Dry deposition of ammonia and ammonium aerosols over heathland

Abstract

The dry deposition velocities of ammonia (NH3) and particulate ammonium were determined using the micrometeorological gradient method. 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 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. The low canopy resistance observed and the lack of any diurnal variation suggest that uptake at the leaf surface is relevant. Measurements and calculations from a surface layer model indicate an annual average 24-hr deposition velocity to heathland of 1.4 cm/s. For particulate ammonium, only in in 10 measurements leads to flux values significantly higher than the detection limit of the method, owing to the low deposition velocity of particulate NH+4. The average flux measures is equivalent to 3.8 kg NH+4/ha/yr. The annual average 24-hr dry deposition velocity for particulate NH+4/ha/yr. is 0.17 cm/s. -from Author