Aqueous secondary organic aerosol formation from the direct photosensitized oxidation of vanillin in the absence and presence of ammonium nitrate

Vanillin (VL), a phenolic aromatic carbonyl abundant in biomass burning emissions, forms triplet excited states (3VL∗) under simulated sunlight leading to aqueous secondary organic aerosol (aqSOA) formation. Nitrate and ammonium are among the main components of biomass burning aerosols and cloud or fog water. Under atmospherically relevant cloud and fog conditions, solutions composed of either VL only or VL with ammonium nitrate were subjected to simulated sunlight irradiation to compare aqSOA formation via the direct photosensitized oxidation of VL in the absence and presence of ammonium nitrate. The reactions were characterized by examining the VL decay kinetics, product compositions, and light absorbance changes. Both conditions generated oligomers, functionalized monomers, and oxygenated ring-opening products, and ammonium nitrate promoted functionalization and nitration, likely due to its photolysis products ( qOH, qNO2, and NO−2 or HONO). Moreover, a potential imidazole derivative observed in the presence of ammonium nitrate suggested that ammonium participated in the reactions. The majority of the most abundant products from both conditions were potential brown carbon (BrC) chromophores. The effects of oxygen (O2), pH, and reactants concentration and molar ratios on the reactions were also explored. Our findings show that O2 plays an essential role in the reactions, and oligomer formation was enhanced at pH<4. Also, functionalization was dominant at low VL concentrations, whereas oligomerization was favored at high VL concentrations. Furthermore, oligomers and hydroxylated products were detected from the oxidation of guaiacol (a non-carbonyl phenol) via VL photosensitized reactions. Last, potential aqSOA formation pathways via the direct photosensitized oxidation of VL in the absence and presence of ammonium nitrate were proposed. This study indicates that the direct photosensitized oxidation of VL may be an important aqSOA source in areas influenced by biomass burning and underscores the importance of nitrate in the aqueous-phase processing of aromatic carbonyls. Published by Copernicus Publications on behalf of the European Geosciences Union. 274 B. R. G. Mabato et al.: Aqueous SOA formation from the direct photosensitized oxidation of vanillin


Introduction
Aqueous reactions can be an important source of secondary organic aerosols (SOAs; Blando and Turpin, 2000;Volkamer et al., 2009;Lim et al., 2010;Ervens et al., 2011;Huang et al., 2011;Lee et al., 2011;, such as highly oxygenated and low-volatility organics (Hoffmann et al., 2018;Liu et al., 2019), which may affect aerosol optical properties due to contributions to brown carbon (BrC; Gilardoni et al., 2016). BrC refers to organic aerosols that absorb radiation efficiently in the near-ultraviolet (UV) and visible regions (Laskin et al., 2015). The formation of aqueous SOA (aqSOA) via photochemical reactions involves oxidation, with the hydroxyl radical ( q OH) usually considered as being the primary oxidant (Herrmann et al., 2010;. The significance of photosensitized chemistry in atmospheric aerosols has recently been reviewed (George et al., 2015). For instance, triplet excited states of organic compounds ( 3 C * ) from the irradiation of light-absorbing organics such as non-phenolic aromatic carbonyls (Canonica et al., 1995;Anastasio et al., 1997;Vione et al., 2006; have been reported to oxidize phenols at higher rates and with greater aqSOA yields compared to q OH (Sun et al., 2010;Yu et al., 2014;Smith et al., 2016). Aside from being an oxidant, 3 C * can also be a precursor of singlet oxygen ( 1 O 2 ), superoxide (O q − 2 ) or hydroperoxyl ( q HO 2 ) radicals, and q OH (via HO q 2 / O q − 2 formation) upon reactions with O 2 and substrates (e.g., phenols; George et al., 2018). The 3 C * concentration in typical fog water has been estimated to be >25 times than that of q OH, making 3 C * the primary photo-oxidant for biomass burning phenolic compounds (Kaur and Anastasio, 2018;Kaur et al., 2019). Recent works on triplet-driven oxidation of phenols have mainly focused on changes in physicochemical properties (e.g., light absorption) and aqSOA yield (e.g., Smith et al., , 2015Smith et al., , 2016, with few reports on reaction pathways and products (e.g., Yu et al., 2014;Chen et al., 2020;Jiang et al., 2021). Inorganic salts such as ammonium nitrate are major components of aerosols and cloud or fog water. In cloud and fog water, the concentrations of inorganic nitrate can vary from 50 to >1000 µM, with higher levels typically noted under polluted conditions (Munger et al., 1983;Collett et al., 1998;Zhang and Anastasio, 2003;Li et al., 2011;Giulianelli et al., 2014;Bianco et al., 2020). Upon photolysis (Vione et al., 2006;Herrmann, 2007;Scharko et al., 2014), inorganic nitrate in cloud and fog water can contribute to BrC (Minero et al., 2007) and aqSOA formation Klodt et al., 2019;Zhang et al., 2021) by generating q OH and q NO 2 (also a nitrating agent). For example, the aqSOA yields from the photo-oxidation of phenolic carbonyls in ammonium nitrate are twice as high as that in ammonium sulfate solution . Nitration is a significant process in the formation of light-absorbing organics or BrC in the atmosphere (Jacobson, 1999;Kahnt et al., 2013;Mohr et al., 2013;Laskin et al., 2015;Teich et al., 2017;Li et al., 2020).
Biomass burning (BB) is a significant atmospheric source of both phenolic and non-phenolic aromatic carbonyls (Rogge et al., 1998;Nolte et al., 2001;Schauer et al., 2001;Bond et al., 2004). Upon exposure to sunlight, aromatic carbonyls are excited to their triplet excited states, which can initiate oxidation leading to aqSOA formation (e.g., ). An example is vanillin (VL; Henry's law constant of 4.56×10 5 M atm −1 ; Yaws, 1994), a phenolic aromatic carbonyl that has been used as a model compound for methoxyphenols, which are abundant in BB emissions (Li et al., 2014;Pang et al., 2019a). The aqueous q OH oxidation and direct photodegradation of VL have been shown to yield low-volatility products, although these findings were based on 254 nm irradiation (Li et al., 2014). Photodegradation kinetics and aqSOA yields have been reported for direct VL photodegradation under simulated sunlight (Smith et al., 2016), with oxygenated aliphatic-like compounds (high H : C, ≥ 1.5 and low O : C, ≤ 0.5 ratios) noted as being the most likely products (Loisel et al., 2021). Additionally, aqueous-phase reactions of phenols with reactive nitrogen species have been proposed to be a significant source of nitrophenols and SOA (Grosjean, 1985;Kitanovski et al., 2014;Kroflič et al., 2015Kroflič et al., , 2021Pang et al., 2019a;. For instance, nitrite-mediated VL photo-oxidation can generate nitrophenols, and the reactions are influenced by nitrite / VL molar ratios, pH, and the presence of q OH scavengers (Pang et al., 2019a). Nitrate and ammonium are also among the main biomass burning aerosol components (Xiao et al., 2020;Zielinski et al., 2020). As BB aerosols are typically internally mixed with other aerosol components (Zielinski et al., 2020), VL may coexist with ammonium nitrate in BB aerosols. The direct photosensitized oxidation of VL in the absence and presence of ammonium nitrate may then reveal insights into the atmospheric processing of BB aerosols. Moreover, the 3 C * of non-phenolic aromatic carbonyls (e.g., 3,4-dimethoxybenzaldehyde -DMB; a nonphenolic aromatic carbonyl; Yu et al., 2014;Jiang et al., 2021) and phenolic aromatic carbonyls (e.g., acetosyringone and VL; Smith et al., 2016) have been shown to oxidize phenols, but the reaction products from the latter are unknown.
