Organic nitrate aerosol formation via NO3 + biogenic volatile organic compounds in the southeastern United States

Abstract

Gas- and aerosol-phase measurements of oxidants, biogenic volatile organic compounds (BVOCs) and organic nitrates made during the Southern Oxidant and Aerosol Study (SOAS campaign, Summer 2013) in central Alabama show that a nitrate radical (${\mathrm{NO}}_{\mathrm{3}}$) reaction with monoterpenes leads to significant secondary aerosol formation. Cumulative losses of ${\mathrm{NO}}_{\mathrm{3}}$ to terpenes are correlated with increase in gas- and aerosol-organic nitrate concentrations made during the campaign. Correlation of ${\mathrm{NO}}_{\mathrm{3}}$ radical consumption to organic nitrate aerosol formation as measured by aerosol mass spectrometry and thermal dissociation laser-induced fluorescence suggests a molar yield of aerosol-phase monoterpene nitrates of 23–44 %. Compounds observed via chemical ionization mass spectrometry (CIMS) are correlated to predicted nitrate loss to BVOCs and show ${\mathrm{C}}_{10}{\mathrm{H}}_{17}{\mathrm{NO}}_{\mathrm{5}}$, likely a hydroperoxy nitrate, is a major nitrate-oxidized terpene product being incorporated into aerosols. The comparable isoprene product ${\mathrm{C}}_{\mathrm{5}}{\mathrm{H}}_{\mathrm{9}}{\mathrm{NO}}_{\mathrm{5}}$ was observed to contribute less than 1 % of the total organic nitrate in the aerosol phase and correlations show that it is principally a gas-phase product from nitrate oxidation of isoprene. Organic nitrates comprise between 30 and 45 % of the NO${}_y$ budget during SOAS. Inorganic nitrates were also monitored and showed that during incidents of increased coarse-mode mineral dust,${\mathrm{HNO}}_{\mathrm{3}}$ uptake produced nitrate aerosol mass loading at a rate comparable to that of organic nitrate produced via ${\mathrm{NO}}_{\mathrm{3}}$ $+$ BVOCs.

Introduction

Secondary organic aerosol (SOA), formed from the oxidation of volatile organic compounds (VOCs) by ozone (${\mathrm{O}}_{\mathrm{3}}$), hydroxyl radical ($\mathrm{OH}$), or nitrate radical (${\mathrm{NO}}_{\mathrm{3}}$), affects visibility as well as regional and global radiative climate forcing . Aerosol has been studied as a source for significant risk factors for pulmonary and cardiac disorders . Organic aerosol (OA) contributes a large fraction of the total tropospheric submicron particulate matter (PM; ). Biogenic volatile organic compounds (BVOCs) are dominant precursors in SOA formation . SOA is a significant fraction of total aerosol mass in the southeastern United States (SEUS) (predicted to be 80–90 % of the organic aerosol load; ; ). Understanding the interaction of anthropogenic pollutants with BVOCs is vital to improving our understanding of the human impact on SOA formation and the associated radiative forcing of climate change .

Nitrogen oxides (NO${}_x$ $=$ $\mathrm{NO}$ $+$ ${\mathrm{NO}}_{\mathrm{2}}$), common byproducts of combustion, are linked to aerosol formation in the troposphere via daytime and nighttime oxidation mechanisms . Total reactive nitrogen, NO${}_y$, consists of NO${}_x$, as well as NO${}_x$ reaction products, including ${\mathrm{NO}}_{\mathrm{3}}$, ${\mathrm{HNO}}_{\mathrm{3}}$, $\mathrm{HONO}$, alkyl nitrates, peroxynitrates and all particulate organic nitrates. Alkyl nitrates produced from oxidation of VOCs are related to tropospheric ozone generation and, via low-volatility products, can lead to formation of SOA . Oxidation of NO${}_x$ to nitric acid (${\mathrm{HNO}}_{\mathrm{3}}$) can also produce inorganic nitrate aerosol via heterogeneous uptake of ${\mathrm{NO}}_{\mathrm{3}}$ onto mineral or sea salt aerosols and via co-partitioning with ammonia to form semi-volatile ${\mathrm{NH}}_{\mathrm{4}}{\mathrm{NO}}_{\mathrm{3}}$ .

Nitrogen oxides are primarily emitted as $\mathrm{NO}$ . $\mathrm{NO}$ is oxidized to ${\mathrm{NO}}_{\mathrm{2}}$ and further to the highly reactive ${\mathrm{NO}}_{\mathrm{3}}$ radical. ${\mathrm{NO}}_{\mathrm{3}}$ is especially predominant at night when loss via photolysis and $\mathrm{NO}$ reaction are at a minimum .

The formation of ${\mathrm{NO}}_{\mathrm{3}}$ and the associated ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ in the atmosphere have been studied in detail . The hydrolysis of ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ to ${\mathrm{HNO}}_{\mathrm{3}}$ can be important in the prediction of the tropospheric oxidant burden with respect to the ${\mathrm{O}}_{\mathrm{3}}$ production, and therefore $\mathrm{OH}$ radical production . However, previous studies in eastern Texas have found ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ uptake into aerosols to be relatively low in the southern United States (TexAQS average $\mathit{\gamma }$ of 0.003) .

${\mathrm{NO}}_{\mathrm{3}}$ is an effective nocturnal oxidizer of VOCs . ${\mathrm{NO}}_{\mathrm{3}}$ oxidation is especially reactive towards unsaturated, non-aromatic hydrocarbons of which BVOCs are major global constituents. ${\mathrm{NO}}_{\mathrm{3}}$ is less reactive towards aromatic compounds and saturated hydrocarbons, which are major compounds of anthropogenic VOCs. Nitrate oxidation of some BVOCs, such as $\mathit{\beta }$-pinene and limonene, lead to rapid production of SOA in laboratory experiments with high yields . Analysis of previous field studies have characterized the loss of ${\mathrm{NO}}_{\mathrm{3}}$ to its major daytime sinks, including reaction with $\mathrm{NO}$ and photolysis, as well as its loss to BVOCs during both daytime and nighttime .

Nitrogen-containing oxidation products include alkyl nitrates (${\mathrm{RONO}}_{\mathrm{2}}$), peroxynitrates (${\mathrm{RO}}_{\mathrm{2}}{\mathrm{NO}}_{\mathrm{2}}$) and nitric acid (${\mathrm{HNO}}_{\mathrm{3}}$) , all of which may partition to the aerosol phase and contribute to SOA (via direct reaction or catalysis) . Ambient concentrations of alkyl nitrates and peroxynitrates can be quantified using laser-induced fluorescence and mass spectrometry methods . Ions and acids (i.e., ${\mathrm{HNO}}_{\mathrm{3}}$) can be quantified using ion chromatography (IC; ) as well as chemical ionization mass spectrometry (CIMS) . The combination of these instruments, as well as others discussed below, allow for the determination of a total ambient-oxidized nitrogen (NO${}_y$) budget, which enables the interpretation of the importance of nitrogen oxides in SOA formation.

have reported that organic aerosol from nitrate radical oxidized monoterpenes are strongly influenced by anthropogenic pollutants and contribute 19–34 % of the total OA content (labeled less-oxidized oxygenated organic aerosols, LO-OOA). Monoterpene oxidation products show a large contribution to LO-OOA year-round . Another aerosol mass spectrometry factor specific to reactive uptake of isoprene oxidation products (e.g., IEPOX), isoprene-OA, is isolated in the warmer summer months in both urban as well as rural areas across the SEUS and contributes 18–36 % of summertime OA . LO-OOA is seen predominantly during nighttime hours, implying ${\mathrm{NO}}_{\mathrm{3}}$ oxidation of monoterpenes, and is strongly correlated specifically with the nitrate functionality in organic nitrates . It is suggested that during the summer months, increasing nighttime LO-OOA balances with increasing daytime isoprene-OA to give the observed constant OA concentration over the diurnal cycle. estimated that the total particle-phase organic nitrates contribute 5–12 % of total OA in the southeastern US in summer.

Map of Alabama with ${\mathrm{SO}}_{\mathrm{2}}$ and NO${}_x$ emissions point sources shown, as well as major roadways (black). Centreville is located in Central Alabama about 55 miles SSW of Birmingham, AL. Major highways, city limits and major contributors to emissions are referenced for Alabama. The size of the emission markers depicts the relative concentrations of the pollutants according to the 2013 EPA Air Markets Program in square-root tons per day, and only sources above 50 square-root tons per day are shown. For reference, the Alabama Power Company Gaston Plant emits 19.52 $\mathrm{kg}{\mathrm{h}}^{-\mathrm{1}}$ ${\mathrm{SO}}_{\mathrm{2}}$ and 6.43 $\mathrm{kg}{\mathrm{h}}^{-\mathrm{1}}$ NO${}_x$.

