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 NH3 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 SO2-4 ions below the fumigated area, relative to the adjacent control area. Comparison of the additional amounts of SO2-4 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 SO2-4–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 SO2-4–S relative to SO2 and the average rate of NH3 release during a sampling period. The results imply that the surface resistance to the dry deposition of SO2 is small relative to the atmospheric resistance during periods of NH3 fumigation, and that the deposition velocity for SO2 is therefore dependent on ambient NH3 concentrations. In regions where NH3 and SO2 concentrations are similar, a knowledge of ambient NH3 concentrations is therefore a requirement for the development of accurate models of the dry deposition of SO2 to vegetation, and the estimation of Critical Loads for S deposition.