An outstanding mineral dust transport event from Aralkum desert to northern Italy

This work presents the analysis of an exceptional event of mineral dust transport which impacted northern Italy at the end of March 2020. The event originated over the Aralkum region, a desert area formed recently owing to the almost complete drying of the Aral Lake (Jin et al., 2017). This event presents multiple unprecedented and interesting characteristics: first of all, the eastern origin and the transport of this dust plume over several kilometres is quite anomalous for mid-latitudes where the dominant circulation is characterized by a westerly component; secondly, the event occurred during the lockdown period imposed to contrast the spread of the SARS-COV2 pandemic in Italy, with relevant decrease of particulate emissions over the receptor area. The event was observed through aerosol number concentrations retrieved by optical particle counters (OPCs) located at three different sites in north-eastern Italy, namely Bologna (44°29’58’’N, 11°21’14’’E, 62 m a.s.l.), Trieste (45°37’29’’N, 13°46’46’’E, 30 m a.s.l.), and the WMO-GAW (World Meteorological Organization-Global Atmosphere Watch) station located on top of Mt. Cimone (MTC; 44°11’37’’N, 10°42’02’’E, 2165 m a.s.l.). The analysis of aerosol number concentration at the three sites (Fig. 1) shows how the event impacted firstly on 27th March the city of Trieste and arrived in Bologna and MTC the day after. The analysis of the vertical distribution of aerosol concentration suggests how mineral dust impacted differently at the higher height site of MTC and was transported at relatively low atmospheric layers. This observation is confirmed by the visual investigation of lidar-ceilometer data collected in Milan and by analyzing back-trajectories calculated for the period at the three receptor sites. The thorough synoptic analysis of the period suggests that the intense easterly winds were originated by a low pressure system to the South of the Aralkum desert, as opposed to the high pressure system observed to the North of the desert; moreover, the analysis of tropopause pressure and specific humidity values over the area between 24th and 27th March 2020 suggest that the intense vertical motions suspending the dust plume over the desert were favored by a tropopause folding event.