Daily measurements of sulfate, nitrate and chloride in PM10 have been made at three geographically separated UK sites over a three year period. Chloride shows a clear seasonal pattern with highest concentrations in winter, whilst sulfate and nitrate both show highest concentrations in the spring, apparently related to weather patterns. Spatial variability of both sulfate and nitrate is low in comparison to temporal variations, with high correlations of both species between all three sites, London ( North Kensington), Harwell and Belfast, despite a geographic separation of 510 km. Both SO42-/SO2 and NO3-/NOx ratios are considerably higher in summer than winter, reflecting a greater oxidising capacity of the atmosphere. SO42-/NO3- ratios are higher in summer than winter, suggesting that aqueous phase oxidation of SO2, expected to be most important in the winter months is not appreciably influencing production of sulfate aerosol, although greater dissociation of ammonium nitrate in summer may also play a role. Regression of concentrations at London, North Kensington with those from the proximate rural site of Harwell is interpreted as showing a similar effect of regional transport at the two sites and a small influence of local formation in the urban atmosphere or primary emissions, averaging 0.46 mu g m(-3) of nitrate and 0.22 mu g m(-3) of sulfate.