Comparison of aqueous secondary organic aerosol (aqSOA) product distributions from guaiacol oxidation by non-phenolic and phenolic methoxybenzaldehydes as photosensitizers in the absence and presence of ammonium nitrate

. Aromatic carbonyls (e

Most previous studies on aqSOA formation via photosensitized non-carbonyl phenol oxidation have examined 3,4dimethoxybenzaldehyde (DMB), a non-phenolic methoxybenzaldehyde, as the photosensitizer Yu et al., 2014Yu et al., , 2016Ye et al., 2019;Chen et al., 2020;Jiang et al., 2021;Ma et al., 2021;Ou et al., 2021;. By contrast, phenolic carbonyls have been mainly studied as aqSOA precursors via q OH-, nitrate-, nitrite-, and 3 DMB*-mediated oxidation (Li et al., 2014;Huang et al., 2018;Pang et al., 2019;. However, strongly lightabsorbing phenolic carbonyls (e.g., molar absorptivity above 300 nm ≥ 7×10 3 M −1 cm −1 ) can also serve as photosensitizers to promote aqSOA formation Mabato et al., 2022). For instance, the direct photosensitized oxidation of phenolic carbonyls (i.e., oxidation of phenolic carbonyls by their 3 C * or 3 C * -derived oxidants) such as vanillin (VL; another methoxybenzaldehyde) efficiently forms lowvolatility products, with aqSOA mass yields of up to 140 % . Moreover, the aqSOA mass yields from the oxidation of syringol by 3 DMB * and 3 VL * are similar (111 % and 114 %, respectively;Smith et al., 2014Smith et al., , 2016. In addition, we recently reported that the direct photosensitized oxidation of VL and guaiacol oxidation by 3 VL * yield similar products (oligomers, functionalized monomers, and oxygenated ring-opening products) as observed with 3 DMB * Mabato et al., 2022). Guaiacol is a noncarbonyl BB methoxyphenol with an emission rate from fireplace wood combustion in the range of 172 to 279 mg kg −1 Simoneit, 2002). The atmospheric reactivity of methoxyphenols has recently been reviewed (Liu et al., 2022). However, our previous experiments  were performed at a concentration (0.1 mM VL) higher than what was typically used for DMB (0.005 to 0.01 mM; Smith et al., 2014Smith et al., , 2015Yu et al., 2014Yu et al., , 2016. Therefore, direct comparisons between photosensitization by 3 DMB * and 3 VL * cannot be made. Despite the above findings, much is still unknown about how aqSOA formation proceeds in systems using phenolic carbonyls as photosensitizers. BB aerosols are typically internally mixed with other aerosol components, such as ammonium nitrate (AN; Zielinski et al., 2020). Hence, aromatic carbonyls and phenols may coexist with AN in BB aerosols. Nitrate and ammonium facilitate the formation of aqSOA and brown carbon (BrC) via a number of pathways. Nitrate photolysis can produce q OH and nitrating agents (e.g., q NO 2 ; Minero et al., 2007;Huang et al., 2018;Mabato et al., 2022;Wang et al., 2022;Yang et al., 2022), and ammonium reacts with carbonyls to yield N-containing heterocycles (e.g., imidazoles) and oligomers capable of UV-Vis light absorption (De Haan et al., 2009Nozière et al., 2009Nozière et al., , 2010Nozière et al., , 2018Shapiro et al., 2009;Yu et al., 2011;Lee et al., 2013;Powelson et al., 2014;Gen et al., 2018;Grace et al., 2019;Mabato et al., 2019). Furthermore, nitrate photolysis may be an important process for SO 2 oxidation and secondary organic aerosol (SOA) formation in the particle phase (Gen et al., 2019a(Gen et al., , 2019bZhang et al., 2020, and it can potentially modify the morphology of atmospheric viscous particles (Liang et al., 2021). Yet, understanding of the effects of inorganic nitrate on aq-SOA formation remains limited. In addition, aqSOA formation studies involving aromatic carbonyls and phenols have probed either photosensitization or nitrate-mediated photooxidation, but these reactions can occur simultaneously. For instance, we previously reported nitrated compounds, including a potential imidazole derivative from the direct photosensitized oxidation of VL in the presence of AN . Accordingly, investigations on reaction systems including both photosensitizers and AN may provide further insights into the aqueous-phase processing of BB aerosols. In this work, we compared aqSOA formation from photosensitized guaiacol (GUA) oxidation by 3 C * of non-phenolic and phenolic methoxybenzaldehydes under identical condi-  . The top of the figure also shows the structures of DMB, VL, and GUA. tions (simulated sunlight and reactant concentration) relevant to cloud and fog waters. The effects of AN on photosensitized aqSOA formation were also examined. In this study, the dominant aqSOA precursor is GUA (Henry's law constant of 9.2 × 10 2 M atm −1 ; Sagebiel et al., 1992), and DMB and VL were used as photosensitizers to oxidize GUA. DMB and VL (Henry's law constants of 7.3 × 10 3 and 4.7 × 10 5 M atm −1 , respectively;Yaws, 1994;EPI Suite version 4.1 -US EPA, 2012;Felber et al., 2021), which are also abundant in BB emissions Li et al., 2014;Chen and Anastasio, 2017;Pang et al., 2019;Mabato et al., 2022) and whose structures differ only by one functional group (−OCH 3 for the former and −OH for the latter, Fig. 1), represented non-phenolic and phenolic methoxybenzaldehydes, respectively. The structures of GUA, DMB, and VL are provided in Fig. 1. Based on their quantum yield of 3 C * formation, DMB and VL have been classified as moderate and poor photosensitizers, respectively (Felber et al., 2021). The photosensitized oxidation of GUA by 3 DMB * and 3 VL * (separately) in the absence (and presence) of AN is referred to as GUA + DMB( + AN) and GUA + VL( + AN), respectively. GUA photo-oxidation by AN alone (GUA + AN) was also explored for comparison with GUA + DMB + AN and GUA + VL + AN. The molar absorptivities of GUA, DMB, VL, and nitrate are shown in Fig. 1. The precursor and photosensitizer decay kinetics, detected products, and absorbance enhancement were used to characterize the reactions. However, it should be noted that we mainly focused on the analyses of the reaction products and product distribution.
