Dobson Units (DU) are used to measure :

Thickness of ozone layer

A Dobson Unit (DU) serves as the fundamental unit of measurement in ozone research. It is defined as the equivalent of 0.01 millimeters of ozone thickness under standard temperature and pressure (STP) conditions. When expressing the thickness of the ozone layer, scientists use Dobson units to convey how thick it would be if it were compressed within Earth's atmosphere.

Ozone in the atmosphere isn’t all packed into a single layer at a certain altitude above the Earth’s surface; it’s dispersed. Even the stratospheric ozone known as “the ozone layer” is not a single layer of pure ozone. It is simply a region where ozone is more common than it is at other altitudes.

The average amount of ozone in the atmosphere is roughly 300 Dobson Units, equivalent to a layer 3 millimeters (0.12 inches) thick—the height of 2 pennies stacked together. What scientists call the Antarctic Ozone “Hole” is an area where the ozone concentration drops to an average of about 100 Dobson Units. One hundred Dobson Units of ozone would form a layer only 1 millimeter thick if it were compressed into a single layer, about the height of a dime.