Adsorption of organic molecules may explain growth of newly nucleated clusters and new particle formation

Description

New particle formation (NPF) in the atmosphere influences the concentrations of atmospheric aerosols and therefore their impact on climate. New particle formation is a two-stage process consisting of homogeneous nucleation of thermodynamically stable clusters followed by growth of these clusters to a detectable size (> 3 nm). Due to the large coagulation rate of clusters smaller than 3 nm with the pre-existing aerosol population, these clusters must grow quickly in order to survive and form new particles. While some previous modeling and field studies have indicated that condensation of low-volatility organic vapor may play an important role in the initial growth of the clusters, due to the small size of the clusters and the relatively high vapor pressure and partial molar volume of even highly oxidized organic compounds, the strong Kelvin effect may prevent typical ambient organics from condensing on these clusters. However, earlier studies did not consider that adsorption of organic molecules on the cluster surface, due to the intermolecular forces between the organic molecule and cluster, may occur and substantially alter the growth process under sub-saturated conditions. Using the Brunauer-Emmett-Teller (BET) isotherm, we show that the adsorption of organic molecules onto the surface of clusters may significantly reduce the saturation ratio (by up to a factor of 100) required for condensation of organics to occur and therefore may provide a physico-chemical explanation for the enhanced initial growth by condensation of organics despite the strong Kelvin effect. The effect of adsorption on initial cluster growth rate and new particle formation will be discussed.