Comment on amt-2022-266

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> 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 (O<span class="inline-formula">₃</span>) 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 O<span class="inline-formula">₃</span> 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 O<span class="inline-formula">₃</span> at different solar zenith angles (SZA) around 90<span class="inline-formula">^∘</span> to O<span class="inline-formula">₃</span> at a SZA of 90<span class="inline-formula">^∘</span> for both sunrise and sunset conditions. These ratios are used to scale the O<span class="inline-formula">₃</span> 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 O<span class="inline-formula">₃</span> concentration at the tangent altitude. We find that at sunrise the retrieved mesospheric O<span class="inline-formula">₃</span> including the diurnal corrections is lower by more than 30 % compared to the archived O<span class="inline-formula">₃</span>. 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 O<span class="inline-formula">₃</span> ratio with the photochemical model prediction.