While several studies on photo-oxidation of BB emissions are available, this work focuses on the comparison between non-phenolic and phenolic methoxybenzaldehydes as photosensitizers in the absence and presence of AN for aqSOA formation. We found that GUA oxidation by 3 DMB * was faster and exhibited greater light absorption relative to GUA + VL. These are likely attributed to the stronger photosensitizing ability of DMB and the −OH group of VL, making it more prone to oxidation and more reactive towards electrophilic aromatic substitution. Oligomerization and functionalization dominated in GUA + DMB and GUA + VL, but functionalization was more significant in GUA + VL due to VL transformation products. Although AN did not significantly influence the oxidation kinetics due to the predominant role of photosensitizer chemistry compared to nitrate, AN promoted the formation of N-containing products. These include N-heterocycles (e.g., imidazoles), suggesting the participation of ammonium in the reactions. Moreover, the product distributions indicate distinct interactions between photosensitization by 3 DMB * and 3 VL * and AN photolysis. In particular, AN generated more N-containing products in GUA + DMB + AN than in GUA + VL + AN and increased the oligomers in GUA + VL + AN. Furthermore, increased nitrated compounds in GUA + DMB + AN and GUA + VL + AN compared to GUA + AN suggest that photosensitized reactions may promote reactions by nitrate photolysis.

Aqueous-phase photo-oxidation experiments
Procedures for the photo-oxidation experiments are presented in detail in our previous study . Experimental solutions were prepared using 0.1 mM guaiacol (GUA, Sigma-Aldrich, ≥ 98.0 %) and 0.01 mM 3,4dimethoxybenzaldehyde (DMB, Acros Organics, 99 + %) or 0.01 mM vanillin (VL, Acros Organics, 99 %, pure), in the absence and presence of ammonium nitrate (1 mM; AN, Acros Organics, 99 + %, for analysis). These GUA and methoxybenzaldehyde concentrations are within the values expected in cloud or fog drops in areas with significant wood combustion (Anastasio et al., 1997;Rogge et al., 1998;Nolte et al., 2001). The AN concentration represents values usually observed in cloud and fog waters (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 must be noted that this study did not intend to identify the AN concentrations that would affect the kinetics but attempted to analyze the effects of AN on photosensitized aqSOA formation. A solution composed of 0.1 mM GUA and 1 mM AN (GUA + AN) was also examined for comparison with GUA + DMB + AN and GUA + VL + AN. Sulfuric acid (H 2 SO 4 ; Acros Organics, ACS reagent, 95 % solution in water) was used to adjust the pH of the solutions to 4, which is within typical cloud pH values (2-7; Pye et al., 2020) and pH values observed in wood-burningimpacted cloud and fog waters (Collett et al., 1998;Raja et al., 2008). The solutions (initial volume of 500 mL) were bubbled with synthetic air (0.5 dm 3 min −1 ) for 30 min be-fore irradiation and throughout the reactions to achieve airsaturated conditions (Du et al., 2011;Chen et al., 2020) and were continuously magnetically stirred. In this study, the reactions can generate 3 DMB * / 3 VL * and secondary oxidants ( 1 O 2 , O q − 2 / q HO 2 , q OH) but not ozone. Solutions contained in a quartz photoreactor were irradiated using a xenon lamp (model 6258, ozone-free xenon lamp, 300 W, Newport) equipped with a long-pass filter (20CGA-305 cut-on filter, Newport) to eliminate light below 300 nm. The reaction temperatures were maintained at 27 ± 2 • C using cooling fans positioned around the photoreactor and lamp housing. The averaged initial photon flux in the reactor measured from 300 to 380 nm was ∼ 3 × 10 15 photons cm −2 s −1 nm −1 (Fig. 1), similar to our previous work . Samples were collected every 30 min for 180 min for offline analyses of (1) GUA, DMB, and VL concentrations using ultra-high-performance liquid chromatography with a photodiode array (UHPLC-PDA) detector and (2) absorbance measurements using UV-Vis spectrophotometry. Moreover, the samples collected before and after irradiation (180 min) were analyzed for (3) reaction products using UHPLC coupled with heated electrospray ionization Orbitrap mass spectrometry (UHPLC-HESI-Orbitrap-MS) operated in positive and negative ion modes and (4) concentrations of small organic acids using ion chromatography (IC). Each experiment was repeated independently at least three times. The reported decay rate constants, small-organic-acid concentration, and absorbance enhancement were averaged from triplicate experiments, and the corresponding errors represent 1 standard deviation. The pseudo-first-order rate constant (k ) for GUA decay was determined using the following equation : where [GUA] t and [GUA] 0 are GUA concentrations at time t and 0, respectively. DMB or VL decay rate constants were calculated by replacing GUA with DMB or VL in Eq. (1). The decay rate constants were normalized to the photon flux measured for each experiment through dividing k by the measured 2-nitrobenzaldehyde (2NB; a chemical actinometer) decay rate constant, j (2NB) . In addition, the decay rate constants were corrected for the internal light screening due to DMB, VL, and AN (Leifer, 1988;Zhang and Anastasio, 2003;Smith et al., 2014Smith et al., , 2015Smith et al., , 2016. The values of the internal light screening factor (S λ ) determined around the peak in the light absorption action spectrum (DMB: 310-335 nm; VL: 304-364 nm; nitrate: 300-331 nm)  for an 8.5 cm cell were 0.95 for GUA + AN, 0.51 for GUA + DMB, 0.54 for GUA + DMB + AN, 0.57 for GUA + VL, and 0.59 for GUA + VL + AN. Moreover, two independently prepared samples for each reaction condition were analyzed using UHPLC-HESI-Orbitrap-MS. Only peaks that were reproducibly detected in both sets of samples were considered. For clarity, the formulas discussed in this work correspond to neutral analytes (e.g., with H + or NH + 4 removed from the ion formula). The details of the analytical procedures are provided in the Supplement (Sects. S1 to S4).

Calculation of normalized abundance of products
Several recent studies have used comparisons of relative abundance of products based on peak areas from mass spectrometry (MS) results (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 (K. . However, comparisons of relative abundance among different compounds can be subject to uncertainties as ionization efficiencies in soft ionization, such as electrospray ionization (ESI), may significantly vary between different compounds (Kebarle, 2000;Schmidt et al., 2006;Leito et al., 2008;Perry et al., 2008;Kruve et al., 2014). In our previous work , we introduced the normalized abundance of products ([P], unitless) (Eq. 2) as a semiquantitative analysis that gives an overview of how the signal intensities changed under different experimental conditions but not the quantification of the absolute product concentration. The calculation assumes equal ionization efficiencies of different compounds, which is commonly used to estimate O : C ratios of SOA (Bateman et al., 2012;Lin et al., 2012;Laskin et al., 2014;De Haan et al., 2019): where A P,t , and A GUA,t are the extracted ion chromatogram (EIC) peak areas of the product P and GUA from UHPLC-HESI-Orbitrap-MS analyses at time t, respectively, and [GUA] t and [GUA] 0 are the GUA concentrations (µM) determined using UHPLC-PDA at time t and 0, respectively. Note that the normalized abundance of products has intrinsic uncertainties due to the variability in ionization efficiencies for various compounds. Moreover, it should be noted that the normalized abundance of products was calculated using only the positive ion mode data as the GUA signal from the negative ion mode was weak and thus may present large uncertainties during normalization. Therefore, products that may not give signals or may have weak signals in the positive ion mode were possibly underestimated in the normalized product abundance. Nevertheless, it enables the comparison of MS results among different experiments. As demonstrated in our previous work  and the current study, a higher normalized abundance of products generally correlates with higher efficiency of oxidation. The reported uncertainties were propagated from the changes in [GUA] measured using UHPLC-PDA and the MS signal intensities.

Results and discussion
Using kinetics data, MS analyses, and absorbance enhancement data, we first examined the differences between GUA + DMB and GUA + VL (Sect. 3.1). Then, we analyzed GUA + DMB + AN, GUA + VL + AN, and GUA + AN (Sect. 3.2) to explore the effects of nitrate photolysis and ammonium on photosensitized aqSOA formation.
3.1 Comparison of photosensitized GUA oxidation by non-phenolic ( 3 DMB * ) and phenolic ( 3 VL * ) methoxybenzaldehydes Prior studies have reported that photosensitized noncarbonyl phenol oxidation in the presence of 3,4dimethoxybenzaldehyde (DMB) and vanillin (VL) (separately) is mainly driven by 3 DMB * and 3 VL * , respectively Mabato et al., 2022), while contributions from secondary oxidants such as 1 O 2 and q OH are likely minor. However, both 3 DMB * and 3 VL * are efficiently quenched by O 2 , suggesting that energy transfer should be considered in evaluating photosensitized processes involving these methoxybenzaldehydes (Felber et al., 2021). Moreover, it was found that 3 DMB * , 1 O 2 , and O q − 2 were the major contributors to the photosensitized oxidation of 4-ethylguaiacol (Chen et al., 2020). Recently, the oxidation of guaiacyl acetone (a non-conjugated phenolic carbonyl) in the presence of DMB has been reported to be initiated by 3 DMB * , 1 O 2 , q OH, or the methoxy radical ( q OCH 3 ) . Further studies are thus required to identify the specific oxidants in these reaction systems. In this study, reactions initiated in the presence of DMB or VL are collectively referred to as photosensitized reactions. The reaction conditions, initial guaiacol (GUA) and DMB or VL decay rate constants, normalized product abundance, and chemical characteristics of aqSOA formed in this work are summarized in Table 1.
