Introduction
In remote continental regions, biogenic volatile organic compounds (BVOCs),
including isoprene and terpenes, are the most reactive organic compounds in
the atmosphere (Guenther, 2013). The oxidative chemistry of BVOCs affects the
distribution of oxidants (OH, O3, NO3) and the lifetime
of NOx (=NO+NO2), creating a feedback
loop that affects oxidant concentrations, the lifetime of BVOCs and secondary
organic aerosol (SOA) formation. Along the pathway to complete oxidation of BVOCs,
reactions with the nitrogen oxide family radicals (NO, NO2 and
NO3) to form organic nitrate products (e.g., Perring et al., 2013)
are an important branch point that sets the importance of this feedback
(Carlton et al., 2018; Mao et al., 2018).
During the day, BVOCs react with the hydroxyl radical (HO) and peroxy
radicals (RO2) are formed. At high and even modest concentrations
of NOx, the peroxy radicals react primarily with NO. The
major products of that reaction are NO2 and an alkoxy radical (RO).
There is also a minor channel (with a branching fraction α) that
results in addition of the NO to the peroxy radical, resulting in an organic
nitrate (RONO2) product. During the night, nitrate radicals
(NO3), the product of the oxidation of NO2 by
O3, are also a major source of RONO2. BVOCs react with
NO3, resulting in the formation of nitrooxy peroxy radicals in high
yields. The radicals subsequently react to form closed shell RONO2,
with branching ratio β.
In the last decade, there have been major updates to our understanding of the
chemical reactions that occur during isoprene oxidation (Paulot et al.,
2009a, b; Crounse et al., 2011; Liu et al., 2013; Peeters et al., 2014;
Nguyen et al., 2014; Wolfe et al., 2016; Mills et al., 2016; Teng et al.,
2017). This understanding includes recognition that the yield of
RONO2 from reaction of isoprene peroxy radicals with NO is
11 %–15 %, which is at the high end of the range reported in earlier
laboratory experiments (Wennberg et al., 2018). The yield of nitrates from
monoterpene oxidation is less clear as laboratory data indicate a very wide
range (e.g., from greater than 1 %, Aschmann et al., 2002, to 26 %,
Rindelaub et al., 2015). For NO3 oxidation of isoprene experimental
data show that the yield, β, is high and varies in the range of
65 %–80 % (Perring et al., 2009a; Rollins et al., 2009; Kwan et al.,
2012).
Once formed, RONO2 can be photolyzed or oxidized to produce
NOx or HNO3 along with an organic partner, or they
can serve as reservoirs of NOx that can be transported or deposited
to the surface. An additional pathway for gas-phase RONO2 loss is
partitioning into aerosol in either an organic phase where vapor pressure
would describe partitioning or an aqueous phase where a Henry's law constant
would describe solubility. In the aerosol, the RONO2 can undergo
liquid-phase reaction. Some RONO2 are rapidly hydrolyzed with
timescales on the order of hours to minutes under environmentally relevant pH
conditions (Jacobs et al., 2014; Boyd et al., 2015; Rindelaub et al., 2016),
while other nitrates are thought to be relatively stable against hydrolysis
in neutral conditions (Hu et al., 2011). The main nitrogen-containing product
of organic nitrate hydrolysis is nitric acid (Darer et al., 2011). Using
measurements of organic nitrates and nitric acid over the Canadian boreal
forest and southeast United States, Browne et al. (2013) and Romer et
al. (2016) provide evidence that hydrolysis of monoterpene and isoprene
nitrates is likely a significant loss process and contributes to
HNO3 production. The short lifetime of HNO3 to deposition
in the boundary layer means that organic nitrate loss through hydrolysis in
the boundary layer is a permanent sink of NOx.
For any organic nitrate, its structure determines the rate of its oxidation
and photolysis as well as the rate of hydrolysis and deposition.
Multifunctional nitrates containing hydroxyl or peroxide groups are likely to
have deposition rates much faster than the rates for monofunctional nitrates
(Shepson et al., 1996). The dry deposition of organic nitrates has been
discussed in the studies by Farmer and Cohen (2008) and Nguyen et al. (2015).
Nguyen et al. (2015) directly measured deposition rates of organic nitrates
from BVOCs and the first-generation isoprene nitrates were observed to have
a daytime dry deposition velocity of ∼2 cm s-1, which is higher
than the values currently used in most models (Ito et al., 2009; Mao et al.,
2013; Browne et al., 2014).
Unlike hydrolysis of organic nitrates in aerosol and deposition of organic
nitrates to the surface, which is considered a sink of nitrogen oxides from
the atmosphere, oxidation and photolysis of RONO2 may recycle
NOx. Different assumptions regarding NOx
recycling during organic nitrate oxidation result in large variations in
simulating NOx and O3 (von Kuhlmann et al., 2004;
Fiore et al., 2005; Wu et al., 2007; Horowitz et al., 2007; Paulot et al.,
2012). For example, Xie et al. (2013) showed that the uncertainty in the
fraction of NOx returned to the atmosphere during isoprene
nitrate oxidation had a larger impact than uncertainty in isoprene nitrate
yield on O3 production. This affirms the need for characterization
of the fate and lifetime of RONO2 in the atmosphere. New clarity is
available for the chemical fate of the first-generation isoprene nitrates
(e.g., Lee et al., 2014; Xiong et al., 2015, 2016), while much less is known
about the fate of organic nitrates formed from monoterpenes. Because few of
these loss processes have been measured, especially for highly oxidized or
monoterpene nitrates, there is large uncertainty associated with any
description of the lifetime of organic nitrates. Several modeling studies
(Paulot et al., 2012; Xie et al., 2013; Mao et al., 2013) have focused
specifically on the fate of isoprene nitrates and have found that how their
chemistry is represented has major consequences for NOx and
O3. Recently, Browne et al. (2014) extended the representation of
organic nitrate chemistry by including in detail the gas-phase chemistry of
monoterpenes and discussed different scenarios for uncertain loss processes
of monoterpene nitrates. Their improved mechanism for BVOC chemistry has been
used as a skeleton for several subsequent modeling studies (e.g., Fisher et
al., 2016 and this work).
However, none of these models has yet combined detailed molecular
representations of individual RONO2 derived from anthropogenic,
isoprene and monoterpene VOC precursors. Here we describe the development of
a gas-phase mechanism along those lines. In a forthcoming paper we couple the
mechanism described here to aerosol and cloud properties. Here we approximate
the effects of aerosols and clouds with simpler parameters representing the
effects of the condensed-phase chemistry. The model calculations are compared
to observations from the SOAS (the Southern Oxidant and Aerosol Study)
campaign in the rural southeastern United States in summer 2013. We explore
the relative contributions of OH and NO3 chemistry to the
production of organic nitrates from BVOCs and investigate the importance of
different organic nitrate loss processes. Then we explore the lifetime of
organic nitrates and consequences of organic nitrate chemistry for
atmospheric NOx. This information helps to understand the
role of RONO2 in the NOx and ozone budgets in the
moderate NOx, BVOC-dominated terrestrial environments that
represent the most common chemical regime on the continents during summer.
