Ozone Measurement

1 Dobson Unit (DU) is defined to be 0.01 mm thickness at STP (standard temperature and pressure). Ozone layer thickness is expressed in terms of Dobson units, which measure what its physical thickness would be if compressed in the Earth's atmosphere.

  In those terms, it's very thin indeed. A normal range is 300 to 500 Dobson units, which translates to an eighth of an inch-basically two stacked pennies. In space, it's best not to envision the ozone layer as a distinct, measurable band. Instead, think of it in terms of parts per million concentrations in the stratosphere (the layer six to 30 miles above the Earth's surface).

Environment Canada

The total ozone maps are based on ground-based measurements available from the World Ozone and Ultraviolet Radiation Data Centre. Preliminary near real-time data from ground-based observations were also used for the most recent maps. Total ozone values are given in Dobson Units. The numbers represent observations taken from ground stations situated at the bottom left corner of the number.

Maps of deviations represent total ozone deviations from the 1978-1988 level estimated using Total Ozone Mapping Spectrometer (TOMS) data for all areas except the Antarctic and from the pre-1980 level estimated using Dobson data over the Antarctic.

Over areas with poor data coverage adjustments are made according to TOMS on Nimbus-7, Meteor-3, ADEOS and Earth Probe satellites. Over the polar night area Dobson and Brewer moon observations and/or NOAA's TIROS Operational Vertical Sounder (TOVS) satellite data are used. TOVS data are also used when the more reliable TOMS data are not available. The mapping algorithm is similar to those used by the WMO Ozone Mapping Centre.

NOAA's TIROS Operational Vertical Sounder(TOVS) 

NOAA's TIROS Operational Vertical Sounder(TOVS) is a suite of three instruments: the Microwave Sounding Unit(MSU), the High resolution Infrared Radiation Sounder(HIRS), and the Stratospheric Sounding Unit(SSU). Each instrument measures radiation emmitted by the Earth at several different wavelengths. The HIRS channel 9 measures Earth's emmitted infrared radiation at 9.7 microns (10^-6 meters). This is a "window channel" meaning that the radiation measured by the HIRS instrument is emmited from the earth's surface (as opposed to radiation being emmitted at other levels of the earth's atmosphere). The amount of radiation reaching the HIRS instrument is dependant upon how much ozone is in the earth's atmosphere (less ozone = more radiation). Therefore, the TOVS Total Ozone algorithm uses this channel (along with information from other HIRS channels) to estimate the total amount of ozone in the earth's atmosphere. The greatest contribution of the emmitted radiation occurs in a region between 200 hPa and 30 hPa (13km to 27km). This "lower stratosphere" region is below the levels where the greatest contribution to the total ozone amount occurs(50hpa to 10hPa or 20km to 30km). Thus the ozone amount measured by the TOVS Total Ozone algorithm is not a true measure of the "total" amount of ozone in the earth's atmosphere. Rather it is a better measure of the ozone amount in the lower stratosphere. To obtain a "total" ozone amount, the TOVS Total Ozone algorithm adjusts the lower stratosphere ozone amount by a climatological amount that is variable with season and latitude.

This is in contrast with satellite instruments which measure the amount of backscattered radiation at various ultraviolet wavelengths. Backscattered radiation levels at wavelengths where ozone absorbtion does and does not take place are compared with the same wavelenghts measured directly from the sun to derive a "total ozone" amount in the earth's atmosphere. This methodology is used by the NASA TOMS and the NOAA SBUV/2 ozone monitoring programs. This methodology provides a truer measure of the total ozone amount in the earth's atmosphere. One drawback is that this method uses "backscattered" sunlight. Which means that data cannot be retrieved in the earth's shadow or polar night regions.

The TOVS Total Ozone algorithm can determine ozone amounts at all times since it is derived from the Earth's emmitted infrared radiation. There are drawbacks to the TOVS infrared methodolgy though. When the earth's surface is either too cold (e.g., the high Antarctic Plateau) too hot (e.g., the Sahara desert) or too obscured (e.g., by heavy tropical cirrus clouds) the accuracy of this methodolgy declines.