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Methods
A special observation schedule was used for the Brewer spectrophotometer operation during the Campo Grande campaign . The same solar zenith angles from 60o to 41o were chosen as values for ds measurements both in the morning and in the afternoon. A total of 20 measurements were made per period, each lasting about 2.5 minutes.
The Langley method was applied to the 20 ds measurements to obtain the atmospheric optical thickness. Atmospheric optical thickness is equal to
t at = t R + t O3 + t SO2 + t ae
where Rayleigh, ozone, sulfur dioxide, and aerosol optical thicknesses are considered, respectively. t R can be obtained from Hansen and Travis (Teillet, 1990)
t R = 0.008569l -4(1 + 0.0113l -2 + 0.00013l -4)p/1013
with l in m m as the wavelength, and p as the atmospheric pressure in mbar at the site. t O3, and t SO2 can be obtained by the general formula
t = s N2.69E16
where s is the absorption cross section (Molina and Molina, 1986; McGee and Burris Jr, 1987; Davidson et al, 1988), and N the gaseous column integral in the atmosphere. This quantity for ozone and sulfur dioxide was obtained by the Brewer.