Predicting photooxidant concentrations in aerosol liquid water based on laboratory extracts of ambient particles

. Aerosol liquid water (ALW) is a unique reaction medium, but its chemistry is poorly understood. For example, little is known of photooxidant concentrations - including hydroxyl radical ( ● OH), singlet molecular oxygen ( 1 O 2 *), and oxidizing triplet excited states of organic matter ( 3 C*) – even though they likely drive much 20 of ALW chemistry. Due to the very limited water content of particles, it is difficult to quantify oxidant concentrations in ALW directly. To predict these values, we measured photooxidant concentrations in illuminated aqueous particle extracts as a function of dilution and used the resulting oxidant kinetics to extrapolate to ALW conditions. We prepared dilution series from two sets of particles collected in Davis, California: one from winter (WIN) and one from summer (SUM). Both periods are influenced by biomass burning, with dissolved organic 25 carbon (DOC) in the extracts ranging from 10 to 495 mg C L -1 . In the winter sample, the ● OH concentration is

We next extrapolated the relationships of oxidant formation rates and sinks as a function of particle mass concentration from our dilute extracts to the much more concentrated condition of aerosol liquid water.Predicted • OH concentrations in ALW (including mass transport of • OH from the gas phase) are (5 -8) × 10 -15 M, similar 35 to those in fog/cloud waters.In contrast, predicted concentrations of 3 C* and 1 O2* in ALW are approximately 10 to 100 times higher than in cloud/fogs, with values of (4 -9) × 10 -13 M and (1 -5) × 10 -12 M, respectively.
Although • OH is often considered the main sink for organic compounds in the atmospheric aqueous phase, the much higher concentrations of 3 C* and 1 O2* in aerosol liquid water suggest these photooxidants will be more important sinks for many organics in particle water.40

Introduction
The chemical processing of organic compounds in cloud/fog water and aerosol liquid water is an important source and sink of secondary organic aerosol (SOA) (Ervens et al., 2011;Gilardoni et al., 2016;Lim et al., 2010;McNeill, 2015).Aerosol liquid water (ALW), i.e., the liquid-phase water on airborne particles, is much less abundant (in terms of liquid water content) and contains much higher concentrations of solutes, compared to clouds and fogs.45 ALW appears to be an efficient and important medium for the production of aqueous SOA (aqSOA) (Ervens and Volkamer, 2010;Faust et al., 2017;Volkamer et al., 2007;Wu et al., 2018;Zhang et al., 2011) and ALW chemistry is often different from that in more dilute cloud and fog drops (Ervens, 2018;Mekic et al., 2018;Zhou et al., 2019).For example, reactions in ALW can more efficiently produce high molecular-weight compounds like oligomers and brown carbon (De Haan et al., 2020;Lim et al., 2010;Renard et al., 2014;Tan et al., 2012;Xia et 50 al., 2018).Modeled rates of aqSOA formation in ALW vary enormously, likely because reactant concentrations and chemical processes in particle water are poorly understood (Ervens and Volkamer, 2010;Ervens, 2018;Lin et al., 2014;Washenfelder et al., 2011).
Due to its limited water content, it is difficult to study chemistry in ALW directly.To get around this problem, Kaur et al. ( 2019) measured • OH, 3 C*, and 1 O2* kinetics as a function of dilution in extracts of a single PM sample 90 and extrapolated the results to aqueous aerosol conditions.However, there are large uncertainties with this extrapolation since the PM extracts were approximately 1000 times more dilute than ALW conditions.In addition, these authors only examined a single sample collected during winter and did not correct their triplet results for probe inhibition by organic compounds.To revisit this approach and, we hope, reduce uncertainty, here we apply the same method but with higher dissolved organic matter concentrations in particle extracts and with correction 95 for triplet probe inhibition.Moreover, in this work we study both a winter PM sample as well as summer wildfire particles to explore differences in oxidant kinetics.