Model description: WRF-Chem model
We use the Weather Research and Forecasting (WRF) model coupled with
Chemistry (WRF-Chem) version 3.5.1 (Grell et al., 2005) with a horizontal
resolution of 12 km over the eastern United States. Our simulation domain is
defined on the Lambert projection, which is centered at 35∘ N,
87∘ W and has 290 and 200 grid points in the west–east and
south–north directions, respectively (see Fig. 3 for the horizontal domain).
The vertical coordinate is hybrid sigma-pressure that covers 30 levels from
the surface to 100 hPa. Near-surface levels follow terrain and gradually
transition to constant pressure at higher levels. Vertical grid spacing
varies with height such that finer spacing is assigned to the lower
atmosphere while coarser vertical spacing is applied at higher levels. In
this analysis, the model predictions are averaged over two lowest model
levels (∼25 m) used for comparison with ground-based measurements
taken from a 20 m walk-up tower. The predicted concentrations in the boundary
layer are described as an average over eight vertical model levels with a height
(∼1000 m) that is comparable with the planetary boundary layer depth
at midday in the southeastern United States in June 2013. Meteorological data for
initial and boundary conditions are driven by the North American Regional
Reanalysis (NARR) and data for the chemical initial and boundary conditions
are taken from MOZART (Emmons et al., 2010). The model simulation period is
from 27 May to 30 June 2013, with the first 5 days as spin-up, similar to
Browne et al. (2014), to remove the impact of initial conditions.
Anthropogenic emissions are based on the US EPA 2011 National Emission
Inventory (NEI) and scaled to 2013 based on the changes in the annual average
emissions from 2011 to 2013. The appropriate scale factors have been derived
from the NEI Air Pollutant Emissions Trend Data. We also adjust
NOx emissions (uniformly reduced by 50 %) following
Travis et al. (2016), who suggest that reduced NOx emissions
can better reproduce the SEAC4RS aircraft measurements for the southeastern
United States. Lightning emissions of NOx are not included
in the model. Lightning NOx is mainly released at the top of
convective updrafts (Ott et al., 2010) and does not strongly impact the
distribution of NO2 in the boundary layer (e.g., Laughner and Cohen,
2017). Biogenic emissions of isoprene, monoterpenes, other BVOCs, oxygenated
VOCs (OVOCs) and nitrogen gas emissions from the soil are parameterized
using the Model of Emissions of Gases and Aerosol from Nature (MEGAN)
(Guenther et al., 2006).
Gas-phase reactions are simulated using the second-generation Regional
Atmospheric Chemistry Mechanism (RACM2) (Goliff et al., 2013), as updated by
Browne et al. (2014) and with additions to the mechanisms as described below,
which is implemented with the kinetic preprocessor (KPP) software (Damian et
al., 2002). The Modal Aerosol Dynamics Model for Europe–Secondary Organic
Aerosol Model (MADE/SORGAM) aerosol module (Ackermann et al., 1998; Schell et
al., 2001) is used to treat organic and inorganic aerosols. This two-product
aerosol scheme does not treat the partitioning of individual chemical species
such as organic nitrates. Therefore, we focus here only on investigating the
impacts of the gas-phase representation of the chemistry and a full
consideration of the gas and aerosol in a coupled framework is a subject of
continuing research.
Chemical mechanism
We base our core chemistry on a modified version of RACM2
(Goliff et al., 2013). The base we begin
with is described by Browne and Cohen (2012) and Browne et
al. (2014) and is referred to
here as the RACM2_Berkeley scheme. Full details of the
RACM2_Berkeley mechanism and a complete list of the compounds
can be found in Browne et
al. (2014). RACM2_Berkeley includes updates to the isoprene oxidation mechanism
(Paulot
et al., 2009a, b; Peeters and Müller, 2010; Lockwood et al., 2010;
Stavrakou et al., 2010; Crounse et al., 2011), an extended mechanism for
anthropogenic-originated organic nitrates (Carter
and Atkinson, 1989; Middleton et al., 1990; Arey et al., 2001), and updates
for monoterpene chemistry. Browne et
al. (2014) evaluated the
RACM2_Berkeley mechanism using aircraft observations over the
Canadian boreal forest.
Schematic representation of the formation of isoprene nitrates (in
green) initiated by OH oxidation. Rerelease of the consumed
NOx to the atmosphere by chemical loss processes of
oxidation, ozonolysis and photolysis of organic nitrate is shown in red. See
Table S2 in the Supplement for species descriptions.
![]()
Schematic representation of the formation of isoprene nitrates (in
green) initiated by NO3 oxidation. For simplification, fates of
only one isomer of hydroxy nitrates (IHNB and IHND) and nitrooxy hydroperoxide
(INPB and INPD) are shown. See Table S2 in the Supplement for species
descriptions.
![]()
In this study, the RACM2_Berkeley scheme is further updated
with recent advances in the representation of OH- and NO3-initiated
BVOC oxidation under both low- and high-NOx conditions, as
well as with improved deposition rates, and is denoted RACM2_Berkeley2 (see Tables S1–S3 in the Supplement). We begin with a more complete
description of recent advances in our understanding of isoprene chemistry
(Fig. 1). The hydroxy peroxy radical (ISOPO2) that is the product of isoprene
oxidation by OH has multiple potential fates. ISOPO2 can undergo unimolecular
isomerization, leading to the production of hydroperoxy aldehydes (HPALD),
among other products. It can react with HO2 to produce isoprene hydroxy
hydroperoxide (ISHP), methyl vinyl ketone (MVK), methacrolein (MACR) and
CH2O. The latter three species can also be formed from the reactions of
ISOPO2 with other (acetyl or methyl) peroxy radicals. ISHP reacts with OH to
form isoprene epoxydiols (IEPOX) and regenerate OH. St. Clair et
al. (2015)
found that the reaction rate of ISHP+OH is approximately 10 % faster
than the rate given by Paulot et al. (2009b) and
indicate the relative role of the different isomers of ISHP. Here we use
kinetics and products of the reactions of three different isomers of ISHP
with OH based on St. Clair et
al. (2015).
We also increase the molar yield of total ISHP from the ISOPO2+HO2
reaction to 93.7 % (Liu
et al., 2013), with a decrease in the yields of MVK, MACR and HOx to
maintain mass balance. We use rates from Bates et
al. (2016) for reactions of three
different isomers of IEPOX with OH.
