THE IMPORTANCE OF OROGRAPHY IN SPATIAL PATTERNS OF RAINFALL ACIDITY IN SCOTLAND

The geographical patterns in rainfall acidity in Scotland show a pronounced gradient from the least polluted areas of the north-west at 5-15-mu-eq H+ l-1 to the south-east where annual rainfall-weighted mean acidity is 45-55-mu-eq H+ l-1. The pattern in mean acidity and wet deposition results partly from depletion of the pollutants with distance from the major source areas of Europe, but also from meteorological and orographic effects. Analysis of rainfall data shows that about half of the north-west-south-east gradient in mean acidity results from orographic effects. These enhance the impacts of relatively unpolluted rain in the west and shelter east coast regions from the same, while relatively polluted rain from easterly trajectories is enhanced on high ground in the east and west coast districts are sheltered. The simple analysis provided quantifies the importance of orographic effects on wet deposition in Scotland and implies that for satisfactory estimates of regional wet deposition from long-range transport models, orographic effects must be incorporated.