Aqueous-Phase Chloramine Production as a Missing Chain in Atmospheric Chlorine Chemistry
YIJING CHEN, Men Xia, Jinghui Zhang, Epameinondas Tsiligiannis, Chao Yan, Runlong Cai, Guangjie Zheng, Junchen Guo, Zhaojin An, Yiran Li, Yuyang Li, Cheng Wu, Qipeng Qu, Xinyan Zhao, Chenjie Hua, Zongcheng Wang, Shuxiao Wang, Yongchun Liu, Lina Cao, Douglas Worsnop, Kebin He, Mattias Hallquist, Tao Wang, Jingkun Jiang, Tsinghua University
Abstract Number: 301
Working Group: Aerosol Chemistry
Abstract
Chlorine radical (Cl·) profoundly impacts atmospheric oxidation capacity, and therefore climate change and air pollution. Chloramines, especially trichloramine (NCl3), is a potential Cl· source. However, their formation mechanism remains elusive. We present evidence of aerosol aqueous-phase production of chloramines with atmospheric observations in Beijing and New Delhi, and develop a comprehensive box model which incorporates 38 aerosol aqueous reactions initiated by Cl2 and NH3 and phase transfer processes. The model well reproduces the diurnal variation of NCl3 by sequential chlorination of NH3, which produces photolabile chlorine while consumes Cl2 and HOCl. Lacking chloramine chemistry in air quality models could significantly overestimate the role of Cl2 and HOCl. Chloramines, especially NCl3, are important Cl· contributors and intermediates in chlorine chemistry while, as a net result, this chloramine chemistry barely changes total chlorine radical production (P(Cl·)). Atmospheric production of chloramines could be ubiquitous globally with the co-existence of acidic aerosols, Cl2, and NH3, as evidenced by observations and our modeling analysis of chloramines in various environments. The contribution of chloramines to P(Cl·) is much higher in clean periods (up to 85%) than polluted periods (~5%) in Beijing, and in clean environments (e.g., ~64% in Toronto) than polluted areas (e.g., ~3% in New Delhi), implying a higher significance of chloramines in a cleaner future along with ongoing global efforts to mitigate air pollution. Overall, chloramine chemistry alters atmospheric Cl· production mechanism and represents a large missing contributor of Cl· in previous observations worldwide.