Seasonal variation and optical properties of brown carbon and nitroaromatic compounds in PM2.5 in the urban area of Beijing

Nitroaromatic compounds (NACs) are important light-absorbing constituents of brown carbon (BrC), yet their seasonal characteristics, particle-size distributions, optical properties, and sources remain insufficiently resolved in highly polluted urban environments. This study presents a year-long field investigation of NACs in PM_2.5 in urban Beijing, focusing on their seasonal variations, size distributions, optical properties, and sources. Eight NAC species were quantified across four seasons, with winter showing the highest concentrations due to enhanced emissions and unfavorable meteorology. Size-resolved analysis revealed fine-mode dominance (0.43–1.1 μm), reflecting secondary formation from aromatic precursors and implying strong radiative and health relevance, while coarse-mode occurrence suggested contributions from primary biomass burning and heterogeneous processes. NACs exhibited distinct light-absorbing properties, with laboratory-calibrated mass absorption efficiency (MAE) and absorption Ångström exponent (AAE) confirming their role as contributors to methanol-soluble brown carbon (MSBrC). On annual average, NACs accounted for 2.7 ± 1.5 % (0.35–8.3 %) of MSBrC absorption at 365 nm, indicating that they are a non-negligible but limited contributor to BrC light absorption. Positive matrix factorization (PMF) identified biomass burning, coal combustion, vehicle emissions, dust, and secondary formation as major sources, with seasonal variations highlighting the importance of mixed combustion sources in summer and autumn. Overall, this work provides a comprehensive molecular- and size-resolved dataset that links NAC concentrations, optical properties, and source apportionment, offering new insights into the atmospheric role of NACs and their implications for urban air quality and radiative forcing.