Previous works on aqSOA formation via triplet-mediated oxidation are mostly based on reactions between phenols and a non-phenolic aromatic carbonyl as triplet precursor (e.g., Yu et al., 2014;Jiang et al., 2021). Also, studies examining the effects of inorganic nitrate on aqSOA formation and properties remain limited. The present study aimed to evaluate aqSOA formation via the direct photosensitized oxidation of a triplet precursor (VL) alone. Furthermore, aqSOA formation via the direct photosensitized oxidation of VL in the presence of ammonium nitrate was also examined. Accordingly, the main goals of this study are (1) to compare aqSOA formation in cloud and fog water via the direct photosensitized oxidation of VL in the absence and presence of ammonium nitrate; (2) to evaluate the influences of O 2 , solution pH, and reactants concentration and molar ratios on the reactions; (3) to investigate the participation of ammonium in the direct photosensitized oxidation of VL in the presence of ammonium nitrate; and (4) to examine aqSOA formation from the oxidation of guaiacol, a non-carbonyl phenol, via photosensitized reactions of VL. To achieve these goals, solutions composed of either VL only or VL in the presence of ammonium nitrate were subjected to simulated sunlight irradiation under atmospherically relevant cloud and fog conditions. Solutions composed of VL in the presence of sodium nitrate were also examined for comparison with the presence of ammonium nitrate. The reactions were characterized based on VL decay kinetics, detected products, and light absorbance changes. Finally, we proposed aqSOA formation pathways via the direct photosensitized oxidation of VL in the absence and presence of ammonium nitrate. This work presents a comprehensive comparison of aqSOA formation from the direct photosensitized oxidation of VL in the absence and presence of ammonium nitrate.

Aqueous-phase photo-oxidation experiments
Photo-oxidation experiments were performed in a custombuilt quartz photo reactor. The solutions (initial volume of 500 mL) were continuously mixed throughout the experiments using a magnetic stirrer. The solutions were bubbled with synthetic air or nitrogen (N 2 ; > 99.995 %; 0.5 dm 3 /min) for 30 min before irradiation to achieve air-or N 2 -saturated conditions, respectively, and the bubbling was continued throughout the reactions (Du et al., 2011;Chen et al., 2020). The aim of the air-saturated experiments was to enable the generation of secondary oxidants ( 1 O 2 , O q − 2 / q HO 2 , and q OH) from 3 VL * as O 2 is present. Conversely, the N 2saturated experiments would inhibit the formation of these secondary oxidants, which can lead to 3 VL * -driven reactions . Comparison of results of air-and N 2 -saturated experiments can yield information on the reaction pathways that require O 2 involved in the direct photosensitized oxidation of VL. In this study, the reactions can generate 3 VL * and secondary oxidants ( 1 O 2 , O q − 2 / q HO 2 , and q OH) but not ozone; hence, we focused on reactions involving the former. Solutions were irradiated through the quartz window of the reactor using a xenon lamp (model 6258; ozone-free xenon lamp; 300 W; Newport) equipped with a longpass filter (20CGA 305 nm cut-on filter; Newport) to eliminate light below 300 nm. Cooling fans positioned around the photo reactor and lamp housing maintained reaction temperatures at 27 ± 2 • C. The averaged initial photon flux in the reactor from 300 to 380 nm, measured using a chemical actinometer (2-nitrobenzaldehyde), was 2.6 × 10 15 photons cm −2 s −1 nm −1 (Fig. S1). Although the concentration of VL in cloud or fog water has been estimated to be <0.01 mM (Anastasio et al., 1997), a higher VL concentration (0.1 mM) was used in this study to guarantee sufficient signals for product identification . The chosen ammonium nitrate (AN) or sodium nitrate (SN) concentration (1 mM) was based on values observed in cloud and fog water (Munger et al., 1983;Collett et al., 1998;Zhang and Anastasio, 2003;Li et al., 2011;Giulianelli et al., 2014;Bianco et al., 2020). It should be noted that this study is not intending to identify the concentrations of ammonium nitrate that would affect the kinetics but to examine the effect of ammonium nitrate on aqSOA formation from the direct photosensitized oxidation of VL. Moreover, the photo-oxidation of guaiacol (GUA; 0.1 mM), a non-carbonyl phenol, in the presence of VL (0.1 mM) was studied. The GUA experiments allowed us to examine aq-SOA formation from the oxidation of phenols by 3 VL * . Samples (10 mL) were collected hourly for a total of 6 h for offline chemical and optical analyses. VL (and GUA) decay kinetics measurements (calibration curves for VL and GUA standard solutions; Fig. S2), product characterization, small organic acids measurements, and absorbance measurements were conducted using ultra-high-performance liquid chromatography (UHPLC) with a photodiode array detector (UHPLC-PDA), a UHPLC coupled with quadrupole time-offlight mass spectrometry (UHPLC-qToF-MS) equipped with an electrospray ionization (ESI) source and operated in the positive ion mode (the negative ion mode signals were too low for our analyses), ion chromatography (IC), and UV-Vis spectrophotometry, respectively. Each experiment was repeated independently at least 3 times, and measurements were done in triplicate. The reported decay rate constants and absorbance enhancement are the average of the results from triplicate experiments, and the corresponding errors represent 1 standard deviation. The mass spectra are based on the average of results from duplicate experiments. The Supplement (Sects. S1 to S6) provides details on the materials and analytical procedures. The pseudo-first-order rate constant (k') for VL decay was determined using the following equation :

Calculation of normalized abundance of products
Comparisons of peak abundance in mass spectrometry have been used in many recent studies (e.g., Lee et al., 2014;Romonosky et al., 2017;Wang et al., 2017;Fleming et al., 2018;Song et al., 2018;Klodt et al., 2019;Ning et al., 2019) to show the relative importance of different types of compounds . However, ionization efficiency may greatly vary for different classes of compounds (Kebarle, 2000;Schmidt et al., 2006;Leito et al., 2008;Perry et al., 2008;Kruve et al., 2014), and so uncertainties may arise from comparisons of peak areas among compounds. In this work, we assumed equal ionization efficiency of different compounds, which is commonly used to estimate O : C ratios of SOA (e.g., Bateman et al., 2012;Lin et al., 2012;Laskin et al., 2014;De Haan et al., 2019) to calculate their normalized abundance. The normalized abundance of a product, [P] (unitless), was calculated as follows: where A P, t and A VL, t are the extracted ion chromatogram (EIC) peak areas of the product P and VL from UHPLC-qToF-MS analyses at time t, respectively.
