three OCO-2 bands, top to bottom: O₂ A-band 0.76 µm · weak CO₂ 1.61 µm · strong CO₂ 2.06 µm
no absorption model
surface
airmass factor — surface pressure — band albedos — continuum radiances —

Sunlight crosses the atmosphere twice before reaching the satellite. In the O₂ A-band the absorber amount is known, so the band measures the light path (surface pressure, clouds, aerosols); in the two CO₂ bands the same path reveals how much CO₂ it crossed. Move XCO₂ and watch the CO₂ lines deepen in the two lower panels while the O₂ band on top stays put. The dashed line is the reflected sunlight without any absorption.