Enhanced surface photochemistry in chloride–nitrate ion mixtures

Abstract

Heterogeneous reactions of sea salt aerosol with various oxides of nitrogen lead to replacement of chloride ion by nitrate ion. Studies of the photochemistry of a model system were carried out using deliquesced mixtures of NaCl and NaNO 3 on a Teflon s substrate. Varying molar ratios of NaCl to NaNO 3 (1 : 9 Cl À : NO 3 À , 1 : 1 Cl À : NO 3 À , 3 : 1 Cl À : NO 3 À , 9 : 1 Cl À : NO 3 À) and NaNO 3 at the same total concentration were irradiated in air at 299 AE 3 K and at a relative humidity of 75 AE 8% using broadband UVB light (270-380 nm). Gaseous NO 2 production was measured as a function of time using a chemiluminescence NO y detector. Surprisingly, an enhanced yield of NO 2 was observed as the chloride to nitrate ratio increased. Molecular dynamics (MD) simulations show that as the Cl À : NO 3 À ratio increases, the nitrate ions are drawn closer to the interface due to the existence of a double layer of interfacial Cl À and subsurface Na + . This leads to a decreased solvent cage effect when the nitrate ion photodissociates to NO 2 +O À , increasing the effective quantum yield and hence the production of gaseous NO 2 . The implications of enhanced NO 2 and likely OH production as sea salt aerosols become processed in the atmosphere are discussed.

Reactions at surfaces in the atmosphere: integration of experiments and theory as necessary (but not necessarily sufficient) for predicting the physical chemistry of aerosols

“…5b and c show the results of MD simulations that predict that the surface chloride ions pull nitrate ions closer to the interface via formation of a double layer with Na + which attracts NO 3 À . 177 Since the quantum yield for NO 2 production in pure nitrate solutions at wavelengths above 300 nm is only B0.01, 139,155,156 even a relatively small shift in nitrate towards the surface could cause a significant increase in the efficiency, as is observed. Thus, theory again provided molecular level insight into what was initially a surprising experimental result.…”

“…169 Fig. 4 shows the measured ratio of i-butane to n-butane as a function of the ratio of i-butane to propane for the pure NaNO 3 case.…”

“…5a, the NO 2 generated per NO 3 À ion in the particles increased as the fraction of chloride ions increased! 177 Again, MD simulations provide key molecular insights into why this is the case. Fig.…”

Surface-Enhanced Nitrate Photolysis on Ice

“…Given these compelling yet somewhat indirect evidences, we are poised to provide direct evidence for enhanced NO3 -photolysis at the surface of ice. However, highlighting any interfacial specificity in molecular-level reaction mechanisms 15,31,32,[34][35][36][37] or in their kinetics [37][38][39][40] requires means of discriminating processes occurring on the surface from those occurring in the bulk. As nitrates photolysis kinetics are very slow, 10,41,42 any specific contribution from surfacebound NO3 -may be obscured by diffusive uptake into the bulk which, however slow, 51 will strongly couple the surface and bulk contributions to the overall photolysis rates and photochemical NO2 fluxes.…”

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

“…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). For each MNO 3 experiment (except for KNO 3 ), the RH in the chamber was B5% above the deliquescence relative humidity (DRH) of that salt ( Table 1), so that the salts were aqueous solutions.…”