Photodissociation of particulate nitrate as a source of daytime tropospheric Cl2

Xiang Peng, Tao Wang*, Weihao Wang, A. R. Ravishankara, Christian George, Men Xia, Min Cai, Qinyi Li, Christian Mark Salvador, Chiho Lau, Xiaopu Lyu, Chun Nan Poon, Abdelwahid Mellouki, Yujing Mu, Mattias Hallquist, Alfonso Saiz-Lopez, Hai Guo, Hartmut Herrmann, Chuan Yu, Jianing DaiYanan Wang, Xinke Wang, Alfred Yu, Kenneth Leung, Shuncheng Lee, Jianmin Chen

*Corresponding author for this work

Research output: Contribution to journalJournal articlepeer-review

28 Citations (Scopus)


Chlorine atoms (Cl) are highly reactive and can strongly influence the abundances of climate and air quality-relevant trace gases. Despite extensive research on molecular chlorine (Cl2), a Cl precursor, in the polar atmosphere, its sources in other regions are still poorly understood. Here we report the daytime Cl2 concentrations of up to 1 ppbv observed in a coastal area of Hong Kong, revealing a large daytime source of Cl2 (2.7 pptv s−1 at noon). Field and laboratory experiments indicate that photodissociation of particulate nitrate by sunlight under acidic conditions (pH < 3.0) can activate chloride and account for the observed daytime Cl2 production. The high Cl2 concentrations significantly increased atmospheric oxidation. Given the ubiquitous existence of chloride, nitrate, and acidic aerosols, we propose that nitrate photolysis is a significant daytime chlorine source globally. This so far unaccounted for source of chlorine can have substantial impacts on atmospheric chemistry.

Original languageEnglish
Article number939
Number of pages10
JournalNature Communications
Publication statusPublished - 17 Feb 2022

Scopus Subject Areas

  • Chemistry(all)
  • Biochemistry, Genetics and Molecular Biology(all)
  • General
  • Physics and Astronomy(all)


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