The nitrate radical (NO 3 ) oxidation of alpha-pinene is a significant source of secondary organic aerosol and organic nitrogen under simulated ambient nighttime conditions

. The reaction of a -pinene with NO 3 is an important sink of both a -pinene and NO 3 at night in regions with mixed biogenic and anthropogenic emissions; however, there is debate on its importance for secondary organic aerosol (SOA) and reactive nitrogen budgets in the atmosphere. Previous experimental studies have generally observed low or zero SOA 10 formation, often due to excessive [NO 3 ] conditions. Here, we characterize the SOA and organic nitrogen formation from a pinene + NO 3 as a function of nitrooxy peroxy ( n RO 2 ) radical fates with HO 2 , NO, NO 3 , and RO 2 in an atmospheric chamber. We show that SOA yields are not small when the n RO 2 fate distribution in the chamber mimics that in the atmosphere, and the formation of pinene nitrooxy hydroperoxide (PNP) and related organonitrates in the ambient can be reproduced. Nearly all SOA from a -pinene + NO 3 chemistry derives from the n RO 2 + n RO 2 pathway, which alone has an SOA mass yield of 65(±9)%. 15 Molecular composition analysis shows that particulate nitrates are a large (60-70%) portion of the SOA, and that dimer formation is the primary mechanism of SOA production from a -pinene + NO 3 under simulated nighttime conditions. We estimate an average n RO 2 + n RO 2 à ROOR branching ratio of ~18%. Synergistic dimerization between n RO 2 and RO 2 derived from ozonolysis and OH oxidation also contribute to SOA formation, and should be considered in models. We report a 58 (±20)% molar yield of PNP from the n RO 2 + HO 2 pathway. Applying these laboratory constraints to model simulations of 20 summertime conditions observed in the Southeast United States (where 80% of a -pinene is lost via NO 3 oxidation, leading to 20% n RO 2 + n RO 2 and 45% n RO 2 + HO 2 ) , we estimate yields of 13% SOA and 9% particulate nitrate by mass, and 26% PNP by mole, from a -pinene + NO 3 in the ambient. These results suggest that a -pinene + NO 3 significantly contributes to the SOA budget, and likely constitutes a major removal pathway of reactive nitrogen from the nighttime boundary layer in mixed biogenic/anthropogenic areas.


Introduction
Monoterpenes (C10H16) are a major class of biogenic hydrocarbons. Although less abundant 35 than isoprene in terms of absolute emission flux of non-methane hydrocarbons, they have a disproportionate importance for the formation of secondary organic aerosol (SOA), accounting for half of the total fine aerosol globally (Zhang et al., 2018), and for nitrogen oxide (NO, NO2, NO3) sequestration through the 40 formation of gaseous and particle-phase organic nitrates (Pye et al., 2015). Thus, monoterpene chemistry plays a prevailing role in aerosol-climate interactions and atmospheric air quality. Of the monoterpenes, a-pinene is the most abundant globally (Sindelarova et al., 2014). This is especially notable over boreal coniferous forests where the a-pinene emission flux alone can overtake isoprene and the combined flux of all other monoterpenes during the summer season (Hakola et al., 2003). The atmospheric abundance, fast reaction rates, and nighttime emission profile of a-pinene conspire for it to dominate the fate of 45 the nitrate radical (NO3) in the dark, and also to play a significant role in the daytime (Ayres et al., 2015). The reaction of apinene + NO3, thus, is one of the most prevalent reactions observed in the summer in mixed biogenic-anthropogenic sites, such as the Southeastern United States.
NO3 reacts with a-pinene by addition to the double bond, mainly producing a nitrooxy alkyl radical in the tertiary position, which is rapidly converted to a nitrooxy peroxy radical (nRO2) upon collisions with molecular oxygen (Scheme 1). 50 the seed particle chosen was ammonium sulfate (9%) or organic (16%). Nah et al. (2016) and Kurtén et al. (2017) also observed minimal SOA formation (mass yields of 3.6% and <1%, respectively) using NO2 + O3 as a source of NO3, and formaldehyde to promote HO2 chemistry. From these observations, it is clear that chamber reaction conditions are highly influential in the observed SOA yields, and that previous studies may have each probed different nRO2 fates. Thus, a systematic investigation of how nRO2 fates dictate reaction outcomes will enable reconciliation of past results and accurate representation of this 70 reaction in atmospheric models. The high initial NO3 concentrations (tens of ppb) used in some previous studies, derived from the decomposition of N2O5, will cause nRO2 + NO3 to dominate in the chamber, when it is negligible in the field ([NO3] is persistently at or below the detection limit of 1 pptv in the rural Southeast United States; Ayres et al., 2015). Kjaergaard and coworkers further illuminated the role of alkoxy radical scission in SOA formation, and predicted low SOA yields from the apinene + NO3 reaction when the RO2 radical is reduced to RO via bimolecular reaction with NO3, RO2, and even HO2 (Kurtén 75 et al., 2017).
