Aerosol dry deposition measured with eddy-covariance technique at Wasa and Aboa, Dronning Maud Land, Antarctica Anna GRO « NLUND, 1 Douglas NILSSON, 1 Ismo K. KOPONEN, 2 Aki VIRKKULA, 3 Margareta E. HANSSON 4 1 Department of Meteorology, Stockholm University, S-106 91 Stockholm, Sweden 2 Department of Physics, University of Helsinki, FIN-00014 Helsinki, Finland 3 Finnish Meteorological Institute, FIN-00880 Helsinki, Finland 4 Department of Physical Geography and Quaternary Geology, Stockholm University, S-106 91 Stockholm, Sweden E-mail: margareta.hansson@natgeo.su.se ABSTRACT . Interpretation of ice-core records in terms of changes in atmospheric con- centrations requires understanding of the various parameters within air^snow transfer func- tions. The dry-deposition velocity is one of these parameters, dependent on local meteorological conditions and thereby also affected by climate changes.We have determined aerosol dry-deposition velocities by measurements of aerosol particle-number concentration and the vertical wind component with an eddy-covariance system close to the Swedish and Finnish research stations Wasa and Aboa in Dronning Maud Land, Antarctica. Measure- ments were performed over a smooth, snow-covered area and over moderately rough, rocky ground during 4 and 19 days, respectively, in January 2000. The median dry-deposition velocity determined 5.25 m above the surface was 0.33 and 0.80 cm s ^1 , respectively. The large difference between the two sites was mainly due to the stratification of the surface boundary layer, the surface albedo and the surface roughness height. The dry-deposition number fluxes were dominated by the particle-size modes defined as ultrafine and Aitken, with mean diam- eters around 14 and 42 nm, respectively. A larger dry-deposition velocity, owing to stronger Brownian diffusion, for the smaller ultrafine mode was verified by the measurements. INTRODUCTION Wet, dry and fog deposition are the mechanisms by which particles and gases are removed from the atmosphere (e.g. Davidson and others,1996). If the deposition takes place on the cold ice sheets of Antarctica and Greenland, the chemical constituents are embedded in the glacial ice when more snow accumulates on top of the previous layer, and records of deposition over time are preserved. Many deposition records have been extracted from polar ice sheets and have contribu- ted much to our present understanding of changes in atmos- pheric composition in the past (e.g. Barnola and others,1987; Legrand and others, 1988; Hansson, 1994; Steffensen, 1997; Fuhrer and others, 1999). However, interpretation of ice-core deposition records spanning large climatic changes requires understanding of the various parameters within the air^snow transfer functions.The dry-deposition velocity is one of these parameters and it is dependent on local meteorological con- ditions and thereby also affected by climate changes.The dry- deposition velocity is strongly size-dependent (Davidson and others,1996), with a clear minimum for particles about 0.1 m m in diameter (i.e. accumulation mode). Larger particles have higher dry-deposition velocities due to sedimentation and impaction processes, and so have the smaller particles due to Brownian diffusion. Particles with higher dry-deposition velocity will therefore be over-represented in the deposition records in the ice compared to the atmospheric concentra- tions (Unnerstad and Hansson,2001). Previously reported data on dry-depositionvelocities over snow surfaces are sparse and highly varying between differ- ent species (e.g. Ibrahim and others,1983; Cadle and others, 1985; Davidson and others, 1985; Hillamo and others, 1993; Bergin and others 1995). Impurities in ice cores are usually measured in the soluble phase (as ion concentrations) or in the insoluble phase (as elemental concentrations, or as total dust mass or number concentrations).These two fractions are combined in the atmosphere, either internally or externally mixed, but most probably interacting and determining the aerosol particle-size distribution together. The aim of this study was to characterize dry deposition of aerosol particles to surfaces in Antarctica in relation to surface roughness, turbulent flow, boundary-layer structure and the aerosol number and size distribution. An eddy-co- variance (EC) method was used which offered a way to meas- ure the turbulent transport (vertical flux) of a compound by correlating the fast fluctuations of its number concentrations with the vertical wind speed. This method has previously been successfully applied with a similar instrumentation in the Arctic over sea and pack ice (Nilsson and Rannik,2001) and over a Scots pine forest in southern Finland (Buzorius and others, 1998, 2000). MEASUREMENTS AND METHODS Sites The Swedish station Wasa and the Finnish station Aboa in Dronning Maud Land (DML), Antarctica, are situated on the nunatak Basen (73 03’ S, 13 25’ W). Basen lies in the northeastern part of Vestfjella, which is a 135 km long range Annals of Glaciology 35 2002 # International Glaciological Society 355