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

Abstract

The photochemistry of nitrate ions in bulk aqueous solution is well known, yet recent evidence suggests that the photolysis of nitrate may be more efficient at the air-water interface. Whether and how this surface enhancement is altered by the presence of different cations is not known. In the present studies, thin aqueous films of nitrate salts with different cations were deposited on the walls of a Teflon chamber and irradiated with 311 nm light at 298 K. The films were generated by nebulizing aqueous 0.5 M solutions of the nitrate salts and the generation of gas-phase NO 2 was monitored with time. The nitrate salts fall into three groups based on their observed rate of NO 2 formation (R NO 2 ): (1) RbNO 3 and KNO 3 , which readily produce NO 2 (R NO 2 4 3 ppb min À1 ), (2) Ca(NO 3 ) 2 , which produces NO 2 more slowly (R NO 2 o 1 ppb min À1 ), and (3) Mg(NO 3 ) 2 and NaNO 3 , which lie between the other two groups. Neither differences in the UV-visible spectra of the nitrate salt solutions nor the results of bulk-phase photolysis studies could explain the differences in the rates of NO 2 production between these three groups. These experimental results, combined with some insights from previous molecular dynamic simulations and vibrational sum frequency generation studies, show that cations may impact the concentration of nitrate ions in the interface region, thereby directly impacting the effective quantum yields for nitrate ions.

Particulate nitrate photolysis in the atmosphere

“…The quantum yields of solid-phase nitrate photolysis are four orders of magnitude lower than those of aqueous phase one. 82 Nonetheless, emissions of gaseous NO 2 at >1 ppb min −1 83 and HONO at < 0.06 ppt min −1 84 are possible from solid-phase nitrate photolysis when thin water films or so-called surface adsorbed water (SAW) are present on the solid surface. While the exact role of SAW in nitrate photolysis remains poorly understood, the presence of SAW likely promotes the dissolution of solid nitrate to form aqueous nitrate in SAW.…”

“…83 Richards et al found that thin films (∼800 nm) of RbNO 3 and KNO 3 produce more gaseous NO 2 than those of Mg(NO 3 ) 2 and NaNO 3 , and Ca(NO 3 ) 2 . 83 Molecular dynamics simulations suggested that cations can regulate the surface propensity of nitrate anions. 66,101 For instance, the concentration of nitrate anion in the interface region of 2 M KNO 3 thin film could be ten times higher than that of 2 M NaNO 3 .…”

“…62 For example, Mg(NO 3 ) 2 solution produces NO 2 three times faster than Ca(NO 3 ) 2 solution because it has 50% more free nitrate at the interface, probably due to less contact ion pairings. 83 Furthermore, a recent computational study suggests that the ion pairs between cations and nitrate in an aqueous solution can also change the molar absorption coefficient, which would affect the nitrate photolysis rate. 122…”

Photoinduced Oxidation Reactions at the Air–Water Interface

“…On the other hand, experiments in thin water films containing nitrate, chlorine, and bromine salts have shown that the photolysis of NO 3 – occurs much faster at interfaces than in the bulk, ,,− with J values ranging between 1.2 × 10 –3 and 3 × 10 –6 s –1 , depending on the features of the interface. ,,, A potential explanation for this enhancement is associated with the solvation of nitrate anion. In bulk water, nitrate is surrounded by a solvent cage that facilitates both the recombination of the NO 2 and OH radicals produced by reaction and their deactivation by collision with solvent molecules, thus decreasing the photolysis quantum yield. ,,, MD simulations have shown that when halide anions (especially bromide) are present, nitrate ions are dragged closer to the interface so that the water solvent cage surrounding them is reduced, , arguably making the escape of NO 2 to the gas phase easier. ,,− Nevertheless, despite the large amount of work done on nitrate photochemistry, open issues remain that are not yet completely understood. For instance, it was recently shown that an important part of nitric acid remains undissociated at the air–water interface, ,− and the relevance of the corresponding photochemistry is still undetermined.…”

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.…”