Owing to their high emissions and high reactivity with the major atmospheric oxidants (O3, OH, NO3), the oxidation of biogenic volatile organic compounds (BVOCs) emitted by vegetation, such as isoprene (C5H8), monoterpenes (C10H16), and sesquiterpenes (C15H24), is believed to be the dominant contributor to global secondary organic aerosol (SOA) formation (e.g., Kanakidou et al., 2005). While this is supported by the observation that ambient organic aerosol is predominantly “modern” and therefore biogenic in origin (Lewis et al., 2004; Schichtel et al., 2008; Marley et al., 2009), there exists an apparent contradiction because ambient organic aerosol is well correlated with anthropogenic tracers (de Gouw et al., 2005; Weber et al., 2007). This apparent discrepancy could be reconciled if anthropogenic pollution influences the atmospheric oxidation of BVOCs and their aerosol formation pathways. The oxidation of BVOCs by nitrate radicals (NO3), formed from the reaction of ozone with NO2, provides a direct link between anthropogenic pollution and the abundance of biogenic carbon in atmospheric aerosol.

Biogenic hydrocarbons react rapidly with nitrate radicals (Atkinson and Arey, 2003a) and the SOA yields are generally higher than in photooxidation and ozonolysis (e.g., Griffin et al., 1999; Hallquist et al., 1999; Spittler et al., 2006; Ng et al., 2008; Fry et al., 2009, 2011, 2014; Rollins et al., 2009). As monoterpene emissions are not entirely light dependent, they are emitted during the day and at night (Fuentes et al., 2000; Guenther et al., 2012) and can contribute substantially to ambient organic aerosol. Monoterpenes have also been found to make up as much as 28 % of non-methane organic carbon emissions from biomass burning in both field and laboratory studies (Akagi et al., 2013; Hatch et al., 2015; Stockwell et al., 2015). Fires from biomass burning are more likely to smolder at night and are therefore more likely to emit monoterpenes, which can then react with nitrate radicals (Akagi et al., 2013). Results from previous field studies provided evidence of aerosol formation from nitrate radical oxidation of BVOCs during both daytime and nighttime (McLaren et al., 2004; Iinuma et al., 2007; Fuentes et al., 2007; Brown et al., 2009, 2013; Rastogi et al., 2011; Rollins et al., 2012; Rollins et al., 2013). Specifically, many of these studies found a significant increase in the amount of monoterpene organic aerosol and oxidation products at night, which could be attributed to nighttime monoterpene oxidation by nitrate radicals (McLaren et al., 2004; Iinuma et al., 2007; Rastogi et al., 2011). Results from recent flight measurements in Houston, TX, also showed that organic aerosol was enhanced in the nocturnal boundary layer at levels in excess of those attributable to primary emissions, implying a source of SOA from the BVOCs+NO3 reaction (Brown et al., 2013).

Global modeling studies showed large variations in the total SOA burden that can be attributed to the oxidation of BVOCs by nitrate radicals, ranging from ∼ 5 to 21 % (Hoyle et al., 2007; Pye et al., 2010). Specifically, Pye et al. (2010) showed that the inclusion of nitrate radical oxidation reaction doubled the total amount of terpene (monoterpenes and sesquiterpenes) aerosol, pointing to the significant contribution of this chemistry to total organic aerosol burden. In these modeling studies, all aerosol formation from the nitrate radical oxidation of terpenes was calculated based on the β-pinene+NO3 SOA yields obtained in Griffin et al. (1999). A recent modeling study by Russell and Allen (2005) determined that as much as 20 % of all nighttime SOA is from the reaction of β-pinene+NO3. Due to the significance of nitrate radical oxidation pathways in SOA formation, it is important that the SOA yields for BVOCs+NO3, and especially that of β-pinene+NO3, are well constrained from fundamental laboratory studies and accurately represented in models.

