Modelling base cations in Europe—sources, transport and deposition of calcium

The deposition of the base cations calcium, magnesium and potassium from the atmosphere needs to be quantified in the calculation of the total deposited acidity in the critical loads approach. Of these base cations, calcium has been found to be the most important in terms of mass deposited. However, the sources of calcium to the atmosphere are not well understood. Recently, the first spatially disaggregated inventory of industrial calcium emissions for Europe was presented by Lee and Pacyna (1998) who estimated a total European emission of 0.7-0.8 Mt yr -1. However, it is thought that wind blown dust from soils contributes a substantial fraction to the deposition of calcium. In this work, the source strength of calcium from arid regions within the EMEP modelling domain was estimated using the global mineral dust emission data base of Tegen and Fung (1994) and an estimation of the calcium content of soils. This results in a "natural" calcium emission of 6 Mt yr -1. A long-range transport model, TRACK, was used to calculate the wet and dry deposition of calcium arising from these industrial and natural sources to the UK which resulted in a total deposition of 29-30 kt yr -1. Of this annual deposition, 0.6-0.7 kt arises from cement manufacturing, 0.02-0.03 kt from iron and steel manufacturing, 0.8-0.83 kt from a large point source power generation, and 28 kt from power generation from a small boiler plant. The natural emissions of calcium from arid regions result in a deposition of calcium to the UK of 0.5 kt yr -1. The measured wet deposition of calcium to the UK is 89 kt yr -1 and the estimated dry deposition 14 kt yr -1. The short-fall in the modelled deposition of calcium is thus of the order of 70 kt yr -1, which is suggested to arise from wind-blown dust from agricultural land in the UK and mainland Europe. The estimated emissions, and thus modelled deposition are rather uncertain, such that estimating deposition of calcium attributable to agricultural soil emissions by differencing has a large uncertainty. However, this is the first such study of its kind for Europe and represents a first step towards understanding the sources of calcium and their contribution to mitigating deposited acidity from acidifying pollutants such as sulphur dioxide, nitrogen oxides and ammonia.