Beforehand, however, another interesting remark will be made concerning the trend component. In Chapter 4 the autocorrelation of the emissions series was analysed. The autocorrelative ability, i.e. the persistence characteristic of the SO2 series, was motive for dealing with algorithms of univariate short-term forecasting.To a certain extent it describes the ‘inertia’ of polutent concentration in the atmospheric system. Its opponents are external factors of influence such as air mass flow, increased or reduced emissions, changing air stratification etc., which interfere with a stable balanced state being achieved.