Particle collection and extraction 105
Fine particles (PM2.5) were collected on the roof of Ghausi Hall on the campus of the University of California, Davis in February and August 2020.Davis air quality in winter is often impacted by residential wood combustion, while the August 2020 samples were impacted by Northern California wildfires.PM2.5 was collected using a highvolume sampler equipped with a PM10 inlet (Graseby Andersen) and two offset, slotted impactor plates (Tisch Environmental, Inc., 230 series) to remove coarse particles.The resulting fine particles were collected onto 110 Teflon-coated borosilicate glass microfiber filters (Pall Corporation, EmFab TM filters, 8 in.× 10 in.) that were precleaned by gently shaking in Milli-Q water for 8 h and then drying at 100 °C.During sampling, the airflow rate was maintained at 68 (±2) m 3 per hour.Particles were either collected for 24 h or up to a week; see Table S1  To prepare particulate matter extracts (PMEs), filters were cut into 2 cm × 2 cm squares on the day of extraction.
Each square was placed in an individual, sealed, 20-mL amber glass vial and extracted with Milli-Q water by shaking for 4 h in the dark.The extracts from the same filter sample were combined, filtered (0.22 µm PTFE; Pall), and adjusted to pH 4.2 with sulfuric acid or sodium hydroxide to mimic the acidity of winter particle water 120 in the Central Valley of California (Parworth et al., 2017).The pH of each extract was measured by a pH microelectrode (MI-414 series, protected tip; Microelectrodes, Inc.).The UV-Vis spectrum of each PME was measured in a 1-cm cuvette immediately after pH adjustment with a Shimadzu UV-2501PC spectrophotometer.
Rates of sunlight absorption between 300 and 450 nm were calculated for midday winter-solstice sunlight in Davis, as described by Kaur et al. (2019).PMEs were divided into 4-mL HDPE bottles and flash-frozen in liquid 125 nitrogen immediately after preparation and were later thawed on the day of experiments.Filter squares were weighed by a microbalance (Sartorius M2P) before and after extraction to determine the PM mass extracted; the resulting PM mass/water mass ratios in the filtered extracts might be overestimated because of removal of insoluble material during filtration.Dissolved organic carbon (DOC) and major ion concentrations (Table S2) in PMEs were measured by a Shimadzu TOC-VCPH analyzer and Metrohm ion chromatographs (881 Compact IC 130 Pro) equipped with conductivity detectors, respectively.
To investigate the relationship between particle dilution and oxidant concentration, filter squares from the same sample were extracted with five different volumes of Milli-Q water: 10, 2, 0.7, 0.4, and 0.3 mL.To obtain enough filter squares for this dilution series, for both the winter (WIN) and summer (SUM) samples we combined extracts from 180 filter squares cut from three sheets of filter that were collected on consecutive days.The same number 135 of squares were cut from each of the three filters in a given sample.We use "PME name-water volume" (e.g., WIN-0.7) to denote the sample and extraction volume.Because it is difficult to extract squares with only 0.4 or 0.3 mL of Milli-Q, for these dilutions we extracted each filter square with 1 mL of Milli-Q and then used a rotary evaporator (Buchi Rotavapor R-110; temperature set no higher than 65 C) to remove water until we obtained the equivalent of a 0.4 or 0.3 mL extract.We define the concentration factor (CF) of an extract as the inverse of the 140 volume used for extraction.For example, WIN-10 has a concentration factor of 0.1.https://doi.org/10.5194/egusphere-2023-566Preprint.Discussion started: 28 March 2023 c Author(s) 2023.CC BY 4.0 License.

Sample illumination and chemical analysis
We illuminated samples with light from a 1000 W xenon arc lamp passed through a water filter, an AM1.0 air mass filter (AM1D-3L, Sciencetech), and a 295 nm long-pass filter (20CGA-295, Thorlabs) to simulate tropospheric sunlight (Kaur and Anastasio, 2017).We first transferred the extract into a silicone-plugged GE 021 145 quartz tube (5 mm inner diameter, 1.0 mL volume) and then spiked it with the photooxidant probe and mixed it.
The entire tube was illuminated at 20 °C and was not stirred.Dark control samples were wrapped in aluminum foil and kept in the same photoreactor chamber.During illumination, approximately 150 μL aliquots were removed from the illuminated and dark tubes at specific time intervals to measure concentrations of probes with high-performance liquid chromatography (HPLC, Shimadzu LC-20AB pump, Thermo Scientific Accucore XL 150 C18 column (50 × 3 mm, 4 μm bead), and Shimadzu-M20A UV-Vis detector).The photon flux on each experiment day was determined by measuring the photolysis rate constant of a 10 µM 2-nitrobenzaldehyde (2NB) solution in the same type of container as samples (Galbavy et al., 2010).

Photooxidant measurements
Details about determining photooxidant concentrations are provided in past papers (Anastasio and McGregor, 155 2001;Kaur and Anastasio, 2017;Kaur et al., 2019) and are only discussed briefly here.

Hydroxyl radical ( • OH)
The production rate, rate constant for loss, and steady-state concentration of • OH were quantified using benzoic acid (BA) and a competition kinetics technique.A 0.020 M stock solution of benzoic acid/benzoate was prepared and adjusted to pH 4.2.For each sample, four 1.0-mL aliquots of PME were spiked with different final 160 concentrations (100 -1200 µM) of BA, keeping PME dilution by the addition of probe to less than 10%.We then illuminated each PME and used HPLC to monitor the formation of p-hydroxybenzoic acid (p-HBA), which is formed from BA oxidation by • OH.The initial rate of p-HBA formation was determined from a regression between concentration and illumination time, using either a linear regression or, for plots with curvature, a threeparameter exponential fit 165 Rates of p-HBA formation were normalized to sunlight conditions at midday on the winter solstice at Davis (solar zenith angle = 62°;  2, = 0.0070 s -1 (Anastasio and McGregor, 2001)), and corrected for internal light screening due to sample absorption, using where   is the internal light screening factor in an individual sample (Table S1) and  2, is the photolysis 175 rate constant of 2NB measured on the experiment day.
We then fitted 1/RP,norm versus 1/[BA] with a linear regression and used the slope and y-intercept to calculate the initial production rate of • OH (POH), the pseudo first-order rate constant of • OH loss by natural sinks (k'OH), and the steady-state • OH concentration Here Yp-HBA (0.18) is the yield of p-HBA from the reaction of BA with • OH (Anastasio and McGregor, 2001) and  +⋅ is the second-order rate constant of BA reacting with • OH at pH 4.2 (5.1 × 10 9 M -1 s -1 ) (Ashton et al., 1995;Wander et al., 1968).• OH measurements are in Table S3.185 https://doi.org/10.5194/egusphere-2023-566Preprint.Discussion started: 28 March 2023 c Author(s) 2023.CC BY 4.0 License.

