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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 3 C, 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 FROM AUTHOR]

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 (1 O 2*), and oxidizing triplet excited states of organic matter (3 C *) – 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, 3 C * 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 1 O 2* concentrations, (0.2–8.5) × 10 -12 M, but in WIN the 1 O 2* concentration increases linearly with DOC, while in SUM 1 O 2* 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 3 C * and 1 O 2* 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 O 2* in aerosol liquid water suggest these photooxidants will be more important sinks for many organics in particle water. [ABSTRACT FROM AUTHOR]

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 15 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) 20 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) x 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* 25 concentrations have a range of (0.7-45) x 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 30 - 7.9) x 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*. [ABSTRACT FROM AUTHOR]