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

The deposition of the base cations calcium, magnesium and potassium from th e atmosphere needs to be quantified in the calculation of the total deposit ed acidity in the critical loads approach Of these base cations, calcium ha s 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 emission s for Europe was presented by Lee and Pacyna (1998) who estimated a total E uropean emission of 0.7-0.8 Mt yr(-1). However, it is thought that wind blo wn dust from soils contributes a substantial fraction to the deposition of calcium. In this work, the source strength of calcium from arid regions wit hin the EMEP modelling domain was estimated using the global mineral dust e mission 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 hit yr (-1). A long-range transport model, TRACK, was used to calculate the wet an d dry deposition of calcium arising from these industrial and natural sourc es to the UK which resulted in a total deposition of 29-30 kt yr(-1). Of th is annual deposition, 0.6-0.7 kt arises from cement manufacturing, 0.02-0.0 3 kt from iron and steel manufacturing 0.8-0.83 kt from a large point sourc e power generation, and 28 kt from power generation from a small boiler pla nt. The natural emissions of calcium from arid regions result in a depositi on of calcium to the UK of 0.5 kt yr(-1). The measured wet deposition of ca lcium 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 ord er of 70 kt yr(-1), which is suggested to arise from wind-blown dust from a gricultural land in the UK and mainland Europe. The estimated emissions, an d thus modelled deposition are rather uncertain, such that estimating depos ition of calcium attributable to agricultural soil emissions by differencin g has a large uncertainty. However, this is the first such study of its kin d for Europe and represents a first step towards understanding the sources of calcium and their contribution to mitigating deposited acidity from acid ifying pollutants such as sulphur dioxide, nitrogen oxides and ammonia. (C) 1999 Elsevier Science Ltd. All rights reserved.