NITROGEN DEPOSITIONS TO THE BALTIC SEA - EXPERIMENTAL AND MODEL ESTIMATES

Air pollution in the Baltic region is being monitored and assessed within the framework of the Helsinki Commission for the Protection of the Marine Environment in the Baltic Sea. Measurements of inorganic nitrogen compounds in precipitation, and to a lesser extent in air, have been performed since the mid-1980s under the supervision of HELCOM's Expert Group on Air Pollution, EGAP. The purpose is to establish a basis for evaluating the airborne pollution load to the Baltic Sea. These data have been used to estimate deposition of nitrogen to the sea surface by using various methods of extrapolating coastal data on concentrations and precipitation amounts. These estimates are supplemented by model calculations with the EMEP models. Estimates for the period 1986-1990 show that the annual wet deposition fluxes vary from about 650 kg N km-2 in the north to about 1106 kg N km-2 in the southern Baltic Sea, resulting in a total wet deposition of about 325 kT N yr-1. Other authors have estimated dry depositions for 1980-1986 at about 60 kT N yr-1. On the other hand, model calculations lead to a total dry and wet deposition of about 275 kT N yr-1. The reasons for the deviations are discussed and used to infer methodological limitations.