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Abstract. Fog/cloud drops and aerosol liquid water are important sites for the transformations of atmospheric species, largely through reactions with photoformed oxidants such as the hydroxyl radical (OH), singlet molecular oxygen (1O2∗), and oxidizing triplet excited states of organic matter (3C∗). Despite their importance, few studies have measured these oxidants or their seasonal variations. To address this gap, we collected ambient PM2.5 from Davis, California, over the course of a year and measured photooxidant concentrations and light absorption in dilute aqueous extracts. Mass absorption coefficients (MACs) normalized by dissolved organic carbon range from 0.4–3.8 m2 per gram C at 300 nm. Concentrations of OH, 1O2∗, and 3C∗ in the extracts range from (0.2–4.7) × 10−15 M, (0.7–45) × 10−13 M, and (0.03–7.9) × 10−13 M, respectively, with biomass burning brown carbon playing a major role in light absorption and the formation of 1O2∗ and 3C∗. Extrapolating photooxidant kinetics from our dilute particle extracts to concentrated aerosol liquid water (ALW) conditions gives an estimated OH concentration of 7 × 10−15 M and ranges for 1O2∗ and 3C∗ of (0.6–7) × 10−12 M and (0.2–1) × 10−12 M, respectively. Compared to the results in Kaur et al. (2019), our ALW predictions show roughly 10 times higher OH, up to 5 times higher 3C, and 1O2∗ concentrations that are lower by factors of 20–100. These concentrations suggest that 3C∗ and 1O2∗ in ALW dominate the processing of organic compounds that react quickly with these oxidants (e.g., phenols and furans, respectively), while OH is more important for less reactive organics. 

Abstract. While gas-phase reactions are well established to have significant impacts on the mass concentration, chemical composition, and optical properties of secondary organic aerosol (SOA), the aqueous-phase aging of SOA remains poorly understood. In this study, we performed a series of long-duration photochemical aging experiments to investigate the evolution of the composition and light absorption of the aqueous SOA (aqSOA) from guaiacyl acetone (GA), a semivolatile phenolic carbonyl that is common in biomass burning smoke. The aqSOA was produced from reactions of GA with hydroxyl radical (•OH-aqSOA) or a triplet excited state of organic carbon (3C∗-aqSOA) and was then photoaged in water under conditions that simulate sunlight exposure in northern California for up to 48 h. The effects of increasing aqueous-phase •OH or 3C∗concentration on the photoaging of the aqSOA were also studied. High-resolution aerosol mass spectrometry (HR-AMS) and UV–Vis spectroscopy were utilized to characterize the composition and the light absorptivity of the aqSOA and to track their changes during aging. Compared to •OH-aqSOA, the 3C∗-aqSOA is produced more rapidly and shows less oxidation, a greater abundance of oligomers, and higher light absorption. Prolonged photoaging promotes fragmentation and the formation of more volatile and less light-absorbing products. More than half of the initial aqSOA mass is lost, and substantial photobleaching occurs after 10.5 h of prolonged aging under simulated sunlight illumination for 3C∗-aqSOA and 48 h for •OH-aqSOA. By performing positive matrix factorization (PMF) analysis of the combined HR-AMS and UV–Vis spectral data, we resolved three generations of aqSOA with distinctly different chemical and optical properties. The first-generation aqSOA shows significant oligomer formation and enhanced light absorption at 340–400 nm.The second-generation aqSOA is enriched in functionalized GA species and has the highest mass absorption coefficients in 300–500 nm, while the third-generation aqSOA contains more fragmented products and is the least light absorbing. These results suggest that intermediately aged phenolic aqSOA is more light absorbing than other generations, and that the light absorptivity of phenolic aqSOA results from a competition between brown carbon (BrC) formation and photobleaching, which is dependent on aging time. Although photoaging generally increases the oxidation of aqSOA, a slightly decreased O/C of the •OH-aqSOA is observed after 48 h of prolonged photoaging with additional •OH exposure. This is likely due to greater fragmentation and evaporation of highly oxidized compounds.Increased oxidant concentration accelerates the transformation of aqSOA and promotes the decay of BrC chromophores, leading to faster mass reduction and photobleaching. In addition, compared with •OH, photoaging by3C∗ produces more low-volatility functionalized products, which counterbalances part of the aqSOA mass loss due to fragmentation and evaporation. 

Abstract. 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 radicals (OH), singlet molecular oxygen (1O2*), and oxidizing triplet excited states of organic matter (3C*) – even though they likely drive much 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 carbon (DOC) in the extracts ranging from 10 to 495 mg C L−1. In the winter sample, the OH concentration is independent of particle mass concentration, with an average value of 5.0 (± 2.2) × 10−15 M, while in summer OH increases with DOC in the range (0.4–7.7) × 10−15 M. In both winter and summer samples, 3C* concentrations increase rapidly with particle mass concentrations in the extracts and then plateau under more concentrated conditions, with a range of (0.2–7) × 10−13 M.WIN and SUM have the same range of 1O2* concentrations, (0.2–8.5) × 10−12 M, but in WIN the 1O2* concentration increases linearly with DOC, while in SUM 1O2* approaches a plateau. 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 to those in fog/cloud waters. In contrast, predicted concentrations of 3C* and1O2* 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 3C* and 1O2* in aerosol liquid water suggest these photooxidants will be more important sinks for many organics in particle water. 

