DOAS is an acronymn for Differential Optical Absorption Spectroscopy. The principle of DOAS is that variations in optical absorption coefficients are used to measure the absorbing compounds. This is the principle of all absorption spectroscopy including measurements in the infrared and microwave regions. The expression DOAS, however, is understood to apply only to working in the visible and ultraviolet. DOAS has been used to measure a restricted but important group of atmospheric molecules and radicals with high sensitivity. This includes H2CO, OH, NO3, SO2, O3 and NO2. DOAS is well reviewed in a recent article by Platt. (Differential Optical Absorption Spectroscopy by U. Platt, pages 27 through 84 in Air Monitoring by Spectroscopic Techniques, M. Sigrist, editor, John Wiley & Sons, Inc., 1994.)

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