[Figure omitted. See PDF]

In this paper, we use the initial products (ex. ${\mathrm{C}}_{10}{\mathrm{H}}_{17}{\mathrm{NO}}_{\mathrm{5}}$), as well as total aerosol-phase organic nitrates, to track ${\mathrm{NO}}_{\mathrm{3}}$ radical contributions to SOA formation during Southern Oxidant and Aerosol Study (SOAS). We analyze the role of ${\mathrm{NO}}_{\mathrm{3}}$ oxidation of BVOCs both at night and during the day. Nitrate sinks have been determined for measured BVOCs and correlations of observed alkyl nitrate products versus these calculated loss rates are discussed.

The 2013 SOAS campaign was a comprehensive field intensive study in central Alabama near Centreville (CTR), in which concentrations of oxidants, BVOCs and aerosol were measured with a particular focus on understanding the effects of anthropogenic pollution on SOA formation. The site was chosen due to its high biogenic VOC emissions as well as its relatively large distance from anthropogenic pollution (Fig. ). County-level monoterpene emissions across the US shows the CTR site gives a regional representation of monoterpene emissions in the SEUS . Furthermore, showed that the CTR site is representative of more-oxidized and less-oxidized oxygenated organic aerosols (MO-OOA and LO-OOA, respectively) loadings across several monitoring stations in the SEUS. Comparison of annual molar emissions in the SEUS (an eight-state region including the CTR site) of BVOCs (estimated from ) to NO${}_x$ emissions (from 2011 NEI database) suggests that NO${}_x$ is the limiting reagent in NO${}_x$-drived BVOCs oxidation throughout the region and demonstrates that the CTR site is regionally representative.

Alabama is home to a number of power plant facilities that are a large point sources of NO${}_x$ capable of being carried long distances. Alabama's non-interstate roadways also have large emissions of NO${}_x$, though a majority of the emissions come from urban areas. Although the NO${}_x$ emissions have been steadily dropping since 1998, they are still substantial (2.70 million tons in reported for SEUS in 1999 to 1.75 million tons in 2008; ). Frequent controlled biomass burning events (crop burning; ), as well as vehicular sources also contribute to local NO${}_x$ emissions and PM concentrations (a full analysis of contributions can be found at the EPA National Emissions Inventory, http://www.epa.gov/ttn/chief/net/2011inventory.html).

In the present study, we investigate the production of SOA species from ${\mathrm{NO}}_{\mathrm{3}}$ reaction with monoterpenes. ${\mathrm{NO}}_{\mathrm{3}}$ loss to BVOCs is calculated and compared to aerosol mass spectrometry (AMS), chemical ion mass spectrometry (CIMS), and thermal dissociation laser-induced fluorescence (TD-LIF) measurements of aerosol-organic nitrates. We compare this to an alternate fate of NO${}_x$, heterogeneous ${\mathrm{HNO}}_{\mathrm{3}}$ uptake to produce inorganic nitrate aerosol, which is considered in detail in a second paper . Both pathways from NO${}_x$ to nitrate aerosol shown in Scheme 1 are produced at various times in the SEUS.

Generalized reaction fate for ${\mathrm{NO}}_{\mathrm{2}}$ in the troposphere. Oxidation of ${\mathrm{NO}}_{\mathrm{2}}$ from atmospheric oxidants leads to two possible paths.

[Figure omitted. See PDF]

Experimental

Measurements for the SOAS campaign took place near the Talladega National Forest, 6 miles southwest of Brent, AL (32.9029${}^{\circ }$ N, 87.2497${}^{\circ }$ W), from 1 June–15 July 2013. The forest covers 157 000 acres to the northwest and southeast of Centreville, AL. Figure  shows a map of the site location as well as nearby point sources of anthropogenic NO${}_x$ and ${\mathrm{SO}}_{\mathrm{2}}$. The site is in a rural area representative of the transitional nature between the lower coastal plain and Appalachian highlands . Wind direction varied during SOAS allowing for periods of urban influence from sources of anthropogenic emissions located near the sampling site, including the cities of Montgomery, Birmingham, Mobile, and Tuscaloosa . The closest large anthropogenic NO${}_x$ emission point sources are the Alabama Power Company Gaston Plant located near Birmingham and the Green County Power Plant southwest of Tuscaloosa (EPA Air Markets Program 2013).

Two cavity ringdown spectrometers (CRDSs) were used to determine ambient mixing ratios of NO${}_x$, ${\mathrm{O}}_{\mathrm{3}}$, NO${}_y$, ${\mathrm{NO}}_{\mathrm{3}}$ and ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ . A CRDS is a high sensitivity optical absorption method based on the decay time constant for light from an optical cavity composed of two high reflectivity mirrors. ${\mathrm{NO}}_{\mathrm{2}}$ is measured using its optical absorption at 405 $\mathrm{nm}$ in one channel, and ${\mathrm{O}}_{\mathrm{3}}$, $\mathrm{NO}$, and total NO${}_y$ are quantitatively converted to ${\mathrm{NO}}_{\mathrm{2}}$ and measured simultaneously by 405 $\mathrm{nm}$ absorption on three additional channels. ${\mathrm{NO}}_{\mathrm{3}}$ is measured at its characteristic strong absorption band at 662 $\mathrm{nm}$. ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ is quantitatively converted to ${\mathrm{NO}}_{\mathrm{3}}$ by thermal dissociation and detected in a second 662 $\mathrm{nm}$ channel with a detection limit of 1 $\mathrm{pptv}$ (30 $\mathrm{s}$, 2 $\mathit{\sigma }$) for ${\mathrm{NO}}_{\mathrm{3}}$ and 1.2 $\mathrm{pptv}$ (30 $\mathrm{s}$, 2 $\mathit{\sigma }$) for ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ .

TD-LIF (${\mathrm{PM}}_{2.5}$ size cut) was used to measure total alkyl nitrates ($\mathrm{\Sigma }$ANs), total peroxy nitrates ($\mathrm{\Sigma }$PNs) and aerosol-phase $\mathrm{\Sigma }$ANs . A high-resolution time-of-flight aerosol mass spectrometry (HR-ToF-AMS, hereafter AMS; ; ${\mathrm{PM}}_{\mathrm{1}}$ size cut) was used to measure submicron organic and inorganic nitrate aerosol composition using the nitrate separation method described in . Organic nitrates in the particle phase ($p$${\mathrm{RONO}}_{\mathrm{2}}$) decompose prior to ionization on the AMS vaporizer to ${\mathrm{NO}}_{\mathrm{2}}^{+}$ organic fragments; hence, $p$${\mathrm{RONO}}_{\mathrm{2}}$ cannot be quantified directly from AMS data. The contribution of $p$${\mathrm{RONO}}_{\mathrm{2}}$ to total particulate nitrate was calculated using the method first discussed in and briefly summarized here. This method relies on the different fragmentation patterns observed in the AMS for organic nitrates vs. ${\mathrm{NH}}_{\mathrm{4}}{\mathrm{NO}}_{\mathrm{3}}$, specifically the ratio of the ions ${\mathrm{NO}}_{\mathrm{2}}^{+}$ to ${\mathrm{NO}}^{+}$. Since this ratio depends on mass spectrometer tuning, vaporizer settings, and history, proposed to interpret the field ratio of these ions in relation to the one recorded for ${\mathrm{NH}}_{\mathrm{4}}{\mathrm{NO}}_{\mathrm{3}}$ (which is done routinely during infield calibrations of the instrument). Using such normalized ratios, most field and chamber observations of pure organic nitrates are consistent with (${\mathrm{NO}}_{\mathrm{2}}^{+}/{\mathrm{NO}}^{+}$) $/$ ((${\mathrm{NO}}_{\mathrm{2}}^{+}/{\mathrm{NO}}^{+}$)$\mathrm{ref}$ of $\mathrm{1}/2.25$ to $\mathrm{1}/\mathrm{3}$ of the calibration ratio. also used this method for the SEUS and discussed the estimated uncertainties. The data reported here were calculated using the $\mathrm{1}/2.25$ ratio derived from and used in , interpolating linearly between pure ammonium nitrate and organic nitrate. It should be noted that (a) the relative ionization efficiency (RIE) for both types of nitrate is assumed to be the same (since similar neutrals are produced) and (b) that the organic part of the molecule will be quantified as OA in the AMS. Therefore, while only equivalent ${\mathrm{NO}}_{\mathrm{2}}$ $p$${\mathrm{RONO}}_{\mathrm{2}}$ can be reported from AMS measurements, this makes the technique well-suited for comparison with the TD-LIF method. These measurements correlate well to one another, but the magnitudes differ by a factor of approximately 2–4 for unknown reasons, with TD-LIF being larger than AMS (see Supplement).

