Dobson Unit (DU): a measurement of column ozone levels
If 100 DU of ozone were brought to the Earth's surface, it would form a layer 1 millimeter thick. In the tropics, ozone levels are typically between 250 and 300 DU year-round. In temperate regions, seasonal variations can produce large swings in ozone levels. For instance, measurements in Leningrad have recorded ozone levels as high as 475 DU and as low as 300 DU. These variations occur even in the absence of ozone depletion, but they are well understood. Ozone depletion refers to reductions in ozone below normal levels after accounting for seasonal cycles and other natural effects.
The standard way to express ozone amounts in the atmosphere. One DU is 2.7 x 10 1016 ozone molecules per square centimeter. One Dobson unit refers to a layer of ozone that would be 0.001 cm thick under conditions of standard temperature (0 degree C) and pressure (the average pressure at the surface of the Earth). For example, 300 Dobson units of ozone brought down to the surface of the Earth at 0 degree C would occupy a layer only 0.3 cm thick in a column. Dobson was a researcher at Oxford University who, in the 1920s, built the first instrument (now called the Dobson meter) to measure total ozone from the ground.
NASA Earth Observatory
Dobson Units - Measurement unit for determining the total amount of ozone present in a vertical column of air above the surface of the earth. An air layer at atmospheric pressure of 1013 hPa and temperature of 298 K which measures 1 mm in thickness and is equivalent to 100 dobson units.
[Science; v266; 217; 1994.]
Atmospheric Chemistry Glossary |
