Abstract
During the last 5 decades the northeastern part of Estonia (the region where oil shale and the chemical industry are located) has been subjected to pollution with acidic compounds. In 1981–1988 the yearly mean nitrogen (N) deposition load was up to 11.1 kg ha–1. This N pollution level combined with the deposition of sulphur (S) could have seriously endangered the environment, but the simultaneous emission of strongly alkaline fly ash restrained acidification processes. After 1989–1991 the situation changed, and in 1994–1996 the N deposition load in northeastern Estonia remained within the range of 2.6 to 6.6 kg ha–1 year–1 and that of S within 2 to 50 kg ha–1 year–1. Because the fly ash deposition is permanently decreasing, more sensitive lichens and mosses can be subjected to critical N+S loads in the future. The proportion of oil shale industry in total emission of NOx in Estonia from stationary sources equals approximately 65 to 75%. During 1996–2000 the yearly mean concentration of NO2 in the air of towns increased from 9 to 12 to 16 to 29 μg m–3. The emission of N compounds was mainly caused by N oxides in flue gases from power plants, as well as ammonia and carbamide discharges from chemical plants. In 1988–1990 the estimated yearly total emission of NOx (as NO2 equivalent) was about 18 to 18.6 thousand t and in 1994–2000, 9.9 to 11.8 thousand t.