Seasonal variations in photooxidant formation and light absorption in aqueous extracts of ambient particles
Abstract. 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.
Lan Ma et al.
Status: open (until 30 Jun 2023)
- RC1: 'Comment on egusphere-2023-861', Anonymous Referee #1, 25 May 2023 reply
Lan Ma et al.
Lan Ma et al.
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The manuscript aims at measuring the seasonal variation in photooxidant formation and concentration in atmospheric water and to predict the lifetime of 5 compounds in the atmosphere. Overall, I found the article well written and would support its publication as it brings interesting information to the community.
I did not find major issues in the article, here is my list of comments and corrections:
Abstract and introduction
The abstract and introduction are clear. In addition to singlet oxygen, excited triplet states and hydroxyl radical, the authors could also mention in the introduction other photooxidants that were not considered in the study but that may play a role in the transformation of some classes of contaminants. E.g., Halides radicals may play a role in the transformation of electron rich compounds (Marine Chemistry 115 (2009) 134–144) or long-lived photooxidant could be important for the transformation of phenols or anilines (Water Research 213 (2022) 118095).
L25. It looks to me that the OH quantum yield value is too high and does not correspond to the values presented in the article (Table S3).
L.79. I would switch organic compounds for DOM as the quoted studies presents correlations between 3DOM* quantum yields and factors correlating with the molecular weight / aromaticity.
Material and methods
L.141. I would indicate the spectrophotometer cuvette pathlength.
l.146 I would add in the SI the arc lamp spectra, that is important to evaluate nitrate photolysis.
Results and discussion
The results are presented in a logical order, I have two main comments on the results:
Figures, the date format may confuse non-American reader (e.g., one can read the first date as November first 2019 or January 11th 2019). I would suggest writing the months to be clearer. Also, the numbers on the y-axis could be written as 1×10-15 (and not 1E-15).
L.306. “fresh BB are fragmented during aging”, it could be noted that ozone exposure also induces and increase of E2/E3 (Leresche et al. quoted in the manuscript) and that ozone indeed also induce a decrease in mean molecular weight indicating that fragmentation occurs during ozonation (Environmental Science & Technology, 2023 57 (14), 5603-5610).
L.347. DDT assay, the abbreviation is not defined, switch for the full name.
L.450. Do the authors think that there are anilines moieties in PME ? I would suggest withdrawing the mention to anilines.
L.508. The second-order rate constant between singlet oxygen and water was reevaluated to be of 2.76*105 M-1 s-1 (Environ. Sci.: Processes Impacts, 2017, 19, 507–516) I would suggest using the more recent value.
L.552. 3C* fraction that produces singlet oxygen (fΔ). This fraction was recently measured for Suwannee River fulvic acid to be of 0.34 (Environ. Sci. Technol. 2017, 51, 13151−13160). The value from McNeill and Canonica is a rule of thumb I believe. It would be worth mentioning this 0.34 value.
L.678. “Estimated concentrations of 1O2, 3C*, and OH in ALW are on the order of 10-12 - 10-11, 10-13 - 10-12 and 10-14 M”. I would suggest putting the respective number range next to the corresponding reactive species, as it is, it is difficult to see which numbers correspond to what.
L.993 and L.66, it should be Hoigné and not Hoigne.