Accounting for the photochemical variation in stratospheric NO2 in the SAGE III/ISS solar occultation retrieval

The Stratospheric Aerosol and Gas Experiment (SAGE) III has been operating on the International Space Station (ISS) since mid-2017. Nitrogen dioxide (<span classCombining double low line"inline-formula"NO2</span>) number density profiles are routinely retrieved from SAGE III/ISS solar occultation measurements in the middle atmosphere. Although <span classCombining double low line"inline-formula"NO2</span> density varies throughout the day due to photochemistry, the standard SAGE <span classCombining double low line"inline-formula" >NO2</span> retrieval algorithm neglects these variations along the instrument's line of sight by assuming that the number density has a constant gradient within a given vertical layer of the atmosphere. This assumption will result in a retrieval bias for a species like <span classCombining double low line"inline-formula"NO2</span> that changes rapidly across the terminator. In this work we account for diurnal variations in retrievals of <span classCombining double low line"inline-formula"NO2</span> from the SAGE III/ISS measurements, and we determine the impact of this algorithm improvement on the resulting <span classCombining double low line"inline-formula"NO2</span> number densities. The first step in applying the diurnal correction is to use publicly available SAGE III/ISS products to convert the retrieved number density profiles to optical depth profiles. The retrieval is then re-performed with a new matrix that applies photochemical scale factors for each point along the line of sight according to the changing solar zenith angle. In general <span classCombining double low line"inline-formula"NO2</span> that is retrieved by accounting for these diurnal variations is more than 10 % lower than the standard algorithm below 30 km. This effect is greatest in winter at high latitudes and generally greater for sunrise occultations than sunset. Comparisons with coincident profiles from the Optical Spectrograph and InfraRed Imager System (OSIRIS) show that <span classCombining double low line"inline-formula"NO2</span> from SAGE III/ISS is generally biased high; however the agreement improves by up to 20 % in the mid-stratosphere when diurnal variations are accounted for in the retrieval. We conclude that diurnal variations along the SAGE III/ISS line of sight are an important term to consider for <span classCombining double low line"inline-formula"NO2</span> analyses at altitudes below 30 km.