Introduction
Organic aerosol (OA) is increasingly recognized as a globally important
component of the fine particulate matter that exerts a large but uncertain
negative radiative forcing on Earth's climate (Myhre et al., 2013) and
adversely affects human health around the world (Lelieveld et al., 2015).
This global importance is complicated by large regional differences in OA
concentrations relative to other sources of aerosol such as black carbon,
sulfate, nitrate and sea salt. OA comprises 20 %–50 % of total fine
aerosol mass at continental mid-latitudes, but more in urban environments and
biomass burning plumes, and up to 90 % over tropical forests (Kanakidou
et al., 2005; Zhang et al., 2007). Outside of urban centers and fresh biomass
burning plumes, the majority of this OA is secondary organic aerosol (SOA)
(Jimenez et al., 2009), produced by oxidation of directly emitted volatile
organic compounds followed by partitioning into the aerosol phase. Forests
are strong biogenic VOC emitters, in the form of isoprene
(C5H8), monoterpenes (C10H16), and
sesquiterpenes (C15H24), all of which are readily oxidized by
the three major atmospheric oxidants, OH, NO3 and O3. The
total global source of biogenic SOA from such reactions remains highly
uncertain, with a view estimating it at 90±90 TgC yr-1
(Hallquist et al., 2009), a large fraction of which may be anthropogenically
controlled (Spracklen et al., 2011; Goldstein et al., 2009; Carlton et al.,
2010; Hoyle et al., 2011). As most NO3 arises from anthropogenic
emissions, OA production from NO3 + isoprene is one mechanism
that could allow for the anthropogenic control of biogenic SOA mass loading.
Isoprene constitutes nearly half of all global VOC emissions to the
atmosphere, with a flux of ∼600 Tg yr-1 (Guenther et al., 2006). As a result, accurate global
biogenic SOA budgets depend strongly on yields from isoprene oxidation.
Recent global modeling efforts find that isoprene SOA is produced at rates
from 14 (Henze and Seinfeld, 2006; Hoyle et al., 2007) to
19 TgC yr-1
(Heald et al., 2008), which implies that it could
constitute 27 % (Hoyle et al., 2007), 48 % (Henze
and Seinfeld, 2006) or up to 78 % (Heald et al., 2008)
of total SOA (based also on varying estimates of total SOA burden in each
study). More recent observational constraints on SOA yield from isoprene
find complex temperature-dependent mechanisms that could affect vertical
distributions (Worton et al., 2013) and suggest that isoprene SOA
constitutes from 17 % (Hu et al., 2015) to 40 % (Kim et al.,
2015) and up to 48 % (Marais
et al., 2016) of total OA in the southeastern United States (SEUS). This large
significance comes despite isoprene's low SOA mass yields, two recent
observational studies estimated the total isoprene SOA mass yield to be
∼3 % (Marais et al., 2016; Kim et al., 2015), and
modeling studies typically estimate isoprene SOA yields to be 4 % to 10 %,
depending on the oxidant, in contrast to monoterpenes' yields of 10 % to
20 % and sesquiterpenes' yields of > 40 % (Pye et al.,
2010). Furthermore, laboratory studies of SOA mass yields may have a
tendency to underestimate these yields, if they cannot access the longer
timescales of later-generation chemistry, or are otherwise run under
conditions that limit oxidative aging of first-generation products
(Carlton et al., 2009).
Laboratory chamber studies of SOA mass yield at OA loadings of
∼10 µg m-3 from isoprene have typically found low
yields from O3 (1 % Kleindienst et al., 2007) and OH
(2 % at low NOx to 5 % at high NOx
Kroll et al., 2006; Dommen et al., 2009; 1.3 % at low
NOx and neutral seed aerosol pH but rising to 29 % in
the presence of acidic sulfate seed aerosol due to reactive uptake of
epoxydiols of isoprene IEPOX Surratt et al.,
2010). One recent chamber study on OH-initiated isoprene SOA formation
focused on the fate of second-generation RO2 radical found
significantly higher yields, up to 15 % at low NOx
(Liu et al., 2016), suggesting that omitting
later-generation oxidation chemistry could be an important limitation of
early chamber determinations of isoprene SOA yields. Another found an
increase in SOA formed with increasing HO2 to RO2 ratios,
suggesting that RO2 fate could also play a role in the variability of
previously reported SOA yields (D'Ambro et al., 2017).
For NO3 oxidation of isoprene, early chamber experiments already
pointed to higher yields (e.g., 12 % Ng et al., 2008) than for OH
oxidation. Ng et al. (2008)
also observed chemical regime differences: SOA yields were approximately two
times larger when chamber conditions were tuned such that first-generation
peroxy radical fate was RO2+RO2 dominated than when it was
RO2+NO3 dominated. In addition, Rollins et al. (2009) observed a
significantly higher SOA yield (14 %) from second-generation NO3
oxidation than that when only one double bond was oxidized (0.7 %). This
points to the possibility that later-generation, RO2+RO2
dominated isoprene + NO3 chemistry may be an even more substantial
source of SOA than what current chamber studies have captured. Schwantes et
al. (2015) investigated the gas-phase
products of NO3 + isoprene in the RO2+HO2 dominated regime
and found the major product to be isoprene nitrooxy hydroperoxide (INP,
75 %–78 % molar yield), which can photochemically convert to isoprene
nitrooxy hydroxyepoxide (INHE), a molecule that might contribute to SOA
formation via heterogeneous uptake similar to IEPOX. Here again, multiple
generations of chemistry are required to produce products that may contribute
to SOA.
As the SOA yield appears to be highest for NO3 radical
oxidation, and isoprene is such an abundantly emitted BVOC, oxidation of
isoprene by NO3 may be an important source of OA in areas with
regional NOx pollution. As the SOA yield with neutral
aerosol seed appears to be an order of magnitude larger than that from other
oxidants, even if only 10 % of isoprene is oxidized by NO3, it
will produce comparable SOA to daytime photo-oxidation. For example, Brown et
al. (2009)
concluded that NO3 contributed more SOA from isoprene than OH over
New England, where > 20 % of isoprene emitted during the
previous day was available at sunset to undergo dark oxidation by either
NO3 or O3. The corresponding contribution to total SOA mass
loading was 1 %–17 % based on laboratory yields (Ng et al., 2017).
Rollins et al. (2012) concluded
that multi-generational NO3 oxidation of biogenic precursors was
responsible for one-third of nighttime organic aerosol increases during the
CalNex-2010 experiment in Bakersfield, CA. In an aircraft study near Houston,
TX, Brown et al. (2013) observed elevated organic aerosol in the nighttime boundary layer, and
correlated vertical profiles of organic and nitrate aerosol in regions with
rapid surface level NO3 radical production and BVOC emissions. From
these observations, the authors estimated an SOA source from NO3 + BVOCs
within the nocturnal boundary layer of 0.05–1 µg m-3 h-1.
Carlton et al. (2009) note the large
scatter in chamber-measured SOA yields from isoprene photooxidation and point
throughout their review of SOA formation from isoprene to the likely
importance of poorly understood later generations of chemistry in explaining
field observations. We suggest that similar differences in multi-generational
chemistry could explain the variation among the (sparse) chamber and field
observations of NO3 + isoprene yields described in the previous
paragraph, and summarized in a recent review of NO3 + BVOC oxidation
mechanisms and SOA formation (Ng et al., 2017).
The initial products of NO3 + isoprene include organic nitrates, some of
which will partially partition to the aerosol phase. Organic nitrates in the
particle phase (pRONO2) are challenging to quantify with online methods,
due to both interferences and their often overall low concentrations in
ambient aerosol. Hence, field datasets to constrain modeled pRONO2 are
sparse (Ng et al., 2017; Fisher et al., 2016). One of the most used methods
in recent studies, used also here, is quantification with the Aerodyne
Aerosol Mass Spectrometer (AMS). Organic nitrates thermally decompose in the
AMS vaporizer and different approaches have been used to apportion the
organic fraction contributing to the total nitrate signal. Allan et al. (2004a)
first proposed the use of nitrate peaks at m/z 30 and 46 to distinguish various nitrate species with the AMS.
