Measurements of ammonia concentrations, fluxes and dry deposition velocities to a spruce forest 1991-1995

The dry deposition velocities and fluxes of ammonia have been estimated fro m measurements of the vertical gradient of ammonia and micrometeorology abo ve a spruce forest in western Jutland, Denmark. Measurements have been made in seven periods, each lasting about one week and covering all seasons and different meteorological situations. Different deposition characteristics were observed, depending on the ammonia concentration and the relative humi dity. At conditions with westerly winds, the wind brings air masses from th e North Sea with low concentration levels of ammonia to the site, while at conditions with easterly winds, the air have passed central Jutland with la rge emission areas. Some of the relatively low deposition velocities or emi ssions were observed during conditions with low ammonia concentration and w esterly winds. These observations might relate to a compensation point of t he forest, i.e. an ammonia concentration below which the trees and/or the s urface emit ammonia due to an equilibrium with the ammonia inside the needl es or on the surface. Emission of ammonia was also observed at relatively h igh ammonia concentration levels (above 2 mu g NH3-N m(-3)), mainly during one measuring period characterized by easterly winds with dry conditions an d high ammonia concentrations, and the emissions might relate to evaporatio n from ammonia saturated surfaces or emission from mineralization in the fo rest soil. In general, relatively high net deposition velocities were obser ved during conditions with relative humidity above 80% or at ammonia concen trations moderate higher than a given (temperature dependent) compensation point. During stable conditions some observations revealed that the gradien t above the canopy not necessarily represents the exchange with the canopy. (C) 1999 Elsevier Science Ltd. All rights reserved.