Given the high relevance of the a-pinene + NO3 reaction, it is critical to place tighter constraints on how this reaction contributes to SOA and organic nitrogen in the ambient environment. In light of the emerging appreciation for the importance of RO2 radical fate in designing chamber experiments (Nguyen et al, 2014a;Xu et al., 2019;Boyd et al., 2015;Teng et al., 2017;Crounse et al., 2013), we re-investigate this reaction to probe the SOA yield and organic nitrate formation from a-pinene 80 + NO3 from each relevant nRO2 reaction channel. While a chamber experiment may never truly replicate the field, and ours certainly are no exception, the nRO2 fate distribution in this work was designed to approach those expected in the ambient nighttime (Ayres et all., 2015;Romer et al., 2018), including any reaction synergies that may occur (Kenseth et al., 2018;Inomata, 2021). Finally, relatively little information is available for the nRO2 compared to their hydroxylated counterparts; this work also constrains the rate coefficients and branching ratios of the a-pinene nRO2 through a combination of chamber 85 reactions and modeling. We demonstrate a new HO2 formation route in the dark chamber that does not require carbon inputs, and thus enables SOA yields to be more accurately measured when probing the RO2 + HO2 pathway from the a-pinene + NO3 reaction.
H2O2 (50 wt.% in H2O, Aldrich) was then injected by flowing 4 L min -1 of ultra zero air through a bubbler warmed to 40 °C in a water bath. After the inorganic gas-phase reactants were introduced, the chamber was allowed to mix for 1-2 h, during which time secondary formation of NO3 (from O3 + NO2) and HO2 (from NO3 + H2O2) could proceed. Seed particles were 115 introduced to the chamber during mixing by atomizing a solution of 0.06 M ammonium sulfate ((NH4)2SO4, ≥99%, Aldrich) through a 210 Po neutralizer. Lastly, liquid standards of α-pinene (Sigma Aldrich, >99%) were injected with gas-tight syringes into an airtight glass bulb, and introduced to the chamber by a 4 L min -1 flow of ultra zero air. The α-pinene reacts quickly and thus was mixed rapidly from pulsed injections of high pressure ultra zero air (100 psi) for 2 min in order to initiate the reaction.
The ozonolysis of α-pinene occurs concurrently with its NO3-initiated oxidation with this experiment design. Ozonolysis of α-120 pinene produces OH, which reacts with H2O2 to be an additional dark formation source of HO2, as well as with α-pinene itself.
Control reactions were performed to accompany each experiment listed in Table 1 that includes ozone, using the same conditions minus NO2, in order to subtract out the SOA and other product formation from the purely ozonolytic reaction.
For experiments that used N2O5 as the NO3 source, injections of a-pinene and other desired inorganic reactants, including seed particles, NO2, and NO (200 ppm ±1% in N2, Praxair), were conducted first. The reactions were then initiated 125 by the rapid injection of gas-phase N2O5, which was previously evaporated into an evacuated 500 mL glass bulb to the desired pressure and backfilled to room pressure with N2. N2O5 was synthesized according to Claflin and Ziemann (2018), verified using FT-IR, and stored in the dark at -20°C prior to use. In these experiments, the decomposition equilibria of N2O5 ⇆ NO2 + NO3 was manipulated via injections of NO2 in order to slow NO3 formation and thus control the nRO2 fate.
During experiments, mixing ratios of oxygenated gas-phase organics were quantified with a custom-built triple-130 quadrupole chemical ionization mass spectrometer (CIMS) using CF3Oas the reagent ion. Instrumental details, including humidity-dependent calibration methods, have been described in detail previously (Crounse et al., 2006;St. Clair et al., 2010;Nguyen et al., 2014b;Praske et al., 2015). The CIMS detects PNP and other polar analytes predominantly without fragmentation as clusters with CF3O -. Although authentic standards of PNP are not available for direct calibration in CIMS, the analytical sensitivity of synthesized organic nitrates of different carbon length and neighboring groups in the CF3O -CIMS 135 were found to be different from each other by a factor of 20-30% (Lee et al., 2014;Teng et al., 2017); thus, sensitivity of PNP and pinene hydroxy-nitrate were assumed to be the same as isoprene hydroxynitrates with 30% uncertainty.
A scanning mobility particle sizer (SMPS), comprised of an electrostatic classifier (TSI 3080) and a condensation particle counter (TSI 3772), was used to measure particle size distributions between 15 nm and 670 nm. Control experiments monitoring the dark decay of ammonium sulfate seed aerosol concentrations in the chamber were used to determine diameter-140 dependent particle wall loss rates (Schwantes et al., 2019), which were then used to correct experiment particle concentrations.
Calculations of particle mass from measured aerodynamic diameter assume a density of 1.2 g cm -3 .