Oxidizing triplet excited states of organic matter ( 3 C*)
Triplets were measured employing syringol (SYR) and (phenylthio)acetic acid (PTA) as probes (Kaur and Anastasio, 2018;Ma et al., 2023).SYR captures both weakly and strongly oxidizing triplets, but its decay can be inhibited by dissolved organic matter (DOM) in PME (Canonica and Laubscher, 2008;Ma et al., 2023;Wenk and Canonica, 2012;Wenk et al., 2015).In contrast, PTA is less sensitive to inhibition by DOM, but it only reacts 190 appreciably with strongly oxidizing triplets (Ma et al., 2023).Two 1.0 mL aliquots of PME were spiked with 10 µM of SYR or PTA, and then illuminated to determine the pseudo-first order rate constants for loss of each probe (k'P,EXP).Next, k'P,EXP values were normalized to Davis winter sunlight conditions and corrected for light screening using an equation analogous to Eq. 3 to obtain rate constant k'P.The contributions of direct photodegradation, • OH, and 1 O2* to probe decay were then subtracted to determine the rate constant for loss of probe due to triplets, 195 Here jP is the probe direct photodegradation rate constant under Davis winter sunlight, and kP+OH and kP+1O2* are the bimolecular rate constants of probe reacting with • OH and 1 O2*, respectively (Table S4).• OH accounts for 2% -35% and 3% -17% of the decay of SYR and PTA, respectively, while 1 O2* accounts for 3% -45% and 2% 200 -10% for SYR and PTA (Tables S5 and S6).Since triplets in PMEs represent the excited states of a complex mixture of brown carbon, there is no single value for the second-order rate constant of 3 C* reacting with probes (kP+3C*).To estimate triplet concentrations, we assume that 3 C* in PME have the same average reactivity as the triplet state of 3,4-dimethoxybenzaldehyde, 3 DMB* (Fleming et al., 2020;Kaur and Anastasio, 2018;Kaur et al., 2019).Unlike our past work (Kaur et al., 2019), we corrected for DOM inhibiting the decays of SYR and PTA, 205 which can cause an underestimate of 3 C* concentrations.To do this, we measured the inhibition factor (IF) in samples (Canonica and Laubscher, 2008;Ma et al., 2023;Wenk et al., 2011) and used it to correct the 3 C* concentration.Details about inhibition factor measurements and [ 3 C*] corrections are in Supplemental Section S1.The 3 C* concentration after inhibition correction is where kP+3DMB* is the second-order rate constant of probe with 3 DMB* (Table S4), and IFP,corr is the inhibition factor of the probe in that extract (Table S7). 3  While our past work indicates that 3 DMB* is a good surrogate for the average oxidizing triplet in Davis drops and particles (Kaur and Anastasio, 2018;Kaur et al., 2019), it is possible that kP+3DMB* is higher than the rate constant for probe with natural triplets.This is the case for surface waters, where the 2,4,6-trimethylphenol (TMP) + 215 3 DMB* rate constant (Ma et al., 2023) is three times higher than the TMP + 3 CDOM* rate constant (Erickson et al., 2018).If this is also the case for our PM extracts, we would be underestimating oxidizing triplet concentrations by roughly a factor of three.

Singlet molecular oxygen ( 1 O2*)
We used FFA as a probe to determine 1 O2* concentrations (Anastasio and McGregor, 2001;Haag et al., 1984).220 1.0 mL of PME sample was divided into two 0.5 mL aliquots, and then one was diluted with 0.5 mL H2O while the other was diluted with 0.5 mL deuterium oxide (D2O).10 µM FFA was spiked into each solution and then both were illuminated.The pseudo-first-order rate constant of FFA loss in H2O-and D2O-diluted PME (k'FFA,H2O and k'FFA,D2O) during illumination was determined as the negative slope of a linear regression between ln([FFA]t/[FFA]0) versus illumination time (t).The 1 O2* concentration in the undiluted PME was determined from 225 the difference of FFA loss rates in H2O and D2O using (Anastasio and McGregor, 2001) where D is the sample dilution factor (i.e., 0.5 for our experiments); kFFA+1O2* is the second-order rate constant of FFA reacting with 1 O2* at 20 °C, 0.96 (± 0.04)×10 8 M -1 s -1 (Appiani et al., 2017); k'H2O and k'D2O are the firstorder rate constants for loss of 1 O2* in 100% H2O (2.2 × 10 5 s -1 ) and D2O (1.6 × 10 4 s -1 ), respectively (Bilski et 230 al., 1997); and χH2O and χD2O are the mole fractions of H2O and D2O in the D2O-diluted solution.Analogous to equation 3, we normalized the experimentally determined 1 O2* concentrations using the light screening factor of each PME and to Davis winter sunlight conditions. 1 O2* measurements are in Table S8.

Testing extraction and rotary evaporation 235
Our winter particle filters were collected in February 2020, when Davis was influenced by residential wood combustion; the average PM2.5 concentration during our sampling was 9.2 µg m -3 .The summer particles were https://doi.org/10.5194/egusphere-2023-566Preprint.Discussion started: 28 March 2023 c Author(s) 2023.CC BY 4.0 License.collected in August 2020, when severe wildfires were occurring approximately 30 km from Davis, resulting in an average PM2.5 concentration of 54 µg m -3 .Figure 1 shows the dissolved organic carbon (DOC) concentrations and rates of light absorption (Rabs) as a function of dilution in the winter (WIN) and summer (SUM) particle extracts.240 We express dilution as the ratio of dry particle mass to liquid water mass in our extracts since we can experimentally measure these quantities for our PMEs and can estimate values for both clouds/fogs and airborne particles.Both DOC and Rabs are directly proportional to particle mass/water mass ratio, indicating that the extractions of filter squares with varying volumes of water achieved the same extraction efficiency.The DOC values of the most concentrated extracts (-0.4 and -0.3) also follow the linear relationship, showing that the rotary 245 evaporation process used for these dilutions did not lead to significant loss of brown carbon or other organic compounds.As shown in Figure S1, UV-Vis spectra of the -0.4 and -0.3 extracts before and after rotovapping are essentially the same, indicating that evaporation did not change the BrC composition significantly.
We also examined if rotovapping affects photooxidant concentrations.First, we extracted one filter either with 0.7 mL water/square (sample PME-NR) or 2 mL water/square followed by rotovapping to the equivalent of 0.7 250 mL/square (sample PME-R).In a second test, we diluted a rotovapped sample (WIN-0.3)by a factor of 6.7 with water to obtain an extract equivalent to 2 mL Milli-Q/square (WIN-0.3D);this diluted, rotovapped sample should be equivalent to WIN-2, a not-rotovapped sample with the same overall dilution.Figure S2