Two chemical ionization mass spectrometers (Caltech's cTOF-CIMS and University of Washington's HR-ToF-CIMS, hereafter both referred to as CIT-CIMS and UW-CIMS, respectively; ) were used to identify specific organic nitrate product ions, specifically monoterpene and isoprene products . The CIT-CIMS measured only gas-phase products while the UW-CIMS employed a Filter Inlet for Gas and AEROsol (FIGAERO) to separate aerosol and gas species . Both spectrometers are capable of resolving ions with different elemental formulae at common nominal $m/z$.

An online cryostat-Gas Chromatography-Mass Spectrometer (GC-MS) was used to measure mixing ratios of gas-phase BVOC species . BVOC emissions at the CTR site are dominated by isoprene, $\mathit{\alpha }$-pinene, $\mathit{\beta }$-pinene, and limonene (Fig. S1 in the Supplement; ). Surface area concentration was calculated from number distribution measurements of a hygroscopicity scanning mobility particle sizer (SMPS) and optical particle sizer (OPS) similar to a dry-ambient aerosol size spectrometer . Boundary layer height was measured using a CHM 15k-Nimbus and the method employs a photon counting of back-scattered pulse of near-IR light (1064 $\mathrm{nm}$) via lidar principle. A Metrohm Monitor for Aerosols and Gases in Ambient Air (MARGA; ; ${\mathrm{PM}}_{2.5}$ size cut), which combines a wet-rotating denuder/steam jet aerosol collector inlet with positive and negative ion chromatograph, measured inorganic ion concentrations at a 1$\mathrm{h}$ time resolution in both the aerosol and gas phases.

Site infrastructure consisted of a 65-foot tower, with the top platform set above the canopy height for sampling to prevent bias between measurements, and seven trailers located in a field $\sim 90$ $\mathrm{m}$ south of the tower. The tower instruments used for this analysis consisted of the two CRDSs, CIT-CIMS, TD-LIF, and a cryostat GC-MS. The field trailers contained the AMS, SMPS, APS (aerodynamic particle sizer), UW-CIMS, and MARGA.

${\mathrm{NO}}_{\mathrm{3}}$ kinetic rate constants and equilibrium constants used to determine losses.

Reaction	A	E/R${}^{\mathrm{a}}$	B${}^{\mathrm{b}}$	k(298 $\mathrm{K}$)	Source
${\mathrm{O}}_{\mathrm{3}}+{\mathrm{NO}}_{\mathrm{2}}\to {\mathrm{O}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{3}}$	$1.2\times {10}^{-13}$	2450
${\mathrm{NO}}_{\mathrm{3}}+{\mathrm{NO}}_{\mathrm{2}}\rightleftharpoons {\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$	$2.7\times {10}^{-27}$		11 000
$\mathrm{NO}+{\mathrm{NO}}_{\mathrm{3}}\to \mathrm{2}{\mathrm{NO}}_{\mathrm{2}}$	$1.5\times {10}^{-11}$	$-17$0
Isoprene $+$ ${\mathrm{NO}}_{\mathrm{3}}\to$ Products	$3.03\times {10}^{-12}$	446
$\mathit{\alpha }$-pinene $+$ ${\mathrm{NO}}_{\mathrm{3}}\to$ Products	$1.19\times {10}^{-12}$	$-490$
$\mathit{\beta }$-pinene $+$ ${\mathrm{NO}}_{\mathrm{3}}\to$ Products				$2.51\times {10}^{-12}$
Camphene $+$ ${\mathrm{NO}}_{\mathrm{3}}\to$ Products				$6.6\times {10}^{-13}$
Myrcene $+$ ${\mathrm{NO}}_{\mathrm{3}}\to$ Products				$1.1\times {10}^{-11}$
Limonene $+$ ${\mathrm{NO}}_{\mathrm{3}}\to$ Products				$1.22\times {10}^{-11}$

${}^{\mathrm{a}}$ Reaction rate constants are reported as $k\left(T\right)=A{e}^{-(E_a/R)/T}$, in units of (${\mathrm{cm}}^{\mathrm{3}}{\mathrm{molecule}}^{-\mathrm{1}}{\mathrm{s}}^{-\mathrm{1}}$)${}^{\mathrm{b}}$ Equilibrium constants are reported as $K_{\mathrm{eq}}=A{e}^{B/T}$, in units of (${\mathrm{cm}}^{\mathrm{3}}{\mathrm{molecule}}^{-\mathrm{1}}$)

Results

Organic NO${}_x$ sink: ${\mathrm{NO}}_{\mathrm{3}}$ $+$ BVOC production of organic nitrate SOA

During the SOAS campaign, we monitored reactant and product species indicative of ${\mathrm{NO}}_{\mathrm{3}}$ $+$ BVOCs, which may partition into the aerosol phase and consequently serve as a source of first-generation SOA. A ${\mathrm{NO}}_{\mathrm{3}}$ reaction with biogenic alkenes forms organic nitrates (Reaction ). $${\mathrm{NO}}_{\mathrm{3}}+\text{BVOC}\to {\mathrm{RONO}}_{\mathrm{2}}+\text{other products}$$ ${\mathrm{NO}}_{\mathrm{3}}$ and ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ (which exists in equilibrium with ${\mathrm{NO}}_{\mathrm{2}}$ $+$ ${\mathrm{NO}}_{\mathrm{3}}$) in the region were consistently low during the campaign. The resulting ${\mathrm{NO}}_{\mathrm{3}}$ mixing ratio was below the detection limit of the cavity ringdown instrument (1 $\mathrm{pptv}$) for the entire campaign. Calculated steady-state ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ was validated against observed measurements (see below) and ${\mathrm{NO}}_{\mathrm{3}}$ predicted from the steady-state approximation was used for all calculations involving ${\mathrm{NO}}_{\mathrm{3}}$ radical mixing ratios. Using the rate constant for ${\mathrm{NO}}_{\mathrm{2}}$ $+$ ${\mathrm{O}}_{\mathrm{3}}$ (Table 1), the production rate of the nitrate radical ($P$(${\mathrm{NO}}_{\mathrm{3}}$)) is given by $$P\left({\mathrm{NO}}_{\mathrm{3}}\right)=k_{{\mathrm{O}}_{\mathrm{3}}+{\mathrm{NO}}_{\mathrm{2}}}\left[{\mathrm{O}}_{\mathrm{3}}\right]\left[{\mathrm{NO}}_{\mathrm{2}}\right].$$ The calculated loss rate of ${\mathrm{NO}}_{\mathrm{3}}$, $\mathit{\tau }$(${\mathrm{NO}}_{\mathrm{3}}$), to reactions with individual BVOCs, $\mathrm{NO}$, and photolysis ($j_{{\mathrm{NO}}_{\mathrm{3}}}$, modeled for clear sky from MCM, ) is $$\mathit{\tau }\left({\mathrm{NO}}_{\mathrm{3}}\right)={\displaystyle \frac{\mathrm{1}}{\left(\sum _i{k}_{{\mathrm{NO}}_{\mathrm{3}}+{\mathrm{BVOC}}_i}\right[\text{BVOC}{]}_i+k_{{\mathrm{NO}}_{\mathrm{3}}+\mathrm{NO}}\left[\mathrm{NO}\right]+j_{{\mathrm{NO}}_{\mathrm{3}}})}}.$$ $j_{{\mathrm{NO}}_{\mathrm{3}}}$ values were calculated from solar zenith angles and ${\mathrm{NO}}_{\mathrm{3}}$ photolysis rates . The values were then adjusted for cloud cover by taking measured solar radiation values (Atmospheric Research and Analysis, Inc.; $\mathrm{W}{\mathrm{m}}^{\mathrm{2}}$) and normalizing their peak values to those of the modeled photolysis data. Peak modeled $j_{{\mathrm{NO}}_{\mathrm{3}}}$ values were 0.175 ${\mathrm{s}}^{-\mathrm{1}}$ for clear sky at the daily solar maximum. After normalizing, typical values of $j_{{\mathrm{NO}}_{\mathrm{3}}}$ were 0.110 ${\mathrm{s}}^{-\mathrm{1}}$ during daytime.

