The accession of Poland to the European Union involved the need or regional air quality assessment and brought radical change in requirements towards the sottware tools used for assessment purposes. According to Polish law, a zone is an agglomeration o rover 250 000 inhabitants, or a poviat (second level or local government administration in Poland), or a group of poviats, and assessment should consider both global and regional inllow or pollutants as well as the impact of local emission sources and significant sources in a voivodeship. These requirements have imposed a model range of over 250 km. Following an analysis or different models operating all over the world, the CALPUFF model together with the CALMET meteorological processor was chosen to be implemented in air quality assessment systems in Polish zones. This paper presents the results or model calculations performed within the air quality assessment in Mazowieckie voivodeship as well as compares them with the measurements obtained at automatic air monitoring stations.