We maintain the overall branching ratio of isoprene nitrates at 11.7 % as
in Browne et al. (2014) while
changing the mix of isomers. Browne et
al. (2014) implemented a scheme
for the reaction of ISOPO2 with NO, based on experiments conducted by Paulot
et al. (2009b), including β- and δ-hydroxy isoprene nitrates (ISOPNB and ISOPND) with yields of 4.7 % and
7.0 %, respectively. Here we update the yield of β versus δ isomers to 10.5 % and 1.2 %, respectively. A theoretical study by
Peeters et al. (2014)
showed that the peroxy radical redissociations are fast and peroxy isomers
may interconvert, so that β isomers comprise ∼95 % of the
radical pool. The experimental findings of Teng et al. (2017) are
also consistent with that idea. Simulations by Fisher et
al. (2016)
showed that an isoprene hydroxy radical distribution leading to 90 %
ISOPNB and 10 % ISOPND is consistent with SOAS observations.
ISOPND and ISOPNB then photolyze or react with O3 or OH to yield
either NOx or second-generation organic nitrates. We update
reaction rates of isoprene hydroxy nitrate oxidation based on Lee et
al. (2014). Compared to the RACM2_Berkeley mechanism (based on Paulot et
al., 2009b and Lockwood et al., 2010), the reaction rates for
ISOPNs+OH are increased and the rate coefficients of
ISOPNs+O3 are decreased. The model represents the products
of the reactions of ISOPND and ISOPNB with OH as ISOPNDO2 and ISOPNBO2. We
update the reaction of ISOPNB+OH to include a small yield of
IEPOX and NO2 (12 %) as found by Jacobs et al. (2014). We also
update the rate constants for reaction of ISOPNDO2 and ISOPNBO2 with NO,
producing second-generation isoprene nitrates following Lee et al. (2014).
Second-generation isoprene nitrates from the OH-initiated pathway include
ethanal nitrate (ETHLN), propanone nitrate (PROPNN), multifunctional isoprene
nitrate (IMONIT), methacrolein nitrate (MACRN) and methyl vinyl ketone nitrate
(MVKN). MACRN and MVKN can also be formed directly from photooxidation of MVK
and MACR under high-NOx conditions. Here, we follow Praske
et al. (2015) to update MVK chemistry under both low- and
high-NOx conditions, resulting in greater recycling of OH
and NO2 and decreased formation of organic nitrates from reactions
with NO or HO2.
In addition to OH chemistry, isoprene is oxidized by NO3. In
RACM2_Berkeley, the isoprene + NO3 chemistry was
parameterized with one generic organic nitrate as the product. Recently,
Schwantes et
al. (2015)
developed a kinetic mechanism for NO3-initiated oxidation of
isoprene in which products, branching ratios and rate constants are
estimated based on recent experimental results. Their suggested products from
NO3 oxidation of isoprene are consistent with organic nitrates
detected in the ambient atmosphere during SOAS. RACM2_Berkeley2 treats
the NO3-initiated oxidation of isoprene in some
detail with formation and subsequent oxidation of isoprene nitrates largely
based on the work of Schwantes et
al. (2015)
and Rollins et
al. (2009)
(Fig. 2). In the first step, NO3 addition to isoprene forms a
nitrooxy peroxy radical (INO2) and then, depending on the radical the
INO2 reacts with, first-generation isoprene nitrates are formed,
namely C5 carbonyl nitrate (ICN), β- and δ-hydroxy nitrate
(IHNB and IHND) and β and δ-nitrooxy hydroperoxide (INPB and
INPD).
We set the ICN yield at 54 % and 72 % for the
INO2+NO3
and INO2+INO2 reactions, respectively. ICN is a
first-generation isoprene nitrate that is reactive towards NO3
(Rollins et al.,
2009). Subsequent oxidation of ICN by NO3 forms second-generation
isoprene nitrates as well as nitric acid at rates and yields based on the Master
Chemical Mechanism (MCM v3.2) (Jenkin
et al., 1997; Saunders et al., 2003). MCM v3.2 uses a reaction rate
coefficient of 1.22×10-14 cm3 molec-1 s-1 at 298 K,
which is 5 times slower than the rate given by Rollins et
al. (2009) and an
order of magnitude faster than the rate given by Schwantes et
al. (2015).
Given the differences in the experimental data, splitting the difference by
using the MCM rate seems a reasonable choice.
IHNB and IHND are also identified in the chamber experiments as products of
the INO2+INO2 reaction. We follow Schwantes et
al. (2015)
and use rate constants and products, respectively, from Lee et
al. (2014) and Jacob et
al. (2014), for the
subsequent fate of IHNB and IHND upon reaction with OH. We note that these
rate constants and products correspond to the rate constants and products for
the reactions of ISOPND and ISOPNB (OH-initiated isoprene hydroxy nitrates)
with OH. In Fig. 2, we only show reactions for one of the isomers.
Nitrooxy hydroperoxides (INPD and INPB) are the dominant products of the
INO2+HO2 reaction with a combined 77 % yield, most
of which is δ isomers (INPD). Schwantes et al. (2015) found that the
total molar yield of INPD and INPB per reacted isoprene is higher than the
yield found in previous studies (Ng et al., 2008; Kwan et al., 2012). In
these previous studies, the carbonyl nitrate (ICN) was the main contributor
to the family of isoprene nitrates produced by NO3-initiated
chemistry and nitrooxy hydroperoxides (INPD and INPB) were a minor fraction.
The difference is likely caused by variation in the fate of the nitrooxy
peroxy radical (INO2+HO2 vs.
INO2+NO3) under different experimental conditions. The
fate of this radical (INO2) in the nighttime atmosphere is still
highly uncertain (Brown and Stutz, 2012; Boyd et al., 2015).
Chamber experiments by Schwantes et
al. (2015) suggested that OH reacts with INPD and INPB to form second-generation
isoprene nitrates and nitrooxy hydroxy epoxides (INHE), a newly identified
product that undergoes a similar heterogeneous chemistry as IEPOX and can
impact secondary organic aerosol formation. We set the yield of the
INHE δ isomer from INPD at 37 % and yield of the INHE β isomer from INPB at 78 %. Those are lower than the IEPOX yield formed
from ISHP. INHE isomers will be further oxidized by OH, leading to recycling
of NOx or forming a later generation of organic nitrates. We
use the same rates that we use for OH oxidation of IEPOX isomers (Schwantes
et al., 2015; Bates et al., 2016). In this scheme, we also assume that INPD
and INPB undergo hydrogen abstraction from the hydroperoxide group with the
same rate constants as those we use for hydrogen abstraction from the
hydroperoxide group of ISHP as suggested by St. Clair et
al. (2015).
