## 6 Experimental OMPS Radiance Assimilation
UV surface reflectance plays an important role in the CRTM simulations because most OMPS NM channels can see the surface under clear-sky conditions. Furthermore, there are large uncertainties in surface reflectance. In the ozone retrieval, one channel at 331 nm is used to estimate surface reflectance, and this reflectance is assigned to all other channels in the retrieval. Using CRTM, we calculate the total transmittance from the surface to the OMPS NM instrument and can calculate the accumulated radiance sensitivity to the ozone in the atmosphere (see Figure 5). As shown in Figure 5, radiance at 331 nm is somewhat affected by atmospheric ozone. This suggests that it may be better to use ozone-independent channels to estimate the surface reflectance. Our proposed two channels (see symbol o in Figure 5) have high transmittance (high sensitivity to surface) and nearly zero sensitivity to ozone in the atmosphere. In fact, one can see from Figure 4 that by using one constant reflectance (black line), the error in the CRTM simulation error increases for wavelengths longer than 331 nm since surface reflected solar radiation increases with wavelength as well. This large error can be reduced (see cyan line in Figure 4) if we use the surface reflectance interpolated or extrapolated from the estimated reflectance at the two wavelengths marked with \"o\" in Figure 5. This indicates that surface reflectance is not constant for the wavelengths between 250 nm and 380 nm. The linear spectral dependence of the surface reflectance can be a good approximation for this study. Based on the linear assumption, we analyze the difference of the normalized radiance between global OMPS NM measurements and the CRTM simulations for a time window of 6 h.  
Figure 4: SNPP normalized radiance. Red line represents the OMPS NM measurements. The black line is the CRTM simulation using a constant surface reflectance by fitting the normalized radiance at 331 nm. The cyan line is the CRTM simulation using linearly interpolated/extrapolated surface reflectance from two reflectance by fitting the normalized radiance at 347.6 nm and 371.8 nm. The scalar radiative transfer can be accurate for OMPS NM radiance simulations with an accurate effective surface reflectance. For this test, we chose a case over a Libya desert calibration site where the surface UV reflectance is low, only about 3% [43]. We first derived the surface reflectance at the wavelengths of 347.6 nm and 371.8 nm using polarized CRTM. Then, we interpolated or extrapolated the two reflectance points for other OMPS NM channels. A total of nine channels were selected to be used in the simulation. The last three of the nine channels are insensitive to ozone in the atmosphere. The other six channels are sensitive to the ozone amount in the atmosphere. We first calculated the absolute difference between the OMPS NM measurement and the CRTM simulation. The error given in this section is the absolute difference divided by the measurement in percentage. As shown in Table 2, the simulated radiances using the polarized CRTM agree with OMPS NM measurements very well (<0.1%). However, using the scalar CRTM model, the error is higher than 3.6% (5 th column in Table 2). This result implies that the difference between the polarized and scalar CRTM simulations can be significant. If we use the same scalar CRTM model to derive the surface reflectance at the two wavelengths mentioned above and interpolate or extrapolate the two reflectance for other channels, the scalar CRTM simulation can also achieve a high accuracy (see the last column of Table 2). The estimated surface reflectance using the polarized CRTM simulation can be off the estimate by a few percent by using the scalar CRTM calculation. We also chose a case over Dome C site that is a radiometric calibration site in Antarctica. The surface reflectance in the UV spectra is very high [44], larger than 0.9. Again, the scalar radiative transfer can be accurate to OMPS NM radiance simulations with the effective surface reflectance.