The dry deposition velocities of ammonia (NH3) and particulate ammonium were determined using the micrometeorological gradient method. About 130 measurements of 1 hour duration were conducted in the Netherlands from 1984 to 1987 in different seasons, mainly over heather/purple moor grass vegetation. The results show that NH3 is deposited more rapidly than other trace gases, such as ozone, sulfur dioxide, or nitrogen dioxide. An average deposition velocity of 1.9 cm/s was observed during the measurements (most of them carried out during the daytime) over all heather locations. The average flux of NH3 during these periods was equivalent to 16 kg NH3/ha/yr. The canopy resistance for NH3 varied between close to zero and 150 s/m, which is low relative to the stomatal resistance to water vapor. The effect of several variables on the canopy resistance was investigated, but only that of surface wetness and air temperature was established clearly. The results indicate that wet vegetation acts as an almost perfect sink for NH3. The low canopy resistance observed and the lack of any diurnal variation suggest that uptake at the leaf surface is relevant.

No clear-cut difference between the various locations was found. The observed variation in the measured deposition velocities could be explained to some 60% with a surface layer resistance mdoel on the basis of fluctuations in meteorological variables, surface wetness, and air temperature. To obtain an estimate of the long-term average deposition velocity of NH3, we used a surface-layer model to account for the effect of nighttime meteorology. Measurements and calculations indicate an annual average 24-hour deposition velocity to heathland of 1.4 cm/s. For particulate ammonium, only one in 10 measurements leads to flux values significiantly higher than the detection limit of the method, owing to the low deposition velocity of particular NH+4. The average flux measured is equivalent to 3.8 kg NH+4/ha/yr. The annual average 24-hour dry deposition velocity for particulate NH+4 is 0.17 cm/s. Under the circumstances encountered in the Netherlands, only about 20% of NHx dry deposition was by way of dry depositon of particulate NH+4. For a typical site in the Netherlands the dry deposition was about 50% of the estimated input by wet deposition. ¿ American Geophysical Union 1994