Session: Aquatic Photochemistry:

Singlet oxygen (¹O₂) is a reactive oxygen species that has recently gained attention as a competitive oxidant in the atmosphere. This excited state of molecular oxygen is formed by indirect photochemistry in the presence of chromophoric dissolved organic matter (DOM) as sensitizers, molecular oxygen and sunlight. The highly reactive intermediate ¹O₂is then capable of degrading many organic atmospheric components, affecting their lifetime in the atmosphere. Despite this influence on atmospheric fate, the spatiotemporal distribution of ¹O₂ in DOM is currently unknown. We hypothesized that biomass burning organic aerosols emitted during winter would lead to higher ¹O₂ production compared to summer. Therefore, to advance atmospheric ¹O₂research, we investigated the ¹O₂ sensitizing ability of organic aerosols sampled on 24-hour PM10 filters. Specifically, these filters were collected throughout 2013 in Frauenfeld and San Vittore in Switzerland, characterized as urban background and rural traffic measurement stations, respectively. We extracted the water-soluble organic components and quantified ¹O₂steady state concentrations as well as ¹O₂ quantum yield. In our ongoing efforts, our results from Frauenfeld so far show a range between 0.38 – 6.05 x 10^-13M for ¹O₂steady state concentrations and quantum yields up to 2.1± 0.5 %. In preliminary experiments, samples from the rural site San Vittore show similar values. The values underline ¹O₂’s potential importance for atmospheric processing, e.g. comparing to Manfrin et al. (ES&T, 2019) who reported ¹O₂steady state concentrations of 3 ± 1 x 10^-14M from secondary organic aerosols extracts. More importantly, the filter extracts analyzed thus far show a strong seasonal trend, with increased ¹O₂values in winter as compared to summer. This result corroborates the hypothesis that there is more chromophoric DOM present in winter, due to a higher fraction of brown carbon emitted e.g. in biomass burning for residential heating. To extend this analysis, we are currently correlating the results for ¹O₂with molecular markers based on mass spectrometry data available from previous filter analysis provided by Daellenbach et al., (ACP, 2017). Finding these correlations will enable the prediction of ¹O₂sensitizing abilities of organic material present in the aerosols both qualitatively and quantitatively. In all, our work will help constrain the seasonal relevance of ¹O₂photochemistry in the atmosphere.

Imad elHaddad

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