A new approach to studying aqueous reactions using diffuse reflectance infrared Fourier transform spectrometry: application to the uptake and oxidation of SO2 on OH-processed model sea salt aerosol

Abstract

Diffuse reflectance infrared Fourier transform spectrometry (DRIFTS) is a powerful technique for analyzing solid powders and for following their reactions in real time. We demonstrate that it can also be applied to studying the uptake and reactions of gases in liquid films. Within the DRIFTS cell, a 10% (w/w) mixture of MgCl₂·6H₂O in NaCl was equilibrated with air at 50% RH, which is above the deliquescence point of the magnesium salt but below that of NaCl. This mixture of NaCl coated with an aqueous magnesium chloride solution was then reacted with gas phase OH to generate hydroxide ions via a previously identified interface reaction. This treatment, hereafter referred to as OH-processing, was sufficient to convert some of the magnesium chloride to Mg(OH)₂ and Mg₂(OH)₃Cl·4H₂O, making the aqueous film basic and providing a reservoir of alkalinity. Subsequent addition of SO₂ to the basic processed mixture resulted in its uptake and conversion to sulfite which was measured by FTIR. The sulfite was simultaneously oxidized to sulfate by HOCl/OCl⁻ that was formed in the initial OH-processing of the salt. Further uptake and oxidation of SO₂ in the aqueous film took place when the salt was subsequently exposed to O₃. These studies demonstrate that DRIFTS can be used to study the chemistry in liquid films in real time, and are consistent with the hypothesis that the reaction of gaseous OH with chloride ions generates alkalinity that enhances the uptake and oxidation of SO₂ under these laboratory conditions.