Ions and light absorption 260
Figure 1 shows that summer and winter PMEs have DOC concentrations in the range of 16 -495 and 10 -336 mg C L -1 , respectively, but WIN has slightly higher particle mass/water ratios, (0.05 -1.6) × 10 -3 µg PM/µg H2O, compared to (0.04 -1.4) × 10 -3 µg PM/µg H2O for SUM.The summer wildfire sample shows a higher average fraction of organic carbon to PM mass, 0.37 (± 0.02), compared to winter (0.20 ± 0.01).The OC/PM ratio in SUM, which represents relatively fresh aerosols from wildfires, is lower than the typical values near 0.5 for biomass 265 burning particles (Reid et al., 2005;Schauer et al., 2001), probably because our water extractions did not fully solubilize non-polar organic compounds from the particles.The winter sample has lower organic carbon but higher concentrations of ions, including nitrate (NO3 -), sulfate (SO4 2-), and ammonium (NH4 + ) (Table S2).For example, nitrate concentrations in WIN range from 0.18 to 5.2 mM and contribute on average (± 1 σ) 20 (± 2) % of the total extracted PM mass.In contrast, NO3 -concentrations in SUM are about five times lower (0.03 -1.0 mM) at the 270 same concentration factor and only contribute an average of 4.4 (± 0.4) % of the SUM PM mass.The sulfate in WIN accounts for 11 (± 4) % of extracted PM mass, with concentrations (0.03 -2.3 mM) around 4 times higher than in SUM (0.02 -0.6 mM, accounting for an average of 4.2 (± 0.6) % of extracted PM mass).NH4 + is also higher in WIN (0.20 -3.6 mM) compared to SUM (0.10 -1.SUM are high and comparable to ammonium, with ranges of 0.2 -3.6 mM and 0.1 -1.7 mM, respectively.275 Concentrations of potassium, a tracer of biomass burning (Andreae, 1983), are 0.03 -0.7 mM in both WIN and SUM, with a K/PM mass ratio of 0.02 (±0.004), in the range reported for biomass burning aerosols, 0.02 to 0.05 (Reid et al., 2005;Urban et al., 2012).
For all PMEs, absorbance declines exponentially with wavelength (e.g., Fig. S1), and WIN and SUM samples have the same average absorption Ångström exponent (AAE, 300 -450 nm) of 7.2 (Table S1), comparable to 280 AEE values (6 -8) previously reported in water extracts of biomass burning particles (Hecobian et al., 2010;Hoffer et al., 2006;Kaur et al., 2019).The pathlength-normalized absorption coefficient at 300 nm (α300) for the summer samples (0.2 -6.7 cm -1 ) is about 2 times higher than winter samples at the same concentration factor (0.1 -3.0 cm -1 ) (Table S1).Thus, summer extracts absorb sunlight at approximately twice the rate as winter extracts (Figure 1).We also calculated the dissolved organic carbon-normalized mass absorption coefficient (MACDOC) 285 of each extract by dividing the absorbance at 300 or 365 nm by the DOC concentration (Kaur et al., 2019).SUM average MACDOC values across all dilutions are 3.1 (± 0.1) and 1.0 (± 0.1) m 2 (g C) -1 at 300 and 365 nm, respectively, which are approximately 1.5 times higher than the WIN values (Table S1).This difference is likely because the SUM sample is dominated by fresh wildfire organic aerosols that are composed of organic compounds with a higher degree of unsaturation, increasing light absorption (Fleming et al., 2020).Meanwhile, the WIN 290 sample may contain a lower fraction of fresh biomass burning aerosols due to oxidation and photobleaching of the brown carbon (Forrister et al., 2015;Wong et al., 2019).Our MAC value for WIN is similar to the average MAC value in the previous Davis winter samples (Kaur et al., 2019).

Photooxidants in PM extracts
In this section we first present our measured oxidant concentrations as a function of particle dilution in the WIN 295 and SUM extracts.We use DOC as the independent variable in our plots because BrC likely dominates the production of 3 C* and 1 O2* and DOC is proportional to concentration factor in each extract series.We then examine how the production rate (POX) and rate constant for loss (k'OX) for each oxidant vary as a function of In the next section (3.4), we extrapolate these kinetic parameters to aerosol liquid water conditions to predict photooxidant concentrations in ALW.

