Raw number and volume concentration time series are presented in Fig. . These comparisons are not directly indicative of relative yields due to differences in initial monoterpene concentrations shown in Table  (see Sect.  for further discussion of aerosol mass yields). However, these comparisons nicely illustrate the vast diversity of the behavior of each monoterpene with respect to systematically changing oxidant conditions, from O3 dominated to NO3 dominated. α-Pinene exhibits a decrease in both the total number of particles produced (Ntot) and total aerosol volume produced (Vtot) with increasing NO2, consistent with the findings of other studies . β-Pinene and Δ3-carene both exhibit a similar decrease in Ntot with addition of NO2 as α-pinene, but at early times in the reaction, the addition of NO2 appears to enhance volume growth relative to the O3-only experiment. Limonene exhibits enhancement in both Ntot and Vtot with addition of NO2. While all three of the monoalkene monoterpenes produce fewer particles at higher [NO2], α-pinene is the only terpene for which the aerosol production seems to be systematically depleted with the addition of NO2. β-Pinene and Δ3-carene, in contrast, seem to level off at comparable Ntot values for the intermediate range of [NO2]. All four monoterpenes exhibit suppression of aerosol formation at the highest [NO2] studied, which is likely the result of RO2+NO2 chemistry becoming kinetically dominant at such high concentrations and producing metastable, less condensable peroxy nitrate products .

While this study lacks direct BVOC measurements and thus is not optimized to rigorously measure aerosol yields, the framework of aerosol mass yields can still be used to compare aerosol formation trends between each experiment while accounting for differing initial hydrocarbon concentrations as well as differing aerosol mass loadings for each experiment. Unitless aerosol mass yields (Y) are defined as the aerosol mass produced per hydrocarbon mass consumed (Y=ΔM/ΔHC). Since the hydrocarbon was not measured online during experiments, ΔHC values were determined using the gas-phase kinetics model described in detail in the Supplement. The modeled cumulative concentration of monoterpene reacted was converted to ΔHC in µg m-3 using the molecular weight of monoterpenes (136.23 g mol-1). In the model, ΔHC is calculated based on how much of each oxidant reacts with the monoterpene. However, NO3 can also react with subsequent RO2 radicals, thus depleting the concentration available to react directly with BVOC. The rate constant used for RO2 + NO3 (2×10-12 cm3 molec-1 s-1) is reasonably well known and constant over a range of RO2 structures (1.8±1.5×10-12 cm3 molec-1 s-1 for C2 - C6 RO2) . The rate constant for RO2 + RO2, the main competing RO2 sink, is far more variable over RO2 structures, though, so the “best estimate” employed in this study spans 3 orders of magnitude (described further in Supplement). Therefore, kRO2+RO2 is the largest source of uncertainty in ΔHC, and aerosol yield ranges are calculated spanning the minimum (10-15 cm3 molec-1 s-1) and maximum (10-12 cm3 molec-1 s-1) values used. Because O3 is not expected to react with RO2 (whereas NO3 does), ΔHC from the O3-only experiments does not vary in response to shifting kRO2+RO2 values.

ΔM was determined by converting the wall-loss-corrected aerosol total volume data to mass, assuming a SOA density of 1.4 g mL-1 . Uncertainty in ΔM was estimated using replicate measurements of α-pinene + O3 (experiments 1 and 2 in Table ) as described in detail in the Supplement. The two ΔM time series were interpolated onto the same ΔHC trace, and time series of the average and standard deviation of ΔM were calculated. The deviation between these two experiments was slightly variable with time, so we conservatively chose the highest stable value -15 % relative error – to use as the ΔM precision estimate. Using the corresponding ΔM and ΔHC time series and respective uncertainties, a time series of mass yields was attainable, as shown in Fig. , plotted against aerosol mass produced (ΔM), where ΔM and ΔHC are calculated relative to the beginning of the experiment. In some cases, namely the β-pinene + O3 + NO2 experiments, the aerosol growth rapidly exceeded the size range of the SEMS (20–800 nm). Aerosol data presented here are truncated as soon as the size distribution begins to exceed the range of the SEMS instrument resulting in these experiments “ending” at quite low mass loadings before the yield curves have flattened.

