Enhancement of the dry deposition of sulphur dioxide to a forest in the presence of ammonia

A section of Scots pine ( Pinus sylvestris) forest in central Scotland was fumigated with NH 3 between March and October 1993 from a point source 5 m above the forest canopy. Measured throughfall composition, sampled weekly, showed enhanced deposition of both NH + 4 and SO 2- 4 ions below the fumigated area, relative to the adjacent control area. Comparison of the additional amounts of SO 2- 4 measured in throughfall in the fumigated area relative to the control area with possible causal factors showed (i) a positive correlation between the average additional SO 2- 4–S flux in a sampling period and the average % time that the surface was wet, and (ii) a positive correlation between the average deposition velocity for the additional SO 2- 4–S relative to SO 2 and the average rate of NH 3 release during a sampling period. The results imply that the surface resistance to the dry deposition of SO 2 is small relative to the atmospheric resistance during periods of NH 3 fumigation, and that the deposition velocity for SO 2 is therefore dependent on ambient NH 3 concentrations. In regions where NH 3 and SO 2 concentrations are similar, a knowledge of ambient NH 3 concentrations is therefore a requirement for the development of accurate models of the dry deposition of SO 2 to vegetation, and the estimation of Critical Loads for S deposition.