Aerosol measurements at L'Aquila EARLINET station in central Italy: Impact of local sources and large scale transport resolved by LIDAR

Surface measurements of multi-channel aerosol mass concentration are analyzed together with LIDAR observations at L'Aquila, a central Italy site part of the EARLINET network (http://www.earlinet.org/), with the main purpose of discriminating aerosol particles originated from in-situ sources and those transported from remote sites, and their effects on local aerosol load and on the aerosol optical depth. Four major episodes of both Saharan desert and forest fire aerosol transport were observed during spring–summer months of 2007. The analysis of these events shows that at the ground surface the aerosol mass concentration increase due to desert dust particles is about 160% of the PMcoarse, whereas forest fires increase the PM1 by about 150%, with respect to typical reference unperturbed conditions during the same time period. Calculation of the aerosol optical depth (AOD) from the LIDAR retrieved aerosol extinction at 351 nm shows that the corresponding increases in AOD are 95% for Saharan dust and 220% in case of forest fires. These results show that in a site impacted by aerosol transport from the desert and frequent forest fires, the first has the bigger effect in terms of local aerosol load in the coarse mode, whereas the latter impacts more the PM1 and the UV aerosol extinction and optical depth. A well tested radiative transfer model (TUV, Madronich and Floke, 1998), extended to the solar near infrared spectrum, has been used to calculate the top-of-atmosphere radiative change due to these transported aerosols: the calculated change is of the order of 0.5–1 W/m2 for forest fire events end 1–2 W/m2 for Saharan dust. The larger impact of desert aerosols is due to their much larger effective radius with respect to forest fire aerosols, whose scattering efficiency rapidly declines for solar wavelengths in the visible and near infrared range.