Uptake of gas-phase alcohol and organic acid molecules by water surfaces

It is now recognized that many important atmospheric reactions occur inside aqueous droplets of clouds and fogs. Recognition of the importance of these heterogeneous processes has stimulated an increasing amount of research in this area and a growing understanding about the nature of heterogeneous processes. Mass accommodation coefficients ({alpha}) have been measured as a function of temperature in the range 260-291 K for methanol, ethanol, 1-propanol, 2-propanol, 2-methyl-2-propanol, ethylene glycol, chloroethanol, bromoethanol, iodoethanol, formic acid, and acetic acid. The experimental method employs a monodispersed train of droplets ({le}230 {mu}m in diameter) in a low-pressure flow reactor. Droplet-trace gas interaction times are in the range (2-10) x 10{sup {minus}3} s. All mass accommodation coefficients show a negative temperature dependence and can be well expressed in terms of an observed Gibbs free energy as {alpha}/(1{minus}{alpha}) = exp({minus}{Delta}G{sub obs}/RT). Of the species studied, iodoethanol has the largest mass accommodation coefficient; for this molecule {alpha} ranges from about 0.04 at 290 K to 0.2 at 260 K. 1-Propanol has the smallest {alpha}; it ranges from about 0.01 at 290 K to 0.07 at 260 K. The results show systematic trends. Most notably, when {Delta}G{sub obs} is expressed as {Delta}G{sub obs} = {Delta}H{sub obs} {minus} T{Delta}S{sub obs}, is observed that the magnitudes of both {Delta}H{sub obs} and {Delta}S{sub obs} correlate and increase in the sequence diols < acids < haloethanols < alkyl alcohols. The results have led to the development of a model for the uptake of gas-phase molecules by liquids.