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. Biomass burning (BB) emits large amounts of brown carbon (BrC); however, theevolutionary behavior of BrC in BB emissions (BB BrC) resulting from complexatmospheric processes is poorly understood. In this study, thetransformation of contents and the chromophoric characteristics of BrC insmoke particles emitted by the burning of rice straw (RS), corn straw (CS), and pinewood (PW) under O3 aging are investigated. The O3 aginginduced the reduction of light absorption and fluorescence for the BB BrC,suggesting the decomposition of chromophores and fluorophores. These changeswere accompanied by a decrease in aromaticity, average molecular weight, andthe light absorption capacity for the chromophores, as well as an increase in humification for the fluorophores. The excitation emission matrix combinedwith a parallel factor analysis revealed that protein-like components (C3)were predominantly decomposed by O3 aging, while the relativedistribution 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 allthe BB BrC samples, which accounted for 84 %–87 % of the totalfluorophores in final O3-aged BB BrC. Two-dimensional correlationspectroscopy (2D-COS) was performed on the synchronous fluorescence, whichsuggested that the RS and CS BrC exhibits the same susceptiblefluorophores changes upon O3 aging. It showed that O3 firstlyreacted with protein-like fractions (263–289 nm) and then with fulvic-likefractions (333–340 nm). In comparison, the changing sequence of susceptiblefluorophores in the PW BrC to O3 was in the order of fulvic-like fluorophores withshorter wavelengths (309 nm), protein-like fluorophores (276 nm), and fulvic-like fluorophores with longer wavelengths (358 nm). The2D-FTIR-COS (2D-COS combined with FTIR) analysis showed conjugated C=O and aromatic C=C and C=Ogroups were the most susceptible functional groups to O3 aging for allBB BrC. Moreover, it also revealed a consistent sequential change, 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. Ourresults provide new insights into the evolutionary behavior of thechromophoric and fluorescent properties of BB BrC during O3 aging,which are of great significance for better understanding the heterogeneousoxidation pathways of BB-derived BrC in the atmospheric environment.

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