Chlorine activation indoors and outdoors via surface-mediated reactions of nitrogen oxides with hydrogen chloride

Gaseous HCl generated from a variety of sources is ubiquitous in both outdoor and indoor air. Oxides of nitrogen (NO_y) are also globally distributed, because NO formed in combustion processes is oxidized to NO ₂, HNO₃, N₂O₅ and a variety of other nitrogen oxides during transport. Deposition of HCl and NO_y onto surfaces is commonly regarded as providing permanent removal mechanisms. However, we show here a new surface-mediated coupling of nitrogen oxide and halogen activation cycles in which uptake of gaseous NO₂ or N ₂O₅ on solid substrates generates adsorbed intermediates that react with HCl to generate gaseous nitrosyl chloride (ClNO) and nitryl chloride (ClNO₂), respectively. These are potentially harmful gases that photolyze to form highly reactive chlorine atoms. The reactions are shown both experimentally and theoretically to be enhanced by water, a surprising result given the availability of competing hydrolysis reaction pathways. Airshed modeling incorporating HCl generated from sea salt shows that in coastal urban regions, this heterogeneous chemistry increases surface-level ozone, a criteria air pollutant, greenhouse gas and source of atmospheric oxidants. In addition, it may contribute to recently measured high levels of ClNO₂ in the polluted coastal marine boundary layer. This work also suggests the potential for chlorine atom chemistry to occur indoors where significant concentrations of oxides of nitrogen and HCl coexist.