transfer reaction-mass spectrometry
(Baasandorj et al., 2015; Hu et al., 2011), and CO by gas chromatography
with a reducing compound photometer (Kim et al.,
2013).
During SOAS, VOCs were measured by gas chromatography-mass spectrometry
(Gilman et al., 2010), while
NO2 was measured by photolytic conversion to NO with subsequent
detection via chemiluminescence in excess ozone (CLD). The limit of
detection (LOD) for NO2 during SOAS was 0.1 ppb, while precision was
±4 % at an overall propagated uncertainty of ±15 %.
O3 measurements during SOAS employed a pressure and temperature
compensated UV absorption instrument (TEI-49i; Thermo Scientific, Franklin,
MA 02038, USA), with absolute calibration based on the known absorption
coefficient for O3 at 254 nm. The LOD for O3 during SOAS was 1.2 ppb,
precision was ±3 %, and overall uncertainty was ±6 %.
HNO3 was determined by difference during SOAS, with one instrument
channel measuring NO via CLD (as for NO2 above) downstream of a
350 ∘C Mo converter that quantitatively reduces ambient HNO3
to NO, and the other channel measuring NO downstream of a
Na2CO3-coated (1 % in deionized water) denuder that removes
nearly 100 % of ambient HNO3. The channels switch every 10 s. During
SOAS, the LOD for HNO3 was 60 pptv, precision was ±14 %, and
the overall uncertainty was ±17 %. SOAS also included mixing height
measurements via lidar (CHM 15k Nimbus ceilometer; Jenoptik AG, 07743 Jena,
Germany). The measurement is based on photon counting of back-scattered
pulses of near–IR light (1064 nm), and we assume here that the aerosol layer
detected closest to the ground represents the mixing height. The measurement
precision was ±4 % at an overall uncertainty of ±13 %.
Distribution of HCOOH over North AmericaVertical profiles of HCOOH and related species
Vertical profiles of HCOOH and related species during summer
over the US Southeast. Measurements from the SENEX campaign are plotted in
black, and are compared to simulated concentrations from GEOS-Chem (in red)
sampled along the flight track at the time of measurement. Horizontal lines
show the standard deviation of concentration in each altitude bin. Fresh
biomass burning (CH3CN > 225 ppt) and pollution
(NOx/ NOy> 0.4 or NO2> 4 ppb) plumes
have been removed prior to plotting. Separate lines are shown for the
simulated abundance of MVK+MACR and MVK+MACR plus isoprene
hydroxyhydroperoxides (a.k.a. ISOPOOH), which can interfere with MVK+MACR
measurements (Liu et al., 2013).
Figure 3 shows average vertical profiles of HCOOH and an ensemble of related
chemical species as measured over the US Southeast during SENEX. HCOOH
mixing ratios average ∼ 2.5 ppb near the surface, decreasing
to 0.25–0.7 ppb in the free troposphere. The measured concentrations
approach those of HCHO (mean of ∼ 4 ppb near the surface),
which is a ubiquitous oxidation product of isoprene and many other VOCs. The
high observed HCOOH concentrations indicate that this compound is a major
component of the reactive carbon budget, and (if secondary in origin) a
central product of VOC oxidation in the atmosphere.
Also shown in Fig. 3 are predicted concentrations from the GEOS-Chem
base-case simulation described above. Here and elsewhere, the model has been
sampled along the aircraft flight track at the time of measurement. We see
that the mean vertical profiles of CO, isoprene, the sum of methyl vinyl
ketone and methacrolein (MVK+MACR; both are isoprene oxidation products),
formaldehyde (HCHO), total monoterpenes (ΣMONOT), and NOx are
all well captured by the model. GEOS-Chem underpredicts CO in the free
troposphere, consistent with a low model bias in the CO background
(Kim et al., 2013), but otherwise the vertical
profiles are in good agreement with observations, in terms of both magnitude
and shape.
Simulated HCOOH binned and plotted as a function of the
observed concentrations during the SENEX campaign over the US Southeast.
Simulated values are shown as a stack plot, with HCOOH from the oxidation of
isoprene (green), other biogenics (blue), and other sources (red) adding to
give the total model amount. Vertical lines show the standard deviation of
the simulated abundance in each bin.
Conversely, simulated concentrations of HCOOH are dramatically low relative
to the aircraft data, averaging only ∼ 1 ppb near the surface.
A similar issue is apparent for CH3COOH. In addition to the boundary
layer bias, we see for both acids a major model underestimate in the free
troposphere: above 600 hPa, observed concentrations for both species average
0.25 ppb or more, whereas those in the model are a factor of 10 less at
10–40 ppt.
The comparisons shown in Fig. 3 do not provide any indication of a major
bias in the simulated emissions of isoprene, monoterpenes, or other HCHO
precursors that could come close to explaining the observed discrepancy for
HCOOH and CH3COOH. Likewise, overly vigorous boundary layer ventilation
is untenable as an explanation, based on the accurate model profile shapes
for the non-acid species, as well as the fact that both acids are biased low
in the model throughout the vertical column. The aircraft measurements
clearly demonstrate that some aspect of the model HCOOH budget is seriously
in error, and this supports other recent studies based on satellite and
in situ measurements (Cady-Pereira et al., 2014; Paulot et al., 2011;
Stavrakou et al., 2012). Since the sources of HCOOH are thought to be mainly
secondary in nature, this points to a significant gap in our current
understanding of hydrocarbon oxidation chemistry. In the next section we
apply tracer–tracer relationships measured and simulated during SENEX to
shed light on potential missing terms in the HCOOH budget.
Relationship between HCOOH and other chemical
tracers
Figure 4 shows the simulated HCOOH mixing ratios during SENEX from (i) isoprene oxidation, (ii) other
exclusively biogenic sources, and (iii) all other
sources, binned and plotted as a function of the observed HCOOH amount.
“Other sources” include photochemical production of HCOOH from primary VOCs
with anthropogenic or mixed origins (e.g., ethene) as well as direct HCOOH
emissions from non-biogenic sources. We see in the figure that when the
measured HCOOH concentrations are high, isoprene oxidation is the
predominant model source. By itself, this is not clear evidence that the
high observed HCOOH concentrations arise from isoprene oxidation, since the
modeled HCOOH from isoprene is strongly correlated with that from other
biogenic sources (R=0.92). On the other hand, the sole secondary HCOOH
source in the model that is purely anthropogenic (acetylene oxidation) has
only a weak correlation (R=0.29) with the HCOOH observations.
The strongest correlation between HCOOH and the extensive array of other
chemicals observed during SENEX, aside from other carboxylic acids, is with
methanol, with R=0.70 for the entire data set and R=0.68 within the
planetary boundary layer (PBL; P> 800 hPa). An independent analysis
(de Gouw et al., 2014) concluded that methanol
variability during SENEX was dominated by emissions from the terrestrial
biosphere, and this is consistent with findings from other studies (Hu et
al., 2011; Millet et al., 2008b; Stavrakou et al., 2011; Wells et al., 2012, 2014). The observed HCOOH–methanol relationship thus
provides an additional indication that biogenic sources are driving the
abundance of atmospheric HCOOH over this region.
Source partitioning of atmospheric HCOOH based on a
regression against methanol (as a biogenic tracer) and MEK (as a
predominantly anthropogenic tracer) during the SENEX aircraft campaign. The
figure shows the resulting attribution of the measured HCOOH abundance
(black) to biogenic (green) and other (anthropogenic + background; red)
sources.
Figure 5 shows a source attribution of atmospheric HCOOH based on multiple
regression of the SENEX observations against concurrent measurements of
methanol (as a biogenic tracer) and methyl ethyl ketone (MEK; as an
anthropogenic tracer), and with the intercept set to the lowest HCOOH
concentrations observed during the campaign (0.01 quantile; 0.1 ppb). MEK
exhibits the strongest correlation with HCOOH of any anthropogenic tracer
(R=0.42 within the PBL). While MEK is known to have some biogenic sources
(de Gouw et al., 1999; Jordan et al., 2009; Kirstine et al., 1998;
McKinney et al., 2011), it is thought to be mainly produced from the
oxidation of butane and other anthropogenic hydrocarbons (Jenkin et al.,
1997; Saunders et al., 2003). As we will see later, observations during
SLAQRS in Greater St. Louis imply a significant anthropogenic contribution
to atmospheric MEK in that location. We use it here as an anthropogenic
tracer; to the degree that MEK is affected by biogenic sources, the
associated anthropogenic HCOOH source fraction may be overstated.
We see in Fig. 5 that the resulting regression captures 74 % of the
variance in atmospheric HCOOH, and on average attributes 86 % of the
observed HCOOH abundance to biogenic sources and less than 15 % to other
sources. A bootstrap analysis gives 95 % uncertainty ranges of 82–90 %
and 10–18 % for the mean biogenic and other contributions, respectively,
while the variance inflation factor (VIF < 5) shows that the
regression is not unduly affected by multicollinearity. Here we have
transformed (squared) the methanol concentrations to give a linear
relationship with HCOOH, using instead the untransformed data yields a
smaller biogenic fraction (69 %) but a slightly degraded fit.
The above considerations point to biogenic VOC oxidation (or conceivably
direct biotic emissions) as the largest source of atmospheric HCOOH over
this part of North America. However, these findings leave room for a
comparable per-reaction yield of HCOOH for both biogenic and anthropogenic
VOCs, when one considers the emission disparity between the two. Biogenic
VOCs have been estimated to account for approximately 88 % of the total
(anthropogenic + biogenic) VOC flux from North America in carbon units
(Millet et al., 2008a), comparable to the mean biogenic contribution to
HCOOH derived above. In fact, similar amounts of HCOOH (∼ 2 ppb)
were recently observed during wintertime in the Uintah Basin (Utah,
US), where hydrocarbon reactivity is dominated by alkanes and aromatics
associated with oil and gas operations, and during summertime in Los Angeles
(CA, US), where isoprene and unsaturated anthropogenic compounds make up the
major part of the reactivity
(Yuan et al.,
2015). Production of HCOOH (and likely other carboxylic acids) appears
therefore to be a ubiquitous feature of atmospheric hydrocarbon oxidation
across a range of precursor types.
Drivers of temporal variability in HCOOHSOAS ground site, Alabama
Timeline of chemical and meteorological measurements during
the SOAS campaign (June–July 2013) near Brent AL, USA. The measured
concentrations of HCOOH in black are compared to the simulated values in
red.
Figure 6 shows concentrations of HCOOH and related species measured at the
SOAS ground site in Alabama during June and July 2013. This site is located
in a mixed deciduous forest 8 km from the small cities of Brent and
Centreville (combined population 7700). HCOOH concentrations ranged from
near-zero to more than 10 ppb during the SOAS study, with strong diurnal
fluctuations. The GEOS-Chem simulation is unable to reproduce the dynamic
range seen in the measurements, and exhibits a low bias (on average
3×) that is consistent with the SENEX comparisons above.