Using Eqs. () and (), a steady-state predicted ${\mathrm{NO}}_{\mathrm{3}}$ mixing ratio ($[{\mathrm{NO}}_{\mathrm{3}}{]}_{\mathrm{SS}}$) can be calculated: $$[{\mathrm{NO}}_{\mathrm{3}}{]}_{\mathrm{SS}}={\displaystyle \frac{P\left({\mathrm{NO}}_{\mathrm{3}}\right)}{\mathit{\tau }({\mathrm{NO}}_{\mathrm{3}}{)}^{-\mathrm{1}}}}.$$ [${\mathrm{NO}}_{\mathrm{3}}{]}_{\mathrm{SS}}$ can then be used to calculate steady-state predicted ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ from the ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ equilibrium (Table 1) and measured ${\mathrm{NO}}_{\mathrm{2}}$ $$[{\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}{]}_{\mathrm{SS}}=K_{\mathrm{eq}}[{\mathrm{NO}}_{\mathrm{2}}\left]\right[{\mathrm{NO}}_{\mathrm{3}}{]}_{\mathrm{SS}},$$ where $K_{\mathrm{eq}}$ is $2.7\times {10}^{-27}$exp${}^{(11000/T)}$ ${\mathrm{cm}}^{\mathrm{3}}{\mathrm{molecule}}^{-\mathrm{1}}{\mathrm{s}}^{-\mathrm{1}}$ (; see Table 1). Comparison of the predicted ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ to the measured ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ mixing ratios for the campaign demonstrates that both timing and magnitude of predicted ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ peaks match observations (Fig. S2). Predicted steady-state ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ tracked observations when the latter were available, and propagation of the error of calculated ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ shows peak measured values fall within uncertainty bounds of the predicted (Fig. S3a); therefore, ${\mathrm{NO}}_{\mathrm{3},\mathrm{SS}}$ is hereafter used as the best estimate of ${\mathrm{NO}}_{\mathrm{3}}$ to calculate production rates of BVOC nitrate products. ${\mathrm{NO}}_{\mathrm{3},\mathrm{SS}}$ peaks at 1.4 $\mathrm{ppt}$ $\pm$ 0.4 $\mathrm{ppt}$. Propagation of errors in rate constants in the ${\mathrm{NO}}_{\mathrm{3},\mathrm{SS}}$ calculation (Fig. S3a) shows that the error spans or is close to a mixing ratio of 0 for ${\mathrm{NO}}_{\mathrm{3}}$ during the entire campaign when data were available. Correlation of measured ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ vs. predicted ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ shows that during periods of high ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$, we overestimate the concentration by a factor of 2 (Fig. S2). Furthermore, propagation of error in the ${\mathrm{NO}}_{\mathrm{3},\mathrm{SS}}$ calculation (Fig. S3b) shows that the error encompasses the measured ${\mathrm{NO}}_{\mathrm{3}}$ during the entire campaign when data were available, showing that predicted ${\mathrm{NO}}_{\mathrm{3},\mathrm{SS}}$ is consistent with the lack of detection of ${\mathrm{NO}}_{\mathrm{3}}$ by CRDSs.

Nitrate radical concentration estimated by the steady-state approximation (red trace) shows several instances where peaks in ${\mathrm{NO}}_{\mathrm{3}}$ concentration correspond to times of $\mathrm{\Sigma }$AN (gaseous $+$ aerosol) buildup (light blue trace) from TD-LIF and particle phase organic nitrate from AMS (dark blue). The black overlay in TD-LIF $\mathrm{\Sigma }$ANs is the aerosol-phase measurement of $\mathrm{\Sigma }$ANs and qualitatively shows that, when data are available, a large portion of the organic nitrates appear to be in the aerosol phase.

[Figure omitted. See PDF]

Diurnally averaged organic and inorganic nitrates show organic nitrates peaking in the early morning and inorganic nitrates peaking midday. Note the AMS had a ${\mathrm{PM}}_{\mathrm{1}}$ size cut, while MARGA and TD-LIF had a ${\mathrm{PM}}_{2.5}$ size cut. See text and Supplement for more details on this comparison.

[Figure omitted. See PDF]

A substantial fraction (30–45 %) of the NO${}_y$ budget is comprised of organic nitrates ($\mathrm{\Sigma }$AN $+$ $\mathrm{\Sigma }$PN; Fig. S4). Measurements of gas-phase and aerosol-phase alkyl nitrates show that a substantial fraction of the organic nitrates are in the aerosol phase (30 % when aerosol-phase AMS is compared to TD-LIF total $\mathrm{\Sigma }$ANs vs. 80 % when comparing aerosol-phase $\mathrm{\Sigma }$ANs to TD-LIF total $\mathrm{\Sigma }$ANs at 17:00 CDT – Central Daylight Time) when total $\mathrm{\Sigma }$AN concentration builds up (Figs.  and ). The average diurnal cycle shown in Fig.  also shows that TD-LIF measured $\mathrm{\Sigma }$ANs are almost completely in the aerosol phase at night, but only about 50 % in the aerosol phase during the day. During peaks in ${\mathrm{NO}}_{\mathrm{3},\mathrm{SS}}$, we see corresponding spikes in the $\mathrm{\Sigma }$ANs concentrations as well as organic nitrate concentration from AMS, all of which occur during nighttime periods (Fig. ). This is consistent with organic nitrates formed by ${\mathrm{NO}}_{\mathrm{3}}$ $+$ BVOCs rapidly partitioning into the aerosol phase.

BVOC measurements show large mixing ratios of isoprene throughout the entire campaign (daytime peaks above 8 $\mathrm{ppb}$), followed by $\mathit{\alpha }$- and $\mathit{\beta }$-pinene (peak nighttime mixing ratios of 0.5–1 $\mathrm{ppb}$, Fig. S1). Using the measured mixing ratios of VOCs, and their reaction rates with ${\mathrm{NO}}_{\mathrm{3}}$, predicted ${\mathrm{NO}}_{\mathrm{3}}$ losses are calculated and compared to organic nitrate aerosol. Figure  shows the diurnally averaged ${\mathrm{NO}}_{\mathrm{3}}$ losses for the entire campaign period (1 June–15 July 2013). Daytime losses include photolysis and reaction with $\mathrm{NO}$. Approximately half the daytime losses are due to reaction of ${\mathrm{NO}}_{\mathrm{3}}$ with BVOCs. (Note, this does not necessarily imply that ${\mathrm{NO}}_{\mathrm{3}}$ reaction is a substantial loss process from the perspective of BVOCs; during the day, $P$($\mathrm{HO}$${}_x$) exceeds $P$(${\mathrm{NO}}_{\mathrm{3}}$) by a factor of 10–70 at SOAS; therefore, $\mathrm{OH}$ will typically dominate.) However, from the standpoint of ${\mathrm{NO}}_{\mathrm{3}}$ lifetime, previous forest campaigns have assumed ${\mathrm{NO}}_{\mathrm{3}}$ $+$ monoterpene reactions to be important only during the night and that photolysis and $\mathrm{NO}$ losses were the dominant ${\mathrm{NO}}_{\mathrm{3}}$ sinks during the day . In this study, we predict significant losses of ${\mathrm{NO}}_{\mathrm{3}}$ to isoprene and monoterpenes during daylight hours.

Average diurnal profile of ${\mathrm{NO}}_{\mathrm{3}}/{\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ losses 1 June–15 July 2013. NO and photolysis losses peak during the daytime (in fact, nighttime ${\mathrm{NO}}_{\mathrm{3}}$ $+$ $\mathrm{NO}$ loss is likely zero, and even [$\mathrm{NO}$] below the instrument detection limits would cause the non-zero rates shown here); however, losses to alkenes are significant during both night and day Terpene losses are calculated from GC-MS data, $\mathrm{NO}$ and ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ data are from CRDSs, and photolysis losses are calculated as described in Sect. 3.1. Uncertainties in rate constants of BVOCs $+$ ${\mathrm{NO}}_{\mathrm{3}}$ range from 2 % for myrcene to $-40$ % for $\mathit{\beta }$-pinene . Uncertainties in rate constants of BVOCs $+$ ${\mathrm{NO}}_{\mathrm{3}}$ range from $\pm 30$ % for $\mathit{\alpha }$-pinene to up to a factor of 2 for isoprene (Calvert et al., 2000); $\mathrm{NO}$ measurements had $\pm 35$ % uncertainty, BVOC measurements $\pm 20$ %, and photolysis $\pm 20$ % based on solar radiation measurement uncertainty.

