Long-term quantification of ammonia exchange between agricultural cropland and the atmosphere—I. Evaluation of a new method based on passive flux samplers in gradient configuration

A new method for quantification of ammonia exchange between agricultural cropland and the atmosphere is described. Ammonia is absorbed by passive flux samplers (glass tubes coated on the inside with oxalic acid), continuously integrating the horizontal flux of ammonia in various heights above the exchange surface. The mean horizontal fluxes are subsequently divided by the mean wind speed in the same heights, giving ammonia concentrations which are finally used for calculating an ammonia concentration gradient with respect to height and the vertical net flux of ammonia. It is shown that passive flux samplers are able to measure NH3 net fluxes over extended periods (up to about 1 week), because NH3 cdncentrations during periods with high wind velocities are weighted higher than NH3 concentrations during periods with low wind velocities, thereby reducing errors due to time-averaging. Emission and deposition fluxes of ammonia down to 0.1 kg NH3 ha−1 week−1 can be measured with a variation coefficient smaller than 20%. The method allows simultaneous measurements at many locations, thus making it possible to obtain more and better information about sources and sinks for atmospheric ammonia and about the significance of ammonia exchange fluxes for atmospheric ammonia pollution and for the nitrogen enrichment of terrestrial ecosystems.