We also see in Fig. 6 substantial day-to-day variability in atmospheric
HCOOH driven by meteorological shifts. HCOOH concentrations are generally
elevated during the warm and sunny conditions that prevailed for much of the
period shown in the figure, but drop dramatically during cooler and cloudy
days (e.g., 185–189). As shown in Fig. 6, these patterns mirror a number
of other biogenic (e.g., methanol) and secondary (e.g., MEK, HNO3)
compounds measured at the site. In fact, the strongest HCOOH correlation at
SOAS is seen for HNO3 with R=0.78, followed closely by methanol at
R=0.74, likely reflecting their common drivers of variability –
sunlight-driven production and surface uptake/deposition.
Diurnal cycle of HCOOH and related species as measured
during SOAS. Error bars indicate ±1 (thick) and ±2 (thin)
standard errors about the observed mean (points).
Figure 7 shows mean diurnal cycles for HCOOH and selected other tracers over
the entire SOAS campaign. In the case of HCOOH, we see a pronounced morning
increase that parallels that of isoprene and appears to slightly precede
that of MVK+MACR. Following a peak in the afternoon, HCOOH concentrations
drop throughout the evening and night at a rate intermediate between O3
and HNO3. The GEOS-Chem simulation does not reproduce this large
diurnal amplitude: HCOOH concentrations are overestimated at night, the
prompt morning increase seen in the data is delayed and much too weak,
daytime concentrations are underestimated, and the strong evening decline is
not captured.
Errors in the model mixing heights may contribute to the above
discrepancies, but cannot be the main explanation. We see in Fig. S3 that
the GEOS-FP mixing heights for this location are generally too high during
the day, and at times appear too low at night. The daytime bias will
exacerbate the model HCOOH underestimate at that time; however, the HCOOH
discrepancy is too great to be rectified by a 30–50 % mixing height
correction. Furthermore, a model underestimate of the nocturnal boundary
layer depth should lead to an overprediction of near-surface HCOOH
deposition and depletion at night, whereas we see in Fig. 7 a clear
underprediction of this sink.
Nighttime measurements of HCOOH and related species over the
US Southeast. Top panel: timeline of HCOOH and altitude measurements during
a SENEX flight on the night of 1–2 July. The HCOOH trace is colored by time
of day. Middle left: map of the SENEX flight track over TN and AL with the
same color coding. The location of the SOAS ground site is indicated by the
grey square. Also plotted (in green) is the percentage tree cover according
to CLM4 (Oleson et al., 2010).
Remaining panels: vertical profiles of HCOOH and related species as measured
(black) and simulated (red) during this flight. Horizontal lines show the
standard deviation of concentration in each altitude bin. Fresh biomass
burning (CH3CN > 225 ppt) and pollution (NOx/ NOy> 0.4 or NO2> 4 ppb) plumes have been removed
prior to plotting. Separate lines are shown for the simulated abundance of
MVK+MACR and MVK+MACR plus isoprene hydroxyhydroperoxides (aka ISOPOOH),
which can interfere with MVK+MACR measurements (Liu et al.,
2013).
The prompt early-morning HCOOH increase seen during SOAS would seem to
implicate direct emissions rather than photochemical production, since the
rise occurs simultaneously with that of isoprene. However, we believe this
behavior is partly driven by a combination of residual layer entrainment and
increasing photochemical production over the course of the morning. The top
panel of Fig. 8 shows HCOOH measurements during a nighttime SENEX flight
on 2 July 2013 over Alabama and Tennessee. In the vicinity of the SOAS site,
HCOOH concentrations aloft are 1.5–2.7 ppb, whereas concentrations at the
ground drop to near-zero over the course of the night due to deposition
within the shallow surface layer. This elevated residual layer HCOOH is then
entrained into the HCOOH-depleted air at the surface as the mixed layer
develops after sunrise. We see similar behavior in Fig. 7 for O3 and
HNO3. In a further manifestation of this dynamic, SOAS featured several
nights with simultaneous enhancements of HCOOH and O3 associated with
episodic downmixing of residual layer air.
Figure 8 (bottom panels) also shows mean vertical profiles for HCOOH and
other VOCs measured during the same SENEX night flight. We see an inverted
HCOOH vertical profile at night, due to surface uptake, that is not present
in the model. The same model-measurement disparity is apparent for
MVK+MACR, HCHO, and CH3COOH, though to a lesser degree. We thus have
a reversal of the vertical gradient of HCOOH and other oxygenated VOCs
between the day and night. This progression can be seen in the top panel of
Fig. 8: early in the evening (prior to 183.10 UTC), low-altitude flight
segments are accompanied by elevated HCOOH, and vice versa. Later in the
night, the situation has reversed, with the high altitude segments generally
associated with more elevated HCOOH concentrations.
The model's inability to capture the evening HCOOH decline (Fig. 7) and
the nocturnal vertical gradient (Fig. 8) implies (i) a substantial
underestimate of the HCOOH surface sink, or (ii) overly vigorous model mixing
with overlying air during the night (thus replenishing HCOOH from aloft).
The former could arise for dynamical (e.g., an overestimate of the
aerodynamic and quasi-laminar resistances to deposition) or chemical (e.g.,
consumption of HCOOH by SCIs or other species) reasons. However, it cannot
be due to an underestimate of the surface resistance for HCOOH itself, since
as shown in Sect. 4.3 replacing the HCOOH deposition velocity with that
for HNO3 does not resolve the model bias in this regard.
Diurnal amplitude of HCOOH, Ox (O3+ NO2)
and MVK+MACR during SOAS. Left column: mean diurnal cycle of these species
as measured (black) and modeled (red) over the course of the campaign. Right
column: comparison of the mean modeled versus measured nighttime (19:00–06:00 LST) decay rates for the same species, plotted on a logarithmic scale.
Numbers inset give the major axis slope with 95 % confidence interval.
Figure 9 compares the modeled and measured nocturnal (19:00–06:00 LST) decay
of HCOOH, Ox (O3+ NO2), and MVK+MACR as an average over
the SOAS field campaign. Data are plotted on logarithmic axes; the slope
then estimates the model : observed ratio of deposition loss frequencies
(assuming deposition is the dominant process driving the nighttime decline).
We see that the effect of deposition on the ambient nighttime HCOOH
concentrations is underestimated in the model by a factor of 4–5. However, a
similar bias is apparent for Ox. This suggests a general dynamical bias
in the model rather than anything particular to HCOOH, or, perhaps, a
chemical sink for both Ox and HCOOH in surface air. A possibility for
the latter is terpenoids that consume O3, generating SCI that can then
consume or produce HCOOH. Figure 9 also shows that, unlike HCOOH and
Ox, the model slope for MVK+MACR is only 30 % lower than observed.
Overall, we see a strong diurnal cycle in HCOOH at the surface, driven by
depletion in a shallow surface layer at night, and entrainment of
HCOOH-enriched residual layer air plus photochemical production/direct
emissions during the day. These dynamics are not captured by the model, but
there is inconsistency in model performance for HCOOH and Ox versus
MVK+MACR. Improved constraints on surface uptake is a major need to
improve our understanding of oxygenated VOCs and other trace gases. If dry
deposition of HCOOH is in fact underestimated by the model, the magnitude of
its missing source becomes proportionately larger.
SLAQRS ground site, Greater St. Louis,
Missouri–Illinois
The SLAQRS study (August–September 2013) was based in East St. Louis, IL, within the
Greater Saint Louis metropolitan area. The edge of the Ozark Plateau, one of
the global hotspots for isoprene emission (and referred to as the “isoprene
volcano”; Wiedinmyer et al., 2005) lies
∼ 35 km to the south and west of the site. To the north and
east lie the predominantly non-isoprene-emitting agricultural landscapes of
northern Missouri, southern Illinois, and Iowa. As Fig. 10 shows, the
transport regime during the study shifted between northeasterly winds (or
stagnant conditions) with low isoprene concentrations, and southwesterly
winds that brought heavily isoprene-impacted air masses into the city (up to
8 ppb of isoprene transported from the Ozarks). Because of these regime
shifts the site provides a unique opportunity for examining the role of
biogenic VOCs in a polluted urban area.
Timeline of chemical measurements during the SLAQRS
campaign (August–September 2013) in Greater St. Louis MO–IL, USA. Green shading
indicates time periods with southwesterly winds (180–270∘; wind speed > 0.5 m s-1) from the Ozark
Plateau.
Diurnal cycle of HCOOH and related chemicals during SLAQRS
as a function of wind direction, with time of day (LST) plotted radially.
Plots generated using open air (Carslaw and Ropkins, 2012).
Figure 11 shows the mean diurnal cycle as a function of wind direction for a
number of species measured during SLAQRS. We see the highest concentrations
of anthropogenic compounds such as CO and benzene when winds are from the
northeast (partly reflecting an association with low wind speeds). On the
other hand, elevated amounts of isoprene and MVK+MACR are specifically
associated with southwesterly winds. Peak concentrations occur at night
because of rapid daytime photooxidation during transit from the Ozarks.
Also plotted in Fig. 11 are HCOOH and CH3COOH. In both cases we see
high concentrations (several ppb) from all sectors, but the highest amounts
clearly occur with the southwesterly winds that also bring elevated isoprene
and other biogenic oxidation products. HCOOH and CH3COOH are
longer lived than isoprene and MVK+MACR and are not depleted to the same
degree during transport; concentrations thus typically peak in the late
afternoon rather than at night.
The hottest conditions during SLAQRS occurred with southwesterly winds,
raising the question of whether the above HCOOH and CH3COOH
enhancements merely reflect accelerated chemistry at high temperatures, or a
correlating dynamical effect (e.g., stagnation) rather than a biogenic
origin. An examination of two other compounds with a substantial (acetone)
to dominant (MEK) secondary anthropogenic source, and comparable
photochemical lifetimes to HCOOH and CH3COOH (> 1 day at OH = 107 molec cm-3), reveals that this is not the case. While we
do see higher amounts of acetone and MEK with southwesterly winds (Fig. 11), the diurnal cycle is strikingly different than HCOOH and CH3COOH.
Here, the concentrations peak following the morning and evening rush hours
along with CO and toluene (Fig. S4), rather than in the late afternoon.
The SLAQRS data set is thus consistent with SENEX and SOAS in pointing
towards a major biogenic source of formic and acetic acids. However, while
the highest ambient levels are clearly linked to biogenic sources,
concentrations of several ppb are seen even when isoprene is low. This is
consistent with other measurements in urban areas (e.g., Le Breton et
al., 2012; Veres et al., 2011) and in an oil and gas producing area
(Yuan et al.,
2015). The overall indication is of a ubiquitous chemical source of HCOOH
(and likely other carboxylic acids) across a range of precursors. Since
biogenic emissions dominate the reactive carbon budget, they would then also
provide the bulk of the precursor material for HCOOH and related compounds.
Potential importance of other HCOOH source and sink pathways
In this section we examine the sensitivity of atmospheric HCOOH to a range
of other possible source and sink pathways, and assess the potential
importance of each in light of the large budget gaps discussed above.
