High Contribution of Secondary Formation to Brown Carbon in China Humid Haze: Enhancing Role of Ammonia and Amines

To better understand the sources and formation mechanisms of atmospheric brown carbon (BrC) in China haze, an intensive field observation was conducted in the North China Plain (NCP) during the 2023 winter. Our results showed that compared to that (28%) in dry haze, the contribution of secondary formation to BrC was significantly enhanced during humid haze, accounting for 46% of BrC production with the aqueous-phase reaction as the dominant formation pathway. The strong correlations between light absorption at λ_365nm and water-soluble organic nitrogen compounds, particularly imidazoles (IMs), indicated a key role of nitrogen-containing organic compounds in the aqueous-phase BrC formation process. In the humid haze, IMs are largely produced by liquid-phase reactions of carbonyls with amines and free ammonia (NH₃(aq)), which accounted for 57% of the total IMs in the humid haze events. Amines produced IMs more efficiently and less pH dependent than NH₃(aq), with alkyl IMs and oxidized IMs being their products, respectively. Both types of BrC increased with increasing levels of amines and NH₃(aq) during humid haze, suggesting their enhancing roles in BrC formation in China haze, which should be accounted for by models for better simulating the physicochemical characteristics and climate effects of atmospheric BrC.