Highly reactive gases are deposited on all natural surfaces at rates which are dependent only on the atmospheric resistances. Deposition surfaces are regarded as perfect and surface and canopy resistance equal to zero. In such cases, the flux estimates are quite straightforward. The field measurements indicate that deposition velocities for HNO 3 and HCl are large. It should be added, however, that the estimation of atmospheric boundary layer resistance poses some problem for these gases for the regions with rough surfaces, such as forests. There are various empirical relationships established for various regions, which contribute to fairly large uncertainties in flux estimates for HNO3 and HCl reaching at least 50%.
The exchange of ammonia between terrestrial surfaces and the atmosphere is more complicated than that for HNO 3 and HCl because ammonia is the most abundant alkaline component in the atmosphere and emission and deposition fluxes are both commonly observed. There are also other complications related to the nitrogen metabolism of crop plants which produces ammonia. In-cell ammonia will seek equilibrium with ambient air concentrations - the so-called compensation point. At ambient ammonia concentrations exceeding this point, ammonia is deposited. Otherwise, ammonia fluxes will be from plant cells and directed away from the surface. Compensation points are specific to habitats and so ideally the ammonia exchange between atmosphere and the terrestrial surfaces should be analyzed for natural and managed vegetation, separately.
Measurements in the Netherlands have concluded that rates of ammonia deposition over the heathland are very large with negligible canopy resistance. The opposite is true for agricultural land with the application of urea. The upward fluxes of ammonia from agricultural soils can be large, up to 2 orders of magnitude larger than the deposition fluxes onto moorland. The upward fluxes can be reduced by the uptake of ammonia by crops.
The bidirectional nature of the ammonia flux has also been investigated for the aquatic ecosystems, particularly in the coastal areas.
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