(a) Concentrations of total organic nitrates for the
average of the model simulation period (June 2013). (b) Fractional
contribution of BVOC-derived organic nitrates to total organic nitrates. The
location of the SOAS CTR ground site is circled in the figure.
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C4 carbonyl hydroperoxy nitrates (R4NO) and C4 carbonyl hydroxy nitrates (R4N) form from
oxidation of the first-generation isoprene nitrates. In reaction with OH,
they produce propanone nitrate. The subsequent fate of PROPNN is
dominated by oxidation by OH and photolysis, resulting in the return of
NOx to the atmosphere. Here, we include a photolysis rate
for PROPNN and other carbonyl nitrates (ICN, ETHLN), MVKN and MACRN that is
faster than that used in some other recent mechanisms following the
recommendations in Müller et al. (2014). These rates are 10 times larger
than the rates given by Paulot et al. (2009a). We use the enhanced photolysis
rate of PROPNN for carbonyl nitrate ICN formed from NO3-initiated
isoprene oxidation and also include photolysis reactions for other new
isoprene nitrate species, following Schwantes et al. (2015) and MCM v3.2. The
fast photolysis reactions of organic nitrates implemented in this work lead
to an increase in NOx concentrations.
Organic nitrate deposition to surfaces
We use the resistance-based approach based on the original formulation of
Wesely (1989) to calculate dry
deposition velocities. Dry deposition rates of RONO2 in
RACM2_Berkeley follow Ito et
al. (2007). In RACM2_Berkeley2, using the same formalism, we update the dry deposition parameters
(effective Henry's Law coefficients, H*; and reactivity factors, f0) for
isoprene nitrates as recommended by Nguyen et
al. (2015), which results
in more rapid removal than in RACM2_Berkeley (Table S3 in the
Supplement). We also update the dry deposition parameters for
monoterpene-derived nitrates that were previously assumed to deposit at a
rate similar to the deposition rate of isoprene hydroxy nitrates (Browne
et al., 2014; Ito et al., 2007). Dry deposition velocities of the nitrooxy
hydroxy epoxides (INHE) are assumed to be similar to the updated depositional
loss rate of IEPOX, given by Nguyen et
al. (2015).
Hydrolysis of tertiary nitrates
In addition to oxidation, photolysis and deposition to the surface, another
possible fate of organic nitrates is uptake to the aerosol phase followed by
hydrolysis. A rapid hydrolysis (Hu
et al., 2011; Jacobs et al., 2014) is recognized for tertiary nitrates, while
non-tertiary nitrates under atmospheric conditions are considered unreactive (Darer
et al., 2011; Boyd et al., 2015). Due to the limitations of the model
representation of organic nitrate aerosol from either aqueous (Marais
et al., 2016) or vapor-pressure-dependent pathways (Pye et al., 2015), we
represent this process for gas-phase organic nitrates by applying a
timescale of 3 h for tertiary nitrates based on the laboratory chamber study by
Boyd et
al. (2015).
The fraction of tertiary (Ftertiary) vs. non-tertiary nitrates is
estimated, depending upon the molecular structure of the nitrate, from MCM
v3.2.
We apply Ftertiary at 41 % for β isomers of isoprene nitrates
from OH oxidation while we assume that all δ isomers are
non-tertiary. Most of the nitrates formed by NO3-initiated
chemistry of isoprene are not tertiary nitrates. The fraction of tertiary
nitrates for monoterpene-derived nitrates is also different for species
formed by OH oxidation than from NO3 oxidation. In
RACM2_Berkeley2, we introduce TONIH (C10 nitrooxy
hydroperoxide), TONIN (saturated) and UTONIN (unsaturated)
monoterpene-derived nitrates from NO3 oxidation, which differ from
the unsaturated (UTONIT) and saturated (TONIT) monoterpene-derived nitrates
from OH oxidation. Since contributions of tertiary limonene, α-pinene
and β-pinene nitrates from NO3 reaction are 35 %,
15 % and 50 % (MCM v3.2), respectively, we define Ftertiary at
35 % as an average for TONIH, UTONIN and TONIN. Ftertiary is
defined at 77 % for UTONIT and TONIT. The value is the average of the
62 % for α-pinene nitrates and 92 % for β-pinene
nitrates and is equal to the 77 % for limonene nitrates from OH
chemistry.
Further changes in the RACM2_Berkeley2 mechanism for
monoterpene nitrate chemistry consist of a revised reaction rate for
API + NO3. The rate constant is calculated as an average of the
rates given in MCM v3.2 for α-pinene and β-pinene, as API in
the mechanism indicates a 50–50 mixture of α-pinene and β-pinene. In our mechanism, following Browne et
al. (2014), first-generation monoterpene nitrates react with O3 and OH and form
second-generation nitrates. Here, we also add reaction of first-generation
monoterpene nitrates with NO3 with the rate constant K =3.15×10-13 exp(-448.0/Temp), following Fisher et
al. (2016).
We assume the second-generation monoterpene nitrate can oxidize, photolyze
and deposit identically to nitric acid (Browne et al., 2014). In
summary, we have described a detailed chemical mechanism tracking individual
organic nitrates in some detail through second-generation products of
isoprene and monoterpene oxidation.
Results and discussion
We evaluate our mechanism by comparison to SOAS observations in Bibb County,
Alabama (32.90∘ N latitude, 87.25∘ W
longitude), in summer 2013 (Carlton
et al., 2018; Mao et al., 2018). These observations together with field
campaign data from the long-term monitoring site in the Southeastern Aerosol
Research and Characterization (SEARCH) Network (Hansen et al., 2003) (at the same
location) provide unique resources for evaluation of our model of organic
nitrate chemistry. The measurements include total and speciated organic
nitrates, gas-phase and aerosol organic nitrates, HOx radicals, a wide
range of VOCs and ozone (Pye et al., 2015; Romer et al., 2016; Lee et al., 2016).
Organic nitrate concentrations
Figure 3a shows the spatial distribution of total organic nitrates for the
24 h average of the model simulation period at the surface. The location of
the SOAS ground site (at Centreville, CTR) is circled in the figure. The campaign area
is in a location with among the highest modeled organic nitrate
concentrations in the region, up to 350 ppt. Figure 3b highlights that the
modeled RONO2 originating from biogenic VOCs dominate over the
organic nitrates with anthropogenic VOC precursors over most of the domain.
In the southeast, up to 80 % of organic nitrates are biogenic. Biogenic
nitrates are 40 %–50 % in the northern portion.
