UC Davis

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.