Molecular characterization and optical properties of methanol-soluble brown carbon chromophores in a metropolitan area of northeastern Asia

Abstract

The light-absorbing chromophores in atmospheric brown carbon (BrC) remain poorly characterized, impeding our understanding of BrC chemistry, its optical properties, emission sources, and accurate representation in models. This knowledge gap hinders the assessment of BrC's role in radiative balance and photochemistry, which is crucial for developing effective mitigation strategies. This study conducted molecular characterization of BrC chromophores in PM_2.5 in a metropolitan area of northeastern Asia. Identified chromophores in all periods included CHO, CHON and CHOS compounds. Phthalic acids, nitroaromatic compounds (NACs) and polycyclic aromatic hydrocarbons (PAHs) were identified and associated with vehicle emissions, biomass burning, or secondary formation, while CHOS compounds were primarily attributed to ship engine exhaust. NACs and PAHs were observed to significantly contribute to light absorption, with nitrophenolic compounds playing a dominant role in near-UV region. The light absorption of the compounds identified in non-polluted winter was most significant at the wavelength of 362 nm, while that in summer was at 314 nm. Moreover, lignin-like compounds were prevalent during polluted periods. This research identified diverse brown carbon chromophores, improving our understanding of their impacts on air quality and climate. While further analysis of unidentified substances remains necessary, this study provide a valuable foundation for future investigations into the sources and formation mechanisms of BrC.