Atmospheric Environment 37 Supplement No. 1 (2003) S167–S177 . Fluxes of ammonia in the coastal marine boundary layer Lise Lotte S^rensen a, *, Ole Hertel b , Carsten Ambelas Skj^th b , Mikael Lund b , Britta Pedersen b a Ris^ National Laboratory, Wind Energi and Atmospheric Physics Department, P.O. Box 49, Frederiksborgvej 399, DK-4000 Roskilde, Denmark b National Environmental Research Institute, P.O. 358, Frederiksborgvej 399, DK-400 Roskilde, Denmark Abstract Concentrations of ammonia in air and ammonium in surface water were measured from a platform in the Southern North Sea close to the Dutch coast. Fluxes were derived from the measurements applying Monin–Obukhov similarity theory and exchange velocities calculated. The fluxes and air concentrations of ammonia were compared to results obtained from the Lagrangian transport-chemistry model ACDEP with and without a parameterisation of outgoing fluxes of ammonia from the sea. The results indicate that the flux may in fact be upward during periods with low atmospheric ammonia concentrations and that the calculated overall ammonia dry deposition may be overestimated by a factor two or more in the coastal region. A more detailed study is needed in order to quantify how this may influence overall deposition to given marine waters. In some cases the deposition may solely be redistributed whereas the total deposition is only marginally influenced. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Ammonia fluxes; Surface exchange; Coastal zone; Reduced nitrogen; Flux divergence; Advection 1. Introduction European coastal zones are areas of great concern due to the frequently observed episodes of algal blooms that in worst case situations lead to oxygen depletion of the bottom waters when the bio mass die and decompose. Algal growth is generally limited by the supply of nitrogen (Kronvang et al., 1993). The increased fre- quency of algal blooms including blooms of harmful algae are therefore believed to be associated with high nitrogen inputs from anthropogenic activities either from river run-off or atmospheric deposition. Atmo- spheric nitrogen deposition has been shown to con- tribute an increasing fraction of the overall nitrogen load. For Danish marine waters atmospheric nitrogen deposition has been estimated to be equal to river run- off (Ambelas Skj^th et al., 2002). It has been estimated that over the period from the beginning of this century and up to the end of the mid-1980s, nutrient fluxes of nitrogen and phosphorous to the Baltic Sea increased by a factor of four and eight, respectively (Larsson et al., 1985). Similar increases over this time period are likely to have taken place for many other European marine waters. The atmospheric contribution to the nitrogen load of the German Bight during 1989–1992 was estimated to be about 30% (Beddig et al., 1997), and estimates for the Baltic Sea points at a 50% contribution from atmospheric deposition (Rosenberg et al., 1990). The atmospheric inorganic input is furthermore directly consumable by the algae, which is only true for parts of the river runoff. The project: ‘‘Atmospheric nitrogen inputs into the coastal ecosystem’’ (ANICE) project addressed the flux of nitrogen species from the atmosphere into the ocean, with emphasis on coastal waters (de Leeuw et al., 2001). The study aimed at identifying the role of the atmo- sphere as a source of biologically essential species to the marine biota. ANICE focused on quantifying the deposition of atmospheric inputs of inorganic nitrogen compounds into the North Sea, and the governing ARTICLE IN PRESS *Corresponding author. E-mail address: lotte.geern@risoe.dk (L.L. S^rensen). URL: http://www.risoe.dk. 1352-2310/03/$-see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S1352-2310(03)00247-4