3.1.1 Kinetic analysis of photosensitization by 3 DMB * and 3 VL * No significant loss of GUA or photosensitizers was observed for dark experiments (p > 0.05). Figure S1 shows the decay of GUA, DMB, and VL under different experimental conditions. Upon irradiation, the GUA decay rate constant in GUA + DMB was ∼ 4 times higher than in GUA + VL.
In GUA + DMB, the decay rate constant of GUA was ∼ 8 times higher than that of DMB, consistent with a previous study . Contrastingly, the decay rate constant of VL was 2.4 times higher than that of GUA in GUA + VL. This VL consumption was also observed in our earlier work using 0.1 mM GUA + 0.1 mM VL . These trends could be explained by the following reasons. First, DMB has a stronger photosensitizing ability than VL based on its higher quantum yield of 3 C * formation and longer lifetime of 3 DMB * compared to 3 VL * (Fel- ber et al., 2021). Second, VL is also a phenolic compound similar to GUA and is therefore highly reactive towards oxidation. For instance, its −OH group can be oxidized by 3 VL * via H-atom abstraction to form phenoxy radicals which can undergo coupling to form oligomers (Kobayashi and Higashimura, 2003;Sun et al., 2010;Mabato et al., 2022). The faster consumption of VL than GUA suggests a competition between ground-state VL and GUA for reaction with 3 VL * .
Moreover, compared to a −OCH 3 group (in DMB), a −OH group (in VL) has a stronger electron-donating ability and is thus more activating towards electrophilic aromatic substitution. It should be noted that the differences in the GUA decay rate constants among different reaction systems are not quantitatively equivalent to photosensitizing efficiencies, and a detailed quantitative analysis of this is beyond the scope of this study. Nonetheless, these results suggested that GUA oxidation in GUA + DMB was overall more efficient than in GUA + VL. Our kinetic analysis focused on the decay rate constants of the aqSOA precursor (GUA) and the photosensitizers (DMB and VL) during photosensitization under the same experimental conditions (same aqSOA precursor and concentration, same photosensitizer concentration, and same lamp photon flux). The effects of other factors (e.g., intersystem crossing efficiency) on the rate constants were not examined. Explicit kinetic studies (e.g., Smith et al., 2014Smith et al., , 2015 that measure second-order rate constants should be conducted in the future to extend the applicability of the kinetic parameters to other conditions.
3.1.2 Product distributions and chemical characteristics of aqSOA from photosensitization by 3 DMB * and 3 VL * The products detected using UHPLC-HESI-Orbitrap-MS were used to characterize the aqSOA formed in this work. The signal-weighted distributions of aqSOA calculated from combined positive (POS) and negative (NEG) ion mode MS  results are summarized in Fig. 2. The signal-weighted distributions calculated separately from POS and NEG ion mode MS results are available in Figs. S2 and S3. It should be noted that in this work, the product distributions for all experiments were based on the same irradiation time of 180 min. An irradiation time of 180 min was chosen as it was sufficient to show the differences in the extent of reaction of GUA among the reaction systems studied. For reaction systems with precursors of different reactivities, chemical analysis at a fixed reaction time may be looking at different generations of products of each precursor, as Yu et al. (2014) reported. Measuring the product distribution at a fixed time might have missed the information on what and/or how many products are formed at similar quantities of precursors reacted. The situation could be even more complicated if different precursors had major differences in pathways and dominant intermediates. However, comparing the product distributions after a certain time of light exposure, as is the case for this study, is useful to evaluate what products would form after a certain time of photosensitization. Oligomers and derivatives of GUA dominated both GUA + DMB and GUA + VL, in agreement with pronounced oligomerization from triplet-mediated oxidation of relatively high phenol concentration (e.g., 0.1 to 3 mM; Li et al., 2014;Yu et al., 2014Yu et al., , 2016Slikboer et al., 2015;Ye et al., 2019;Mabato et al., 2022). Figure 3 schematically depicts the main differ-ences between photosensitized GUA oxidation by 3 DMB * and 3 VL * in the absence and presence of AN. As shown in Fig. 3, 3 DMB * and 3 VL * can oxidize GUA via H-atom abstraction to form phenoxy radicals which undergo coupling to form oligomers (Kobayashi and Higashimura, 2003;Sun et al., 2010;Mabato et al., 2022). The higher oligomer contribution in GUA + DMB is likely due to the better photosensitizing ability of DMB than VL and partly the lower abundance of 3 VL * due to fast VL consumption. VL was consumed faster than DMB during GUA oxidation ascribable to the −OH group of VL, making it more susceptible to oxidation and more reactive towards electrophilic aromatic substitution. In addition, the normalized product abundance for GUA + DMB was ∼ 4 times higher than that for GUA + VL (Table 1), further suggesting more efficient photosensitized GUA oxidation by 3 DMB * than by 3 VL * . The oxidation of GUA or transient organic intermediates by secondary oxidants (e.g., 1 O 2 and q OH) from 3 DMB * or 3 VL * and the fragmentation of larger compounds generate highly oxidized ring-opening products Huang et al., 2018;Chen et al., 2020). GUA + DMB had a higher contribution of ring-opening products than GUA + VL, likely due to the greater availability of secondary oxidants in the former and fast VL consumption lowering the production of these species in GUA + VL. The IC analyses also indicate the formation of small organic acids (e.g., formic acid), which ap-peared to have higher concentrations in the presence of DMB than in VL (Fig. S4). Although no data are available for the concentration changes (every 30 min) of small organic acids during the reaction, it is likely that an increasing trend would be