Deposition of atmospheric pollutants on forests

Abstract

Forests have long been considered as efficient sinks for atmospheric pollutants. The potential for large rates of deposition is provided by the turbulent structure of air above and within forest canopies. Large rates of deposition of pollutant gases, however, are only found for the very reactive gases HNO 3 , HCl and NH 3 . In contrast, the pollutants SO 2 and O 3 are deposited on forests and short vegetation at similar rates under the control of stomatal resistance. Deposition of sub-micrometre aerosol particles on forests appears to be inefficient but at high elevations in the United Kingdom (up to 500 m) these aerosols are frequently activated into cloud droplets in the size range 5-10 µm (radius). These droplets are efficiently captured by forest canopies and this deposition pathway may make a large contribution to annual inputs at high elevation sites. The effects of afforestation on inputs of pollutants to catchments are illustrated by model calculations for inputs of sulphur and nitrogen to Kielder forest in northern England. Inputs of sulphur and nitrogen to this area as moorland are estimated at 17.5 kg ha -1 (1 hectare = 10 4 m 2 ) and 12.4 kg ha -1 annually, respectively. Afforestation of the moorland increases sulphur and nitrogen inputs by 30% and 90%, respectively.