Chloramine chemistry as a missing link in atmospheric chlorine cycling

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.