When GOME looks down through the Earth's air, only about one in a million of all the molecules in its line of sight is ozone. Agents that attack the ozone are usually less than one part in a billion. They are disproportionately effective because they act like catalysts, carrying out the same ozone- destroying reactions again and again. GOME sees these small traces by differential absorption spectroscopy. Decoding the words reveals the simple idea behind the forbidding name.
Spectroscopy measures the intensities of light at different wavelengths.
Seen from space, the Earth's surface shines by reflected sunlight. Four gratings then decompose segments for a second time, and silicon light- detectors register 1024 different wavelengths in each segment. Absorption means the interception of some of the earthshine by atmospheric molecules. The wavelengths of missing earthshine identify the molecules responsible, while the extent of the absorption measures their abundance. Ozone has bands of strong absorption in three of the four segments of GOME's spectrum.