Measurements of the O2A- and B-bands for determining temperature and pressure profiles from ACE-MAESTRO: Forward model and retrieval algorithm

The ACE-MAESTRO (Atmospheric Chemistry Experiment-Measurement of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation) instrument on the SCISAT satellite is able to measure solar occultation absorption in the A- and B-bands of molecular oxygen with a spectral resolution of 2 nm. Profiles of total atmospheric density are derived by exploiting the constant and known mixing ratio of O2, and are used to determine profiles of pressure and temperature using hydrostatic balance and the ideal gas law. A highly accurate combined fast-line-by-line and extended correlated-k technique is implemented to fast forward model MAESTRO's O2 absorption measurements, which ensures that errors in pressure and temperature resulting from the forward model approximation are essentially negligible. Estimated errors in pressure and temperature are determined for the A-, B-, and combined A-B-band retrievals for a typical retrieval, and demonstrate that pressure profiles should be derivable to within 1% and temperature to within 2 K over most altitudes using a combined A-B retrieval. The combined retrieval provides an improvement of up to 0.25% in pressure and 0.5 K in temperature over the A-band retrieval alone. The B-band could also be used alone below about 50 km, where its independent retrieval produces error estimates of 1-1.5% in pressure and 2-3 K in temperature.