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.

Fig. 1.

Reaction of HCl with NO₂ on SiO₂ studied by infrared spectroscopy. (A) Nitrosyl chloride (ClNO) is formed efficiently when a stream of gas containing 3.7–5.5 ppm (vol:vol) NO₂, 5–10 ppm (vol:vol) HCl and H₂O (≈0.5% relative humidity) flows over a bed of high-surface area (330 m²) SiO₂ (shaded regions); the reaction does not occur when the stream is diverted through an empty reaction cell (clear regions). (B) The concentration of ClNO formed per NO₂ reacted increases from 25% in the absence of added water vapor to 50% at a relative humidity (RH) ≥5%; the triangle marks the average ClNO yield derived from the experiment described in panel A. The maximum yield expected based on reaction [5] is 0.5. (C) The addition of water vapor has a dramatic effect on gaseous NO₂ initially present in a reaction cell containing SiO₂ pellets. Sequential addition of water vapor (1.6, 3.1, and 3.2 Torr, indicated by green regions) leads to enhanced uptake of NO₂ on the SiO₂ surfaces and formation of nitrous acid (HONO). Introducing HCl to the chamber (blue region) leads to rapid ClNO formation with a yield of 47%, relative to the amount of NO₂ initially present. Units of concentration are parts per million by volume (ppmv).

Fig. 2.

Snapshots along an ab initio molecular dynamics “on-the-fly” trajectory of ClNO formation show how the reaction of HCl and ONONO₂ is catalyzed by 1 water molecule. (A) Weakly bound reactive complex formed upon geometry optimization of ONONO₂, HCl and H₂O. (B) First proton transfer from HCl to H₂O to form Cl^- and H₃O⁺. (C) Second proton transfer from H₃O⁺ to NO₃⁻ to form H₂O and HNO₃. (D) Product ClNO is formed. Calculations are carried out at the MP2/cc-pVDZ level of theory. Depicted atomic distances are given in Ångströms. The amount of time elapsed along the trajectory is provided below each structure.

Fig. 3.

Nitronium nitrate (NO₂⁺NO₃⁻) is the key intermediate in reaction of N₂O₅ with HCl on surfaces. ATR-FTIR was used to obtain the spectra of species adsorbed to a ZnSe crystal exposed to 10 Torr of N₂O₅ (solid line) and subsequently 20 Torr of HCl (dashed line) in the absence of added water vapor. (Inset) An expanded spectral region where the NO₂⁺ infrared absorption band occurs.

Fig. 4.

Geometry optimizations of N₂O₅ in the absence and the presence of 1, 2 and 3 water molecules conducted at MP2/cc-pVDZ level of theory show how ionization and dissociation of N₂O₅ occurs on water clusters. Depicted atomic distances are given in Ångströms; Mulliken charges on the NO₂^δ+ and NO₃^δ− moieties are provided in the table.

Fig. 5.

Reaction of N₂O₅ with HCl in absence and then in the presence of added water vapor. (A) Nitryl chloride (ClNO₂) is formed essentially quantitatively (96% yield) as 300 ppm of anhydrous HCl is added at t = 0 to a reaction chamber (volume 64 cm³) containing 126 ppm N₂O₅. Rapid uptake of N₂O₅ onto walls leads to the drop in N₂O₅ levels initially. (B) The rate of reaction of N₂O₅ (300 ppm) with HCl vapor (400 ppm) is increased in the presence of water vapor (≈25% relative humidity). Note the different time scales in A and B. Units of concentration are parts per million by volume (ppmv). (C) Ab initio calculations show that the reaction between HCl and NO₂⁺ is catalyzed by water. Minima are obtained by optimizations carried out in the absence and presence of 1 and 2 water molecules at MP2/cc-pVDZ level of theory. The number of water molecules was increased by placing an additional water molecule 4 Å apart from previously obtained structure. Depicted atomic distances are given in Ångströms.

Fig. 6.

The overlap between the UV-vis absorption spectra of ClNO (104, 105) and ClNO₂ (104) and emission spectra from the Sun (106) and fluorescent lamps typical of indoor settings.

Fig. 7.

Impact of HCl-NO₂ chemistry on ambient air quality in the South Coast Air Basin (SoCAB) of California. Shown are the domain-wide time maxima mixing ratios in parts-per-billion by volume (ppbv) for the base case (A). The peaks occur at 14:00, 6:00, and 12:00 for O₃, ClNO, and HCl, respectively. The increases over the values in A for O₃, ClNO, and HCl at the same hours for O₃, ClNO, and HCl are shown in B, i.e., these are the additional concentrations on top of those predicted for the base case due to inclusion of the heterogeneous HCl-to-ClNO conversion.