Kinetic modelling
We use a kinetic model (Mech. S1) to simulate gas-phase chemistry for each experiment in the environmental chamber. The mechanism uses reaction parameters from the JPL Chemical Kinetics and Photochemical Data Evaluation 145 (Burkholder et al., 2019), and is run on Matlab (MathWorks, Inc.). We also include reactions of a-pinene with OH, O3, and NO3, and isomer-specific reactions of the subsequently produced peroxy radicals with HO2, NO, NO3, and other RO2 radicals (individually represented). Product yields from each pathway, along with rates of RO2 + RO2 reactions, are adjusted to fit the experimental data. We initialize simulations with the inorganic species listed in Table 1, and allow the model to run for the allotted mixing time before instantaneously adding pinene. The model does not include any wall deposition of vapors or gas-150 particle interactions, and is used only to estimate the concentrations of gas-phase species and the contributions of each peroxy radical reactive pathway.

SOA composition analysis by high-resolution mass spectrometry (HRMS)
SOA were collected for composition analysis using Omnipore hydrophilic Teflon filters (0.2 µm diameter pore, Millipore Corp.) that is compatible with polar and non-polar organics. The filters were gently extracted using LC-MS grade 155 acetonitrile (Optima, Fisher Scientific) to mass concentrations on the order of 100 µg/mL, depending on the experiment, by several ultrasound pulses of duration 1 s in order to limit cavitation in the ultrasonic bath that may alter analyte compositions.

Filter extracts were directly infused into a linear-trap-quadrupole (LTQ) Orbitrap XL mass spectrometer (Thermo Instrument
Corp.) using positive and negative ion mode electrospray ionization (ESI) at 4 kV spray voltage and a mass resolving power of 60,000 m/Δm at m/z 400. An external calibration was performed in both ion modes using commercial mass standards in the 160 range of 100 -2000 m/z (Pierce™ LTQ ESI Positive and Pierce™ LTQ ESI Negative calibration solutions, Fisher Scientific), and the data were recalibrated until the mass accuracy obtained from standard solutions was < 1 ppm.
The data analysis was performed similarly to our previous works (Nguyen et al., 2010;Nguyen et al., 2011). Briefly, the raw data were de-convoluted using Decon2LS (freeware from Pacific Northwest National Laboratory), and background subtracted for peaks present in the solvent. The m/z peaks were assigned to molecular formulas (CcHhOoNn) based on a custom 165 Matlab script that applies Lewis and Senior rules (Kind and Fiehn, 2007) and a Kendrick Mass Defect analysis (base CH2; Roach et al., 2011) that have been demonstrated on SOA mixtures. The prevalent ionization mechanism for this specific analyte mixture was found to be sodium cluster formation (M+Na + ) in the positive mode, which occurs preferentially for carbonyls (Kruve et al., 2013), and nitrate cluster formation (M+NO3 -) in the negative mode, which is efficient for organic nitrates, alcohols, and other functional groups (Sisco and Forbes, 2015;Mathis and McCord, 2005). The nitrate anion, prominently 170 detected in the mass analyzer at m/z 61.988, was not purposefully introduced but likely formed either from in-source fragmentation of organic nitrates or from HNO3 produced during the hydrolysis of tertiary organonitrates from the aqueous LC solvents, and fortuitously acted as a reagent ion for chemical-assisted electrospray. While peak heights correlate well with concentration in direct-infusion ESI HRMS when the analyte matrix is similar (Chan et al., 2020), the correlation coefficients are unknown for each analyte in the mixture; thus, the HRMS data is qualitative. Furthermore, due to the labile -ONO2 groups, 175 the organonitrate observations from HRMS likely represent a lower limit.

SOA and PNP yields from different nRO2 fates
To investigate the dependence of aerosol and gaseous PNP yields on the nRO2 reaction partner, we performed a series of environmental chamber experiments (experiments 1-16, Table 1) with various starting conditions designed to isolate or 180 maximize the contributions of each RO2 reaction pathway. Example time profiles from three representative experiments are shown in Figure 1. nRO2 + NO3, nRO2 + NO, and nRO2 + RO2 chemistry were isolated in experiments using N2O5 as the NO3 source. Using excess N2O5 as a source causes a rapid initial spike of NO3, making the nRO2 + NO3 pathway easy to isolate in Experiment 13 (Fig. 1, left). The nRO2 + NO pathway is similarly easy to isolate with excess NO added before N2O5 injection, as in Experiment 15. nRO2 + nRO2 chemistry can be maximized by injecting excess a-pinene prior to N2O5 addition; the NO3 185 is thus dominantly consumed by reaction with a-pinene, leaving the subsequently produced nRO2 to react with each other, as in Experiment 26 (Fig. 1, middle), for which we calculate that 79% of nRO2 reacted with other nRO2.
