## 3 Retrieval
### Inversion Experiments
#### 3.2.2 Scenarios  
The geophysical _baseline_ scenario has a spectrally flat albedo of 0.2, 0.2, 0.1, and 0.05 in the SIF, O\({}_{2}\), wCO\({}_{2}\), and sCO\({}_{2}\) fit window; values which have also been used by, e.g., [PERSON] et al. [4]. It does not include chlorophyll fluorescence, scattering by aerosols, clouds, or Rayleigh. Its temperature, pressure, and water vapor (XH\({}_{2}\)O = 3031 ppm \(\widehat{=}\) 19.52 kg/m\({}^{2}\)) profiles are taken from an ECMWF analysis of 28 August 2015, 12:00 UTC, 9\({}^{\circ}\)E, 53\({}^{\circ}\)N. Its CO\({}_{2}\) profile is calculated with SECM2016 and corresponds to an XCO\({}_{2}\) value of about 395 ppm. Note that ECMWF and SECM2016 are also used to compute the first guess and a priori H\({}_{2}\)O and CO\({}_{2}\) profiles (Table 1). All other scenarios are descendants of the _baseline_ scenario.  
Each scenario is analyzed for three solar zenith angles (20\({}^{\circ}\), 40\({}^{\circ}\), and 60\({}^{\circ}\)) and for two directions of polarization (parallel and perpendicular to the SPP). The satellite zenith angle is set to 0\({}^{\circ}\) (nadir).  
The _SIF_ scenario adds 1 mW/m\({}^{2}\)/sr/nm chlorophyll fluorescence at 760 nm to the simulated measurement of the _baseline_ scenario. The _XCO\({}_{2}\)+6 ppm_ scenario has an increased CO\({}_{2}\) concentration of 15 ppm, 10 ppm, and 5 ppm in the three lowermost layers, so that the column-average concentration is enhanced by 6 ppm.  
All scattering related scenarios are more complex for the retrieval because of FOCAL's scattering approximations. The _Rayleigh_ scenario adds Rayleigh scattering to the baseline scenario; the Rayleigh optical thickness at 760 nm for this scenario is about 0.026. _Rayleigh_+_Aerosol BG_ additionally includes a (primarily) stratospheric background aerosol with an AOT (aerosol optical thickness at 760 nm) of 0.019 (0.003 at 1600 nm and 0.001 at 2050 nm). _Rayleigh_+_Aerosol_ cont adds a continental aerosol to the boundary layer so that the total AOT becomes 0.158 (0.060 at 1600 nm and 0.037 at 2050 nm). _Rayleigh_+_Aerosol_ urban adds a strong contamination with urban aerosol to the boundary layer and the total AOT becomes 0.702 (0.245 at 1600 nm and 0.151 at 2050 nm).  
The scenarios _Rayleigh_+_Dark surface_, _Rayleigh_+_Bright surface_, and _Rayleigh_+_Ocean glint_ distinguish from the _Rayleigh_ scenario only by their surface reflection properties. _Rayleigh_+_Dark surface_ and _Rayleigh_+_Bright surface_ correspond to the _Rayleigh_ scenario but with an albedo multiplied with 0.7 and 1.4, respectively. The _Rayleigh_+_Ocean glint_ scenario deviates from the assumption of a Lambertian surface bidirectional reflectance distribution function (BRDF); it includes an ocean surface at a wind speed of 5 m/s, 37\({}^{\circ}\) to the solar principal plane (SPP). Additionally, the satellite zenith angle of this scenario is set to 0.75 times the solar zenith angle so that the satellite looks near the glint spot of specular reflectance.  
Two cloud scenarios (_Rayleigh+Aerosol BG+Water cloud_ and _Rayleigh+Aerosol BG+Ice cloud_) add a sub-visible water or ice cloud to the _Rayleigh+Aerosol BG_ scenario. The water cloud has a height of 3 km, droplets with an effective radius of 12 um, and a COT (cloud optical thickness at 500 nm) of 0.039. The ice cloud is made of fractal particles with an effective radius of 50 um, has a height of 8 km, and a COT of 0.033.  
Appendix B lists important input parameters which have been used to perform the SCIATRAN RT calculations for all scenarios.