Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 3 4 2003 10.5194/acpd-3-3493-2003 http://www.atmos-chem-phys-discuss.net/3/3493/2003/ http://www.atmos-chem-phys-discuss.net/3/3493/2003/acpd-3-3493-2003.html http://www.atmos-chem-phys-discuss.net/3/3493/2003/acpd-3-3493-2003.pdf 3493 3523 2003-07-14 Operational mapping of atmospheric nitrogen deposition to the Baltic Sea O. Hertel C. Ambelas Skjøth J. Brandt J. H. Christensen L. M. Frohn J. Frydendall National Environmental Research Institute, Department of Atmospheric Environment, P.O. Box 358, Frederiksborgvej 399, 4000 Roskilde, Denmark A new model system for mapping and forecasting nitrogen deposition to the Baltic Sea has been developed. The system is based on the Lagrangian variable scale transport-chemistry model ACDEP (Atmospheric Chemistry and Deposition model), and aims at delivering deposition estimates to be used as input to marine ecosystem models. The system is tested by comparison of model results to measurements from monitoring stations around the Baltic Sea. The comparison shows that observed annual mean ambient air concentrations and wet depositions are well reproduced by the model. Diurnal mean concentrations of NH_x (sum of  NH₃ and NH₄) and NO₂ are fairly well reproduced, whereas concentrations of total nitrate (sum of HNO₃ and NO₃^−) are somewhat overestimated by the model. Wet depositions of nitrate and ammonia are fairly well described for annual mean values, whereas the discrepancy is high for the monthly mean values and the wet depositions are rather poorly described concerning the diurnal mean values. The model calculations show that the atmospheric nitrogen deposition has a pronounced south – north gradient with depositions in the range about 1.0 tonnes N km^−2 in south and 0.2 tonnes N km^−2 in north. The model results show that in 2000 the maximum deposition to the Danish waters appeared during the summer in the algae growth season. For the northern parts of the Baltic the highest depositions were distributed over most of the year.