The absorption a satellite sees depends on how far the light travelled through the atmosphere, not only on how much CO₂ is there. A low sun or a slanted view lengthens the path, by the airmass factor m = 1/cos(SZA) + 1/cos(VZA). Light scattered back by aerosol or thin cloud never reaches the ground, so it crosses less CO₂ and makes the column look too small; an opaque cloud blocks the path entirely and the sounding is discarded. Oxygen has a known, constant amount, so the O₂ A-band reveals the true path length; dividing it out is what makes XCO₂ accurate to a fraction of a percent.