Abstract
Abstract. 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 O3 and NO2 absorption have a negligible impact. For AOD >0.1, a small but non-negligible percentage (∼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 ∼3 % lower at 380 nm and ∼2 % lower at 500 nm compared with AERONET-Cimel. The comparison of the Ångström 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.
Highlights
In recent decades there has been a growing interest in the role played by atmospheric aerosols in the radiation budget and the Earth’s hydrological cycle, mainly through their physical and optical properties (IPCC, 2013)
Concerning mean bias (MB) and root-mean-square error (RMSE) associated with aerosol optical depth (AOD) differences, our results show quite similar skill scores to those found at Mauna Loa, USA, for AOD500 nm (Kim et al, 2008), the number of data pairs used at Izaña Observatory (IZO) (∼ 71 000) is much higher and the AOD
While Global Atmosphere Watch precision-filter radiometers (GAW-PFR) is the World Meteorological Organization (WMO)-defined global AOD reference, being directly linked to WMO-Commission for Instruments and Methods of Observation (CIMO), and was designed to detect long-term AOD trends, AERONETCimel is the densest AOD measurement network globally, and the network most frequently used for aerosol characterization and for model and satellite observation evaluation
Summary
In recent decades there has been a growing interest in the role played by atmospheric aerosols in the radiation budget and the Earth’s hydrological cycle, mainly through their physical and optical properties (IPCC, 2013). The most comprehensive and important parameter that accounts for the optical activity of aerosols in the atmospheric column is the aerosol optical depth (AOD) (WMO, 2003, 2005). Two global ground-based radiometer networks provide aerosol optical properties of the atmospheric column using centralized data processing procedures based on their respective standard criteria and centralized protocols for calibration and quality control, linking all network instruments. These are GAW-PFR (Global Atmosphere Watch precision-filter radiometer; http: //www.pmodwrc.ch/worcc/; last access: 5 September 2018) and AERONET-Cimel (AErosol RObotic NETwork Cimel Electronique radiometer; https://aeronet.gsfc.nasa.gov; last access: 1 September 2018) networks. These include, for example, SKYNET (SKYradiometer NETwork) and its seven associated subnetworks, which use the Prede-POM sky radiometer to investigate aerosol–cloud–solar radiation interactions (e.g. Campanelli et al, 2004; Nakajima et al, 2007; Takamura and Nakajima, 2004)
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