Regular measurements of the ozone content of the atmosphere were initiated in the early 1920s by G.M.B. Dobson using spectrographic instruments. He built his first version of what is now known as the Dobson ozone spectrophotometer in 1927.(1-5)
It makes precise measurements of the relative intensity of sunlight at pairs of wavelengths in the UV spectrum; the total ozone column6 is deduced from these measurements and the known absorption coefficients of ozone at the wavelengths of measurement. Canada acquired a Dobson in 1948. Until the 1950s there were only about 20 regularly operating stations in the world (see figure below), most of which were located in the northern hemisphere. Some of these had already interrupted their measurement programs in the late 1950s.
At this time the reason for studying ozone was the hope that research into stratospheric motion, of which ozone was a useful tracer, would lead to improved models for forecasting the weather. It is for this reason that we have such a comprehensive data base of ozone measurements for the 1960s and 1970s, well before ozone depletion became a scientific issue. Systematic ozone observations using common observational methods and calibration procedures laid out by the International Ozone Commission in collaboration with the World Meteorological Organization (WMO) were started in preparation for the International Geophysical Year (1957–1958). These standard procedures formed the basis of the Global Ozone Observing System, which presently has more than 160 continuously operating stations. Three main types of instruments are used for routine total ozone observations: Dobson and Brewer spectrophotometers that can measure total ozone with up to 1% accuracy [e.g., Kerr et al.1988], and satellite instruments, such as the Ozone Monitoring Instrument (OMI) on board the NASA Aura satellite (7). Vertical profiles of ozone though the atmosphere are measured using ozone sondes (radio sondes using a small balloon to carry aloft an ozone sensor) and ozone LIDARs. The World Ozone and UV Radiation Data Centre, the international repository for all ground-based measurements of ozone, is housed at Environment Canada in Toronto.(8)
Total Ozone distribution
The main features of the global total ozone distribution were discovered during the first years of regular observations in the 1920s [Dobson et al. 1926, 1929, 1930]. The most important feature is a strong latitudinal gradient of total ozone, with lower values over the equator and tropics and higher values over middle and high latitudes.
This gradient has a well-pronounced annual cycle, reaching a maximum in spring and a minimum in fall. The annual cycle in total ozone at a given point thus has an amplitude that is a function of latitude, with maximum amplitudes found at about 60˚ north and south latitude. In the tropics seasonal variations are small, with ozone maxima in summer; in the equatorial region there are essentially no seasonal variations.
The explanation for this behaviour, also deduced by Dobson and confirmed by Brewer  from aircraft measurements of water vapour, is that the high values at extra-tropical latitudes are a result of transport from the tropical source region. Ozone is formed, primarily in the tropics at altitudes above about 30 km, via the action of ultraviolet light from the sun upon molecular oxygen. While the highest relative concentrations are found here, the more moderate values of 1–2 ppmv found at about 15 km actually make a much greater contribution to the total column ozone, because the density of the atmosphere is greater by an order of magnitude at the lower altitude. Mixing ratios in the lower stratosphere are higher at mid-latitudes and toward the poles.
This latitudinal distribution is made possible by the relatively long lifetime (months to years) of ozone in the lower stratosphere, and the Brewer-Dobson circulation that transports stratospheric ozone from the tropics toward the poles and downwards at high latitudes. This circulation is both strongest and most variable in winter, and the highest total column amounts and the most variability are found in the winter stratosphere below 20 km.
- Dobson’s original ozone spectrometer, 1926. – – Science Museum
- M. B. Dobson – March 1968 / Vol. 7, No. 3 / APPLIED OPTICS (pps. 387 – 405). http://www.esrl.noaa.gov/gmd/ozwv/dobson/papers/Applied_Optics_v7_1968.pdf
- A discussion of the instrument can be found here: ftp://ftp.cmdl.noaa.gov/dobson/Papers/
- A copy of the instrument handbook is available from the WMO: http://www.wmo.int/pages/prog/arep/gaw/documents/GAW183-Dobson-WEB.pdf
- Equivalent thickness of the ozone layer, expressed as a vertical column of pure ozone. The standard unit is the Dobson Unit, equal to 10-5 m at 0˚C and sea level pressure (STP). The global average of total ozone is approximately 300 DU, equivalent to a thickness of 3 mm.
- NASA Aura OMI page
- Home – World Ozone and Ultraviolet Radiation Data Centre (WOUDC) – [Meteorological Service of Canada – WOUDC]