[VL] t and [VL] 0 are the VL concentrations (µM) determined using UHPLC-PDA at time t and 0, respectively. Here, we relied on the direct quantification of [VL] using UHPLC-PDA (see Fig. S2 for the VL calibration curve). We emphasize that the normalized abundance of products in this study is a semiquantitative analysis intended to provide an overview of how the signal intensities changed under different experimental conditions but not to quantify the absolute concentration of products. Also, as it is based on comparisons of peak abundance from UHPLC-qToF-MS analyses, the normalized abundance of products in this study is associated with intrinsic uncertainties due to the variability in ionization efficiencies for various compounds. Moreover, the major products detected in this study are probably those with high concentration or high ionization efficiency in the positive ESI mode. The use of relative abundance (product peaks are normalized to the highest peak; e.g., Lee et al., 2014;Romonosky et al., 2017;Fleming et al., 2018;Klodt et al., 2019) would yield the same major products reported. Typical fragmentation behavior observed in MS/MS spectra for individual functional groups from Holčapek et al. (2010) are outlined in Table S1.

Results and discussion
3.1 Kinetics, mass spectrometric, and absorbance changes analyses during the direct photosensitized oxidation of VL in the aqueous phase For clarity purposes, the reactions involving reactive species referred to in the following discussions are provided in Table 1. Table 2 summarizes the reaction conditions, initial VL (and GUA) decay rate constants, normalized abundance of products, and average carbon oxidation state ( OS c ) of the 50 most abundant products. In general, the 50 most abundant products contributed more than half of the total normalized abundance of products and can facilitate the discussions of reaction pathways and calculation of the OS c . As shown in Fig. S3, VL underwent oxidation both directly and in the presence of ammonium (and sodium) nitrate upon simulated sunlight illumination. VL absorbs light and is promoted to its excited singlet state ( 1 VL * ) and then undergoes inter-system crossing (ISC) to the excited triplet state, 3 VL * . In principle, 3 VL * can oxidize ground-state VL (type I photosensitized reactions) via H atom abstraction or electron transfer, and form O q − 2 or HO q 2 in the presence of O 2 (George et al., 2018), or react with O 2 (type II photosensitized reactions) to yield 1 O 2 via energy transfer or O q − 2 via electron transfer (Lee et al., 1987;Foote et al., 1991). q HO 2 , and q OH) from 3 VL * . However, as discussed later, we found that the direct photosensitized oxidation of VL under air-saturated conditions in this study is mainly governed by 3 VL * . Moreover, q OH, q NO 2 , and NO − 2 / HNO 2 , i.e., N(III), generated via nitrate photolysis (Reactions 1-3; Table 1), can also oxidize or nitrate VL. In this work, the direct photosensitized oxidation of VL in the absence (VL-only experiments) and presence of ammonium nitrate are referred to as VL * and VL + AN, respectively.

VL photo-oxidation under N 2 and air-saturated conditions
As previously stated, the air-saturated experiments can enable the generation of secondary oxidants ( 1 O 2 , O q − 2 / q HO 2 , and q OH) from 3 VL * as O 2 is present. In contrast, the N 2saturated experiments would inhibit the formation of these secondary oxidants from 3 VL * , facilitating 3 VL * -driven reactions . Moreover, Chen et al. (2020) reported that, for experiments conducted under three saturated gases (air, O 2 , and N 2 ), the rate constant for 4-ethylguaiacol (a non-carbonyl phenol) loss by 3 DMB * decreased in the order of air > N 2 > O 2 . The higher rate constant under N 2saturated conditions compared to that under O 2 -saturated conditions indicates that 3 DMB * is a more important con- ; φ OH, 300 = 6.7(±0.9) % Fischer and Warneck (1996); Mack and Bolton (1999); Pang et al. (2019a) Goldstein and Czapski (1995); Pang et al. (2019a) tributor than 1 O 2 for 4-ethylguaiacol degradation. The highest rate constant noted under air-saturated conditions was attributed to the presence of O 2 , resulting in a synergistic effect of 1 O 2 and 3 C * . The differences in air-saturated and N 2saturated experiments can then be used to infer the role of reaction pathways that require O 2 in the direct photosensitized oxidation of VL. The photosensitized oxidation of VL under both N 2 -and air-saturated conditions ( Fig. S3a) were carried out at pH 4, which is representative of moderately acidic aerosol and cloud pH values (Pye et al., 2020). No significant VL loss was observed for dark experiments. The oxidation of ground-state VL by 3 VL * via H atom abstraction or electron transfer can form phenoxy (which is in resonance with a carbon-centered cyclohexadienyl radical that has a longer lifetime) and ketyl radicals (Neumann et al., 1986a, b;Anastasio et al., 1997). The coupling of phenoxy radicals or phenoxy and cyclohexadienyl radicals can form oligomers, as observed for both air-saturated and N 2 -saturated experiments (see discussions later). The VL decay rate constant for VL * under air-saturated conditions was 4 times higher than under N 2 -saturated conditions ( Table 2). As mentioned earlier, secondary oxidants q HO 2 , and q OH) can be generated from 3 VL * when O 2 is present (under air-saturated conditions). However, the direct photosensitized oxidation of VL in this study is likely governed by 3 VL * , and these secondary oxidants have only minor participation. For instance, 1 O 2 is also a potential oxidant for phenols (Herrmann et al., 2010;Minella et al., 2011;, but 1 O 2 reacts much faster (by ∼ 60 times) with phenolate ions than neutral phenols (Tratnyek and Hoigne, 1991;Canonica et al., 1995;McNally et al., 2005). Under the pH values (pH 2.5 to 4) considered in this study, the amount of phenolate ion is negligible (pK a of VL = 7.9), so the reaction between VL and 1 O 2 should be slow. Interestingly, however, both 3 C * and 1 O 2 have been shown to be important in the photo-oxidation of 4-ethylguaiacol (pK a = 10.3) by 3 DMB * (solution with pH of ∼ 3; Chen et al., 2020). Furthermore, while the irradiation of other phenolic compounds can produce H 2 O 2 , a precursor for q OH (Anastasio et al., 1997), the amount of H 2 O 2 is small. Based on this, only trace amounts of H 2 O 2 were likely generated from VL * (Li et al., 2014) under airsaturated conditions, suggesting that the contribution from q OH was minor. Overall, these suggest that the direct photosensitized oxidation of VL in this study is mainly driven by 3 VL * .
