Aerosol direct radiative effect in the Po Valley region derived from AERONET measurements

Abstract. The aerosol direct radiative effect (ADRE) affecting the Po Valley and the adjacent North Adriatic Sea is studied using 10-year series of measurements collected at two AERONET sites located in the western part of the Valley (Ispra), and on a platform (AAOT) offshore Venice. This region is characterized by a high, mostly continental, aerosol load with comparable average aerosol optical thickness τ_a at both locations (0.21 at 500 nm) and more absorbing aerosols at Ispra. A dynamic aerosol model accounting for the changes in scattering phase function with τ_a is used for radiative transfer calculations, together with boundary conditions representative of terrestrial and marine surfaces. A sensitivity analysis allows the construction of an error budget for the daily ADRE estimates, found to be of the order of 20% and mostly due to uncertainties on aerosol single scattering albedo and τ_a. The daily radiative efficiencies, normalized by τ_a at 500 nm, increase from December to June, from −17 to −24 W m⁻² τ_a⁻¹ at top-of-atmosphere (TOA) and −33 to −72 W m⁻² τ_a⁻¹ at surface for the Po Valley, and from −15 to −32 (TOA) and −35 to −65 W m⁻² τ_a⁻¹ (surface) for the AAOT site. The average of log-transformed ADRE for TOA, surface and atmosphere are −5.2, −12.2 and +6.8 W m⁻² for the Po Valley case, and −6.5, −13.0 and +6.5 W m⁻² for the AAOT site but these values can be much higher for individual days. Concurrent clear-sky days give indications on the regional atmospheric heating spatial gradients. Differences between the atmospheric ADRE at the two locations average 6.3 W m⁻² with a gradient positive towards the inner valley in 65% of the cases. This study confirms the importance of duly considering the radiative impact of aerosols on the regional climate.