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 NaNO₃ on a Teflon® substrate. Varying molar ratios of NaCl to NaNO₃ (1 : 9 Cl⁻ : NO₃⁻, 1 : 1 Cl⁻ : NO₃⁻, 3 : 1 Cl⁻ : NO₃⁻, 9 : 1 Cl⁻ : NO₃⁻) and NaNO₃ at the same total concentration were irradiated in air at 299 ± 3 K and at a relative humidity of 75 ± 8% using broadband UVB light (270–380 nm). Gaseous NO₂ production was measured as a function of time using a chemiluminescence NO_y detector. Surprisingly, an enhanced yield of NO₂ was observed as the chloride to nitrate ratio increased. Molecular dynamics (MD) simulations show that as the Cl⁻ : NO₃⁻ ratio increases, the nitrate ions are drawn closer to the interface due to the existence of a double layer of interfacial Cl⁻ and subsurface Na⁺. This leads to a decreased solvent cage effect when the nitrate ion photodissociates to NO₂+O˙⁻, increasing the effective quantum yield and hence the production of gaseous NO₂. The implications of enhanced NO₂ and likely OH production as sea salt aerosols become processed in the atmosphere are discussed.