SCI reaction with carboxylic acids
Recent advances in synthesizing and detecting SCIs (Taatjes et al., 2008;
Welz et al., 2012) have enabled significant (and quickly evolving) progress
in understanding their atmospheric chemistry. For instance, SCIs may also
provide a sink as well as a source of HCOOH and other carboxylic acids: Welz
et al. (2014) measured rate
coefficients for a set of SCI + HCOOH and SCI + CH3COOH reactions
and derived values ranging from 1.1–5 × 10-10 cm3 molec-1 s-1. Based on their results, we performed a sensitivity
simulation that includes this chemistry with rate coefficients of 1.1, 2.5,
and 1.0 × 10-10 cm3 molec-1 s-1, respectively,
for the reaction of CH2OO, CH3CHOO, and other SCIs with carboxylic
acids. This chemistry has the effect of both increasing the sink and
decreasing the source of HCOOH, since fewer SCIs go on to produce carboxylic
acids. However, we find that the overall effects are modest. Globally, the
relative importance of chemical loss and deposition as HCOOH sinks is
shifted slightly, with the former increasing by ∼ 11 % and
the latter decreasing by ∼ 6 % compared to the amounts in
Fig. 1. The overall HCOOH source is diminished slightly (4 %). Figure 12
shows that the SCI + carboxylic acid chemistry reduces the mean simulated
HCOOH abundance in surface air by 10–20 % for SENEX and SOAS relative to
the base model. We also find that this chemistry cannot explain the rapid
nighttime decay of HCOOH observed during SOAS: the rate of decline is not
significantly changed from the base-case slope in Fig. 9.
Sensitivity of atmospheric HCOOH to selected sources and
sinks in GEOS-Chem. Shown in black is the mean HCOOH vertical profile
observed during SENEX (top panel) and the mean HCOOH diurnal cycle observed
during SOAS (bottom panel). Colored lines show the corresponding simulated
amounts from GEOS-Chem for selected sensitivity runs described in the text.
Base: base-case simulation; SCI+Acids: including reactions between SCIs
and carboxylic acids (Welz et
al., 2014); Incr Dry Dep: setting the HCOOH dry deposition velocity equal to
that of HNO3; CH3O2+OH: including reaction between
CH3O2 and OH (Bossolasco et al., 2014) with a 100 %
yield of CH2OO; No GLYC/HAC: excluding HCOOH from isoprene-derived
glycoaldehyde and hydroxyacetone. Grey lines show example model adjustments
that can fit the mean observed SENEX profile. Solid grey: scaled source from
isoprene oxidation (3× base-case); dot-dashed grey: scaled source
from direct biogenic emissions (26× base-case); dotted grey: scaled
source from isoprene (1.8× base-case) combined with a ubiquitous
chemical source of HCOOH; dashed grey: scaled source from isoprene
(2.3× base-case) combined with a 56 % CH2OO yield from
CH3O2+ OH. See text for details.
Along with these atmospheric implications, the Welz et al. (2014) findings may also imply
that reported HCOOH yields from laboratory ozonolysis studies are biased low
(particularly for experiments done under dry conditions), due to suppression
of secondary organic acids by SCIs.
SCI reaction with water vapor and self-reaction
The predominant sink of the CH2OO SCI in our base-case simulation is
reaction with water vapor, and estimates of the kCH2OO+H2O rate
coefficient vary by 2–3 orders of magnitude (Newland et al., 2015; Stone
et al., 2014). However, we find that replacing our default rate
(kCH2OO+H2O=10-17 cm3 molec-1 s-1; Jenkin et
al., 1997; Saunders et al., 2003) with recent higher (1.3 × 10-15 cm3 molec-1 s-1; Newland et al.,
2015) and lower (5.4 × 10-18 cm3 molec-1 s-1;
Stone et al., 2014) estimates has a negligible impact on the simulated
HCOOH budget. This is because the competing model sinks for CH2OO
(SO2, CO, NO, NO2 – rates follow MCMv3.2; Jenkin et al., 1997;
Saunders et al., 2003) are sufficiently slow that reaction with water
dominates, even at kCH2OO+H2O=5.4×10-18 cm3 molec-1 s-1. For the same reason, including the CH2OO
self-reaction at 8 × 10-11 cm3 molec-1 s-1
(Ting et al., 2014) has no appreciable effect on the simulated
distribution of HCOOH.
It is worth pointing out, however, that Welz et al. (2012) and
Stone et al. (2014) were not able to directly measure the CH2OO +
H2O rate coefficient, instead reporting an upper limit (< 4 × 10-15 and < 9× 10-17 cm3 molec-1 s-1, respectively). If the actual rate is significantly
slower than the values applied here (or the rates for
competing SCI reactions are faster; e.g., Welz et al., 2012) then the role
of CH2OO + H2O as a source of HCOOH would decrease. Likewise,
the importance of the SCI + carboxylic acid reactions above will depend
directly on the rate of competing SCI + water vapor reactions.
Dry deposition
The strong temporal decline (Fig. 9) and vertical gradient (Fig. 8) of
HCOOH at night measured during SOAS and SENEX are not captured by the model,
perhaps indicating an underestimate of the HCOOH deposition velocity. To
test whether this is the case, we carried out a sensitivity analysis with
the HCOOH deposition velocity for each time and model location set to the
corresponding value computed for HNO3 (a highly soluble gas that
undergoes rapid and irreversible deposition). This leads to a 5 % drop in
the global HCOOH burden and a 20–25 % decrease in the mean surface air
concentrations simulated for SENEX and SOAS (Fig. 12). However, we also
see in Fig. 12 that the rapid nighttime decrease is still not captured by
the model, and the corresponding decay rate is statistically unchanged from
the base-case simulation (Fig. 9). It therefore is not feasible to rectify
this issue simply based on the modeled HCOOH surface resistance to
deposition. This, combined with the fact that the nighttime decay in Ox
during SOAS is underestimated by a similar amount, suggests a more general
dynamical bias in the model related to the diurnal cycle of boundary layer
mixing (or possibly a vigorous nighttime chemical sink for both species).
Additional sources of HCOOH : HCHO + HO2, CH3O2+ OH
Reversible addition of HO2 to HCHO produces the HOCH2OO radical,
which can go on to form HCOOH (Jenkin et al.,
2007). We find that implementing this chemistry in the same manner as Paulot
et al. (2011) leads to a
∼ 4 % global increase in the photochemical source of HCOOH.
Because of the strong temperature dependence of the reverse reaction
(Atkinson et al., 2006), the HCOOH increase manifests
mainly in the upper troposphere where its lifetime is long, and the increase
in the simulated global burden (17 %) is thus larger than that in the
source. Simulated concentrations in the lower free troposphere and in the
boundary layer are not significantly affected by the HO2+ HCHO
reaction, and the model-measurement comparisons discussed above are
statistically unchanged compared to the base-case run.
On the other hand, the CH3O2+ OH reaction is a major potential
lever on the atmospheric abundance of HCOOH, depending on the reaction
products. Including this reaction at the recently reported rate of
kCH3O2+OH=2.8×10-10 cm3 molec-1 s-1
(Bossolasco et al., 2014), and assuming that the CH2OO
Criegee intermediate is formed with 100 % yield, leads to a > 5× increase in the simulated global burden of HCOOH. The free
tropospheric HCOOH bias seen in the base-case simulation during SENEX is
eliminated (Fig. 12). However, the HCOOH source from CH3O2+
OH is spatially diffuse and cannot account for the enhanced boundary layer
concentrations measured during SOAS, SENEX, or SLAQRS. The daytime model
underestimate during SOAS is reduced while the nighttime overestimate is
increased (Fig. 12), and there is a slight degradation in the model
correlation with the airborne SENEX data set (R=0.66) compared to the
base-case run (R=0.71).
The above impacts depend both on the rate of the CH3O2+ OH
reaction and on the product yields, which have yet to be directly measured.
Here we have assumed that the reaction proceeds exclusively through
H-abstraction to yield CH2OO, and this was shown to have a large impact
on the atmospheric HCOOH budget. However, other reaction pathways are
possible: O-atom abstraction to yield CH3O + HO2, and SN2
substitution to yield CH3OH + O2 (Archibald et al., 2009;
Fittschen et al., 2014). The former would lead immediately to HCHO
production (as in the CH3O2+ NO pathway), while if the latter
dominates it would require a major revision to present understanding of the
global methanol budget (Millet et al., 2008b; Stavrakou et al., 2011;
Wells et al., 2014). The CH2OO yield for the analogous CH3O2+ Cl reaction has been estimated at 50 %
(Jungkamp et al., 1995; Maricq et al., 1994)
to 90 % (Daële and Poulet, 1996), while that for the
CH3O2+ BrO reaction was recently estimated at ∼ 80 % (Shallcross et al., 2015).
Vertical profiles of CH3OOH and the
NOx: CH3OOH ratio over North America. Shown are the mean measured
(black) and simulated (colored) values during the INTEX-A
(Singh et al., 2006) and INTEX-B
(Singh et al., 2009) flight campaigns. Red lines show
results from the GEOS-Chem base-case simulation, while the blue lines show
results from a sensitivity run that includes the CH3O2+ OH
reaction at kCH3O2+OH= 2.8 × 10-10 cm3 molec-1 s-1 (Bossolasco et al., 2014). Horizontal lines indicate
±1 (thick) and ±2 (thin) standard errors about the mean.
In any case, based on the Bossolasco et al. (2014) results,
oxidation of CH3O2 by OH would be an important reaction in low-NO
environments where we would otherwise expect CH3O2+ HO2 to
predominate, yielding CH3OOH + O2. Figure 13 shows mean vertical
profiles of CH3OOH and the NOx: CH3OOH ratio for two aircraft
campaigns where such data are available (INTEX-A and INTEX-B; Lee et
al., 1995; Singh et al., 2006; Singh et al., 2009). We see that including
the CH3O2+ OH reaction at 2.8 × 10-10 cm3 molec-1 s-1 degrades the model simulation of both CH3OOH and
the NOx: CH3OOH relationship for these data sets. This suggests
either that the reported CH3O2+ OH rate is too high, or the
presence of some offsetting model error in the base-case run. Clearly, this
chemistry has the potential to significantly alter our understanding of
several key chemical budgets, and developing better constraints on the
CH3O2+ OH rate coefficient and the resulting products should be
a high priority for future research.
Isoprene chemistry
Oxidation of glycoaldehyde and hydroxyacetone provides the largest
isoprene-derived HCOOH source in the model, and our representation of this
chemistry is based on the findings of Butkovskaya et al. (2006a, b). However, more recent work has
failed to detect any significant HCOOH production by these pathways
(Orlando et al., 2012). Excluding this source in the model decreases
the mean surface concentrations during SENEX by 40–45 % (Fig. 12), and
approximately doubles the model-measurement discrepancy (i.e., regression
slope) seen during this campaign. This would imply an even larger missing
source of atmospheric HCOOH than is otherwise required.
