Aerosol Optical Depth comparison between GAW-PFR and AERONET-Cimel radiometers from long term (2005–2015) 1-minute synchronous measurements [Discussion paper]

Abstract

A comprehensive comparison of more than 70000 synchronous 1-minute aerosol optical depth (AOD) data from three Global Atmosphere Watch-Precision Filter Radiometer (GAW-PFR) and 15 Aerosol Robotic Network-Cimel (AERONET-Cimel) radiometers was performed for the four nearby wavelengths (380, 440, 500 and 870nm) in the period 2005–2015. The goal of this study is to assess whether, despite the marked differences between both networks and the number of instruments used, their long term AOD data are comparable and consistent. AOD traceability established by the World Meteorological Organization (WMO) consists in determining the percentage of synchronous data within specific limits. If, at least, 95% of the AOD differences of an instrument compared to the WMO standards lie within these limits, both data populations are considered equivalent. The percentage of traceable data is 92.7% (380nm), 95.7% (440nm), 95.8% (500nm) and 98.0% (870nm). When small misalignments in GAW-PFR sun-pointing were fixed (period 2010–2015), the percentage of traceable data increased. The contribution of calibration related aspects to comparison outside the 95% traceability limits is insignificant in all channels, except in 380nm. The simultaneous failure of both cloud screening algorithms might occur only under the presence of cirrus, or altostratus clouds on the top of a dust-laden Saharan air layer. Differences in the calculation of the optical depth contribution due to Rayleigh scattering, and O3 and NO2 absorption have a negligible impact. For AOD >0.1, a non-negligible percentage (≈1.9%) of the AOD data outside the 95% traceability limits at 380nm can be partly assigned to the different field of view of the instruments. The comparison of the Angström exponent (AE) shows that under non-pristine conditions (AOD >0.03 and AE <1) the AE differences remain <0.1. The excellent traceability in this study has been obtained using well calibrated Master instruments.