Contrastingly, the minimal decay of VL under N 2saturated conditions can be attributed to the phenoxy (which is in resonance with a carbon-centered cyclohexadienyl radical that has a longer lifetime) and ketyl radicals formed upon oxidation of ground-state VL by 3 VL * decaying via backhydrogen transfer to regenerate VL (Lathioor et al., 1999). A possible explanation for this is the involvement of O 2 in the secondary steps of VL decay. For instance, a major fate of the ketyl radical is a reaction with O 2 (Anastasio et al., 1997). In the absence of O 2 , radical formation occurs, but the forward reaction of ketyl radical and O 2 is blocked, leading to the regeneration of VL, as suggested by the minimal VL decay. Aside from potential inhibition of secondary oxidants generation , N 2 purging may have also hindered the secondary steps for VL decay.
The VL decay rate constant for VL + AN under airsaturated conditions was also higher (6.6 times) than under N 2 -saturated conditions, possibly due to reactions facilitated by nitrate photolysis products that may have been enhanced in the presence of O 2 (Vione et al., 2005;Kim et al., 2014;Pang et al., 2019a). As shown later, more N-containing species were observed for VL + AN under air-saturated conditions than under N 2 -saturated conditions. An example is enhanced VL nitration likely from increased q NO 2 formation, such as from the reaction of q OH and O q − 2 with NO − 2 (Reactions 4 and 5, respectively; Table 1) or the autoxidation of q NO from NO − 2 photolysis (Reactions 6-9; Table 1) a Irradiation time for VL * (0.01 mM, A10) was 3 h. b The data fitting was performed in the initial linear region. Each value is the average of results from triplicate experiments. Errors represent 1 standard deviation. Kinetic measurements were not performed for experiments marked with NA (not available). c Ratio of the normalized abundance of the 50 most abundant products to that of total products, except for direct GUA photodegradation and GUA + VL (A13 and A14), whose ratios are based on the absolute signals of products. d The normalized abundance of products was calculated using Eq.
(2). The samples for experiments without nitrate (marked with NA) were not analyzed for N-containing compounds. For the GUA experiments, the normalized abundance of products was calculated only for GUA + VL as the GUA signal from the UHPLC-qToF-MS in the positive ion mode was weak, which may introduce large uncertainties during normalization. e OS c of the 50 most abundant products.
in aqueous solutions (Pang et al., 2019a). Nevertheless, the comparable decay rate constants for VL * and VL + AN imply that 3 VL * chemistry still dominates, even at 1 : 10 molar ratio of VL / AN. This can be attributed to the much higher molar absorptivity of VL compared to that of nitrate ( Fig. S1) and the high VL concentration (0.1 mM) used in this study. The quantification of the oxidants in our reaction systems is not explored here and requires additional work. In essence, the N 2 -saturated experiments suggest that the secondary steps for VL decay via 3 VL * may require O 2 to proceed. Nonetheless, further study on the impact of O 2 on the reactive intermediates involved is required to understand the exact mechanisms occurring under air-saturated conditions.
The products from VL * under N 2 -saturated conditions were mainly oligomers (e.g., C 16 H 14 O 4 ; Fig. 1a), consistent with triplet-mediated oxidation forming higher molecular weight products, with less fragmentation relative to oxidation by q OH Chen et al., 2020). A threefold increase in the normalized abundance of products was noted upon addition of AN (VL + AN under N 2 -saturated conditions; Fig. 1b), and in addition to oligomers, functionalized monomers (e.g., C 8 H 6 O 5 ) and N-containing compounds (e.g., C 8 H 9 NO 3 ; no. 3 in Table S2) were also observed, which is in agreement with q OH-initiated oxidation yielding more functionalized/oxygenated products compared to triplet-mediated oxidation Chen et al., 2020). Oligomers, functionalized monomers (e.g., demethylated VL; Fig. S4), and N-containing compounds (e.g., C 16 H 10 N 2 O 9 ; no. 4 in Table S2; for VL + AN) had higher normalized abundance under air-saturated conditions (Fig. 1c, d), which are attributable to efficient 3 VL * -initiated oxidation and enhanced VL nitration in the presence of O 2 . For both VL * and VL + AN under air-saturated conditions, the most abundant product was C 10 H 10 O 5 (no. 5 in Table S2), which is a substituted VL. Irradiation of VL by a 254 nm lamp has also been reported to lead to VL dimerization and functionalization via ring-retaining pathways, as well as small oxygenates formation, but only when q OH from H 2 O 2 were involved (Li et al., 2014). In this work, small organic acids (e.g., formic acid) were observed from both VL * and VL + AN under air-saturated conditions (Fig. S5) due to simulated sunlight that could access the 308 nm VL band (Smith et al., 2016). Interestingly, we observed a potential imidazole derivative (C 5 H 5 N 3 O 2 ; no. 6 in Table S2) from VL + AN under air-saturated conditions (Fig. 1d), which may have formed from reactions induced by ammonium. This compound was not observed in a parallel experiment in which AN was replaced with SN ( Fig. S6a; see Sect. 3.1.3 for the discussion).
The potential aqSOA formation pathways via the direct photosensitized oxidation of VL in the absence and presence of AN in this study are summarized in Fig. 2. At pH 4, 3 VL * -initiated reactions yielded oligomeric species such as C 16 H 12 O 6 and C 22 H 22 O 6 . Earlier works on phenolic aq-SOA formation have reported that oligomers can form via the coupling of phenoxy radicals or phenoxy and cyclohexadienyl radicals (Sun et al., 2010;Yu et al., 2014;. In this work, phenoxy radicals (in resonance with a carbon-centered cyclohexadienyl radical) can be generated from several processes, such as the oxidation of ground-state VL by 3 VL * via H atom abstraction or electron transfer coupled with proton transfer from the phenoxyl radical cation or from solvent water (Neumann et al., 1986a, b;Anastasio et al., 1997) and photoinduced O-H bond breaking (Berto et al., 2016). Also, similar reactions can be initiated by q OH (Gelencsér et al., 2003;Hoffer et al., 2004;Chang and Thompson, 2010;Sun et al., 2010), which, in this study, can be generated from the reaction between 3 VL * and O 2 , as well as nitrate photolysis. Trace amounts of H 2 O 2 could be formed during VL photodegradation (Li et al., 2014), similar to the case of other phenolic compounds (Anastasio et al., 1997). In addition, ring-opening products (Fig. S5) from fragmentation in both VL * and VL + AN may have reacted with VL or dissolved ammonia to generate C 10 H 10 O 5 (no. 5 in Table S2; Pang et al., 2019b) or a potential imidazole derivative (C 5 H 5 N 3 O 2 ; no. 6 in Table S2), respectively. Moreover, nitrate photolysis products promoted functionalization and nitration (e.g., C 16 H 10 N 2 O 9 ; no. 4 in Table S2). The molecular transformation of VL upon photosensitized oxidation was examined using the van Krevelen diagrams (Fig. S7). For all experiments (A1-14; Table 2) in this study, the O : C and H : C ratios of the products were similar to those observed from the aging of other phenolic compounds  and BB aerosols (Qi et al., 2019). Under N 2 -saturated conditions, oligomers with O : C ratios ≤ 0.6 were dominant in VL * , while smaller molecules (n c ≤ 8) with higher O : C ratios (up to 0.8) were also observed for VL + AN. In contrast, more products with higher O : C ratios (≥ 0.6) were noted under air-saturated conditions for both VL * and VL + AN. For experiments A5 to A8, H : C ratios were mostly around 1.0, and double bond equivalent (DBE) values were typically (58 % of the 50 most abundant products) >7, indicating that the products were mainly oxidized aromatic compounds . Compounds with H : C ≤ 1.0 and O : C ≤ 0.5 are common for aromatic species, while compounds with H : C ≥ 1.5 and O : C ≤ 0.5 are typical for more aliphatic species Kourtchev et al., 2014;Jiang et al., 2021). In contrast, Loisel et al. (2021) reported mainly oxygenated aliphatic-like compounds from the direct irradiation of VL (0.1 mM), attributable to their use of ESI in the negative ion mode, which has higher sensitivity for detecting compounds such as carboxylic acids (Holčapek et al., 2010;Liigand et al., 2017) and the solid-phase extraction (SPE) procedure causing the loss of some oligomers (LeClair et al., 2012;Zhao et al., 2013;Bianco et al., 2019). Among experiments A5 to A8, VL + AN under air-saturated conditions (A7) had the highest normalized abundance of products and OS c , probably due to the combined influence of 3 VL * and enhanced VL nitration in the presence of O 2 . Our measured OS c for all experiments range from −0.28 to +0.12, while other studies on phenolic aqSOA formation reported OS c ranging from −0.14 to +0.47 (Sun et al., 2010) and 0.04 to 0.74 . The OS c in this study were likely lower estimates since we excluded contributions from ring-opening products, which may have higher OS c values, as these products are not detectable in the positive ion mode. In brief, more oxidized aqSOA and higher normalized abundance of products, such as high O : C ratio oligomers and functionalized monomers, were noted under air-saturated conditions due to efficient VL oxidation by 3 VL * in the presence of O 2 . Compared to N 2 -saturated conditions, the higher normalized abundance of N-containing products under air-saturated conditions for VL + AN (at pH 4) suggests a potential enhancement of VL nitration in the presence of O 2 .
