Aerosol optical depth comparison between GAW-PFR and AERONET-Cimel radiometers from long-term (2005-2015) 1 min synchronous measurements

A comprehensive comparison of more than 70 000 synchronous 1 min aerosol optical depth (AOD) data from three Global Atmosphere Watch precision-filter radiometers (GAW-PFR), traceable to the World AOD reference, and 15 Aerosol Robotic Network Cimel radiometers (AERONET-Cimel), calibrated individually with the Langley plot technique, was performed for four common or "near" wavelengths, 380, 440, 500 and 870 nm, in the period 2005-2015. The goal of this study is to assess whether, despite the marked technical differences between both networks (AERONET, GAW-PFR) and the number of instruments used, their long-term AOD data are comparable and consistent. The percentage of data meeting the World Meteorological Organization (WMO) traceability requirements (95% of the AOD differences of an instrument compared to the WMO standards lie within specific limits) is > 92% at 380 nm, > 95% at 440 nm and 500 nm, and 98% at 870 nm, with the results being quite similar for both AERONET version 2 (V2) and version 3 (V3). For the data outside these limits, the contribution of calibration and differences in the calculation of the optical depth contribution due to Rayleigh scattering and O-3 and NO2 absorption have a negligible impact. For AOD > 0.1, a small but non-negligible percentage (similar to 1.9 %) of the AOD data outside the WMO limits at 380 nm can be partly assigned to the impact of dust aerosol forward scattering on the AOD calculation due to the different field of view of the instruments. Due to this effect the GAW-PFR provides AOD values, which are similar to 3% lower at 380 nm and similar to 2% lower at 500 nm compared with AERONET-Cimel. The comparison of the Angstrom exponent (AE) shows that under non-pristine conditions (AOD > 0.03 and AE < 1) the AE differences remain < 0.1. This long-term comparison shows an excellent traceability of AERONET-Cimel AOD with the World AOD reference at 440, 500 and 870 nm channels and a fairly good agreement at 380 nm, although AOD should be improved in the UV range.