The majority of the previous laboratory studies of the BVOCs+NO3 chemistry were performed under dry conditions (Berndt and Boge, 1997a, b; Wängberg et al., 1997; Griffin et al., 1999; Hallquist et al., 1999; Bonn and Moorgat, 2002; Spittler et al., 2006; Ng et al., 2008; Rollins et al., 2009; Fry et al., 2009, 2011, 2014; Perraud et al., 2010; Kwan et al., 2012; Jaoui et al., 2013). The effect of relative humidity on SOA formation, however, could potentially be important for nighttime (where NO3 radicals dominate) and early morning chemistry as the ambient relative humidity (RH) is typically higher at these times. Several recent studies have investigated the effect of water on SOA formation from the nitrate radical oxidation pathways but the results are inconclusive. For instance, Spittler et al. (2006) found that the SOA yield is lower at 20 % RH compared to dry conditions, suggesting that water vapor may alter the gas-phase oxidation mechanism and/or partitioning into the particle phase, thus shifting the equilibrium partitioning of organic compounds. However, other studies showed that the presence of water vapor did not affect particle size distributions and SOA formation (Bonn and Moorgat, 2002; Fry et al., 2009). Thus, the role of water in SOA formation from nitrate radical oxidation of BVOCs is still unclear.

Another important parameter in SOA formation from BVOCs+NO3 is the fate of peroxy radicals, which directly determines the oxidation products, SOA yields, and aerosol chemical and physical properties (Kroll and Seinfeld, 2008; Orlando and Tyndall, 2012; Ziemann and Atkinson, 2012). Previous studies regarding the effects of peroxy radical fates on SOA formation from BVOCs typically focused on photooxidation and ozonolysis systems (e.g., Presto et al., 2005; Kroll et al., 2006; Ng et al., 2007a; Eddingsaas et al., 2012; Xu et al., 2014) and isoprene+NO3 chemistry (Kwan et al., 2012; Ng et al., 2008; Nguyen et al., 2014). To our knowledge, the effects of differing peroxy radical branching on SOA formation from nitrate radical oxidation of monoterpenes have not been investigated. The relative importance of different peroxy radical reaction channels concerning BVOCs+NO3 chemistry in the atmosphere is not well established (Brown and Stutz, 2012). While earlier studies by Kirchner and Stockwell (1996) suggested that RO2+NO3 is more important in the nighttime atmosphere, a recent study by Mao et al. (2012) showed that the HO2 mixing ratios are often on the order of 10 ppt at night. It is therefore possible that RO2+HO2 pathways could be important pathways in nighttime oxidation of BVOCs.

Nitrate radical chemistry is expected to produce a substantial amount of organic nitrate compounds, owing to direct addition of nitrate radical via reaction with a double bond. Organic nitrates have been observed to form a substantial portion of atmospheric aerosol in field studies (Brown et al., 2009; Day et al., 2010; Zaveri et al., 2010; Beaver et al., 2012; Rollins et al., 2012, 2013; Fry et al., 2013; Brown et al., 2013; Xu et al., 2015a). Organic nitrate formation has a significant impact on total NOx lifetime, especially in NOx-limited regions where NOx lifetime is sensitive to the formation rates of organic nitrates (Browne and Cohen, 2012). Ambient organic nitrates can be formed through photooxidation of volatile organic compounds (VOCs) in the presence of NOx (Chen et al., 1998; Arey et al., 2001; Yu et al., 2008) and through nitrate radical addition (Spittler et al., 2006; Perring et al., 2009; Rollins et al., 2009; Kwan et al., 2012). One removal mechanism for atmospheric organic nitrates is hydrolysis in the particle phase (e.g., Sato, 2008; Szmigielski et al., 2010; Darer et al., 2011; Hu et al., 2011; Liu et al., 2012; Rindelaub et al., 2015). Modeling studies have assumed that the majority (75 %) of the organic nitrates formed in the day are composed of tertiary nitrates based on results from the photooxidation of α-pinene and β-pinene in the presence of NOx (Browne et al., 2013). However, the organic nitrates formed from photooxidation and nitrate radical oxidation could have different chemical structures (primary, secondary, and tertiary) and need to be investigated to better constrain the fates of organic nitrates (e.g., hydrolysis lifetime) in the atmosphere over their entire life cycle (both day and night).