nRO2 + HO2 chemistry is more difficult to isolate due to the scarcity of clean HOx sources in the dark. Here, we describe a method that can achieve a domination of nRO2 + HO2 chemistry without additional carbon inputs, initiated by the slow production of NO3 in situ via NO2 + O3, which will introduce three reaction partners for a-pinene (NO3, O3, OH). Alkene 190 ozonolysis has been used previously to produce dark OH in chamber experiments (Xu et al., 2019); Leveraging the ozonolysis of a-pinene already occurring in the chamber, we can amplify HO2 production by injecting excess H2O2 prior to a-pinene to scavenge OH. Thus, the OH + H2O2 à HO2 + H2O reaction simultaneously produces HO2 and suppresses the side chemistry of a-pinene + OH. This HO2 source is relevant to the nighttime atmosphere, as ozonolysis is always occurring with NO3 reaction in the ambient due to the major source chemistry from NO2 + O3. 195 The added H2O2 provides an additional benefit -its reaction with NO3 is a source of HO2 prior to a-pinene injection (Fig. S1). The H2O2 + NO3 à HO2 + HNO3 reaction has been estimated by Burrows, Tyndall, and Moortgat (1985) to have an upper limit of <2×10 -15 cm 3 molec -1 s -1 -too slow for atmospheric relevance, but sufficient to produce significant HO2 in 200 chambers when H2O2 is in excess. The rate coefficient of this reaction was further constrained from its upper limit based on the ratio of kCH2O+OH/kCH2O+NO3 (Burkholder et al., 2019), resulting in a rate coefficient of 1.1×10 -16 cm 3 molec -1 s -1 used in our simulations. The formation of PNP was not adequately reproduced if HO2 is assume to originate from H2O2 + 205 OH alone, i.e., omitting the H2O2 + NO3 reaction in the kinetic model; the yield of PNP would need to be unphysically high to reconcile the difference.
The a-pinene + NO3 reaction is modeled to be the major a-pinene sink in experiments initiated by NO2 + O3 (e.g., 73% in Experiment 22, Fig.   2, Table 2). Even so, the background chemistry from a-pinene + O3 and a-210 pinene + OH (from the ozone control experiments without NO2) are substantial sources of SOA and require careful subtraction (Fig. 2). Caveats to this approach include: (1) in the controls, ozone and OH are larger sinks for a-pinene due to a lack of competition from NO3; thus, a larger fraction of a-pinene produces SOA from ozonolysis in the control compared to the 215 experiment and the subsequent subtraction obtains a lower-limit SOA yield; The difference (green) shows the contribution from a-pinene + NO3.
(2) synergistic reactions between RO2 intermediates from the different oxidation pathways are not possible to isolate, and contributed roughly 20% to the analytical signal from the SOA composition analysis (Section 3.3); however, it is now appreciated that these synergies also occur in the ambient and are not realistic to ignore in laboratory and modeling studies (Kenseth et al., 2018;Inomata, 2021). 220 These 'simulated nighttime' experiments (e.g. Experiment 20, Fig. 1, right) are termed as such because they provide an atmospherically relevant balance of reactive pathways, in which fractional contributions are comparable to those on summer 225 nights in the Southeast United States (60-80% apinene + NO3, 20-40% a-pinene + O3; 30-50% nRO2 + HO2, 30-50% nRO2 + RO2; Ayres et al., 2015;Romer et al., 2018). This is corroborated by gas phase data from the CIMS (Fig. 3), which show that 230 NO3-initiated products are the major volatile products formed in the simulated nighttime experiments. Furthermore, these experiments are able to reproduce gaseous organonitrates observed during the Southern Oxidant and Aerosol Study (SOAS; Carlton et al., 2018), including PNP -a major monoterpene 235 semivolatile at the SOAS site in rural Alabama -and an unknown compound at m/z 314 that has also been observed in the ambient with similar time profiles to PNP (Fig. S2).
Product yield results from different nRO2 reaction pathways are shown in Table 2, along with the modeled contributions of each nRO2 reaction pathway. We observe low SOA formation in experiments maximizing nRO2 + NO3 (3(±3)% mass yield, Experiment 13) and nRO2 + NO (12(±7)%, Experiment 15). This is consistent with previous observations 240 of 0-16% SOA mass yields from N2O5-initiated experiments (in which the nRO2 + NO3 pathway tends to dominate), where the larger yields were obtained only with slow continuous introductions of N2O5 (Bell et al., 2021;Fry et al., 2014;Hallquist et al., 1999;Moldanova et al., 2000;Muutzel et al., 2021;Spittler et al., 2006). We also observe no correlation between SOA yields in 'simulated nighttime' experiments and the fractional contribution of the nRO2 + HO2 pathway (Fig. S3), suggesting no SOA production from this pathway, consistent with Kurten et al. (2017) and Nah et al. (2016). This result conforms to 245 expectations as nRO2 + HO2 produces either the RO radical (Iyer et al., 2018) leading to the volatile pinonaldehyde (Kurtén et al., 2017) or the hydroperoxide, which has high enough volatility to be observed by the CIMS. We do, however, observe high yields of SOA in experiments targeting nRO2 + nRO2 chemistry (up to 82% mass yield, Experiment 26). This includes both N2O5-initiated experiments with excess a-pinene and O3 + NO2 + H2O2-initiated 'simulated nighttime' experiments, during which we measured SOA mass yields of 19-55%. 250   Across all experiments, the modeled fractional contribution of the nRO2 + nRO2 pathway correlates strongly with SOA yield (Fig. 4a). An error-weighted linear regression (York et al., 2004) of ozonolysis-corrected SOA yield against the modeled nRO2 255 + nRO2 contribution suggests a 67% mass yield (R 2 = 0.54) from this pathway alone. With such a large number of experiments spanning a range of different pathway contributions, however, we can improve upon this simple fit by performing a multivariate linear regression to estimate SOA yields from all pathways, with coefficients limited to be ≥0. This results in a including the ozonolysis and OH pathways as additional independent variables, also gives coefficients indistinguishable within error (0% for a-pinene + OH, 18% for a-pinene + O3).
