Production of gas phase NO2 and halogens from the photolysis of thin water films containing nitrate, chloride and bromide ions at room temperature

Abstract

aNitrate and halide ions coexist in particles generated in marine regions, around alkaline dry lakes, and in the Arctic snowpack. Although the photochemistry of nitrate ions in bulk aqueous solution is well known, there is recent evidence that it may be more efficient at liquid-gas interfaces, and that the presence of other ions in solution may enhance interfacial reactivity. This study examines the 311 nm photolysis of thin aqueous films of ternary halide-nitrate salt mixtures (NaCl-NaBr-NaNO 3 ) deposited on the walls of a Teflon chamber at 298 K. The films were generated by nebulizing aqueous 0.25 M NaNO 3 solutions which had NaCl and NaBr added to vary the mole fraction of halide ions. Molar ratios of chloride to bromide ions were chosen to be 0.25, 1.0, or 4.0. The subsequent generation of gas phase NO 2 and reactive halogen gases (Br 2 , BrCl and Cl 2 ) were monitored with time. The rate of gas phase NO 2 formation was shown to be enhanced by the addition of the halide ions to thin films containing only aqueous NaNO 3 .] r 1.0, the NO 2 enhancement was similar to that observed for binary NaBr-NaNO 3 mixtures, while with excess chloride NO 2 enhancement was similar to that observed for binary NaCl-NaNO 3 mixtures. Molecular dynamics simulations predict that the halide ions draw nitrate ions closer to the interface where a less complete solvent shell allows more efficient escape of NO 2 to the gas phase, and that bromide ions are more effective in bringing nitrate ions closer to the surface. The combination of theory and experiments suggests that under atmospheric conditions where nitrate ion photochemistry plays a role, the impact of other species such as halide ions should be taken into account in predicting the impacts of nitrate ion photochemistry.

The effect of cations on NO2 production from the photolysis of aqueous thin water films of nitrate salts

“…In the interface region, nitrate ions are less solvated than those in the bulk, 86 resulting in increased NO 2 production yields. 33,[46][47][48]94 Computational modeling and X-ray photoelectron spectroscopy show that cations (Na + and Rb + ) have a strong impact on Br À surface enhancement, affecting the ion's partitioning of halide ions at the interface. 32 96 The coated chambers were then filled with synthetic air (Scott-Marrin Inc., NO x o 0.001 ppm, SO 2 o 0.001 ppm, Riverside, CA) that flowed through a water bubbler and mixed with dry air to adjust the final relative humidity (RH).…”

Aqueous phase oxidation of bisulfite influenced by nitrate photolysis

“…And, the reaction of HOBr with Br -(or Cl -) can also lead to the production of Br2 (or BrCl), as described by R30 and R31 (Mozurkewich, 1995;Vogt et al, 1996). Moreover, •OH can oxidize both Cland Brto form photochemically active halogen molecules such as Br2, BrCl and Cl2 (R32-R34) (Alexander et al, 2003;Richards and Finlayson-Pitts, 2012;Richards-Henderson et al, 2013). In addition, Brcan be oxidized to Br2 by nitrate under acidic conditions in the dark (R35), and the formed HONO further oxidizes Br - (Richards-Henderson et al, 2013).…”

“…Moreover, •OH can oxidize both Cland Brto form photochemically active halogen molecules such as Br2, BrCl and Cl2 (R32-R34) (Alexander et al, 2003;Richards and Finlayson-Pitts, 2012;Richards-Henderson et al, 2013). In addition, Brcan be oxidized to Br2 by nitrate under acidic conditions in the dark (R35), and the formed HONO further oxidizes Br - (Richards-Henderson et al, 2013). These nonradical halogen oxidant products may contribute to subsequent oxidation of HSO3 -(R36, R37), e.g., rapid S(IV) oxidation by HOCl and HOBr (R38, R39) (Richards and Finlayson-Pitts, 2012;Vogt et al, 1996).…”

“…Cl -, Br -, I -) are ubiquitous in the atmosphere via the transport of sea salt aerosols, the emission of fossil fuel combustion and biomass burning (Richards et al, 2011;Cahill et al, 1992;Cheng et al, 2000). The influence of halide ions on the formation of NO2 during the photochemistry of halide-nitrate ion mixtures has been extensively studied (Richards et al, 2011;Richards and Finlayson-Pitts, 2012;Richards-Henderson et al, 2013;Wingen et al, 2008;Custard et al, 2017). For example, Wingen et al (2008) observed enhanced NO2 production from photolysis of deliquesced nitrate aerosols containing chloride ions, and the reason was suggested that halide ions can draw nitrate ions closer to the interface.…”

Introductory lecture: atmospheric chemistry in the Anthropocene

“…[294][295][296][297][298][299] For example, nitrate ion photolysis at interfaces is more efficient than in the bulk, and is inuenced by the presence of halide ions. [294][295][296][297][298][299] However, it should be noted that whether nitrate ions are enhanced at the interface in water is somewhat controversial, [300][301][302] and could depend on cluster size, for example.…”