Takamura, T., and H. Irie, 2019: Forward scattering effect on the estimation of the aerosol optical thickness for Sun photometry. J. Meteor. Soc. Japan, 97,
Early Online Release Graphical Abstract with highlights
Plain Language Summary: Sun photometry is one of the most popular methods to estimate optical thickness of aerosols (AOT), and has a long history because it is simple and easy to use. The observed direct normal irradiance, however, includes a part of forward scattering by aerosols so that the resultant AOT has some errors to be underestimated. Such erroneous AOT can lead an increase of ambiguity when estimating the radiative forcing of aerosols. This is dependent on an optical geometry of the measuring instrument and aerosol characteristics. In this report, the forward scattering effects on the AOT estimation are assessed by using several typical aerosol types and opening angles (OA) of instrumental geometry.
Forward scattering ratio γλ.fwd, which is defined as a ratio of the forward scattering part to the true direct normal irradiance (Iλ), by Iλ.obs=Iλ(1+γλ.fwd), is calculated as functions of AOT (τλ.aer), OA, and relative air mass(m), where several typical aerosol types are used in the simulation. As a result, the ratio is approximately expressed by the following term, ελωλτλaerm, where ελ is a proportional coefficient and ωλ is a single scattering albedo of aerosols. Figure 1 shows an example of the variation of the coefficient ελ.. These patterns are primarily dependent on the size distribution of aerosols.
- Forward scattering ratio (γλ.fwd), defined by γλ.fwd = ΔIλ.fwd /Iλ, is approximately proportional to the product of the optical thickness (τλ.aer), the single scattering albedo (ωλ) of aerosols and the relative air mass (m), i.e., γλ.fwd ≈ ελωλτλaerm.
- Then, the error of the estimate of τλ.aer can be approximately expressed by Δτλ≈ -ελωλτλaer.
- The proportional coefficient ελ is dependent on the aerosol characteristics as well as the opening angle of measurement instruments.