The potential for long-range transboundary atmospheric transport

A simple generic procedure was developed to determine the potential for long-range transboundary atmospheric transport of substances by means of the residence time of the substance in air. The atmospheric residence time, τ a, is defined here as the time period in which the mass of the pollutant in the atmospheric boundary layer is reduced by 50%. This was calculated using first-order and time-averaged reaction rates for the removal processes of dry and wet deposition, and degradation in air. The dry deposition process is described by means of an effective deposition velocity which accounts for the possible re-emission of the substance. Examples of the atmospheric residence time for a number of pesticides, chemicals and combustion products are presented. The atmospheric residence time is recommended as a criterion in screening methods and risk assessments that consider the spatial extent of atmospheric long-range transport of substances.