Aerosol stratification, optical properties and radiative forcing in Venice (Italy) during ADRIEX

This study presents ground-based measurements of aerosol optical properties and solar radiation performed in Venice (Italy) within the Aerosol Direct Radiative Impact Experiment (ADRIEX). The measurement site is of particular interest due to its location at the eastern border of the polluted Po Valley and constituting the northern edge of the Mediterranean basin. Lidar observations showed a stratified structure of aerosols in the lower troposphere (0-4 km), with more than 40% of the aerosol optical depth due to aerosol above the local planetary boundary layer (about I kin thick). Different aerosol vertical distributions and optical and microphysical properties were observed to develop under the influence of the various advection regimes encountered during ADRIEX. Sunphotometer measurements indicated fine particles (radius <1 mu m) as dominating the light extinction, confirming the main aerosol source in the region as anthropogenic. Photometric data also revealed a relatively low aerosol absorption (i.e. a single-scattering albedo >0.92 at 440 nm), this being comparable to those found in other urban/industrial sites in Europe and USA, and slightly lower than those characterizing central America and Asia. Overall, the aerosol optical properties measured at Venice during ADRIEX translate into a summertime mean daily aerosol forcing between -10 and -14 W m(-2) at the surface and between -4 and -8 W m(-2) at the top of the atmosphere. Copyright (C) 2007 Royal Meteorological Society.