A unit of atmospheric column abundance. One Dobson unit corresponds to approximately molecules per square centimetre. Multiplying a column decrement by an absorption cross section gives its contribution to a monochromatic optical-depth decrement.
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A Dobson Unit (DU) is a measure of the amount of ozone in a column of the Earth's atmosphere. It is defined as the number of ozone molecules in a column of air that is 1 square centimeter in cross-section and extends from the surface of the Earth to the top of the atmosphere, weighted to a standard pressure and temperature. Specifically, 1 Dobson Unit corresponds to a total column of ozone that would create a layer of pure ozone 0.