HEAVILY-CONTAMINATED SNOWFALLS IN THE REMOTE SCOTTISH HIGHLANDS - A CONSEQUENCE OF REGIONAL-SCALE MIXING AND TRANSPORT

Heavily-contaminated (black) snowfalls frequently occur in the Cairngorm Mountains (Scotland), in spite of their relative remoteness from major source regions. The frequency of such pronounced events has been characterized throughout one complete snow season. Daily samples of snow were collected at 1100 m under sub-arctic conditions, and the reported results from the analyses of major ions and a large number of trace elements (dissolved and particulate phases) represent a novel precipitation chemistry data set. This paper focuses on the major ion, black carbon and particulate concentrations, but formic acid concentrations are also reported. The heavily-contaminated snowfalls are associated with back-trajectories which originate in the east (Eastern Europe, European U.S.S.R. and the Baltic) and, in spite of providing a small proportion (30%) of the total snowfall, produce the bulk of the pollutant wet deposition (e.g. 70% of non-marine SO4(2-); 80% of black carbon). The covariances of the constituent concentrations indicate that airborne pollution is relatively aged and well-mixed before removal by the snowflakes. Back-trajectory, synoptic and upper-air analyses have been combined to produce a qualitative model of pollutant transport from source to receptor region. Once air is advected out of a (typically) long-lived anticyclone over Eastern Europe (which produces the regional-scale mixing), transport westwards of the 'regional plume' is usually associated with an elevated stable layer which can effectively decouple the plume from the surface and inhibit dispersion. Over the North Sea-NE Scotland region, horizontal convergence, convection and topographic uplift can all contribute to precipitation formation and pollutant wet deposition. Although there are indications that the NE Scottish Highlands are particularly susceptible to these 'black' snowfalls, comparison with data from monitoring stations within 35 km of the study site, and 600-800 m lower elevation, does not indicate enhancement of ion concentration in snowfall with increasing altitude.