Seasonal variations in photooxidant formation and light absorption in aqueous extracts of ambient particles

Atmospheric waters – including fog/cloud drops and aerosol liquid water – are important sites for the transformations of atmospheric species, largely 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, and very little information about how oxidant levels vary with season or particle type. To address this gap, we collected ambient PM2.5 from Davis, California over the course of a year and measured photooxidant concentrations in dilute aqueous extracts of the particles. We categorized samples into four groups: Winter & Spring (Win-Spr), Summer & Fall (Sum-Fall) without wildfire influence, fresh biomass burning (FBB), and aged biomass burning (ABB). FBB contains significant amounts of brown carbon (BrC) from wildfires, and the highest mass absorption coefficients (MAC) normalized by dissolved organic carbon, with an average (± 1 σ) value of 3.3 (± 0.4) m2 (g C)−1 at 300 nm. Win-Spr and ABB have similar MAC averages, 1.9 (± 0.4) and 1.5 (± 0.3) m2 (g C)−1, respectively, while Sum-Fall has the lowest MACDOC (0.65 (± 0.19) m2 (g C)−1). ●OH concentrations in extracts range from (0.2–4.7) × 10−15 M and generally increase with concentration of dissolved organic carbon (DOC), although this might be because DOC is a proxy for extract concentration. The average quantum yield for ●OH formation (ΦOH) across all sample types is 3.7 (± 2.4) %, with no statistical difference among sample types. 1O2* concentrations have a range of (0.7−45) × 10−13 M, exhibiting a good linearity with DOC that is independent of sample type (R2 = 0.93). Fresh BB samples have the highest [1O2*] but the lowest average Φ1O2*, while Sum-Fall samples are the opposite. Φ1O2* is negatively correlated with MACDOC, indicating that less light-absorbing samples form 1O2* more efficiently. We quantified 3C* concentrations with two triplet probes: syringol (SYR), which captures both strongly and weakly oxidizing triplets, and (phenylthio)acetic acid (PTA), which is only sensitive to strongly oxidizing triplets. Concentrations of 3C* are in the range of (0.03–7.9) × 10−13 M and linearly increase with DOC (R2 = 0.85 for SYR and R2 = 0.80 for PTA); this relationship for [3C*]SYR is independent of sample type. The average ratio of [3C*]PTA/[3C*]SYR is 0.58 (± 0.38), indicating that roughly 60 % of oxidizing triplets are strongly oxidizing. Win-Spr samples have the highest fraction of strongly oxidizing 3C*, with an average of 86 (± 43) %. Φ3C*,SYR is in the range of (0.6–8.8) %, with an average value, 3.3 (± 1.9) %, two times higher than Φ3C*,PTA. FBB has the lowest average Φ3C*, while the aging process tends to enhance Φ3C*, as well as Φ1O2*. To estimate photooxidant concentrations in particle water, we extrapolate the photooxidant kinetics in our dilute particle extracts to aerosol liquid water (ALW) conditions of 1 µg PM/µg H2O for each sample type. The estimated ALW ●OH concentration is 7 × 10−15 M when including mass transport of gas-phase ●OH to the particles. 1O2* and 3C* concentrations in ALW have ranges of (0.6–7) × 10−12 M and (0.08–1) × 10−12 M, respectively. In the Win-Spr and Sum-Fall samples, photooxidant concentrations increase significantly from lab particle extracts to ALW, while the changes for the FBB and ABB samples are minor. The small increases in 1O2* and 3C* from extract to ALW for the biomass burning particles are likely due to the high amounts of organic compounds in the extracts, which lead to strong quenching of these oxidants even under our dilute conditions. Compared to the photooxidant concentration estimates in Kaur et al. (2019), our updated ALW estimates show higher ●OH (by roughly a factor of 10), higher 3C* (by factors of 1–5) and lower 1O2* concentrations (by factors of 20–100). Our results indicate that 3C* and 1O2* in ALW dominate the processing of organic compounds that react quickly with these oxidants (such as phenols and furans, respectively), while ●OH is more important for less reactive organics.