Discussion and implications
The analyses above clearly show a major model underestimate of HCOOH (and
CH3COOH) sources, a finding supported by other recent work (Le
Breton et al., 2012; Stavrakou et al., 2012; Veres et al., 2011; Yuan et
al., 2015). The fact that these organic acids are present in such large
amounts in the continental troposphere, with sources that from available
evidence are mainly photochemical, implies that some central aspect of the
atmospheric VOC oxidation chain is not currently understood.
Based on the observed patterns of variability and tracer–tracer correlations
discussed earlier, we infer that the missing HCOOH sources have a majority
biogenic component. However, elevated HCOOH amounts are seen even in
anthropogenically dominated air masses, and in the free troposphere,
suggesting that other processes are also at play. To illustrate the
magnitude of the inferred missing source, Fig. 12 shows (in grey) four
example model adjustments that can fit the mean observed SENEX profile.
These were derived by regressing the model-measurement residuals against the
simulated source contributions to atmospheric HCOOH.
Explaining the boundary layer HCOOH measured during SENEX based solely on isoprene oxidation requires a ∼ 3×
increase in the model HCOOH yield (which is 13 % is the base-case simulation) as shown in Fig. 12 (solid grey line). Accommodating such
a large HCOOH yield would require a significant revision to other product yields in order to conserve carbon in the isoprene oxidation chain.
Alternatively, explaining the SENEX observations solely on the basis of direct biogenic emissions of HCOOH would
require on the order of a 26× increase in its biotic flux. Extrapolating this over the North American domain
of Fig. S1 yields HCOOH emissions that are 27 % those of isoprene on a mass basis (8 % on a carbon basis).
However, we see in Fig. 12 that of these first two scenarios neither can explain the elevated free tropospheric
concentrations of HCOOH, nor can they account for the high HCOOH seen during low-isoprene periods in SLAQRS and in
the Uintah Basin during winter (Yuan et al., 2015).
An increased biogenic source of HCOOH (direct or secondary from isoprene/monoterpenes) plus some contribution
from CH3O2+ OH (yielding CH2OO and subsequently HCOOH) could potentially explain the SENEX
profile. For example, the dashed grey line in Fig. 12 shows the mean simulated HCOOH profile resulting from a 2.3×
increase in the source from isoprene and a 56 % CH2OO yield from CH3O2+ OH (or equivalently
a 100 % CH2OO yield and a 44 % reduction in kCH3O2+OH; Bossolasco et al., 2014). We are still
left, however, with substantial anthropogenic HCOOH enhancements (e.g., as observed recently in St. Louis, London, and Utah)
that cannot be explained by a diffuse source that is most important at low NOx.
Figure 12 also shows the model HCOOH profile that would result from an increase in the source from isoprene
combined with ubiquitous HCOOH production throughout the VOC oxidation cascade. Here, a 1.8× increase in the
HCOOH yield from isoprene and an aggregate weighted yield of 2 % from the ensemble of RO2+ NO reactions achieves an approximate fit to the mean SENEX profile.
Overall, it appears that an increased HCOOH source from isoprene (or other
biogenic source) combined with a widespread chemical source across a range
of precursor types is most tenable as an explanation for the full suite of
available atmospheric observations.
There are a number of key sources of uncertainty that need to be resolved to
close the HCOOH budget and thus improve our overall understanding of VOC
chemistry in the atmosphere. (i) The role of isoprene, and of other biogenic
compounds such as monoterpenes, in HCOOH production needs to be quantified.
There is conflicting laboratory evidence for the importance of
glycoaldehyde/hydroxyacetone chemistry in this context, and as we have seen
this has major implications for the HCOOH budget. As shown above, available
atmospheric observations leave room for a substantially larger HCOOH source
from isoprene oxidation than the ∼ 13 % used here as
base case (and which includes production from glycoaldehyde and
hydroxyacetone). The analyses here cannot segregate isoprene oxidation from
some other correlating HCOOH source such as direct surface emissions or
terpene oxidation. In fact, Stavrakou et al. (2012) postulated on the basis of satellite
observations from the Infrared Atmospheric Sounding Interferometer (IASI) that terpenoid emissions from boreal
forests were a large source of atmospheric HCOOH. However, given the
magnitude of the required HCOOH source inferred here, isoprene as (by far)
the largest source of reactive carbon to the atmosphere appears a probable
candidate. (ii) The atmospheric chemistry of SCIs (in particular their rates of
reaction with water vapor) needs to be better constrained in order to define
their importance as carboxylic acid sources and sinks, and as oxidants of
other critical atmospheric species (e.g., SO2). (iii) Reaction between
CH3O2 and OH (Bossolasco et al., 2014) could be of
significant importance for atmospheric chemistry, with implications for a
number of key chemical budgets. Including this chemistry degrades the model
simulation of CH3OOH and NOx: CH3OOH, though this might
reflect some offsetting model error (e.g., in the CH3OOH lifetime).
Reducing the uncertainty in the CH3O2+ OH reaction rate, and
determining the product yields, is a high priority for future research.
(iv) Finally, we have seen here that GEOS-Chem cannot capture the impact of
deposition on surface air concentrations of HCOOH. A similar issue is seen
for Ox, which may imply a common dynamical or chemical issue rather
than a deposition underestimate specific to HCOOH. In any case, developing a
more robust representation of surface deposition (and associated boundary
layer coupling) is needed to improve our understanding of land–atmosphere
interactions and our ability to relate observed concentrations to sources.
The Supplement related to this article is available online at doi:10.5194/acp-15-6283-2015-supplement.
Acknowledgements
This research was supported by the National Science
Foundation (grants #1148951 and 0937004) and by the Minnesota
Supercomputing Institute. We are indebted to Jean-François Müller,
John Orlando, Carl Percival, Andrew Rickard, Paul Shepson, Domenico
Taraborrelli, and Paul Wennberg for a number of illuminating discussions
that benefited this work. We thank John Holloway, Thomas Hanisco, Glenn
Wolfe, and Frank Keutsch for providing CO and HCHO measurements during
SENEX, as well as Ron Cohen, Bill Brune, David Tan, and Brian Heikes for
providing NO, NO2, and CH3OOH measurements during INTEX-A and
INTEX-B. SOAS measurements used here were performed at the Centreville, AL,
SEARCH site, which is funded by Southern Company and EPRI. We thank Bob
Yantosca for his work developing compatibility for GEOS-FP within GEOS-Chem.
We also thank Jay Turner as well as Dhruv Mitroo and the rest of the ACT Lab
at WUStL for their help during the SLAQRS deployment. BJW acknowledges the
US EPA Science to Achieve Results (STAR) program (grant #R835402) for
support during SLAQRS. HCHO measurements during SENEX were also supported by
EPA STAR (grant #83540601). This research has not been subjected to any
EPA review and therefore does not necessarily reflect the views of the
Agency, and no official endorsement should be
inferred.Edited by: J. Williams
References
Alam, M. S., Camredon, M., Rickard, A. R., Carr, T., Wyche, K. P., Hornsby,
K. E., Monks, P. S., and Bloss, W. J.: Total radical yields from tropospheric
ethene ozonolysis, Phys. Chem. Chem. Phys., 13, 11002–11015, 2011.Amos, H. M., Jacob, D. J., Holmes, C. D., Fisher, J. A., Wang, Q., Yantosca,
R. M., Corbitt, E. S., Galarneau, E., Rutter, A. P., Gustin, M. S., Steffen,
A., Schauer, J. J., Graydon, J. A., Louis, V. L. St., Talbot, R. W.,
Edgerton, E. S., Zhang, Y., and Sunderland, E. M.: Gas-particle partitioning
of atmospheric Hg(II) and its effect on global mercury deposition, Atmos.
Chem. Phys., 12, 591–603, 10.5194/acp-12-591-2012, 2012.Andreae, M. O., Talbot, R. W., Andreae, T. W., and Harriss, R. C.: Formic and
acetic acid over the central Amazon region, Brazil: 1. Dry season, J.
Geophys. Res., 93, 1616–1624, 1988.Andrews, D. U., Heazlewood, B. R., Maccarone, A. T., Conroy, T., Payne, R. J.,
Jordan, M. J. T., and Kable, S. H.: Photo-tautomerization of acetaldehyde to
vinyl alcohol: A potential route to tropospheric acids, Science, 337,
1203–1206, 2012.Archibald, A. T., McGillen, M. R., Taatjes, C. A., Percival, C. J., and
Shallcross, D. E.: Atmospheric transformation of enols: A potential secondary
source of carboxylic acids in the urban troposphere, Geophys. Res. Lett., 34,
L21801, 10.1029/2007GL031032, 2007.Archibald, A. T., Petit, A. S., Percival, C. J., Harvey, J. N., and Shallcross,
D. E.: On the importance of the reaction between OH and RO2 radicals,
Atmos. Sci. Lett., 10, 102–108, 2009.Atkinson, D. B. and Spillman, J. L.: Alkyl peroxy radical kinetics measured
using near-infrared CW-cavity ring-down spectroscopy, J. Phys. Chem. A, 106,
8891–8902, 2002.Atkinson, R., Baulch, D. L., Cox, R. A., Crowley, J. N., Hampson, R. F.,
Hynes, R. G., Jenkin, M. E., Rossi, M. J., Troe, J., and IUPAC Subcommittee:
Evaluated kinetic and photochemical data for atmospheric chemistry: Volume II
– gas phase reactions of organic species, Atmos. Chem. Phys., 6, 3625–4055,
10.5194/acp-6-3625-2006, 2006.Baasandorj, M., Millet, D. B., Hu, L., Mitroo, D., and Williams, B. J.:
Measuring acetic and formic acid by proton-transfer-reaction mass
spectrometry: sensitivity, humidity dependence, and quantifying
interferences, Atmos. Meas. Tech., 8, 1303–1321,
10.5194/amt-8-1303-2015, 2015.Bannan, T. J., Bacak, A., Muller, J. B. A., Booth, A. M., Jones, B., Breton,
M. L., Leather, K. E., Ghalaieny, M., Xiao, P., Shallcross, D. E., and
Percival, C. J.: Importance of direct anthropogenic emissions of formic acid
measured by a chemical ionisation mass spectrometer (CIMS) during the Winter
ClearfLo Campaign in London, January 2012, Atmos. Environ., 83, 301–310,
2014.Bohn, B., Siese, M., and Zetzschn, C.: Kinetics of the OH + C2H2
reaction in the presence of O2, J. Chem. Soc., Faraday Trans., 92,
1459–1466, 1996.Bossolasco, A., Faragó, E. P., Schoemaecker, C., and Fittschen, C.: Rate
constant of the reaction between CH3O2 and OH radicals, Chem. Phys.
Lett., 593, 7–13, 2014.Brophy, P. and Farmer, D. K.: A switchable reagent ion high resolution
time-of-flight chemical ionization mass spectrometer for real-time
measurement of gas phase oxidized species: characterization from the 2013
Southern Oxidant and Aerosol Study, Atmos. Meas. Tech. Discuss., 8,
3199–3244, 10.5194/amtd-8-3199-2015, 2015.Buras, Z. J., Elsamra, R. M. I., and Green, W. H.: Direct determination of the
simplest Criegee intermediate (CH2OO) self reaction rate, J. Phys. Chem.
