The evolutionary behavior of chromophoric brown carbon during ozone aging of fine particles from biomass burning

Abstract. Biomass burning (BB) emits large amounts of brown carbon (BrC), however, little is known about the evolutionary behavior of BrC produced by BB (BB-BrC) in atmospheric processes. In this study, the transformation of levels and the chromophoric characteristics of BrC in smoke particles emitted by the burning of rice straw (RS), corn straw (CS) and pine wood (PW) under O3 aging are investigated. The O3 aging induced the reduction of light absorption and fluorescence for the BB-BrC, suggesting the decomposition of chromophores and fluorophores. These changes were accompanied by a decrease of aromaticity, average molecular weight and the light absorption capacity for the chromophores, and an increase of humification for the fluorophores. The excitation emission matrix combined with a parallel factor analysis revealed that protein-like components (C3) were predominantly decomposed by O3 aging, while the relative distribution of a humic-like component with highly oxygenated chromophores (C4) gradually increased. In general, the humic-like substances (C1+C2+C4) were transformed to be the most abundant fluorophores for all the BB-BrC samples, which accounted for 84 %–87 % of the total fluorophores in final O3-aged BB-BrC. Two dimensional correlation spectroscopy (2D-COS) was performed on the synchronous fluorescence, which suggested that the RS- and CS- BrC exhibits the same susceptible fluorophores changes upon O3 aging. It showed that O3 firstly reacted with protein-like fractions (263–289 nm), and then with fulvic-like fractions (333–340 nm). In comparison, the changing sequence of susceptible fluorophores in the PW-BrC to O3 were in the order of fulvic-like with shorter wavelengths (309 nm) > protein-like fluorophores (276 nm) > fulvic-like fluorophores with longer wavelengths (358 nm). The 2D-FTIR-COS analysis showed conjugated C=O and aromatic C=C and C=O groups were the most susceptible functional groups to O3 aging for all BB-BrC. Moreover, it also revealed a consistent sequential changes, which is in the order of aromatic OH, conjugated C=O groups and aromatic C=O, aromatic COO−, and finally lignin-derived C-C, C-H and C-O groups. Our results provide new insights into the evolutionary behavior of the chromophoric and fluorescent properties of BB-BrC during O3 aging. They have important implications for the heterogeneous oxidation of BB emission to form BrC, and are of great significance for improving the accuracy of climate models and source apportionment models parameterized by the optical properties of BrC.