Sticking coefficient and processing of water vapor on organic-coated nanoaerosols

Organic-based aerosols may play an unexpectedly important role in the atmospheric processing of water vapor. In this paper, we report results of a molecular dynamics (MD) study on the sticking coefficient of water vapor on a coated particle (water droplet coated with fatty acid of radius similar to 4 nm). The sticking coefficient was found to be almost a constant (11 - 16%) for incident speeds around the most probable speed as opposed to 100% for water vapor incident on a pure water droplet. The sticking coefficient was found to increase with the size of the water cluster (water-Nmer) impinging the surface of the particle and was seen to approach 1 for impinging water clusters consisting of 10 molecules or more. We also computed the average energy transferred per collision for monomers impinging the surface of the coated particle and found the value to be 4.4581 kJ/mol. Most important perhaps was the fact that despite the lower sticking coefficient, the equilibrium vapor pressure of water over these inverted micelles was considerably lower due to the surface tension effects of the fatty acid layer. As such, these coated particles act as effective substrates for water vapor condensation and may play a role as cloud condensation nuclei.