Nitrate (NO3-) and ammonium (NH4 +) ions are important determinants of the ionic strength and p H of precipitation1. The deposition of these ions on vegetation and on the ground also constitutes a significant source of nutrients for plants. This is particularly the case in polluted regions, for example, Europe, where the deposition of nitrogen compounds is enhanced2. There are indications (for example, increased run-off of nitrogen3) that forests in some parts of Europe have reached saturation levels with respect to nitrogen deposition; and excessive wet and dry deposition of nitrogen compounds has been suggested as a possible cause of the extensive damage to forests in Europe 4. Data from Rothamsted, UK, indicate that the mean concentration of NO3- in precipitation doubled between the 1850s and I960 (ref. 5); similarly, the NH4+ concentration probably increased between the 1880s and 1960. A review of precipitation chemistry data from Europe indicates an approximately threefold increase in NO3 - deposition between the 1890s and the late 1970s6. The present analysis of data from the European Air Chemistry Network reveals that the NO3- concentration at most stations approximately doubled between the late 1950s and early 1970s, with a less pronounced increase in NH4+. Detailed analysis of data from 12 Swedish stations showed no further increase of NO3- or NH 4+ between 1972 and 1984. In contrast, the sulphate concentration decreased by ~30% for the same locations and time period.
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