The goal of this study is to determine the aerosol yields and characterize the mechanisms and chemical composition of SOA formation from the β-pinene+NO3 system. Laboratory chamber experiments are performed in the dark under dry and humid conditions. To investigate the effects of peroxy radical fates on SOA yields and chemical composition, the experiments are designed to probe the “RO2+NO3” vs. “RO2+HO2” reaction pathways. Aerosol yields are obtained over a wide range of initial β-pinene mixing ratios. Based on the measured gas-phase and particle-phase oxidation products, mechanisms for SOA formation from β-pinene+NO3 are proposed. Results from this study are used to evaluate the contributions of nitrate radical oxidation of monoterpenes to ambient organic aerosol measured in the southeastern United States (US), including the Southern Oxidant and Aerosol Study (SOAS) and the Southeastern Center for Air Pollution and Epidemiology (SCAPE) study.

Gas-phase oxidation and aerosol growth is observed to be a rapid process in the β-pinene+NO3 reaction. Peak aerosol growth is typically observed within 10–15 min for all reaction conditions except in humid (RH = 50, 70 %) “RO2+HO2 dominant” experiments, where aerosol reaches peak growth in about 30 min. Figure S1 in the Supplement shows a typical mass spectrum for the CIMS data. Specifically, the major gas-phase products are detected at m/z 342, 356, 358, and 372 (which correspond to MW = 215, 229, 231, 245 amu, respectively). These compounds likely correspond to organic nitrate species with molecular assignments of C10H17NO4, C10H15NO5, C10H17NO5, and C10H15NO6, respectively. Figure 2 shows the time series of these species and the aerosol growth over the course of a typical “RO2+HO2 dominant” experiment in dry conditions. The products at m/z 356 and 358 (MW = 229 and 231 amu) decrease over the course of the experiment. While this can be attributed to vapor phase wall loss, it is also possible that these gas-phase compounds undergo further reaction. This is further supported by the increase in the species at m/z 372 (MW = 245 amu). The proposed gas-phase oxidation mechanism and formation of compounds at m/z 372 from compounds at m/z 356 will be discussed further in Sect. 4.1.

Although all the above gas-phase species are observed under all reaction conditions, m/z 358 (MW = 231 amu) is significantly higher in the “RO2+HO2 dominant” experiments than in the “RO2+NO3 dominant” experiments (Fig. S2), which is indicative of differences in the gas-phase chemistry depending on the RO2 fate. Under both “RO2+HO2 dominant” and “RO2+NO3 dominant” conditions, experiments conducted under dry conditions have significantly higher N2O5 concentrations than humid conditions (by at least a factor of 2) as measured by CIMS. This is likely due to N2O5 uptake (loss) on the wet chamber surfaces and/or seed aerosol. The relative abundance of N2O5 under different experimental conditions is important in terms of β-pinene reaction rate and aging of aerosol, which are discussed in Sect. 4.2.2 and 4.4, respectively.

