At elevated concentrations, air pollutants exert direct effects (both acute and chronic) on plants. High ozone concentrations in the summer, for example, inflict visible damage on leaves and needles. More prolonged high-level exposure to ozone results in reduced growth, which in agricultural crops can translate into lower yields.
But air pollutants are also transported over great distances and eventually deposited - in a wet or dry form - in sensitive aquatic and terrestrial ecosystems. Inputs of sulphur and nitrogen, for example, give rise to acidification not only of alpine mountain lakes and higher-altitude rivers, but also of forest floors at any altitude. As a result of overfertilization, elevated nitrogen inputs also adversely affect a wide range of nitrogen-sensitive ecosystems such as forests, species-rich natural pastures and dry grassland, alpine heathland, raised bogs and fens. One example of what can happen when forests are overburdened with nitrogen is that nitrogen may be leached from the forest floor (in the form of nitrate) into the groundwater. Around two thirds of the nitrogen inputs currently affecting sensitive Swiss ecosystems are attributable to ammonia released from agriculture, while approximately one third can be traced back to nitrogen oxides from combustion processes.
Even today, internationally recognized critical loads and critical levels established by the UNECE for inputs of acids, nitrogen and ozone continue to be exceeded over large areas of Switzerland.