Chloramine chemistry as a missing link in atmospheric chlorine cycling

Yijing Chen^{1

2}, Men Xia^{3

4

5}, Jinghui Zhang⁶, Epameinondas Tsiligiannis², Cheng Wu², Chao Yan⁵, Runlong Cai^{3

7}, Guangjie Zheng¹, Yuyang Li¹, Junchen Guo¹, Zhaojin An^{1

8}, Yiran Li¹, Xinyan Zhao¹, Qipeng Qu¹, Chenjie Hua⁴, Zongcheng Wang⁴, Shuxiao Wang¹, Yongchun Liu⁴, Lina Cao⁶, Kebin He¹, Markku Kulmala^{3

4

5}, Mattias Hallquist², Tao Wang⁹, Douglas Worsnop^{3

10}, Jingkun Jiang¹

Affiliations

¹ State Key Laboratory of Regional Environment and Sustainability, School of Environment, Tsinghua University, Beijing 100084, China.
² Department of Chemistry and Molecular Biology, University of Gothenburg, Gothenburg 40530, Sweden.
³ Institute for Atmospheric and Earth System Research/Physics, Faculty of Science, University of Helsinki, Helsinki 00014, Finland.
⁴ Aerosol and Haze Laboratory, Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China.
⁵ Nanjing-Helsinki Institute in Atmospheric and Earth System Sciences, Nanjing University, Nanjing 210023, China.
⁶ Department of Intelligent Edge Cloud, Chinatelecom Cloud Technology Co. Ltd., Beijing 100010, China.
⁷ Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP3), Department of Environmental Science and Engineering, Fudan University, Shanghai 200438, China.
⁸ School of Engineering and Applied Sciences, Harvard University. Cambridge, MA 02138, USA.
⁹ Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hong Kong 999077, China.
¹⁰ Aerodyne Research Inc., Billerica, MA 01821, USA.

PMID: 41160677
PMCID: PMC12571055
DOI: 10.1126/sciadv.adv4298

Chloramine chemistry as a missing link in atmospheric chlorine cycling

Yijing Chen et al. Sci Adv. 2025.

. 2025 Oct 31;11(44):eadv4298.

doi: 10.1126/sciadv.adv4298. Epub 2025 Oct 29.

Authors

Affiliations

¹ State Key Laboratory of Regional Environment and Sustainability, School of Environment, Tsinghua University, Beijing 100084, China.
² Department of Chemistry and Molecular Biology, University of Gothenburg, Gothenburg 40530, Sweden.
³ Institute for Atmospheric and Earth System Research/Physics, Faculty of Science, University of Helsinki, Helsinki 00014, Finland.
⁴ Aerosol and Haze Laboratory, Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China.
⁵ Nanjing-Helsinki Institute in Atmospheric and Earth System Sciences, Nanjing University, Nanjing 210023, China.
⁶ Department of Intelligent Edge Cloud, Chinatelecom Cloud Technology Co. Ltd., Beijing 100010, China.
⁷ Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP3), Department of Environmental Science and Engineering, Fudan University, Shanghai 200438, China.
⁸ School of Engineering and Applied Sciences, Harvard University. Cambridge, MA 02138, USA.
⁹ Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hong Kong 999077, China.
¹⁰ Aerodyne Research Inc., Billerica, MA 01821, USA.

PMID: 41160677
PMCID: PMC12571055
DOI: 10.1126/sciadv.adv4298

Abstract

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.

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Conflict of interest statement

The authors declare that they have no competing interests.

Figures

**Fig. 1.. Field characterizations of primary and secondary productions of NCl₃ in summer Beijing.**
(A) Comparison of measured and simulated daily averages of NCl₃ in the base scenario and with the secondary formation mechanism considered. The shaded area represents the 10th and 90th percentiles of the measured NCl₃. The orange line denotes the measured average diurnal variation in NO₂ photolysis frequency [j(NO₂)]. A case indicates (B) primary emission dominates and (C) secondary production dominates the occurrence of NCl₃. (D) Wind rose plot color-coded by NCl₃ mixing ratios. Cases shown in (B) and (C) were marked as triangles in (D) accordingly. A NCl₃ spike (case 1) is identified when its deviation from the local median of hourly mixing ratios within an 8-hour moving window (centered on the spike, ±4 hours) exceeds three times the median of the absolute differences from the median of the observations, considering only positive deviations. The 8-hour moving window is applied to account for the background variability. The others are classified into case 2.

**Fig. 2.. The critical role of Cl₂ and RH in driving NCl₃ formation.**
(A) Correlation between the predicted and measured NCl₃ in Beijing based on machine learning. (B) Impacts of top 10 influencing factors on the NCl₃ level, ranked by their overall importance (mean absolute SHAP value) in descending order. Dependency of measured nocturnal NCl₃ on (C) Cl₂ and (D) RH. Average PM_2.5 concentrations in each interval are shown as dots in (C) and (D). Sensitivity tests of nocturnal (E) Cl₂ and (F) RH on the output NCl₃ in the box model. Winter nighttime is defined as 00:00 to 7:00 and 17:00 to 24:00, while summer nighttime is 00:00 to 5:00 and 19:00 to 24:00.

**Fig. 3.. Sources, transformations, and impacts of atmospheric chloramines.**
Average aqueous production and loss rates of (A) NCl₃, (B) NHCl₂, and (C) NH₂Cl from different reaction pathways in the box model. Reactions that contribute less than 5% to the total budgets, such as phase transfer, are not shown. (D) Comparison of the average total reactive chlorine levels with or without (w/o) incorporating the proposed secondary mechanism. (E) Schematic of atmospheric evolutions of chloramines. The proposed mechanism is illustrated in red.

**Fig. 4.. Substantial contributions of chloramines to Cl· production in multiple locations.**
Average contribution of chloramines to P(Cl·) in (A) Toronto, (B) Beijing, and (C) New Delhi. (D) PM_2.5 dependency of chloramine contribution to P(Cl·) in Beijing and New Delhi. The solid lines indicate the average values, and the shaded areas indicate the 25th and 75th percentiles. Molec, molecules. (E) Worldwide enhancement of P(Cl·) with and without chloramines being observed. “Others” in (E) means the combination of all other Cl· production pathways except for ClNO₂-, Cl₂-, NCl₃-, and NHCl₂-related processes (e.g., ClO· + NO). In solid rectangles where chloramine measurement data are available, the average summertime P(Cl·) is shown for Beijing and Toronto, and the average wintertime P(Cl·) is shown for New Delhi. Simulated chloramine mixing ratios are used in the dashed rectangles. References (, , , , , , , –61) are presented as superscripts in (E).

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Chloramine chemistry as a missing link in atmospheric chlorine cycling

Affiliations

Chloramine chemistry as a missing link in atmospheric chlorine cycling

Authors

Affiliations

Abstract

Conflict of interest statement

Figures

References

LinkOut - more resources

Full Text Sources