Marcolli et al. (2006), in the first reported tentative assignment of aerosol organic nitrate
using AMS data, used cluster analysis to analyze data from the 2002 New
England Air Quality Study. In that study, cluster analysis identified two
categories with high m/z 30 contributions. One of these peaked in the
morning when NOx was abundant and was more prevalent in
plumes with the lowest photochemical ages, potentially from isoprene oxidation
products. The second was observed throughout the diurnal cycle in both fresh
and aged plumes, and contained substantial m/z 44 contribution (highly
oxidized OA). A subsequent AMS laboratory and field study discussed and
further developed methods for separate quantification of organic nitrate (in
contrast to inorganic nitrate) (Farmer et al., 2010). A
refined version of one of these separation methods, based on the differing
NO2+ / NO+ fragmentation ratio for organic vs. inorganic nitrate,
was later employed to quantify organic nitrate aerosol at two forested rural
field sites where strong biogenic VOC emissions and relatively low
NOx
combined to make substantial organic nitrate aerosol concentrations (Fry
et al., 2013; Ayres et al., 2015). Most recently, Kiendler-Scharr et al. (2016)
used a variant of this method to conclude
that across Europe, organic nitrates comprise ∼40 % of
submicron organic aerosol. Modeling analysis concluded that a substantial
fraction of this organic nitrate aerosol is produced via NO3 radical
initiated chemistry. Chamber studies have employed this fragmentation ratio
method to quantify organic nitrates (Boyd et al., 2015; Bruns et al.,
2010; Fry et al., 2009, 2011; Rollins et al., 2009), providing the
beginnings of a database of typical organonitrate fragmentation ratios from
various BVOC precursors.
Measurements conducted at the SOAS ground site in Centreville, Alabama in
2013 found evidence of significant organonitrate contribution to SOA mass
loading. Xu et al. (2015) reported that organic nitrates constituted 5 %
to 12 % of total organic aerosol mass from AMS data applying a variant of
the NO2+ / NO+ ratio method. They identify a
nighttime-peaking “LO-OOA” AMS factor, which they attribute to mostly
NO3 oxidation of BVOC (in addition to O3 + BVOC). They
estimated that the NO3 radical oxidizes 17 % of isoprene,
20 % of α-pinene, and 38 % of β-pinene in the
nocturnal boundary layer at this site. However, applying laboratory-based SOA
yields to model the predicted increase in OA, Xu et al. (2015) predict only
0.7 µg m-3 of SOA would be produced, substantially lower than
the measured nighttime LO-OOA production of 1.7 µg m-3. The
more recent analysis of Zhang et al. (2018) found a strong correlation of
monoterpene SOA with the fraction of monoterpene oxidation attributed to
NO3, even for non-nitrate containing aerosol, suggesting an influence
of NO3 even in pathways that ultimately eliminate the nitrate
functionality from the SOA, such as hydrolysis or NO2 regeneration.
Ayres et al. (2015) used a correlation of overnight organonitrate aerosol
buildup with calculated net NO3 + monoterpene and isoprene
reactions to estimate an overall NO3 + monoterpene SOA mass yield
of 40 %–80 %. The factor of two range in this analysis was based on
two different measurements of aerosol-phase organic nitrates. These authors
used similar correlations to identify specific CIMS-derived molecular
formulae that are likely to be NO3 radical chemistry products of
isoprene and monoterpenes, and found minimal contribution of identified
first-generation NO3 + isoprene products to the aerosol phase (as
expected based on their volatility). Lee et al. (2016) detected abundant
highly functionalized particle-phase organic nitrates at the same site, with
apparent origin both from isoprene and monoterpenes, and both daytime and
nighttime oxidation, and estimated their average contribution to submicron
organic aerosol mass to be between 3 %–8 %. For the same ground
campaign, Romer et al. (2016) found evidence of rapid conversion from alkyl
nitrates to HNO3, with total alkyl nitrates having an average daytime
lifetime of 1.7 h.
Xie et al. (2013) used a model
constrained by observed alkyl nitrate correlations with O3 from the
INTEX-NA/ICARTT 2004 field campaign to determine a range of isoprene nitrate
lifetimes between 4 and 6 h, with 40 %–50 % of isoprene nitrates formed
by NO3 + isoprene reactions. Laboratory studies show that not all
organic nitrates hydrolyze to HNO3 equally rapidly: primary and
secondary organic nitrates were found to be less prone to aqueous hydrolysis
than tertiary organic nitrates (Darer et al., 2011; Hu et al., 2011; Fisher
et al., 2016; Boyd et al., 2015). This suggests that field-based estimates of
the contribution of organic nitrates to SOA formation could be a lower limit,
if they are based on measurement of those aerosol-phase nitrates. This is
because if hydrolysis is rapid, releasing HNO3 but leaving behind the
organic fraction in the aerosol phase, then that organic mass would not be
accurately accounted for as arising from nitrate chemistry. This was
addressed in a recent modeling study of SOAS (Pye
et al., 2015) in which modeled hydrolysis products of particulate organic
nitrates of up to 0.8 µg m-3 additional aerosol mass loading in the
southeastern U.S. were included in the estimate of change in OA due to changes
in NOx. Another recent GEOS-Chem modeling study using
gas- and particle-phase organic nitrates observed during the SEAC4RS and
SOAS campaigns similarly found RONO2 to be a major sink of
NOx across the SEUS region (Fisher et al., 2016; Lee et
al., 2016).
Complementing these SOAS ground site measurements, the NOAA-led SENEX
(Southeast Nexus) aircraft campaign conducted 18 research flights focused in
part on studying the interactions between biogenic and anthropogenic
emissions that formed secondary pollutants between 3 June and 10 July 2013
(Warneke et al., 2016). Flight instrumentation focused on measurement of
aerosol precursors and composition enable the present investigation of SOA
yields using this aircraft data set. Edwards et al. (2017)
used data from the SENEX night flights to evaluate the nighttime oxidation of
BVOC, observing high nighttime isoprene mixing ratios in the residual layer
that can undergo rapid NO3 oxidation when sufficient
NOx is present. These authors suggest that past
NOx reductions may have been uncoupled from OA trends due to
NOx not having been the limiting chemical species for OA
production, but that future reductions in NOx may decrease
OA if NO3 oxidation of BVOC is a substantial regional SOA source.
As isoprene is ubiquitous in the nighttime residual layer over the
southeastern United States and the NO3 + isoprene reaction is rapid,
NO3 reaction will be dominant relative to O3 in places with
anthropogenic inputs of NOx (Edwards et al., 2017
conclude that when NO2 / BVOC > 0.5, NO3 oxidation
will be dominant). Hence, a modest NO3 + isoprene SOA yield may
constitute a regionally important OA source.
Several modeling studies have investigated the effects of changing
NOx on global and SEUS SOA. Hoyle et al. (2007) found an increase
in global SOA production from 35
to 53 Tg yr-1 since preindustrial times, resulting in an increase in
global annual mean SOA mass loading of 51 %, attributable in part to
changing NOx emissions. Zheng et al. (2015) found only moderate SOA
reductions from a 50 % reduction in NO emissions: 0.9 %–5.6 % for
global NOx or 6.4 %–12.0 % for southeastern US
NOx, which they attributed to buffering by alternate
chemical pathways and offsetting tendencies in the biogenic vs. anthropogenic
SOA components. In contrast, Pye et al. (2015) find a 9 % reduction in total organic aerosol in Centreville,
AL for only 25 % reduction in NOx emissions. A simple
limiting-reagent analysis of NO3 + monoterpene SOA from power plant
plumes across the United States found that between 2008 and 2011, based on
EPA-reported NOx emissions inventories, some American power
plants shifted to the NOx-limited regime (from 3.5 % to
11 % of the power plants), and showed that these newly
NOx-limited power plants were primarily in the southeastern
United States (Fry et al., 2015). The effect of changing
NOx on SOA burden is clearly still in need of further study.
Here, we present aircraft transects of spatially discrete
NOx plumes from electric generating units (EGU), or power
plants (PP), as a method to specifically isolate the influence of NO3
oxidation. These plumes are concentrated and highly enriched in
NOx over a scale of only a few km
(Brown et al., 2012), and have nitrate radical
production rates (P(NO3)) 10–100 times greater than those of
background air. The rapid shift in P(NO3) allows direct comparison
of air masses with slow and rapid oxidation rates attributable to the nitrate
radical, effectively isolating the influence of this single chemical pathway
in producing SOA and other oxidation products. Changes in organic nitrate
aerosol (pRONO2) concentration and accompanying isoprene titration
enable a direct field determination of the SOA yield from NO3 + isoprene.
