Broadband extinction method to determine atmospheric aerosol optical properties

The equivalent wavelength (λ_E), at which the aerosol optical depth (AOD) is equal to broadband AOD (BAOD), can change in a wide range from 0.619 μm to 1.575 μm in the usual aerosol conditions. By using the least squares technique and some empirical corrections, a parameterized relationship of λ_E with BAOD, Ångström wavelength exponent (α), solar zenith angle (θ₀) and H₂O amount is developed. Using this relationship, and based on the strong sensitivity of BAOD on θ0 when θ0>70°, the broadband extinction method to derive the spectral AOD and α is further proposed. As shown in comparative simulations to retrieve AOD by the present, Molineaux et al. and Gueymard methods, the present method has the best accuracy in most simulations using Junge, MODTRAN, log-normal and Deirmendjian aerosol models. A key question of the pyrheliometer method to determine wavelength-dependent AODs is the effect of uncertainty in the aerosol size istribution. It is found that the AOD solution around λ_E is less sensitive to the uncertainty. The wavelength exponent α is derived using an assumption of the stable atmospheric turbidity. If the pyrheliometer data from θ0=85° to 70° are used and the change of the turbidity is ±10%, the error of solutionα is usually within ±0.32. If the variation of the turbidity is random, the mean value of a lot of the measurements of α would be very reasonable.