Maximum reservoir capacity of vegetation for persistent organic pollutants: Implications for global cycling

The concept of maximum reservoir capacity (MRC) or "equilibrium capacity ratio,'' the ratio of the capacities of the vegetation and of the atmospheric mixed layer (AML) to hold chemical under equilibrium conditions, is applied to selected persistent organic pollutants (POPs) in vegetation in order to assess its importance for the global cycling of POPs. Vegetation is found to have a significant storage capacity, and because of its intimate contact with the atmosphere may play an important role in global cycling of POPs. The vegetation MRC is calculated for some representative PCB congeners (PCB-28; -152; -180) at the global scale with a spatial resolution of 0.25degrees x 0.25degrees. It is shown to be comparable to that of the skin layer of the soil and to vary over many orders of magnitude, between compounds, locations, and time (seasonally/diurnally), depending on the vegetation type and on the temperature. The highest MRC values are observed in areas with low temperatures and coniferous forests (e.g., Siberia, Canada, Scandinavia), while the lowest values are typically located in warm and desert areas (e.g., Sahara). Large differences were also observed at the regional scale. Implications for the global cycling and long-range atmospheric transport (LRAT) of POPs are discussed, including comparisons with soil and ocean MRCs, which will drive net transfers of POPs between media and regions.