Insights into the spatiotemporal differences of brown carbon in North China plain: Composition, optical properties and sources

To understand the spatiotemporal difference of atmospheric brown carbon (BrC) in the North China Plain (NCP), the chemical composition of PM_2.5 and optical properties in Beijing during winter and summer of 2023 were investigated, along with parallel measurements at a mountainous site (Xinglong, ∼1000 m, a.s.l.). Although the light-absorption of BrC (Abs₃₆₅) in Xinglong was comparable to that in Beijing, its light-absorptivity (MAE₃₆₅) was much stronger; And both parameters exhibited higher values in winter than in summer at each site. Notably, the light absorption of BrC relative to black carbon aloft was 1.7-fold of that at the surface during the whole campaign, indicating an enhanced role of BrC in aerosol absorption in upper boundary layer. The source apportionment identified that biomass burning was the predominant source for the BrC at both sites (31 % ∼ 39 %), yet the fractional contribution of secondary formation aloft (34 %) was moderately enhanced by ∼4 % relative to that at the surface. As the important BrC chromophores, the nitrated aromatic compounds (NACs) in Xinglong were more abundant relative to these in Beijing, especially during wintertime. Among the detected NACs, 4-nitrophenol and 4-nitrocatechol were the most abundant species at the surface, while the mass fraction of 5-nitroguaiacol significantly enhanced aloft, even comparable to 4NP in summer. Additionally, aqueous chemistry could be an important pathway to NACs formation, even accounting for 80 % of the total NACs in a typical haze episode. Our findings provide more insight into the spatiotemporal variability in the physicochemical properties of BrC in NCP, which will facilitate the improvement of the model to accurately quantify its climatic impacts.