National Oceanic and Atmospheric Administration (NOAA), Earth System Research Laboratory (ESRL)

Total Column Ozone is the amount of ozone in the atmosphere above the station (MLO). As the vast majority (~90%) of the ozone is in the stratosphere, the value is representative of the thickness of the ozone layer. The value is expressed in Dobson Units (DU) – the thickness of a separate layer of pure zone at sea level temperature (15 degrees Celsius) and pressure. A representative value for MLO is 275 DU (a layer of pure ozone 2.75 millimeters thick).

The Dobson Spectrophotometer 76 measures how fast Ultraviolet (UV) radiation decreases in the 300-340 Nanometer wavelength range. The more ozone, the greater the difference in intensity between the longer and shorter wavelengths. The Dobson is measured three times per day on weekdays at MLO.

Ozone controls the amount of UV radiation reaching the earth’s surface. It contributes to the heat balance of the atmosphere, and can be used as an air mass tracer.

The plot below shows the running Dobson record at MLO.


The plot below shows the yearly Dobson Ozone cycle at MLO with the UV correlation.


The plot below shows a monthly average total ozone at MLO from 1964-2005. Similar data from other CMDL stations is available on a pdf document. (Download PDF)

Ozone is a major player in the radiative balance of the stratosphere. Long term changes in the amount and distribution of stratospheric ozone are both indicators and results of temperature and circulation changes.

Dobson Dome with sunset at MLO
(courtesy of Forrest M. Mims III)