Hydroxyl radical in PM extracts
As shown in Fig. 2a, the most dilute sample in the WIN dilution series, WIN-10, has the lowest • OH concentration, while in the other dilutions [ • OH] is noisy but appears independent of DOC.This result, that • OH concentration 305 is essentially independent of particle mass concentration, is similar to what Kaur et al. (2019) observed for winter samples (green points in Fig. 2), although our • OH concentrations are approximately 10 times higher.Kaur et al. (2019) found that the • OH photoproduction rate (POH) and sink (k'OH) both linearly increase with concentration factor, leading to a roughly constant • OH concentration since the concentration is equal to the ratio POH/k'OH (Eq.10).To explore this in our samples, we determined POH and k'OH in all of the WIN and SUM 310 extracts; we start by considering the WIN results.As shown in Fig. 3a, POH and k'OH both increase linearly with DOC, consistent with the winter PM extract observations of Kaur et al. (2019), though our samples have a higher slope for POH but a lower one for k'OH.This higher • OH production rate, coupled with a lower rate constant for • OH loss, is responsible for the roughly 10 times higher [ • OH] in this work, but we do not know why these parameters are so different between the previous and current winter particle samples.POH in WIN ranges from 315 0.02 ×10 -8 to 4.8 ×10 -8 M s -1 , significantly higher than typical values (approximately 10 -10 M s -1 ) in cloud and fog waters (Arakaki et al., 2013;Kaur and Anastasio, 2018;Tilgner and Herrmann, 2018).In Davis fog samples, the major source of • OH is photolysis of nitrate and nitrite (Anastasio and McGregor, 2001;Kaur and Anastasio, 2017).However, in our winter PM extracts, nitrate accounts for 10% or less of POH (Table S3), while the nitrite contribution is negligible.Instead, we hypothesize that our samples might contain higher concentration of 320 transition metals, contributing to • OH production (Vidrio et al., 2009).While DOC photoreactions also can be a source of • OH (Badali et al., 2015), it seems likely that POH is correlated with DOC primarily because DOC is a proxy for concentration factor in the extracts.As for • OH sinks in our WIN extracts, k'OH is in the range (0.2 -9.9) ×10 6 s -1 , higher than previous Davis fog values ((0.4 -1.3) ×10 6 s -1 ; (Kaur and Anastasio, 2017)).The lowest k'OH (in WIN-10, the most dilute extract) is comparable to the field blank values (Table S3), suggesting that [ • OH] 325 in WIN-10 may be artificially low because of background contamination.We also calculated the rate constant of organics reacting with • OH (kDOC+OH) for the winter samples; our average WIN value, 2.4 (± 0.7) × 10 8 L (mol C) -

345
Unlike WIN, • OH in the summer samples linearly increases with concentration factor or DOC, with an • OH concentration range of (0.4 -7.7) ×10 -15 M (Fig. 2a).This indicates that either POH or k'OH does not increase linearly with DOC.As shown in Fig. 3b, k'OH is linear with DOC, but POH is proportional to the DOC concentration squared.Our interpretation is that • OH production in SUM is a bimolecular reaction rather than a first-order 350 photolysis.The most likely candidate is the photo-Fenton reaction involving soluble reduced iron and hydrogen https://doi.org/10.5194/egusphere-2023-566Preprint.Discussion started: 28 March 2023 c Author(s) 2023.CC BY 4.0 License.peroxide (or organic peroxides) (Paulson et al., 2019;Zepp et al., 1992), where the concentrations of both reactants increase with concentration factor, as does [DOC].Therefore, although WIN and SUM have roughly similar • OH concentrations, they apparently have different mechanisms governing • OH formation.POH in SUM is in the range (0.03 -8.2) ×10 -8 M s -1 , with the value in SUM-0.3 nearly double that of WIN-0.3.In contrast, • OH sinks for the 355 summer and winter samples are similar (Fig. 3) and the average kDOC+OH value in SUM is 2.9 (± 1.1) × 10 8 L (mol C) -1 s -1 , not significantly different from the WIN value.

