Evaluating solar aureole radiance measurements to identify cirrus clouds and dust in the AERONET database

A novel cloud screening method was introduced by Giles et al., 2019, to improve removal of cirrus clouds in the Aerosol Robotic Network (AERONET) aerosol optical depth (AOD) database. The Cimel sun-sky radiometer performs solar aureole measurements at the 1020 nm wavelength during angular sky scans. Empirical thresholds of the solar aureole angular characteristics were based on MPLNET lidar cirrus cloud identification and enabled the application of this new cirrus cloud screening method in the AERONET Version 3 database. The present study explores a wider application of the AERONET solar aureole measurements with respect to improving identification of cirrus clouds and dust as well as the determination of approximate aerosol size. MPLNET lidar cloud categorization combined with geostationary satellite data allows for matching AERONET measurements up to 60 degrees solar zenith angle to determine the existence of homogeneous cirrus clouds over AERONET sites at larger optical air mass. Further, spectral solar aureole curvatures are analyzed to determine the dominant aerosol size independently of AOD and are compared to Angstrom exponent and size distribution parameters. During high aerosol loading dust events, the solar aureole curvature parameters show a maximum near solar noon and minima in the early morning and late afternoon due to increased forward scattering. As a result, this diurnal dependence of solar aureole curvature could be used as a potential method to improve dust event retention in the AERONET AOD data set. Giles, D. M., et al.: Advancements in the Aerosol Robotic Network (AERONET) Version 3 database automated near-real-time quality control algorithm with improved cloud screening for Sun photometer aerosol optical depth (AOD) measurements, Atmos. Meas. Tech., 12, 169209, https://doi.org/10.5194/amt-12-169-2019, 2019.