Atmospheric reactive halogens reshaped by the clean energy policy and agricultural activity in a rural area of the North China Plain

Abstract

Reactive halogen species (RHS) play important roles in air pollution and climate change. Observational evidence has identified coal and biomass burning as an important source of RHS in polluted continental regions, including the North China Plain (NCP). Over the past ten years, the Chinese government has enacted various mitigation measures to control air pollutant emissions, including a clean energy initiative in the NCP. Here we report recent measurements of RHS at an NCP’s rural site where extraordinary levels of RHS were observed during the winter of 2017. We show that reactive bromines like BrCl and Br2 largely diminished after the implementation of the clean energy policy, but high levels of reactive chlorine persisted. A surprising finding in the recent field study is a potentially new chlorine source, likely from chlorine-based fertilizers. Moreover, the changes in aerosol acidity and the NO3 production rate led to a large increase in ClNO2 production with an inhibition of Cl2. The high ClNO2 levels (average: 150 pptv, peak: 3.8 ppbv) accounted for 43% of the oxidation of alkanes, increased conventional radicals (OH, HO2, RO2) by 4–8%, and net ozone production by 8–11%. Our study suggests more attention to crop fertilization as a potentially important source of atmospheric chlorine.