A variety of factors may contribute to the absolute numerical values of these yields differing from yields reported in the literature. For example, vapor-phase wall losses were not accounted for , and the chamber size, mixing, and conditioning of walls differ from other studies. Since these experiments were conducted without seed particles, rather than having a constant particle distribution for vapors to condense onto, size distributions emerged as freshly nucleated particles that proceeded through full growth curves until they exceeded the range of the SEMS and eventually were removed through the constant outflow of the chamber. This combination of growth and dilution led to an oscillatory behavior of periodic full growth curves as the condensational sink was changing, thus preventing a true steady state from ever being achieved. The yield curves shown in Fig.  highlight a single growth curve for each experiment, but these yields may be more indicative of kinetically limited growth than thermodynamic partitioning, causing them to differ from other studies. Additionally, and perhaps most importantly, the chemistry itself (including both first-generation oxidation and peroxy radical fate) is expected to differ substantially in these mixed oxidant conditions compared to single oxidant studies in the literature. With all of those factors in mind, the precision reflected in the error ranges in Fig.  gives us confidence that the relative yield comparisons between individual experiments in this study are robust.

Figure  enables yield comparisons at comparable mass loadings and also accounts for the fact that each experiment began with somewhat variable BVOC concentrations. We still see similar trends as were observed in the Vtot panels of Fig. . Figure  illustrates that increasing [NO2] substantially depletes aerosol formation from α-pinene, whereas β-pinene and Δ3-carene have comparable yields over the full range of oxidant conditions, and limonene exhibits enhancement of aerosol formation at higher [NO2]. It should be noted that yield calculations were only performed on the O3-only and lowest two [NO2] studied for each monoterpene due to difficulties in reliably reproducing ΔHC in the kinetics model for the highest [NO2] experiments. The model is constrained using the observed O3 decay, but these high NO2 experiments react nearly all the BVOC by way of NO3, leaving the O3 decay nearly unaffected. Furthermore, we expect the full duration of these experiments to be kinetically dominated by the RO2+ NO2 reservoir (peroxy nitrates), thus hindering SOA production. For these reasons, the high NO2 experiments are not included in yield comparisons.

Gas-phase kinetics modeling of the steady-state conditions in the chamber yielded the time series of relative O3 and NO3 (and OH) contributions to BVOC oxidation. Since each experiment starts with O3, NO2, NO3, and N2O5 at their equilibrium concentrations, initial BVOC oxidation will be dominated by NO3, which reacts orders of magnitude faster than O3 (Table ). Eventually, as concentrations of precursors change over time, rates to each oxidant change and O3 starts to contribute. We assume OH is produced from stabilized Criegee intermediates from ozonolysis according to the monoterpene-dependent yields found in and described in the Supplement. The timing and relative contribution of O3 depends on the relative rate constants of O3 and NO3 with each monoterpene and thus the influence of each oxidant varies for all conditions tested.

This feature of staggered oxidant contributions is convenient to test the hypothesis that observed yield differences between different oxidant conditions applied to a single monoterpene can be attributed to distinct contributions from NO3 and O3 oxidation. For β-pinene (Fig. ), Δ3-carene (Fig. S8), and limonene (Fig. S8), the fact that any aerosol mass is observed during the beginning of the NO2 experiments when oxidation exclusively goes by way of NO3 indicates that qualitative yield differences relative to the O3 experiment can be attributed at least in part to NO3 oxidation products. In contrast, Fig.  shows that neither of the α-pinene experiments with NO2 produce any aerosol mass until O3 starts to contribute. This observation is consistent with the hypothesis that the observed suppression of aerosol formation from α-pinene with increasing concentrations of NO2 can be attributed to its (near) 0 % yield with NO3 observed in other studies .

For β-pinene, Δ3-carene, and limonene, while it is clear that aerosol mass forms from NO3 oxidation products in the NO2 experiments, a time lag becomes apparent during which BVOC is reacting but no aerosol is formed. As soon as aerosol formation is initiated, however, the mass rapidly increases. The kinetics model used in this study assumes three options for RO2 fate: reaction with RO2, NO3, or NO2. In Sect. , we propose that high NO2 experiments yield low mass concentrations due to the formation of less stable peroxy nitrates. This explanation likely accounts for the lag time observed in each NO2 experiment before aerosol is able to form. Indeed, from the model we can calculate a ratio of RO2+ NO2 products relative to the sum of RO2+ RO2, RO2+ NO3, and RO2+ NO2 products present in the chamber at each time in the experiment. When this ratio time series is overlaid onto the plots in Fig. , a minimum in RO2+ NO2 products appears at approximately the same time that aerosol formation is initiated. This timing, shown in Fig.  and Fig. S8, indicates that even the low NO2 experiments have enough NO2 present that formation of relatively volatile peroxy nitrates may kinetically dominate experiments at early times until RO2+ RO2 and RO2+ NO3 products start to compete.

