Diurnal changes in the distribution of dicarboxylic acids, ketocarboxylic acids and dicarbonyls in the urban Tokyo atmosphere

Abstract

Secondary organic aerosol in the urban atmosphere is an important issue of not only local air pollution but also regional and global atmospheric chemistry. To better understand the production of water-soluble organic aerosols, we analyzed time-series aerosol samples collected in Tokyo every 3 h during summer and winter campaigns for homologous dicarboxylic acids (C2–C10), ketocarboxylic acids (C2–C4) and α-dicarbonyls (C2–C3) using a GC and GC/MS. Here, we report, for the first time, diurnal distributions of dicarboxylic acids, ketoacids and dicarbonys in the urban atmosphere. Oxalic acid was found as the most abundant species, followed by malonic, succinic, phthalic, pyruvic and glyoxylic acids. Total concentrations of diacids, ketoacids and dicarbonyls showed a concentration maximum in daytime (11–14 h) with a dynamic change in their molecular composition especially in summer samples. Relative abundances of unsaturated (maleic, methylmaleic and phthalic) and some saturated (methylsuccinic and adipic) species increased in the morning (8–11 h) whereas those of oxalic acid decreased down to 35% in 8–11 h but increased up to 58% toward 17–20 h. The diurnal variations are interpreted by photochemical production of diacids, where unsaturated and some saturated species are preferentially generated early in the morning via photo-oxidation of aromatic hydrocarbons and cyclic olefins and further oxidized to oxalic acid in the afternoon.