Field campaign and experimental and modeling methods
The Southeast Nexus (SENEX: http://esrl.noaa.gov/csd/projects/senex/, last access: 8 August 2018)
campaign took place 3 June through 10 July 2013 as the NOAA WP-3D aircraft
contribution to the larger Southeast Atmospheric Study (SAS:
http://www.eol.ucar.edu/field_projects/sas/, last access: 8 August 2018), a large, coordinated
research effort focused on understanding natural and anthropogenic emissions,
oxidation chemistry and production of aerosol in the summertime atmosphere in
the southeastern United States. The NOAA WP-3D aircraft operated 18 research
flights out of Smyrna, Tennessee, carrying an instrument payload oriented
towards elucidating emissions inventories and reactions of atmospheric trace
gases, and aerosol composition and optical properties (Warneke et al.,
2016). One of the major goals of the larger SAS study is to quantify the
fraction of organic aerosol that is anthropogenically controlled, with a
particular focus on understanding how OA may change in the future in response
to changing anthropogenic emissions.
The subset of aircraft instrumentation employed for the present analysis of
nighttime NO3 + isoprene initiated SOA production includes
measurements used to determine NO3 radical production rate
(P(NO3) = kNO2+O3(T) [NO2] [O3]), isoprene and
monoterpene concentrations, other trace gases for plume screening and
identification, aerosol size distributions and aerosol composition. The
details on the individual measurements and the overall aircraft deployment
goals and strategy are described in Warneke et al. (2016). Briefly, NO2
was measured by UV photolysis and gas-phase
chemiluminescence (P-CL) and by cavity ringdown spectroscopy, (CRDS), which
agreed within 6 %. O3 was also measured by both gas-phase
chemiluminescence and CRDS and agreed within 8 %, within the combined
measurement uncertainties of the instruments. Various volatile organic
compounds were measured with several techniques, including for the isoprene
and monoterpenes of interest here, proton reaction transfer mass
spectrometry (PTR-MS) and canister whole air samples and post-flight GC-MS
analysis (iWAS/GCMS). A comparison of PTR-MS and iWAS/GCMS measurements of
isoprene during SENEX has high scatter due to imperfect time alignment and
isoprene's high variability in the boundary layer, but the slope of the
intercomparison is 1.04 (Warneke et al., 2016); for more details on the
VOC intercomparisons; see also Lerner et al. (2017).
Acetonitrile from the PTRMS was used to screen for the influence of biomass
burning. Sulfur dioxide (SO2) was used to identify emissions from
coal-fired power plants. All gas-phase instruments used dedicated inlets,
described in detail in the supplemental information for Warneke et al. (2016).
Aerosol particles were sampled downstream of a low turbulence inlet
(Wilson et al., 2004), after which they were dried by
ram heating, size-selected by an impactor with 1 µm aerodynamic diameter
size cut-off, and measured by various aerosol instruments (Warneke et al.,
2016). An ultra-high-sensitivity aerosol sizing spectrometer (UHSAS, Particle
Metrics, Inc., Boulder, CO Brock et al., 2011; Cai et al., 2008) was used
to measure the dry submicron aerosol size distribution down to about 70 nm.
Data for the UHSAS are reported at 1 Hz whereas AMS data were recorded
roughly every 10 s. The ambient (wet) surface areas were calculated
according to the procedures described in Brock et al. (2016). A pressure-controlled inlet
(Bahreini et al., 2008) was
employed to ensure that a constant mass flow rate was sampled by a compact
time-of-flight aerosol mass spectrometer (C-ToF-AMS), which measured the
non-refractory aerosol composition (Drewnick et al., 2005). The
aerosol volume transmitted into the AMS was calculated by applying the
measured AMS lens transmission curve (Bahreini et al., 2008)
to the measured particle volume distributions from the UHSAS. For the entire
SENEX study, the mean, calculated fraction of aerosol volume behind the 1
micron impactor that was transmitted through the lens into the AMS instrument
was 97 % (with ±4 % standard deviation), indicating that most of
the submicron aerosol volume measured by the sizing instruments was sampled
by the AMS.
After applying calibrations and the composition-dependent collection
efficiency following Middlebrook et al. (2012), the limits of detection for the flight analyzed here were
0.05 µg m-3 for nitrate, 0.26 µg m-3 for organic mass, 0.21 µg m-3
for ammonium, and 0.05 µg m-3 for sulfate,
determined as three times the standard deviation of 10 s filtered air
measurements obtained for 10 min during preflight and 10 min during
postflight (110 datapoints). Note that the relative ionization efficiency for
ammonium was 3.91 and 3.87 for the two bracketing calibrations and an average
value of 3.9 was used for the flight analyzed here. An orthogonal distance
regression (ODR-2) of the volume from composition data (AMS mass plus
refractory black carbon) using a mass weighted density as described by
Bahreini et al. (2009) vs. the volume based on the sizing instruments (after correcting for
AMS lens transmission as above) had a slope of 1.06 for the entire SENEX
study and 72 % of the data points were within the measurements' combined
uncertainties of ±45 % (Bahreini et al., 2008). However, for
the flight analyzed here, the same regression slope was 1.58, which
is slightly higher than the combined uncertainties. It is unclear why the two
types of volume measurements disagree more for this flight. This does not
change the conclusions of this work because this has been incorporated into
the error in aerosol organic nitrate, which still show positive enhancements
in pRONO2 for these plumes (see Fig. 4 below). These complete error
estimates are also used in Fig. 5 to clearly show the uncertainties in the
yields. The volume comparison is discussed further in the Supplement and shown
for the plumes of interest in Fig. S1.
The C-ToF-AMS is a unit mass resolution (UMR) instrument and the mass
spectral signals that are characteristic of aerosol nitrate at m/z30 and 46
(NO+ and NO2+) often contain interferences from organic
species such as CH2O+ and CH2O2+, respectively.
Here, the m/z 30 and 46 signals have been corrected for these interferences by
using correlated organic signals at m/z 29, 42, 43, and 45 that were derived
from high-resolution AMS measurements during the NASA SEAC4RS campaign
that took place in the same regions of the SE US shortly after SENEX (see
and Fig. S2). The corrections were applied to the
individual flight analyzed here from 2 July. All of the corrections were
well correlated with each other for the SEAC4RS dataset and we used the
organic peak at m/z 29 (from CHO+) and the peak at m/z 45 (from
CHO2+), respectively, as those corrections were from peaks
closest (in m/z) to those being corrected. Once corrected, the nitrate mass
concentrations in the final data archive for this flight were reduced by
0–0.24 µg sm-3, an average reduction of 0.11 µg sm-3
or 32 % from the initial nitrate mass concentrations. The organic
interferences removed from the m/z 30 and m/z 46 signals are linearly correlated
with the total organic mass concentrations, corresponding to an average
1.3 % increase in the total organic mass.
The ratio of the corrected NO2+ / NO+ signals was then used to
calculate the fraction of aerosol nitrate that was organic (pRONO2) or
inorganic (ammonium nitrate) based on the method described first in Fry et al. (2013). Here we used an
organic NO2+ / NO+ ratio that was equal to the ammonium nitrate
NO2+ / NO+ ratio from our calibrations divided by 2.8. This
factor was determined from multiple datasets (see discussion in the Supplement). The ammonium
nitrate NO2+ / NO+ ratio was
obtained from the two calibrations on 30 June and 7 July that bracketed the
flight on 2 July, which is analyzed here. It was 0.514 and 0.488,
respectively, and for all of the data from both calibrations it averaged
0.490. Hence, the organic nitrate NO2+ / NO+ ratio was
estimated to be 0.175. This is the first time, to our knowledge, that UMR
measurements of aerosol nitrate have been corrected with HR correlations and
used to apportion the corrected nitrate into inorganic or organic nitrate
species.
The time since emission of intercepted power plant plumes was estimated from
the slope of a plot of O3 against NO2. For nighttime emitted
NOx plumes that consist primarily of NO (Peischl et al., 2010), O3 is negatively
correlated with NO2 due to the rapid reaction of NO with O3 that
produces NO2 in a 1 : 1 ratio:
NO+O3→NO2+O2.
