Efficient production of singlet oxygen and organic triplet excited states in aqueous PM2.5 in Hong Kong, South China
Abstract. Photooxidants drive many atmospheric chemical processes. The photoexcitation of light-absorbing organic compounds (i.e., brown carbon (BrC)) in atmospheric waters can lead to the generation of reactive organic triplet excited states (3C∗), which can undergo further reactions to produce other photooxidants such as singlet oxygen (1O2). To determine the importance of these aqueous photooxidants in SOA formation and transformation, we must know their steady-state concentrations and quantum yields. However, there has been limited measurements of aqueous 3C∗ and 1O2 in atmospheric samples outside of North America and Europe. In this work, we report the first measurements of the steady-state concentrations and quantum yields of 3C∗ and 1O2 produced in aerosols in South China. We quantified the production of 3C∗ and 1O2 in illuminated aqueous extracts of PM2.5 collected in different seasons at two urban sites and one coastal semi-rural site during a year-round study conducted in Hong Kong, South China. The mass absorption coefficients at 300 nm for BrC in the aqueous PM2.5 extracts ranged from 0.49 × 104 to 2.01 × 104 cm2 g-C−1 for the three sites. Both 1O2 and 3C∗ were produced year-round. The steady-state concentrations of 1O2 ([1O2]ss) in the illuminated aqueous extracts spanned two orders of magnitude, ranging from 1.56 × 10−14 to 1.35 × 10−12 M, with a study average of (4.02 ± 3.52) × 10−13 M. The steady-state concentrations of 3C∗ ([3C∗]ss) in the illuminated aqueous extracts spanned two orders of magnitude, ranging from 2.93 × 10−16 to 8.08 × 10−14 M, with a study average of (1.09 ± 1.39) × 10−14 M. The [1O2]ss and [3C∗]ss correlated with the concentration and absorbance of BrC, thus implying that the amount of BrC drives the steady-state concentrations of these photooxidants. The locations (urban vs. semi-rural) did not have a significant effect on [3C∗]ss and [1O2]ss, which indicated that BrC from local sources did not have a significant influence on the year-round 3C∗ and 1O2 production. 3C∗ and 1O2 production were found to be the highest in winter and the lowest in summer for all three sites. The observed seasonal trends of 1O2 and 3C∗ production could be attributed to the seasonal variations in long-range air mass transport. Our analysis highlighted the key role that regional sources play in influencing the composition and concentrations of water-soluble BrC in winter PM2.5 in Hong Kong, which contributed to their highest 3C∗ and 1O2 production. The current results will be useful for modeling seasonal aqueous organic aerosol photochemistry in the South China region.
Yuting Lyu et al.
Status: open (until 01 Jun 2023)
- RC1: 'Comment on egusphere-2023-739', Anonymous Referee #1, 12 May 2023 reply
- RC2: 'Comment on egusphere-2023-739', Cort Anastasio, 20 May 2023 reply
Yuting Lyu et al.
Yuting Lyu et al.
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The authors present an observational study on PM2.5 aqueous extract photochemistry. They measured singlet oxygen and excited triplet states on PM2.5 collected in urban and rural sites in Hong Kong. Not much is known about singlet oxygen and excited triplet states observations outside of Europe and North America and I think that this manuscript add interesting observations on PM2.5 properties.
Abstract and Introduction
The abstract and introduction are clearly written and present well the singlet oxygen and organic triplet literature.
The methods section is well documented, and the experiments well described. I have one small technical comment about the determination of the rate of light absorption (equation 2):
In equation 2, it is not clear what the authors used for the optical pathlength, is it the diameter of the quartz tubes or some average path length through the tubes ? A side question on that point is if an incorrect pathlength in the solution would influence the singlet oxygen quantum yield determination. I am wondering about the presented singlet oxygen quantum yield numbers that are higher than in previous studies. The Rayonet reactor used by the authors has reflecting side walls and the effective path length could be longer than the measured one due to photon passing multiple time through the experiment’s solution.
Results and discussion
Reading the results and discussion, I was left a little wondering about the reasons for the observed seasonality. Maybe the authors could elaborate a little more. Here are some of my thoughts on the subject:
The authors did not see significant differences in the extracts between the three sampling sites (and attributed that to the brown carbon source being mostly not local) but observed a seasonal difference in extracts characteristics. Reading the article, I understand that the authors attributed the winter brown carbon to mainland China sources. I was left wondering about what the summer brown carbon sources are. Are the authors attributing the summer provenance to local sources, marine emissions or on lands further apart from Hong Kong ? It would be worth being clearer about this point.
This could have some implications if the summer aerosols are older than the winter ones and could explain some of the seasonal differences. Literature on photobleaching indicate that light exposure induces a loss of sensitizing properties (Water Research Volume 66, 1 December 2014, Pages 140-148 Photobleaching-induced changes in photosensitizing properties of dissolved organic matter) and a loss in absorbance (Environ. Sci. Technol. 2021, 55, 13152−13163).
The authors observed an increased singlet oxygen quantum yield for the fall and winter extracts. If the summer extracts were older and more exposed to sunlight that could explain part of the observed seasonal difference.
A last thought on the high singlet oxygen quantum yield observed is that ozone exposure may induce an increase in singlet oxygen quantum yield (Environ. Sci. Technol. 2019, 53, 5622−5632). If the authors think that their extracts were more exposed to ozone than extracts from other (north American and European studies), that could be a possible explanation.
Here are now some more specific comments than need to be addressed:
1) Line 254 and Lines 268-269: “due to the presence of aromatic compounds (e.g., polycyclic aromatic hydrocarbons) from local vehicle emissions” and “These results implied that the water-soluble BrC in PM2.5 was weakly influenced by local emission sources near the sites.” These two sentences look to be saying first that local emission sources are important and second that it is not important.
2) Line 332: “On average, Rabs was about 20 times higher than the sum of Rf,1O2 and Rf,3C∗ . This indicated that majority of the (photo) energy absorbed by the illuminated extracts in the photochemical experiments were dissipated by non-reactive pathways” this paragraph is misleading. Rf,3C∗ is a very small subset of the total triplets. The total triplets rate of light absorption can be estimated to be around 3 times Rf,1O2. The factor 3 coming from the estimate of the yield of triplet state conversion to singlet oxygen found in Environ. Sci. Technol. 2017, 51, 13151−13160. Also, the authors should not sum Rf,1O2 and Rf,3C∗ as singlet oxygen if formed from the triplet states.