Validation of AERONET estimates of atmospheric solar fluxes and aerosol radiative forcing by ground-based broadband measurements

The AErosol RObotic NETwork (AERONET) estimates of instantaneous solar broadband fluxes (F) at surface have been validated through comparison with ground-based measurements of broadband fluxes at Mauna Loa Observatory (MLO) and by the Baseline Surface Radiation (BSRN) and the Solar Radiation Networks (SolRad-Net) during the period 1999-2005 and 1999-2006, respectively. The uncertainties in the calculated aerosol radiative forcing (Delta F) and radiative forcing efficiency (Delta F-eff) at the bottom of the atmosphere were also assessed. The stations have been selected attempting to cover different aerosols influences and hence radiative properties: urban-industrial, biomass burning, mineral dust, background continental, maritime aerosols and free troposphere. The AERONET solar downward fluxes at surface agree with ground-based measurements in all situations, with a correlation higher than 99% whereas the relation of observed to modeled fluxes ranges from 0.98 to 1.02. Globally an overestimation of 9 +/- 12 Wm(-2) of solar measurements was found, whereas for MLO (clear atmosphere) the differences decrease noticeably up to 2 +/- 10 Wm(-2). The highest dispersion between AERONET estimates and measurements was observed in locations dominated by mineral dust and mixed aerosols types. In these locations, the F and Delta F uncertainties have shown a modest increase of the differences for high aerosol load, contrary to Delta F-eff which are strongly affected by low aerosol load. Overall the discrepancies clustered within 9 +/- 12 Wm(-2) for Delta F and 28 +/- 30 Wm(-2) per unit of aerosol optical depth, tau, at 0.55 mu m for Delta F-eff, where the latter is given for tau(0.44 mu m) >= 0.4. The error distributions have not shown any significant tendency with other aerosol radiative properties as well as size and shape particles.