The importance of nitrogen oxides for the exceedance of critical thresholds in the Nordic countries

Impacts of air pollutants and especially acidification in ecosystems have b een of serious concern in the Nordic countries since the 1970s. The current approach to assess several pollutants (sulfur and nitrogen oxides, ammonia , volatile organic compounds) and their effects (acidification, eutrophicat ion and ground-level ozone) simultaneously is extremely complex. This study explored the relative role of nitrogen oxides in environmental impacts in the Nordic countries. The share of NOx in the exceedances of critical loads , the long-term ecosystem protection targets, was found to be roughly 25% i n acidification and 50% in eutrophication. The contribution of NOx emission s to ground-level ozone fort-nation was considered important, as NOx is the limiting precursor in ozone formation in the Nordic countries. The compari son of observed and modeled accumulated ozone concentrations (AOT40) for th e early 1990s shows noticeable differences in the Nordic area, partly due t o the sensitivity of the AOT40 indicator to the 40 ppb threshold value.