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

What does this program measure?

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).

How does this program work?

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.

Why is this research important?

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.

Are there any trends in the data?

The plot below shows the running Dobson record at MLO.

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

MLO Yearly Dobson Cycle
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)

Monthly Average Ozone at MLO

How does this program fit into the big picture?
What is it's role in global climate change?

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.

Comments and References

Dobson Dome with Mauna Loa Observatory Sunset (Courtesy of Forrest M. Mims III)Dobson Dome with sunset at MLO
(courtesy of Forrest M. Mims III)

Lead Investigator(s):

Sam Oltmans
Robert Evans


MLO Contact(s):

David Nardini
808-933-6965 (x229)
Darryl Kuniyuki
808-933-6965 (x236)

Web Site(s)

Date Started


Related Programs

ESRL Dobson Ozone
Ozone / Water Vapor


Dobson Dome at MLODobson dome

Zenith sky hatchZenith sky hatch

R-Dial lensR-Dial (right) and lens (left)

Q1 and Q2 leversQ1 and Q2 levers

Laser pointer, sun director, r-dialLaser pointer (right), sun director (middle), r-dial (left)

Electronics BoardElectronics board

Dobson Instrument Front ViewSun director