An analysis of spatial and temporal properties of daily sulfate, nitrate and chloride concentrations at UK urban and rural sites.

Daily measurements of sulfate, nitrate and chloride in PM(10) 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 SO/SO(2) and NO/NO(x) ratios are considerably higher in summer than winter, reflecting a greater oxidising capacity of the atmosphere. SO(4)(2-)/NO(3)(-) ratios are higher in summer than winter, suggesting that aqueous phase oxidation of SO(2), 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 microg m(-3) of nitrate and 0.22 microg m(-3) of sulfate.

Particulate sulphate and nitrate in Southern England and Northern Ireland during 2002/3 and its formation in a photochemical trajectory model.

Daily measurements of sulphate and nitrate are reported from Harwell in southern England and Belfast in Northern Ireland for the period 2002/3. When the higher percentiles are compared with the mean concentration, nitrate reveals considerably greater episodicity than either sulphate or PM(10) (measured by TEOM). A photochemical trajectory model using the Master Chemical Mechanism scheme has been used to predict daily concentrations of both nitrate and sulphate aerosol over the period March to August 2002 at the Belfast and Harwell sites. This has been carried out for daily samples using 72, 96 and 120 h air mass back trajectories obtained from both the British Atmospheric Data Centre and the HYSPLIT on-line service. Additionally, model simulations have been conducted for 5 trajectories generated through clustering of the trajectories for individual days. This reveals an under-prediction of the model associated particularly with trajectories originating from the European mainland. In general, the model performs reasonably well in simulating concentrations of both nitrate and sulphate, which is surprising given that the model does not account for processes requiring the presence of liquid water. This suggests that aqueous phase oxidation processes may not make a major contribution to airborne sulphate concentrations in the U.K. in the spring and summer months. It appears that inclusion of explicit ammonium nitrate formation chemistry may be essential to reliable prediction of episodic nitrate peaks.