Can clouds enhance long-range transport of low volatile, ionizable and surface-active chemicals?

Atmospheric partitioning and transport of low volatile organic compounds is strongly influenced by the presence of water (e.g. clouds) and its deposition velocity (e.g. rainfall, snow). It was identified that the assumption of continuous rainfall underestimates the residence time and the transport potential of non-volatile substances. The liquid water content of clouds and the high specific surface of frozen or liquid cloud droplets can significantly contribute to the total activity capacity (i.e. the capacity to sorb chemicals) of the atmosphere for non-volatile, ionizable and surface active substances. A modified version of the regional multimedia activity model for ionics MAMI, including twolayered atmosphere with atmospheric boundary layer (ABL) and lower/middle troposphere (LMT), interface partitioning, intermittent rainfall and variable cloud coverage was applied to a selection of ten low volatile or ionizable chemicals to investigate the potential of clouds to enhance the atmospheric transport potential. Probability density functions were derived for input substance properties and environmental parameters to quantify uncertainty and variability and probabilistic simulations at steady state were run for a constant emission to the atmospheric boundary layer to identify key model inputs. The degradation rate, the duration of dry and wet periods and the parameters describing air-water bulk partitioning (KAW and T) and ionization (pKa and pH) determine the residence time in the ABL. In the LMT, however, the residence time depends also on the water content of clouds and on interface partitioning. In some cases the residence time and its variability range is similar in the two compartments, while some compounds (e.g. diazinon, 2,4-D, perfluorooctanoic acid) are more persistent in the LMT. The longer residence time predicted for some compounds in the LMT is due to the capacity of clouds to sorb non-volatile molecules in the liquid water and at the interface of cloud droplets. The efficiency of wet deposition to remove low volatile organic pollutants from the atmosphere is limited primarily by the duration of the dry interval between precipitation events. During dry periods persistent non-volatile chemicals can be transported to the troposphere. Here, the high capacity of of tropospheric clouds to sorb non-volatile and surface active chemicals limits oxidation and wet deposition rates and increases the potential for long-range transport