Reaction (R1) goes rapidly (NO pseudo first order loss rate coefficient of
0.03 s-1 at 60 ppb O3) to completion, so that all
NOx is present as NO2, as long as the plume NO does not
exceed background O3 after initial mixing of the plume into background
air. Subsequent oxidation of NO2 via Reaction (R2) leads to an
increasingly negative slope of O3 vs. NO2:
NO2+O3→NO3+O2.
Equation (1) then gives plume age subsequent to the completion of
(R1) in terms of the observed slope, m, of O3 vs. NO2 (Brown et
al., 2006).
tplume=ln1-S(m+1)Sk1O3‾
Here S is a stoichiometric factor that is chosen for this analysis to be 1
based on agreement of plume age with elapsed time in a box model run
initialized with SENEX flight conditions (see below); k1 is the
temperature dependent bimolecular rate constant for NO2+O3
(Reaction R2) and O3‾ is the average O3 within the plume.
We calculate plume ages using both a stoichiometric factor of 1 (loss of
NO3 and N2O5 dominated by NO3 reactions) and 2
(loss dominated by N2O5 reactions), although we note that the
chemical regime for NO3+N2O5 loss may change over the lifetime
of the plume, progressing from 1 to 2 as the BVOC is consumed. We use S=1
values in the analysis that follows. As the more aged plumes are more
likely to have S approach 2, this means that some of the older plumes may
have overestimated ages. Figure S3 shows the
plume age calculated by Eq. (1) using modeled NOx,
NOy and O3 concentrations for S=1 and S=2, from nighttime simulations
of plume evolution using an observationally constrained box model. This
confirms that for nighttime plumes, S=1 plume ages match modeled elapsed
time well. The model used for this calculation, and those used to assess
peroxy radical lifetimes and fates in Sect. 4.3, was the Dynamically Simple
Model of Atmospheric Chemical Complexity (DSMACC Emmerson
and Evans, 2009) containing the Master Chemical Mechanism v3.3.1 chemistry
scheme (Jenkin et al., 2015). More details on the
model approach are provided in the Supplement.
Nighttime flight selection
There were three nighttime flights (takeoffs on the evenings of 19 June,
2 and 3 July 2013, Central Daylight Time)
conducted during SENEX, of which one (2 July) surveyed regions surrounding
Birmingham, Alabama, including multiple urban and power plant plume
transects. As described in the introduction, these plume transects are the
focus of the current analysis as they correspond to injections of
concentrated NO (and subsequently high P(NO3)) into the regionally
widespread residual layer isoprene. The nighttime flight on 3 July, over
Missouri, Tennessee and Arkansas sampled air more heavily influenced by
biomass burning than biogenic emissions. The 19 June night flight sampled
earlier in the evening, in the few hours immediately after sunset, and
sampled more diffuse urban plume transects that had less contrast with
background air. Therefore, this paper uses data exclusively from the 2 July
flight, in which nine transects of well-defined NOx plumes
from power plants emitted during darkness can be analyzed to obtain
independent yields measurements.
A map of the 2 July flight track is shown in Fig. 1a. After takeoff at 8:08
pm local Central Daylight Time on 2 July, 2013 (01:08 a.m. UTC 3 July 2016),
the flight proceeded towards the southwest until due west of Montgomery, AL,
after which it conducted a series of east-west running tracks while working
successively north toward Birmingham, AL. Toward the east of Birmingham, the
aircraft executed overlapping north-south tracks at six elevations to sample
the E. C. Gaston
power plant. During the course of the flight, concentrated
NOx plumes from the Gaston, Gorgas, Miller and Greene City
power plants were sampled. Around 01:30 and 02:30 a.m. Central Daylight Time
(05:30 and 06:30 a.m. UTC), two transects of the Birmingham, AL urban plume were
measured prior to returning to the Smyrna, TN airport base.
The flight track is shown colored by the nitrate radical production rate,P(NO3), to show the points of urban and/or power plant plume
influence:
P(NO3)=k2(T)[NO2][O3].
Here, k2 is again the temperature-dependent rate coefficient for
reaction of NO2+O3 (Atkinson et al., 2004), and the square
brackets indicate concentrations. Figure 1b further illustrates the selection
of power plants plumes: sharp peaks in P(NO3) are indicative of
power plant plume transects, during which isoprene mixing ratios also are
observed to drop from the typical regional residual layer background values
of ∼1 ppb, indicative of loss by NO3 oxidation (an individual
transect is shown in more detail below in Fig. 2). Also shown in Fig. 1b are
measured concentrations of isoprene and monoterpenes throughout the flight,
showing substantial residual layer isoprene and supporting the assumption
that effectively all NO3 reactivity is via isoprene (see calculation
in next section). Residual layer concentrations of other VOCs that could
produce SOA (e.g., aromatics) are always below 100 pptv, and their reaction
rates with NO3 are slow. Edwards et al. (2017) have shown that
NO3 and isoprene mixing ratios for this and other SENEX night flights
exhibit a strong and characteristic anticorrelation that is consistent with
nighttime residual layer oxidation chemistry.
(a) Map of northern Alabama, showing the location of the
flight track of the 2 July 2013 night flight used in the present analysis,
with plume numbers labeled and wind direction shown. Although the wind
direction changed throughout the night, these measurements enable us to
attribute each plume to a power plant source (see labels in Fig. 1b and
Table 2). Color scale shows P(NO3) based on aircraft-measured
[NO2] and [O3], while power plants discussed in the text are
indicated in blue squares with marker size scaled to annual
NOx emissions for 2013 (scale not shown). Isoprene emissions
are widespread in the region (Edwards et al., 2017). Panel (b) shows
time series data from the same flight, with plume origins and numbers
labeled, showing aircraft-measured isoprene and monoterpene concentrations,
altitude, and P(NO3) determined according to Eq. (2) (log scale),
showing that the isoprene was uniformly distributed (mixing ratios often in
excess of 1 ppbv), while the more reactive monoterpenes were present at
mixing ratios below 100 ppt except at the lowest few hundred meters above
ground in the vertical profiles (not used in the present analysis). Figure 1b
also shows that sharp peaks in nitrate radical production rate occur both at
the lowest points of these vertical profiles, when the aircraft approached
the surface, but also frequently during periods of level flight in the
residual layer, which correspond to the power plant plume transects analyzed
in this paper.
![]()
Results
Selection of plumes
Figure 2 shows a subset of the 2 July flight time series data, illustrating
three NOx plumes used for analysis. The large NO3
source and isoprene loss was accompanied by an increase in organic nitrate
aerosol mass, which we attribute to the NO3 + isoprene reaction
based on prior arguments. We observed each plume as a rapid and brief
perturbation to background conditions, in the order of 10–50 s, or 1–5 km in
spatial scale. Each plume's perturbed conditions can correspond to different
plume ages, depending on how far downwind of the power plant the plume
transect occurred.
Three representative plume transect observations from the
2 July 2013 flight (plumes are identified by the peaks in P(NO3),
listed in Table 1 at times 02:18, 02:20, and 02:21 UTC). Note the difference
in sulfate enhancement in the three plumes, which is largest in the third
plume, and is accompanied by increases in ammonium. In all three cases, the
isoprene concentration drops in the plumes, accompanied by a clear increase
in organic nitrate, no changes in the inorganic nitrate, and modest changes
in organic aerosol mass concentrations.