Lett., 5, 2224–2228, 2014.Butkovskaya, N. I., Pouvesle, N., Kukui, A., Mu, Y., and Le Bras, G.:
Mechanism of the OH-initiated oxidation of hydroxyacetone over the
temperature range 236–298 K, J. Phys. Chem. A, 110, 6833–6843, 2006a.Butkovskaya, N. I., Pouvesle, N., Kukui, A., and Le Bras, G.: Mechanism of
the OH-initiated oxidation of glycoaldehyde over the temperature range
233–296 K, J. Phys. Chem. A, 110, 13492–13499, 2006b.Cady-Pereira, K. E., Chaliyakunnel, S., Shephard, M. W., Millet, D. B., Luo,
M., and Wells, K. C.: HCOOH measurements from space: TES retrieval algorithm
and observed global distribution, Atmos. Meas. Tech., 7, 2297–2311,
10.5194/amt-7-2297-2014, 2014.Carslaw, D. C. and Ropkins, K.: openair – An R package for air quality data
analysis, Environ. Modell. Softw., 27–28, 52–61,
10.1016/j.envsoft.2011.09.008, 2012.Cazorla, M., Wolfe, G. M., Bailey, S. A., Swanson, A. K., Arkinson, H. L.,
and Hanisco, T. F.: A new airborne laser-induced fluorescence instrument for
in situ detection of formaldehyde throughout the troposphere and lower
stratosphere, Atmos. Meas. Tech., 8, 541–552, 10.5194/amt-8-541-2015,
2015.Chao, W., Hsieh, J.-T., Chang, C.-H., and Lin, J. J.-M.: Direct kinetic
measurement of the reaction of the simplest Criegee intermediate with water
vapor, Science, 347, 751–754, 2015.Chebbi, A. and Carlier, P.: Carboxylic acids in the troposphere,
occurrence, sources, and sinks: A review, Atmos. Environ., 30, 4233–4249,
1996.Chhantyal-Pun, R., Davey, A., Shallcross, D. E., Percival, C. J., and
Orr-Ewing, A. J.: A kinetic study of the CH2OO Criegee Intermediate
self-reaction, reaction with SO2 and unimolecular reaction using cavity
ring-down spectroscopy, Phys. Chem. Chem. Phys., 17, 3617–3626, 2015.Clubb, A. E., Jordan, M. J. T., Kable, S. H., and Osborn, D. L.:
Phototautomerization of acetaldehyde to vinyl alcohol: A primary process in
UV-irradiated acetaldehyde from 295 to 335 nm, J. Phys. Chem. Lett., 3,
3522–3526, 2012.Crounse, J. D., Paulot, F., Kjaergaard, H. G., and Wennberg, P. O.: Peroxy
radical isomerization in the oxidation of isoprene, Phys. Chem. Chem. Phys.,
13, 13607–13613, 2011.Crounse, J. D., Knap, H. C., Ornso, K. B., Jorgensen, S., Paulot, F.,
Kjaergaard, H. G., and Wennberg, P. O.: Atmospheric fate of methacrolein. 1.
Peroxy radical isomerization following addition of OH and O2, J. Phys.
Chem. A, 116, 5756–5762, 2012.Daële, V. and Poulet, G.: Kinetics and products of the reactions of
CH3O2 with Cl and ClO, J. Chim. Phys. PCB, 93, 1081–1099, 1996.da Silva, G.: Carboxylic acid catalyzed keto-enol tautomerizations in the
gas phase, Angew. Chem. Int. Ed., 49, 7523–7525, 2010.de Gouw, J. A., Howard, C. J., Custer, T. G., and Fall, R.: Emissions of
volatile organic compounds from cut grass and clover are enhanced during the
drying process, Geophys. Res. Lett., 26, 811–814, 1999.de Gouw, J. A. and Warneke, C.: Measurements of volatile organic compounds in
the Earth's atmosphere using proton-transfer-reaction mass spectrometry, Mass
Spectrom. Rev., 26, 223–257, 2007.de Gouw, J. A., Middlebrook, A., Brock, C., Gilman, J., Graus, M., Holloway,
J., Lerner, B., Liao, J., Trainer, M., Warneke, C., and Welti, A.: Formation
of organic aerosol in the outflow from urban areas in the southeastern United
States, Goldschmidt Abstracts, 520, 2014.DiGangi, J. P., Boyle, E. S., Karl, T., Harley, P., Turnipseed, A., Kim, S.,
Cantrell, C., Maudlin III, R. L., Zheng, W., Flocke, F., Hall, S. R.,
Ullmann, K., Nakashima, Y., Paul, J. B., Wolfe, G. M., Desai, A. R., Kajii,
Y., Guenther, A., and Keutsch, F. N.: First direct measurements of
formaldehyde flux via eddy covariance: implications for missing in-canopy
formaldehyde sources, Atmos. Chem. Phys., 11, 10565–10578,
10.5194/acp-11-10565-2011, 2011.Duncan, B. N., Logan, J. A., Bey, I., Megretskaia, I. A., Yantosca, R. M.,
Novelli, P. C., Jones, N. B., and Rinsland, C. P.: Global budget of CO,
1988–1997: Source estimates and validation with a global model, J. Geophys.
Res., 112, D22301, 10.1029/2007JD008459, 2007.Eliason, T. L., Aloisio, S., Donaldson, D. J., Cziczo, D. J., and Vaida, V.:
Processing of unsaturated organic acid films and aerosols by ozone, Atmos.
Environ., 37, 2207–2219, 2003.Faragó, E. P., Viskolcz, B., Schoemaecker, C., and Fittschen, C.:
Absorption spectrum and absolute absorption cross sections of CH3O2
radicals and CH3I molecules in the wavelength range 7473–7497
cm-1, J. Phys. Chem. A, 117, 12802–12811, 2013.Fischer, E. V., Jacob, D. J., Millet, D. B., Yantosca, R. M., and Mao, J.: The
role of the ocean in the global atmospheric budget of acetone, Geophys. Res.
Lett., 39, L01807, 10.1029/2011GL050086, 2012.Fittschen, C., Whalley, L. K., and Heard, D. E.: The reaction of
CH3O2 radicals with OH radicals: A neglected sink for
CH3O2 in the remote atmosphere, Environ. Sci. Technol., 48,
7700–7701, 2014.Galloway, J. N., Likens, G. E., Keene, W. C., and Miller, J. M.: The composition
of precipitation in remote areas of the world, J. Geophys. Res., 87,
8771–8786, 1982.Gilman, J. B., Burkhart, J. F., Lerner, B. M., Williams, E. J., Kuster, W.
C., Goldan, P. D., Murphy, P. C., Warneke, C., Fowler, C., Montzka, S. A.,
Miller, B. R., Miller, L., Oltmans, S. J., Ryerson, T. B., Cooper, O. R.,
Stohl, A., and de Gouw, J. A.: Ozone variability and halogen oxidation within
the Arctic and sub-Arctic springtime boundary layer, Atmos. Chem. Phys., 10,
10223-10236, 10.5194/acp-10-10223-2010, 2010.Glasius, M., Wessel, S., Christensen, C. S., Jacobsen, J. K., Jorgensen, H. E.,
Klitgaard, K. C., Petersen, L., Rasmussen, J. K., Hansen, T. S., Lohse, C.,
Boaretto, E., and Heinemeier, J.: Sources to formic acid studied by carbon
isotopic analysis and air mass characterization, Atmos. Environ., 34,
2471–2479, 2000.
Glasius, M., Boel, C., Bruun, N., Easa, L. M., Hornung, P., Klausen, H. S.,
Klitgaard, K. C., Lindeskov, C., Moller, C. K., Nissen, H., Petersen, A. P.
F., Kleefeld, S., Boaretto, E., Hansen, T.S., Heinemeier, J., and Lohse, C.:
Relative contribution of biogenic and anthropogenic sources to formic and
acetic acids in the atmospheric boundary layer, J. Geophys. Res., 106,
7415–7426, 2001.Goode, J. G., Yokelson, R. J., Ward, D. E., Susott, R. A., Babbitt, R. E.,
Davies, M. A., and Hao, W. M.: Measurements of excess O3, CO2, CO,
CH4, C2H4, C2H2, HCN, NO, NH3, HCOOH,
CH3COOH, HCHO, and CH3OH in 1997 Alaskan biomass burning plumes by
airborne fourier transform infrared spectroscopy (AFTIR), J. Geophys. Res.,
105, 22147–22166, 2000.Guenther, A., Karl, T., Harley, P., Wiedinmyer, C., Palmer, P. I., and Geron,
C.: Estimates of global terrestrial isoprene emissions using MEGAN (Model of
Emissions of Gases and Aerosols from Nature), Atmos. Chem. Phys., 6,
3181–3210, 10.5194/acp-6-3181-2006, 2006.Guenther, A. B., Jiang, X., Heald, C. L., Sakulyanontvittaya, T., Duhl, T.,
Emmons, L. K., and Wang, X.: The Model of Emissions of Gases and Aerosols
from Nature version 2.1 (MEGAN2.1): an extended and updated framework for
modeling biogenic emissions, Geosci. Model Dev., 5, 1471–1492,
10.5194/gmd-5-1471-2012, 2012.Hasson, A. S., Orzechowska, G., and Paulson, S. E.: Production of stabilized
Criegee intermediates and peroxides in the gas phase ozonolysis of alkenes:
1. Ethene,trans-2-butene, and 2,3-dimethyl-2-butene, J. Geophys. Res., 106,
34131–34142, 2001.Hatakeyama, S. and Akimoto, H.: Reactions of Criegee intermediates in the
gas phase, Res. Chem. Intermed., 20, 503–524, 1994.Hatakeyama, S., Washida, N., and Akimoto, H.: Rate constants and mechanisms
for the reaction of OH (OD) radicals with acetylene, propyne, and 2-butyne in
air at 297 ± 2 K, J. Phys. Chem., 90, 173–178, 1986.Hatch, C. D., Gough, R. V., and Tolbert, M. A.: Heterogeneous uptake of the C1 to C4 organic acids on a swelling clay mineral, Atmos. Chem. Phys., 7, 4445–4458, 10.5194/acp-7-4445-2007, 2007.Heald, C. L., Ridley, D. A., Kreidenweis, S. M., and Drury, E. E.: Satellite
observations cap the atmospheric organic aerosol budget, Geophys. Res.
Lett., 37, L24808, 10.1029/2010GL045095, 2010.Holloway, J. S., Jakoubek, R. O., Parrish, D. D., Gerbig, C., Volz-Thomas, A.,
Schmitgen, S., Fried, A., Wert, B., Henry, B., and Drummond, J. R.: Airborne
intercomparison of vacuum ultraviolet fluorescence and tunable diode laser
absorption measurements of tropospheric carbon monoxide, J. Geophys. Res.,
105, 24251–24261, 2000.Hottle, J. R., Huisman, A. J., Digangi, J. P., Kammrath, A., Galloway, M. M.,
Coens, K. L., and Keutsch, F. N.: A laser induced fluorescence-based
instrument for in-situ measurements of atmospheric formaldehyde, Environ.
