Dobson units

These maps ot stratospheric ozone concentration over the North Pole snow how the ozone was depleted from 2001 to 2003. Red areas represent ozone concentrations greater than 500 Dobson units (DU) the concentrations decrease through green, yellow, and blue, to purple at less than 270 DU. Normal ozone concentration at temperate latitudes is about 350 DU. 

The total ozone integrated through a column in the atmosphere from the earth s surface is often used as a measure of stratospheric ozone, since as seen in Fig. 12.1, about 85-90% of the total ozone is found in this region. Dobson units are used to express the amount of total column ozone. One Dobson unit (DU) is the height of the layer of pure gaseous ozone in units of 10"5 m that one would have if one separated all of the atmospheric 03 and compressed it into a layer at 1 atm and 273 K. That is, 100 DU is equivalent to a layer of pure ozone of thickness of 1 mm. 

FIGURE 12.16 Average total column ozone measured in October at Halley Bay, Antarctica, from 1957 to 1994 [DU = Dobson units (see text)] (adapted from Jones and Shanklin, 1995). 

These phenomena are illustrated in Fig. 14.21a, which shows a model calculation of the change in global surface temperature when 10 Dobson units (DU) of 03 (10 DU = total column 03 equivalent to a layer of thickness 0.1 mm at 273 K and a pressure of 1 atm see Chapter 12.A) are added one at a time to each of 33 vertical layers of the atmosphere (assuming no feedbacks). An increase in the global surface temperature is predicted when the ozone is added in layers up to... 

Mean atmospheric 03 at Halley in Antarctica in October. Dobson units are defined in Problem 18-13. [From J. D. Shanklin, British Antarctic Survey, http //www.antarctica.ac.uk/mel/ids/ozone/]... 

The model was initially compared with LOWTRAN 7 (5) results and with Brewer spectrophotometer measurements carried out at Lisbon in 1990. LOWTRAN-MESTRad results show a reasonable good agreement over 290-400 nm range with a mean difference of about 10%. Figure 4 shows de Model/LOWTRAN ratios for direct, diffuse and global spectral irradiances computed for the same conditions of ozone (332 Dobson Units), solar zenith angle (45.2°) and aerosol optical depth (0.0). 

Figure 4. MESTRad/LOWTRAN ratios for direct, diffuse and global spectral irradiances computedfor the same conditions of ozone (332 Dobson Units), solar zenith angle (45.2 °) and aerosol optical depth at 500 nm...

Figure 6. Measured (Brewer) and computed (original and corrected) global irradiance spectra for Lisbon on l July of 1990 (Z=48.3°, 03=312 Dobson units and Tm> =0.3.

Figure 8.a) Amplitudes of first three harmonics (m=l, 2,3 correspondingly columns from left to right) of total ozone for period December 1991- May 1992 in Dobson units [DU], and b) of net ozone healing rates in March at height 53 km (m=-l /solid/, m=2 /dotted/, m=3 /dashed/ lines) in K. degree per day [K/day]. 

Figure 11.5 This diagram comes from NASA s ozone monitoring programme TOMS (Total Ozone Mapping Spectrometer). The ozone hole over the Antarctic (shown in purple and pink on the diagram) is largest in the Antarctic spring. Note Dobson Units are a measure of the total amount of ozone in a vertical column from the ground to the top of the atmosphere.

Ozone forms in the upper stratosphere from molecular oxygen under the influence of UV solar radiation. In the lower stratosphere and troposphere, the source of ozone is the decomposition of nitrogen dioxide under the influence of UV and visible radiation. The formation of the vertical profile of ozone concentration is connected with its meridional and vertical transport. The general characteristic of this profile is the total amount of ozone measured by the thickness of its layer given in Dobson units (1 DU = 0.001 cm). 

A Dobson unit (DU) is a measure of the total amount of ozone over our heads 1 DU is equivalent to an ozone thickness of 0.01 mm at 0°C and 1 atm pressure. What is the total mass of ozone present in the atmosphere if the average overhead amount is 200 DU You may assume 0°C and 1 atm pressure. 

Lowest values of ozone measured from satellite each year in the ozone hole. Global average ozone is about 300 Dobson Units. Before 1980 ozone less than 200 Dobson Units was rarely seen. In recent years ozone near 100 Dobson Units has become normal in the ozone hole. Ozone in the year 2002 ozone hole was higher than we have come to expect because of unusually high temperatures in the Antarctic stratosphere (from NASA data)... 

