Wavelength dependence of aerosol optical depth and the fit of the Ångström law |
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| Authors: | Z D Adeyewa E E Balogun |
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| Institution: | (1) Department of Meteorology, Federal University of Technology, Akure, Nigeria, NG;(2) Department of Physics, Obafemi Awolowo University, Ile-Ife, Nigeria, NG |
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| Abstract: | Summary ?The dependence of aerosol optical depth on wavelength as well as the fit of the ?ngstr?m approximation have been investigated
under different air masses at a sub-Arctic location (Abisko, Sweden; 68° 21′ N, 18° 49′ E) and a tropical environment (Ife,
Nigeria; 7° 30′ N, 4° 31′ E). The study is based on spectral data acquired with a high resolution spectral radiometer (spectral
range: 300–1100 nm) 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 Pinatubo
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 ?ngstr?m equation, has been shown to be a good index
of the general fit of the ?ngstr?m 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 ?ngstr?m law is highest under intense
harmattan conditions with considerably high β and low α, it has been observed that the ?ngstr?m fit was good in many highly
turbid conditions at the tropical site. Hence, apart from the level of turbidity, the applicability of the ?ngstr?m approximation
is strongly dependent on aerosol characteristics and source region.
Formerly Adeyefa.
Received May 18, 2001; revised June 20, 2002; accepted August 5, 2002 |
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