Elemental Composition and Air Trajectories of African Dust Transported in Northern Italy

Abstract

Transport of dust of African origin over Northern Italy and the Alps is a well known phenomenon rather frequent in spring and summer months. In March 1991 at Stelvio National Park, during a field campaign aimed at assessing the concentration and composition of aerosol particles in the Alpine region, two episodes of African dust transport were pointed out.

In these occasions the composition of particulate matter changed drastically with a significant increase of the concentration of typical crustal elements such as Al, Si, K, Ca, Ti, Fe, Mn, while the concentrations of elements such as S, Zn, Pb, Br, typically associated with industrial and vehicular traffic sources both of local and regional origin, remain constant or decrease.

Titanium results a good tracer-element, the Ti/Ca, Ti/Fe and Al/Si concentration ratios are also good signatures of these events. Backward air mass trajectories, calculated on the basis of wind field provided by the European Centre for Medium Range Weather Forecast and of “3D Trajectory” computer package, indicate a direct transport from Africa. The analysis of the meteorological situation indicates that the thermodynamic instability associated with a cold front passing over the Sahara, coupled with upper strong southern winds is a decisive situation favourable to transport. The complete elemental characterization of a time series of particulate samplings supplemented by air mass trajectory reconstruction proved to be an effective method for the identification of dust transport phenomena.