Forward Scattering Effect on the Estimation of the Aerosol Optical Thickness for Sun Photometry

Abstract

 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 direct normal irradiance. Consequently, the estimated optical thickness of aerosols can be underestimated 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 γ_λ.fwd, which is defined as a ratio of the forward scattering part to the true direct normal irradiance (I_λ), by I_λ.obs = I_λ(1 + γ_λ.fwd), is approximately proportional to the product of the optical thickness (τ_λ.aer) and the single scattering albedo (ω_λ) of aerosols and the relative air mass (m), γ_λ.fwd ≈ ε_λω_λτ_λaerm. The coefficient ε_λ 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 ε_λ is tabulated for several aerosol types and opening angles. Then, the error of the estimate of τ_λaer can be approximately expressed by Δτ_λ ≈ −ε_λω_λτ_λaer.