Attenuation by aerosols

To investigate the attenuation of spectral global irradiance by aerosols, cloudless sky spectra were normalized to model calculations for an aerosol-free atmosphere. The figure shows the calculated reduction of erythemal and UVA irradiances as a function of the optical depth at 340 nm. The slope of the linear regression is about -10% per optical depth of 0.5. A small increase of the slope towards lower wavelengths and larger solar zenith angles was found from the data. At polluted sites with more absorbing aerosols (lower single-scattering albedo) the slope of the decrease is expected to be larger.

In consequence, a description of the aerosol is required to model spectral irradiance with high accuracy. The figure shows the ratio of measured and calculated erythemal irradiance, for three degrees of aerosol parameterization: (top) no aerosol in the model; (middle) visibility at 550 nm, used to estimate the optical depth in the UV; (bottom) measured spectral UV aerosol optical depth. Knowledge of the spectral optical depth improves the agreement considerably. At polluted sites with more absorbing aerosols, some information about the absorbing properties of the aerosol (single-scattering albedo) would be required additionally. Here, a clean-continental aerosol with a high single-scattering albedo of about 0.95 was assumed.