A simple engineering method for calculating the aerosol optical thickness at 500 nm (AOT500) is proposed for Estonian summer conditions. The method is expressed by a single formula and represents a simplified version of a more complicated model developed at Moscow University, which assumes fulfilment of the Ångström formula. For the input our method uses three parameters: (1) the Ångström wavelength exponent (a), which can be given as its seasonal climatological mean; (2) broadband Bouguer coefficient (p2) of atmospheric transparency for optical mass m = 2 (solar elevation » 30°); and (3) the amount of columnar precipitable water vapour (W). The method was evaluated using the AERONET Tõravere data on aerosol optical thickness for three summer months (JJA) during 2002–2004. Because of the absence of high quality data on precipitable water, it was estimated approximately using surface water vapour pressure. Evaluation of the method demonstrated a good overall agreement with the observed AERONET 418 summer values of the AOT500 from 2002–2004. The evaluation also raised doubt that precipitable water is underestimated in our model compared to the AERONET Level 2 Version 1 observations. Due to its simplicity the method can be used for express estimations of the AOT500 under summer conditions in regions with similar climates to the Estonian one. Transition to other wavelengths is available using the Ångström formula.
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