Concentration-dependent deposition velocities for ammonia: moving from lab to field

Estimates of the dry deposition of ammonia (NH3) gas in a field fumigation experiment on an ombrotrophic bog have been made by the inferential technique, using measured wind speed at 2 m, and air concentrations at two heights above the vegetation. The parameters for a concentration-dependent surface resistance term were derived from flux measurements over the same vegetation in a chamber study, separating stomatal from non-stomatal resistances (Jones et al., Atmos Environ, 41, 2049-1061, 2007). Application of these values to the field experiment led to estimates of NH3-N deposition from 3 kg N ha−1y−1 in ambient air, with an NH3 concentration at 0.5 m above the canopy of 0.7 μg m−3, to 70 kg N ha−1y−1 where annual average air concentrations were 100 μg m−3 and concentrations during fumigation were up to 1600 μg m−3. The equivalent deposition velocities (at z=0.5 m) were 0.016 m s−1 in ambient air and 0.003 m s−1 at100 μg m−3. The differences between annual deposition estimates made from independent air concentration data at 0.1 m and 0.5 m above the canopy were small for distances more than 10 m from the source, after vertical mixing was complete. Over 4 years (2003 to 2006) and at 8 sampling points more than 10 m from the NH3 source, the mean difference between the dry deposition estimates, using NH3 concentrations measured at 0.1 m and 0.5 m above the canopy, was 2%. Use of a constant surface resistance, with no concentration dependence, as commonly used in inferential models of dry deposition, would have predicted deposition up to 8 times too large for this experimental examination of the direct effects of NH3 on bog vegetation.