Air pollution control describes measures to achieve an air quality that has no unacceptable effects on people or the environment. Nitrogen and sulphur compounds as well as ozone are particularly important for forests. Due to strict limits and technological innovations, the pollution from air contaminants has decreased in recent decades, and their deposition in forests has been reduced.

Measurements from the Level-II programme are important for showing the success of air pollution control measures and documenting their impact on ecosystems. The Level-II programme was established in 1994 as part of the Geneva Convention on Long-Range Transboundary Air Pollution (CLRTAP) to analyze the type and intensity of environmental influences. Today, the data are used to meet various national and international reporting obligations, for example, under the European Directive on National Emission Ceilings (NEC-D).

At the open sites associated with the Level-II areas, the concentration of key pollutants in the air is recorded.

The atmospheric deposition of substances is measured using deposition collectors. Surveys are conducted both in open areas and within the forest stands.