Surface Exchange of Ammonia over Grazed Pasture

Summary

From July 1992 until July 1994 continuous measurements of the exchange of ammonia between surface and atmosphere over cultivated pasture were carried out at Zegveld in the west of the Netherlands. Fluxes were determined according to the profile method, using thermodenuders and continuous flow denuders for NH₃ measurements. Concentrations showed a dominating diurnal trend of high values at night and lower values during the day, due to differences in mixing intensity with clean air from higher layers of the atmosphere. In the grazed season average concentrations were higher than in the ungrazed season, due to grazing, fertilizer and manure application and plant activity in the region. In the grazed season emission dominated during daytime, while at night and also during daytime in the ungrazed season deposition was usually observed.

Exchange at this location can be roughly described with a model combining bidirectional exchange with the stomata, and deposition to (water layers on) the leaf surface.

Additional SO₂ measurements showed efficient deposition to the outer leaf surface, probably due to uptake in water layers on this surface, according to a correlation with humidity. No clear evidence of co-deposition with NH₃ was found, although the slightly lower average SO₂ deposition in winter compared to summer could indicate the effect of a lower NH₃/SO₂ ratio.