Deposition of air pollutants (2.1)

Atmospheric pollutants impair the vitality of forests by affecting trees and other organisms both directly and indirectly, as well as through soil. Trees suffering from pollutants are also susceptible to damage from extreme weather conditions and to destruction.

Gases resulting from the combustion of fossil fuels contain sulphur and nitrogen oxides which react chemically in the atmosphere and fall to the earth as acid deposition. Thanks to international efforts in reducing emissions, sulphur deposition began to decrease conspicuously in the late 1980s and has continued to do so, being now 40% to 60% of what it was at its peak. Nitrogen deposition has also decreased, but not as much as sulphur.

Figure 2.1 Total deposition of sulphate sulphur and nitrogen (nitrate nitrogen and ammonium nitrogen) in 2009.
Source: Finnish Environment Institute

A considerable part of acid deposition comes with long-range pollution transport. Around 2000, 71% of nitrogen deposition and 83% of sulphur deposition measured in Finland originated abroad.

There is as yet no information on the long-term cumulative effects of acid deposition on soils, and the capacity of forest soils to neutralise acid is not completely known.

The decrease in sulphur deposition is the result of reductions of emissions from energy production, reduced use of industrial fuel oil, the introduction of new alternative energy sources, and improvements in the production methods of pulp and paper plants and metal and chemical industries.