observed because fragmentation, which leads to the decomposition of initially formed oligomers and the generation of smaller oxygenated products, becomes important at longer irradiation times . This trend has also been observed in our previous work on the direct photosensitized oxidation of VL , as well as other studies on photosensitized oxidation of non-carbonyl phenols and phenolic carbonyls (e.g., Yu et al., 2016;Jiang et al., 2021). The reactions of secondary oxidants or ringopening products with GUA can form functionalized products. Notably, the contribution of monomers in GUA + VL was almost twice as high as in GUA + DMB, ascribable to VL transformation products. We previously showed that for the direct photosensitized oxidation of VL, functionalization prevails over oligomerization at 0.01 mM VL, the [VL] used in this work, while oligomerization dominates at higher [VL] (0.1 mM; Mabato et al., 2022). It has been reported that oligomerization could occur during the electrospray ionization process (Yasmeen et al., 2010). In this work, it was confirmed that the oligomers observed were generated in the solutions via aqueous reactions instead of being artifacts of HESI-MS. This is based on the absence of dimers and higher oligomers in the HESI mass spectra of dark control solutions acquired by direct infusion (Yu et al., 2016).
The higher OS C for GUA + VL than in GUA + DMB (Table 1) was probably due to the significant functionalization in the former. Moreover, the distributions of OS C and carbon number (Fig. S6a-d) show that these aqSOA products have similar elemental composition to those of low-volatility oxygenated organic aerosol (LV-OOA) and semi-volatile oxygenated organic aerosol (SV-OOA) and somewhat similar elemental composition to those of biomass burning organic aerosol (BBOA) (Kroll et al., 2011). Further discussions on van Krevelen diagrams (Fig. S5a-d) and OS C vs. carbon number plots (Fig. S6a-d) for GUA + DMB and GUA + VL aqSOA are available in the Supplement (Sect. S5). In brief, 3 DMB*-initiated GUA oxidation was faster and yielded higher normalized product abundance than oxidation by 3 VL * . This is likely due to the stronger photosensitizing ability of DMB than VL and the −OH group of VL facilitating its rapid consumption. In addition, oligomerization and functionalization dominated in both GUA + DMB and GUA + VL, as reported in similar studies (Yu et al., , 2016Chen et al., 2020;Jiang et al., 2021;Mabato et al., 2022). However, functionalization was more prominent in the latter, attributable to the transformation of VL. Nonetheless, it must be noted that for phenolic aqSOA, fragmentation will ultimately be more predominant at longer irradiation times (Yu et al., 2016;Huang et al., 2018;Mabato et al., 2022).

Light absorption of aqSOA from
photosensitization by 3 DMB * and 3 VL * The absorbance enhancement of phenolic aqSOA generated via reactions with 3 C * has been linked to the formation of conjugated structures due to oligomerization and functionalization (e.g., additions of hydroxyl and carbonyl groups; Yu et al., 2014Yu et al., , 2016Smith et al., 2016;Ye et al., 2019;Chen et al., 2020;Jiang et al., 2021;Ou et al., 2021;Mabato et al., 2022;Wang et al., 2022). Moreover, the aqueousphase photo-oxidation of BB emissions can enhance BrC absorbance via the formation of aromatic dimers and functionalized products (Hems et al., 2020). The increase in light absorption throughout 180 min of irradiation and the change in the rate of sunlight absorption ( R abs )  from 350 to 550 nm at 180 min during typical clear and haze days in Beijing, China, for all the reaction systems studied are provided in Fig. 4. Figure S7 shows the absorption spectra after 180 min of irradiation for each reaction system studied. In this work, the absorbance enhancement of GUA + DMB and GUA + VL (Fig. 4a) could be due to oligomers and functionalized monomers, which are the highest contributors to the product signals. Identifying the chromophores responsible for the absorbance enhancement may be beneficial in understanding the impact of aqSOA on the Earth's radiative balance and determining the reactions that affect light absorption by aqSOA . How-ever, the detected products did not exhibit distinct peaks in the UHPLC-PDA chromatograms, likely due to the concentration of the chromophores being below the detection limit of the PDA. Nevertheless, the higher absorbance enhancement and R abs for GUA + DMB compared to GUA + VL were probably due to the higher contribution and normalized abundance (by ∼ 6 times) of oligomers in the former. Additional information about aqSOA light absorption can be deduced from the plots of the double-bond equivalent (DBE) values vs. carbon number (n C ) . Figure S8 shows these plots along with the DBE reference values of fullerene-like hydrocarbons (Lobodin et al., 2012), catacondensed polycyclic aromatic hydrocarbons (PAHs; Siegmann and Sattler, 2000), and linear conjugated polyenes with a general formula C x H x+2 . The shaded area indicates a sufficient level of conjugation for visible light absorption, and species within this region are potential BrC chromophores. GUA + DMB and GUA + VL aqSOA exhibited a significant overlap in the DBE vs. n C space; nearly all products from both systems, including the high-relative-abundance species, are potential BrC chromophores. GUA + DMB had more oligomeric products with high relative abundance (n C ≥ 12 and DBE ≥ 8). For GUA + VL, high-relative-abundance products also include monomeric species (n C = 7 to 8 and 4 to 5 DBE) corresponding to hydroxylated products (e.g., C 7 H 8 O 4 and C 8 H 8 O 5 ; nos. 28 and 35; Table S2). These observations further indicate the importance of oligomerization and functionalization for the absorbance enhancement of aq-SOA generated via photosensitization by 3 DMB * and 3 VL * . In summary, 3 DMB * and 3 VL * can oxidize GUA, resulting in aqSOA and BrC formation, but GUA + DMB products exhibited stronger light absorption. In GUA + VL, the extent of GUA oxidation was limited by significant VL consumption.