[Figure omitted. See PDF]

To assess heterogeneous losses of ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ to particles, an uptake rate coefficient of ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ into deliquesced aerosols is estimated using PM surface area ($S_A$, ${\mathrm{nm}}^{\mathrm{2}}{\mathrm{cm}}^{-\mathrm{3}}$), the molecular speed of ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ ($\overline{c}$, $\mathrm{m}{\mathrm{s}}^{-\mathrm{1}}$), and the uptake coefficient (${\mathit{\gamma }}_{{\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}}$). $$k_{\mathrm{het}}={\displaystyle \frac{\mathrm{1}}{\mathrm{4}}}\times {\mathit{\gamma }}_{{\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}}\times {\overline{c}}_{{\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}}\times S_A$$ Conditions of high relative humidity in the SEUS necessitated a higher $\mathit{\gamma }$ of 0.02 as the uptake coefficient , which represents an upper limit from previous field studies . We predict heterogeneous ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ uptake to be very small over the campaign despite high relative humidity. When ${\mathrm{PM}}_{2.5}$ concentration was at its highest in mid-July, the calculated uptake rate coefficient was calculated at $1.6\times {10}^{-\mathrm{3}}$ ${\mathrm{s}}^{-\mathrm{1}}$ in mid-July, representing less than 1 % of the loss of ${\mathrm{NO}}_{\mathrm{3}}$.

Sample calculation of $({\mathrm{NO}}_{\mathrm{3},\mathrm{loss}}{)}_{\mathrm{integ}}$ overlaid against aerosol ${\mathrm{RONO}}_{\mathrm{2}}$ measured by AMS (red). The monoterpene maxima correlate well with the AMS maxima (black dots). Minima and maxima (black dots) were chosen for the beginning and end of AMS buildup periods, respectively. The time period shown is arbitrarily chosen.

[Figure omitted. See PDF]

Calculation of ${\mathrm{NO}}_{\mathrm{3}}$ loss to BVOCs

Using literature ${\mathrm{NO}}_{\mathrm{3}}$ $+$ BVOC rate coefficients and calculated ${\mathrm{NO}}_{\mathrm{3},\mathrm{SS}}$, we calculate instantaneous ${\mathrm{NO}}_{\mathrm{3}}$ loss rates ((${\mathrm{NO}}_{\mathrm{3},\mathrm{loss}}{)}_{\mathrm{inst}}$) for the campaign. $$({\mathrm{NO}}_{\mathrm{3},\mathrm{loss}}{)}_{\mathrm{inst}}=\sum _i{k}_{{\mathrm{NO}}_{\mathrm{3}}+{\mathrm{VOC}}_{\mathrm{i}}}\left[\text{VOC}{]}_i\right[{\mathrm{NO}}_{\mathrm{3}}{]}_{\mathrm{SS}}$$ BVOC mixing ratios from GC-MS and rate constants shown in Table 1 were used to calculate the time-integrated nitrate loss to reactions with BVOCs. $$({\mathrm{NO}}_{\mathrm{3},\mathrm{loss}}{)}_{\mathrm{integ}}=\sum _{i,t}({\mathrm{NO}}_{\mathrm{3},\mathrm{loss}}{)}_{\mathrm{inst},i}\times \mathrm{\Delta }t$$ Specifically, time loss of ${\mathrm{NO}}_{\mathrm{3}}$ radical to reaction with BVOCs ($({\mathrm{NO}}_{\mathrm{3},\mathrm{loss}}{)}_{\mathrm{integ}}$) were calculated during periods of increasing ${\mathrm{RONO}}_{\mathrm{2}}$ concentrations as monitored by CIMS or aerosol-phase ${\mathrm{RONO}}_{\mathrm{2}}$ monitored by AMS or TD-LIF during SOAS. The beginning and end of the buildup periods were chosen as the approximate trough and peak values for the individual analyses (CIMS, AMS, and TD-LIF). This buildup of aerosol ${\mathrm{RONO}}_{\mathrm{2}}$ was only observed after sunset with one buildup event per night. The boundary layer during night hours is relatively stable, such that NO${}_x$ and BVOC measurements can be considered an area-wide average and this simple box model can be used to calculate $({\mathrm{NO}}_{\mathrm{3},\mathrm{loss}}{)}_{\mathrm{integ}}$ (Eqs. , ).

Under the assumption of a constant nighttime boundary layer height and an approximately uniform, area-wide source that limits the time rate of change due to horizontal advection (i.e., a nighttime box), the time integrals of ${\mathrm{RONO}}_{\mathrm{2}}$ produced provide estimates of the evolution of ${\mathrm{RONO}}_{\mathrm{2}}$ concentrations at night (this assumption was verified using $\mathrm{CO}$ to minimize first-order effects of dilution from changes in the boundary layer ). Time periods of CIMS ${\mathrm{RONO}}_{\mathrm{2}}$ or aerosol buildup were chosen to determine time intervals for calculation of $({\mathrm{NO}}_{\mathrm{3},\mathrm{loss}}{)}_{\mathrm{integ}}$ when data were available.

$({\mathrm{NO}}_{\mathrm{3},\mathrm{loss}}{)}_{\mathrm{integ}}$ is the calculated time integral of the reaction products of ${\mathrm{NO}}_{\mathrm{3}}$ with individual or combined mixing ratios of BVOCs, and $\mathrm{\Delta }t$ is the time step between each calculated value of ${\left({\mathrm{NO}}_{\mathrm{3},\mathrm{loss}}\right)}_{\mathrm{inst},i}$. Data are averaged to 30 $\min$ increments, a time step sufficient to resolve the observed rate of change. Figure  shows an example of the resulting calculated integrated ${\mathrm{NO}}_{\mathrm{3}}$ losses from Eq. () to both isoprene and summed monoterpenes. These nightly loss values are correlated with organic nitrate gas- and aerosol-phase measurements and linear fits, and correlation coefficients were calculated to aid in the interpretation of gas- and aerosol-phase organic nitrate formation. Note that these peak times occur during nighttime hours when the boundary layer is shallow (Fig. S5).

Scatter plots of aerosol ${\mathrm{RONO}}_{\mathrm{2}}$ (AMS and TD-LIF) compared to $({\mathrm{NO}}_{\mathrm{3},\mathrm{loss}}{)}_{\mathrm{cum}}$. The magnitudes of the two particle phase organic nitrate measurements differ by a factor of $\approx$2–4 for unknown reasons; however, the slope can be used as a relative molar yield of ${\mathrm{NO}}_{\mathrm{3}}$ loss to monoterpenes. Time period for AMS comparison is 9 June–15 July 2013 and for TD-LIF is 27 June–15 July 2013.

[Figure omitted. See PDF]

Scatter plots of selected molecules' concentration buildups against time-integrated monoterpene losses to ${\mathrm{NO}}_{\mathrm{3}}$ radical, during periods of observed organic nitrate buildup measured by CIMS. Panels (a) and (d) show particle phase ${\mathrm{C}}_{10}{\mathrm{H}}_{15}{\mathrm{NO}}_{\mathrm{5}}$ and ${\mathrm{C}}_{10}{\mathrm{H}}_{17}{\mathrm{NO}}_{\mathrm{5}}$ measured by the UW FIGAERO; (b) and (e) show gas-phase ${\mathrm{C}}_{10}{\mathrm{H}}_{15}{\mathrm{NO}}_{\mathrm{5}}$ and ${\mathrm{C}}_{10}{\mathrm{H}}_{17}{\mathrm{NO}}_{\mathrm{5}}$ also measured by UW; (c) and (f) show the same gas-phase species measured by the CIT-CIMS, with calibrated concentrations. Panels (g) and (h) show gas-phase ${\mathrm{C}}_{10}{\mathrm{H}}_{17}{\mathrm{NO}}_{\mathrm{4}}$ measured by both CIMS. The gas-phase correlations with calibrated mixing ratios measured by the CIT-CIMS (c, f, h) allow for a rough estimation of the lower limit molar yields via the slopes: ${\mathrm{C}}_{10}{\mathrm{H}}_{15}{\mathrm{NO}}_{\mathrm{5}}$, 0.4 %; ${\mathrm{C}}_{10}{\mathrm{H}}_{17}{\mathrm{NO}}_{\mathrm{5}}$, 3 %; and ${\mathrm{C}}_{10}{\mathrm{H}}_{17}{\mathrm{NO}}_{\mathrm{4}}$, 3 %.