Oxidizing triplet excited states of organic matter in PM extracts
We determined oxidizing triplet concentrations using two probes.SYR is highly reactive towards both strongly and weakly oxidizing triplets, but its decay by 3 C* can be inhibited by DOM, leading to an underestimate of 3 C* 360 concentrations (Canonica and Laubscher, 2008;Maizel and Remucal, 2017;Wenk et al., 2011).PTA that has a higher oxidation potential (1.47 V vs. SHE, estimated using the Marcus equation) SYR (~1.17 V vs. SHE) (Canonica et al., 2000;Chellamani and Sengu, 2008), is less reactive than SYR with weakly oxidizing triplets (and thus does not capture the whole oxidizing triplet pool), but its advantage is that it is more resistant to inhibition by DOM (Klein et al., 2006;Ma et al., 2023).For both probes, we correct for probe inhibition by 365 measuring the inhibition factor (IF) and using it to correct 3 C* concentrations (Section S1 and Table S7).
Inhibition factors of SYR are as low as 0.13 (± 0.03) in the most concentrated sample (WIN-0.3),indicating that approximately 87 (± 20) % of SYR decay is inhibited by DOM in this sample, which would lead to a 3 C* concentration that is 7.5 (± 1.7) times lower than the actual value if there was no correction for inhibition.As for PTA, IF values are all greater than 0.9, indicating little inhibition.For simplicity, we only show 3 C* concentrations 370 after inhibition factor correction; uncorrected values are given in Tables S5 and S6. 3 C* concentrations as a function of DOC are in Figure 2.With SYR as the triplet probe (Fig. 2b), the [ 3 C*]SYR range is (0.5 -7.1) ×10 -13 M in WIN and (1.6 -6.8) ×10 -13 M in SUM.At the same DOC, [ 3 C*]SYR values in summer and winter are similar, despite the differences in sample composition (Table S5).Oxidizing triplet concentrations in our samples are generally higher than those from Kaur et al. (2019) (Fig. 2c, green points), 375 which can be attributed to higher DOC in our samples and our correction for SYR inhibition.From PTA, the [ 3 C*]PTA range is (0.2 -3.9) ×10 -13 M in WIN and (0.4 -2.9) ×10 -13 M in SUM, with WIN having higher values than SUM at the same concentration factor (Fig. 2c).For both probes, the 3 C* concentration initially increases with DOC but then approaches or reaches a plateau under more concentrated conditions.Kaur et al. (2019) observed the same trend.Their interpretation was that in dilute solutions O2 is the dominant sink for triplets, while under more concentrated conditions DOM becomes the major sink.Therefore, 3 C* production and loss are both functions of DOC, as described by The dashed lines in Figs.2b and 2c show the regression fitting results of Equation 11 to the experimental data.
From the fitted parameter b (Table S9), we can determine krxn+Q,3C* (Eqn.S6), the total rate constant of triplet physical quenching and chemical reaction with DOC.Values from our Fig. 2 fittings are 7.6 (± 6.8) ×10 7 L (mol C) -1 s -1 for WIN and 1.2 (±0.5) ×10 8 L (mol C) -1 s -1 for SUM (Table S10).Kaur et al. (2019) obtained 9.3 (±1.3) ×10 7 L (mol C) -1 s -1 for Davis winter particle extracts, but they did not correct for SYR inhibition, which should 395 be more significant at higher DOC, leading to an earlier plateau and higher apparent rate constant.Despite this, the three values are not significantly different, possibly because the Kaur samples had much lower DOC and thus were less affected by SYR inhibition.Wenk et al. (2013) obtained a range of values of (1.3 -3.9) ×10 7 L (mol C) - 1 s -1 for surface water DOM quenching and reacting with 2-acetonaphthone and 3-methoxyacetophenone triplets; their lower values imply that atmospheric DOM, at least in our samples, more efficiently quenches triplets than 400 does DOM in surface waters.
The DOC quenching and reaction rate constants from our PTA-derived triplet concentrations are 5.7 (±1.2) ×10 7 and 6.6 (±1.0) ×10 7 L (mol C) -1 s -1 for WIN and SUM, respectively.These values are lower than those obtained using SYR, as reflected by the weaker curvature of the PTA dashed lines (Figure 2c) compared to SYR (Figure 2b).The similar values of krxn+Q,3C* from PTA in WIN and SUM suggest that this rate constant is insensitive to 405 particle type.Therefore, the higher [ 3 C*]PTA in WIN compared to SUM at the same DOC level can be attributed https://doi.org/10.5194/egusphere-2023-566Preprint.Discussion started: 28 March 2023 c Author(s) 2023.CC BY 4.0 License.
to differences in 3 C* production.This is consistent with the differences in apparent quantum yields: the WIN yield of triplets is 1.8 (±0.3)%, more than double the SUM value of 0.8 (±0.1)% (Table S6).

Singlet molecular oxygen in PM extracts 410
The final photooxidant we measured is singlet molecular oxygen.As shown in Fig. 2d, winter and summer samples have similar 1 O2* concentrations, in the range of (0.2 -8.5) ×10 -12 M, with values increasing with DOC.
The lowest values, in the most dilute extracts, are comparable to fog water concentrations, while our highest concentrations are approximately four times higher than those in previous Davis winter particle extracts (Anastasio and McGregor, 2001;Kaur and Anastasio, 2017;Kaur et al., 2019). 1 O2* is the most abundant oxidant 415 in our PMEs, with concentrations roughly 10 times higher than 3 C* and 1000 times higher than • OH.In both series of samples, the 1 O2* concentration increases with DOC, as seen in Kaur et al. (2019).Since brown carbon is the source of 1 O2*, the 1 O2* production rate increases with DOC.In contrast, in dilute samples (e.g., our extracts) the dominant sink for 1 O2* is water, whose concentration is independent of sample concentration factor.All three sets of samples in Fig. 2d exhibit very similar relationships between 1 O2* and DOC, suggesting DOC 420 concentration might be a good predictor of 1 O2* concentrations in atmospheric waters.Apparent quantum yields of 1 O2* are 3.0 (± 0.2)% for WIN and 2.0 (± 0.4)% for SUM (Table S8), which are in the range of typical values for atmospheric waters (Bogler et al., 2022;Kaur and Anastasio, 2017;Kaur et al., 2019;Manfrin et al., 2019) and surface waters (Ossola et al., 2020).For WIN, 1 O2* is linearly related to DOC throughout the dilution series, but in SUM the singlet oxygen concentration exhibits a linear relationship at low DOC and then starts to level off 425 in the more concentrated extracts (Fig. 2d).Kaur et al. (2019) predicted this plateauing under aerosol liquid water conditions as the very high concentration of organics can become the dominant 1 O2* sink.However, a better explanation for this curvature in our samples is that the concentration of 3 C* (the precursor of 1 O2*) plateaus at high DOC, which slows the production rate of 1 O2*.In the summer sample of Figure 2d, the curvature of 1 O2* is more likely a consequence of 3 C* plateauing, rather than DOC becoming an important 1 O2* sink, because 1 O2* 430 generally has lower reactivity than triplets with most organics (Arnold, 2014;Canonica et al., 2000;Wilkinson et al., 1995).Based on rough estimates of the composition and reactivity of particulate organics from biomass burning (Kaur et al., 2019), we estimate that for organics to be an important sink of 1 O2*, the DOC concentration would need to be ~5000 mg C L -1 ; this is 10 times higher than the maximum DOC in our extracts, suggesting that organics are a negligible sink of 1 O2* in our extracts.Assuming 3 C* is responsible for the 1 O2* curvature in the This equation gives a good fit to the SUM data, as shown by the red dashed line in Fig. 2d.From the parameter b, we calculate that the rate constant for DOC reacting and physically quenching 1 O2*-producing triplet states 440 (krxn+Q,3C*) is 2.1 (± 0.3) ×10 7 L (mol C) -1 s -1 .This is lower than the values acquired from [ 3 C*]SYR and [ 3 C*]PTA, which is reasonable since the 1 O2*-derived value represents the whole triplet pool (i.e., all triplets that can undergo energy transfer with dissolved oxygen), which is a larger pool than oxidizing triplets.Our results suggest that the non-oxidizing triplets are less reactive with organics than are oxidizing triplets, leading to a lower rate constant for reaction and quenching by DOC, as seen previously by Canonica et al. (2000).445

