Four years of aerosol absorption coefficient measurements at Mt. Cimone: study of processes affecting their variability

The Mediterranean basin is globally considered as a hot-spot region for climate change and air-quality issues. The variability of trace gases and aerosols in this region is driven by the transport of pollutant air-masses from Europe and other continents, especially during summer, when the intense vertical transport is favoured by meteorological conditions.Mt. Cimone Global GAW Station (CMN, 44°11' N, 10°42' E), at 2165 m a.s.l. in the Northern Apennines, with a 360° free horizon, is considered representative of the background conditions of the Southern Europe/Mediterranean basin. It is particularly suitable to detect and characterize several air-masses originated in different areas (e.g. North Africa, Central Europe and Eastern Europe). In order to better investigate the variability of aerosols and their optical properties in the Mediterranean basin, aerosol absorption measurements by means of a 7-wavelength Aethalometer (AE-30, Magee Scientific) are continuously carried out at CMN since 2011. Simultaneously, also an Optical Particle Counter (OPC, GRIMM 1108) and a Multi Angle Absorption Photometer (MAAP 5012, Thermo Scientific) have been used. The calculation of the absorption coefficients at different wavelengths, as well as the value of the Ångström exponent of the absorption coefficient (?, Fig. 1) has been performed for the June 2011 - December 2014 period, for evaluating their seasonal and diurnal variations, and to attribute their variability. In fact, the different ? values allowed detecting pollution events of distinct origin (e.g. Saharan dust events, firewood burning), over the whole sampling period (see e.g. Sandradewi et al., 2008; Collaud Coen et al., 2004). To better explain the ? variability and to assess the processes influencing these variations, we selected specific events affected by pollution episodes, open fire emissions (see methodology in Putero et al., 2014, an example is shown in Fig. 2) and Saharan dust transport (see methodology in Duchi et al., 2014). In the first two cases, the pollution events have been selected according to a statistical exceedance of black carbon (BC) values from MAAP in respect to the seasonal values; then we coupled satellite observations (MODIS fire product, "filtered" by the MODIS land cover) and deterministic modelling system (HYSPLIT back-trajectories) to identify which ones have been likely influenced by open fires emissions. As a further step, for specifically evaluating the contribution from different areas, four regions were arbitrarily picked out (shown in Fig. 2), for evaluating their contribution in terms of BC and aerosol absorption coefficient. To identify the events influenced by Saharan dust transport, the same statistical method has been applied to the number of coarse particles, and then coupled with the presence of air-masses that originated (or passed) over North Africa, as deduced by the back-trajectories from the FLEXTRA model.