Review of dry deposition measurements of ammonia and nitric acid to forest

The paper describes some general aspects of the estimation of dry deposition fluxes and reviews the measurements of dry deposition of atmospheric gaseous ammonia and nitric acid in relation to forests. The dry deposition of ammonia is shown to be a bi-directional surface flux, i.e, the forest receives a nitrogen input from ammonia in the air, though some ammonia apparently is also emitted from the forest. Still, atmospheric ammonia originating from agricultural activities contributes with large amounts of nitrogen to the ecosystems. Nitric acid seems to deposit very fast, though only very few flux measurements above forest are reported. The method for estimating the fluxes of nitric acid might be questionable for some conditions. For the investigations reported the concentration levels of nitric acid have been relatively low and this compound has contributed with moderate loads of nitrogen to the ecosystem. (C) 1999 Elsevier Science B.V. All rights reserved.