Hydration-influenced sorption of organic compounds by model and atmospheric humic-like substances (HULIS)

Atmospheric humic-like substances (HULIS) constitute a major fraction of the water soluble organic carbon of aerosol particles. We investigated sorption and desorption of water and two model organic contaminants (toluene and benzyl alcohol) on HULIS and a standard humic substance (Suwannee River fulvic acid; SRFA) under varying relative humidity using a quartz crystal microbalance. Simultaneous sorption of water and benzyl alcohol (capable of specific interactions like hydrogen bonding or charge transfer) on HULIS and SRFA shows significant, humidity-dependent, cooperative sorption at intermediate water activity, as well as a dependence of sorption distribution coefficient on the wetting-drying pathway. In contrast, sorption of toluene (capable of only nonspecific interactions) was humidity-independent. Atmospheric HULIS is thus found to have several sorption features in common with terrestrial and aquatic humic substances and soil organic matter. These features are consistent with the link solvation model (LSM), whereby water assists in cooperative sorption of specifically interacting compounds by the organic matter sorbent, and subsequent changes in sorbent structure result in sorption hysteresis. Sorption of compounds capable of only nonspecific interactions is unaffected by hydration status. Such sorption features can lead to considerable uncertainty in predicting and modeling transport of organic pollutants in the atmosphere.