The Air/Surface Adsorption Equilibrium of Organic Compounds Under Ambient Conditions

The environmental fate of volatile and semivolatile organic pollutants strongly depends on their partitioning between the gas phase and condensed phases. This partitioning includes absorption in bulk phases like water or organic matter and adsorption on ambient surfaces. Often, the latter process is neglected due to the limited knowledge of adsorption as compared to absorption. The first part of this work discusses how the adsorption equilibrium can be assessed as a function of surface and adsorbate properties, temperature, and relative humidity. It is shown that the traditional approach of correlating adsorption constants with the volatility of the considered compounds or with their octanol/air partitioning does not provide a reasonable understanding of the experimental data. Instead, a model is needed that is based on a simple description of the relevant intermolecular interactions. The second part of this work analyses the relative importance of adsorption to surfaces for the overall sorption of airborne organic compounds in the environment. This discussion is focused on gas/particle partitioning and on the exchange between soil surfaces and the atmosphere.