Comparison of photosensitized GUA oxidation by
non-phenolic ( 3 DMB * ) and phenolic ( 3 VL * ) methoxybenzaldehydes in the presence of AN 3.2.1 Kinetic analysis of photosensitization by 3 DMB * and 3 VL * in the presence of AN Ammonium nitrate (AN) did not significantly affect (p > 0.05) the decay rate constants of GUA, DMB, and VL for both GUA + DMB + AN and GUA + VL + AN (Table 1), likely due to the higher molar absorptivities of the photosensitizers compared to that of nitrate. This implies that the chemistry of 3 DMB * and 3 VL * dominated that of nitrate. In this work, the GUA decay rate constants decreased in the order of GUA + DMB and/or GUA + DMB + AN > GUA + VL and/or GUA + VL + AN > GUA + AN (Table 1). Note that as the molar absorptivities of the photosensitizers are higher than that of nitrate, the kinetics data were also analyzed on a per-photon-absorbed basis for a more appropriate comparison of reaction efficiency (Sect. S6).
3.2.2 Product distributions and chemical characteristics of aqSOA from photosensitization by 3 DMB * and 3 VL * in the presence of AN For both GUA + DMB + AN and GUA + VL + AN, AN had no significant effect on the normalized product abundance ( Table 1), but it induced the formation of N-containing products composed of N-heterocycles (e.g., imidazoles and pyridines) and oligomers, as well as nitrated species. Similarly, we previously reported a potential imidazole derivative from the direct photosensitized oxidation of VL in the presence of AN, which was attributed to the reaction of ring-opening products with dissolved ammonia . Oligomers remained the highest signal contributors in the presence of AN (Fig. 2), but interactions between photosensitization by 3 DMB * and 3 VL * and AN photolysis were distinct. First, nitrated species had similar contributions in both cases, but the contribution and normalized abundance of all N-containing products in GUA + DMB + AN were 2 and ∼ 14 times higher, respectively, than in GUA + VL + AN. This difference can be attributed to the higher contribution of N-heterocycles and N-containing oligomers in GUA + DMB + AN. Compared to GUA + VL, GUA + DMB had a higher contribution of ring-opening products which can react with ammonia, as discussed earlier (Figs. 2 and 3). Second, the decrease in oligomers in GUA + DMB + AN may be due to their fragmentation induced by q OH from nitrate photolysis and then conversion to N-containing products. Correspondingly, the contribution of possibly ring-retaining N-containing products in GUA + DMB + AN (18.6 %) was ∼ 3 times higher than that in GUA + VL + AN (6.5 %). While fragmentation of oligomers likely occurred in GUA + VL + AN as well, the increase in oligomers suggests that other reactions have taken place. For GUA + VL + AN, q OH or q NO 2 from nitrate photolysis may have initiated H-atom abstraction from the −OH group of VL, generating phenoxy radicals which can undergo coupling to form more oligomers (Kobayashi and Higashimura, 2003;Sun et al., 2010;Mabato et al., 2022). This may also explain the more significant decrease in monomers in GUA + VL + AN (∼ 3 times) compared to GUA + DMB + AN (∼ 2 times). Similarly, we previously observed an increase in oligomers during the direct photosensitized oxidation of 0.01 mM VL , the [VL] used in this work, upon adding 1 mM AN. These findings indicate that photosensitization by non-phenolic and phenolic methoxybenzaldehydes may interact differently with AN photolysis. q HO 2 , q OH) can be formed from 3 DMB * and 3 VL * upon reactions with O 2 and GUA (George et al., 2018;Chen et al., 2020;. The structures shown are examples of the major products (Tables S1 to S4) for different product classifications. GUA + AN mainly formed oligomers, similarly to q OHmediated phenol oxidation (Yu et al., , 2016, followed by N-containing products. The normalized product abundance of GUA + AN was the lowest among all experiments, likely due to the lower GUA decay constant relative to photosensitized oxidation. Moreover, the normalized abundance of N-containing products in GUA + AN was ∼ 12 times lower than that in GUA + DMB + AN but comparable to that in GUA + VL + AN. This discrepancy for GUA + VL + AN might be due to the weaker signals of its N-containing products in the positive compared to the negative ion mode. As previously mentioned, the normalized product abundance was calculated using only the positive ion mode data as the GUA signal from the negative ion mode was weak and thus may present large uncertainties during normalization. Interestingly, the contributions from nitrated species in GUA + DMB + AN and GUA + VL + AN were higher than in GUA + AN, suggesting possible enhancement of nitration reactions. This is likely due to the increased formation of q NO 2 , for instance, via the reactions of q OH and O q − 2 (from 3 DMB * or 3 VL * ) with NO − 2 (Pang et al., 2019;Mabato et al., 2022). Similarly, we previously reported enhanced nitration via the direct photosensitized oxidation of VL in the presence of AN under air-saturated conditions (O 2 is present) relative to nitrogen-saturated conditions . These findings imply that photosensitization may promote reactions induced by nitrate photolysis.
