Wintertime aerosol dynamics and chemical composition across the mixing layer over basin valleys

Aerosol size distributions and chemical compositions were characterized during extensive balloon soundings over the Terni valley (central Italy). The evolution of aerosol size distributions across the mixing layer, and further up, revealed features similar in many aspects to those observed over Milan (the Po Valley) and attributed to sedimentation and ageing dynamics. Sedimentation led to a lower vertical mixing of coarse particles than of fine ones. This also resulted in a decline in the mean volume of coarse particles across the mixing height (-50.5 ± 15.1% and -47.2 ± 12.4% over Terni and Milan, respectively) accompanied by a reduction in crustal components. Conversely, fine particles were subject to ageing, resulting in an increase in their mean volume above the mixing height (+10.9 ± 4.8% and +4.0 ± 3.1% over Terni and Milan, respectively); this process was accompanied by an increase in secondary aerosol components, and a greater correlation between different aerosol size-classes. These results were obtained over basins of different sizes and geographical location; as such, they corroborate the presence of common forms of behaviour driven by comparable meteorological and orographic conditions, which seem to characterize polluted basin valleys in general