Validation of POAM III Aerosols: Comparison to SAGE II and HALOE

We describe statistical comparisons between Polar Ozone and Aerosol Measurement (POAM) III and Stratosphere Aerosol and Gas Experiment (SAGE) II measurements of aerosol extinction in 1998 and 1999. SAGE II and POAM III are in qualitative agreement, and show that since the launch of POAM III in March of 1998, stratospheric aerosol extinctions at visible and near-IR wavelengths have remained at background levels. We present quantitative differences between the SAGE II and POAM III extinctions at 1020 nm and 450 nm for temporally and spatially coincident measurements. At 1020 nm the instruments agree to within about 30% from 10 to 22 km, where most of the aerosol extinction lies. Differences at 450 nm are similar to 1020 nm in the southern hemisphere, but much larger in the northern hemisphere. We have also compared the POAM and SAGE slant optical depths (the fundamental measurement quantity for both instruments) at both 1020 nm and 450 nm. The differences at 1020 nm are very similar to the extinction differences, suggesting that the retrieved extinction differences are primarily the result of fundamental measurement differences. However, at 450 nm there are statistically significant differences between the extinction and optical depth comparisons (extinction differences are much larger), suggesting that at least part of the extinction differences are the result of retrieval differences (aerosol separation, Rayleigh scattering specification, modeling of refraction, etc.) rather than fundamental measurement differences. Finally, we have compared derived surface areas and volume densities from POAM III, SAGE II, and the Halogen Occultation Experiment (HALOE). We find good qualitative agreement between all three instruments. Statistical differences can be explained by the aerosol extinction differences in the SAGE II comparisons, and by the lack of sensitivity on the part of POAM III and SAGE II to very small aerosols in the HALOE comparisons.