Seasonal evolution of the tropospheric aerosol vertical profile in the central Mediterranean and role of desert dust

The seasonal evolution of the aerosol vertical distribution in the Central Mediterranean is studied using measurements made in the period 1999-2008 at Lampedusa with an aerosol Lidar and a multi filter rotating shadowband radiometer (MFRSR). Measurements show that the aerosol vertical distribution is largely influenced by Saharan dust, which produces a strong annual cycle both in aerosol vertical extension and optical depth. Dust layers are present in the profile in 38% of the cases throughout the year, and in 57% in summer. The dust top altitude peaks in late spring, up to 9 km. The monthly average optical depth at 500 nm for dust cases shows a main peak in July (0.38), and values exceeding 0.2 throughout March-September. Conversely, non-dust cases show a very limited seasonality, both in vertical distribution and aerosol optical depth. The monthly average optical depth for non-dust cases is smaller than 0.17 throughout the year. During winter, the vertical distribution and optical depth are very similar for both dust and non-dust cases. The seasonal average extinction coefficient profiles for dust and non-dust cases show remarkable differences in spring and summer, when values of the extinction coefficient exceed 0.5 x 10(-4) m(-1) throughout the altitude range 0-4.5 km for dust cases, and 0-1 km altitude for non-dust cases, respectively. Estimates of the Lidar Ratio are derived by combining Lidar and MFRSR measurements. The average Lidar Ratio at 532 nm is about 30 sr.