The Dobson Unit serves as the standard unit of measurement for quantifying the total amount of a trace gas within a vertical column of the Earth’s atmosphere. It is primarily applied to the study of atmospheric ozone, which is highly dispersed at various altitudes. This standardized unit allows scientists to make global comparisons of ozone concentrations across different locations and over long periods of time.
Defining the Dobson Unit
The Dobson Unit (DU) represents the total number of ozone molecules overhead in an atmospheric column extending from the ground to space. This measurement is derived by imagining all the ozone in that column is compressed into a single, pure layer at the Earth’s surface. The unit is named after Gordon Dobson, one of the first scientists to investigate atmospheric ozone in the 1920s.
One Dobson Unit is precisely defined as a layer of pure ozone 0.01 millimeters thick at standard temperature and pressure (STP). STP is set at 0 degrees Celsius and 1 atmosphere (average sea-level pressure). Therefore, 100 DU of ozone would form a layer just one millimeter thick if compressed under these conditions.
The Ozone Layer and DU’s Purpose
The Dobson Unit is necessary because the ozone layer is not a distinct, measurable band but rather a region in the stratosphere, approximately 6 to 30 miles above the surface, where ozone concentration is highest. The DU measures the Total Column Ozone (TCO), which is the cumulative amount of ozone in a column stretching from the ground to the top of the atmosphere. This integrated measurement quantifies the overall effectiveness of the atmospheric shield against harmful solar ultraviolet (UV) radiation.
The vast majority of the ozone measured in Dobson Units is found in the stratosphere, where it absorbs biologically damaging UV-B radiation. The single TCO value expressed in DU provides a standardized metric to track the health of this planetary shield. The measurement includes both stratospheric ozone and the smaller amounts of ozone found near the ground, which is a component of smog.
Measuring Ozone Concentration
Scientists use specialized instruments to acquire TCO values, beginning with the original Dobson spectrophotometer. This ground-based instrument measures the intensity of solar ultraviolet radiation. It works by comparing the ratio of two different UV wavelengths: one strongly absorbed by ozone and one that is not. The difference in the measured intensity ratio is then used to calculate the amount of ozone present overhead.
Modern monitoring includes automated instruments and satellite-based sensors that provide daily global coverage. The Total Ozone Mapping Spectrometer (TOMS) was a predecessor to current instruments like the Ozone Monitoring Instrument (OMI), launched aboard NASA’s Aura satellite in 2004. OMI uses hyperspectral imaging to observe solar backscatter radiation, continuing the long-term record for total ozone and providing high-resolution global data.
Interpreting Dobson Unit Values
The interpretation of Dobson Unit values connects this measurement directly to environmental phenomena, particularly the state of the ozone layer. The worldwide average TCO is approximately 300 DU, which translates to a pure ozone layer about three millimeters thick at the surface. Values across the globe range between 230 and 500 DU, with concentrations naturally lower near the equator and increasing toward the poles.
The most significant interpretation of low DU values relates to the “ozone hole,” defined by a severe reduction in stratospheric ozone over Antarctica. Scientists established a threshold of 220 DU to define the boundary of the ozone hole, as values below this level were not observed before 1979. During the austral spring, ozone concentrations within the hole often drop significantly lower, sometimes reaching averages around 100 DU. Geographical and seasonal factors influence these readings, and the Antarctic ozone depletion is a clear example of a seasonally recurring low-value event.