Article ID: 2019-059
The accurate aerosol optical thickness is indispensable for estimating the radiative forcing of aerosols in the atmosphere. Sun photometry is one of the most popular methods, which is simple and easy to use, but it should be noted that some errors due to forward scattering effect can be introduced in the observation of the direct normal irradiance. Consequently, the estimated optical thickness of aerosols can be under-estimated even if the calibration constant is correct. This possibility depends on an optical geometry of the measuring instrument as well as aerosol characteristics. This report assesses these effects by assuming several aerosol types and instrumental parameters quantitatively.Forward scattering ratio γλfwd, which is defined as a ratio of the forward scattering part to the true direct normal irradiance (Iλ), by Iλobs=Iλ(1+γλfwd), is approximately proportional to the product of the optical thickness (τλaer) and the single scattering albedo (ωλ) of aerosols and the relative air mass (m), γλfwd≈ελωλτλaerm. The coefficient ελ is a proportional constant which is dependent on the opening angle of the instrument as well as the optical characteristics of aerosols. The variation of ελ is tabulated for several aerosol types and opening angles. Then the error for the estimate of τλaer can be approximately expressed by Δτλ≈ -ελωλτλaer.