These fits intrinsically depend on kinetic model parameters 270 such as bimolecular RO2 reaction rates, some of which are uncertain.
While we are unable to fully quantify these rates, we find that certain ratios between rates are constrained by our experimental outcomes.
For example, the negligible SOA yield in the high-NO3 Experiment 13 suggests that nRO2 + nRO2 chemistry cannot play a major role in 275 that experiment. This can only be achieved if the bulk rate coefficient for nRO2 + nRO2 reactions (kRO2+RO2) is approximately two orders of magnitude slower than that of nRO2 + NO3 reactions (kRO2+NO3).
We therefore set the bulk kRO2+RO2 to 1×10 -13 cm 3 molecule -1 s -1 (with the corresponding bulk kRO2+NO3 equal to 1×10 -11 cm 3 molecule -1 s -1 ), 280 but we note that this rate likely represents a weighted mean of different rates for the different nRO2 isomers (Orlando and Tyndall, 2012), and that it remains uncertain. Higher kRO2+RO2, which has been suggested elsewhere for pinene (Berndt et al., 2018), would necessitate a much higher kRO2+HO2 rate in 'simulated nighttime' experiments, a value that is better constrained (Boyd et al., 285 2003). A slower bulk rate of 1×10 -14 cm 3 molecule -1 s -1 would scale the contribution of nRO2 + nRO2 chemistry in the 'simulated nighttime' experiments by a factor of 0.67, which in turn would require scaling SOA yields from this pathway up by 50%.
To constrain the sequestration of reactive nitrogen under ambient conditions from the a-pinene + NO3 reaction, we also quantify PNP yields in our simulated nighttime experiments. Figure 5 (top) shows measured PNP molar yields as a function of the nRO2 + HO2 fate contribution. Regression analysis (York et al., 2004) suggests a branching ratio for PNP 290 formation of 58(±2)% from nRO2 + HO2 (R 2 = 0.88), which we implement in our kinetic model (Fig. 5, bottom). Including the uncertainty in the CIMS calibration increases the uncertainty bounds to 58(±20)%. The bulk yield likely represents a combination of different branching ratios to PNP formation from the different nRO2 isomers. We hypothesize that the secondary (minor) nRO2 produces exclusively PNP 295 in its reaction with HO2, which would imply a branching ratio of 37% for PNP formation from the tertiary (major) nRO2 + HO2, assuming an initial branching of 65:35 major:minor isomers from a-pinene + NO3 (Jenkin et al., 1997;Saunders et al., 2003). Our measured bulk PNP yield is somewhat higher than the 30% estimated in the FIXCIT studies 300 (Kurtén et al., 2017), which used similar instrumentation but slightly different chamber conditions (formaldehyde instead of H2O2 and slow addition of a-pinene to minimize RO2 + RO2 chemistry). Some of this discrepancy might be explained by an overestimate of the contribution of nRO2 + HO2 chemistry in the FIXCIT experiment, where they 305 assumed that all NO3 produced by O3 + NO2 reacted with a-pinene and that all nRO2 reacted with HO2 (Kurtén et al., 2017).
We also detect a-pinene hydroxy nitrate (PHN), a product of nRO2 + nRO2 chemistry, and a-pinene dinitrate (PDN), a product of nRO2 + NO or nRO2 + NO3 chemistry, by CF3O -CIMS. Based on similar 310 regression analyses of observations and modeling results (Fig. S4), we estimate a bulk PHN yield from nRO2 + nRO2 of 11.7(±3.3)%. However, because the major nRO2 isomer (with the tertiary peroxy radical) is unable to donate an a-hydrogen for PHN formation, a bulk yield of 11.8% represents a 34% branching ratio from the minor (secondary) 315 nRO2 isomer (assuming an initial ratio of 65:35 major:minor isomers from a-pinene + NO3; Jenkin et al., 1997;Saunders et al., 2003). While observed PDN correlates with nRO2 + NO3 chemistry, its low signal and uncertain CIMS sensitivity makes quantification difficult.
