MODELLING ISSUES The modeling area covers the territory of Poland (900 km x 750 km). The adopted computational grid constitutes a part of the MSC-W EMEP (Meteorological Syntesizing Centre-West, Co-operative Programme for Monitoring and Evaluation of the Long Range Transmission of Air Pollutants in Europe) model grid, only with finer spatial resolution (30 km x 30 km). The polar stereographic projection is applied. A grid designated in such a way facilitates comparison of the results obtained with other European models results. Sulphur dioxide atmospheric dispersion and deposition have been modeled by two different models: the POLSOX-I Eulerian grid model and the REGFOR Lagrangian trajectory model. Model's codes combine transport, diffusion, chemistry and deposition processes (dry and wet) in the study area. Two chemical compounds has been modeled: SO2 and SO42-. The necessity of applying two different models arises form the fact that sulphur dioxide concentrations in Poland are caused by three groups of emission sources: Sources from abroad (mainly from Germany and the Czech Republic), Polish sources from industrial and municipal sectors, Polish sources from the public power generation sector (power plants and cogeneration plants). SO2 concentrations caused by first two groups of sources are defined in the system as background concentrations; these sources and are not subjected for control procedure. The third group of sources is subjected for real-time energy production control procedure. Resulting concentration of SO2 in each computational grid element is the sum of background concentration and concentration caused by third group of sources.