Dobson unit

DU

A unit to measure the total amount of ozone in a vertical column above the Earth’s surface (total column ozone). The number of Dobson units is the thickness in units of 10^–5 m that the ozone column would occupy if compressed into a layer of uniform density at a pressure of 1,013 hPa and a temperature of 0°C. One DU corresponds to a column of ozone containing 2.69 × 1,020 molecules per square metre. A typical value for the amount of ozone in a column of the Earth’s atmosphere, although very variable, is 300 DU.

Definition courtesy of IPCC AR4.