[Figure omitted. See PDF]

Gas-phase CIMS data correlated to predicted isoprene $+$ ${\mathrm{NO}}_{\mathrm{3}}$, during periods of buildup of these ${\mathrm{C}}_{\mathrm{5}}$ and ${\mathrm{C}}_{\mathrm{4}}$ nitrates as measured by each CIMS. Panels (a) and (b) show ${\mathrm{C}}_{\mathrm{5}}{\mathrm{H}}_{\mathrm{9}}{\mathrm{NO}}_{\mathrm{5}}$, which is well correlated to predicted isoprene $+$ ${\mathrm{NO}}_{\mathrm{3}}$ suggesting this is a ${\mathrm{NO}}_{\mathrm{3}}$ gas-phase product, with the calibrated mixing ratios measured by CIT enabling estimation of an approximate lower limit molar yield of 7 %. Panel (c) shows that ${\mathrm{C}}_{\mathrm{5}}{\mathrm{H}}_{\mathrm{9}}{\mathrm{NO}}_{\mathrm{4}}$ is poorly correlated to isoprene $+$ ${\mathrm{NO}}_{\mathrm{3}}$ suggesting that this product comes (at least in part) from another oxidative source (ex. ${\mathrm{RO}}_{\mathrm{2}}$+NO). Panel (d), ${\mathrm{C}}_{\mathrm{4}}{\mathrm{H}}_{\mathrm{7}}{\mathrm{NO}}_{\mathrm{5}}$, also shows a poorer correlation than panels (a) and (b), suggesting it is not exclusively a product of ${\mathrm{NO}}_{\mathrm{3}}$ oxidation, or has rapid losses.

[Figure omitted. See PDF]

Implied organic nitrate and SOA yields

The correlation slopes in Fig.  are in ${\mathrm{ppb}}_{\mathrm{aerosol}}$/${\mathrm{ppb}}_{({\mathrm{NO}}_{\mathrm{3},\mathrm{loss}}{)}_{\mathrm{integ}}}$ and indicate the average molar organic nitrate aerosol yield. The AMS and TD-LIF measurements of aerosol-phase organic nitrates suggest a molar yield of 23 and 44 %, respectively (Fig. ). This calculation uses all available data from each instrument and assumes no other processes are taking place. We note that without knowledge of the average molecular weight of the aerosol-organic nitrate, only molar yield estimates are possible. Several chamber studies have measured organic nitrate yields from ${\mathrm{NO}}_{\mathrm{3}}$ oxidation of individual terpenes: and both found 10–15 % total organic nitrate (ON) yield for $\mathit{\alpha }$-pinene; found 45 % total ON yield for $\mathit{\beta }$-pinene under humid conditions, found 22 % under dry conditions, and found aerosol-organic nitrate to comprise 45–74 % of OA produced from ${\mathrm{NO}}_{\mathrm{3}}$ $+$ $\mathit{\beta }$-pinene; found 30 % total ON yield while found 54 % for limonene. A mix of these chamber organic nitrate yields are consistent with the observed molar yield range reported here, which uses only ${\mathrm{NO}}_{\mathrm{3}}$ losses to monoterpenes.

To derive an estimated SOA mass yield from these correlations, we propose the following rough calculation. Conversion of the reported molar yield to an SOA mass yield requires assuming $\mathrm{1}:\mathrm{1}$ reaction stoichiometry of ${\mathrm{NO}}_{\mathrm{3}}$ with monoterpenes (MW $=$ 136 $\mathrm{g}{\mathrm{mol}}^{-\mathrm{1}}$) and estimating the average molecular weight (250 $\mathrm{g}{\mathrm{mol}}^{-\mathrm{1}}$) of the condensing organic nitrates. Using the range of molar yields determined here (23–44 %), this conversion gives an SOA mass yield range from 42 to 81 %. These apparent aggregated yields of SOA from ${\mathrm{NO}}_{\mathrm{3}}$ $+$ monoterpene are higher than one might expect from laboratory-based yields from individual monoterpenes, particularly since ${\mathrm{NO}}_{\mathrm{3}}$ $+$ $\mathit{\alpha }$-pinene SOA yields are essentially zero and $\mathit{\alpha }$-pinene is the dominant monoterpene in this region. For $\mathit{\beta }$-pinene, , and found SOA mass yields in the 30–50 % range at relevant loading and relative humidity, and , and found a limonene SOA yield of 25–57 %. Because the actual average molecular weight of the condensing species is unknown (we do not include sesquiterpene oxidation products and higher molecular weight BVOC products as reported by , with which we would calculate larger SOA mass yields), this comparison is not straightforward, but it appears that the aggregate SOA yield suggests higher ultimate SOA mass yields than simple chamber experiments dictate, perhaps suggesting that post-first-generation products create more condensable species.

Since nitrate product buildup occurs over multiple hours (Fig. ), the rapid particulate organic nitrate losses (timescale of 2–4 $\mathrm{h}$) found by researchers at the University of Washington are a lower limit. This also does not take into account heterogeneous hydrolysis , photolysis , or reaction with the hydroxyl radical ($\mathrm{OH}$) . Because understanding of these nitrate loss processes is poor, a quantitative estimate of how this would affect derived molar yields would be premature.

Finally, because this yield is based on total ambient monoterpene concentrations, it incorporates nitrate radical loss to $\mathit{\alpha }$-pinene, which is known to produce very modest yields of SOA (0–10 %) from ${\mathrm{NO}}_{\mathrm{3}}$ reaction . This suggests effective overall SOA yields from other BVOCs must be large.

Organic nitrate product analysis

Observations of ${{\mathrm{NO}}_{\mathrm{3}}}_{,\mathrm{SS}}$ compared to TD-LIF and AMS (Fig. ) suggest aerosol-organic nitrates are dominated by nighttime ${\mathrm{NO}}_{\mathrm{3}}$ $+$ BVOCs, rather than other known nitrate-producing reactions (e.g., ${\mathrm{RO}}_{\mathrm{2}}$ $+$ $\mathrm{NO}$), which would dominate during the daytime and would not coincide with peaks in [${\mathrm{NO}}_{\mathrm{3}}$].

Researchers at University of Washington described the observation of particle phase ${\mathrm{C}}_{10}$ organic nitrate concentrations peaking at night during SOAS , consistent with high SOA yield from ${\mathrm{NO}}_{\mathrm{3}}$ + monoterpenes. Observed ${\mathrm{C}}_{10}$ organic nitrates include many highly oxidized molecules, suggesting that substantial additional oxidation beyond the first-generation hydroxynitrates occurs . Specific first-generation monoterpene organic nitrate compounds were identified and measured in the gas and aerosol phases . Using the $({\mathrm{NO}}_{\mathrm{3},\mathrm{loss}}{)}_{\mathrm{integ}}$ calculations, another correlation analysis is conducted to identify key gas- and aerosol-phase products of ${\mathrm{NO}}_{\mathrm{3}}$ oxidation. Observed buildups in gas- and aerosol-phase organic nitrate concentrations from each CIMS are scattered against predicted $({\mathrm{NO}}_{\mathrm{3},\mathrm{loss}}{)}_{\mathrm{integ}}$ to monoterpenes (Fig. ). The generally good correlations suggest that all of the molecular formulae shown here have contributions from ${\mathrm{NO}}_{\mathrm{3}}$ chemistry. Comparisons of observed $R^{\mathrm{2}}$ values and slopes for each of these correlation plots may then provide some mechanistic insight. For example, the species with larger $R^{\mathrm{2}}$ (${\mathrm{C}}_{10}{\mathrm{H}}_{17}{\mathrm{NO}}_{\mathrm{5}}$) may indicate a greater contribution to these species from nitrate radical chemistry. If we assume the same sensitivity across phases in the cases where the same species is observed (Fig. a/b and d/e), we can estimate the relative amount in each phase by the ratio of the slopes. This would suggest that ${\mathrm{C}}_{10}{\mathrm{H}}_{15}{\mathrm{NO}}_{\mathrm{5}}$ partitions preferentially to the particle phase, while ${\mathrm{C}}_{10}{\mathrm{H}}_{17}{\mathrm{NO}}_{\mathrm{5}}$ partitions preferentially to the gas phase.