Sci. Technol., 43, 790–795, 2009.Hu, L., Millet, D. B., Mohr, M. J., Wells, K. C., Griffis, T. J., and Helmig,
D.: Sources and seasonality of atmospheric methanol based on tall tower
measurements in the US Upper Midwest, Atmos. Chem. Phys., 11, 11145–11156,
10.5194/acp-11-11145-2011, 2011.Hu, L., Millet, D. B., Baasandorj, M., Griffis, T. J., Travis, K. R., Tessum,
C. W., Marshall, J. D., Reinhart, W. F., Mikoviny, T., Müller, M.,
Wisthaler, A., Graus, M., Warneke, C., and de Gouw, J.: Emissions of
C6-C8 aromatic compounds in the United States: Constraints from
tall tower and aircraft measurements, J. Geophys. Res., 120, 826–842,
10.1002/2014JD022627, 2015a.Hu, L., Millet, D. B., Baasandorj, M., Griffis, T. J., Turner, P., Helmig, D.,
Curtis, A. J., and Hueber, J.: Isoprene emissions and impacts over an
ecological transition region in the US Upper Midwest inferred from tall tower
measurements, J. Geophys. Res., 120, 3553–3571, 10.1002/2014JD022732,
2015b.Jacob, D. J.: Chemistry of OH in remote clouds and its role in the production
of formic acid and peroxymonosulfate, J. Geophys. Res., 91, 9807–9826, 1986.Jenkin, M. E., Cox, R. A., Emrich, M., and Moortgat, G. K.: Mechanisms of the
Cl-atom-initiated oxidation of acetone and hydroxyacetone in air, J. Chem.
Soc., Faraday Trans., 89, 2983–2991, 1993.Jenkin, M. E., Saunders, S. M., and Pilling, M. J.: The tropospheric
degradation of volatile organic compounds: A protocol for mechanism
development, Atmos. Environ., 31, 81–104, 1997.Jenkin, M. E., Hurley, M. D., and Wallington, T. J.: Investigation of the
radical product channel of the CH3COO2+ HO2 reaction in
the gas phase, Phys. Chem. Chem. Phys., 9, 3149–3162, 2007.Jordan, C., Fitz, E., Hagan, T., Sive, B., Frinak, E., Haase, K., Cottrell,
L., Buckley, S., and Talbot, R.: Long-term study of VOCs measured with PTR-MS
at a rural site in New Hampshire with urban influences, Atmos. Chem. Phys.,
9, 4677–4697, 10.5194/acp-9-4677-2009, 2009.Jungkamp, T. P. W., Kukui, A., and Schindler, R. N.: Determination of rate
constants and product branching ratios for the reactions of CH3O2
and CH3O with Cl atoms at room temperature, Ber. Bunsen. Phys. Chem.,
99, 1057–1066, 1995.Kaiser, J., Li, X., Tillmann, R., Acir, I., Holland, F., Rohrer, F., Wegener,
R., and Keutsch, F. N.: Intercomparison of Hantzsch and
fiber-laser-induced-fluorescence formaldehyde measurements, Atmos. Meas.
Tech., 7, 1571–1580, 10.5194/amt-7-1571-2014, 2014.Karton, A.: Inorganic acid-catalyzed tautomerization of vinyl alcohol to
acetaldehyde, Chem. Phys. Lett., 592, 330–333, 2014.Kawamura, K., Ng, L. L., and Kaplan, I. R.: Determination of organic acids
(C1-C10) in the atmosphere, motor exhausts, and engine oils, Environ. Sci.
Technol., 19, 1082–1086, 1985.Keene, W. C. and Galloway, J. N.: Organic acidity in precipitation of North
America, Atmos. Environ., 18, 2491–2497, 1984.Keene, W. C., Galloway, J. N., and Holden, J. D.: Measurement of weak organic
acidity in precipitation from remote areas of the world, J. Geophys. Res.,
88, 5122–5130, 1983.Kesselmeier, J.: Exchange of short-chain oxygenated volatile organic
compounds (VOCs) between plants and the atmosphere: A compilation of field
and laboratory studies, J. Atmos. Chem., 39, 219–233, 2001.Kesselmeier, J. and Staudt, M.: Biogenic volatile organic compounds (VOC):
An overview on emission, physiology and ecology, J. Atmos. Chem., 33, 23–88,
1999.Kesselmeier, J., Bode, K., Gerlach, C., and Jork, E. M.: Exchange of
atmospheric formic and acetic acids with trees and crop plants under
controlled chamber and purified air conditions, Atmos. Environ., 32,
1765–1775, 1998.Kim, S. Y., Millet, D. B., Hu, L., Mohr, M. J., Griffis, T. J., Wen, D., Lin,
J. C., Miller, S. M., and Longo, M.: Constraints on carbon monoxide emissions
based on tall tower measurements in the U.S. Upper Midwest, Environ. Sci.
Technol., 47, 8316–8324, 2013.Kirstine, W., Galbally, I., Ye, Y. R., and Hooper, M.: Emissions of volatile
organic compounds (primarily oxygenated species) from pasture, J. Geophys.
Res., 103, 10605–10619, 1998.Kuhn, U., Rottenberger, S., Biesenthal, T., Ammann, C., Wolf, A., Schebeske,
G., Oliva, S. T., Tavares, T. M., and Kesselmeier, J.: Exchange of
short-chain monocarboxylic acids by vegetation at a remote tropical forest
site in Amazonia, J. Geophys. Res., 107, 8069, 10.1029/2000JD000303,
2002.Larsen, B. R., Di Bella, D., Glasius, M., Winterhalter, R., Jensen, N. R., and
Hjorth, J.: Gas-phase OH oxidation of monoterpenes: Gaseous and particulate
products, J. Atmos. Chem., 38, 231–276, 2001.Le Breton, M., McGillen, M. R., Muller, J. B. A., Bacak, A., Shallcross, D. E., Xiao, P., Huey, L. G., Tanner, D., Coe, H., and Percival,
C. J.: Airborne observations of formic acid using a chemical ionization mass spectrometer, Atmos. Meas. Tech., 5, 3029–3039, 10.5194/amt-5-3029-2012, 2012.Lee, A., Goldstein, A. H., Keywood, M. D., Gao, S., Varutbangkul, V.,
Bahreini, R., Ng, N. L., Flagan, R. C., and Seinfeld, J. H.: Gas-phase
products and secondary aerosol yields from the ozonolysis of ten different
terpenes, J. Geophys. Res., 111, D07302, 10.1029/2005JD006437, 2006a.Lee, A., Goldstein, A. H., Kroll, J. H., Ng, N. L., Varutbangkul, V., Flagan,
R. C., and Seinfeld, J. H.: Gas-phase products and secondary aerosol yields
from the photooxidation of 16 different terpenes, J. Geophys. Res., 111,
D17305, 10.1029/2006JD007050, 2006b.Lee, B. H., Lopez-Hilfiker, F. D., Mohr, C., Kurtén, T., Worsnop, D. R.,
and Thornton, J. A.: An iodide-adduct high-resolution time-of-flight
chemical-ionization mass spectrometer: Application to atmospheric inorganic
and organic compounds, Environ. Sci. Technol., 48, 6309–6317, 2014.Lee, J. H., Leahy, D. F., Tang, I. N., and Newman, L.: Measurement and
speciation of gas phase peroxides in the atmosphere, J. Geophys. Res., 98,
2911–2915, 1993.Lee, M., Noone, B. C., O'Sullivan, D., and Heikes, B. G.: Method for the
collection and HPLC analysis of hydrogen peroxide and C1 and C2
hydroperoxides in the atmosphere, J. Atmos. Oceanic Technol., 12,
1060–1070, 1995.Lelieveld, J. and Crutzen, P. J.: The role of clouds in tropospheric
chemistry, J. Atmos. Chem., 12, 229–267, 1991.Lin, J.-T. and McElroy, M.: Impacts of boundary layer mixing on pollutant
vertical profiles in the lower troposphere: Implications to satellite remote
sensing, Atmos. Environ., 44, 1726–1739, 10.1016/j.atmosenv.2010.02.009,
2010.Lin, S.-J. and Rood, R. B.: Multidimensional flux form semi-Lagrangian
transport schemes, Mon. Weather Rev., 124, 2046–2070, 1996.Liu, J., Zhang, X., Parker, E. T., Veres, P. R., Roberts, J. M., Gouw, J. A. D.,
Hayes, P. L., Jimenez, J. L., Murphy, J. G., Ellis, R. A., Huey, L. G., and
Weber, R. J.: On the gas-particle partitioning of soluble organic aerosol in
two urban atmospheres with contrasting emissions: 2. Gas and particle phase
formic acid, J. Geophys. Res., 117, D00V21, 10.1029/2012JD017912, 2012.Liu, Y. J., Herdlinger-Blatt, I., McKinney, K. A., and Martin, S. T.:
Production of methyl vinyl ketone and methacrolein via the hydroperoxyl
pathway of isoprene oxidation, Atmos. Chem. Phys., 13, 5715–5730,
10.5194/acp-13-5715-2013, 2013.Mao, J., Jacob, D. J., Evans, M. J., Olson, J. R., Ren, X., Brune, W. H.,
St. Clair, J. M., Crounse, J. D., Spencer, K. M., Beaver, M. R., Wennberg, P.
