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

Abstract

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 , HNO 3 , N 2 O 5 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 NO 2 or N 2 O 5 on solid substrates generates adsorbed intermediates that react with HCl to generate gaseous nitrosyl chloride (ClNO) and nitryl chloride (ClNO 2 ), 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 ClNO 2 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.

Catalytic Role for Water in the Atmospheric Production of ClNO

“…Attempts to generate ClNO from the reaction of HCl on silica previously exposed to HONO were unsuccessful. 8 Model calculations provide unique insights into the mechanism of ClNO formation. The investigated reactions and corresponding reaction rate constants at 250 K and room temperatures are summarized in Figure 6.…”

“…Furthermore, the calculations elucidate the temperature dependence of the reactions, which is critical because this chemistry may occur in polar regions as well, where very active chemistry and photochemistry involving oxides of nitrogen has been observed. [30][31][32][33] This chemistry potentially contributes to chemistry indoors as well 8 and thus may be of general environmental importance.…”

Heterogeneous reaction of N<sub>2</sub>O<sub>5</sub> with airborne TiO<sub>2</sub> particles and its implication for stratospheric particle injection

“…One previous study (Molina et al, 1997) investigated the uptake of ClONO 2 (1-10 × 10 −7 Torr) onto aluminum oxide and Pyrex glass in the presence of HCl (1-10 × 10 −6 Torr) at 210-220 K, and suggested that this process is very efficient, with an uptake coefficient of 0.02, which is > 10 times larger than that onto stratospheric sulfuric acid aerosols. In addition, the uptake of N 2 O 5 onto HCldoped sulfuric acid and SiO 2 (Raff et al, 2009) leads to the formation of ClNO 2 , whose photolysis will release Cl atoms and therefore represent a pathway for chlorine activation (Ghosh et al, 2012). Heterogeneous chlorine activation is not included in the modelling work because of the lack of reliable kinetics data.…”

Introductory lecture: atmospheric chemistry in the Anthropocene

“…24a, the peak disappeared on exposure to gas phase HCl. 585 The yields of ClNO 2 increased with water vapor concentration up to the predicted maxima (DClNO 2 /DN 2 O 5 ¼ 1), which was surprising since water was expected to compete with the HCl reaction, forming HNO 3 instead:…”

“…24b). 585 In addition, ab initio molecular dynamics simulations show that for water lms on surfaces, N 2 O 5 can be stabilized at the air-water interface in such a way that reaction with Cl À via an S N 2 reaction can occur. 588 Baergen et al 242 proposed that similar chemistry could occur in "urban grime" which contains both oxides of nitrogen and chloride ions, as well as a complex mixture of organics.…”