Illumination of phenolic aromatic carbonyls with high molar absorptivities (ε λmax ; ∼ 8 to 22×10 3 M −1 cm −1 ) leads to an overall loss of light absorption, but increased absorbance at longer wavelengths (>350 nm), where the carbonyls did not initially absorb light (Smith et al., 2016). Figure 3a illustrates the changes in total absorbance from 350 to 550 nm of VL * and VL + AN under N 2 -and air-saturated conditions. The absorption spectra of VL * under air-and N 2 -saturated conditions (pH 4) at different time intervals are shown in Fig. S8. For both VL * and VL + AN, evident absorbance enhancement was observed under air-saturated conditions, while the absorbance changes under N 2 -saturated conditions Figure 1. Reconstructed mass spectra of assigned peaks from (a) VL * pH 4 (N 2 saturated; A6), (b) VL + AN pH 4 (N 2 saturated; A8), (c) VL * pH 4 (air saturated; A5), (d) VL + AN pH 4 (air saturated; A7), (e) VL * pH 3 (air saturated; A3), (f) VL + AN pH 3 (air saturated; A4), (g) VL * pH 2.5 (air saturated; A1), and (h) VL + AN pH 2.5 (air saturated; A2) after 6 h of simulated sunlight irradiation. The normalized abundance of products was calculated using Eq. (2). The 50 most abundant products contributed more than half of the total normalized abundance of products, and they were identified as monomers (blue), dimers (green), trimers (red), and tetramers (orange). Gray peaks denote peaks with low abundance or an unassigned formula. Examples of high-intensity peaks were labeled with the corresponding neutral formulas. Note the different scales on the y axes.
were minimal, consistent with the VL decay trends. Dimers and functionalized products have been shown to contribute to chromophore formation for the aqueous photo-oxidation of guaiacyl acetone (another aromatic phenolic carbonyl) by 3 DMB * . Based on this, the higher normalized abundance of oligomers, which have large, conjugated π electron systems (Chang and Thompson, 2010), and hydroxylated products (Li et al., 2014;Zhao et al., 2015) observed under air-saturated conditions have contributed to the absorbance enhancement. However, it is worth noting that the products detected may not have contributed significantly to the total products formed and, hence, may not be the primary contributors to the absorbance enhancement. As mentioned earlier, the major products detected in this study are probably those with high concentration or high ionization efficiency in the positive ESI mode. In other words, the absorbance enhancement may not necessarily correlate directly with the products detected. Correlating speciated chromophores with absorbance changes may be useful in demonstrating how aqSOA influence the Earth's radiative balance and identifying chemical reactions that can affect the overall light absorption by aqSOA. This can be accomplished by using liquid chromatography (LC) coupled with photodiode array (PDA) detector and high-resolution mass spectrometry (HRMS; LC/PDA/HRMS platform; e.g., Lin et al., 2017;. In our experiments, VL (and GUA) concentration measurements, product characterization, and absorbance measurements were performed using UHPLC-PDA, UHPLC-qToF-MS, and UV-Vis spectrophotometry, respectively. A similar approach is then possible using the current methods in this work by matching the retention time (RT) of the products detected using UHPLC-ToF-MS with that in the PDA. However, the concentration of the chromophores in this study is below the detection limit of the PDA, based on the lack of distinct PDA signals from the products. Absorbance increase at >350 nm has also been reported for the photosensitized oxidation of phenol and 4-phenoxyphenol (De Laurentiis et al., 2013a, b) and direct photolysis of tyrosine and 4-phenoxyphenol (Bianco et al., 2014) in which dimers have been identified as initial substrates. The continuous absorbance enhancement throughout 6 h of irradiation correlated with the observation of oligomers and nitrated compounds after irradiation. However, the increasing concentration of small organic acids (Fig. S5) throughout the experiments suggests that fragmentation, which results in the decomposition of initially formed oligomers and formation of smaller oxygenated products , is important at longer irradiation times. Overall, these trends establish that compared to N 2 -saturated conditions, VL oxidation by 3 VL * under airsaturated conditions (O 2 is present) enabled the efficient formation of light-absorbing compounds from both VL * and VL + AN.