![]()
Candidate plumes were initially identified by scanning the time series flight
data for any period where the production rate of nitrate radical
(P(NO3)) rose above 0.5 ppbv h-1. This threshold was
chosen to be above background noise and large enough to isolate only true
plumes (see Fig. 1a). The value is thus subjectively chosen, but was
consistently applied across the dataset. For each such period, a first
screening removed any of these candidate plumes that occurred during missed
approaches or other periods where radar altitude above ground level (a.g.l.) was
changing, because in the stratified nighttime boundary layer structure,
variations in altitude may result in sampling different air-masses, rendering
the adjacent out of plume background not necessarily comparable to in-plume
conditions. A second criterion for rejection of a plume was missing isoprene
or AMS data during brief plume intercepts. No selected plumes on 2 July
showed enhanced acetonitrile or refractory black carbon, indicating no
significant biomass burning influence. Finally, two plumes downwind of the
Gaston power plant (at 03:10 and 03:14) were removed from the present
analysis, because (03:10) the background isoprene was changing rapidly,
preventing a good baseline measurement, and (03:14) there was no observed
decrease in isoprene concentration in-plume (as well as no increase in
nitrate aerosol). The 03:14 plume was apparently too recently emitted to have
undergone significant nighttime reaction; its O3 / NO2 slope was
unity, within the combined measurement error of O3 and NO2
(Eq. 1). After this filtering, there are 9 individual plume observations for
determination of NO3 + isoprene SOA yields (see Table 1). The rapid
increases in P(NO3) appeared simultaneously with significant
decreases in isoprene and increases in aerosol nitrate. The aerosol and
isoprene measurements (taken at data acquisition rates < 1 Hz) were
not exactly coincident in time, which leads to some uncertainty in the yield
analysis below.
Derivation of SOA yields from observed changes in isoprene and aerosol mass
in plumes depends on two conditions, and has several caveats that will be
discussed in the text that follows (see Table 3 below for a summary of these
caveats). The two conditions are (1) that the majority of VOC mass
consumed by NO3 in plumes is isoprene (rather than monoterpenes or
other VOC), and then that (2a) the change in aerosol organic
mass concentration during these plumes is due to NO3 + isoprene
reactions, and/or (2b) the change in aerosol nitrate mass concentration
is due to NO3 + isoprene reactions. There are separate considerations
for each of these conditions.
For the first condition, we note that the isoprene to monoterpenes ratio just
outside each plume transect was always high (a factor of 10 to 70, on average
26). With the 298 K NO3 rate constants of ∼5×10-12 cm3 molec-1 s-1 for monoterpenes and 6.5×10-13 cm3 molec-1 s-1 for isoprene (Calvert et al., 2000),
isoprene (∼2 ppb) will always
react faster with nitrate than monoterpenes (∼0.04 ppbv). At these
relative concentrations, even if all of the monoterpene is oxidized, the
production rate of oxidation products will be much larger for isoprene.
Contribution to aerosol by N2O5 uptake is also not important in
these plumes. Edwards et al. (2017)
calculated the sum of NO3 and N2O5 loss throughout this
flight and showed that it is consistently NO3 + BVOC dominated (Fig. S4 of
that paper). As isoprene depletes, N2O5 uptake will increasingly
contribute to NO3 loss, but as shown below, we are able to rule out a
substantial source of inorganic nitrate for most plumes. We also know that
despite increased OH production in-plume, the isoprene loss is still
overwhelming dominated by NO3 (Fig. S5 in Edwards, et al., 2017).
The second condition requires that we can find an aerosol signal that is
attributable exclusively to NO3 + isoprene reaction products, whether it
be organic aerosol (OA) or organic nitrate aerosol (pRONO2) mass
loading, or both. We note that the ratio of in-plume aerosol organic mass
increase to pRONO2 mass increase is noisy (see discussion below at Fig. 6),
but indicates an average in-plume ΔOA to ΔpRONO2
ratio of about 5. The large variability is primarily due to the fact that the
variability in organic aerosol mass between successive 10 s data points
for the entire flight is quite large (of order 0.75 µg m-3) and
comparable to many of the individual plume ΔOA increases, far
exceeding the expected organonitrate driven increases in OA, which are
roughly twice the pRONO2 mass increases. It is also possible that in
these plumes, where total aerosol mass is elevated, semivolatile organic
compounds may repartition to the aerosol phase, contributing a
non-pRONO2 driven variability in ΔOA. For example, if some gas
phase IEPOX is present in the residual layer, it may be taken up into the
highly acidic aerosol from the power plants. Alternatively, very polar
gas-phase compounds could partition further into the higher liquid water
associated with the sulfate in the plume. Therefore, in-plume organic aerosol
increases cannot be attributed clearly to NO3 + isoprene SOA
production, so we do not use them in the SOA yield calculations.
This leaves consideration 2b, whether all increase in nitrate mass is due to
NO3 + isoprene reactions. Here we must evaluate the possibility of
inorganic nitrate aerosol production in these high-NOx
plumes. Fine-mode aerosol inorganic nitrate can be formed by the (reversible)
dissolution of HNO3(g) into aqueous aerosol. In dry aerosol samples,
inorganic nitrate is typically in the form of ammonium nitrate
(NH4NO3), when excess ammonium is available after neutralization of
sulfate as (NH4)2SO4 and NH4(HSO4). Due to the
greater stability of ammonium sulfate salt relative to ammonium nitrate, in
high-sulfate plumes with limited ammonium, inorganic nitrate aerosol will
typically evaporate as HNO3(g) (Guo
et al., 2015) (Reaction R3):
2NH4NO3(aq)+H2SO4(aq)(NH4)2SO4(aq)+2HNO3(g).
Inorganic nitrate can also form when crustal dust (e.g., CaCO3) or
sea salt (NaCl) are available. Uptake of HNO3 is rendered favorable by
the higher stability of nitrate mineral salts, evaporating CO2 or HCl.
Inorganic nitrate can also be produced by the heterogeneous uptake of
N2O5 onto aqueous aerosol; Edwards et al. (2017) demonstrated that
this process is negligible relative to NO3 + BVOC for the 2 July
SENEX night flight considered here.
There are several lines of evidence that the observed nitrate aerosol is
organic and not inorganic. First, examination of the
NO2+ / NO+ (interference-corrected
m/z 46 : m/z 30) ratio measured by the aircraft AMS (Fig. 3) shows a
ratio throughout the 2 July flight, including the selected plumes, that is
substantially lower than that from the bracketing ammonium nitrate
calibrations. This lower AMS measured NO2+ / NO+
ratio has been observed for organic nitrates (Farmer et al., 2010), and some
mineral nitrates (e.g., Ca(NO3)2 and NaNO3; Hayes et
al., 2013 ), which are not important in this case because aerosol was
dominantly submicron. As described above, we can separate the observed AMS
nitrate signal into pRONO2 and inorganic nitrate contributions. These
mass loadings are also shown in Fig. 3, indicating dominance of
pRONO2 throughout the flight.
For the flight under consideration, the estimated relative
contributions of ammonium and organic nitrate to the total corrected nitrate
signal (a) was calculated from the ratios of the corrected peaks at
m/z 30 and 46 (b). Each of the plumes is identified here by plume
number. The ratios of NO2+ / NO+ (black data in b)
from the corrected peaks at m/z 46 and 30, respectively, are compared to
the ratios expected for ammonium nitrate (AN Calibration Ratio, blue
horizontal line at 0.49) or organic nitrate (pRONO2 Ratio, green
horizontal line at 0.175), which is estimated from the AN calibration ratio
using multiple data sets (see discussion in the Supplement). The
measured ratio for most of the flight is more characteristic of organic
nitrate than ammonium nitrate.
![]()
We can also employ the comparison of other AMS-measured aerosol components
during the individual plumes to assess the possibility of an inorganic
nitrate contribution to total measured nitrate. Figure S5a shows that the
in-plume increases in sulfate are correlated with increases in ammonium with
an R2 of 0.4. The observed slope of 5.4 is characteristic of primarily
(NH4)HSO4, which indicates that the sulfate mass is not fully
neutralized by ammonium. However, we note that if the largest observed
aerosol nitrate increase is due solely to ammonium nitrate, the ammonium
increase would be only 0.11 µg m-3, which would be difficult to
discern from the NH4 variability of order 0.11 µg m-3.
However, the slope is consistent with incomplete neutralization of the
sulfate by ammonium, which would make HNO3(g) the more thermodynamically
favorable form of inorganic nitrate. The ion balance for the ammonium nitrate
calibration particles and the plume enhancements are shown in Fig. S5b.
Complete neutralization of the calibration aerosols is nearly always within
the gray 10 % uncertainty band for the relative ionization efficiency of
ammonium (Bahreini et al., 2009). In contrast, many of the plume enhancements
are near the 1 : 2 line (as primarily ammonium bisulfate) within the combined
10 % ammonium and 15 % sulfate uncertainty error bars or without
ammonium (sulfuric acid). Thus, NH4NO3 is unlikely to be stable
in the aerosol phase under the conditions of these plumes, consistent with
the AMS observations.
