Light absorption properties and composition of water-soluble brown carbon in North China Plain: implication for an enhancing role of nitrogenous organic compounds

Abstract. Brown carbon (BrC), an efficiently light-absorbing carbonaceous aerosol, exerts significant impacts on the global energy budget and regional climate, attracting growing scientific attention. To advance understanding of the spatial variability of atmospheric BrC and its dominant formation pathways in the North China Plain (NCP), light absorption properties, chemical composition and formation process of the water-soluble BrC in 2023 winter were investigated by conducting simultaneous measurements at five sites across the NCP, namely, Beijing, Tianjin, Luancheng (rural site), Handan and Jinan. Our results showed that the average light absorption coefficient at 365 nm (abs365) in Luancheng was approximately 1.1–3.5 times higher than those in urban ones; While mass absorption efficiency displayed a distinctly different spatial pattern, with the strongest light-absorptivity (1.40 ± 0.02 m2 g−1) recorded in Jinan. Notably, average abs365 in four urban sites exhibited a decline of ~ 45 % from 2018 to 2023 compared to those previous observations. Furthermore, the light-absorptivity of BrC was enhanced from clean to haze period at the most sampling sites along with the increasing N:C ratio, indicating that nitrogenous organic compounds (NOCs) were the important BrC chromophores in the NCP. Additionally, more than 50 % of NOCs were confirmed to be secondarily formed; and the ammonia-driven aqueous reactions were identified as the predominant pathway governing the secondary formation of these NOCs. These results elucidate the substantial contribution of NOCs to atmospheric BrC in the NCP, and further confirm the importance of ammonia emission for alleviating haze and BrC pollution in this region.