The temperature dependence of mass accommodation of sulfur dioxide and hydrogen peroxide on aqueous surfaces

Abstract

The authors report the laboratory determination of the temperature-dependent mass accommodation coefficients of SO{sub 2} and H{sub 2}O{sub 2} on aqueous surfaces over the range 260-292 K. Mass accommodation kinetics involve fundamental chemical interactions of liquid surfaces about which little is known. Uptake of SO{sub 2} and H{sub 2}O{sub 2} by cloud droplets is believed to be critical to S(IV) oxidation in the troposphere. Our experimental method combines a monodisperse train of droplets (200 {mu}m in diameter) and a low-pressure flow reactor. Uptake rates of trace gases are determined by measuring changes in trace gas number density as a function of exposed liquid surface area. Experiments with systematic variation of water vapor and rare gas partial pressures permit deconvolution of gas diffusion and temperature-dependent mass accommodation. In the case of SO{sub 2}, variation of droplet-gas interactions on millisecond time scales resolved pH-dependent saturation effects at the aqueous surface. Results for SO{sub 2} show {gamma}{sub 273} = 0.11 {plus minus} 0.02 with no significant temperature variation. H{sub 2}O{sub 2} shows a strong temperature dependence consistent with an attractive well depth of at least 26 kJ mol{sup {minus}1} with {gamma}{sub 273} = 0.18 {plus minus} 0.02. These results are discussed in termsmore » of the different aqueous solubilities of SO{sub 2} and H{sub 2}O{sub 2}.« less