Ammonia dry deposition over grassland - micrometeorological flux-gradient measurements and bidirectional flux calculations using an inferential model

For a 17-day period in late summer 1995 ammonia (NH3) fluxes over an extens ive area of semi-natural ungrazed short grassland were measured and modelle d. The measurements were carried out at the Melpitz field research station near Torgau in the state of Saxony in eastern Germany (86 m above sea level , 51 degrees 31'N, 12 degrees 56'E). For the calculation of the fluxes the micrometeorological flux-gradient technique was applied using a sensitive c ontinuous-flow wet annular denuder system for NH3 at three heights. Both do wnward fluxes to the canopy and upward fluxes from the grassland to th; atm osphere were observed. On the basis of the direction of the flux in relatio n to air concentrations, the ammonia canopy compensation point was estimate d to vary within the range 0.2 to 3.0 mug m(-3). Because the simple 'canopy /surface resistance' (R-c) model presupposes a surface concentration of zer o for the observed trace gas, which cannot explain bidirectional fluxes, th e results of the measurements were compared with a static 'canopy compensat ion point-cuticular resistance' (chi (c) - R-w) model, which is able to qua ntify the stomatal compensation point (chi (s)) and allows bidirectional fl uxes. The agreement between the model and the measured fluxes for the whole period was relatively good. Given realistic ratios of ammonium to proton c oncentrations (Gamma = [NH4+]/[H+]) for the intercellular fluid of between 150 and 1000 the minimum in the difference was found for Gamma approximate to 1000 (correlation coefficient r(2) = 0.23) and even bidirectional fluxes were obtained. Possible reasons for the differences between the measuremen ts and model estimates are discussed.