Trends in the Acidity of Rain in Europe: A Re-Examination of European Atmospheric Chemistry Network Data

Summary

Data from the European Atmospheric Chemistry Network over the period 1956–76has been evaluated statistically to establish what trends are apparent in precipitation acidity. Out of 120 sites with 5 or more years data 29 show a significant trend of increasing annual average acidity in precipitation during the period and 5 show a decrease. For these sites substantially the same result arises for the hydrogen ion concentration calculated from ionic balance based on detailed chemical analysis of the precipitation samples. Examination of the monthly data showsthat the increased annual average levels for H⁺, where they do occur, arise from an increased frequency of intermittent high monthly values. At several stations this appears to have started quite suddenly around 1965 leading to an apparent step change in annual average acidity. Data from most of the sites does not appear to fit the simple picture of an expanding area receiving acid precipitation over the period in question.