Light Absorption and Chromophore Characteristics of Brown Carbon in PM Emitted From Agricultural Machinery in China

The extensive utilization of agricultural machinery in China has made it a prominent contributor to particulate matter (PM). However, there still exist significant knowledge gaps in understanding optical characteristics and molecular composition of chromophores of brown carbon (BrC) in PM emitted from agricultural machinery. Therefore, BrC in PM from six typical agricultural machines in China were measured to investigate the light absorption, chromophore characteristics, and influencing factors. Results showed that the average emission factors of methanol-soluble organic carbon (MSOC) and water-soluble organic carbon (WSOC) were 0.96 and 0.21 g (kg fuel)⁻¹, respectively, exhibiting clear decreasing trends with increasing engine power and improving emission standards. Despite the light absorption coefficient of methanol-extracted BrC (Abs_365,M) being approximately 2.2 times higher than that of water (Abs_365,W), mass absorption efficiency of water-extracted BrC (MAE_365,W) exhibited significantly greater values than MAE_365,M. Among the detected chromophores, nitro-aromatic compounds (NACs) exhibited the highest contribution to light absorption that was about 14.5 times more than to total light absorption compared to their mass contributions to MSOC (0.04%), followed by polycyclic aromatic hydrocarbons (PAHs) and oxygenated PAHs (OPAHs). Besides, the average integrated simple forcing efficiency values were estimated to be 1.5 W g⁻¹ for MSOC and 3.7 W g⁻¹ for WSOC, indicating significant radiative forcing absorption of agricultural machinery. The findings in this study not only provide fundamental data for climate impact estimation of but also propose effective strategies to mitigate BrC emissions, such as enhancing emission standards and promoting the adoption of high-power agricultural machinery.