Columnar aerosol optical properties in the Po Valley, Italy, from MFRSR data

Abstract

The Po Valley is one of the most polluted areas in Europe. In order to provide a further contribution to the characterization of aerosol optical properties in this area, a more than 2 yr set of Multi Filter Rotating Shadow‐band Radiometer (MFRSR) measurements performed from September 2006 to May 2009 at Bologna, in northern Italy, were analyzed, paying particular attention to calibration, cloud screening, and uncertainty assessment of the measurements. The results show that daily mean aerosol optical depth (AOD) at 500 nm varies between 0.02 and 0.78, with an overall average value of 0.20 ± 0.11, and that daily mean values of Ångström's exponent α range between 0.21 and 1.86, with an overall average value of 1.23 ± 0.30. The annual variations of these parameters are evidence of the large variability of aerosol content and size distribution shape parameters throughout the year. Interannual seasonal statistics show a cyclic behavior of AOD, with monthly median values increasing from winter to autumn. In late spring and summer, several episodes of Saharan dust transport from North Africa were associated with high values of AOD and low values of α, contributing to an appreciable modification of the seasonal mean values of these two optical parameters. The high temporal resolution of daily field measurements allowed the definition of the average features of daily cycles for both parameters AOD and α, on a seasonal basis. Comparison of MFRSR results with those obtained from three AERONET stations in the Po Valley shows good agreement for both optical parameters. Absolute values of biases and root‐mean‐square differences between the MFRSR results and those of the AERONET stations were found to be lower than 0.05 and 0.20, respectively, for AOD at 500 nm and lower than 0.30 and 0.44 for α, indicating that aerosol optical characteristics are quite homogeneous over the whole region.