3.2 SOA yield dependence on seed surface area 320 SOA from gas-particle partitioning tends to exhibit a strong dependence on seed aerosol surface area, because of competition between particle surfaces and chamber walls (Schwantes et al., 2019;Zhang et al., 2014;Zhang et al., 2015). This effect can cause an underestimation of SOA yields in atmospheric chambers if insufficient seed aerosol is used. To quantify  . 6). No SOA formation was observed in the 330 absence of seed aerosol, indicating that α-pinene + NO3 does not nucleate. This is in contrast to our ozonolysis control experiments where nucleation was observed, in agreement with other accounts (Burkholder et al., 2007;Hoppel 335 et al., 2001;Inomata, 2021;Takeuchi et al., 2019). The lack of nucleation is consistent with the expectation that the α-pinene + NO3 reaction dominates the nighttime experiments over ozonolysis. SOA yields reached a maximum when initial seed surface area reached 143 µm 2 cm -3 (equivalent under experimental conditions to 5.3 µg m -3 ). This threshold is at least an order of magnitude lower than those measured by Schwantes et al. (2019) for SOA from isoprene OH oxidation under high-NO conditions and by Zhang et al. (2014) for SOA 340 from toluene oxidation, suggesting much lower volatility for the α-pinene + NO3 products.

SOA molecular composition
Filters collected following SOA formation in Experiments 26 (high nRO2 + nRO2), 27 (simulated nighttime), and the ozonolysis control (without NO2) for Experiment 27 were analyzed by HRMS to determine the SOA molecular composition (Figure 7). As expected, the mass spectrum from the 'simulated nighttime' experiment (#27, Fig. 7B) exhibit substantial 345 overlap with those from the N2O5-initiated experiment in which the nRO2 + nRO2 pathway dominated (#26, Fig. 7D), corroborating the results in Section 3.1 that suggested the nRO2 + nRO2 pathway is responsible for most SOA formation from a-pinene + NO3 chemistry. The SOA from nRO2 + nRO2 chemistry is dominated by compounds containing 17-20 carbon atoms, suggesting that dimerization is the primary mechanism by which nRO2 + nRO2 reactions lead to SOA formation.
Masses in the 'simulated nighttime' SOA that are also observed in the nRO2 + nRO2 control experiment (#26) account 350 for 39% of the negative mode peak signal and 29% of the positive mode peak signal in the SOA from the 'simulated nighttime' experiment. Negative mode may overestimate the contributions from dinitrates dimers from nRO2 + nRO2, whereas positive mode may underestimate them. In the positive mode, it was also found that 34% of the peaks in the 'simulated nighttime' experiment can also be found from ozonolysis chemistry (including OH reaction), 14% of peaks are found in both ozonolysis and nRO2 + nRO2 chemistry, and 24% of peaks are completely unique to the nighttime chemistry. SOA from a-pinene 355 ozonolysis is dominated by oxygenated monomers in the negative mode and lacks organonitrate functionalities, making it easy Figure 6. Measured ozonolysis-corrected SOA yields from seed area experiments (#17-25) plotted against initial seed surface area. Error bars denote experimental uncertainty from SMPS measurements.
to distinguish from the mass spectrum of NO3-oxidized SOA. The peaks from a-pinene ozonolysis can thus be subtracted off from the 'simulated nighttime' SOA mass spectrum, leaving a spectrum attributable to a-pinene + NO3 under 'simulated nighttime' conditions (Fig. 7C).

360
Figure 7. Mass spectra of filter samples from Experiments 26 (D) and 27 (B), along with the ozonolysis-only control for Experiment 27 (A) and the signal difference between B and A (C). Peaks attributable to a-pinene ozonolysis alone are shown in red, while those attributable to a-pinene + NO3 followed by RO2 + RO2 chemistry are shown in blue. Pie charts at right denote the fraction of a-pinene reacting via each pathway for a given experiment. Formation mechanisms for the key dimer species (a-d) identified in the bottom panels are shown in Schemes 2-3. 365 The signal-weighted composition of SOA from the 'simulated nighttime' reaction ( Fig. 7B), which includes background ozonolysis, is 60-70% nitrogen-containing organics with 1-3 nitrate groups, as determined in both negative and positive mode (where organic nitrogen species are not enhanced). Of the organic nitrogen, it appears that 2N species are the most abundant and comprise ~30% of the SOA on average. This suggests dimerization from nRO2 + nRO2 to form C14-20 370 compounds is highly important to SOA formation. Previous analyses of the molecular composition of SOA from a-pinene + NO3 have also found a dominant contribution of dimers in the particle phase (Bell et al., 2021;Takeuchi et al., 2019). The 0N, 1N, and 3N species may also be dimers (4N were not highly observed), as each of the expected RO2s from all channels can have 0-2 N (Fig. 8). There is also a clear population of trimers from α-pinene SOA that is observed here (Fig. 7, m/z centered around 700, with associated carbons C21-C30) and elsewhere (Claflin and Ziemann, 2018;Romonosky et al., 2017) that may 375 suggest RO2s can react with neutral compounds to propagate RO2 radicals that then terminate with another RO2.
The data show that SOA formation from a-pinene + NO3 originates from a varied cohort of distinct nRO2 isomers.