The O : C ratios for GUA + DMB + AN and GUA + VL + AN were lower than those in the absence of AN (Table 1), possibly due to the formation of Nheterocycles, altering the elemental ratios. The O : C and H : C ratios were comparable in GUA + DMB + AN and GUA + VL + AN, but the N : C ratio for the former was higher, implying a greater extent of reactions involving AN. Relative to GUA + DMB + AN and GUA + VL + AN, GUA + AN had a higher N : C ratio, as can be expected given that AN was the only oxidant source. The lower OS C values of GUA + DMB + AN and GUA + VL + AN compared to GUA + AN may be attributed to triplet-initiated oxidation generating higher-molecular-weight products with less fragmentation compared to q OH-mediated oxidation Chen et al., 2020). Nonetheless, AN generally increased the OS C values for both GUA + DMB and GUA + VL, with a more noticeable increase for the former, suggesting more oxidized products. Similarly, in a previous work, the more oxygenated and oxidized aqSOA from the photo-oxidation of phenolic carbonyls in AN solutions than in ammonium sulfate solutions has been ascribed to nitrate photolytic products promoting the reactions . Furthermore, GUA + DMB + AN and GUA + VL + AN aqSOA had mainly similar features in the OS C vs. n C plots to those observed in the absence of AN (Fig. S6). More information on van Krevelen diagrams (Figs. S5e-h and S9) and OS C vs. n C plots (Figs. S6e-h and S10) for GUA + DMB + AN, GUA + VL + AN, and GUA + AN aqSOA is provided in the Supplement (Sect. S7). In essence, AN had no significant effect on the decay kinetics ascribable to photosensitizer chemistry prevailing over nitrate, but it induced the formation of N-containing products. Moreover, AN modified the product distributions, albeit in different ways (Figs. 2 and 3). In particular, N-containing products were more abundant in GUA + DMB + AN, probably due to more extensive fragmentation in GUA + DMB than in GUA + VL. In GUA + VL + AN, AN promoted oligomer formation likely via the −OH group of VL. Furthermore, GUA + DMB + AN and GUA + VL + AN had more nitrated products than GUA + AN, suggesting that photosensitized reactions may promote nitrate-photolysisinitiated reactions.
3.2.3 Light absorption of aqSOA from photosensitization by 3 DMB * and 3 VL * in the presence of AN The presence of AN also did not appreciably affect the absorbance enhancement and R abs for both GUA + DMB + AN and GUA + VL + AN (Fig. 4). For GUA + DMB + AN, the N-containing products may have offset the decrease in oligomers to maintain the absorbance enhancement observed from GUA + DMB. Wang et al. (2022) reported that nitration might contribute sig-nificantly to absorbance enhancement for methoxyphenols in sodium nitrate. In GUA + VL + AN, the decrease in monomers may have counteracted the increased oligomers and the generated N-containing products. Compared to GUA + DMB + AN, the N-containing products from GUA + VL + AN probably had less impact on the absorbance enhancement based on their smaller signal contribution.
Similarly to in experiments without AN, CHO species from GUA + DMB + AN and GUA + VL + AN mainly overlapped in the DBE vs. n C space (Fig. S8c, d) and were mostly potential BrC chromophores. In both systems, GUA dimers were the products with the highest relative abundance. For GUA + DMB + AN, products with high relative abundance also include a CHN species, while two CHON species had high n C (18, 20) and DBE (16,14) values. In GUA + VL + AN, products with high relative abundance include a CHON species (n C = 11 and 9 DBE). Approximately 30 % and 43 % of the N-containing products for GUA + DMB + AN and GUA + VL + AN, respectively, were among the potential BrC chromophores. This suggests the possible significance of N-containing products for light absorption of aqSOA from photosensitization by methoxybenzaldehydes and AN photolysis. Correspondingly, nitroaromatic compounds and N-heterocycles are frequently noted in BBOA Kitanovski et al., 2012;Kourtchev et al., 2016) and have been proposed to be potential contributors to BrC light absorption (Laskin et al., 2015). Relative to GUA + DMB + AN and GUA + VL + AN, only 19 % of the N-containing products in GUA + AN were potential BrC chromophores (Fig. S8e, f), and these did not include CHN species. These indicate that the N-containing products formed in the presence of both photosensitizers and AN may be more significant contributors to the light absorption of phenolic aqSOA than those formed in AN only.

