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

Abstract

Chlorine radical (Cl·) profoundly impacts atmospheric oxidation capacity, and therefore climate change and air pollution. Chloramines, especially trichloramine (NCl3), 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 Cl2 and NH3 and multiple phase transfer processes. The model can well reproduce the diurnal variation of NCl3. We show that this atmospheric production of chloramines could be ubiquitous globally with the co-existence of aerosol water, Cl2, and NH3, while NCl3 formation is mainly influenced by Cl2 and relative humidity as NH3 is typically in great excess. The contribution of chloramines (mostly NCl3) 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·.