Long term measurements of the land-atmosphere exchange of ammonia over moorland

Measurements of the exchange of ammonia between the atmosphere and moorland vegetation using micrometeorological methods at a site in southern Scotland are reported. The median concentration of continuous measurements over seven months of the log-normally distributed ammonia data at the site is 0.45 mu g NH3 m(-3) with a geometric standard deviation of 3.4. The measurements made using a continuous wet annular denuder sampling system with a separate denuder al three heights above the surface provided satisfactory fluxes by the aerodynamic flux-gradient method for over 70% of the time between January and August 1995. The site which consists of unimproved moorland vegetation represented an efficient sink for ammonia during the winter months with deposition velocities (V-d) in the range 15 to 30 mm s(-1) and canopy resistances (R-c) close to zero. The monthly median V-d declined with time from 25 mm s(-1) in February to 5 mm s(-1) in August as the result of a steady increase of the canopy resistance in the dry, warmer, summer conditions to typically 100 s m(-1) during the daytime. The canopy compensation point increased from zero during winter months to about 0.2 mu g m(-3) in August and short periods of NH3 emission were observed whenever ambient concentration fell below 0.2 mu g m(-3) during the summer. The annual input of gaseous NH3 to this moorland surface is approximately 2 kg N ha(-1), and is similar to the input of NH4+-N in wet deposition. (C) 1998 Elsevier Science Ltd. All rights reserved.