In Scheme 2 we suggest multiple pathways involving bimolecular reactions, b-scission, and intramolecular isomerization that can produce different C7-C10 nRO2 isomers following the initial reaction of a-pinene with NO3. These pathways are consistent with RO2 chemistry previously suggested from theoretical (Kurtén et al., 2017) or lab studies (Xu et al., 2019) of similar 380 systems. Some reactions of nRO2 with NO, NO3, or HO2 shown in Scheme 2 propagate nRO2 radicals due to unimolecular decomposition rather than forming stable products; this implies that dimer-containing SOA from nRO2 can originate from all nRO2 fate pathways. Figure 8C shows the most abundantly observed dimers in the SOA from nRO2 + nRO2 chemistry (labeled peaks a-d in Fig. 7D), which include C20H30O4 (a-a, Fig 7D.a; presumably following the loss of 2 HONO from dimer C20H32N2O8 in the ESI source), C17H26N2O10 (d-a, Fig. 7D.b), C20H32N2O10 (b-b, Fig. 7D.c), and C20H31N3O13 (e-a, Fig 7D.d). 385 Bell et al. (2021) also observed C20H32N2O8 and C20H32N2O10 among the most abundant compounds in SOA from a-pinene + NO3 chemistry in which nRO2 + nRO2 reactions were thought to dominate.
Dimers from the ozonolysis control experiments are notably absent from the 'simulated nighttime' experiment. This is likely because the peroxy radicals from a-pinene ozonolysis, which would have reacted with each other to dimerize in the Scheme 2. Proposed mechanisms for the formation of stable products (red) from a-pinene + NO3 oxidation, including nitrooxy-hydroperoxide (PNP), a-pinene hydroxynitrate (PHN), pinene dinitrate (PDN), the oxidation product observed by CIMS at m/z 314, and pinonaldehyde, as well as a-pinene-derived peroxy radicals (nRO2a-f, blue) that may contribute to particle-phase dimers (Fig. 8) following the dominant secondary addition of NO3 to a-pinene. "X" stands in for any of RO2, HO2, NO, or NO3 as bimolecular reaction partners.
ozonolysis-only experiment, instead react with the more abundant nRO2 radicals from a-pinene + NO3 in the 'simulated 390 nighttime' experiment. This dimerization from cross-reactions of peroxy radicals from both oxidation pathways is responsible for the ~ 24% additional HRMS signals in the a-pinene + NO3 SOA spectrum (Fig. 7C, black peaks) but not in the nRO2 + nRO2 SOA spectrum (Fig. 7D). Figure 8D show how RO2s formed from ozonolysis (i-iii; Iyer et al., 2021), initially via the vinylhydroperoxide channel, and OH reactions (1-2; Xu et al., 2019) can couple with nRO2s to produce synergistic dimers during nighttime oxidation of α-pinene. This type of synergy is missing when lab studies cleanly isolate reaction pathways, 395 but may be an important part of SOA formation in the ambient as reactions occur simultaneously (Teng et al., 2017). These results underscore the importance of conducting chamber experiments under atmospherically relevant conditions. While we do not seek to represent each individual nRO2 and dimerization pathway in our kinetic model, we adjust the product yields from the first-generation nRO2 + nRO2 reactions to provide a reasonable estimate of dimer formation. The observed combined positive and negative mode peak signal attributable to C14-20 compounds (presumed dimers) in nRO2 + 400 nRO2 SOA (Experiment 27, Fig. 6D) constitutes 40-60% of the total signal. To produce this 40-60% of the 65% SOA mass yield estimated from nRO2 + nRO2 chemistry in Section 3.1 using dimers, assuming a mean dimer molar mass of 485 g mol -1 (the average of the positive mode and negative mode peak-signal-weighted mean), requires a molar yield of 0.18 dimer molecules from the nRO2 + nRO2 reaction (0.09 per nRO2). This bulk effective yield represents an average across the various nRO2 + nRO2 isomer permutations; branching ratios from individual nRO2 + nRO2 isomer reactions may differ substantially, 405 Figure 8. Structures and molecular formulas for (A) nRO2 from NO3-initiated chemistry (Scheme 2), (B) RO2 from OH or O3 chemistry (Xu et al., 2019;Iyer et al., 2021), (C) prominent dimers observed from nRO2 self/cross reactions, and (D) dimers from synergistic nRO2 + RO2 couplings observed in the 'simulated nighttime; reactions. Dimers with chemical formulas written in pink are uniquely observed in the 'simulated nighttime' experiments; those marked with a * have moderate-to-high signal (40>S/N). but in our kinetic model we simply apply the bulk effective yield to all peroxy radical self-and cross-reactions. This 18% dimer yield is somewhat higher than has previously been measured in other systems (Orlando and Tyndall, 2012), including from a-pinene ozonolysis (Zhao et al., 2018), but fits with recent evidence suggesting that increased molecular size and functionalization can significantly increase dimer branching ratios (Molteni et al., 2019;Berndt et al., 2018).

