Temporal evolution of nitrogen compounds in Swedish precipitation since 1955

Abstract

Nitrate (NO₃⁻) and ammonium (NH₄⁺) ions are important determinants of the ionic strength and pH 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 Europe⁴. 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 NH₄⁺ between 1972 and 1984. In contrast, the sulphate concentration decreased by ∼30% for the same locations and time period.