All SOA growth data are corrected for particle wall loss by applying size-dependent coefficients determined from wall loss experiments at GTEC following the methodology described in Keywood et al. (2004). The size-dependent particle wall loss rates calculated for both chambers at GTEC are shown in Fig. S3. Figures 3 and 4 show the SOA yields for “RO2+NO3 dominant” and “RO2+HO2 dominant” experiments over a wide range of aerosol mass loadings (ΔMo= 5.1–216.1 µg m-3). The SOA yields lie in the range of 27.0–104.1 % over the conditions studied. Aerosol mass yield (Y) is defined as the aerosol mass concentration produced (ΔMo) divided by the mass concentration of hydrocarbon reacted (ΔHC), Y=ΔMo/ΔHC (Odum et al., 1996; Bowman et al., 1997; Odum et al., 1997a, b). For all experiments, aerosol mass concentration is obtained from the SMPS aerosol volume concentration (averaged over 30 min at peak growth) and the calculated aerosol density. The aerosol density is calculated from the SMPS volume distribution and the HR-ToF-AMS mass distribution in the nucleation experiments (Bahreini et al., 2005). The densities of the organic aerosol generated in nucleation experiments under dry and humid (RH = 50, 70 %) conditions are determined to be 1.41 g cm-3 and 1.45 g cm-3 for the “RO2+NO3 dominant” experiments and 1.54 and 1.61 g cm-3 for the “RO2+HO2 dominant” experiments.

It can be seen from Fig. 3 that the aerosol yields in the “RO2+NO3 dominant” experiments under dry vs. humid conditions in the presence of (NH4)2SO4 seed are similar. The presence of the more acidic (NH4)2SO4+H2SO4 seed does not appear to enhance SOA production in the “RO2+NO3 dominant” experiments (Fig. S4). Therefore, we fit the Odum two-product model (Odum et al., 1996, 1997a) to all of our experimental data shown in Fig. 3 to obtain a single yield curve. The SOA yield parameters are given in Table 2. Shown in Fig. 4 are the aerosol yields from “RO2+HO2 dominant” experiments under dry vs. humid (RH = 70 %) conditions. The SOA yield curve (solid red line) for the “RO2+NO3 dominant” experiments is also shown for comparison.

For comparison, SOA yields from previous β-pinene+NO3 laboratory chamber studies (Griffin et al., 1999; Fry et al., 2009) are also shown in Fig. 3. Without adding HCHO as an additional HO2 source, it is likely that the experiments in Griffin et al. (1999) and Fry et al. (2009) are more similar to our “RO2+NO3 dominant” experiments. Specifically, Fry et al. (2009) noted that the β-pinene+NO3 reaction likely does not produce significant concentrations of HO2 radicals and therefore has a low HO2 / RO2 ratio. As Griffin et al. (1999) assumed an aerosol density of 1.0 g cm-3, the experimental data from Griffin et al. (1999) shown in Fig. 3 have been multiplied by the density calculated in our study for “RO2+NO3 dominant” experiments under dry conditions (i.e., 1.41 g cm-3). The data shown in Fig. 3 from Fry et al. (2009) have also incorporated a particle density of 1.6 g cm-3calculated in their study. In addition to correcting for density, the equilibrium partitioning coefficient, K, from Griffin et al. (1999) has been adjusted from 306 to 298 K using an enthalpy of vaporization of 42 kJ mol-1 for comparison to results from our study (Chung and Seinfeld, 2002). It is noted that the SOA yields obtained in the current study are higher than those in Griffin et al. (1999) and Fry et al. (2009), particularly at lower aerosol mass loadings that are more relevant to ambient environments. These results are discussed in more detail in Sect. 4.2.

