Distribution and stable carbon isotopic composition of dicarboxylic acids, ketocarboxylic acids and <i>α</i>-dicarbonyls in fresh and aged biomass burning aerosols

Abstract

Biomass burning (BB) is a significant source of dicarboxylic acids (diacids) and related compounds that play important roles in atmospheric chemistry and climate change. In this study, a combustion chamber and oxidation flow reactor were used to generate fresh and aged aerosols from burned rice, maize and wheat straw to investigate atmospheric aging and the stable carbon isotopic (δ13C) composition of these emissions. Succinic acid (C4) was the most abundant species in fresh samples, while oxalic acid (C2) became dominant after atmospheric aging. Of all diacids, C2 had the highest aged to fresh emission ratios (A/F), suggesting that C2 is largely produced through secondary photochemical processes. Compared with fresh samples, the emission factors of ketocarboxylic acids and α-dicarbonyls increased after 2 d but decreased after 7 d aging, indicating a short residence time and further atmospheric degradation from 2 to 7 d. The δ13C values of C2 for aged biomass samples were higher than those of urban aerosols but lower than marine or mountain aerosols, and the δ13C values of C2 became isotopically heavier during aging. Relationships between the reduction in volatile organic compounds (VOCs), such as toluene, benzene and isoprene, and increase in diacids after 2 d aging indicate that these VOCs led to the formation of diacids. However, no significant correlation was found between decreases in VOCs and increases in 7 d aged diacids. In addition, the A/F of C2 was 50.8 at 2 d and 64.5 at 7 d, indicating that the conversion of VOCs to C2 was almost completed within 2 d. For the longer aging times, the particulate-phase compounds may undergo further degradation in the oxidation processes.