A shortcut from broadband to spectral aerosol optical depth

The concept behind the shortcut idea is a close correlation between column broadband aerosol optical depth (BAOD) and aerosol optical depth at 500 nm (AOD500). The method uses only two input parameters: (a) the Bouguer broadband coefficient of column transparency for optical mass m = 2 (solar elevation about 30°) and (b) integrated column precipitable water vapour which can be roughly estimated using surface water vapour pressure. In creating the method, a large database, including almost 20 000 complex, spectral and broadband direct solar beam observations at Toravere, Estonia, during all seasons of a 8-year period, 2002-2009, was used. The AOD500 observations were performed by the NASA project AERONET and the broadband direct beam ones by the Estonian Meteorological and Hydrological Institute. Analysis of this database revealed a high correlation between BAOD and AOD500 which enabled transition from broadband to spectral AOD. Almost 82% of the observations in the database belonged to lower turbidities when AOD500 0.6, relative RMSD remained 9%. For comparison, the same database was used to test Gueymard's broadband parameterization based on his SMARTS2 classic model. The last one, apparently due to problems with circumsolar radiation, slightly but systematically underestimated the AOD500. However, there was a close correlation between our shortcut results and Gueymard's broadband parameterization.