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. (C) 2003 Elsevier Science Ltd. All rights reserved.