This study presents the new v7.2 NO\(_2\) profile retrieval for the Optical Spectrograph and InfraRed Imager System (OSIRIS) aboard the Odin platform. The study first addresses the changes made to the profile retrieval, which are designed to improve the sensitivity of the NO\(_2\) product in the upper troposphere–lower stratosphere and to reduce low biases observed in previous OSIRIS NO\(_2\) products. The differences in measured NO\(_2\) between the v7.2 and the previous v6.0 profiles are summarized in Figure 1, which shows the relative differences between the two products.

Figure 1: Mean percent difference between v6.0 and v7.2 OSIRIS NO\(_2\) for the period November 2001 to May 2020.

Following the presentation of the new NO\(_2\) product, it is then compared against coincident measurements of NO\(_2\) from the Atmospheric Chemistry Experiment–Fourier Transform Spectrometer (ACE-FTS) and the Stratospheric Aerosol and Gas Experiment III on the International Space Station (SAGE III/ISS). To account for the diurnal cycle in NO\(_2\), the PRATMO photochemical box model was used to account for differences in measurement times, with all datasets scaled to the same local solar time. The number of coincident profile measurements between OSIRIS and ACE-FTS and SAGE III/ISS, is shown in Figure 2 for an altitude of 25.5 km, and the results of the comparisons are shown in Figures 3 and 4 for ACE-FTS and SAGE III/ISS. For the comparisons, ACE-FTS and SAGE III/ISS are compared against the v6.0 and v7.2 NO\(_2\) products, with an additional comparison against the v7.2 product after an averaging-kernel-based filter is applied to the data to ensure the sensitivity of the retrievals.

Figure 2: Number of coincident profiles at 25.5 km for ACE-FTS and SAGE III/ISS with OSIRIS. The results are separated based on whether they are sunset occultations (SS) or sunrise occultations (SR).

Figure 3: Mean percent difference between coincident profiles from ACE-FTS v4.1 and OSIRIS. Results are shown for sunrise occultations (top row) and sunset occultations (bottom row). The first row shows comparisons made with OSIRIS v6.0 NO\(_2\), while the middle shows comparisons with the v7.2 product. In the last column, comparisons are shown after an averaging kernel filter has been applied to the OSIRIS v7.2 profiles. The dashed lines indicate the tropopause altitude, and the dotted line the average 380 K potential temperature altitude.

Figure 4: Same as Figure 3 but for SAGE III/ISS.

From these comparisons, it is noted that the coincident ACE-FTS and SAGE III/ISS measurements were found to agree to within 20 % of the OSIRIS NO\(_2\) measurements throughout much of the stratosphere. In general OSIRIS NO\(_2\) measurements agrees better with ACE-FTS than with SAGE III/ISS, although this could be due to the larger number of coincidences with ACE-FTS. Overall, based on the comparisons the OSIRIS v7.2 NO\(_2\) product is an improvement over the previous version, particularly below 30 km, as the bias between OSIRIS v7.2 and both ACE-FTS and SAGE III/ISS has generally decreased from v6.0.