Global surface-based sun photometer network for long-term observations of column aerosol optical properties: Intercomparison of aerosol optical depth

Comparisons of aerosol optical depths (AODs) determined from several types of Sun photometers operating side by side as part of four different networks (GAW PFR, AERONET, SKYNET, and NOAA/ESRL aerosol monitoring programs) were made at 6 different environmental stations to evaluate the different types of current state-of-the-art instruments under different aerosol loading conditions. A comparison between AERONET CIMEL and GAW PFR at a high altitude calibration site, Mauna Loa, shows an excellent agreement with 0.001 bias for 500 nm AOD. AODs obtained from direct Sun-pointing instruments are within 0.01 bias, though these results are similar to or slightly larger than those given in previous short-term intensive studies. These results suggest that well-maintained networks of direct Sun-pointing instruments developed by different companies/institutions can provide quality-assured AOD data across the globe to the aerosol-climate research community. The poorer agreement between a hemispherical field-of-view (FOV) MFRSR and the finite FOV Sun-pointing instrument is found to be due to uncertainty in the angular characterization of the MFRSR optics.