Abstract
Abstract. Photochemical reactions represent the main pathway for the removal of non-methane volatile organic compounds (VOCs) in the atmosphere. VOCs may react with hydroxyl radical (OH), the most important atmospheric oxidant, or they can be photolyzed by actinic radiation. In the presence of clouds and fog, VOCs may partition into the aqueous phase where they can undergo aqueous photolysis and/or reaction with dissolved OH. The significance of direct aqueous photolysis is largely uncertain due to the lack of published absorption cross sections and photolysis quantum yields. In light of this, we strive to identify atmospherically relevant VOCs where removal by aqueous photolysis may be a significant sink. The relative importance of different photochemical sinks is assessed by calculating the ratios of the removal rates inside air parcels containing cloud and fog droplets. This relative approach provides useful information in spite of the limited aqueous photolysis data. Results of this work should help guide researchers in identifying molecules that are the most likely to undergo aqueous OH oxidation and photolysis. For example, we find that out of the 27 atmospherically relevant species investigated, the removal of glyceraldehyde and pyruvic acid by aqueous photolysis is potentially an important sink. We also determine the relative magnitudes of these four chemical sinks for the set of relevant organic compounds.
Highlights
Organic compounds play a key role in the chemistry of the atmosphere
It is estimated that 130–270 Tg C yr−1 of emitted volatile organic compounds (VOCs) are removed by dry and wet deposition, while the remaining 1030–1170 Tg C yr−1 are transformed by chemical reactions
Out of the oxygenated organic compounds identified by Saxena and Hildemann (1996) we choose a subset of 27 compounds for which there is the required experimental data published in the literature
Summary
Organic compounds play a key role in the chemistry of the atmosphere. Global emissions of non-methane volatile organic compounds (VOCs) total approximately 1300 Tg C yr−1 (Atkinson and Arey, 2003; Goldstein and Galbally, 2007).It is estimated that 130–270 Tg C yr−1 of emitted VOCs are removed by dry and wet deposition, while the remaining 1030–1170 Tg C yr−1 are transformed by chemical reactions. Oxidation by hydroxyl radical is the most important atmospheric sink for many VOCs (Andreae and Crutzen, 1997). Gas phase photolysis can be important for certain classes of compounds including carbonyls (Moortgat, 2001), peroxides (Lee et al, 2000), and organic nitrates (Atkinson, 1990). In the presence of clouds and fog, VOCs may partition into the aqueous phase to various extents depending on their solubility. As in the gas phase, OH is the main oxidant for the transformation of aqueous organics in cloud and fog droplets (Ervens et al, 2011). Direct photolysis in the aqueous phase can be an important sink depending on the chemical functionality of the species (Vione et al, 2006; Graedel and Goldberg, 1983)
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