Trends in atmospheric ammonia and particulate ammonium concentrations in Sweden and its causes

Daily average air concentrations of ammonia plus particulate ammonium, nitric acid plus particulate nitrate, sulphur dioxide and particulate sulphate have been measured at four EMEP (European Monitoring and Evaluation Programme) sites in Sweden since 1986. From 1993 sampling was made with a filter pack that separates the gaseous and particulate forms. The separation is not perfect, but fairly good when compared to a denuder. The same filter pack was used for monthly measurements at four high altitude sites in the Swedish mountain areas. The concentration of particulate ammonium decreases more rapidly than the ammonia emissions in Sweden and neighbouring countries while the ammonia concentration increases. Particulate ammonium is mainly long-range transported from the continent to the southern parts of Sweden, while gaseous ammonia comes from all directions. The particulate ammonium concentrations are charged balanced with particulate sulphate and nitrate concentrations and the long-range transport is therefore limited by the availability of these anions.