Median diurnal cycles of observed (black) and simulated (red) total
organic nitrates at Centreville during the 2013 SOAS campaign. The vertical
bars show the interquartile range of the hourly data. The panel includes the
mean of the simulated and observed organic nitrates. Diurnal cycles of the
OH-initiated, NO3-initiated and second-generation organic nitrate
concentrations are shown as the stacked bars.
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Figure 4 compares the median diurnal cycle of observed total organic nitrates
from the SOAS campaign to the model simulation during the simulation period.
Total organic nitrates in both the gas and particle phase were measured by
TD-LIF (thermal dissociation laser-induced fluorescence, Day et al., 2002).
Temporal variability in the total organic nitrates for the entire time series
is reproduced with little bias (r2=0.8 and normalized mean bias
(NMB) = 32 %). Although the mean of the simulated organic nitrates
(0.26±0.19) slightly overestimates the mean of the observations
(0.20±0.1), the medians are found to be within variability of the
observations. The simulated mean of total RONO2 in this study is in
the range of two other recent modeling studies over the southeastern United
States in summer 2013 that simulated 200 ppt (Fisher et al., 2016) and
270 ppt (Li et al., 2018). However, in both of these studies RONO2
derived from anthropogenic VOC precursors were not included. In our
simulation, these organic nitrates represent ∼20 % of total
RONO2. Specific sources of the differences include the slightly
smaller yield of 10 % for isoprene nitrates and application of a
3 h hydrolysis lifetime only for ISOPNB in Li et al. (2018). Fisher et
al. (2016) apply a faster hydrolysis rate (1 h) for all organic nitrates and
a lower yield (9 % for isoprene nitrates).
The highest bias in the model median values and variability is observed
after sunset to sunrise, which is likely caused by mismatch in vertical
turbulent mixing in the simulated and actual boundary layers. Inclusion of
hydrolysis as a possible fate for tertiary organic nitrates results in
significant improvement of the simulations compared to the observations (not
shown here). Tertiary nitrates have shorter lifetime against hydrolysis under
atmospheric conditions, compared to the lifetime against deposition (Fig. S1
in the Supplement), making them the most important sink of nitrates. Li et
al. (2018)
also showed, by introducing the hydrolysis of ISOPNB, the model relative bias
of total RONO2 was reduced by 18 % during ICARTT (International Consortium for Atmospheric
Research on Transport and Transformation, summer 2004)
over the southeastern United States.
Diurnal cycles of measured and simulated RONO2 have maximum values
at 320 and 370 ppt around 10:00 CDT, respectively, with a slow decline through
the rest of the day. Throughout the night, the mixing ratios were observed
and modeled to remain nearly constant. Around 10:00, when the highest total
organic nitrates are observed during SOAS, the simulated OH-initiated and
second-generation organic nitrate concentrations both reach their maximum.
Second-generation nitrates do not show sharp variability over day and night
because of their longer lifetime but they do slightly increase after sunrise
(around 07:00). OH-initiated organic nitrates that can remain in the residual
layer overnight contribute to the total organic nitrate during the morning.
At sunrise, when OH and NO begin to increase (Fig. S2 in the Supplement),
OH-initiated organic nitrates increase until they reach their maximum at
around 10:00. In contrast, NO3-initiated organic nitrates reach
their peak mixing ratio before sunrise, and, immediately after sunrise, they
decline sharply to a minimum concentration during the day. As the sun sets,
NO drops to near zero and NO3 production initiates the formation of
organic nitrates. OH- and NO3-initiated reactions occur out of phase
in the diurnal cycle, resulting in the relatively flat diurnal profile for
total organic nitrate throughout the night. Observations of individual
molecules are predicted to have more strongly varying diurnal cycles,
consistent with observations (Xiong et al., 2015).
Organic nitrate composition
The composition of the simulated organic nitrates by our model during SOAS at
the Centreville site is shown in Fig. 5a. Monoterpene nitrates are
calculated to be one-third of total organic nitrates, which is comparable to
the Browne et al. (2014)
calculation for boreal regions of North America. We define total
isoprene-derived nitrates in WRF-Chem as the sum of isoprene hydroxy
nitrates, isoprene carbonyl nitrates, MVK and MACR nitrates, isoprene
nitrooxy hydroperoxides, ethanal nitrate, propanone nitrate and
multifunctional isoprene nitrates. We find the contribution of the total
isoprene-derived nitrates to total organic nitrates to be 44 %. This is
consistent with the range of 25 %–50 % observed from SEAC4RS airborne
measurements taken onboard the NASA DC-8 in August–September 2013 over the
southeastern United States (Fisher
et al., 2016). However, it is in contrast to other recent modeling studies
over the southeastern United States by Mao et
al. (2013) and Xie et
al. (2013) that
suggested, respectively, more than 90 % and 60 % of total organic
nitrates are from isoprene oxidation. This discrepancy is likely due to the
simulated longer lifetime of these nitrates as well as omission of organic
nitrates produced from monoterpenes and anthropogenic VOCs in those models.
The composition of the (a) simulated organic nitrates by
WRF-Chem using RACM2_Berkeley2 and (b) observed organic nitrates
during SOAS at CTR.
![]()
The observed RONO2 composition during SOAS is shown in Fig. 5b. The
sums of the individual isomers of isoprene nitrates, terpene hydroxy nitrates
and terpene nitrooxy hydroperoxides were measured in the gas phase by chemical
ionization time-of-flight mass spectrometry using CF3O- reagent ion
(Crounse
et al., 2006; Schwantes et al., 2015; Teng et al., 2015), and ethyl and
isopropyl nitrates were measured by gas chromatography–mass spectrometry
(de Gouw et al., 2003). Similar to the model results, the largest contributions to the total
organic nitrates in the observations are isoprene oxidation products, which
represent 22 % against 44 % in the model. Carbonyl isoprene nitrates
including ICN, ETHLN and PROPNN as a fraction of total RONO2
(8 % in the model and <7 % in the observations) and their
concentrations (Fig. S3a and b in the Supplement) are reproduced well by the
model. However, the model overestimates the fraction of RONO2 that
is isoprene hydroxy nitrates and MVKN + MACRN (21 % modeled vs. 13 %
observed). Isoprene hydroxy nitrates from NO3-initiated chemistry
are a small portion of the total simulated isoprene hydroxy nitrates
(∼15 %). The difference between the modeled and observed
contribution of isoprene hydroxy nitrates to total organic nitrates is thus
more likely a result of differences between the modeled and observed nitrates
that are products of OH-initiated chemistry. Insights from very recent
studies by Teng et
al. (2017) and Wennberg et
al. (2018) suggest a larger
Ftertiary for OH-initiated isoprene hydroxy nitrates than the value we
calculated from MCM. That suggests a larger fraction of these nitrates is
subject to hydrolysis and thus it perhaps explains part of the discrepancy
between the model simulations and observations.
