## 2 Data and Methods
### Oceanic RT Model  
To calculate underwater irradiances, we use a lookup table of scalar and planar irradiances computed with the Hydrolight RT model [11] with our extensions for ocean IOPs into the UV wavelength range [10]. We use the default boundary conditions for Hydrolight. The incident sky radiance is calculated by the incorporation of a clear-sky spectral irradiance model, a cloud cover correction, and a sky radiance distribution model [11]. According to [PERSON] [11], \"These models have proven sufficiently accurate for most HydroLight applications. In particular, apparent optical properties (AOPs) are only weakly dependent on the details of the sky radiance distribution--that is, after all, one of the main reasons for using AOPs\".  
The sea surface is assumed to be covered with waves that are parameterized as a function of the surface wind speed. We assume the wind speed to be equal to 5 m/s. In our atmospheric RT model, we do not consider a separation of the incident light into a clear-sky fraction and a cloudy fraction. This approach is similar to that described in [PERSON] et al. [42].  
We scale the input surface irradiance for Hydrolight computations by our OMI-retrieved spectral irradiance. We do not apply the default cloud cover correction in the calculations because this correction has effectively been incorporated into the \(C_{T}\) factor.  
The IOP model used here is an extension of the Case I water model [43] to the spectral region 290-400 nm. The model has been described in detail in [PERSON] et al. [10; 44]; here, we briefly describe the main features of the IOP model and its verification in Appendix A. The only input parameter of the model is chlorophyll concentration.  
The RT computations with Hydrolight were conducted for vertically homogeneous waters. We do not account for non-elastic Raman scattering because its contribution to the in-water radiation field in the UV region is small. This is due to a relatively large wavelength shift between the vibrational Raman excitation and emission bands, which is about 40-50 nm. Due to the sharp decrease of solar radiation at shorter wavelengths in the UV region, the vibrational Raman excitation is significantly reduced at shorter wavelengths, and thus the vibrational Raman emission at longer wavelengths is small. This fact substantially limits the spectral performance of algorithms developed to estimate UV underwater light attenuation from satellite hyperspectral observations from the in-water vibrational Raman scattering signal [19]. Reliable results of the light attenuation retrieval can be expected for longer wavelengths of the UV-A spectral range only. The attenuation of the harmful UV-B radiation seems to be hardly estimated using the in-water Raman signal from the theoretical point of view. It should also be noted that the Raman-based approach for retrieving the light attenuation has another limitation; it is applicable for cloud fractions less than 0.05 only [19]. Additionally, the Raman effect is significantly smoothed due to the relatively low spectral resolution of OCI. The Raman effect strongly depends on the spectral resolution of an instrument. Even at the OMI spectral resolution of about 0.5 nm, the Raman effect does not exceed a few percent in the deepest Ca II Fraunhofer lines [45].  
Lookup tables for in-water downward and upward irradiances were generated for the UV wavelength range of 290-400 nm, for chlorophyll concentrations that varied from 0.01 to 10 mg/m\({}^{3}\), for SZAs ranging from 0\({}^{\circ}\) to 80\({}^{\circ}\), and ozone column amounts from 150 to 550 DU.