Conclusions and atmospheric implications
The photosensitized oxidation of guaiacol (GUA) by triplet excited states of 3,4-dimethoxybenzaldehyde ( 3 DMB * ) and vanillin ( 3 VL * ) (separately) in the absence and presence of ammonium nitrate (AN) was compared under identical conditions (simulated sunlight and reactant concentration) relevant to atmospheric cloud and fog waters. Compared to GUA + VL, faster GUA oxidation and stronger light absorption by the products were observed in GUA + DMB. Moreover, VL was consumed faster relative to DMB, limiting the extent of GUA oxidation in GUA + VL. These differences are rooted in DMB having a better photosensitizing ability than VL and the −OH group of VL, making it more susceptible to oxidation and more reactive towards electrophilic aromatic substitution. Both GUA + DMB and GUA + VL generated aqSOA (including potential BrC chromophores) com-posed of oligomers, functionalized monomers, oxygenated ring-opening products, and N-containing products in the presence of AN. The major aqSOA formation processes for GUA + DMB and GUA + VL were oligomerization and functionalization, but functionalization appeared to be more significant in GUA + VL due to VL transformation products. The photochemical evolution of aqSOA from GUA + DMB has been reported by Yu et al. (2016). Similar experiments for aqSOA from GUA + VL should be conducted in the future to better understand photosensitized reactions involving phenolic carbonyl photosensitizers.
AN did not significantly affect the decay kinetics due to the predominant effect of 3 DMB * and 3 VL * chemistry compared to nitrate, but it promoted the formation of Ncontaining products; these are composed of N-heterocycles (e.g., imidazoles) and oligomers and nitrated species. The observation of N-heterocycles agrees with our previous findings that ammonium participates in photosensitized oxidation of phenolic compounds in the presence of AN . These results also suggest that photosensitized oxidation of phenolic compounds in the presence of AN might be an important source of N-heterocycles and nitrated products. Identifying the sources of N-heterocycles and nitrated compounds is important due to their environmental and health impacts (Laskin et al., 2009). Moreover, photosensitized reactions by non-phenolic and phenolic methoxybenzaldehydes may be differently influenced by AN photolysis. For instance, the more extensive fragmentation in GUA + DMB than in GUA + VL possibly resulted in more N-containing products in GUA + DMB + AN. Furthermore, the increased oligomers in GUA + VL + AN may be due to VL-derived phenoxy radicals induced by q OH or q NO 2 from nitrate photolysis. In addition, more nitrated compounds observed in GUA + DMB + AN and GUA + VL + AN than in GUA + AN imply that photosensitized reactions may promote nitrate-mediated photolytic reactions. On a related note, the significance of photosensitization by BrC (via formation of solvated electrons; Y.  and by marine dissolved organic matter (via O q − 2 formation; Garcia et al., 2021) in enhanced nitrite production from nitrate photolysis has been reported. A recent study from our group has shown that glyoxal photo-oxidation mediated by both nitrate photolysis and photosensitization can significantly enhance the atmospheric sink of glyoxal . Further studies are needed to improve our understanding of the interplay between photosensitized reactions and nitrate photolysis.
This study demonstrates that the structural features of photosensitizers affect aqSOA formation via non-carbonyl phenol oxidation. The VL results are broadly relevant to other phenolic carbonyls, but the effects of different functional groups should still be considered. For instance, the aldehyde-ketone pair components of syringaldehyde and acetosyringone, both phenolic carbonyls, have been reported to have equal reactivity towards direct photosensitized oxi-dation. This is due to the greater light absorption by the aldehyde form but higher quantum efficiency for loss for the ketone form . However, more aqSOA was observed from syringaldehyde than acetosyringone (in either AN or ammonium sulfate; Huang et al., 2018). Our findings also imply that while the contributions of photosensitization by 3 VL * (and other phenolic carbonyls with similar photosensitizing abilities) to aqSOA formation would be relatively low compared to that of 3 DMB * (and other nonphenolic carbonyls with similar photosensitizing abilities), these are not negligible. As both non-phenolic and phenolic carbonyls such as the methoxybenzaldehydes examined in this work are emitted in large quantities from biomass burning, future experiments should probe the aqSOA contribution of a wider variety of photosensitizers. Moreover, further experiments on photosensitized reactions in authentic particulate matter (PM) samples should be conducted in the future. Multicomponent reactions such as GUA + DMB + AN and GUA + VL + AN should also be explored for a more accurate simulation of ambient conditions. These would be useful in assessing the overall impact of photosensitized reactions and AN photolysis on aqSOA formation in areas impacted by biomass burning and high AN concentrations, as well as for their better representation in aqSOA models. Data availability. The data used in this publication are available to the community and can be accessed by request to the corresponding author.
Author contributions. BRGM designed and conducted the experiments; BRGM and CKC wrote the paper. All co-authors contributed to the discussion of the manuscript.

Competing interests.
The contact author has declared that none of the authors has any competing interests.
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