Extrapolating photooxidant concentrations to ALW conditions
In the dilution experiments above, we investigate oxidant kinetics and concentrations as a function of concentration i.e., particle mass/water mass ratio.In this section we extrapolate these relationships from our dilute extract conditions (with PM mass/water mass ratios of (0.04 -1.6) × 10 -3 µg PM/µg H2O) to the much more concentrated conditions of aerosol liquid water (up to ~ 1 µg PM/µg H2O).450

Hydroxyl radical in ALW
To estimate [ • OH] in particle water for WIN, we apply the linear relationships of POH and k'OH with DOC that we determined in our extracts (Fig. 3a), along with the relationship of [DOC] to particle mass/water mass ratio, to predict kinetics under more concentrated particle water conditions.Parameters used in the extrapolation are provided in Table S11.Extrapolating to an ALW of 1 µg PM/µg H2O yields an estimated POH of 2.7 ×10 -5 M s -1 , 455 and k'OH of 5.0 × 10 9 s -1 .However, since our aqueous experiments do not include • OH transferred from the gas phase (POH,gas), we added POH,gas estimated by Kaur et al. (2019) to our extrapolated POH to calculate POH,tot.We then estimate [ • OH] as POH,tot divided by k'OH (Eq.10).Estimating [ • OH] for the SUM sample is more complicated since POH initially increases with DOC squared.We simulate the • OH production rate as a function of DOC by using photo-Fenton reaction rate constants and setting soluble iron and hydrogen peroxide concentrations to fit 460 measured values (Section S3).We then apply this simple model to predict POH for SUM out to ALW conditions.
For k'OH in SUM, we use the measured linear dependence on DOC (Fig. 3b). Figure 4a shows the predicted hydroxyl radical steady-state concentrations for SUM and WIN across a wide range of liquid water content, from dilute cloud/fog drops to concentrated aqueous particle conditions.We also include the winter PM • OH predictions from Kaur et al. (2019) for comparison.For WIN, [ • OH] slowly decreases from 465 1 ×10 -14 M in cloud/fog waters (at 3 × 10 -5 µg PM/µg H2O) to 6 × 10 -15 M in ALW (at 1 µg PM/µg H2O).Calculated [ • OH] values are higher than measured values, especially under the most dilute conditions, because • OH from gas-phase mass transfer is included in our extrapolation.The • OH trend for WIN is consistent with the result of Kaur et al. (2019), but our concentrations are 6 -12 times higher.This is because WIN has a slope of POH vs. DOC around 4 times higher than that in Kaur et al. (2019), while the slope for k'OH in WIN is slightly lower (Fig. 470 3a).For our winter sample under dilute conditions, aqueous processes are as important an • OH source as is gasphase transfer (Fig. 4b).However, the aqueous production rate rises more rapidly with PM mass concentration than does gas-phase mass transfer, making aqueous reactions the dominant source of • OH under ALW conditions, where they account for more than 90% of • OH production.This slower increase of POH,gas is also responsible for the decreasing [ • OH] with increasing PM mass concentration.475 For SUM, predicted [ • OH] is approximately constant at 4 × 10 -15 M under dilute conditions (Fig. 4a), with gasphase mass transport being the major source of • OH (Fig. 4c).[ • OH] then increases to 1 × 10 -14 M at 1 × 10 -3 µg PM/µg H2O as the aqueous production rate (POH,aq) increases rapidly and aqueous reactions dominate • OH production.When moving to more concentrated conditions, [ • OH] plateaus because we assume the aqueous H2O2 concentration reaches a maximum of 100 μM due to equilibrium with the gas phase (Section S3).Thereafter, 480 POH,aq increases linearly, but more slowly, with PM mass/water mass ratio; since k'OH also increases linearly with concentration factor, [ • OH] remains nearly constant at 9 ×10 -15 M for PM/water ratios of roughly 10 -3 to 1 µg PM/µg H2O.For both WIN and SUM, our measured • OH concentrations in the most concentrated extracts are approximately an order of magnitude higher than in Kaur et al. (2019) and this difference is maintained throughout the predicted [ • OH] to ambient particle water conditions.

