National air pollution assessments based on the MATCH-Sweden model – Results for the period 1999-2002 The MATCH-Sweden model system is used as a tool for mapping of air pollution deposition and concentration over Sweden and for air pollution assessment studies. The system includes a simplified type of data assimilation of background atmospheric chemistry observations in Sweden and Norway. Over the entire period 1999-2002 the annual sulphur and nitrogen deposition consistently shows a maximum over Southwest Götaland and decreases northwards. An annual air pollution budget for Sweden is determined. During the period of study the Swedish emissions account for 5-7% of the total annual deposition of sulphur in Sweden and for 11-13% of the corresponding NOx-deposition. The Swedish contribution to the total deposition of NHx-nitrogen is, according to the present calculations, about 16%. However, the uncertainty in this value is larger than for sulphur and NOx-nitrogen. Comparisons between results for the year 2000 obtained with MATCH-Sweden and the new “Unified EMEP Eulerian model” (EMEP, 2003) respectively, show good agreement for most parameters. The agreement is much better than observed in earlier comparisons with the old EMEP model. Large differences are found only for long-range transport contribution to the NHx-deposition over Northern Sweden, where MATCH-Sweden calculations indicate larger depositions than EMEP. However, the MATCH-Sweden system – including simplified data assimilation - suffers from a lack of representative air and precipitation NHx-measurements in that area. A tentative estimate of deposition trends over Sweden indicates a 50-55% decrease for sulphur between 1991 and 2002. For NOx- and NHx-nitrogen the corresponding decrease in deposition is only about 15%each.