## 2 Data and Methods
### OMI Data  
The Ozone Monitoring Instrument (OMI) is a nadir-viewing spectrometer aboard the United States National Aeronautics and Space Administration's (NASA's) Earth Observing System (EOS) atmospheric composition (chemistry) observatory (Aura) that flies in a sun-synchronous orbit with a mean local equator crossing time of about 13:45 (ascending node). Aura is part of the afternoon \"A-train\" satellite constellation. OMI measures the solar irradiance daily and Earth backscattered radiance from 270-500 nm with a spectral resolution of approximately 0.5 nm [25; 26]. It provides near global coverage with a nadir pixel size of 13 \(\times\) 24 km\({}^{2}\) (along and across the swath, respectively) and a swath width of about 2600 km. OMI made daily global measurements at the start of the mission, but later the global coverage period increased to two days, presumably due to a partial obstruction of the Earth-view port by insulation material on the outside of the instrument. Radiance and irradiance measurements from OMI Collection 3 Level 1B (product names OML1 BRUG and OML1 BIRR, respectively) [27; 28; 29] are used in this work. These are obtained from the Collection 3 products at the Goddard Earth Sciences Data and Information Services Center (GES DISC). OMI TOA radiance measurements used in this study are from sensor band 2 (UV-2) and band 3 (Vis). A small angular offset in pointing of the two bands results in differences in FOV location of 2-3 km in rows 10-40 and 15-25 km at the very edges of the OMI swath. These larger differences at swath edges are predominantly in the cross-track direction.  
Figure 1: Extraterrestrial spectral irradiance at different spectral resolutions shown for (**a**) the entire UV spectral range and (**b**) the OCI spectral range shown in the light blue cutout in (**a**). The color bar represents the boxcar smoothing width in nanometers.  
To carry out atmospheric RT computations in UV, we need to know the total column ozone amounts and surface reflectance. The total ozone fields used as input to the atmospheric RT computations are obtained from the NASA OMI Total Ozone Mapping Spectrometer (TOMS)-like product (OMTO3) that has been validated extensively up to a solar zenith angle (SZA) of 80\({}^{\circ}\)[30]. We use operational L2 total column ozone measurements from OMI in Dobson Units (1 DU = \(2.69\times 10^{16}\) molecules/cm\({}^{2}\)). The OMI-TOMS V8.5 total ozone data [31] are also obtained from the GES DISC.  
As a proxy of the surface reflectance we use a monthly climatology of the so-called minimum Lambertian Equivalent Reflectivity (LER). Given the measured top-of-atmosphere (TOA) radiance, \(I_{m}\), the LER is defined by inverting the following exact equation:  
\[I_{m}=I_{0}+RT/(1-RS_{b}), \tag{1}\]  
where \(R\) is the surface LER, \(I_{0}\) is the TOA path radiance calculated for a black surface, \(T\) is the total (direct + diffuse) solar irradiance reaching the surface converted to the ideal Lambertian reflected radiance (dividing by \(\pi\)) and then multiplied by the transmittance of the reflected radiation between the surface and TOA in the direction of a satellite instrument and \(S_{b}\) is the diffuse flux reflectivity of the atmosphere for the case of its isotropic illumination from below. OMI-derived multi-year gridded LER data are used to create the climatology of the minimum LER (OMLER) [32; 33].