Singlet molecular oxygen in ALW
Lastly, we consider the extrapolation of 1 O2* concentrations from our dilute experimental solutions to ALW conditions.To do this, we consider the production of 1 O2* by 3 C* as well as H2O and DOM as sinks for singlet oxygen.In terms of 1 O2* sources, we first assume the O2 concentration is constant at all conditions, i.e., not 530 considering a solute effect on O2 solubility.Next, we assume the plateauing of [ 3 C*] at high concentration factors results in a plateauing of the 1 O2* production rate, as evidenced in the curvature of [ 1 O2*] in SUM (Fig. 2d).To account for this effect, we fit [ 1 O2*] versus DOC using an equation analogous to Eq. 11 and calculate the 1 O2* production rate (P1O2*) with the fitted parameters (Eq.S11).This process does not work for WIN, however, since it shows no curvature of [ 1 O2*].So to predict the 3 C* effect for this sample, we adjusted the regression parameters 535 so that the fitted line passed through just the first four data points (Figure S5).In terms of modeling DOM as a sink for 1 O2*, this effect does not appear in our lab extracts (due to their relatively low DOC content), but we expect it will happen under more concentrated conditions.To incorporate this effect, we estimated the secondorder rate constant for loss of 1 O2* by DOC (k1O2*+DOC) using the same approach from Kaur et al. (2019) but determined a lower value (1 × 10 5 L (mol C) -1 s -1 ) based on our 1 O2* concentration data versus DOC .We then 540 calculate the first-order sink for 1 O2* due to DOC as the product of this second-order rate constant and the DOC concentration.The resulting predictions for 1 O2* concentrations, along with the production rate and sink rate constants for the summer sample, are in Figure 6. Figure 6a shows that our predictions of 1 O2* under ALW conditions are roughly 10 to 100 times lower than those in Kaur et al. (2019); this is because we include the effect of plateauing 3 C* 555 concentration on the 1 O2* production rate, which decreases 1 O2* concentrations under ALW conditions.In Fig. 6a, [ 1 O2*] for SUM starts at 4 × 10 -13 M in dilute drops, peaks at 1 ×10 -11 M at 1.0 ×10 -2 µg PM/µg H2O (where P1O2* first plateaus; Fig. 6b), and then starts to decrease.This decrease is because the production rate for 1 O2* (P1O2*) is constant while the 1 O2* sink from DOC (k'1O2*,DOC) increases with particle mass concentration and becomes the dominant 1 O2* sink; the result is a singlet oxygen concentration of 1 × 10 -12 M at 1 µg PM/µg H2O. 560 This concentration is only 1.4 times higher than [ 3 C*]SYR under the same condition (Fig. S7).For WIN, [ 1 O2*] starts at 1 ×10 -13 M in dilute drops, reaches a maximum of 3 × 10 -11 M at 4.0 ×10 -2 µg PM/µg H2O, and then decreases to 5 ×10 -12 M at 1 µg PM/µg H2O (Fig. S6).Under ALW conditions, WIN has a maximum [ 1 O2*] that is 3 times higher than SUM because measured [ 1 O2*] in WIN presents much less curvature than SUM, i.e., the organics in WIN appear to be less reactive with 1 O2*-producing triplet states compared to those in the SUM 565 sample.Therefore, the plateau of P1O2* in WIN shows up only under more concentrated conditions compared to SUM (Fig. S6).

Conclusions and uncertainties
We measured concentrations of three photooxidants -hydroxyl radical, oxidizing triplet excited states of organic matter, and singlet molecular oxygen -as a function of particle dilution in aqueous extracts of winter particles 570 (influenced by residential wood combustion) and summer particles (strongly influenced by wildfires).The extracts contain high amounts of organic matter, with dissolved organic carbon concentrations ranging from 10 to 495 mg C L -1 .DOC-normalized mass absorption coefficients at 300 nm are 2.1 (±0.2) m 2 (g C) -1 in winter and 3.1 (±0.1) m 2 (g C) -1 in summer, with absorption Ångström exponents of 7.2 for both, indicating significant amounts of brown carbon.575 In the winter sample, the measured • OH concentration appears to be independent of extract concentration, while in the summer sample • OH increases with concentration factor.In both WIN and SUM, measured 3 for https://doi.org/10.5194/egusphere-2023-566Preprint.Discussion started: 28 March 2023 c Author(s) 2023.CC BY 4.0 License.details.Upon collection, each sample was wrapped in aluminum foil (baked previously at 500 °C for 8 h), sealed in a Ziploc bag, and frozen at −20 °C.Field blanks were obtained in an identical manner as samples, including 115 loading the clean filters into the sampler and turning on the pump for 2 min.
presents photooxidant concentrations in the two tests.In each test, the concentrations are essentially the same in the rotovapped and not rotovapped samples, indicating a negligible effect of rotary evaporation on photooxidant kinetics.255 https://doi.org/10.5194/egusphere-2023-566Preprint.Discussion started: 28 March 2023 c Author(s) 2023.CC BY 4.0 License.

Figure 1 .
Figure 1.Dependence of dissolved organic carbon (DOC, circles) and rate of sunlight absorption between 300 -450 nm (Rabs, diamonds) on particle mass/water mass ratio (i.e., aqueous particle concentration) in summer (red) and winter (blue) particle extracts.

Figure 2 .
Figure 2. Steady-state concentrations of (a) hydroxyl radical, oxidizing triplet excited states of brown carbon determined by (b) SYR and (c) PTA, and (d) singlet molecular oxygen in WIN (blue) and SUM (red) samples as a function of dissolved organic carbon.WIN-0.3Dresults are also included.Previous measurements in Davis winter 335

Figure 4 .
Figure 4. (a) Dependence of hydroxyl radical concentration on particle mass/water mass ratio in winter (blue) and summer (red) extracts.Solid circles are measured values, while lines are extrapolations to the ambient aqueous aerosol conditions, including contributions from aqueous • OH formation and • OH mass transport from the gas phase.Previous measurements and extrapolation with Davis winter particle extracts are shown in green (Kaur et al., 2019).