Solar occultation measurement of mesospheric ozone by SAGE III/ISS: impact of variations along the line of sight caused by photochemistry

Abstract

Twilight gradients in the concentration of atmospheric species with short photochemical lifetimes influence the transmission data obtained in a solar occultation instrument, such as the Stratospheric Aerosol and Gas Experiment III aboard the International Space Station (SAGE III/ISS). These photochemically induced changes result in nonlinear asymmetries in the species distribution near the tangent altitude along the line of sight (LOS). The bias introduced by neglecting the effects of twilight variations in the retrieval of mesospheric ozone is the focus of this study. Ozone (O3) in the mesosphere exhibits large variations near the terminator during sunrise and sunset based on current understanding of the photochemistry of this altitude region. The algorithm used in the SAGE III/ISS standard retrieval procedure for mesospheric ozone does not include the effects of these gradients. This study illustrates a method for implementing a correction scheme to account for the twilight variations in mesospheric O3 and gives an estimate of the bias in the standard retrieval. We use the results from a diurnal photochemical model conducted at different altitudes to develop a database of ratios of mesospheric O3 at different solar zenith angles (SZA) around 90∘ to O3 at a SZA of 90∘ for both sunrise and sunset conditions. These ratios are used to scale the O3 at levels above the tangent altitude for appropriate SZA in the calculation of the optical depth along the LOS. In general, the impact of the corrections due to twilight variations is to increase the contribution of the overlying layers to the optical depth thereby reducing the retrieved O3 concentration at the tangent altitude. We find that at sunrise the retrieved mesospheric O3 including the diurnal corrections is lower by more than 30 % compared to the archived O3. We show the results obtained for different latitudes and seasons. In addition, for nearly collocated sunrise and sunset scans, we note that these corrections lead to better qualitative agreement in the sunrise to sunset O3 ratio with the photochemical model prediction.