Conclusions and atmospheric relevance 410
Contrary to previous chamber experimental results, we have shown here that the reaction of a-pinene with NO3 can form high mass yields of SOA (>21%, at ~30 ppbv of a-pinene) under RO2 fate branching ratios designed to mimic those of summer nights in the Southeast United States. Much of this SOA originates from the self-and cross-reactions of nitrated peroxy radicals (nRO2 + nRO2); we estimate that this pathway alone has an SOA mass yield of 65%, with an associated dimer branching ratio of ~ 18%, while SOA formation from other nRO2 pathways is negligible within uncertainty. This hypothesis 415 is also supported by the dominance of dimers in the ozonolysis-subtracted mass spectra of SOA collected from 'simulated nighttime' experiments, the diversity of which suggests that many different peroxy radical rearrangements and dimerization channels contribute to SOA formation from nRO2 + nRO2 chemistry.
The magnitude of a-pinene concentrations in the Southeast United States will be lower than that used in our chamber; thus, we model the a-pinene + NO3 chemistry at nighttime concentrations measured during the SOAS campaign in rural 420 Alabama in the summer (1 ppb a-pinene, 1 ppb other terpenes, 20 ppb ozone, 1 ppt NO3, and 2 ppt HO2; Ayres et al., 2015;Romer et al., 2018) and assuming steady-state for nRO2. At atmospheric concentrations, our kinetic model predicts that 80% of a-pinene reacts with NO3. Of the nRO2 formed, approximately 20% is predicted to react with a-pinene-derived nRO2 or O3RO2 at kRO2+RO2 to 1×10 -13 cm 3 molecule -1 s -1 . Combined with our measured 65% SOA mass yield from nRO2 + RO2, this suggests an overall SOA mass yield of 13% from a-pinene + NO3 chemistry under the ambient conditions of SOAS. This SOA 425 yield corresponds to a 9% particulate nitrate yield for compounds that did not hydrolyze. The mass of the -ONO2 group alone represents 13% of the SOA mass. A rough calculation predicts that the nighttime reaction of 1 ppbv (~ 5.5 µg/m 3 ) a-pinene may produce ~0.65 µg/m 3 SOA (~0.44 µg/m 3 of which is particulate nitrates) through its reaction with NO3. These calculations neglect cross reactions of nRO2 with other terpene RO2, estimated to comprise another 20% of the nRO2 fate, which may also lead to SOA formation. Although the yields in this work would predict a-pinene + NO3 represents a significant fraction of 430 aerosol and particulate nitrate mass observations during SOAS, previous modeling efforts did not track the SOA or organic nitrates from a-pinene + NO3 due to belief that this reaction does not produce aerosol (Pye et al., 2015;Ayres et al., 2015).
Atmospheric models should therefore include SOA formation from a-pinene + NO3, ideally with a dependence on nRO2 fate.
Since HRMS analysis suggests that most compounds in a-pinene + NO3 SOA retain their organonitrate functionality, they will become a permanent sink of NOx following particle deposition. The a-pinene + NO3 chemistry can also have 435 important implications for reactive nitrogen budgets in the gas phase. The high yield of PNP (58%) from nRO2 + HO2 will temporary sequester NOy, which may become a permanent sink when the PNP undergoes deposition (Nguyen et al., 2015) or retains its nitrate functionality following further oxidation. Approximately 45% of nRO2 is expected to react with HO2 during the conditions of SOAS; thus, the nighttime reaction of 1 ppbv a-pinene is expected to produce ~260 pptv of volatile PNP.
During SOAS, PNP is observed with maximum concentrations of ~30 pptv, possibly limited by its atmospheric deposition and 440 transport, among other fates (Nguyen et al., 2015). Together, both particle and gas-phase organonitrate formation and deposition from a-pinene + NO3 chemistry may amount to a substantial removal pathway for atmospheric NOx.
Our results also highlight the necessity of performing chamber experiments under conditions that more closely match those in the ambient atmosphere. Measured SOA yields depend heavily on the reactive fate of RO2 in chamber experiments.
Previous experiments using N2O5 as a source of NO3 to study SOA formation from a-pinene + NO3 likely observed low mass 445 yields due to the dominance of nRO2 + NO3 reactions, from which we observe little-to-no SOA formation. Similarly, experiments that maximize nRO2 + HO2 may observe primarily volatile products. Only in experiments designed to allow sufficient nRO2 + nRO2 reactions was SOA formation observed. Furthermore, HRMS analysis shows that a large fraction of the compounds in SOA formed under 'simulated nighttime' conditions are not observed in either ozonolysis-only or nRO2 + nRO2-only SOA, suggesting that synergistic reaction pathways can enhance SOA formation when multiple oxidation channels 450 occur simultaneously. Because ozonolysis and NO3 oxidation occur together at night in the atmosphere, chamber experiments isolating one or the other will necessarily be biased in their representation of nighttime SOA from a-pinene and potentially other volatile hydrocarbons.
Code and data availability. Chamber experiment data are available online at the Index of Chamber Atmospheric Research in 455 the United States (ICARUS; https://icarus.ucdavis.edu/). The chemical mechanism for kinetic modelling can be found in the Supplement (Mech. S1).