Changes in SBUV ozone profiles near Natal, Brazil, from 1979 to 1985

Abstract

Ozone column amounts from Natal, Brazil, are calculated from radiances measured by the solar backscatter ultraviolet (SBUV) spectrometer aboard Nimbus 7 during March/April of 1979 and 1985. These amounts are calculated at altitudes from 36 to 47 km, a region where ozone varies exponentially with altitude. Calculations are made with the algorithm of Thomas and Holland (1977), providing ozone values from five SBUV channels at wavelengths from 255 to 292 nm. Each of the five column amounts is essentially independent of measurements from the other SBUV channels but not independent of the assumption of exponential ozone change with altitude. SBUV ozone amounts are significantly lower in 1985 than in 1979, with a distinct skew between the ozone profiles from the two data sets. The differences between the two SBUV data sets increase from 3.6% to at 4.7 mbar (36 km) to 15.9% at 1.2 mbar (47 km). There are corresponding changes in the backscattered radiances from SBUV for these data sets, with a distinct wavelength dependence for the changes. SBUV radiances from 1985 are higher by 2.3% at 292 nm, increasing to 9.6% at 255 nm. There is a direct relationship between radiance differences and differences in the ozone column amounts. The structure of the ozone changes does not agree with current calculations of ozone reduction due to fluorocarbons. Using SBUV values alone, it is not possible to separate atmospheric and instrumental changes. Published ROCOZ‐A ozone profiles from Natal in March/April 1985 show reasonable agreement with the 1979 SBUV measurements there. On an absolute basis the ROCOZ‐A measurements at Natal in 1985 do not agree with the 1985 SBUV profiles. Several tests can be performed to resolve these differences between instruments. Until these tests are completed, it would seem prudent to exercise caution before attributing changes in the SBUV radiances to the atmosphere.