Bulk aerosol composition from the experiments is characterized by the HR-ToF-AMS. A typical high-resolution mass spectrum for aerosol formed under dry conditions where the RO2+NO3 pathway is dominant (Experiment 5 in Table 1) is shown in Fig. 5. A key feature of the mass spectrum is the high intensity of the nitrate ions at NO+ and NO2+, which make up about 11 % of the combined organics and nitrate signals. The majority (> 90 %) of the nitrogen atoms are detected at these two ions with the remaining nitrogen-containing ions detected at higher masses as CxHyOzN. The mass spectra for the aerosol generated in the “RO2+HO2 dominant” and “RO2+NO3 dominant” experiments are similar. One notable difference between the “RO2+HO2 dominant” and “RO2+NO3 dominant” experiments is the NO+ : NO2+ ratio for the organic nitrates (R-ON), which ranges from 4.8 to 10.2 in all experiments. While the NO+ : NO2+ ratio averages 6.5 for “RO2+NO3 dominant” experiments, it averages 8.6 for “RO2+HO2 dominant” experiments. Since the values of R-ON may depend on the instrument, we normalize the R-ON to the NO+ : NO2+ ratio of ammonium nitrate (R-AN), which is expected to be a better metric (Farmer et al., 2010). In our study, multiple measurements of R-AN are obtained from the ionization efficiency (IE) calibrations and the average value is 1.8 (range of 1.2–2.7). Applying the R-AN that is measured closest in time to each chamber experiment, we calculate the average R-ON : R-AN ratio to be 3.2 for “RO2+NO3 dominant” experiments and 4.8 for “RO2+HO2 dominant” experiments.

For both types of experiments, there is a negligible difference in the mass spectrum of the aerosol produced in dry or high humidity (RH = 50, 70 %) conditions. In Fig. 5, nitrate and organic ions are each assigned a different color to indicate an individual AMS HR ion family. There are a few notable ions in the aerosol mass spectrum. The signals at m/z 67 (C5H7+) and m/z 91 (C7H7+), while not significant in the high-resolution mass spectra of several biogenic SOA systems (Ng et al., 2008; Chhabra et al., 2010), are relatively large for β-pinene+NO3 SOA. These ions also make up a larger fraction of the HR-ToF-AMS signal for SOA formed from the ozonolysis of β-caryophyllene (Chen et al., 2015) when compared to other biogenic SOA. Therefore, m/z 67 (C5H7+) and m/z 91 (C7H7+) could potentially serve as useful indicators for SOA formed from monoterpene/sesquiterpene oxidation in ambient aerosol mass spectra. However, more studies of SOA formed from the oxidation of biogenic VOCs are necessary to apportion ambient organic aerosol (OA) based on these fragments.

Figure 6 shows the time evolution of the major organic families relative to sulfate measured by the HR-ToF-AMS for a typical dry “RO2+NO3 dominant” experiment (Experiment 5 in Table 1). Sulfate is used to normalize the decay of the organic families because it is non-volatile and any decrease in sulfate is reflective of particle wall loss and changes in aerosol collection efficiency (CE) in the HR-ToF-AMS (Henry and Donahue, 2012). Any change of each organic family relative to sulfate is therefore interpreted as a change in organic mass unrelated to particle wall loss or CE. Non-oxidized fragments (CH Family in green) decrease more rapidly relative to sulfate than the more oxidized fragments (CHO1 Family in purple; CHOgt1 (fragments with greater than 1 oxygen atom) Family in pink). The change in mass for each organic family is determined over a 2.5 h period following peak aerosol growth (at t ∼ 15 min) in each “RO2+NO3 dominant” experiment (dry and humid). We find that the CHOgt1 Family increases by 4 % in dry experiments and remains relatively constant in humid experiments. This is consistent with a larger extent of aerosol aging in the dry experiments and is further discussed in Sect. 4.4.

Figure 7 shows the time evolution of HR-ToF-AMS nitrate-to-organics ratio in the “RO2+NO3 dominant” experiments at RH = 50 % normalized by that in the corresponding dry experiments with the same initial hydrocarbon concentration. For simplicity, we refer to this ratio as (nitrate : org)norm. Normalizing the nitrate-to-organics ratio obtained from the humid experiments to the dry experiments allows for determining the extent of possible organic nitrate hydrolysis under humid conditions. Since only the relative change in the (nitrate : org)norm ratio is important for comparison purposes, the maximum (nitrate : org)norm measurement for each experiment is set to be unity. Nitrate mass is defined here as the sum of the mass of the NO+ and NO2+ ions. This does not account for the CxHyOzN fragments, but these fragments only account for less than 10 % (by mass) of the nitrate functional groups detected by HR-ToF-AMS. As the experiment progresses, the (nitrate : org)norm ratio decreases and stabilizes at a value of about 0.9, indicating that there is no further decrease in the mass of nitrate relative to the mass of organics beyond this point. From our particle wall loss experiments, we establish that the particles are lost to the chamber wall with comparable rates under dry and humid conditions, suggesting that the observed decrease in the (nitrate : org)norm ratio is not a result of differing particle wall loss in dry and humid experiments. Instead, the decrease under humid conditions is attributed to hydrolysis of organic nitrate compounds in the particle phase. This is further discussed in Sect. 4.3.2.