Although the gas-phase monoterpene nitrate product correlations display substantial scatter, likely due to their multiple possible sources and rapid partitioning to the aerosol phase, we can use the calibrated mixing ratios measured by the CIT-CIMS to calculate approximate lower limit molar yields for ${\mathrm{C}}_{10}{\mathrm{H}}_{15}{\mathrm{NO}}_{\mathrm{5}}$ (0.4 %), ${\mathrm{C}}_{10}{\mathrm{H}}_{17}{\mathrm{NO}}_{\mathrm{5}}$ (3 %), and ${\mathrm{C}}_{10}{\mathrm{H}}_{17}{\mathrm{NO}}_{\mathrm{4}}$ (3 %) from ${\mathrm{NO}}_{\mathrm{3}}$, based on the slope of correlations shown in panels (c), (f), and (h). We estimate these to be lower limits, because no losses of these species during the period of buildup is taken into account in this correlation analysis.

The median particulate fraction of ${\mathrm{C}}_{\mathrm{5}}{\mathrm{H}}_{\mathrm{9}}{\mathrm{NO}}_{\mathrm{5}}$ (particle phase/total) observed by the UW-CIMS was less than 1 %, and ${\mathrm{C}}_{\mathrm{5}}{\mathrm{H}}_{\mathrm{9}}{\mathrm{NO}}_{\mathrm{5}\left(\mathrm{p}\right)}$ comprised less than 1 % of total particulate organic nitrate . Those ${\mathrm{C}}_{\mathrm{5}}$ species that are observed in the particle phase constitute less than 12 % of total particulate organic nitrate mass (as measured by the UW-CIMS; , Supplement), and are more highly oxidized molecules, inconsistent with first-generation ${\mathrm{NO}}_{\mathrm{3}}$ $+$ isoprene products. This suggests that most (especially first-generation) isoprene nitrate products remain in the gas phase. The correlation of gas-phase first-generation isoprene nitrate concentrations with ${\mathrm{NO}}_{\mathrm{3}}$ loss again provides evidence about the oxidative sources of these molecules (Fig. ). ${\mathrm{C}}_{\mathrm{5}}{\mathrm{H}}_{\mathrm{9}}{\mathrm{NO}}_{\mathrm{5}}$ (panels a and b) shows the strongest correlation with $({\mathrm{NO}}_{\mathrm{3},\mathrm{loss}}{)}_{\mathrm{integ}}$ to isoprene among all the individual molecules ($R^{\mathrm{2}}=0.54$ for UW and 0.70 for CIT), suggesting that this compound is a product of ${\mathrm{NO}}_{\mathrm{3}}$ oxidation. The better correlations of these ${\mathrm{C}}_{\mathrm{5}}$ species than those observed in Fig.  may be due to slower gas-phase losses of organic nitrates relative to the semi-volatile ${\mathrm{C}}_{10}$ species. Using the calibrated mixing ratios from CIT for ${\mathrm{C}}_{\mathrm{5}}{\mathrm{H}}_{\mathrm{9}}{\mathrm{NO}}_{\mathrm{5}}$, we calculate an approximate lower limit molar yield of 7 %. The ${\mathrm{C}}_{\mathrm{5}}{\mathrm{H}}_{\mathrm{9}}{\mathrm{NO}}_{\mathrm{4}}$ and ${\mathrm{C}}_{\mathrm{4}}{\mathrm{H}}_{\mathrm{9}}{\mathrm{NO}}_{\mathrm{5}}$ isoprene products (panels c and d) show poorer correlation with $({\mathrm{NO}}_{\mathrm{3},\mathrm{loss}}{)}_{\mathrm{integ}}$ to isoprene ($R^{\mathrm{2}}=0.11$ and 0.35, respectively), suggesting that these products are not (exclusively) a ${\mathrm{NO}}_{\mathrm{3}}$ $+$ isoprene product, and may instead be a photochemically or ozonolysis produced organic nitrate, via ${\mathrm{RO}}_{\mathrm{2}}$ $+$ $\mathrm{NO}$.

We note that the two CIMS for which data are shown in Figs.  and were located at different heights: the CIT-CIMS was atop the 20 $\mathrm{m}$ tower, collocated with the measurements used to determine $[{\mathrm{NO}}_{\mathrm{3}}{]}_{\mathrm{ss}}$, while the UW-CIMS measured at ground level. Particularly at nighttime, it is possible that this lower 20 $\mathrm{m}$ of the nocturnal surface layer can become stratified, so some scatter and differences in correlations between instruments arising from this occasional stratification are not unexpected.

Comparison to inorganic NO${}_x$ sink: ${\mathrm{NO}}_{\mathrm{3}}^{-}$ aerosol production from heterogeneous uptake of ${\mathrm{HNO}}_{\mathrm{3}}$

Partitioning of semi-volatile ammonium nitrate into aerosol represented a small fraction of aerosol contribution throughout the campaign based on AMS and MARGA data . A more important route of NO${}_x$ conversion to nitrate aerosol occurred via ${\mathrm{HNO}}_{\mathrm{3}}$ heterogeneous reaction on the surface of dust or sea salt particles (Scheme ). This process, which was observed to be especially important during periods of high mineral or sea salt supermicron aerosol concentrations, is described in detail in a companion paper . Briefly, we observe that while concentrations of organic and inorganic nitrate aerosol are generally comparable (Fig. ), the inorganic nitrate is more episodic in nature. Periods of highest ${\mathrm{NO}}_{\mathrm{3}}^{-}$ concentration as measured by the MARGA were observed during two multi-day coarse-mode dust events, from 9 to 15 and 23 to 30 June, while organic nitrates have a more regular diurnal pattern indicative of production from locally available reactants, with most of the organic nitrate present in the condensed phase (Fig. ).

In order to estimate the fluxes of NO${}_x$ loss to aerosol via the two pathways shown in Scheme 1, we calculate the reactive losses of ${\mathrm{NO}}_{\mathrm{2}}$ to organic nitrate (limiting rate is taken to be $\sum _i{k}_i\left[{\mathrm{NO}}_{\mathrm{3}}\right][\text{BVOC}{]}_i$, with the included terpenes $\mathit{\alpha }$-pinene, $\mathit{\beta }$-pinene, limonene, and camphene) and to inorganic nitrate via heterogeneous ${\mathrm{HNO}}_{\mathrm{3}}$ uptake . A substantial fraction of the surface area is in the transition regime, so ${\mathrm{HNO}}_{\mathrm{3}}$ uptake is reduced due to diffusion limitations. To account for this, a Fuchs–Sutugin correction is applied : $$\mathrm{\text{Rate}}=\sum ^{R_p}{\displaystyle \frac{S_a}{R_p}}{D}_g\left({\displaystyle \frac{0.75\mathit{\alpha }(\mathrm{1}+Kn)}{K{n}^{\mathrm{2}}+Kn+0.283Kn\mathit{\alpha }+0.75\mathit{\alpha }}}\right)\left[{\mathrm{HNO}}_{\mathrm{3}}\right],$$ where $S_a$ is surface area, $R_p$ is the radius, $D_g$ is the diffusivity of ${\mathrm{HNO}}_{\mathrm{3}}$ in air (0.118 ${\mathrm{cm}}^{\mathrm{2}}{\mathrm{s}}^{-\mathrm{1}}$), and $\mathit{\alpha }$ is estimated at 0.1 for an upper limit.

Over the campaign, similar magnitudes of the rate of formation of organic and inorganic nitrate aerosol (according to the pathways shown in Scheme 1) are observed, though peaks occur at different times.

[Figure omitted. See PDF]

Since we have seen that the organic nitrates are present predominantly in the condensed phase, we take this comparison to be the relative rate of production of organic nitrate aerosol vs. inorganic nitrate aerosol (Fig. ), and we see that over the summer campaign, the rates are comparable in magnitude, but peak at different times. This analysis suggests that substantial nitrate aerosol (peak values of 1 $\mathrm{\mu }\mathrm{g}{\mathrm{m}}^{-\mathrm{3}}{\mathrm{h}}^{-\mathrm{1}}$, with average rates 0.1 $\mathrm{\mu }\mathrm{g}{\mathrm{m}}^{-\mathrm{3}}{\mathrm{h}}^{-\mathrm{1}}$ for both inorganic and organic nitrate rates) is produced in the SEUS by both inorganic and organic routes (depicted in Scheme ), converting local NO${}_x$ pollution to particulate matter. We note that this calculation accounts only for the production rates of these two types of nitrate aerosol and does not account for the subsequent chemistry that may deplete one faster than the other; hence, relative mass concentrations are not necessarily expected to correlate directly to these relative production rates.