The percentage of BVOC reacted by each of the three oxidants was modeled and is shown in Table . Comparisons were made 2 h into the reaction after the initial buildup of NO3 and N2O5 was depleted and chemical production of NO3 more realistically competes with O3 oxidation of BVOCs. Even at this point in time, NO3 dominates the initial oxidation pathway for all NO2 concentrations and all monoterpenes, further indicating that if NO3 oxidation contributes to SOA mass, as is certainly the case for β-pinene, Δ3-carene, and limonene, these NO3 oxidation products are plentiful enough throughout the full experiment to contribute significantly to observed yield differences between those experiments relative to the O3-only experiments.

Filter samples from experiments that yielded sufficient aerosol mass (all experiments in Table  except 1, 5, 9, 13) were collected and analyzed offline by HPLC-ESI-MS at Colorado State University. Because electrospray ionization is a soft ionization technique, this method has been shown to be especially useful for detecting a wide range of m/z products – including oligomer species that are likely to be significant SOA constituents . Although quantitative comparisons of products are not possible due to differences in mass loadings and a lack of calibration standards, qualitative differences in product distributions were readily apparent and consistent with observed aerosol yield trends.

Introducing NO2 into ozonolysis of monoterpenes influences the composition of resulting SOA in two different ways: first, by forming NO3 that can either oxidize BVOC directly or react with NO3- or O3-initiated RO2, or second, by directly reacting with RO2 or other products and reaction intermediates as NO2. A visual comparison of the total ion chromatograms from ozonolysis of β-pinene with no NO2 and the two lowest concentrations of NO2 (Fig. ) shows that several new products form once NO2 is added, and that in general increasing [NO2] simply increases the intensity of those products rather than changing product identities substantially. For ease of interpretation, results from all of the NO2-containing experiments were combined into a single product distribution from “NO3-influenced oxidation.” We can then compare those product distributions to those of the O3-only experiments. A complete list of compound formulae detected (> 1.5 % relative intensity, see Supplement) in the O3 and NO3 dominated oxidation of each monoterpene is compiled in Table S2.

To best highlight qualitative differences in the identity of molecules that make SOA for each set of precursors, every unique compound (distinct either in mass, retention time, or both) was accounted for once, not normalized by peak intensity. A variety of average bulk composition parameters were calculated for each experiment, highlighted in Table , including average number of C, O, and N atoms per compound, molecular weight, and total number of products. Some artifacts may remain in this data set, such as impurities not captured by the background subtraction or product fragments that do not reflect the original identity of the SOA product. The former should affect all samples uniformly in this analysis and thus will not influence qualitative comparisons, and the latter will either affect multiple samples and thus be irrelevant in comparisons or only affect single samples and thus still provide interesting qualitative differences.

A direct correlation between any of the average parameters (MWavg, Cavg, Oavg, Navg) in Table  and absolute aerosol yields is not obvious. α-Pinene ozonolysis, for example, produced the highest aerosol mass of all the conditions tested, and while its average MW and number of C atoms are higher than ozonolysis from all the other monoterpenes, those same values are comparable to each of the NO3 experiments and substantially lower than those values for limonene + NO3. However, the difference in average values, defined as the difference in each average parameter between O3 and NO3 dominated oxidation for each monoterpene (Δavg), are consistent with O3 vs. NO3 yield comparisons. β-Pinene and Δ3-carene have similar Δavg values for each parameter (as well as similar absolute values for each oxidant condition), suggesting that the addition of NO3 affects the product distribution of these two monoterpenes similarly. The Δavg values for limonene are much higher than any other monoterpene in this study, consistent with it having the highest NO3 aerosol yields. Again, perhaps most notably, the Δavg parameters hover near 0 for α-pinene, suggesting that the aerosol composition does not differ much between the two oxidants – consistent with all of α-pinene's aerosol production coming exclusively from O3 oxidation.

