The aging behaviors of chromophoric biomass burning brown carbon during dark aqueous hydroxyl radical oxidation processes in laboratory studies

This study was conducted to explore the aging behavior of biomass burning (BB) emitted brown carbon (BrC) during an aqueous hydroxyl radical (OH) oxidation process in the absence of light. The evolution characteristics of rice straw (RS) and pine wood (PW) emitted BrC were monitored by UV–vis, synchronous fluorescence and excitation-emission matrix (EEM) spectroscopies combined with two-dimensional correlation spectroscopy (2D-COS) and parallel factor (PARAFAC) analysis. The results indicated that OH radical oxidation could greatly weaken the absorption and fluorescence intensity of both BB BrC, implying the significant degradation of chromophoric BB BrC. EEM-PARAFAC analysis identified one humic-like (C2) and two protein-like (C1, C3) components from BB BrC. The intensities of these fluorescent components all steadily decreased with oxidation, in which the tryptophan-like substance (C1) presented pronounced reduction for both BB BrC. However, the C2/C1 and C3/C1 intensity ratios exhibited an increasing trend with oxidation time, suggesting that the distribution of humic-like (C2) and tyrosine-like (C3) components within BrC could be enhanced during dark OH oxidation processes. These findings suggested that the levels and types of chromophores and fluorophores within BB BrC had been transformed once they were emitted to atmosphere, resulting in the change of optical properties of BB BrC. 2D-absorption-COS demonstrated that the decomposition of chromophores within the high ultraviolet region (275–310 nm) occurred preferentially to those within the low ultraviolet region (225–260 nm). In addition, the 2D-COS combined with fluorescence spectra for both BrC showed that the oxidation of types of fulvic-like fluorophores (340–380 nm) occurred before protein-like fractions (250–300 nm). These results provide detailed knowledge of the aqueous OH radical oxidation process of BB BrC in the atmosphere and are of great significance for understanding the climate and environmental effects of BB BrC and refining the atmospheric models established on the optical properties of BrC.