## 3 Ozone Mapping and Profiler Suite  
The OMPS heritage from the NASA Solar Backscatter Ultraviolet (SBUV) and Total Ozone Mapping Spectrometer (TOMS) series of instruments began operationally with the Nimbus-7 TOMS and the NOAA SBUV/2 series of instruments [12]. The advanced suite is composed of three hyperspectral instruments: a nadir mapper (OMPS-NM) for total column ozone, a nadir profiler (OMPS-NP), and a limb profiler (OMPS-LP) for ozone profiling [13]. The OMPS instrument suite has two telescopes and two solar diffusers. Of the solar diffusers, one solar diffuser works bi-weekly. The other solar diffuser serves as reference, working semiannually. One telescope is used by the limb profiler. The OMPS-NM and OMPS-NP grating spectrometers share the second telescope [14; 15]. A dichroic filter reflects the 250-310 nm light to the NP spectrometer and transmits the 300-380 nm light to the NM spectrometer. The NP and NM spectrometers overlap from 300-310 nm, where the dichroic filter transitions from reflection at 300 nm to transmission at 310 nm. Two-dimensional (340 pixels along the spectral dimension and 740 pixels in the cross-track spatial dimension) charge-coupled devices (CCD) are located at the NM spectrometer's focal plane. The nadir profile spectrometer deploys a two-dimensional CCD of 340 (spectral) by 390 (spatial). The entire CCD is not used, but part of it is sensitive to photons within defined spectral ranges.  
The nadir mapper has a 2800 km cross-track swath divided into 35 fields of view of nearly equal angular extent. The NM SDR data contain 196 channel radiances. The nadir profiler SDR data content of SNPP and NOAA-20 is different. In SNPP OMPS-NP SDR, there are 147 channels, and the spatial resolution is 250 by 250 km\({}^{2}\). NOAA-20 NM SDR includes 151 channels, and the spatial resolution is enhanced to 50 by 50 km\({}^{2}\). The geolocation uncertainty is less than 5 km.  
Together, NM and NP spectrometers can measure reflected UV radiances between 250 nm and 380 nm. Figure 3 shows the optical depths of absorbing gases: O\({}_{3}\), SO\({}_{2}\), NO\({}_{2}\), O\({}_{4}\), Bro, and CIO. Below 340 nm, O\({}_{3}\) absorption dominates. Both NM and NP measurements have high sensitivity to the ozone profile in the atmosphere. The global mean value of SO\({}_{2}\) is about 1 ppm or 0.1 Dobson Units (DU). The effect for such a low SO\({}_{2}\) on OMPS measurements is negligible. However, the eruption and outgassing of volcanoes can dramatically increase the local SO\({}_{2}\) amount in the atmosphere. The local SO\({}_{2}\) amount can be larger than 10 DU. The volcanic ash and SO\({}_{2}\) gas can affect stratospheric temperatures and atmospheric chemistry as well as cause aviation hazards [16]. The olive-colored line in Figure 3 represents the optical depth for the SO\({}_{2}\) of 10 DU. One can use OMPS NM measurement to retrieve the high SO\({}_{2}\) amount [17].  
In this study, we only assimilate OMPS-NM SDR and geolocation data. Latitude, longitude, sensor and solar zenith and azimuth angles, date and time, and quality flags from the NM geolocation granule file are used. The Earth-view radiances, solar irradiances, and channel wavelengths as well as quality flags from the NM SDR granule file are also used. The wavelengths in both the NM and NP measurements are changed slightly depending on the date/time and orbit. The solar irradiance is measured weekly with a reflective diffuser at the entrance aperture. It may be worth mentioning that the solar irradiance in the SDR data file is corrected for a 1 AU Sun and Earth distance. In the CRTM calculation, the Sun-Earth distance is considered in the outputted solar irradiance.