VL photo-oxidation under varying pH conditions
The reactions of 3 C * (Smith et al., , 2016, aromatic photonitration by nitrate (Machado and Boule, 1995;Dzengel et al., 1999;Vione et al., 2005;Minero et al., 2007), and N(III)-mediated VL photo-oxidation (Pang et al., 2019a) have been demonstrated to be pH dependent. In this study, the effect of pH on the direct photosensitized oxidation of VL was investigated over the pH range of 2.5 to 4, which is within typical cloud pH values (2-7; Pye et al., 2020). The decay rate constants for both VL * and VL + AN increased by 1.6 and 1.4 times, respectively, as pH decreased from 4 to 2.5 (Table 2). These differences in the decay rate constants are small but statistically significant (p<0.05). The pK a for the 3 VL * has been reported to be 4.0 (Smith et al., 2016). As there is a greater fraction of 3 VL * that are protonated at pH 2.5 (0.96) than at pH 4 (0.50), it is possible that the pH dependence of the VL decay rate constants observed in this study is due to 3 VL * being more reactive in its protonated form. Smith et al. (2016) also observed a pH dependence for the direct photodegradation of VL (0.005 mM; rate constants at pH ≤ 3 are ∼ 2 times lower than at pH ≥ 5) which has been attributed to the sensitivity of the excimer of VL (i.e., the charge-transfer complex formed between an excited state VL molecule and a separate ground state VL molecule; Birks, 1973;Turro et al., 2010) to acid-base chemistry. The opposite trend observed in this study for 0.1 mM VL may be due to the reactivities of the protonated and neutral forms of 3 VL * being dependent on VL concentration (Smith et al., 2016). The quantum yield for direct VL photodegradation is higher at pH 5 than at pH 2 for 0.005 mM VL, but they are not statistically different for 0.03 mM VL (Smith et al., 2016). As pH decreases, the higher reactivity of 3 VL * and sensitivity of the excimer of VL to acid-base chemistry may have led to faster VL photo-oxidation. Similar to pH 4 experiments, comparable decay rate constants between VL * and VL + AN were also noted at pH <4, again suggesting the predominant role of 3 VL * chemistry compared to nitrate, likely due to the high VL concentration (0.1 mM) used in this study.
As pH decreased, the normalized abundance of products, particularly oligomers and functionalized monomers, was higher for both VL * and VL + AN, consistent with 3 VL * potentially being more reactive in its protonated form. The most abundant products observed were a substituted VL (C 10 H 10 O 5 ; no. 5 in Table S2) and VL dimer (C 16 H 14 O 6 ; no. 7 in Table S2) at pH 4 and pH <4, respectively ( Fig. 1ch). Furthermore, a tetramer (C 31 H 24 O 11 ) was observed only in VL * at pH 2.5. For VL + AN, the normalized abundance of N-containing compounds was also higher at lower pH (Table 2), likely due to increased q OH and q NO 2 formation, which may be caused by the dependence of N(III) (NO − 2 + HONO) speciation on solution acidity (Pang et al., 2019a). At pH 3.3, half of N(III) exists as HONO (Fischer and Warneck, 1996;Pang et al., 2019a), which has a higher quantum yield for q OH formation than that of NO − 2 in the near-UV region (Arakaki et al., 1999;Kim et al., 2014). Also, NO − 2 / HONO can generate q NO 2 via oxidation by q OH (Reactions 4 and 10; Table 1; Pang et al., 2019a). At pH <4, 3 VL * likely have higher reactivity, as suggested by the increased normalized abundance of oligomers (e.g., C 16 H 14 O 6 ; no. 7 in Table S2; C 31 H 24 O 11 ) and Ncontaining compounds (e.g., C 16 H 10 N 2 O 9 ; no. 4 in Table S2; C 13 H 14 N 2 O 10 ; Fig. 2). The most abundant product at pH <4, C 16 H 14 O 6 (no. 7 in Table S2) is likely a C-O coupled dimer.
In previous studies on phenolic aqSOA formation, the generation of phenolic dimers has been proposed to occur via C-C or C-O coupling of phenoxy radicals (Sun et al., 2010;Yu et al., 2014;Huang et al., 2018;Chen et al., 2020;. Similarly, functionalized monomers, such as C 7 H 6 O 3 (demethylated VL; no. 8 in Table S2) and hydroxylated products (e.g., C 8 H 8 O 4 ; no. 9 in Table S2), also had increased normalized abundance for both VL * and VL + AN.
The formation of C 7 H 6 O 3 (no. 8 in Table S2), which varies from the structure of VL by CH 2 , can be explained by q OH addition at the carbon containing the methoxy group, succeeded by the elimination of a methoxy radical ( q OCH 3 ; Yee et al., 2013). This reaction has also been postulated for the q OH oxidation of syringol (2,6-dimethoxyphenol; Yee et al., 2013) and transformation of DMB in a system composed of guaiacyl acetone and 3 DMB * (Misovich et al., 2021). The potential imidazole derivative (C 5 H 5 N 3 O 2; no. 6 in Table S2) was observed only at pH 4, following the pH dependence of ammonium speciation (pK a = 9.25). Imidazole formation requires the nucleophilic attack of ammonia on the carbonyl group , and at pH 4, the concentration of dissolved ammonia in VL + AN was about 10 or 30 times higher than that at pH 3 or pH 2.5, respectively. For the pH values considered in this study, the O : C and H : C ratios in VL * and VL + AN had no significant differences (Figs. S7cd and S9), but molecules with higher O : C ratios (>0.6) were more abundant at pH <4. In addition, the OS c at pH <4 for both VL * and VL + AN were higher than that at pH 4, consistent with higher OS c observed at pH 5 compared to pH 7 for the q OH-mediated photo-oxidation of syringol (Sun et al., 2010). Essentially, the higher reactivity of 3 VL * and predominance of HONO over nitrite at lower pH may have resulted in higher normalized abundance of products mainly composed of oligomers and functionalized monomers.
Higher absorbance enhancement for both VL * and VL + AN (Fig. 3b) was observed as pH increased. To determine whether the pH dependence is due to the acid-base chemistry of the products or of the reactions, the changes in the UV-Vis absorption spectra of the aqSOA formed from VL * at pH 4 and 2.5 were measured over a range of pH conditions from 1.5 to 10.5 (Fig. S10). For both cases, the intensity of absorption at longer wavelengths significantly increased as the pH of the solutions was raised. Moreover, the changes in the UV-Vis absorption spectra for the two solutions of varying pH are comparable, suggesting that the observed pH dependence is rooted in the acid-base chemistry of the reactions involving 3 VL * or the excimer of VL (Smith et al., 2016), as discussed earlier.

