Dobson Units are the unit of measurement for ozone concentration in the atmosphere:

1 Dobson Unit (DU) is 0.01 mm thickness at STP (O ^oC and 1 ATM) and spreads out evenly over the area, it would form a slab approximately 3mm thick corresponding about 300DU. The unit is named after G.M.B.Dobson, one of the first scientists to investigate atmospheric ozone (1920-1960). He designed the Dobson spectrometer which it is the standard instrument used to measure ozone from the ground. The Dobson spectrometer measures the intensity of solar UV-radiation at 4 wavelengths, two of which are absorbed by ozone and two of which are not.