The modeled and observed correlations between Ox
(=O3+NO2) and organic nitrate concentrations during
daytime at SOAS. The lines indicate linear regression (intercept fixed at 0)
and confidence intervals. The panel includes slopes of the lines and root
mean square errors (RMSE).
![]()
The largest difference between the modeled and observed contribution of
isoprene nitrates to total organic nitrates is due to the modeled gas-phase
multifunctional isoprene nitrates and isoprene nitrooxy hydroperoxides.
Aerosol- and gas-phase second-generation multifunctional isoprene nitrates
and aerosol-phase isoprene nitrooxy hydroperoxides were not individually
measured during SOAS. Instead, total aerosol-phase organic nitrates were
measured by TD-LIF, using an activated charcoal denuder to remove gas-phase
organic nitrates and found to contribute around 40 % of total organic
nitrates at the SOAS CTR site (Fig. 5b). Ng et
al. (2008)
and Rollins et
al. (2009) found
isoprene oxidation can form 4 %–23 % nitrate aerosol yields and showed
multifunctional nitrates to be a dominant nitrate aerosol. If the isoprene
nitrooxy hydroperoxides are favored to partition to aerosol this would
explain the model-measurement discrepancy for the calculated contribution of
multifunctional isoprene nitrates and isoprene nitrooxy hydroperoxide. They
are simulated in the gas phase using RACM2_Berkeley2 but we
might interpret them as contributing to particle-phase organic nitrate. That
is consistent with the Lee et
al. (2016) finding from observations
of speciated particle organic nitrates during the SOAS campaign. They showed
particle isoprene nitrates have a dominant contribution from highly
functionalized isoprene nitrates containing between six and eight oxygen
atoms.
The modeled and observed correlations between CH2O and
organic nitrates during daytime at SOAS. The slope shows the best-fit line,
with an intercept allowed to differ from zero to consider the possibility of
background CH2O. The panel includes slopes of the lines and root
mean square errors (RMSE).
![]()
Nitrate aerosol yields for monoterpene oxidation reactions from different
laboratory chamber experiments, field measurements and modeling studies have
been reported to be very high (up to 100 %)
(Russell
and Allen, 2005; Fry et al., 2009, 2011, 2013; Pye et al., 2015; Boyd et al.,
2015). Among monoterpene nitrates, NO3-initiated nitrates (Ayres
et al., 2015) and functionalized nitrates (Lee et al., 2016) have been shown
to be an especially significant fraction of the total particle organic
nitrate source at SOAS site. These findings imply that the remainder of the
measured particle organic nitrates can be attributed to mono- or
sesquiterpene-derived RONO2 including NO3-initiated
terpene hydroxy nitrates, terpene nitrooxy hydroperoxides and multifunctional
terpene nitrates, which are simulated and present in the gas phase in our
mechanism. If we interpret the aerosol nitrates to be these compounds, then
we find a rough correspondence between the model and observations (see
Fig. 5a and b). However, Li et
al. (2018) estimated a smaller contribution of gas-phase NO3-initiated monoterpene
nitrates to total RONO2 due to a lower molar yield (10 % vs. 70 % for
high-reactivity monoterpenes and 10 % for low-reactivity monoterpenes in
this study). In contrast, due to other differences in the mechanism they
found a larger contribution of OH-initiated monoterpene nitrates to total
RONO2 than our finding in this study.
Diurnal cycle of fractional organic nitrate (a) production
and (b) concentrations simulated by WRF-Chem averaged over the
boundary layer at the CTR site during SOAS.
![]()
In RACM2_Berkeley2, the contribution from organic nitrates of anthropogenic
origin is simulated to be 21 % of total RONO2 (referred as
Other at Fig. 5a). That is higher than the 10 % inferred from the
observations of the measured anthropogenic organic nitrates at SOAS. Some
caution should be taken in the interpretation of such a comparison, as the
observations at SOAS do not represent the same species as the modeled ones
and include only ethyl and propyl nitrates (Fig. 5b). In RACM2_Berkeley2, a
wide range of the organic nitrates of anthropogenic origin
(C1–C5 nitrates) is categorized into four groups including
monofunctional saturated, multifunctional unsaturated, multifunctional
saturated and aromatic-derived nitrates that are partitioned from the lumped
precursors including alkanes, aromatics, alcohols and alkenes. However, the
remaining unspeciated measured RONO2 contributes 27 % of the
total organic nitrates observed by TD-LIF. One hypothesis to explain this
difference is that the rest of the simulated organic nitrates of
anthropogenic origin might be a portion of the unspecified measured
RONO2.
Relationships between <inline-formula><mml:math id="M189" display="inline"><mml:mrow class="chem"><mml:msub><mml:mi mathvariant="normal">RONO</mml:mi><mml:mn mathvariant="normal">2</mml:mn></mml:msub></mml:mrow></mml:math></inline-formula>, <inline-formula><mml:math id="M190" display="inline"><mml:mrow class="chem"><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mi>x</mml:mi></mml:msub></mml:mrow></mml:math></inline-formula> and <inline-formula><mml:math id="M191" display="inline"><mml:mrow class="chem"><mml:msub><mml:mi mathvariant="normal">CH</mml:mi><mml:mn mathvariant="normal">2</mml:mn></mml:msub><mml:mi mathvariant="normal">O</mml:mi></mml:mrow></mml:math></inline-formula>
During daytime, ozone and organic nitrates are produced in a common reaction
with branches that yield one or the other. Therefore, their observed and
modeled correlation provides an additional constraint on our understanding of
organic nitrates. The sum of O3 and NO2 is conserved on
longer timescales than O3 alone; accordingly we use
Ox=O3+NO2 in this analysis. As shown in Fig. 6, during
the daytime (from 08:00 to 18:00 local time) the modeled and observed
correlations between Ox and RONO2 are nearly identical. A
linear fit to the observations yields a line with slope of 129±4 ppbv(Ox) ppbv(RONO2)-1 and a fit to the model output yields
125±4 ppbv(Ox) ppbv(RONO2)-1.
This slope has typically been used to estimate the approximate branching
ratio of the entire VOC mixture (αeff) with an assumption that
photochemical production is rapid as compared to removal processes; αeff is
inversely proportional to the Ox vs. RONO2 slope
(Perring et al., 2013). The quantified α's in the laboratory for BVOCs
are much higher than typical α's for anthropogenic VOCs (Perring et
al., 2013, and references therein). Therefore, for regions
like the southeastern United States where BVOCs dominate the VOC mixture, a much
lower slope than our calculated value is expected. We conclude that the
observed slope is reflecting the short lifetime of organic nitrates at SOAS.
