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Chloramine chemistry as a missing link in atmospheric chlorine cycling

Chen, Yijing1; Xia, Men2; Zhang, Jinghui3; Tsiligiannis, Epameinondas4; Wu, Cheng4; Yan, Chao2; Cai, Runlong5; Zheng, Guangjie1; Li, Yuyang1; Guo, Junchen1; An, Zhaojin1; Li, Yiran1; Zhao, Xinyan1; Qu, Qipeng1; Hua, Chenjie6; Wang, Zongcheng6; Wang, Shuxiao1; Liu, Yongchun6; Cao, Lina3; He, Kebin1; Kulmala, Markku7; Hallquist, Mattias4; Wang, Tao8; Worsnop, Douglas7; Jiang, Jingkun1

Published Oct 13, 2025 on Dryad. https://doi.org/10.5061/dryad.c866t1gmf

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Abstract

Chlorine radicals (Cl·) profoundly impact atmospheric oxidation capacity. Chloramines, especially trichloramine (NCl3), 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 NCl3 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 (Cl2) and ammonia (NH3) in atmospheric aerosols are a major source of NCl3. The NCl3 produced in aerosols is a source of gaseous NCl3 and serves as an intermediate during the conversion from Cl2 to Cl·, while direct NCl3 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.