The influence of the dry deposition on the lead concentration in aerosols dependent on the particle size over the Baltic Sea

Abstract

In recent studies concerning modelling aerosol transport and deposition over Europe widely emission estimates of heavy metals, associated with particles, integrated over the size distribution have been used. This is a crucial simplification of the real conditions and reflects an important weakness of the emission inventories. Also the dry deposition parameterization often performed by a single value of the dry deposition velocity representative for mean particle-size distributions can be expected to be rather uncertain. The results of many investigations which provide estimates of atmospheric depositions are based on such assumptions. However compared to the emissions about the dry deposition process there is a much better knowledge which can be used for modelling. The idea in this study was to combine the best available information about particle size dependent emissions and a dry deposition scheme applicable for different particle sizes. The motivation was the evaluation of impactor measurements of particle size segregated lead concentrations in aerosols at the coast of the Baltic Sea. In order to determine the dry deposition over the Baltic Sea the deposition module of the ADOM model was adapted for different sizes of lead containing particles. The model referred to as ADOM (Acid Deposition and Oxidant Model) originally has been developed under the sponsorship of Environment Canada, Atmospheric Environment Service(AES), the Ontario Ministry of Environment and Energy (OMEE), the German Umweltbundesamt and the Electric Power Research Institute (EPRI) for simulating long-range transport and deposition of acidic pollutants in Europe and North America (Venkatram et al., 1988). Presently it is modifyed for heavy metals (Krüger and Petersen, 1996). The dry deposition module of ADOM is formulated by means of an electrical resistance analog. The basis for the electrical analog is the Ohm's law which relates voltage to resistance and current. In the analogous relation for the dry deposition the concentration is analogous to voltage the flux is analogous to current and the inverse of the deposition velocity is analogous to total resistance. For the estimation of the flux the pollutant transfer is thought to occur in the constant flux layer and the deposition layer. In the dry deposition scheme small particles (diameter < 0.05μm) within the deposition layer are transported much in the same way like gases. The Brownian diffusion is considered as the primary transport mechanism. The deposition of larger particles is parameterized by the settling velocity and the stopping time, which are dependent on the Stokes number and collection efficiencies. The roughness lengths of 12 land use categories identical with those of the PHOXA land use data base for Europe are taken into account.The ADOM dry deposition module was applied to air mass movements over the Baltic Sea. Typical trajectories for the receptor point Kap Arkona located in the North East of Germany were investigated. As initial concentrations emission data for lead which are documented in van den Hout, K.D. (1994) were used for the calculations. This inventory contains a general separation of the aerosol size distribution into the 5 diameter intervals (<0.95μm, 0.95–4μm, 4–10μm, 10–20μm, >20μm). The meteorological data along the trajectories were taken from the HIRLAM weather forecast model (Källen, 1996). The dry deposition was calculated along air mass back trajectories for the receptor point Kap Arkona. The results indicate that there is a pronounced decrease of the initial concentration during the first hours of transport. Consequently after some hour simulation time the lead is only concentrated in aerosols of diameters lower than 4μm (figure 1). These findings are quite different compared to the lead concentrations found in size distributions measured at Kap Arkona. The present interpretation of the investigations is that interactions of aerosol particles of different sizes seem to have a strong influence. Possibly particles from sea spray play an important role during the transport over the sea.▪