| Heterogeneous interactions play a significant role in the transformations of gases and aerosols both in the troposphere and stratosphere. However, at this point very little is known about the physical and chemical properties of organic atmospheric aerosols. In the present work, 1-octanol was selected as a surrogate for the hydrophobic oxygenated organic compounds and 1-methylnapthalene was selected as a surrogate for hydrophobic and polycyclic aromatic hydrocarbons found in such aerosols. Using a droplet train apparatus, the uptake of several organic gas phase species and gas phase HX (X = Cl, Br, I) was measured to probe the nature of hydrophobic organic surfaces as a function of relative humidity and temperature. These measurements yielded the mass accommodation coefficient (alpha) which is the probability that a gas phase molecule striking the liquid surface enters the bulk liquid. In some cases the Henry's law coefficient was also obtained.; The uptake of the following organic gas phase species was studied: m-xylene, ethylbenzene, butylbenzene, alpha-pinene, gamma-terpinene, p-cymene, 2-methyl-2-hexanol, and acetic acid. The measured uptake of the organic gas phase species is in accord with expectations. The uptake increases with decreasing temperature and is independent of relative humidity. On the other hand, the observed uptake of the gas phase HX acids on octanol is highly surprising. In the absence of water vapor, alpha for both HBr and HI is unity, however, alpha for HCl is much smaller, on the order 0.015. The values of alpha change dramatically as a function of relative humidity (i.e. the density of water vapor). As the relative humidity increases, the alpha values for HBr and HI decrease and alpha for HCl increases. At a relative humidity of about 50%, alpha for all three species reaches values measured earlier on pure water (alpha between 0.15 and 0.3, depending on temperature). These results are discussed in terms of the nucleation model for mass accommodation and a mechanism is proposed to explain the surprising results related to the uptake of the hydrogen halides. |
