Deposition of atmospheric pollutants to the LOIS area

UK national deposition models were used to estimate deposition of a range of pollutant compounds to the river cotchments in eastern Britain as part of the LOIS programme. Nitrogen dioxide and ammonia concentrations in air were monitored for two catchments, the Tweed and the Ouse. The nitrogen dioxide concentrations in the Ouse were lower than expected from the UK concentration maps, but the ammonia concentrations supported the UK mapped values in both catchments. The uncertainties in the deposition estimates are discussed and the estimates are compared with other available data. The uncertainty in estimated deposition of oxidised nitrogen was of concern. Comparison with river chemical load data suggested that modelled atmospheric inputs accounted for 20% of the sulphate river load and probably less than 30% of the chloride river load. In the Humber area there was evidence of substantial, but unquantified, deposition of hydrogen chloride gas. It appears that the atmosphere is not a major source of calcium, magnesium or the heavy metals. The complexity of vegetation and soil processes with nitrogen compounds makes interpretation difficult. The results, however, suggest a substantial non-atmospheric input of sulphate and chloride in all catchments in the LOIS area.