Wavelength dependence of aerosol optical depth and the fit of the Angstrom law

The dependence of aerosol optical depth on wavelength as well as the fit of the Angstrom approximation have been investigated under different air masses at a sub-Arctic location (Abisko, Sweden; 68degrees 21'N, 18degrees 49'E) and a tropical environment (Ife, Nigeria; 7degrees 30'N, 4degrees 31'E). The study is based on spectral data acquired with a high resolution spectral radiometer (spectral range: 300-1100nm) in absorption-free regions. The wavelength dependence of the aerosols under different air mass conditions at the sub-arctic location offer significant contrasts to aerosols of Saharan origin at Ife. A general characteristic of the aerosol optical depth spectra after the Pina-tubo volcanic eruption was a much weaker wavelength dependence relative to pre-Pinatubo conditions. Categorising the features of the optical depth spectra according to their wavelength dependence, three main groups were observed at Abisko, while two main classes have been discussed for the harmattan season in the tropical climate of Ife and environs. For the first two groups in Abisko (and the first group at Ife), aerosol optical depth generally decreased with wavelength while the third group (second group at Ife) exhibited strong curvatures. The correlation coefficient obtained from the regression equation of the Angstrom equation, has been shown to be a good index of the general fit of the Angstrom approximation for the three groups at Abisko, but much weaker for the harmattan conditions at the tropical location. Although the probability of systematic deviations from the Angstrom law is highest under intense harmattan conditions with considerably high beta and low alpha, it has been observed that the Angstrom fit was good in many highly turbid conditions at the tropical site. Hence, apart from the level of turbidity, the applicability of the Angstrom approximation is strongly dependent on aerosol characteristics and source region.