As solar radiation reaches the Earth's atmosphere and surface a certain percentage is reflected or backscattered out to space. The degree to which incident light is backscattered depends on the albedo of the surface the light encounters, for example, clouds in the atmosphere have a high albedo so a large proportion of sunlight is reflected back.
GOME-2 is able determine amounts of certain gases present in the atmosphere through the principles of spectrometry, where, in incoming light is split into its spectral components to reveal absorption lines, which correspond to the various gases present the observed sample.