Assessment of SAGE version 6.1 ozone data quality

Abstract

The Stratospheric Aerosol and Gas Experiment (SAGE) II V6.1 ozone retrievals are shown to be of better precision at all levels and to be much more accurate than previous retrievals in the lower stratosphere below 20 km altitude. A filtering procedure for removing anomalous ozone profiles associated with volcanic aerosol/cloud effects and other identified artifacts in V6.1 ozone is described. The agreement between SAGE and ozonesondes in the mean is shown to be approximately 10% down to the tropopause. Relative to the sondes, SAGE tends to slightly overestimate ozone (less than 5%) between 15 and 20 km altitude and systematically underestimates ozone in the troposphere by approximately 30% in the regions between 8 km altitude and 2 km below the tropopause. The precisions (random errors) of SAGE ozone retrievals above 25 km altitude are estimated to be 4% or better; they are a factor of 10 worse below 16 km altitude. Linear trends in the differences between coincident SAGE and ozonesondes measurement are generally less than 0.3%/yr and not significantly different from zero in 95% confidence intervals. Compared to V5.96 retrievals, ozone trend differences between 20 and 50 km altitude are approximately 0.1%/yr; below 20 km altitude the SAGE II trends are more positive by approximately 0.2%/yr. For the 1984-1999 period, the SAGE II shows a localized ozone loss of -0.4 ± 0.25%/yr (2σ) in the tropics at 20 km altitude. In the lower stratosphere, between 16 and 22 km altitudes, the SAGE shows significant ozone losses in the midlatitudes in both hemispheres during the 1979-1999 periods. The ozone trends range from -0.24 ± 0.18%/yr to -0.77 ± 0.46%/yr (2σ). However, in the 1984-1999 period, the downward trends are smaller (-0.07%/yr to -0.25%/yr) in this altitude range, and the trends in the integrated column from 12 to 17 km altitude in midlatitudes (35°-60°) are not significantly different from zero (0.1 ± 0.6%/yr (2σ)). Averaged over the tropics (20°S-20°N), the ozone column above 15 km altitude exhibit a trend of -0.12 ± 0.08%/yr (2σ).