Results from this study provide the fundamental information to evaluate the extent to which nitrate radical oxidation of monoterpenes contributes to ambient organic aerosol. This reaction provides a direct mechanism for linking anthropogenic and biogenic emissions, and is likely substantial in the southeastern United States, where both types of emissions are high. A recent field campaign, SOAS, took place in Centreville, Alabama, from 01 June to 15 July 2013 to investigate the effects of anthropogenic pollution in a region with large natural emissions. Based on positive matrix factorization (PMF) analysis of the HR-ToF-AMS data obtained in SOAS, Xu et al. (2015b) identified an OA subtype termed as less-oxidized oxygenated organic aerosol (LO-OOA), which accounted for 32 % of the total OA at Centreville. LO-OOA peaks at night and is well correlated with particle-phase organic nitrates. These suggest that LO-OOA is produced predominantly from nighttime monoterpene+NO3 chemistry, especially from β-pinene+NO3 as β-pinene has a high nighttime concentration (Xu et al., 2015b). Results from the current laboratory chamber study provide the relevant fundamental data for estimating the amount of aerosol produced from monoterpene+NO3 in SOAS. The campaign-averaged loading of non-refractory PM1 in SOAS is about 8 µg m-3 and it has been determined that the aerosol is highly acidic (pH = 0.94 ± 0.59) and contains a large amount of particulate water (5.09 ± 3.76 µg m-3) (Cerully et al., 2014; Guo et al., 2015). At night, the RH can reach up to 90 % during the SOAS measuring period (Guo et al., 2015). The current chamber study is designed to probe SOA formation from nitrate radical oxidation under atmospherically relevant loadings, under high humidity, and in the presence of seed aerosol of different acidity. The fates of peroxy radicals at night are highly uncertain, which mainly arises from the lack of constraints on the reaction rates of the peroxy radicals with other species, such as RO2+NO3 (Brown and Stutz, 2012). In our study, the experiments are conducted under both “RO2+NO3 dominant” and “RO2+HO2 dominant” regimes to explore the effects of peroxy radial fates on SOA formation. Using a SOA yield of 50 % (for a mass loading of 8 µg m-3 obtained from the yield curve) in the presence of acidic seed at RH = 70 % obtained from “RO2+HO2 dominant” experiments, Xu et al. (2015b) estimated that about 50 % of nighttime OA production could be due to the reaction of monoterpenes with nitrate radicals in SOAS, a large fraction of which is from β-pinene+NO3 reaction.

It is noted that the LO-OOA factor is also resolved at both rural and urban sites around the greater Atlanta area in all seasons, where HR-ToF-AMS measurements were conducted as part of the SCAPE (Verma et al., 2014; Xu et al., 2015a, b). It is found that LO-OOA made up 18–36 % of the total OA in rural and urban areas, suggesting that a fairly large fraction of total OA in the southeastern United States could arise from nitrate radical oxidation of monoterpenes.

