Acid deposition is a phenomenon resulting from complex reactions between OH, NO2 – Nitrogen dioxide, and SO2. While the natural pH value of rain is lower than 7 (mainly due to dissolved CO2 – Carbon dioxide), the term acid rain refers to precipitation with a pH value less than 5, which occurs in regions with large amounts of anthropogenic pollution. Aerosol and rain are expected to be slightly acidic in the presence of natural sources of SO2 and NO2, such as in tropical rain forests. However, atmospheric pollution enhances this phenomenon. Many soils and water surfaces can neutralise it, but their acid neutralising capacity is often linked to the presence of ammonia NH3, another nitrogen containing pollutant. Regions with high ammonia emission from agriculture (e.g. the Netherlands) suffer less consequences from acid rain than Scandinavia or Canada.
Acid deposition can be derived from the oxidation of sulphur material leading to the formation of sulfur dioxide SO2 which then reacts with OH and hydrogen per oxide H2O2 to produce sulfuric acid H2SO4 (Wayne, 2000). Additionally, nitric acid HNO3 can be produced from NO2, either directly by reaction with OH, or indirectly by first the reaction with the nitrate radical NO3 leading to the acid anhydride, dinitrogen pentoxide N2O5. HNO3 has a high solubility property in water releasing H+ (low pH), and therefore accumulates in aerosols and cloud droplets. It is also rained out.
Eutrophication phenomenon is driven by nitrogen fertilization of soils and waste water from rivers. Overgrowth of surface water algae and reduction of oxygen for deeper-lying biosystems due to excess deposition of NOx related species is stimulated by nutrient pollution.
Acid deposition and eutrophication have become an important environmental issue and research area for the last decade (Heij and Erisman, 1995). European public recognition emerged in the middle of 1980s after discovering the acidification of Scandinavian lakes. Consequently, measures have been implemented to reduce the NOx and SO2 emissions such as catalytic converters in cars, improved design of furnaces in power plants (in addition to sulfur scrubbers), chemical removal of acid gases. While these measures were originally driven by the acid rain problem, they also resulted in a reduction of aerosol concentrations.