Trends in sulphate and nitrate wet deposition over the United Kingdom: 1986–1999

Abstract

Since 1986 annual wet deposition of non-sea-salt sulphur has decreased over all regions of the United Kingdom, with the largest decrease in eastern areas. Nationally wet deposited sulphur has decreased by about one-third compared with an approximately two-thirds decrease in sulphur dioxide emissions. Over the same period daily measurements of sulphur dioxide and particulate sulphate concentrations at a small number of sites showed a statistically significant decrease at all but the most remote locations. The comparative magnitude of the changes in UK emissions and wet deposition suggests some change in the partitioning between wet and dry sulphur deposition. Trajectory analyses of precipitation events show that background sulphate deposition has not changed significantly in northwestern coastal regions, where it accounts for approximately half of the total. Wet nitrate deposition has decreased at a slower rate and increased slightly in some remote western areas. Sector analyses suggest that this increase is linked with increased deposition during easterly airflow while particulate sulphate concentrations in these airmasses have decreased. Nitrate now accounts for at least one-third of the acidifying effect of the combined wet deposition of non-sea-salt sulphate and nitrate over all parts of the United Kingdom.