O., Cubison, M. J., Jimenez, J. L., Fried, A., Weibring, P., Walega, J. G.,
Hall, S. R., Weinheimer, A. J., Cohen, R. C., Chen, G., Crawford, J. H.,
McNaughton, C., Clarke, A. D., Jaeglé, L., Fisher, J. A., Yantosca, R. M.,
Le Sager, P., and Carouge, C.: Chemistry of hydrogen oxide radicals (HOx) in
the Arctic troposphere in spring, Atmos. Chem. Phys., 10, 5823–5838,
10.5194/acp-10-5823-2010, 2010.Mao, J., Paulot, F., Jacob, D. J., Cohen, R. C., Crounse, J. D., Wennberg,
P. O., Keller, C. A., Hudman, R. C., Barkley, M. P., and Horowitz, L. W.:
Ozone and organic nitrates over the eastern United States: Sensitivity to
isoprene chemistry, J. Geophys. Res., 118, 11256–11268,
10.1002/jgrd.50817, 2013a.Mao, J., Fan, S., Jacob, D. J., and Travis, K. R.: Radical loss in the
atmosphere from Cu-Fe redox coupling in aerosols, Atmos. Chem. Phys., 13,
509–519, 10.5194/acp-13-509-2013, 2013b.Maricq, M. M., Szente, J. J., Kaiser, E. W., and Shi, J.: Reaction of chlorine
atoms with methylperoxy and ethylperoxy radicals, J. Phys. Chem., 98,
2083–2089, 1994.McKinney, K. A., Lee, B. H., Vasta, A., Pho, T. V., and Munger, J. W.:
Emissions of isoprenoids and oxygenated biogenic volatile organic compounds from a New England mixed forest, Atmos. Chem. Phys., 11, 4807–4831, 10.5194/acp-11-4807-2011, 2011.Millet, D. B., Jacob, D. J., Boersma, K. F., Fu, T. M., Kurosu, T. P.,
Chance, K., Heald, C. L., and Guenther, A.: Spatial distribution of isoprene
emissions from North America derived from formaldehyde column measurements by
the OMI satellite sensor, J. Geophys. Res., 113, D02307,
10.1029/2007JD008950, 2008a.Millet, D. B., Jacob, D. J., Custer, T. G., de Gouw, J. A., Goldstein, A. H.,
Karl, T., Singh, H. B., Sive, B. C., Talbot, R. W., Warneke, C., and
Williams, J.: New constraints on terrestrial and oceanic sources of
atmospheric methanol, Atmos. Chem. Phys., 8, 6887–6905,
10.5194/acp-8-6887-2008, 2008b.Millet, D. B., Guenther, A., Siegel, D. A., Nelson, N. B., Singh, H. B., de
Gouw, J. A., Warneke, C., Williams, J., Eerdekens, G., Sinha, V., Karl, T.,
Flocke, F., Apel, E., Riemer, D. D., Palmer, P. I., and Barkley, M.: Global
atmospheric budget of acetaldehyde: 3-D model analysis and constraints from
in-situ and satellite observations, Atmos. Chem. Phys., 10, 3405–3425,
10.5194/acp-10-3405-2010, 2010.Molina, M. J., Ivanov, A. V., Trakhtenberg, S., and Molina, L. T.: Atmospheric
evolution of organic aerosol, Geophys. Res. Lett., 31, L22104,
10.1029/2004GL020910, 2004.Myneni, R. B., Yang, W., Nemani, R. R., Huete, A. R., Dickinson, R. E.,
Knyazikhin, Y., Didan, K., Fu, R., Negron Juarez, R. I., Saatchi, S. S.,
Hashimoto, H., Ichii, K., Shabanov, N. V., Tan, B., Ratana, P., Privette, J.
L., Morisette, J. T., Vermote, E. F., Roy, D. P., Wolfe, R. E., Friedl, M.
A., Running, S. W., Votava, P., El-Saleous, N., Devadiga, S., Su, Y., and
Salomonson, V. V.: Large seasonal swings in leaf area of Amazon rainforests,
P. Natl. Acad. Sci. USA, 104, 4820–4823, 2007.Neeb, P., Horie, O., and Moortgat, G. K.: Gas-phase ozonolysis of ethene in
the presence of hydroxylic compounds, Int. J. Chem. Kinet., 28, 721–730,
1996.Neeb, P., Sauer, F., Horie, O., and Moortgat, G. K.: Formation of
hydroxymethyl hydroperoxide and formic acid in alkene ozonolysis in the
presence of water vapour, Atmos. Environ., 31, 1417–1423, 1997.Neuman, J. A., Huey, L. G., Dissly, R. W., Fehsenfeld, F. C., Flocke, F.,
Holecek, J. C., Holloway, J. S., Hübler, G., Jakoubek, R., Nicks Jr., D.
K., Parrish, D. D., Ryerson, T. B., Sueper, D. T., and Weinheimer, J.:
Fast-response airborne in situ measurements of HNO3 during the Texas
2000 Air Quality Study, J. Geophys. Res., 107, 4436,
10.1029/2001JD001437, 2002.Neuman, J. A., Nowak, J. B., Huey, L. G., Burkholder, J. B., Dibb, J. E.,
Holloway, J. S., Liao, J., Peischl, J., Roberts, J. M., Ryerson, T. B.,
Scheuer, E., Stark, H., Stickel, R. E., Tanner, D. J., and Weinheimer, A.:
Bromine measurements in ozone depleted air over the Arctic Ocean, Atmos.
Chem. Phys., 10, 6503–6514, 10.5194/acp-10-6503-2010, 2010.Newland, M. J., Rickard, A. R., Alam, M. S., Vereecken, L., Muñoz, A.,
Ródenas, M., and Bloss, W. J.: Kinetics of stabilised Criegee
intermediates derived from alkene ozonolysis: reactions with SO2,
H2O and decomposition under boundary layer conditions, Phys. Chem. Chem.
Phys., 17, 4076–4088, 2015.Ngwabie, N. M., Schade, G. W., Custer, T. G., Linke, S., and Hinz, T.:
Abundances and flux estimates of volatile organic compounds from a dairy
cowshed in Germany, J. Environ. Qual., 37, 565–573, 2008.Oleson, K. W., Lawrence, D. M., Bonan, G. B., Flanner, M. G., Kluzek, E.,
Lawrence, P. J., Levis, S., Swenson, S. C., Thornton, P. E., Dai, A., Decker,
M., Dickinson, R., Feddema, J., Heald, C. L., Hoffman, F., Lamarque, J.,
Mahowald, N., Niu, G., Qian, T., Randerson, J., Running, S., Sakaguchi, K.,
Slater, A., Stockli, R., Wang, A., Yang, Z., Zeng, X., and Zeng, X.:
Technical Description of version 4.0 of the Community Land Model (CLM),
National Center for Atmospheric Research, Technical Note NCAR/TN-478+STR,
10.5065/D6FB50WZ, Boulder, CO, 2010.Olivier, J. G. J., van Aardenne, J. A., Dentener, F. J., Pagliari, V.,
Ganzeveld, L. N., and Peters, J. A. H. W.: Recent trends in global greenhouse
gas emissions: Regional trends 1970-2000 and spatial distribution of key
sources in 2000, Environ. Sci., 2, 81–99, 10.1080/15693430500400345,
2005.Orlando, J. J., Nozière, B., Tyndall, G. S., Orzechowska, G. E., Paulson,
S. E., and Rudich, Y.: Product studies of the OH- and ozone-initiated
oxidation of some monoterpenes, J. Geophys. Res., 105, 11561–11572, 2000.Orlando, J. J., Tyndall, G. S., and Taraborrelli, D.:, Atmospheric
oxidation of two isoprene by-products, hydroxyacetone and glycoaldehyde,
Abstract A33L-0315, in 2012 Fall Meeting, AGU, San Francisco, Calif., 3–7
December, 2012.Orzechowska, G. E. and Paulson, S. E.: Photochemical sources of organic
acids. 1. Reactions of ozone with isoprene, propene, and 2-butenes under dry
and humid conditions using SPME, J. Phys. Chem. A, 109, 5358–5365, 2005.Pan, X., Underwood, J. S., Xing, J.-H., Mang, S. A., and Nizkorodov, S. A.: Photodegradation of secondary organic aerosol generated from
limonene oxidation by ozone studied with chemical ionization mass spectrometry, Atmos. Chem. Phys., 9, 3851–3865, 10.5194/acp-9-3851-2009, 2009.Park, J., Gomez, A. L., Walser, M. L., Lin, A., and Nizkorodov, S. A.:
Ozonolysis and photolysis of alkene-terminated self-assembled monolayers on
quartz nanoparticles: implications for photochemical aging of organic aerosol
particles, Phys. Chem. Chem. Phys., 8, 2506–2512, 2006.Paulot, F., Crounse, J. D., Kjaergaard, H. G., Kroll, J. H., Seinfeld, J. H.,
and Wennberg, P. O.: Isoprene photooxidation: new insights into the
production of acids and organic nitrates, Atmos. Chem. Phys., 9, 1479–1501,
10.5194/acp-9-1479-2009, 2009a.Paulot, F., Crounse, J. D., Kjaergaard, H. G., Kurten, A., St Clair, J. M.,
Seinfeld, J. H., and Wennberg, P. O.: Unexpected epoxide formation in the
gas-phase photooxidation of isoprene, Science, 325, 730–733, 2009b.Paulot, F., Wunch, D., Crounse, J. D., Toon, G. C., Millet, D. B., DeCarlo,
P. F., Vigouroux, C., Deutscher, N. M., González Abad, G., Notholt, J.,
Warneke, T., Hannigan, J. W., Warneke, C., de Gouw, J. A., Dunlea, E. J., De
Mazière, M., Griffith, D. W. T., Bernath, P., Jimenez, J. L., and Wennberg,
P. O.: Importance of secondary sources in the atmospheric budgets of formic
and acetic acids, Atmos. Chem. Phys., 11, 1989–2013,
10.5194/acp-11-1989-2011, 2011.Pollack, I. B., Lerner, B. M., and Ryerson, T. B.: Evaluation of ultraviolet
light-emitting diodes for detection of atmospheric NO2 by photolysis –
chemiluminescence, J. Atmos. Chem., 65, 111–125, 2010.Pushkarsky, M. B., Zalyubovsky, S. J., and Miller, T. A.: Detection and
characterization of alkyl peroxy radicals using cavity ringdown spectroscopy,
J. Chem. Phys., 112, 10695–10698, 2000.R'Honi, Y., Clarisse, L., Clerbaux, C., Hurtmans, D., Duflot, V., Turquety,
S., Ngadi, Y., and Coheur, P.-F.: Exceptional emissions of NH3 and HCOOH
in the 2010 Russian wildfires, Atmos. Chem. Phys., 13, 4171–4181,
10.5194/acp-13-4171-2013, 2013.Ryerson, T. B., Williams, E. J., and Fehsenfeld, F. C.: An efficient photolysis
system for fast-response NO2 measurements, J. Geophys. Res., 105,
26447–26461, 2000.Ryzhkov, A. B. and Ariya, P. A.: A theoretical study of the reactions of
carbonyl oxide with water in atmosphere: The role of water dimer, Chem. Phys.
Lett., 367, 423–429, 2003.Sander, R.: Compilation of Henry's law constants (version 4.0) for water as
solvent, Atmos. Chem. Phys., 15, 4399–4981, 10.5194/acp-15-4399-2015,
2015.Sanhueza, E. and Andreae, M. O.: Emission of formic and acetic acids from
tropical savanna soils, Geophys. Res. Lett., 18, 1707–1710, 1991.Sauer, F., Beck, J., Schuster, G., and Moortgat, G. K.: Hydrogen peroxide,
organic peroxides and organic acids in a forested area during FIELDVOC'94,
Chemosphere, 3, 309–326, 2001.Saunders, S. M., Jenkin, M. E., Derwent, R. G., and Pilling, M. J.: Protocol
for the development of the Master Chemical Mechanism, MCM v3 (Part A):
tropospheric degradation of non-aromatic volatile organic compounds, Atmos.