A plot of the calculated plume enhancements from the derived apportionment
into organic (pRONO2) and inorganic (ammonium) nitrate is shown in Fig. 4.
The increases in aerosol nitrate for nearly all of the plumes appear to be
mostly due to enhancements in pRONO2. Based on these considerations, we
conclude that in-plume pRONO2 mass increases are a consequence (and thus
a robust measure) of organic nitrate aerosol produced from NO3 + isoprene.
As each isoprene molecule condensing will have one nitrate
group, the ratio of these increases to isoprene loss is a direct measure of
the molar organic aerosol yield from NO3-isoprene oxidation.
The contribution of each species to the nitrate enhancements in each
of the plumes, showing that the enhancements in most of the plumes are mainly
due to enhancements in organic nitrate, with the exception of Plume 8, which
had enhancements in both organic and ammonium nitrate. Error bars are
estimated from the measurement variability, the UMR corrections to the
nitrate signals, apportionment between organic and inorganic nitrate, and the
total nitrate uncertainty (see Supplement).
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Table 1 shows the selected plumes to be used for yield analysis. Wherever
possible, multiple points have been averaged for in-plume and background
isoprene and nitrate aerosol concentrations; in each case the number of
points used is indicated and the corresponding standard deviations are
reported. In two cases (02:20 and 03:03 plumes), the plumes were so narrow
that only a single point was measured in-plume at the 10 s time resolution
of the PTR-MS and AMS; for these “single-point” plumes it is not possible
to calculate error bars. Error bars were determined using the standard
deviations calculated for in-plume and background isoprene and nitrate
aerosol concentrations, accounting also for the additional uncertainty in
the AMS measurement described in the caption to Fig. 4, and propagated
through the yield formula detailed in the following section.
List of plumes used in this NO3 + isoprene SOA yield analysis.
For each plume, the delta values listed indicate the difference between
in-plume and outside-plume background in average observed concentration, and
the standard deviations (SD) are the propagated error from this subtraction.
(For ΔNO3 from pRONO2, the standard deviations also
include error propagated as described in the caption for Fig. 4) After each
plume number, the numbers of points averaged for isoprene (10 s resolution)
and AMS (10 s resolution) are listed, respectively. As the isoprene
data were reported at a lower frequency, these numbers are typically lower to
cover the same period of time. Plume numbers annotated with * indicate brief
plumes for which only single-point measurements of in-plume aerosol
composition were possible. Additional AMS and auxiliary data from each plume
is included in the Supplement, Table S3.
Plume number
7/2/13 plume
P(NO3)
ΔISOP (ppt)
ΔNO3,aero
ΔNO3 from pRONO2
ΔNO3 from NH4NO3
[#isop/#AMS]
time (UTC)
(ppbv h-1)
[±SD]
(µg m-3) [±SD]
(µg m-3) [±SD]
(µg m-3) [±SD]
Typical variability (µg m-3)
0.05
0.05
0.05
1
02:18
0.9
-335
0.107
0.086
0.022
[2/3]
[128]
[0.039]
[0.047]
[0.012]
2
02:20
0.8
-404
0.079
0.079
0
[*]
[0.049]
3
02:21
1.2
-228
0.067
0.074
-0.007
[4/5]
[121]
[0.039]
[0.043]
[0.027]
4
03:03
1.4
-453
0.118
0.166
-0.049
[*]
[0.088]
5a
03:55
1.0
-255
0.046
0.045
0.002
[3/4]
[251]
[0.019]
[0.026]
[0.015]
6
04:34
0.6
-713
0.072
0.107
-0.035
[2/2]
[219]
[0.031]
[0.059]
[0.029]
7
04:37
0.8
-298
0.100
0.080
0.021
[5/6]
[197]
[0.082]
[0.051]
[0.034]
8b
04:39
0.9
-443
0.354
0.201
0.153
[2/3]
[75]
[0.058]
[0.120]
[0.057]
9
05:04
0.6
-293
0.172
0.227
-0.055
[7/8]
[131]
[0.048]
[0.115]
[0.042]
a Plume 5 has the smallest ΔNO3,aero and may be
affected by background pRONO2 variability. b Plume 8 has a measurable
increase in inorganic nitrate as well as organic.
SOA yield analysis
A molar SOA yield refers to the number of molecules of aerosol
organic nitrate produced per molecule of isoprene consumed. In order to
determine molar SOA yields from the data presented in Table 1, we convert the
aerosol organic nitrate mass loading differences to mixing ratio differences
(ppt) using the NO3 molecular weight of 62 g mol-1 (the AMS
organic nitrate mass is the mass only of the -ONO2 portion of the
organonitrate aerosol). At standard conditions of 273 K and 1 atm (all
aerosol data are reported with this STP definition), 1000 ppt NO3 = 2.77 µg m-3,
so each ΔMpRONO2 is multiplied by 361 ppt
(µg m-3)-1 to determine this molar yield:
YSOA,molar=pRONO2plume±SDpRONO2plume-pRONO2bkg±SDpRONO2bkg-[isopplume±SDisopplume-isopbkg±SDisopbkg].×361pptNO3µgm-3
The SOA molar yields resulting from this calculation are shown in Table 2,
spanning a range of 5 %–28 %, with uncertainties indicated based on
the SDs in measured AMS and isoprene concentrations. In addition to this
uncertainty based on measurement precision and ambient variability, there is
an uncertainty of 50 % in the AMS derived-organic nitrate mass loadings
(see Supplement) and 25 % in the PTR-MS isoprene concentrations (Warneke
et al., 2016). The average molar pRONO2 yield across all plumes,
with each point weighed by the inverse of its standard deviation, is 9 %.
(As noted below, the yield appears to increase with plume age, so this
average obscures that trend.) An alternate graphical analysis of molar SOA
yield from all nine plumes plus one “null” plume (03:14, in which no
isoprene had yet reacted and is thus not included in Tables 1 and 2) obtains
the same average molar yield of 9 % (Fig. 5). Here, the molar yield is
the slope of a plot of plume change in pRONO2 vs. plume change in
isoprene. The slope is determined by a linear fit with points weighted by the
square root of the number of AMS data points used to determine in-plume
pRONO2 in each case. This slope error gives a rather narrow
uncertainty range for the slope (0.0930±0.0011); to obtain an upper
limit in the uncertainty of this molar yield we apply the combined
instrumental uncertainties, based on adding in quadrature the PTR-MS
uncertainty of 5 % and the AMS uncertainty of 50 %. This gives an
overall uncertainty of 50.2 %, resulting in upper and lower limit slopes
of 0.140 and 0.046, respectively; we use this maximum uncertainty estimate to
report the average molar yield as 9 % (±5 %). We have not
corrected the calculated yields for the possibility of NO3
heterogeneous uptake, which could add a nitrate functionality to existing
aerosol. Such a process could be rapid if the uptake coefficient for
NO3 were 0.1, a value characteristics of unsaturated substrates (Ng
et al., 2017), but would not contribute measurably at more conventional
NO3 uptake coefficients of 0.001 (Brown and Stutz, 2012).