To illustrate some of the finer detail of these product distributions, Fig.  shows histograms where each observed product is binned by compound mass in 50 amu intervals. Every experiment shows some contribution from oligomer products (m/z > 246 according to ; > 300 according to ), but this contribution is most pronounced from NO3 oxidation of β-pinene, Δ3-carene, and limonene. In particular, we observe substantially more distinct products > 400 amu from β-pinene, Δ3-carene, and limonene with the O3 / NO2 / NO3 mixture than from O3 alone. In this region, the mass distributions for α-pinene in both oxidant conditions are identical. Since mass is an important contributing factor to volatility (e.g., ), these high-mass products are likely important in aerosol formation and growth and thus may be explanatory of the observed yield differences from NO3 oxidation. If oligomerization is an important pathway leading to SOA formation and growth from NO3-initiated chemistry, α-pinene's lack of oligomer products with NO3 may be responsible for its 0 % aerosol yield. In contrast, comparison of the four O3-only histograms shows relatively small contributions of high MW oligomers for any monoterpene, in spite of quite high aerosol yields in some cases, indicating that aerosol formation by ozonolysis may not require oligomerization.

Recent studies of SOA nucleation and growth from ozonolysis of α-pinene have shown that highly oxidized and/or oligomeric species are likely important in nucleation and early growth, but that growth beginning around 20 nm is dominated by lower MW products (140–380 amu) . This latter MW range is consistent with the ozonolysis products we observe for all four monoterpenes, indicating that high MW products may dominate only early stages of growth and are thus not detectable at the high-mass loadings in this study. NO3 oxidation, however, seems to provide a weaker source of low volatility compounds contributing to nucleation and early growth, as seen in the decrease of Ntot with increasing [NO2] in Fig.  (with the exception of limonene), but produces oligomers throughout the full time period of aerosol growth, leading to total aerosol mass concentrations that rival ozonolysis (with the exception of α-pinene), as seen in Fig. . Further supporting this observed difference in products from ozonolysis compared to NO3 oxidation is the difference in the reaction rate of each process. O3 + BVOC is much slower than NO3 + BVOC, which means that RO2 is produced more slowly from ozonolysis and thus the RO2 lifetime is much longer with respect to other radical species. Longer RO2 lifetimes are more conducive to isomerization processes like autoxidation , which may be responsible for the initial high MW nucleating species observed in other ozonolysis studies. In contrast, NO3 oxidation produces RO2 much more rapidly, therefore increasing the likelihood of RO2 + RO2 oligomerization.

Mass spectra alone provide limited compositional information since they do not distinguish between different functional groups. However, in this system, one functional group that can be easily parsed out of the data is the nitrate group. From the NO3 initiated oxidation chemistry, we expect that any nitrogen present in a molecule is a part of a nitrate functional group. (Some instances of -NO and -ONO have been found in the compound list, causing relatively high Navg values for α-pinene + O3, for example, where we expect any nitrogen is due to impurities.) The Δavg values in Table  for Navg provide an approximate estimate of relative aerosol organic nitrate yield. β-Pinene, Δ3-carene, and limonene all exhibit a substantial increase in average number of N per molecule with the addition of NO2, consistent with the relatively high organic nitrate yields observed from NO3 oxidation of those species in other studies . α-Pinene produces comparatively fewer nitrogen-containing SOA products in the presence of NO2. While the organic nitrate products from α-pinene may be relatively volatile and thus not partition appreciably into the aerosol phase, it is clear that this is not a universal characteristic of C10 organic nitrates, as many do partition into the aerosol phase for all three other monoterpenes studied – even those with relatively low total aerosol mass loading.

We note that the products observed here from ozonolysis vs. NO3 oxidation are consistent with proposed mechanisms in the literature. Table S3 includes proposed structures for several masses that have been observed in other studies, including several monomeric carboxylic acids and aldehydes from ozonolysis as well as multi-functional monomeric nitrates from NO3 oxidation , some of which have been included in Fig.  to highlight relative intensities across different NO2 conditions. Several more speculative structures are shown in the Supplement to indicate that observed oligomeric masses can be reasonably achieved from dimers of first-generation oxidation products.