Influence of organic films on the hygroscopicity of ultrafine sulfuric acid aerosol

In ambient air, organic chemicals are common constituents of aerosol particles, and many of them are surface-active. If these compounds are present as surface films on aqueous droplets, mass transfer of gaseous molecules into or out of the droplets could be impeded, and the hygroscopic growth of acidic droplets could be reduced. In this work, the effects of organic films on the hygroscopic growth rate of ultrafine H2SO4 aerosols (40-120 nm) were investigated under relative humidity conditions of 20-85% and over time scales of 6 and 10 s. Lauric (C-12 saturated),stearic(C-18,saturated),and oleic acids were chosen as the film coating materials. The experimental results showed that the presence of monolayer films of some fatty acids retards the hygroscopic growth rate of H2SO4 aerosol due to the obstructive effect of the films on the permeation of water vapor. The retardation effects depend on the original film coating thickness. Molecular structure of the film-forming compounds is an important factor for determining the effectiveness of the film. Linear molecules, such as lauric and stearic acids, have stronger effects on mass transport of water vapor than those of nonlinear molecules, such as oleic acid.