Guaiacyl acetone (GA) is a phenolic carbonyl emitted in significant quantities by wood combustion that undergoes rapid aqueous-phase oxidation to produce aqueous secondary organic aerosol (aqSOA). We investigate the photosensitized oxidation of GA by an organic triplet excited state (3C*) and the formation and aging of the resulting aqSOA in wood smoke-influenced fog/cloud water. The chemical transformations of the aqSOA were characterized in situ using a high-resolution time-of-flight aerosol mass spectrometer. Additionally, aqSOA samples collected over different time periods were analyzed using high-performance liquid chromatography coupled with a photodiode array detector and a high-resolution Orbitrap mass spectrometer (HPLC-PDA-HRMS) to provide details on the molecular composition and optical properties of brown carbon (BrC) chromophores. Our results show efficient formation of aqSOA from GA, with an average mass yield around 80%. The composition and BrC properties of the aqSOA changed significantly over the course of reaction. Three generations of aqSOA products were identified via Positive Matrix Factorization analysis of the AMS data. Oligomerization and functionalization dominated the production of the first-generation aqSOA, whereas fragmentation and ring-opening reactions controlled the formation of more oxidized second- and third-generation products. Significant formation of BrC was observed in the early stages of the photoreaction, while organic acids were produced throughout the experiment. High-molecular-weight molecules (m/z > 180) with high aromaticity were identified via HPLC-PDA-HRMS and were found to account for a majority of the UV-vis absorption of the aqSOA. 

Abstract. While photooxidants are important in atmospheric condensed phases, there arevery few measurements in particulate matter (PM). Here we measure lightabsorption and the concentrations of three photooxidants – hydroxyl radical(⚫OH), singlet molecular oxygen (1O2*),and oxidizing triplet excited states of organic matter (3C*) –in illuminated aqueous extracts of wintertime particles from Davis,California. 1O2* and 3C*, which are formedfrom photoexcitation of brown carbon (BrC), have not been previously measuredin PM. In the extracts, mass absorption coefficients for dissolved organiccompounds (MACDOC) at 300 nm range between 13 000 and30 000 cm2 (g C)−1 are approximately twice ashigh as previous values in Davis fogs. The average (±1σ)⚫OH steady-state concentration in particle extracts is4.4(±2.3)×10-16 M, which is very similar to previous valuesin fog, cloud, and rain: although our particle extracts are moreconcentrated, the resulting enhancement in the rate of ⚫OHphotoproduction is essentially canceled out by a corresponding enhancement inconcentrations of natural sinks for ⚫OH. In contrast,concentrations of the two oxidants formed primarily from brown carbon (i.e.,1O2* and 3C*) are both enhanced in theparticle extracts compared to Davis fogs, a result of higher concentrationsof dissolved organic carbon and faster rates of light absorption in theextracts. The average 1O2* concentration in the PM extractsis 1.6(±0.5)×10-12 M, 7 times higher than past fogmeasurements, while the average concentration of oxidizing triplets is 1.0(±0.4)×10-13 M, nearly double the average Davis fog value.Additionally, the rates of 1O2* and 3C*photoproduction are both well correlated with the rate of sunlightabsorption. Since we cannot experimentally measure photooxidants under ambient particlewater conditions, we measured the effect of PM dilution on oxidantconcentrations and then extrapolated to ambient particle conditions. As theparticle mass concentration in the extracts increases, measuredconcentrations of ⚫OH remain relatively unchanged,1O2* increases linearly, and 3C* concentrations increase lessthan linearly, likely due to quenching by dissolved organics. Based on ourmeasurements, and accounting for additional sources and sinks that should beimportant under PM conditions, we estimate that [⚫OH] inparticles is somewhat lower than in dilute cloud/fog drops, while [3C*]is 30 to 2000 times higher in PM than in drops, and [1O2*] isenhanced by a factor of roughly 2400 in PM compared to drops. Because ofthese enhancements in 1O2* and 3C* concentrations,the lifetimes of some highly soluble organics appear to be much shorter inparticle liquid water than under foggy/cloudy conditions. Based onextrapolating our measured rates of formation in PM extracts, BrC-derivedsinglet molecular oxygen and triplet excited states are overall the dominantsinks for organic compounds in particle liquid water, with an aggregate rateof reaction for each oxidant that is approximately 200–300 times higherthan the aggregate rate of reactions for organics with ⚫OH. Forindividual, highly soluble reactive organic compounds it appears that1O2* is often the major sink in particle water, which is a newfinding. Triplet excited states are likely also important in the fate ofindividual particulate organics, but assessing this requires additionalmeasurements of triplet interactions with dissolved organic carbon innatural samples.