28 Sep 2022
28 Sep 2022
Status: this preprint is currently under review for the journal AMT.

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

Murali Natarajan, Robert Damadeo, and David Flittner Murali Natarajan et al.
  • Science Directorate, NASA Langley Research Center, 21 Langley Blvd., Mail Stop 401-B, Hampton, VA 23681, USA

Abstract. Twilight gradients in the concentration of atmospheric species with short photochemical lifetimes influence the transmission data obtained in a solar occultation instrument like 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. 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 error 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 zenith angles around 90° to O3 at a zenith angle of 90° for both sunrise and sunset conditions. These ratios are used to scale the O3 at levels above the tangent altitude for appropriate zenith angles in the calculation of the optical depth along the LOS. In general, the impact of the twilight variations is to increase the optical depth thereby reducing the retrieved O3 concentration at the tangent altitude. We find that at sunrise the mesospheric O3 including the diurnal corrections is lower by more than 20 % compared to the archived O3. We show the results obtained for different latitudes and seasons. In addition, for nearly co-located 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.

Murali Natarajan et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on amt-2022-266', Anonymous Referee #1, 31 Oct 2022
  • RC2: 'Comment on amt-2022-266', Anonymous Referee #2, 03 Nov 2022

Murali Natarajan et al.

Murali Natarajan et al.


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Short summary
Photochemically induced changes in mesospheric O3 concentration at twilight can cause asymmetries in the distribution along the line of sight of a solar occultation observation, variations that must be considered in the retrieval algorithm. Correction factors developed from diurnal photochemical model simulations were used to modify the archived SAGE III/ISS mesospheric O3. For June 2021, the bias caused by the neglect of diurnal variations is more than 30 % at 64 km and low latitudes.