Dry deposition of ammonia in a coastal dune area: Measurements and modeling

Ammonia deposition is a threat to many natural ecosystems, including coastal dune areas, because of eutro-phication and acidification. Direct measurements of ammonia fluxes are nevertheless scarce. In this paper we present a full year of measurements to derive the ammonia dry deposition flux in a Dutch coastal dune ecosystem, based on the aerodynamic flux-gradient method (AGM). We found a mean ammonia flux of-7.1 +/- 1.7 ng m- 2 s- 1, and an annual ammonia deposition flux of-132 +/- 32 mol ha- 1 yr- 1 (equivalent to 1.8 +/- 0.4 kg N ha- 1 yr- 1), which is at the low end of the range from estimates from literature made with inferential methods. Modeling the fluxes with the DEPAC module resulted in a mean flux of-17.0 ng m- 2 s- 1. The model over-estimated the deposition fluxes, but diurnal variations of the fluxes derived from measurements were well captured by the model. We propose to change certain DEPAC parameters, like the leaf area index, to values more applicable for a dune ecosystem and show that this improves the agreement between model and measurements.