Atmospheric waters – including fog/cloud drops and aerosol liquid water (ALW) – are important sites for the transformations of atmospheric species and the formation of aqueous secondary organic aerosols (aqSOA). These chemical processes largely occur through reactions with photoformed oxidants such as hydroxyl radical (●OH), singlet molecular oxygen (1O2*), and oxidizing triplet excited states of organic matter (3C*). Despite this, there are few measurements of these photooxidants, especially in extracts of ambient particles. Moreover, ALW – which has high solute concentrations – has great potential to produce aqSOA, but the chemistry in ALW is poorly understood. To address this gap, in this work we measured photooxidant concentrations and formation kinetics in dilute aqueous particle extracts and extrapolated the results to ALW conditions. We also studied the kinetics of triplet reactions with biomass burning phenols under ALW conditions.In chapter 2, we investigated kinetics of six highly substituted phenols that are potentially important in ALW with the triplet excited state of 3,4-dimethoxybenzaldehyde. Second-order rate constants at pH 2 are all fast, (2.6-4.6) × 10^9 M-1 s-1, while values at pH 5 are 2-5 times smaller. Rate constants are reasonably described by a quantitative structure-activity relationship with phenol oxidation potentials. Triplet-phenol kinetics are unaffected by ammonium sulfate, sodium chloride, galactose (a biomass-burning sugar), or Fe(III). In contrast, ammonium nitrate increases the rate of phenol loss by making ●OH, while Cu(II) inhibits phenol decay. Our results suggest that phenols with high KH can be an important source of aqSOA in ALW, with 3C* typically the dominant oxidant. In chapter 3, we evaluated how dissolved organic matter (DOM) and Cu(II) inhibit 3C* probe decay, which can cause an underestimation of 3C* concentrations. Our overarching goal is to find a triplet probe that has low inhibition by DOM and Cu(II), and low sensitivity to 1O2*. We tested 12 potential probes from a variety of compound classes and found that (phenylthiol)acetic acid (PTA) seems well suited for ALW conditions, with mild inhibition and fast rate constants with triplets. We evaluated the performance of both PTA and syringol (SYR) as triplet probes in several aqueous extracts of particulate matter. While PTA is less sensitive to inhibition than SYR, it measures lower triplet concentrations, possibly because it is less sensitive to weakly oxidizing triplets. In chapter 4, we measured photooxidant concentrations in illuminated aqueous particle extracts as a function of particle dilution and used the resulting oxidant kinetics to extrapolate to ALW conditions. Extracts were prepared from winter (WIN) and summer (SUM) particles from Davis, California. As particle mass gets more concentrated, ●OH concentrations in WIN remain relatively unchanged, while they increase in SUM. In both winter and summer samples, 3C* concentrations increase rapidly with particle mass concentrations and then plateau under more concentrated conditions. 1O2* in WIN increases linearly with particle mass while in SUM it approaches a plateau. Under ambient aerosol liquid water conditions we predict a ratio of concentrations of 1O2*: 3C*: ●OH of 10^3 – 10^2: 10^2: 1 with ●OH concentrations on the order of 10^-14 M (including mass transport from the gas phase). Although ●OH is often considered the main sink for organic compounds in the aqueous phase, the much higher concentrations of 3C* and 1O2* in aerosol liquid water suggest these photooxidants are more important sinks for many organics in particle water. In chapter 5, we investigate the seasonal variation of photooxidants by measuring their concentrations in dilute aqueous extracts of ambient particles. Based on UV/Vis and aerosol mass spectrometer data, we categorized our 18 samples into four groups: Winter & Spring (Win-Spr) influenced by residential wood combustion; Summer & Fall (Sum-Fall) without wildfire influence; summer fresh biomass burning particles from wildfires (FBB), and aged biomass burning plumes (ABB). The concentration ratio of 1O2*: 3C*: ●OH in dilute particle extracts is (150 – 1500) : (10 – 300) : 1, respectively, while this ratio in ALW is predicted to be (100 – 1000): (10 – 150): 1. FBB has the highest mass absorption coefficient (MAC) and highest average 1O2* concentration but lowest average quantum yields (Φ) for formation of all three photooxidants. Win-Spr and ABB have similar MAC and higher average Φ of oxidants, indicating aging tends to enhance quantum yields. Sum-Fall has the lowest 1O2* due to low DOC, but highest Φ1O2*. 1O2* and 3C*determined by SYR are linearly correlated with DOC and appear to be independent of sample type.

Biomass burning (BB) is an important source of fine particulate matter and reactive volatile gases that affect air quality, human health, and climate. Phenols and furans are two classes of organic gases from BB that react with various gas-phase oxidants to form secondary organic aerosol (SOA). These gases can also partition into aqueous phases including clouds, fog drops, and aerosol liquid water (ALW) and undergo reaction with aqueous photooxidants, such as hydroxyl radical (●OH), triplet excited organic states of organic compounds (3C* or triplets), and singlet molecular oxygen (1O2*). While a significant amount is known about SOA formation in the gas phase, less is known about the reactions of phenols and furans in the aqueous phase. To investigate this, we studied the aqueous-oxidation kinetics of these two classes of organics and evaluated the potential for both gas- and aqueous-phase reactions of phenols and furans to form SOA across a wide range of liquid water contents.In chapter 2, we investigated the oxidation kinetics of six highly substituted phenols with ●OH since these reactions might be an important source of SOA in ALW. The second-order reaction rates are fast, with rate constants in the range (1.9 – 14) × 109 M-1 s-1 at pH 2 and (14 – 20) × 109 M-1 s-1 at pH 5 and 6. ALW solutes have a range of impacts on phenol oxidation by •OH: a BB sugar and some inorganic salts were found to suppress oxidation, while a nitrate salt and transition metals were observed to enhance oxidation. Our results suggest formation of aqueous-SOA (aqSOA) is significant across clouds/fog drops and ALW, although gas-phase •OH becomes dominant under ALW conditions. In chapter 3, we evaluated how aqueous reactions of phenols with ●OH and 3C* might be a source of light-absorbing organic carbon, i.e., brown carbon (BrC). Past work found these reactions form low-volatility, high-molecular-weight products, but the light absorption and lifetimes of the BrC formed are poorly understood. To study this, we monitored the formation and loss of light absorption by the aqSOA. While the parent phenols absorb very little tropospheric sunlight, they are oxidized to aqSOA that can absorb significant amounts of sunlight. The extent of light absorption by the aqSOA depends on both the ArOH precursor and oxidant: more light-absorbing aqSOA is formed from more highly substituted phenols and from triplet reactions rather than ●OH. Estimated lifetimes of light-absorbing phenolic aqSOA range from 3 to 17 hours in cloud/fog drops, where ●OH is the major sink, and from 0.7 to 8 hours in ALW, where triplet excited states are the major sink. In chapter 4, we measured the oxidation kinetics of 17 furans with singlet oxygen. Second-order rate constants for furans with 1O2* widely vary with chemical substitution, ranging from 3.7 × 103 to 5.9 × 108 M-1 s-1. Inorganic species have a range of impacts on singlet oxygen oxidation kinetics, with ammonium sulfate greatly suppressing oxidation, especially at high concentrations. We estimated the rate of SOA formation from the reaction of 2-methylfuran-3,4-dicarboxylic acid, which suggest that aqueous reactions of highly substituted furans with 1O2* could be a significant source of aqSOA but that triplet excited states and hydroxyl radical can also be significant oxidants.