Water and formic acid aggregates: a molecular dynamics study.

Water adsorption around a formic acid aggregate has been studied by means of molecular dynamics simulations in a large temperature range including tropospheric conditions. Systems of different water contents have been considered and a large number of simulations has allowed us to determine the behavior of the corresponding binary formic acid-water systems as a function of temperature and humidity. The results clearly evidence a threshold temperature below which the system consists of water molecules adsorbed on a large formic acid grain. Above this temperature, formation of liquid-like mixed aggregates is obtained. This threshold temperature depends on the water content and may influence the ability of formic acid grains to act as cloud condensation nuclei in the Troposphere.