Yijing Chen, Men Xia, J. N. Zhang, Epameinondas Tsiligiannis, Cheng Wu, Chao Yan, Runlong Cai, Guangjie Zheng, Yuyang Li, Junchen Guo, Zhaojin An, Yiran Li, Xinyan Zhao, Qipeng Qu, Chenjie Hua, Zongcheng Wang, Shuxiao Wang, Yongchun Liu, Lina Cao, Kebin He, Markku Kulmala, Mattias Hallquist, Tao Wang, Douglas R. Worsnop, Jingkun Jiang
Chlorine radicals (Cl · ) profoundly affect atmospheric oxidation capacity. Chloramines, especially trichloramine, are emerging precursors of Cl · . However, their sources and roles in the atmosphere remain elusive. This study presents field evidence of primary emissions and explicit secondary production pathways of atmospheric trichloramine in Beijing, supplemented by observations from New Delhi and reanalysis of measurements in Toronto. We demonstrate that the sequential chlorination reactions initiated by molecular chlorine and ammonia in atmospheric aerosols are a major source of trichloramine. The trichloramine produced in aerosols is a source of gaseous trichloramine and serves as an intermediate during the conversion from molecular chlorine to Cl · , while direct trichloramine emissions constitute a previously overlooked source of Cl · . Overall, chloramine chemistry alters the Cl · production mechanism and represents a crucial missing pathway to Cl · worldwide.