Uptake of Common Atmospheric Gases by Organic-Coated Water Droplets

Abstract

Organic matter is ubiquitous in the atmosphere. It can form coatings that directly influence both the physical and chemical properties of the atmospheric water droplets and can have important but nontrivial consequences of climate changes. To understand the effects of organic coatings on gas uptake by atmospheric water droplets, we studied the uptake of three common atmospheric gases (H2O, O3, and SO2) by benzoic acid (BA)- or lauric acid (LA)-coated water droplets in both equilibrium and nonequilibrium molecular dynamics simulations. Free energy ΔF profiles for gas uptake by these coated systems were compared with the uncoated results in our previous work. We found that the changes in Coulomb energy dominate the changes in potential energy ΔU in the BA-coated systems, whereas the changes in Lennard-Jones energy play an important role in the LA-coated systems due to the long alkyl chains. We also found a clear positive correlation between the entropic contribution TΔS and the mobility of water and organic molecules. To study the dynamics and kinetics, we investigated how the organic coatings affect the mass accommodation coefficient α in nonequilibrium gas impinging runs. While the effects of the organic coating on α can be different for different gases, we found a general explanation for the change in α in terms of the relative positions of the air–organic interface and ΔF minimum.