Multi wavelength measurements of atmospheric turbidity and determination of the fluctuations in total ozone over Antarctica

Abstract

Sun photometer measurements have been made at λ = 310, 368 and 500 nm during a number of Indian expeditions to Antarctica. The measurements have also been made at λ = 368 and 500 nm over the ocean on a cruise to one of the expeditions. Both the optical depth ( a H ) and the turbidity ( T ) due to atmospheric haze aerosols have been computed over the ocean and over Antarctica. In general, the values of the optical depth are higher at λ = 368 nm that at λ = 500 nm. The values of the optical depth (and turbidity) are also higher over the ocean than over Antarctica. Based on the measurements made at λ = 368 and 500 nm, the exponent α = 2 describes the wavelength dependence of the optical depth. This wavelength dependence of the optical depth has been used to obtain the optical depth at λ = 310 nm. The corresponding value of the selective absorption due to ozone ( kx ≈ 1.1, total ozone x = 0.3 atm-cm) is much greater than the aerosol optical depth a H (≈ 0.11). The optical depth due to Rayleigh scattering ( a R ≈ 1.03) being constant. The relative variability (rms/mean), due to fluctuations on time-scales of a few hours to daily variations, in the computed values of the optical depth a H has been observed to be about 6% during January 1988. The relative variability in the measured intensity at λ = 310 nm, at a zenith angle of 60°, during this period is observed to be about 14%. The relative variability in the total ozone obtained from in situ measurements with the Total Ozone Mapping Spectrometer (TOMS) for the same period is about 3%. It is shown that the wavelength λ = 310 nm, which is towards the upper limits of the u.v.-B band that is highly absorbed, is perhaps best suited to monitor fluctuations in the total ozone (x). This technique is likely to be more effective during the local spring period of September–November over Antarctica, where the phenomenon of the ozone ‘hole’ occurs.