Abstract
Abstract. To better understand the photochemical processing of dicarboxylic acids and related polar compounds, we conducted batch UV irradiation experiments on two types of aerosol samples collected from India, which represent anthropogenic (AA) and biogenic (BA) aerosols, for time periods of 0.5 to 120 h. The irradiated samples were analyzed for molecular compositions of diacids, oxoacids and α-dicarbonyls. The results show that photochemical degradation of oxalic (C2), malonic (C3) and other C8–C12 diacids overwhelmed their production in aqueous aerosols, whereas succinic acid (C4) and C5–C7 diacids showed a significant increase (ca. 10 times) during the course of irradiation experiments. The photochemical formation of oxoacids and α-dicarbonyls overwhelmed their degradation during the early stages of experiment except for ω-oxooctanoic acid (ωC8), which showed a similar pattern to that of C4. We also found a gradual decrease in the relative abundance of C2 to total diacids and an increase in the relative abundance of C4 during prolonged experiment. Based on the changes in concentrations and mass ratios of selected species with the irradiation time, we hypothesize that iron-catalyzed photolysis of C2 and C3 diacids controls their concentrations in Fe-rich atmospheric waters, whereas photochemical formation of C4 diacid (via ωC8) is enhanced with photochemical processing of aqueous aerosols in the atmosphere. This study demonstrates that the ambient aerosols contain abundant precursors that produce diacids, oxoacids and α-dicarbonyls, although some species such as oxalic acid decompose extensively during an early stage of photochemical processing.
Highlights
Dicarboxylic acids and related polar compounds constitute a significant fraction of water-soluble organic aerosols in the atmosphere (Saxena and Hildemann, 1996; Kawamura and Sakaguchi, 1999; Pavuluri et al, 2010)
Ation followed by a gradual decrease in the prolonged experiment with the exception of ωC8 acid, which showed a pattern similar to C4 diacid, and methylglyoxal, which remained relatively abundant from 24 h to the end of the experiment in AA
The mass ratios of C2 diacid to its precursors, i.e., glyoxylic acid, pyruvic acid, α-dicarbonyls and C3, showed a considerable increase with irradiation, while those of C4 to C5–C7 diacids and ωC8 acid and methylglyoxal to glyoxal in AA showed a significant increase with irradiation
Summary
Dicarboxylic acids and related polar compounds constitute a significant fraction of water-soluble organic aerosols in the atmosphere (Saxena and Hildemann, 1996; Kawamura and Sakaguchi, 1999; Pavuluri et al, 2010). Diacids and related compounds have an impact on the indirect radiative forcing and hydrological cycle (Twomey, 1977; Albrecht, 1989). They involve in a series of reactions occurring in gas phase, aerosols and atmospheric waters Oxoacids and α-dicarbonyls can be directly emitted into the atmosphere from incomplete combustion of fossil fuels (Kawamura and Kaplan, 1987) and biomass burning (Narukawa et al, 1999), they are mainly formed by secondary processes of volatile organic compounds of anthropogenic and biogenic origin (Kawamura et al, 1996a; Kawamura and Sakaguchi, 1999; Kanakidou et al, 2005)
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