Machine-learning-assisted chemical characterization and optical properties of atmospheric brown carbon in Nanjing, China

Abstract. The light-absorbing organics, namely brown carbon (BrC), can significantly affect atmospheric visibility and radiative forcing, yet current knowledge of the chemical composition of BrC is largely limited to a number of certain classes of compounds; the chemical and optical properties, and particularly linkage between the two, remain poorly understood. To address this, a comprehensive analysis was conducted on the particulate matter (PM2.5) samples collected in Nanjing, China, during 2022–2023, with a focus on the identification of key BrC molecules. Several important clues related to BrC were found. First, the water-soluble organic aerosol (WSOA) was more oxygenated during the cold season (CS) due to a highly oxidized secondary OA (SOA) factor that was strongly associated with aqueous and heterogenous reactions, especially during the nighttime, while the WSOA during the summer season (SS) was less oxygenated and the SOA was mainly from photochemical reactions. Fossil fuel combustion hydrocarbon-like OA was the largest and dominant contributor to the light absorption during CS (55.6 %–63.7 %). Secondly, our observations reveal that aqueous oxidation can lead to notable photo-enhancement during CS, while photochemical oxidation on the contrary caused photo-bleaching during SS. Both water-soluble and methanol-soluble organics had four key fluorophores, including three factors related to humic-like substances (HULIS) and one protein-like component. Thirdly, molecular characterization shows that CHON compounds were the most abundant species overall, followed by CHO and CHN compounds, and the significant presence of organosulfates in CS samples reaffirmed the importance of aqueous-phase formation. Finally, building upon the molecular characterization and light absorption measurement results, a machine learning approach was applied to identify the key BrC molecules and 31 compounds including polycyclic aromatic hydrocarbons (PAHs), oxyheterocyclic PAHs, quinones, and nitrogen-containing species, which can be a good reference for future studies.