Weekly measurements over one year of ammonia concentrations and surface exchange fluxes at a Danish heathland with passive wind-vane flux samplers in an aerodynamic gradient configuration

The present study aims to establish the annual NH₃ deposition to an inland heathland in Denmark using a micro-meterological approach with passive wind-vane flux samplers. The integrating samplers were replaced at weekly intervals from May 1995 to May 1996. The average concentration, 2.05 μg m^{- 3} at the heathland is at a moderate level when compared to heathlands in other parts of Europe. The average deposition velocity was 0.83 cm s^-1 which is within the range of depositon velocities found for other heathlands in Europe. The average canopy resistance was found to be relatively high, 61 s^-1. The measurements yielded a total NH₃-N deposition of 2.4 (± 0.9) kg ha^-1 yr^-1 with a data coverage of 71% for 1995/1996. In 40% of this time the flux is regarded as zero because the flux is not significant different from zero. In 60% of this time the significant fluxes varied firm -0.052 μg m^-2 s^-1 (deposition equal 16.4 kg N ha^-1 yr^-1) to 0.089 μg m^-2 s^{- 1} (emission equal 28.2 kg N ha^-1 yr^-1). The method is only able to direct measure significant fluxes down to the equivalence of 0.010 μg m^-2 s^-1 (approximately 3.2 kg ha^-1 yr^-1). Therefore the exact deposition cannot be determined by the applied method at very low deposition sites such as a coastal heathland in Denmark. In a high-deposition area as in the central Netherlands the method gave significant fluxes with a 100% data coverage fora two month period.