Implications of ${\mathrm{NO}}_{\mathrm{3}}$ oxidation on SOA formation in the SEUS

The importance of the ${\mathrm{NO}}_{\mathrm{3}}$ $+$ BVOC reaction SOA has only recently been recognized . showed that including ${\mathrm{NO}}_{\mathrm{3}}$ radical oxidation increased predicted SOA yields from terpenes by 100 % and total aerosol concentrations by 30 % . The results of this study underscore the importance of ${\mathrm{NO}}_{\mathrm{3}}$ in SOA formation. Measured aerosol-organic nitrate concentrations are correlated with the reaction of ${\mathrm{NO}}_{\mathrm{3}}$ with BVOCs. This pathway is especially important before sunrise when competing oxidants (${\mathrm{O}}_{\mathrm{3}}$ and $\mathrm{OH}$) are at a minimum.

We can estimate the contribution of this ${\mathrm{NO}}_{\mathrm{3}}$ $+$ BVOC mechanism to total particulate matter using the 2011 NEI data for the states included in the 2004 Southern Appalachian Mountain Initiative study (SAMI; ): Kentucky, Virginia, West Virginia, North Carolina, South Carolina, Tennessee, Alabama, and Georgia (http://www.epa.gov/ttn/chief/net/2011inventory.html). In this eight-state region, the NEI reported emissions of 2.3 $\mathrm{Tg}{\mathrm{yr}}^{-\mathrm{1}}$ ($2.5\times {10}^{\mathrm{6}}$ $\mathrm{t}{\mathrm{yr}}^{-\mathrm{1}}$) of nitrogen oxides and 0.8 $\mathrm{Tg}{\mathrm{yr}}^{-\mathrm{1}}$ ($\mathrm{9}\times {10}^{\mathrm{5}}$ $\mathrm{t}{\mathrm{yr}}^{-\mathrm{1}}$) of ${\mathrm{PM}}_{2.5}$ in 2011. We can estimate the fraction of the NO${}_x$ emitted that is converted to PM using several assumptions. ${\mathrm{NO}}_{\mathrm{2}}$ is estimated to contribute 50 % of the NO${}_y$ budget (Fig. S4), so we multiply the NO${}_x$ emission by 0.5 to account for half of the instantaneous NO${}_x$ residing in the atmosphere as other NO${}_y$ species at any given time. An average lifetime of 16 $\mathrm{h}$ for ${\mathrm{O}}_{\mathrm{3}}$ $+$ ${\mathrm{NO}}_{\mathrm{2}}$ reaction was calculated ($\mathrm{1}/k\left[{\mathrm{O}}_{\mathrm{3}}\right]$) and, with an average nighttime length of 9 $\mathrm{h}$, we estimate about 55 % of ${\mathrm{NO}}_{\mathrm{2}}$ is converted to ${\mathrm{NO}}_{\mathrm{3}}$ overnight. Using the average molar organic nitrate aerosol yield of 30 % determined in this study and an estimated molecular weight of 250 $\mathrm{g}{\mathrm{mol}}^{-\mathrm{1}}$ for oxidized product (terpene hydoxynitrate with two additional oxygen functional groups; ), we convert from molar yield to mass yield of organic nitrate aerosol. This assumes that NO${}_x$ is the limiting reagent for SOA production from this chemistry; as noted in the introduction, comparison of regional NO${}_x$ and BVOC emissions rates supports this assumption. Finally, using the summed NEI NO${}_x$ emissions data for the SAMI states, we calculate a source estimate of 0.6 $\mathrm{Tg}{\mathrm{yr}}^{-\mathrm{1}}$ of ${\mathrm{NO}}_{\mathrm{3}}$-oxidized aerosol. Adding this to the NEI primary ${\mathrm{PM}}_{2.5}$ emissions estimate of 0.8 $\mathrm{Tg}{\mathrm{yr}}^{-\mathrm{1}}$ gives a total 1.4 $\mathrm{Tg}{\mathrm{yr}}^{-\mathrm{1}}$, showing that ${\mathrm{NO}}_{\mathrm{3}}$ initiated SOA formation would contribute a substantial additional source of ${\mathrm{PM}}_{2.5}$ regionally, nearly doubling primary emissions. Model calculations by for the SAMI states estimated 1 $\mathrm{Tg}{\mathrm{yr}}^{-\mathrm{1}}$ of total ${\mathrm{PM}}_{2.5}$ in 2010, including primary and secondary sources. Their modeled ${\mathrm{PM}}_{2.5}$ emissions are lower than our rough estimate here, despite the fact that actual 2010 NO${}_x$ emissions were 2.3 $\mathrm{Tg}{\mathrm{yr}}^{-\mathrm{1}}$ rather than the 3 $\mathrm{Tg}{\mathrm{yr}}^{-\mathrm{1}}$ projected at that time. Hence, despite successful reduction of regional NO${}_x$ emissions , this work suggests that secondary ${\mathrm{PM}}_{2.5}$ production from ${\mathrm{NO}}_{\mathrm{3}}$ oxidation of regionally abundant BVOCs remains a substantial anthropogenic source of pollution in the SEUS.

Conclusions

The contribution of ${\mathrm{NO}}_{\mathrm{3}}$ $+$ BVOCs to SOA formation is found to be substantial in the terpene-rich SEUS. An estimated 23–44 % of nitrate radical lost to reaction with monoterpenes becomes aerosol-phase organic nitrate. Predicted nitrate losses to isoprene and to monoterpenes are calculated from the steady-state nitrate and BVOC mixing ratios and then time integrated during evenings and nights as ${\mathrm{RONO}}_{\mathrm{2}}$ aerosol builds up. Correlation plots of AMS, TD-LIF, and CIMS measurements of gas- and aerosol-phase organic nitrates against predicted nitrate losses to monoterpenes indicate that ${\mathrm{NO}}_{\mathrm{3}}$ $+$ monoterpenes contribute substantially to observed nitrate aerosol. Two specific ${\mathrm{C}}_{10}$ structures measured by CIMS are shown to be ${\mathrm{NO}}_{\mathrm{3}}$ radical products by their good correlation with cumulative $({\mathrm{NO}}_{\mathrm{3},\mathrm{loss}}{)}_{\mathrm{integ}}$; their semi-volatile nature leads to their variable partitioning between gas and aerosol phase. Calibrated gas-phase mixing ratios of selected organic nitrates allow for estimation of lower limit molar yields of ${\mathrm{C}}_{\mathrm{5}}{\mathrm{H}}_{\mathrm{9}}{\mathrm{NO}}_{\mathrm{5}}$, ${\mathrm{C}}_{10}{\mathrm{H}}_{17}{\mathrm{NO}}_{\mathrm{4}}$, ${\mathrm{C}}_{10}{\mathrm{H}}_{17}{\mathrm{NO}}_{\mathrm{5}}$ from ${\mathrm{NO}}_{\mathrm{3}}$ reactions (7, 3, and 3 % respectively). The fact that these molar yields of monoterpene nitrates are substantially lower than the aggregated aerosol-phase organic nitrate yield may suggest that further chemical evolution is responsible for the large SOA yields from these reactions, consistent with . The ${\mathrm{NO}}_{\mathrm{3}}$ $+$ BVOC source of nitrate aerosol is comparable in magnitude to inorganic nitrate aerosol formation, and is observed to be a substantial contribution to regional ${\mathrm{PM}}_{2.5}$.

The Supplement related to this article is available online at doi:10.5194/acp-15-13377-2015-supplement.

Acknowledgements

We would like to acknowledge Anne Marie Carlton, Jim Moore and all of the colleagues that helped to set up this study. B. R. Ayres, H. M. Allen, D. C. Draper, and J. L. Fry gratefully acknowledge funding from the National Center for Environmental Research (NCER) STAR Program, EPA no. RD-83539901 and NOAA NA13OAR4310063. D. A. Day, P. Campuzano-Jost, and J. L. Jimenez thank NSF AGS-1243354 and NOAA NA13OAR4310063; R. C. Cohen thanks NSF AGS-1120076 and AGS-1352972. Edited by: A. Nenes