Oxidized Nitrogen-Containing Organic Compounds Formation Enhanced the Light Absorption of PM_2.5 Brown Carbon

Brown carbon (BrC) is known to have a great impact on atmospheric radiative forcing, but its absorption characteristics at the molecular level is not well understood. This study investigated the seasonal variations of light absorption characteristics and molecular composition of BrC in Xi'an, China. Results showed that BrC exhibited higher light absorption capacity in cold (autumn and winter) than warm seasons (spring and summer). Nitrogen-containing organic compounds were identified as important BrC chromophores. Oxidized-N compounds originated from biomass burning emissions and NO_x/NO₃⁻ mediated oxidation reactions were predominant in cold seasons, whereas reduced-N compounds mainly formed from NH₃/NH₄⁺ mediated reactions were abundant in warm seasons. These results contribute to a better understanding of formation mechanisms and light absorption characteristics of nitrogen-containing BrC chromophores in PM_2.5.