Analysis of Mixed Formic and Acetic Acid Aggregates Interacting With Water: A Molecular Dynamics Simulation Study

Water adsorption on aggregates made of equimolar mixtures of formic and acetic acid molecules has been studied by means of molecular dynamics simulations between 150 and 275 K, covering thus a large range of atmospheric temperatures, as a function of both the water content and temperature. Calculations have shown that both the temperature and the water content have a strong influence on the behavior of the corresponding system. Two opposite situations have been evidenced for the acid water aggregates at sufficiently high water content, namely, water adsorption on large acid grains at low-temperatures, and formation of,a water droplet with; an: acid coating at high temperatures. In this latter case, acetic acid molecules have been found at the surface of the water droplet with their hydrophobic methyl group being as far as possible from the water surface, whereas part of the formic acid molecules were in the inner side of this interface. At low water content and high temperatures, this acid-water mixing was even more pronounced because the number of water molecules was not enough to completely dissociate the acid-aggregate on which, water molecules were adsorbed. Comparison with previous simulations on water interacting with one component acid aggregates did not show any significant effect of the formic acid acetic acid interactions on the behavior of the mixed systems. The results: of the present simulations show how molecular scale approaches can help at better understanding the behavior of organic aerosols in the atmosphere. They also,emphasize the need for further experiments and simulations to achieve a better characterization of the effects of temperature and humidity on the behavior of these aerosols.