Atmospheric fate of non-volatile and ionizable compounds.

A modified version of the Multimedia Activity Model for Ionics MAMI, including two-layered atmosphere, air-water interface partitioning, intermittent rainfall and variable cloud coverage was developed to simulate the atmospheric fate of ten low volatility or ionizable organic chemicals. Probabilistic simulations describing the uncertainty of substance and environmental input properties were run to evaluate the impact of atmospheric parameters, ionization and air-water (or air-ice) interface enrichment. The rate of degradation and the concentration of OH radicals, the duration of dry and wet periods, and the parameters describing air-water partitioning (K(AW) and temperature) and ionization (pK(a) and pH) are the key parameters determining the potential for long range transport. Wet deposition is an important removal process, but its efficiency is limited, primarily by the duration of the dry period between precipitation events. Given the underlying model assumptions, the presence of clouds contributes to the higher persistence in the troposphere because of the capacity of cloud water to accumulate and transport non-volatile (e.g. 2,4-D) and surface-active chemicals (e.g. PFOA). This limits the efficiency of wet deposition from the troposphere enhancing long-range transport.