The probability density functions were used to forecast the air quality in several monitoring sites located in Worek Żytawski region. Prediction of percentile 99.8 value (the maximum allowable 30-minute pollutant concentration) was made on the basis of the frequency function, which represented the annual distribution of SO2. Analysing the historical SO2 concentration data sets identified the form of distribution. The parameters of the distribution were estimated basing upon the annual average concentration prognosis. Then, the preliminary predicted distribution of S02 was verified every month by taking into account successively upcoming concentrations values recorded during the analysed period. The results proved that the air quality estimation obtained with the presented method of verification at the end of September and the ones typically calculated at the end of the year were similarly accurate.