SOA Yields for each plume observation, estimated plume age, and
likely origin. See text for description of uncertainty estimates. For the
mass yields, the calculated SOA mass increase includes both the organic and
(organo)nitrate aerosol mass; the measurements for OA increases shown in
Fig. 6 do not include the nitrate mass.
plume number
plume time
SOA molar yield
SOA mass yield
plume age from
Likely NOx
(UTC)
(fraction)
(fraction)
O3 / NO2
origin &
[±SD]
[±SD]
clock assuming S=1 (h)
altitude (m)
1
7/2/13 2:18
0.09
0.25
2.5
Greene County
[0.06]
[0.17]
@ 540 m
2
7/2/13 2:20
0.07
0.21
1.5
ibid
3
7/2/13 2:21
0.12
0.32
1.5
ibid
[0.10]
[0.25]
4
7/2/13 3:03
0.13
0.36
1.5
Gaston
@ 720 m
5
7/2/13 3:55
0.06
0.17
1.4
Miller/Gorgas
[0.07]
[0.20]
@ 690 m
6
7/2/13 4:34
0.05
0.15
2
ibid
[0.03]
[0.09]
7
7/2/13 4:37
0.10
0.26
5.5
ibid
[0.09]
[0.24]
8
7/2/13 4:39
0.16
0.45
5.8
Miller/Gorgas
[0.10]
[0.28]
@ 1120 m
9
7/2/13 5:04
0.28
0.77
6.3
Gaston
[0.19]
[0.52]
@ 1280 m
SOA molar yield can be determined as the slope of ΔpRONO2 vs. Δ isoprene, both in mixing ratio units. The
linear fit is weighted by square root of number of points used to determine
each in-plume pRONO2, with intercept held at zero. The slope
coefficient ± one standard deviation is 0.0930±0.0011. Larger
“outside” high and low limits of the slope (shown as dashed red and blue
lines) are obtained by adding and subtracting from this slope the combined
instrumental uncertainties, based on adding in quadrature the PTR-MS
uncertainty of 5 % and the AMS uncertainty of 50 %. This gives an
overall uncertainty of 50.2 %, resulting in upper and lower limit slopes
of 0.140 and 0.046, respectively. Points are colored by plume age, and size
scaled by square root of number of points (the point weight used in linear
fit). This plot and fit includes the nine plumes listed in Tables 1 and 2, as
well as the 03:14 “unreacted” plume (at Δ isoprene = -84 ppt). Error bars on isoprene are the propagated
standard deviations of the (in plume–out plume) differences, for plumes in
which multi-point averages were possible. Error bars on pRONO2 are the same
as in Fig. 4, converted to ppt. The points without error bars are
single-point plumes.
![]()
To estimate SOA mass yields, we need to make some assumption about
the mass of the organic molecules containing the nitrate groups that lead to
the observed nitrate aerosol mass increase. The observed changes in organic
aerosol are too variable to be simply interpreted as the organic portion of
the aerosol organic nitrate molecules. We conservatively assume the organic
mass to be approximately double the nitrate mass (62 g mol-1), based on
an “average” molecular structure of an isoprene nitrate with three additional
oxygens: e.g., a tri-hydroxynitrate (with organic portion of formula
C5H11O3, 119 g mol-1), consistent with
2nd-generation oxidation product structures suggested in Schwantes et
al. (2015). Based on this assumed
organic to nitrate ratio, all plumes' expected organic mass increases would
be less than the typical variability in organic of 0.75 µg m-3.
This assumed structure is consistent with oxidation of both double bonds,
which appears to be necessary for substantial condensation of isoprene
products, and which structures would have calculated vapor pressures
sufficiently low to partition to the aerosol phase (Rollins et al., 2009). Another possible
route to low vapor pressure products is intramolecular H rearrangement
reactions, discussed below in Sect. 4.3, which would not require oxidant
reactions at both double bonds. In the case of oxidant reactions at both
double bonds, it is difficult to understand how the second double bond would
be oxidized unless by another nitrate radical, which would halve these
assumed organic to nitrate ratios (assuming the nitrate is retained in the
molecules). In contrast, any organic nitrate aerosol may lose
NO3 moieties, increasing the organic to nitrate ratio. Given these
uncertainties in both directions, we use the assumed “average” structure
above to guess an associated organic mass of double the nitrate mass. Thus,
to estimate SOA mass yield, we multiply the increase in organic nitrate
aerosol mass concentration by three (i.e., 2×ΔMpRONO2+ΔMpRONO2), and divide by the observed decrease in isoprene,
converted to µg m-3 by multiplying by 329 ppt (µg m-3)-1,
the conversion factor based on isoprene's molecular weight of
68.12 g mol-1.
YSOA,mass=pRONO2plume±SDpRONO2plume-pRONO2bkg±SDpRONO2bkg-isopplume±SDisopplume-isopbkg±SDisopbkg×3×329pptµgm-3
Note that the SOA mass yield reported here is based on the (assumed) mass of
organic aerosol plus the (organo)nitrate aerosol formed in each plume. If
instead the yield were calculated using only the assumed increase in
organic mass (i.e., 2×ΔMpRONO2 instead
of 3×ΔMpRONO2), which would be consistent with the method used
in Rollins, et al. (2009)
and Brown et al. (2009), the mass yields would be two-thirds of the values reported here. However,
as
SOA mass yield is typically defined based on the total increase in aerosol
mass, we use the definition with the sum of the organic and nitrate mass
here. This results in an average SOA mass yield of 27 %, with propagated
instrumental errors (see caption to Fig. 5) giving a range of 27%±14%.
We note also that correlation of in-plume increases in OA with pRONO2
(Fig. 6) point to a substantially larger 5 : 1 organic-to-nitrate ratio; if
this were interpreted as indicating that the average molecular formula of the
condensing organic nitrate has 5 times the organic mass as nitrate, this
would increase the SOA mass yields reported here. However, due to the
aforementioned possibility of additional sources of co-condensing organic
aerosol, which led us to avoid using ΔOA in determining SOA yields,
we do not consider this to be a direct indication of the molecular formula of
the condensing organic nitrate. Including OA in the SOA yield determination,
based on this 5 : 1 slope rather than the assumed 2 : 1 OA : pRONO2, would
give 2.5 times larger SOA mass yields than reported here.
Correlation of organic aerosol mass concentration with
pRONO2 mass concentration for the full 2 July flight (grey points and
red fit line, fitted slope and thus average OA / pRONO2 mass ratio of
∼30) and for the points during the selected plumes (colored points,
colored by plume age, average OA / pRONO2 mass ratio of ∼5).
![]()
Several caveats to the present SOA yields analysis are listed below,
alongside the expected direction each would adjust the estimated yields.
As we do not know whether or how much each process may have occurred in
the studied plumes, we cannot quantitatively assess the resulting
uncertainties, so we simply list them here. See text above for more detailed
discussion.
Process
Effect on determined SOA yield
Organic nitrate aerosol loses NO3 functional group
Larger, because the non-nitrate OA would not be counted in this analysis
Both double bonds in isoprene are oxidized by NO3: two nitrates per condensing molecule
Smaller, because the assumed organic to nitrate mass ratio assumes one nitrate per molecule
NO3 oxidizes daytime isoprene oxidation products (e.g., ISOPOOH) to make new aerosol
Smaller, because this would produce organic nitrate aerosol without corresponding decrease in isoprene, so that some of existing SOA production is misattributed to isoprene + NO3
Assumed organic to nitrate mass ratio is incorrect
Unknown direction of effect, depends on whether assumed ratio is high or low
Daytime-produced IEPOX uptake onto acidic particles
No effect (only changes ΔOA, not nitrate)
Suppression of O3 + monoterpene or O3 + isoprene SOA in plumes
No effect (only changes ΔOA, not nitrate)
Finally, the large range in observed yields can be interpreted by examining
the relationship to estimated plume age. Using the slope of O3 to
NO2 (Eq. 1) to estimate plume age as described above, a weak positive
correlation is observed (Table 2, Fig. S4), suggesting that as the plume
ages, later-generation chemistry results in greater partitioning to the
condensed phase of NO3 + isoprene organonitrate aerosol products. This
is consistent with the observation by Rollins et al. (2009) that second-generation
oxidation produced substantially higher SOA yields than the oxidation of the
first double bond alone, but we note that these mass yields (averaging
27 %, would be 18 % using the organic mass only) are higher than even
the largest yield found in that chamber study (14 %, used organic mass
only).
Simulated peroxy radical concentration (a, d),
loss rates (b, e),
and lifetime (c, f), using the MCM v3.3.1 chemical mechanism, for conditions
typical of a nighttime intercepted power plant plume (a–c) and the same plume
initial conditions run for daytime simulation (d–f, local noon occurs at 5 h). Included are total peroxy radical concentration and losses, as well as
the highlighted subclasses HO2, CH3O2, total
nitrooxy-isoprene-RO2 and the total hydroxy-isoprene-RO2
produced from OH oxidation. The righthand panels show HO2,
CH3O2 and the dominant hydroxy-isoprene-RO2
ISOPBO2 and ISOPDO2 (β-hydroxy-peroxy radicals from OH attack at carbons
1 and 4, respectively) lifetime on the left axis and
nitrooxy-isoprene-RO2 on the right axis, showing nighttime lifetimes
an order of magnitude longer than daytime for this NO3 + isoprene
derived RO2 radical (NISOPO2).
