Aqueous-phase chloramine production as a missing chain in atmospheric chlorine chemistry

Chlorine radical (Cl•) profoundly impacts atmospheric oxidation capacity, and therefore climate change and air pollution.Chloramines, especially trichloramine (NCl 3 ), is a potentially important Cl• source, while their formation mechanism remains elusive.Here, we present evidence of aerosol aqueous-phase production of chloramines with observations in Beijing and develop a comprehensive box model which incorporates ~40 aerosol aqueous reactions initiated by Cl 2 and NH 3 and multiple phase transfer processes.The model can well reproduce the diurnal variation of NCl 3 .We show that this atmospheric production of chloramines could be ubiquitous globally with the co-existence of aerosol water, Cl 2 , and NH 3 , while NCl 3 formation is mainly influenced by Cl 2 and relative humidity as NH 3 is typically in great excess.The contribution of chloramines (mostly NCl 3 ) to Cl• production is much higher in clean periods (up to 85%) than polluted periods (~5%) in Beijing, and in clean environments (e.g., ~64% in Canada) than polluted areas (e.g., ~1% in India).Therefore, chloramines can play an increasingly important role in a cleaner future and may partially offset the on-going efforts on air pollution mitigation.The missing representation of chloramines in current models could significantly underestimate the climate, health and ecological impacts of Cl•.