Ozone layer thickness

Data available indicate that ultraviolet-B radiation (UVB) levels have increased significantly over the past 10-15 years at mid-latitude areas of the northern and southern hemispheres (Crutzen, 1992 Kerr McElroy, 1993 Madronich etal., 1995 Wardle etal., 1997). These increases in UVB are linked to reductions in stratospheric ozone (Kerr McElroy, 1993 Madronich etal., 1995 Wardle etal., 1997). Severe seasonal reductions in ozone layer thickness have been recorded over the Arctic (Fergusson Wardle, 1998 Goutail etal., 1999). 

Ozone s presence in the atmosphere (amounting to the equivalent of a layer 3 mm thick under ordinary pressures and temperatures) helps prevent harmful ultraviolet rays of the sun from reaching the earth s surface. Pollutants in the atmosphere may have a detrimental effect on this ozone layer. Ozone is toxic and exposure should not exceed 0.2 mg/m (8-hour time-weighted average - 40-hour work week). Undiluted ozone has a bluish color. Liquid ozone is bluish black and solid ozone is violet-black. 

Measurements of ozone (O3) concentrations in the atmosphere are of particular importance. Ozone absorbs strongly in the ultraviolet region and it is this absorption which protects us from a dangerously high dose of ultraviolet radiation from the sun. The vitally important ozone layer lies in the stratosphere and is typically about 10 km thick with a maximum concentration about 25 km above the surface of the earth. Extreme depletion of ozone in a localised part of the atmosphere creates what is known as an ozone hole. 

The ozone (03) layer aver the southern hemisphere stratosphere in August 2007. The bar at the bottom indicates the color-coding used to indicate the thickness of the layer. The thickness is measured in Dobsons (= 0.01 mm thick). The normal ozone layer for the stratosphere is 360 Dobsons (color coded as green). The ozone "hole" shown in pink is 200-220 Dobsons. The "hole" will increase when another reading is taken in September. 

Due to its gaseous nature it may have an effect on the stratospheric ozone layer [281, 402, 404]. After injection into soil for fumigation, methyl bromide rapidly diffuses through the soil pore space to the soil surface and then into the atmosphere [159,162,163,405,406]. Since a plastic sheet typically covers the soil surface, the rate of emission into the atmosphere depends upon the thickness and density of the plastic, if other conditions are the same [159, 406]. Other routes of disappearance from soil include chemical hydrolysis, methylation to soil organic matter through free radical reactions, and microbial degradation [ 136,159,405,407]. Several reports appeared on the study of the microbial transformations of methyl bromide, summarized as follows ... 

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. 

Thickness of the stratospheric ozone layer over Halley Bay, Antarctica, measured each October over a 40-year period. Notice the rapid decline that begins in the mid-1970s. 

Recently much research has been devoted to the possible climatic consequences of man-induced changes in atmospheric compounds and landscape features. Examples are increases in the concentration of C02 and other greenhouse gases and dust or the thickness of the ozone layer, as well as large-scale deforestation and desertification. Opinions vary as to the... 

This reaction is slow, and if it were the only mechanism for ozone loss, the ozone layer would be thicker than it really is. Certain trace chemical species, mainly free radicals such as the oxides of nitrogen (NO and NOj), atomic hydrogen (H ), oxygen species ( OH and HO ), and chlorine species (Cl, CIO and CIO ) are responsible for catalyzing the recombination reaction. The thickness of the ozone layer is then the result of a competition between the photodissociation and recombination mechanisms. 

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. 

Besides the dust clouds, changes of contents of carbon dioxide and certain other components in the air can also affect the mean temperature of the atmosphere close to the earth s surface. The assumed increase of the surface temperature induced by increasing concentration of certain trace components of the atmosphere is shown in Table 5.6. The problem of effects of increasing CO2 emissions is discussed in more detail in Section 5.2.4.3, where possible effects of CF2CI2 and CFCI3 on the thickness of the ozone layer in the stratosphere are also considered. 

Reduction in thickness of the ozone layer permits increased levels of damaging ultraviolet B radiation to reach the earth s surface. The potential medical consequences of this are shown in Table 1 and have been comprehensively reviewed elsewhere (3-5). 

The concentration of ozone in the stratosphere is quite small. If all of the ozone there were subjected to the amount of atmospheric pres.siire pre.sent at Earth s surface, the ozone layer would be only 3 millimeters thick instead of 10 kilometers Nevertheless, this ozone layer absorbs more than 95 percent of the ultraviolet radiation that comes to our planet from the sun. It is the safety blanket of Planet Earth. 

The Royal Academy of Sciences in Sweden which awards this prize indicated that, by explaining the chemical mechanisms which alter the thickness of the stratospheric ozone layer, the three scientists had contributed towards saving the world from a global environmental problem which was likely to have catastrophic consequences for all mankind. 

How thick would the stratospheric ozone layer be if it were compressed to atmospheric pressure at Earth s average temperature ... 

If the ozone layer over the US were compressed into a layer of temperature 0°C at 1 atmosphere pressure, it would have a thickness of only 3 mm. As a 0.01 mm thickness of an ozone layer is defined to be 1 Dobson Unit (DU), the average thickness of the ozone layer over the US is about 300 DU. 

Rowland, Molina, and Paul Crutzen (a Dutch chemist at the Max Planck Institute for Chemistry in Germany) were awarded the 1995 Nobel Prize for chemistry. As the Royal Swedish Academy of Sciences noted in awarding the prize, "By explaining the chemical mechanisms that affect the thickness of the ozone layer, these three researchers have contributed to our salvation from a global environmental problem that could have catastrophic consequences."... 

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