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We observe increasing SOA yield, from a molar yield of around 10 % at
1.5 h up to 30 % at 6 h of aging. The lowest yields observed are found
in the most recently emitted plumes, suggesting the interpretation of the
higher yields as a consequence of longer aging timescales in the atmosphere.
Mechanistic considerations
These larger SOA mass yields from field determinations (average 27 %)
relative to chamber work (12 %–14 %; see introduction) may arise for
several reasons. We first assess the volatility of assumed first- and
second-generation products using group contribution theory in order to
predict partitioning. After a single oxidation step, with a representative
product assumed to be a C5 hydroperoxynitrate, the saturation vapor
pressure estimated by group contribution theory (Pankow and Asher,
2008) at 283 K would be 2.10×10-3 Torr (saturation mass concentration C*=1.7×104 µg m-3 for MW = 147 g mol-1),
while a double-oxidized isoprene molecule (assuming a C5 dihydroxy dinitrate) has an estimated vapor
pressure of 7.95×10-8 Torr (C*=1.01 µg m-3 for MW = 226 g mol-1).
This supports the conclusion that while the first
oxidation step produces compounds too volatile to contribute appreciably to
aerosol formation, oxidizing both double bonds of the isoprene molecule is
sufficient to produce substantial partitioning, consistent with Rollins et
al. (2009). This is also
true if the second double bond is not oxidized by nitrate (group contribution
estimate Pvap for a C5 tri-hydroxy nitrate is 7.7×10-8 Torr,
C*=0.79 µg m-3 for MW = 181 g mol-1). These
C* saturation concentration values suggest that no dimer formation or
oligomerization is required to produce low-enough volatility
products to condense to the aerosol phase; however, such oligomerization
would result in more efficient condensation. The fact that Rollins et al.
(Rollins et al., 2009) did not observe
larger mass yields may indicate that it takes longer than a typical chamber
experiment timescale to reach equilibrium, or that this absorptive
partitioning model did not accurately capture those experiments, or that
substantial loss of semivolatiles to the chamber walls (e.g., Krechmer et al., 2016) suppressed apparent yields.
Determination of yields from ambient atmospheric data differs from chamber
determinations in several additional respects. First, ambient measurements do
not suffer from wall loss effects, such that no corrections are necessary for
loss of aerosol or semi-volatile gases (Matsunaga and Ziemann,
2010; Krechmer et al., 2016). Second, ambient measurements take place on the
aging time scale of the atmosphere rather than a time scale imposed by the
characteristics of the chamber or the choice of oxidant addition. Third, the
typical lifetime of the initially produced nitrooxy-isoprene-RO2
radical is more representative of the ambient atmosphere rather than a
chamber. The unique conditions of a high NOx power plant
plume affect lifetime and fates of peroxy radicals, as described below.
To help interpret these in-plume peroxy radical lifetimes, a box model
calculation using the MCM v3.3.1 chemistry scheme was run (see details in
the Supplement). This box model shows substantially longer peroxy
radical lifetimes during nighttime than daytime, initializing with identical
plume-observed conditions. These long peroxy radical lifetimes may have
consequences for comparison to chamber experiments: for example, in
Schwantes et al.'s (2015) chamber experiment
on the NO3 + isoprene reaction mechanism, the HO2-limited
nitrooxy-RO2 lifetime was at maximum 30 s. In the plumes investigated
in this study, peroxy radical lifetimes are predicted to be substantially
longer (> 200 s early in the night; see Fig. 7), allowing for the
possibility of different bimolecular fates, or of unimolecular
transformations of the peroxy radicals that may result in lower-volatility
products (e.g., auto-oxidation to form highly oxidized molecules Ehn et
al., 2014).
The typically assumed major fate of nighttime RO2 in the atmosphere
is reaction with HO2 to yield a hydroperoxide, NO3-ROOH. This
is shown in the model output above as the green reaction, and is responsible
for half of early RO2 losses in the MCM modeled plume. Schwantes
et al. (2015) proposed
reaction of these nighttime derived hydroperoxides with OH during the
following day as a route to epoxides, which in turn can form SOA via reaction
with acidic aerosol. Reaction of hydroperoxides with nighttime generated OH
may similarly provide a route to SOA through epoxides, albeit more slowly
than that due to photochemically generated OH.
The predicted longer nighttime peroxy radical lifetimes may enable unique
chemistry. For example, if nitrooxy-isoprene-RO2 self-reactions are
substantially faster than assumed in the MCM, as suggested by Schwantes
et al. (2015), RO2+RO2 reactions may compete with the HO2 reaction even
more than shown in Fig. 7, and dimer formation may be favored at night,
yielding lower volatility products. The 5 : 1 AMS organic to nitrate ratio
observed in the SOA formed in Rollins et al. (2009), and consistent with
aggregated observations reported here, may suggest that in some isoprene
units the nitrate is rereleased as NO2 in such oligomerization
reactions. We note that this larger organic to nitrate ratio would mean
higher SOA mass yields than estimated in Table 2.
Alternatively, longer nighttime peroxy radical lifetimes may allow sufficient
time for intramolecular reactions to produce condensable products. This
unimolecular isomerization (auto-oxidation) of initially formed peroxy
radicals is a potentially efficient route to low-volatility, highly
functionalized products that could result in high aerosol yields. For
OH-initiated oxidation of isoprene, laboratory relative rate experiments
found the fastest 1,6-H-shift isomerization reaction to occur for the
hydroxy-isoprene-RO2 radical at a rate of 0.002 s-1
(Crounse et al., 2011), meaning that peroxy radicals must have
an ambient lifetime of > 500 s for this process to be dominant. As
shown in Fig. 7, the simulated power plant plume peroxy radical lifetimes are
long (> 200 s), so an isomerization reaction at this rate may play
a significant role. However, a recent study has demonstrated that
OH-initiated and NO3-initiated RO2 radicals from the same
precursor VOC can have very different unimolecular reactive fates due to
highly structurally sensitive varying rates of reactions of different product
channels (Kurtén et al., 2017). A similar theoretical
study on the rate of unimolecular autooxidation reactions of
nitrooxy-isoprene-RO2 radicals would be valuable to help determine
under what conditions such reactions might occur, and this knowledge could be
applied to comparing chamber and field SOA yields.
Atmospheric implications and needs for future work
As this paper proposes higher SOA yield for the NO3 + isoprene
reaction than measured in chamber studies, we conclude with some discussion
of the implications for regional aerosol burdens, and further needs for
investigation in the NO3 + isoprene system.
Using an isoprene + NO3 yield parameterization that gave a 12 %
SOA mass yield at 10 µg m-3, Pye et al. (2010) found that adding
the NO3 + isoprene oxidation pathway increased isoprene SOA mass
concentrations in the southeastern United States by about 30 %, increases
of 0.4 to 0.6 µg m-3. The larger NO3 + isoprene SOA mass
yields suggested in this paper, with average value of 30 %, could double
this expected NO3 radical enhancement of SOA production. Edwards et
al. (2017) concluded that the southeastern US is currently in transition
between NOx-independent and NOx-controlled
nighttime BVOC oxidation regime. If NO3-isoprene oxidation is a
larger aerosol source than currently understood, and if future
NOx reductions lead to a stronger sensitivity in nighttime
BVOC oxidation rates, regional SOA loadings could decrease by a substantial
fraction from the typical regional summertime OA loadings of
5±3 µg m-3 (Saha et al., 2017).
Analysis of the degree of oxidation and chemical composition of NO3 + isoprene
SOA would help to elucidate mechanistic reasons for the
different field and lab SOA yields. For example, the potential contribution
of the uptake of morning-after OH + NISOPOOH produced epoxides, discussed
above in Sect. 4.3, onto existing (acidic) aerosol could be quantified by
measurement of these intermediates or their products in the aerosol phase.
Assessment of degree of oxidation could help determine whether
auto-oxidation mechanisms are active. Future similar field studies would
benefit from the co-deployment of the complementary tool of a Chemical
Ionization Mass Spectrometer (CIMS) to detect NO3 + isoprene products
such as organic nitrates (Slade et al., 2017; Lee et al., 2016). Due
to the potentially large effect on predicted SOA loading in regions of high
isoprene emissions, a better mechanistic understanding of these observed
yields is crucial.