Figure 10 shows a comparison of the aerosol mass spectrum from a typical β-pinene+NO3 experiment from this study and the LO-OOA factor obtained from SOAS data. As LO-OOA could have other sources in addition to monoterpene+NO3, the two spectra are not in perfect agreement but they do show similar features above m/z 60. Most noticeable of these are m/z 67 (C5H7+) and m/z 91 (C7H7+) with a ratio of these two ions (C5H7+: C7H7+) of about 2.9 (ranging from 2.5 to 3.6 in other experiments). The mass spectra for the other SOA-forming systems predicted to be of importance at SOAS, namely, α-pinene ozonolysis (Chhabra et al., 2010), isoprene photooxidation (Chhabra et al., 2010), and nitrate radical-initiated isoprene chemistry (Ng et al., 2008), do not show significant intensities at either of these two ions. Therefore, it is likely that high signals at C5H7+ and C7H7+ in ambient aerosol mass spectrum could be indicative of the presence of β-pinene+NO3 reaction products. We note that the average NO+ : NO2+ ratio for aerosol measured at SOAS is 7.1, consistent with the high NO+ : NO2+ ratio from the SOA formed from nitrate radical oxidation of β-pinene in this study.

The gas-phase oxidation products detected by the CIMS in this study can also be used to help interpret ambient data to evaluate the possible contribution of β-pinene+NO3 reaction. For instance, a significant amount of gas-phase organic nitrate species with MW of 215 amu and 231 amu have been observed during the Biosphere Effects of Aerosols and Photochemistry Experiment (BEARPEX) campaign in fall 2009 (Beaver et al., 2012). As these species exhibited a nighttime peak, Beaver et al. (2012) suggested that they could arise from nighttime oxidation of α-pinene or β-pinene by nitrate radicals. The proposed mechanism for β-pinene+NO3 (Fig. 8) show multiple reaction pathways to form species with MW = 215 amu and MW = 231 amu. Therefore, the oxidation of β-pinene by nitrate radicals represents one possible pathway for the formation of the species detected by Beaver et al. (2012). As the β-pinene+NO3 reaction has shown to be important at SOAS (Xu et al., 2015b), it is expected that the gas-phase compounds observed in this chamber study could help explain some of the species detected by the multiple CIMS deployed during the SOAS study.

Although photooxidation is expected to be the major oxidation pathway for atmospheric VOCs, nitrate radical oxidation can account for as much as 20 % of global BVOC oxidation and is predicted to lead to an aerosol mass increase by as much as 45 % when compared to the modeled case where this chemistry is excluded (Pye et al., 2010). Due to high SOA yields, evaluating the mass of aerosol produced by nitrate radical-initiated chemistry is essential to estimate the total organic aerosol burden, both on regional and global scales. Currently, the aerosol yields from nitrate radical oxidation of monoterpenes in most models are assumed to be the same as those determined from β-pinene+NO3 reactions in Griffin et al. (1999) (Pye et al., 2010). In this study, we systematically investigate SOA formation from the nitrate radical oxidation of β-pinene under various reaction conditions (dry, humid, differing radical fate) and a wide range of initial hydrocarbon concentrations that are atmospherically relevant. We determine that the SOA yields from the β-pinene+NO3 systems are consistent with Griffin et al. (1999) for mass loadings > 45 µg m-3, but as much as a factor of 4 higher than those reported in Griffin et al. (1999) for lower mass loadings. The lower SOA yields reported in Griffin et al. (1999) could arise from uncertainties in extrapolating data from higher mass loadings to lower mass loadings in that study, as well as from slower reaction rates and vapor wall loss effects (Zhang et al., 2014). While it is likely that the SOA yields from the nitrate radical oxidation of various monoterpenes are different (Fry et al., 2014), updating SOA formation from β-pinene+NO3 with the new yield parameters in future modeling studies would lead to a more accurate prediction of the amount of aerosol formed from this reaction pathway.

