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

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. 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 MgCl 2 Á 6H 2 O 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 Mg 2 (OH) 3 Cl Á 4H 2 O, making the aqueous film basic and providing a reservoir of alkalinity. Subsequent addition of SO 2 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 À that was formed in the initial OH-processing of the salt. Further uptake and oxidation of SO 2 in the aqueous film took place when the salt was subsequently exposed to O 3 . 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 SO 2 under these laboratory conditions.

Kinetic regimes and limiting cases of gas uptake and heterogeneous reactions in atmospheric aerosols and clouds: a general classification scheme

“…For example, the reaction of Cl 2 with Br − has a strong surface component, especially at low Cl 2 gas-phase concentrations (Hu et al, 1995). Similarly, Knipping et al (2000) as well as Knipping and Dabdub (2002) suggested a surface reaction between the OH radical and Cl − under atmospheric conditions via formation of a surface complex (Laskin et al, 2003;Shaka' et al, 2007).…”

Effects of temperature on the heterogeneous oxidation of sulfur dioxide by ozone on calcium carbonate

“…Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) has been used successfully for the analysis of solids, and it has applied extensively to study the kinetics and mechanisms of the heterogeneous chemistry on particle surfaces (Vogt and Finlayson-Pitts, 1994;Shaka et al, 2007;Li et al, 2006;Finlayson-Pitts, 2000). The advantage of DRIFTS is that it can be used to measure the reaction products in situ without interrupting the reaction processes (Finlayson-Pitts, 2000).…”

High-Precision Measurements of 33S and 34S Fractionation during SO2 Oxidation Reveal Causes of Seasonality in SO2 and Sulfate Isotopic Composition

“…European and North American sites may show high levels of TMIs in winter due to increased power generation and emissions, recent results have shown that natural TMIs are likely to be much stronger catalysts of SO 2 oxidation than anthropogenic TMIs (19). At coastal Chinese sites, observations below the modelled line may also point to the importance of oxidation by HOCl and HOBr (61)(62)(63); this pathway has α < 1 (50) and its importance in the marine boundary layer is not well-quantified (61,64,65). Little or no seasonality occurs in the Chinese data.…”