XMED-dry – a cross-Mediterranean dry deposition measurement network : first results

Abstract

Dry deposition is one of the two major pathways for aerosol particles to be removed from the atmosphere and be transferred into another compartment. Depending on the precipitation pattern and thus, locality, it can dominate then total aerosol flux. Still, measurements of dry deposition with sub-weekly time resolution are rare, as usually the mass collected during this period is too low. For XMed-Dry, a set of seven new dry deposition-only collectors were installed at different locations across the Mediterranean (Huelva, Barcelona, Spain; Île-Rousse, France; Gozo Island, Malta; Lecce, Italy; Athens, Greece; Nicosia, Cyprus) to capture spatial and temporal variability. Sampling was performed on a 3-times-per-week schedule. Particles deposited on a 25 mm pure carbon adhesive protected from wet deposition by a shelter and an active closing mechanism during rain. The carbon adhesive was subject to electron microscopy with energy-dispersive X-ray analysis to obtain size, shape and elemental composition of single particles. Several hundred to several thousand particles were analyzed for each sample. First results show that deposition consists of a highly variable mixture of sea-salt, sulfate, mineral dust, metal oxides and biological material, depending on location, season and meteorological situation. Moreover, different state of ageing of sea-salt was detected by the single particle analysis. Statistical back-trajectory analyses allowed for the discrimination of potential source regions for different compounds. The latter showed that particles of similar chemical composition, but different size can have different origin, e.g., in Gozo large (d>4μm) iron-rich particles (possibly fly-ashes) have probably SE European origin, whereas small iron-rich ones originate from the Saharan desert. Also, for the same receptor location, S-rich sea-salt particles were originating from Italy and SE Europa, while the more pristine ones were produced locally or coming from the Atlantic Ocean.