Currently, the fate of peroxy radicals (RO2+HO2 vs. RO2+NO3, etc.) in the nighttime atmosphere is still highly uncertain (Brown and Stutz, 2012), though recent studies showed that the HO2 mixing ratio is often on the order of 10 ppt (Mao et al., 2012). Thus, RO2+HO2 could be the dominant nighttime fate of peroxy radicals. In this study, we examine the effect of RO2 fate on aerosol yields for the β-pinene+NO3 system. Although more ROOH species are produced through the RO2+HO2 channel, the SOA yields in the “RO2+NO3 dominant” and “RO2+HO2 dominant” experiments are comparable. This indicates that for this system, the overall product chemical composition and volatility distribution may not be very different for the different peroxy radical fates. This is in contrast to results from nitrate radical oxidation of smaller biogenic species, such as isoprene, which have large differences in SOA yields depending on the RO2 fate (Ng et al., 2008). This suggests that the fates of peroxy radicals in nitrate radical experiments for larger BVOCs (such as monoterpenes and sesquiterpenes) may not be as important as it is for small compounds (such as isoprene) and in photooxidation and ozonolysis experiments (e.g., Presto et al., 2005; Kroll et al., 2006; Ng et al., 2007a; Eddingsaas et al., 2012; Xu et al., 2014); this warrants further studies.

The results from this study provide the first insight for the specific organic nitrate branching ratio on the β-pinene+NO3 system. We determine that about 90 and 10 % of the organic nitrates formed from the β-pinene+NO3 reaction are primary organic nitrates and tertiary organic nitrates, respectively. As primary and tertiary organic nitrates hydrolyze at drastically different rates, the relative contribution of primary vs. tertiary organic nitrates determined in this work would allow for improved constraints regarding the fates of organic nitrates in the atmosphere. Specifically, we find that the primary organic nitrates do not appear to hydrolyze and the tertiary organic nitrates undergo hydrolysis with a lifetime of 3–4.5 h. Updating the branching ratio (primary vs. tertiary) with organic nitrates formed by the NO3-initiated oxidation of BVOCs will improve model predictions of hydrolysis of organic nitrates. Hydrolysis of organic nitrates has the potential to create a long-term sink for atmospheric nitrogen in the form of nitric acid. Organic nitrates that do not hydrolyze, however, can potentially be photolyzed or oxidized by OH radicals to release NOx back into the atmosphere (Suarez-Bertoa et al., 2012) or lost by dry or wet deposition.

Results from this chamber study are used to evaluate the contributions from the nitrate radical oxidation of BVOCs to ambient OA in the southeastern United States, where this chemistry is expected to be substantial owing to high natural and anthropogenic emissions in the area. Factor analysis of HR-ToF-AMS data from SOAS and SCAPE field measurements identified an OA subtype (LO-OOA) at these sites which is highly correlated with organic nitrates (Xu et al., 2015a, b). The β-pinene+NO3 SOA yields obtained under reaction conditions relevant to these field studies are directly utilized to estimate the amount of ambient OA formed from this reaction pathway (Xu et al., 2015b). Specifically, it is estimated that 50 % of nighttime OA production occurs through the reaction of monoterpenes with nitrate radicals in SOAS (Xu et al., 2015b). Using the average R-ON : R-AN ratio obtained from this study and prior literature values, Xu et al. (2015a) estimated that organic nitrates contribute 5–12 % of total organic aerosol in the southeastern United States in summer, indicating organic nitrates are important components in ambient aerosol. Results from this study and Xu et al. (2015a, b) illustrate the substantial insights one can gain into aerosol formation chemistry and ambient aerosol source apportionment through coordinated fundamental laboratory studies and field measurement studies. Further, multiple gas-phase organic nitrate species are identified in this chamber study, which could be used to help interpret ambient gas-phase composition data obtained from the large suite of gas-phase measurements in SOAS. Owing to difficulties in measuring complex atmospheric processes, laboratory studies are critical in generating fundamental data to understand and predict SOA formation regionally and globally. In this regard, it is imperative not to view laboratory studies as isolated efforts, but instead to make them essential and integrated parts of research activities in the wider atmospheric chemistry community (e.g., field campaigns).