ENHANCEMENT OF THE DRY DEPOSITION OF SULFUR-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 sou rce 5 m above the forest canopy. Measured throughfall composition, sam pled weekly, showed enhanced deposition of both NH4+ and SO42- ions be low the fumigated area, relative to the adjacent control area. Compari son of the additional amounts of SO(4)(2-)measured in throughfall in t he fumigated area relative to the control area with possible causal fa ctors showed (i) a positive correlation between the average additional SO42--S flux in a sampling period and the average % time that the sur face was wet, and (ii) a positive correlation between the average depo sition velocity for the additional SO42--S relative to SO2 and the ave rage rate of NH3 release during a sampling period. The results imply t hat the surface resistance to the dry deposition of SO2 is small relat ive to the atmospheric resistance during periods of NH3 fumigation, an d that the deposition velocity for SO2 is therefore dependent on ambie nt NH3 concentrations. In regions where NH3 and SO2 concentrations are similar, a knowledge of ambient NH3 concentrations is therefore a req uirement for the development of accurate models of the dry deposition of SO2 to vegetation, and the estimation of Critical Loads for S depos ition. (C) 1998 Elsevier Science Ltd. All rights reserved.