Participation of ammonium in the direct photosensitized oxidation of VL in the presence of AN
Ammonium salts are an important constituent of atmospheric aerosols particles , and reactions between dicarbonyls (e.g., glyoxal) and ammonia or primary amines form BrC (De Haan et al., 2009Nozière et al., 2009;Shapiro et al., 2009;Lee et al., 2013;Powelson et al., 2014;Gen et al., 2018;Mabato et al., 2019). Imidazole and imidazole derivatives are the major products of glyoxal and ammonium sulfate reactions at pH 4 (Galloway et al., 2009;Yu et al., 2011;Sedehi et al., 2013;Gen et al., 2018;Mabato et al., 2019). Here, we compared VL + AN and VL + SN at pH 4 under air-saturated conditions to confirm the participation of ammonium in the photosensitized oxidation of VL. The presence of ammonium did not appear to influence the kinetics of VL decay and light absorbance changes based on VL + AN and VL + SN, thus having no statistically significant difference (p>0.05) with respect to VL decay rate constants (Table 2) and yielding comparable absorbance enhancement (Fig. 3a), respectively. However, it is important to note that this may not be the case for lower concentrations of VL. As previously stated, the reactions in this study were dominated by 3 VL * chemistry, likely due to the higher molar absorptivity of VL than that of nitrate and the high VL concentration used. Similarly, the normalized abundance of products was comparable in both experiments (A7 and A9; Table 2), with C 10 H 10 O 5 (no. 5 in Table S2) as the most abundant product (Figs. 1d and S6a), but in VL + SN, there was a significant amount of a VL dimer (C 15 H 12 O 8 ; no. 10 in Table S2). The normalized abundance of N-containing compounds was also similar for VL + AN and VL + SN, but the detected N-containing compounds were distinct. Aside from the potential imidazole derivative (C 5 H 5 N 3 O 2 ; no. 6 in Table S2), C 8 H 9 NO 3 (no. 3 in Table S2), possibly an aminophenol, was also observed from VL + AN -but only under N 2saturated conditions (Fig. 1b), probably due to further oxidation by 3 VL * . Relative to VL + AN, the products from VL + SN had higher O : C ratios (e.g., C 7 H 4 N 2 O 7 ; no. 11 in Table S2), OS c , and OS c values (Table 2). In summary, while the VL decay kinetics and absorbance enhancement for VL + AN and VL + SN were similar, the product analysis supports the participation of ammonium in the aqueousphase reactions. Figure S11 plots the DBE values vs. number of carbons (n C ; Lin et al., 2018) for the 50 most abundant products from pH 4 experiments under air-saturated conditions, along with reference to DBE values corresponding to fullerene-like hydrocarbons (Lobodin et al., 2012), cata-condensed polycyclic aromatic hydrocarbons (PAHs; Siegmann and Sattler, 2000), and linear conjugated polyenes with a general for-mula C x H x+2 . As light absorption by BrC requires uninterrupted conjugation across a significant part of the molecular structure, compounds with DBE / n C ratios (shaded area in Fig. S11) greater than that of linear conjugated polyenes are potential BrC compounds . Based on this criterion and the observed absorbance enhancement at >350 nm (Fig. 3), the majority of the 50 most abundant products from pH 4 experiments under air-saturated conditions were potential BrC chromophores composed of monomers and oligomers up to tetramers. However, as ESI-detected compounds in BB organic aerosols has been reported to be mainly molecules with n c <25 , there may be higher oligomers that were not detected in our reaction systems.

Effect of reactants concentration and molar ratios on the direct photosensitized oxidation of VL in the aqueous phase
To examine the influence of VL and AN concentration and their molar ratios on the direct photosensitized oxidation of VL, we also characterized the reaction products from lower [VL] (0.01 mM VL * ; A10;  Table S2) and C 10 H 10 O 5 (no. 5 in Table S2). For both VL * and VL + AN, the contribution of <200 m/z to the normalized abundance of products was higher at low [VL] than at high [VL], while the opposite was observed for >300 m/z (Fig. S12d). This indicates that functionalization was favored at low [VL], as supported by the higher OS c , while oligomerization was the dominant pathway at high [VL], consistent with more oligomers or polymeric products reported from high phenols concentration (e.g., 0.1 to 3 mM; Li et al., 2014;Slikboer et al., 2015;Ye et al., 2019). As the formation mechanism of dimers and higher oligomers during aqueous-phase reactions of phenolic compounds involves the coupling of phenoxy radicals (Kobayashi and Higashimura, 2003;Sun et al., 2010), the enhanced oligomerization at high [VL] can be attributed to an increased concentration of phenoxy radicals (in resonance with a carbon-centered cyclohexadienyl radical) at high [VL], promoting radical-radical polymerization (Sun et al., 2010;Li et al., 2014). At low [VL], the contribution of <200 m/z to the normalized abundance of products was higher for 1 : 1 than 1 : 100 VL / AN molar ratio, suggesting the prevalence of functionalization for the former. In addition, 1 : 1 VL / AN (A11; Table 2) had higher OS c than 1 : 100 VL / AN (A12; Table 2), indicating the forma-tion of more oxidized products but fewer N-containing compounds compared to the latter. A possible explanation is that at 1 : 1 VL / AN, VL may compete with NO − 2 for q OH (from nitrate or nitrite photolysis; Reaction 4; Table 1) and indirectly reduce q NO 2 . Similarly, hydroxylation has been suggested to be a more important pathway for 1 : 1 VL / nitrite than in 1 : 10 VL / nitrite (Pang et al., 2019a). Fragmentation, which leads to the decomposition of previously formed oligomers and generation of small, oxygenated products such as organic acids

Oxidation of guaiacol by photosensitized reactions of VL
The oxidation of phenols by 3 C * has been mainly studied using non-phenolic aromatic carbonyls (Anastasio et al., 1997;Smith et al., , 2015Yu et al., 2014;Chen et al., 2020) and aromatic ketones (Canonica et al., 2000) as triplet precursors. Recently, 3 VL * has been shown to oxidize syringol (Smith et al., 2016), a non-carbonyl phenol, although the reaction products remain unknown. In this section, we discussed the photo-oxidation of guaiacol (GUA), a noncarbonyl phenol that is also a lignocellulosic BB pollutant (Kroflič et al., 2015), in the presence of VL (GUA + VL).
The dark experiments did not show any substantial loss of VL or GUA (Fig. S3c). Due to its poor light absorption in the solar range, GUA is not an effective photosensitizer Yu et al., 2014). Accordingly, direct GUA photodegradation resulted in minimal decay, which plateaued after ∼ 3 h. In the presence of VL, the GUA decay rate constant was 2.2 times higher due to the oxidation of GUA by 3 VL * . The decay rate constant of VL in GUA + VL (A14; Table 2) was 3 times slower than that of VL * (A5; Table 2), which may be due to competition between ground-state VL and GUA for reactions with 3 VL * or the increased conversion of 3 VL * back to the ground state through the oxidation of GUA (Anastasio et al., 1997;. For GUA experiments, the normalized abundance of products was calculated only for GUA + VL (2.2; Table 2) as the GUA signal from the UHPLC-qToF-MS in the positive ion mode was weak, which may introduce large uncertainties during normalization. Nonetheless, the number of products detected from these experiments (178 and 844 for direct GUA photodegradation and GUA + VL, respectively) corroborates the kinetics results. The major products (Fig. 4a) from direct GUA photodegradation were C 14 H 14 O 4 (no. 13 in Table S2), a typical GUA dimer, and C 21 H 20 O 6 (no. 14 in Table S2), a trimer which likely originated from photoinduced O-H bond breaking (Berto et al., 2016). In general, higher absolute signal intensities were noted for oligomers (e.g., C 14 H 14 O 4 and C 21 H 20 O 6 ; nos. 13 and 14 in Table S2, respectively) and hydroxylated products (e.g., C 7 H 8 O 4 ) in GUA + VL, similar to those observed from GUA oxidation by 3 DMB * or q OH (from H 2 O 2 photolysis; Yu et al., 2014;Jiang et al., 2021). Also, a potential GUA tetramer (C 28 H 26 O 8 ; no. 15 in Table S2) was observed only in GUA + VL, consistent with more efficient oligomer formation from the triplet-mediated oxidation of phenols relative to direct photodegradation . The products from the direct GUA photodegradation and GUA + VL had mostly similar OS c values (−0.5 to 0.5; Fig. 4b, c), falling into the criterion of biomass burning organic aerosol (BBOA) and semivolatile oxygenated organic aerosol (SV-OOA; Kroll et al., 2011). The corresponding absorbance changes for the GUA experiments (Fig. 3c) were consistent with the observed VL and GUA decay trends and detected products. While minimal absorbance changes, which also plateaued after ∼ 3 h, were observed for direct GUA photodegradation, significant and continuous absorbance enhancement was noted for GUA + VL. Compared to direct GUA photodegradation, GUA oxidation by photosensitized reactions of VL occurred rapidly and yielded higher absolute signal intensities for oligomers and hydroxylated products, which likely contributed to the pronounced absorbance enhancement.

