Aerosol optical thickness retrieval from Sun photometer measurements

Aerosol optical thickness (AOT) is usually retrieved from Sun photometer (SP) signal in three steps: (1) SP calibration (determination of “zero signal” I₀); (2) retrieval of the total optical thickness; (3) removal of the molecular scattering and absorption. This procedure may result in biased I₀ and erroneous AOT for shortwave SP filters of finite width and those contaminated by gaseous absorption. Applying the linear Langley plot after correction of the SP signal for the effects of gaseous scattering and absorption might appear superior for those channels as compared to the traditional procedure, Reagan et al. (1987). The present authors use the LOWTRAN7 calculations to compare the results of both approaches for all channels used in the SP oceanic campaign on board the Research Vessel Akademik Vernadsky in summer of 1991, Ignatov et al. (1993). The scheme for processing the SP data is outlined, and experimental AOT for shortwave channels are presented. Statistical examination of the retrieved AOT in different SP channels allows one to estimate both systematic and random errors in the retrieved AOT