||Laboratory photochemical processing of aqueous aerosols : formation and degradation of dicarboxylic acids, oxocarboxylic acids and alpha-dicarbonyls
Pavuluri, C. M. ,
Kawamura, K. ,
Mihalopoulos, N.Swaminathan, T.
Atmospheric chemistry and physics
8012 , 2015-09-17 , Copernicus Gesellschaft Mbh
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 alpha-dicarbonyls. The results show that photochemical degradation of oxalic (C-2), malonic (C-3) and other C-8-C-12 diacids overwhelmed their production in aqueous aerosols, whereas succinic acid (C-4) and C-5-C-7 diacids showed a significant increase (ca. 10 times) during the course of irradiation experiments. The photochemical formation of oxoacids and alpha-dicarbonyls overwhelmed their degradation during the early stages of experiment except for omega-oxooctanoic acid (omega C-8), which showed a similar pattern to that of C-4. We also found a gradual decrease in the relative abundance of C-2 to total diacids and an increase in the relative abundance of C-4 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 C-2 and C-3 diacids controls their concentrations in Fe-rich atmospheric waters, whereas photochemical formation of C-4 diacid (via omega C-8) 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 alpha-dicarbonyls, although some species such as oxalic acid decompose extensively during an early stage of photochemical processing.