An outstanding Saharan dust event at Mt. Cimone (2165 m a.s.l., Italy) in March 2004

Abstract

A severe PM 10 episode was observed at the high elevation observatory of Mt. Cimone (2165m a.s.l.) in the period of 13th-15th March 2004; during the event PM 10 reached the maximum concentration (80μgm -3 against an average of 8.8±8.0μgm -3) between 1998 and 2011. Meteo-synoptical analysis allowed to ascribe this event to a long lasting and highly coherent Saharan dust outbreak, starting at the beginning of March. The peculiar synoptic configuration causing this massive transport of dust was characterized by a steep gradient between an upper level trough extending to low latitudes with a minimum centred over the North-Western Algerian coast and a Saharan high extending all over the Mediterranean Sea with an elongated north-eastward tongue, whose synergic effect led to a peculiar funnel-shaped dust plume. During the period Mt. Cimone was located exactly along its main axis. The event was first analysed in association with simultaneous more or less conventional compositional parameters such as 7 Be, 210 Pb, and ozone. Subsequently, it was characterized in details both in terms of time and space evolution. The former aspect was investigated using number densities of fine and coarse particles obtained through an Optical Particle Counter which allowed to follow the event evolution at the sub-daily time scale while PM 10 membrane gravimetric analysis was limited by the 48-h sampling schedule suggesting the value of 80μgm -3 recorded is even potentially smoothed down by sampling duration. Besides precise timing, optical counting enabled to detect the inception and development of the event through a steep and simultaneous increase of both coarse and fine particle number densities. Although the former increase was much more relevant, the latter occurrence is much less frequently documented for Saharan Dust events: a clear increase of particles in all the diameter ranges from 0.3μm (lower limit of an OPC) up to 5.0μm was observed during the event. The spatial extension of the event was also examined by means of the analysis of the AERONET ground-based sun photometer data from the Venice station for the event. Results confirmed a relevant increase of coarse particles over a distance of more than 150km. Interestingly AERONET data indicates a more significant variation in the scattering properties of the aerosol rather than in the absorbing ones in connection with the arrival of the Saharan dust, an observation that within the intrinsic limitations of inverse methods to derive aerosol's optical properties is in agreement with some previous observations showing that dust in the Saharan desert region is much less absorbing than previously measured.