Acidity and the multiphase chemistry of atmospheric aqueous particles and clouds

The acidity of aqueous atmospheric solutions is a key parameter driving both the partitioning of semi-volatile acidic and basic trace gases and their aqueous-phase chemistry. In addition, the acidity of atmospheric aqueous phases, e.g., deliquesced aerosol particles, cloud, and fog droplets, is also dictated by aqueous-phase chemistry. These feedbacks between acidity and chemistry have crucial implications for the tropospheric lifetime of air pollutants, atmospheric composition, deposition to terrestrial and oceanic ecosystems, visibility, climate, and human health. Atmospheric research has made substantial progress in understanding feedbacks between acidity and multiphase chemistry during recent decades. This paper reviews the current state of knowledge on these feedbacks with a focus on aerosol and cloud systems, which involve both inorganic and organic aqueous-phase chemistry. Here, we describe the impacts of acidity on the phase partitioning of acidic and basic gases and buffering phenomena. Next, we review feedbacks of different acidity regimes on key chemical reaction mechanisms and kinetics, as well as uncertainties and chemical subsystems with incomplete information. Finally, we discuss atmospheric implications and highlight the need for future investigations, particularly with respect to reducing emissions of key acid precursors in a changing world, and the need for advancements in field and laboratory measurements and model tools.

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) x 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 x 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(-) 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.