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

• Published in: Proceedings of the National Academy of Sciences - PNAS 2009-08, Vol.106 (33), p.13647-13654
• Main Authors: Raff, Jonathan D, Njegic, Bosiljka, Chang, Wayne L, Gordon, Mark S, Dabdub, Donald, Gerber, R. Benny, Finlayson-Pitts, Barbara J
• Format: Article
• Language: English
• Subjects: Air Pollutants - analysis; Air pollution; Atmospheric chemistry; Atmospherics; Boundary layers; Cadmium; Chemical reactions; Chlorides; Chlorine; Chlorine - chemistry; Combustion; Environmental Monitoring - methods; Gases; Greenhouse gases; Human computer interaction; Hydrochloric acid; Hydrochloric Acid - chemistry; Hydrogen; Hydrolysis; Indoor environments; Marine chemistry; Models, Chemical; Molecular Conformation; Molecules; Nitric oxide; Nitrogen; Nitrogen - chemistry; Nitrogen oxides; Nitrogen Oxides - chemistry; Oxidants; Oxidants - chemistry; Oxidation; Oxides; Ozone; Photochemicals; Physical Sciences; Salts; Spectroscopy, Fourier Transform Infrared; Surface Properties; Water vapor
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.0904195106

Gaseous HCl generated from a variety of sources is ubiquitous in both outdoor and indoor air. Oxides of nitrogen (NOy) are also globally distributed, because NO formed in combustion processes is oxidized to NO₂, HNO₃, N₂O₅ and a variety of other nitrogen oxides during transport. Deposition of HCl and NOy 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 N₂O₅ 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.