Conclusions and atmospheric implications
In this study, the direct photosensitized oxidation of VL in the absence and presence of AN under atmospherically relevant cloud and fog conditions have been shown to generate aqSOA composed of oligomers, functionalized monomers, oxygenated ring-opening products, and nitrated compounds (from VL + AN). The oligomers from these reaction systems may be rather recalcitrant to fragmentation, based on their high normalized abundance, even at the longest irradiation time used in this study. Nonetheless, the increasing concentration of small organic acids over time implies that fragmentation becomes important at extended irradiation times. The reactions were observed to be influenced by O 2 , pH, and re-actants concentration and molar ratios. Our results suggest that O 2 could be required for the secondary steps in VL decay (e.g., the reaction of ketyl radical and O 2 ) via 3 VL * to proceed. Compared to N 2 -saturated conditions, 3 VL * -initiated reactions under air-saturated conditions (O 2 is present) proceeded rapidly, promoted the formation of more oxidized aqSOA, and generated products (e.g., oligomers, functionalized monomers, and N-containing compounds) with higher normalized abundance which exhibited stronger light absorption. For pH 4 experiments, the presence of both O 2 and nitrate resulted in the highest normalized abundance of products (including N-containing compounds) and OS c , which is attributed to O 2 promoting VL nitration. Nevertheless, further work on the effect of O 2 on the reactive intermediates involved in the reactions is necessary to elucidate the mechanisms of direct photosensitized oxidation of VL under air-saturated conditions. Additionally, the formation of oligomers from the direct photosensitized oxidation of VL was promoted at low pH (<4). Low VL concentration favored functionalization, while oligomerization prevailed at high VL concentration, consistent with past works (Li et al., 2014;Slikboer et al., 2015;Ye et al., 2019). Hydroxylation was observed to be important at equal molar ratios of VL and nitrate, likely due to VL competing with nitrite for q OH. Furthermore, the GUA experiments indicate that, in mixed biomass burning aerosols, triplet excited states of phenolic aromatic carbonyls can oxidize phenols, forming oligomers and hydroxylated products. Aromatic carbonyls and nitrophenols have been reported to be the most significant classes of BrC in cloud water heavily affected by biomass burning in the North China Plain (Desyaterik et al., 2013). Correspondingly, the most abundant products from our reaction systems (pH 4 and air-saturated solutions) are mainly potential BrC chromophores. These suggest that aqSOA generated in cloud and fog water from the oxidation of biomass burning aerosols via direct photosensitized reactions and nitrate photolysis products can impact aerosol optical properties and radiative forcing, particularly for areas where biomass burning is intensive.
Ammonium (and sodium) nitrate was not found to substantially affect the VL decay rate constants, likely due to the much higher molar absorptivity of VL than nitrate and high VL concentration used in this work. However, the presence of ammonium (and sodium) nitrate promoted functionalization and nitration, indicating the significance of nitrate photolysis for aqSOA formation from biomass-burning-derived compounds. This work demonstrates that nitration, which is an important process for producing light-absorbing organics or BrC (Jacobson, 1999;Kahnt et al., 2013;Mohr et al., 2013;Laskin et al., 2015;Teich et al., 2017;, can also affect the aqueous-phase processing of triplet-generating aromatics. In addition, a potential imidazole derivative observed from VL + AN at pH 4 reveals that ammonium participates in aqSOA formation from the photooxidation of phenolic aromatic carbonyls. This observation also suggests that the photosensitized oxidation of phenolic aromatic carbonyls in the presence of AN could be a source of imidazoles in the aqueous phase. It is important to understand the source of imidazoles due to their possible effects on human health, their photosensitizing potential, and their effect on aerosol optical properties as BrC compounds (Teich et al., 2016).
A recent work (Ma et al., 2021) mimicking phenol oxidation by 3 DMB * (a non-phenolic aromatic carbonyl) in more concentrated conditions of aerosol particles containing high AN concentration (0.5 M) increased the photodegradation rate constant for guaiacyl acetone (an aromatic phenolic carbonyl with high Henry's law constant; 1.2×10 6 M atm −1 ; McFall et al., 2020) by >20 times, which was ascribed to q OH formation from nitrate photolysis (Brezonik and Fulkerson-Brekken, 1998;Chu and Anastasio, 2003). The same study also estimated that reactions of phenols with high Henry's law constants (10 6 to 10 9 M atm −1 ) can be important for SOA formation in aerosol particles, with mechanisms mainly governed by 3 C * and 1 O 2 (Ma et al., 2021). Likewise, Zhou et al. (2019) reported that the direct photodegradation of acetosyringone was faster by about 6 times in the presence of 2 M NaClO 4 . However, the opposite was noted for the photodegradation of VL in sodium sulfate or sodium nitrate, which would occur slower (∼ 2 times slower in 0.5 M sodium sulfate and ∼ 10 times slower in 0.124 M sodium nitrate) in aerosol particles relative to dilute aqueous phase in clouds (Loisel et al., 2021), implying that the nature of inorganic ions may have an essential role in the photodegradation of organic compounds in the aqueous phase. The concentrations of VL and nitrate can be significantly higher in aqueous aerosol particles than what we have used to mimic cloud and fog water. As a major component of aerosols, nitrate can have concentrations as high as sulfate . More studies should then explore the direct photosensitized oxidation of other biomass-burningderived phenolic aromatic carbonyls, particularly those with high molar absorption coefficients. Based on our findings, the presence of nitrate should be considered for examining aqSOA formation from these reactions. The influences of reaction conditions should also be investigated to better understand the oxidation pathways. As aerosols comprise more complex mixtures of organic and inorganic compounds, it is worthwhile to explore the impacts of other potential aerosol constituents on aqSOA formation and photo-oxidation studies. This can also be beneficial for understanding the interplay among different reactions during photo-oxidation. Considering that biomass burning emissions are expected to increase continuously, further studies on these aqSOA formation pathways are strongly suggested.
Data availability. The data used in this publication are available to the community and can be accessed on request to the corresponding author.