The simulated concentrations of first (blue) and second (red)
generation organic nitrates versus their loss rates during daytime at SOAS.
Slopes of the linear fit give their lifetimes. The concentrations and loss
rates of first-generation nitrates are divided by 2.
![]()
Contribution of different fates to (a) the first and
(b) second generation of isoprene and monoterpene nitrates loss.
![]()
Formaldehyde (CH2O) is another co-product to RONO2 and, as
Perring et al. (2009b) discussed, the slope of the RONO2/CH2O correlation is related
to the ratio of the production of both species, as both have similar
lifetimes (Perring et
al., 2009b). We would expect the slope could provide a constraint on the
yield of isoprene nitrates, especially since in much of the domain isoprene
is the dominant source of both RONO2 and CH2O. Figure 7 shows
the correlation between observed CH2O and RONO2 during SOAS.
The slope of the best-fit line, with an intercept allowed to differ from zero
to consider the possibility of background CH2O that mixes in from the
free troposphere, is found to be 0.116, consistent with previous estimates by
Perring et al. (2009b), who observed a slope of 0.119 during INTEX-NA (Intercontinental Chemical Transport
Experiment) in 2004. The slope would imply
an OH-initiated isoprene nitrate yield of 12 %
(Perring et
al., 2009b) if we use a lifetime of 1.7 h at SOAS for RONO2 as
reported by Romer et
al. (2016).
This is nearly identical to the yield used in the mechanism described in this
manuscript. However, the correlation of modeled CH2O and modeled total
RONO2 has a smaller slope of 0.085. The discrepancy between the
slopes from the simulated and observed data can be attributed to model
overestimation of CH2O (Fig. S4 in the Supplement). In Fig. S5, we also
provide additional model evaluation for isoprene and monoterpene
concentrations.
Organic nitrate formation
Figure 8a shows the diurnal cycle of fractional RONO2 production
simulated using RACM2_Berkeley2 averaged over the boundary
layer at the CTR site during SOAS. During the day, production of organic
nitrates is dominated by reaction of isoprene with OH. This is consistent
with the OH reactivity (OHR) of individually measured compounds at SOAS which
was dominated by reaction with isoprene (Kaiser
et al., 2016; Romer et al., 2016). In contrast, the vast majority of
RONO2 production at night is of monoterpene nitrates, which are formed
as a result of NO3 oxidation of nighttime monoterpene emissions.
NO3 chemistry of isoprene leading to isoprene nitrate formation is
also found to be significant during the nighttime. This fraction from
isoprene that accumulates in the boundary layer in late afternoons can reach
35 % of the total organic nitrates formed at night. In addition to
investigating the relative importance of instantaneous organic nitrate
production based on the VOC precursors, we calculate the fraction of isoprene
nitrates produced from NO3 chemistry to the total isoprene nitrate
production (∼44 %, Fig. S5 in the Supplement) over day and
night of the entire modeling period confirming the relative importance of
this pathway for producing isoprene nitrates versus OH oxidation of isoprene.
This finding is consistent with the modeling result (∼40 %) from Xie et
al. (2013).
The fraction of anthropogenic VOC-derived organic nitrates to total
simulated production of organic nitrates is estimated to be negligible;
however their contribution to the simulated concentrations of organic
nitrates is much higher and reached up to 0.25 (Fig. 8b). This is due to
their relatively long lifetime (>100 h lifetime to oxidation by
1×106 molecules cm-3 of OH at 298 K and a similarly long
lifetime to deposition – Henry's law constant of ∼1 Matm-1; Browne et al., 2014, and references therein)
that causes them to persist longer in the atmosphere. Similarly, the fraction
of second-generation nitrates formed from oxidation of first-generation
isoprene and monoterpene nitrates (Fig. 8a) is predicted to be
∼0.04, which is much less than the calculated contribution of their
concentrations to total organic nitrate concentrations (∼0.3)
(Fig. 8b). In the next subsection we will discuss more about the loss
processes and lifetime of these organic nitrates.
Organic nitrate lifetime
We define the lifetime of organic nitrates as the concentration of
RONO2 divided by the combined loss rate via all proposed loss
mechanisms. The loss mechanisms include chemical loss processes (oxidation,
photolysis and hydrolysis of RONO2) and deposition. The nighttime
lifetime of organic nitrates might be longer than the daytime value (and
might be similar to or longer than the length of a single night). Because of
uncertainties associated with simulation of the boundary layer height and
organic nitrate concentrations at nighttime, we focus on the daytime lifetime
as a guide for thinking about the organic nitrate fate. Figure 9 shows the
estimated daytime lifetime of 2 h for first-generation biogenic organic
nitrates and a longer lifetime for the second-generation organic nitrates
(3.2 h). Including organic nitrates from anthropogenic sources we estimate a
fairly short overall lifetime of 2.6 h for total RONO2. This short
lifetime results in less efficient transport of organic nitrates to the free
troposphere and over large distances from sources. Using the SOAS field
observations, Romer et al. (2016) suggested ∼1.7 h for the atmospheric
lifetime of RONO2. They calculated the lifetime by the assumption
that RONO2 are near steady state in the afternoon. If we constrain
our calculation to 12:00–16:00 and give an intercept of 40 ppt as Romer et
al. (2016) did, the overall estimated lifetime in the model is estimated to
be 2.9 h, but using the production rates of organic nitrates instead of the
loss rates (by assumption of the atmospheric steady state condition applied
in Romer et al., 2016) our result remarkably shows very good agreement with
their finding (identical value, Fig. S6 in the Supplement). GEOS-Chem
simulations by Fisher et al. (2016) reported a similar short lifetime by
assuming a hydrolysis lifetime of 1 h for all tertiary and
non-tertiary nitrates and not including the longer-lived small alkyl
nitrates. However, Li et al. (2018) estimated much longer lifetimes for
individual nitrates except ISOPNB, which they assumed to be hydrolyzed.
The simulated instantaneous production of NOx from
loss of organic nitrates versus the instantaneous loss of
NOx to production of organic nitrates. The slope shows the
NOx recycling efficiency.
![]()
Accurate determination of the lifetime of organic nitrates is a major
challenge for assessing the influence of organic nitrates on atmospheric
chemistry. However, the estimated lifetime of ∼3 h for
organic nitrates found here, as well as in other studies over the southeastern
United States
(Perring
et al., 2009b; Romer et al., 2016; Fisher et al., 2016), is less than the
range of NOx lifetimes (5.5–11 h) calculated by
observational studies (e.g.,
Valin et al., 2013; Romer et al., 2016). Organic nitrates should therefore
generally be categorized as short-lived NOx reservoirs,
which remove NOx in a plume, but act as a source of
NOx in remote regions.
