Characterization of atmospheric aerosol absorption properties at different sites in the Po Valley during ACTRIS-2 experiment.

Aerosol optical properties are one of the major sources of uncertainty in radiative forcing and climate models. During a field campaign performed within ACTRIS-2, optical properties and concentration of absorbing particles have been measured with optical and thermo-optical techniques, at three sites in the Po Valley: Bologna, Monte Cimone and San Pietro Capofiume. The attenuation values, obtained from two Aethalometers (AE31, AE33) were compared with those of Multi Angle Absorption Photometer (MAAP), to derive the C0 factor, necessary to convert the attenuation coefficients into absorption. Obtained values ranged between 2.8 and 4.3 in accordance with previous observations and the 3.5 recommended by GAW/WMO. Comparing equivalent black carbon (BC) and elemental carbon (EC by thermo-optical technique), we derived the mass absorption cross-section (MAC), closely linked with aging and mixing state of the aerosol population. We found a values of 9.2±1.4 m2/g in San Pietro and 9.4±1.4 m2/g in Bologna, higher than that considered by default in the instrument. Among the three sites, the highest concentrations were found in Bologna, the lowestat the mountain site, while intermediate ones at San Pietro. The diurnal variability is in accordance with the location: the urban and rural sites show a close dependence on anthropogenic emissions, while Monte Cimone shows a concentration enhancement during the afternoon, when the boundary layer is higher and the valley breezes can favor the transport of pollution. Low values of single scattering albedo (SSA) in the low sites reflects the influence of freshly generated particles from combustion sources, while the higher values at Monte Cimone indicate that aerosols reaching the mountain site are mixed with non absorbing components and more aged. A comparison between the Absorption and Scattering Angstrom Exponent reveals that the main absorbing components are BC and BrC for all the three sites, with few episodes of dust transport.