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 from measurements of the vertical gradient of ammonia and micrometeorology above 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 humidity. At conditions with westerly winds, the wind brings air masses from the North Sea with low concentration levels of ammonia to the site, while at conditions with easterly winds, the air have passed central Jutland with large emission areas. Some of the relatively low deposition velocities or emissions were observed during conditions with low ammonia concentration and westerly winds. These observations might relate to a compensation point of the forest, i.e. an ammonia concentration below which the trees and/or the surface emit ammonia due to an equilibrium with the ammonia inside the needles or on the surface. Emission of ammonia was also observed at relatively high ammonia concentration levels (above 2 mu g NH3-N m(-3)), mainly during one measuring period characterized by easterly winds with dry conditions and high ammonia concentrations, and the emissions might relate to evaporation from ammonia saturated surfaces or emission from mineralization in the forest soil. In general, relatively high net deposition velocities were observed during conditions with relative humidity above 80% or at ammonia concentrations moderate higher than a given (temperature dependent) compensation point. During stable conditions some observations revealed that the gradient above the canopy not necessarily represents the exchange with the canopy. (C) 1999 Elsevier Science Ltd. All rights reserved.