<inline-formula><mml:math id="M252" display="inline"><mml:mrow class="chem"><mml:msub><mml:mi mathvariant="normal">NO</mml:mi><mml:mi>x</mml:mi></mml:msub></mml:mrow></mml:math></inline-formula> recycling efficiency
To determine the fraction of NOx converted to
RONO2 and then released back to the gas phase as
NOx, the relative importance of different loss pathways of
organic nitrates must be known. Oxidation and photolysis of organic nitrates
recycle NOx but hydrolysis and deposition cause permanent
removal of NOx from the atmosphere. Recent field studies
suggested that isoprene nitrates are removed quickly by dry deposition
(Nguyen et al., 2015) and
some have concluded that deposition is the primary sink of nitrates (Rosen
et al., 2004; Horii et al., 2006; Horowitz et al., 2007), while others
estimated that oxidation or ozonolysis is the dominant loss mechanism of
isoprene nitrates (Shepson
et al., 1996; Ito et al., 2007; Perring et al., 2009a; Browne et al., 2013).
Similar uncertainty for the fate and dominant loss processes of monoterpene
nitrates was found by Browne et
al. (2014). Here, we update
possible fates of organic nitrates in WRF-Chem from recent findings including
photolysis (Müller et al., 2014),
oxidation and ozonolysis
(Lee et al., 2014),
deposition (Nguyen et
al., 2015) and hydrolysis
(Boyd
et al., 2015) and then estimate the contribution of different fates to first-
and second-generation isoprene and monoterpene nitrates (Fig. 10a and b). We
note that our calculation represents the loss of the nitrate functionality
and does not include the fraction of loss processes of first-generation
organic nitrates by oxidation and ozonolysis that retain the nitrate
functionality by forming second-generation organic nitrates.
Figure 10b shows that the loss of second-generation organic nitrates is
dominated by deposition (60 %), causing permanent loss of
NOx. That is due to the assumed rapid depositional loss of
second-generation monoterpene nitrates (deposited as fast as HNO3)
in this study following Browne et
al. (2014). Fractional
contributions of photolysis (∼25 %) and oxidation
(∼15 %) are not negligible and are much larger than those
estimated by Browne et al. (2014),
which is a consequence of using the rapid photolysis rates of PROPNN and
ETHLN as second-generation isoprene nitrates
(Müller et al., 2014). In contrast,
the loss of first-generation nitrates occurs largely by the sum of chemical
mechanisms that recycle NOx to the atmosphere: reaction with
ozone (24 %), OH (11 %) and photolysis (20 %), with additional
loss by deposition (15 %) and hydrolysis (∼29 %).
Fisher et
al. (2016)
predicted much larger losses of RONO2 by aerosol hydrolysis
(∼60 % of total nitrate losses), reflecting the short
lifetime of nitrates with respect to hydrolysis applied in their study for
all nitrates. However, with our representation of hydrolysis of nitrates in
aqueous solutions (slower rate and only tertiary nitrates), hydrolysis is
still important (the calculated loss rate for nitrates is ∼3
to 11 pptv h-1), accounting for one-third of the organic nitrate loss
and leading to a large increase in HNO3 production in the
atmosphere. Assessing the impact of hydrolysis of nitrates on the budget of
nitric acid is beyond the scope of this work. We note that the relative
contribution of nitrate hydrolysis in aqueous solutions differs widely for
individual RONO2 species due to their different structures.
Including loss of RONO2 from anthropogenic sources, we find that
loss of overall RONO2 via hydrolysis with an additional
contribution from deposition becomes comparable to loss via other processes
that return NOx to the atmosphere. Figure 11 shows the
NOx recycling efficiency, defined as the ratio between the
instantaneous production of NOx from loss of organic
nitrates and the instantaneous loss of NOx to production of
organic nitrates. Since chemical degradation of nitric acid is much slower
than deposition, the slope of 0.52 is interpreted as the fraction of the
sequestered NOx that can be exported away from its emission
region and released downwind through organic nitrate chemistry. Our finding
is consistent with a recycling of about half of isoprene nitrates to
NOx calculated by Paulot et al. (2009a) and Horowitz
et al. (2007). Of this total, we
calculated ∼38 % of NOx cycled back
relatively quickly while first-generation nitrates are oxidizing and
producing second-generation nitrates.
Conclusions
The lifetimes of organic nitrates with respect to hydrolysis, oxidation and
deposition play an important role in the NOx budget and
formation of O3 and secondary organic aerosols. Analyses from
recent field studies in the southeastern United States found a lifetime of
∼2–3 h for organic nitrates. By incorporating new findings
from recent laboratory, field and modeling studies into a gas-phase mechanism
we provide a state-of-the-science representation of expanded organic nitrates
in the WRF-Chem model. Using the updated model we are able to reproduce a
short organic nitrate lifetime (2.6 h), similar to that observed during SOAS.
After adding hydrolysis as a possible fate of tertiary gas-phase biogenic
organic nitrates in our mechanism and in combination with all other loss
mechanisms, we find that the lifetime of second-generation organic nitrates is
longer that the lifetime of first-generation nitrates. We find dry deposition
is the dominant loss process for second-generation organic nitrates and
chemical mechanisms of ozonolysis, photolysis and oxidation that can recycle
NOx to the atmosphere have a more important role in loss of
first-generation organic nitrates from the atmosphere. The contribution of
tertiary nitrate hydrolysis to total organic nitrate removal from the
atmosphere is found to be 30 %. We find, therefore, that 52 % of the
NOx sequestered by production of organic nitrates can be
cycled back to the atmosphere.
To accurately estimate organic nitrate lifetime, the production, loss and
fate of these compounds must be well constrained. Evaluation of our updated
mechanism using SOAS observations in summer 2013 indicates the model
represents much of the important chemistry governing organic nitrates. We
show that the simulated concentrations of total organic nitrates,
correlations with CH2O and ozone, and the contribution of individual
RONO2 to total organic nitrates are in fairly good agreement with
observations at the SOAS CTR ground site. We find the largest difference
between the modeled and observed contributions of individual organic nitrate
compounds to total RONO2 is for highly functionalized isoprene
nitrates and monoterpene nitrates. We attribute this difference to possible
high aerosol yields of these organic nitrate species, which are represented
in the gas phase in our mechanism. Future analysis for developing a complete
representation of organic nitrate chemistry including an organic nitrate
aerosol formation mechanism, either from aqueous-phase uptake or
vapor-pressure partitioning onto preexisting organic aerosol, in addition to
the detailed gas-phase mechanism described here will benefit the model
approximation.