Growth of the average area of the ozone hole from 1979 to 2003. The ozone hole is defined as the area for which ozone is less than 220 Dobson Units, a value rarely seen under normal conditions. It shows that the ozone in this area hardly occurred at all in 1980, but by year 2000 covered an area larger than North America, 26.5 million km (from NASA data)... 

Typically, stratospheric ozone (O3) concentrations are about 0.2-0.4 ppm (parts per million), compared with about 0.03 ppm in unpolluted situations close to ground level in the troposphere. Stratospheric ozone concentrations are also measured in Dobson units (DU). A Dobson unit is equivalent to the amount of ozone that, if accumulated from the entire atmosphere and spread evenly over the surface of the earth at a pressure of one atmosphere and a temperature of about 68°F (20°C), would occupy a thickness of 10 mm (0.01 m or 0.4 in). Typically, stratospheric zone occurs at a couceutratiou of about 350 DU, equivalent to a layer of only 3.5 mm (0.14 in). 

In the 1920s, G.M.B. Dobson began measuring the amount of ozone in the atmosphere. Although ozone is formed in the higher regions of the stratosphere, most of it is stored in the lower stratosphere, where it can be measured by instruments on the ground or in balloons, satellites, and rockets. Dobson measured levels of stratospheric ozone of more than 300 Dobson units (DU). His measurements serve as a basis for comparison with recent measurements. 

FIGURE 20.29 This false color image shows total stratospheric ozone amounts over the southern hemisphere for September 24, 2006, as recorded by the Ozone Monitoring Instrument (OMI) mounted on the Aura spacecraft. The dramatic depletion of the ozone layer over Antarctica is revealed with the help of the false color scale at the bottom of the figure. Ozone amounts are commonly expressed in Dobson units 300 Dobson units is a typical global average over the course of a year. The size of the Antarctic ozone hole was near a record high and the levels of ozone near a record low on this date. 

Stratospheric ozone is produced at maximum rates in equatorial regions, where solar radiation is most intense. Ozone does not really occur as a layer, but instead as a broadly distributed gas whose peak concentration occurs in midstratosphere. The total amount of ozone present in the atmosphere is small, typically between 200 and 400 Dobson units. A Dobson unit is the amount of ozone that, if gathered together in a thin layer covering Earth s surface at a pressure of 1 atm, would occupy a thickness of 1/100 of a millimeter (10 gm). The entire ozone shield, which protects life on Earth from damage by the UV-B radiation of the Sun (ultraviolet radiation in the 280-320 nm range), is equivalent to a layer of ozone only 2 to 4 mm thick at sea level pressure. 

Dobson unit The standard way to express ozone amounts in the atmosphere. One DU is 2.7 x 1016 ozone molecules per square centimeter. One Dobson unit refers to a layer of ozone that would be 0.001 cm thick under conditions of standard temperature (0°C) and pressure (the average pressure at the surface of the Earth). For example, 300 Dobson units of ozone brought down to the surface of the Earth at 0°C would occupy a layer only 0.3 cm thick in a column. Dobson was a researcher at Oxford University who, in the 1920s, built the first instrument (now called the Dobson meter) to measure total ozone from the ground. 

Fig. 3.6 Mean October levels of total ozone above Halley Bay (76°S), Antarctica, since 1957. The 1986 value is anomalous due to deformation of the ozone hole, which left Halley Bay temporarily outside the circumpolar vortex (a tight, self-contained wind system). Dobson units represent the thickness of the ozone layer at sealevel temperature and pressure (where 1 Dobson unit is equivalent to 0.01 mm). Data courtesy of the British Antarctic Survey. Inset shows seasonally averaged (Sep.-Nov.) ozone partial pressure at about 17 km at 70°S. Data courtesy of G. Konig-Langlo.

Ozone absorbs ultraviolet radiation from the sun while at the same time heating the gases in the stratosphere. Ozone depletion potential (ODP) is derived from its ability to deplete the ozone layer using trifluoromethane as a standard. Ozone absorbs short wavelength ultraviolet radiation from the sun in the stratosphere, it is measured in Dobson units (DUs). One DU is equal to the amount of ozone that would be 10 pm thick under standard temperature and pressure. The average DU has been about 300 for the last few decades however, this number has decreased in recent times. 

FIGURE 9.1 Ozone measurements in Dobson units per year. 

---