Chem. Phys., 3, 161–180, 10.5194/acp-3-161-2003, 2003.Schultz, M. G., Backman, L., Balkanski, Y., Bjoerndalsaeter, S., Brand, R.,
Burrows, J. P., Dalsoeren, S., Vasconcelos, M. D., Grodtmann, B.,
Hauglustaine, D. A., Heil, A., Hoelzemann, J. J., Isaksen, I. S. A., Kaurola,
J., Knorr, W., Ladstaetter-Weißenmayer, A., Mota, B., Oom, D., Pacyna,
J., Panasiuk, D., Pereira, J. M. C., Pulles, T., Pyle, J., Rast, S., Richter,
A., Savage, N., Schnadt, C., Schulz, M., Spessa, A., Staehelin, J., Sundet,
J. K., Szopa, S., Thonicke, K., van het Bolscher, M., Noije, T. V.,
Velthoven, P. V., Vik, A. F., and Wittrock, F.: REanalysis of the
TROpospheric chemical composition over the past 40 years – A long-term
global modeling study of tropospheric chemistry, available at:
http://retro.enes.org/ (last access: 1 January 2012), 5th EU framework
programme, Jülich/Hamburg, Germany, 2007.Shallcross, D. E., Leather, K. E., Bacak, A., Xiao, P., Lee, E. P. F., Ng, M.,
Mok, D. K. W., Dyke, J. M., Hossaini, R., Chipperfield, M. P., Khan, M. A.
H., and Percival, C. J.: Reaction between CH3O2 and BrO radicals: A
new source of upper troposphere lower stratosphere hydroxyl radicals, J.
Phys. Chem. A, 119, 4618–4632, 10.1021/jp5108203, 2015.Singh, H. B., Brune, W. H., Crawford, J. H., Jacob, D. J., and Russell, P. B.:
Overview of the summer 2004 intercontinental chemical transport experiment –
North America (INTEX-A), J. Geophys. Res., 111, D24S01,
10.1029/2006JD007905, 2006.Singh, H. B., Brune, W. H., Crawford, J. H., Flocke, F., and Jacob, D. J.:
Chemistry and transport of pollution over the Gulf of Mexico and the Pacific:
spring 2006 INTEX-B campaign overview and first results, Atmos. Chem. Phys.,
9, 2301–2318, 10.5194/acp-9-2301-2009, 2009.So, S., Wille, U., and da Silva, G.: Atmospheric chemistry of enols: A
theoretical study of the vinyl alcohol + OH + O2 reaction mechanism,
Environ. Sci. Technol., 48, 6694–6701, 2014.Stavrakou, T., Guenther, A., Razavi, A., Clarisse, L., Clerbaux, C., Coheur,
P.-F., Hurtmans, D., Karagulian, F., De Mazière, M., Vigouroux, C.,
Amelynck, C., Schoon, N., Laffineur, Q., Heinesch, B., Aubinet, M., Rinsland,
C., and Müller, J.-F.: First space-based derivation of the global
atmospheric methanol emission fluxes, Atmos. Chem. Phys., 11, 4873–4898,
10.5194/acp-11-4873-2011, 2011.Stavrakou, T., Müller, J.-F., Peeters, J., Razavi, A., Clarisse, L.,
Clerbaux, C., Coheur, P.-F., Hurtmans, D., Mazière, M. D., Vigouroux, C.,
Deutscher, N. M., Griffith, D. W. T., Jones, N., and Paton-Walsh, C.:
Satellite evidence for a large source of formic acid from boreal and tropical
forests, Nat. Geosci., 5, 26–30, 2012.Stone, D., Blitz, M., Daubney, L., Howes, N. U., and Seakins, P.: Kinetics of
CH2OO reactions with SO2, NO2, NO, H2O and CH3CHO as
a function of pressure, Phys. Chem. Chem. Phys., 16, 1139–1149, 2014.Su, Y.-T., Lin, H.-Y., Putikam, R., Matsui, H., Lin, M. C., and Lee, Y.-P.:
Extremely rapid self-reaction of the simplest Criegee intermediate CH2OO
and its implications in atmospheric chemistry, Nature Chem., 6, 477–483,
2014.Taatjes, C. A., Meloni, G., Selby, T. M., Trevitt, A. J., Osborn, D. L.,
Percival, C. J., and Shallcross, D. E.: Direct observation of the gas-phase
Criegee intermediate (CH2OO), J. Am. Chem. Soc., 130, 11883–11885,
2008.Talbot, R. W., Beecher, K. M., Harriss, R. C., and Cofer, W. R.: Atmospheric
geochemistry of formic and acetic acids at a mid-latitude temperate site, J.
Geophys. Res., 93, 1638–1652, 1988.Ting, W. L., Chang, C. H., Lee, Y. F., Matsui, H., Lee, Y. P., and Lin, J. J.:
Detailed mechanism of the CH2I + O2 reaction: Yield and
self-reaction of the simplest Criegee intermediate CH2OO, J. Chem.
Phys., 141, 104308, 10.1063/1.4894405, 2014.Valverde-Canossa, J., Ganzeveld, L., Rappenglück, B., Steinbrecher, R.,
Klemm, O., Schuster, G., and Moortgat, G. K.: First measurements of
H2O2 and organic peroxides surface fluxes by the relaxed
eddy-accumulation technique, Atmos. Environ., 40, S55–S67, 2006.van der Werf, G. R., Randerson, J. T., Giglio, L., Collatz, G. J., Mu, M.,
Kasibhatla, P. S., Morton, D. C., DeFries, R. S., Jin, Y., and van Leeuwen,
T. T.: Global fire emissions and the contribution of deforestation, savanna,
forest, agricultural, and peat fires (1997–2009), Atmos. Chem. Phys., 10,
11707–11735, 10.5194/acp-10-11707-2010, 2010.Vereecken, L., Harder, H., and Novelli, A.: The reaction of Criegee
intermediates with NO, RO2, and SO2, and their fate in the
atmosphere, Phys. Chem. Chem. Phys., 14, 14682–14695, 2012.Veres, P. R., Roberts, J. M., Cochran, A. K., Gilman, J. B., Kuster, W. C.,
Holloway, J. S., Graus, M., Flynn, J., Lefer, B., Warneke, C., and de Gouw,
J.: Evidence of rapid production of organic acids in an urban air mass,
Geophys. Res. Lett., 38, L17807, 10.1029/2011GL048420, 2011.Vlasenko, A., George, I. J., and Abbatt, J. P. D.: Formation of volatile
organic compounds in the heterogeneous oxidation of condensed-phase organic
films by gas-phase OH, J. Phys. Chem. A, 112, 1552–1560, 2008.Walser, M. L., Park, J., Gomez, A. L., Russell, A. R., and Nizkorodov, S. A.:
Photochemical aging of secondary organic aerosol particles generated from the
oxidation of d-limonene, J. Phys. Chem. A, 111, 1907–1913, 2007.Wang, Y. H., Jacob, D. J., and Logan, J. A.: Global simulation of tropospheric
O3-NOx-hydrocarbon chemistry: 1. Model formulation, J. Geophys.
Res., 103, 10713–10725, 1998.Weinstein-Lloyd, J. B., Lee, J. H., Daum, P. H., Kleinman, L. I., Nunnermacker,
L. J., Springston, S. R., and Newman, L.: Measurements of peroxides and
related species during the 1995 summer intensive of the Southern Oxidants
Study in Nashville, Tennessee, J. Geophys. Res., 103, 22361–22373, 1998.Wells, K. C., Millet, D. B., Hu, L., Cady-Pereira, K. E., Xiao, Y., Shephard, M. W.,
Clerbaux, C. L., Clarisse, L., Coheur, P. F., Apel, E. C., de Gouw, J., Warneke, C., Singh, H. B., Goldstein, A. H., and Sive, B. C.:
Tropospheric methanol observations from space: retrieval evaluation and constraints on the seasonality of biogenic emissions, Atmos. Chem. Phys., 12, 5897–5912, 10.5194/acp-12-5897-2012, 2012.Wells, K. C., Millet, D. B., Cady-Pereira, K. E., Shephard, M. W., Henze, D.
K., Bousserez, N., Apel, E. C., de Gouw, J., Warneke, C., and Singh, H. B.:
Quantifying global terrestrial methanol emissions using observations from the
TES satellite sensor, Atmos. Chem. Phys., 14, 2555–2570,
10.5194/acp-14-2555-2014, 2014.
Welz, O., Savee, J. D., Osborn, D. L., Vasu, S. S., Percival, C. J., Shallcross,
D. E., and Taatjes, C. A.: Direct kinetic measurements of Criegee
Intermediate (CH2OO) formed by reaction of CH2I with O2,
Science, 335, 204–207, 2012.Welz, O., Eskola, A. J., Sheps, L., Rotavera, B., Savee, J. D., Scheer, A. M.,
Osborn, D. L., Lowe, D., Murray Booth, A., Xiao, P., Anwar, H. K. M.,
Percival, C. J., Shallcross, D. E., and Taatjes, C. A.: Rate coefficients of
C1 and C2 Criegee intermediate reactions with formic and acetic acid near the
collision limit: Direct kinetics measurements and atmospheric implications,
Angew. Chem. Int. Ed., 53, 4547–4550, 2014.Wesely, M. L.: Parameterization of surface resistances to gaseous dry
deposition in regional-scale numerical models, Atmos. Environ., 23,
1293–1304, 1989.Wiedinmyer, C., Greenberg, J., Guenther, A., Hopkins, B., Baker, K., Geron,
C., Palmer, P. I., Long, B. P., Turner, J. R., Petron, G., Harley, P.,
Pierce, T. E., Lamb, B., Westberg, H., Baugh, W., Koerber, M., and Janssen,
M.: Ozarks Isoprene Experiment (OZIE): Measurements and modeling of the
“isoprene volcano”, J. Geophys. Res., 110, D18307,
10.1029/2005JD005800, 2005.Wu, S. L., Mickley, L. J., Jacob, D. J., Logan, J. A., Yantosca, R. M., and Rind,
D.: Why are there large differences between models in global budgets of
tropospheric ozone?, J. Geophys. Res., 112, D05302,
10.1029/2006JD007801, 2007.Yevich, R. and Logan, J.A.: An assessment of biofuel use and burning of
agricultural waste in the developing world, Global Biogeochem. Cy., 17,
1095, 10.1029/2002GB001952, 2003.Yuan, B., Veres, P. R., Warneke, C., Roberts, J. M., Gilman, J. B., Koss, A.,
Edwards, P. M., Graus, M., Kuster, W. C., Li, S.-M., Wild, R. J., Brown, S.
S., Dubé, W. P., Lerner, B. M., Williams, E. J., Johnson, J. E., Quinn, P.
K., Bates, T. S., Lefer, B., Hayes, P. L., Jimenez, J. L., Weber, R. J.,
Zamora, R., Ervens, B., Millet, D. B., Rappenglück, B., and de Gouw, J. A.:
Investigation of secondary formation of formic acid: urban environment vs.
oil and gas producing region, Atmos. Chem. Phys., 15, 1975–1993,
10.5194/acp-15-1975-2015, 2015.Zander, R., Duchatelet, P., Mahieu, E., Demoulin, P., Roland, G., Servais,
C., Auwera, J. V., Perrin, A., Rinsland, C. P., and Crutzen, P. J.: Formic
acid above the Jungfraujoch during 1985–2007: observed variability,
seasonality, but no long-term background evolution, Atmos. Chem. Phys., 10,
10047–10065, 10.5194/acp-10-10047-2010, 2010.