Stratospheric ozone concentration

Depletion of the Ozone Layer. As a constituent of the atmosphere, ozone forms a protective screen by absorbing radiation of wavelengths between 200 and 300 nm, which can damage DNA and be harmful to life. Consequently, a decrease in the stratospheric ozone concentration results in an increase in the uv radiation reaching the earth s surfaces, thus adversely affecting the climate as well as plant and animal life. Pot example, the incidence of skin cancer is related to the amount of exposure to uv radiation. 

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. 

In recent years, there has been considerable concern over reductions in stratospheric ozone concentrations resulting from human activities. Since stratospheric ozone is the primary screen of solar ultraviolet radiation, ozone reduction would increase ultraviolet radiation (UV-B, wavelengths between 280 and 320 nm) reaching the earth s surface. 

Increasing carbon dioxide concentration and decreasing stratospheric ozone concentration of the atmosphere may alter global radiation fluxes. Presumably a primary result of more carbon dioxide in the air will be warming. While incoming solar radiation is not absorbed... 

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). 

The Antarctic ozone holes typically develop at altitudes of 7.4-16 mi (12-25 km). The average decreases in springtime stratospheric ozone concentrations over Antarctica have been 30-40%. However, in some years the decrease in ozone has been over 60%. In the worst years, the ozone concentration over Antarctica was only 120 DU. In October 1999, the ozone concentrations were less than 50% of what they were in the 1960s. 

In parallel with their own research programs, the manufacturers, through the FPP also jointly fimded research to study the atmospheric chemistry of CFCs in order to assess the extent of any risk they might pose. Independent research workers of imiversities and research institutes worldwide were contracted to measure the rates of reactions, which were essential input data for the complex computer models needed to predict the rate of ozone depletion. This value could not be measured directly in the 1970s because the large daily and seasonal fluctuations in stratospheric ozone concentrations swamped the modest depletion expected from CFCs. 

Although this simple mechanism accounts for the presence of O3 in the stratosphere, close examination of the stratospheric ozone concentration and the rate constants indicates that the loss term for ozone (reaction 4) is not large enough to balance the rate at which ozone is produced. 

In many respects, N2O is analogous to CO2. It has the same linear structure, the same number of electrons (isoelectronic), and a similar (low) reactivity however, CO2 is more soluble in water as a result of the acid-base reaction of CO2 and water. It is the low reactivity of N2O that results in a long tropospheric lifetime, and therefore its eventual transport to the stratosphere, where it is believed to be a primary control on the concentration of ozone in the stratosphere. Concentrations of N2O in the troposphere are increasing, and this has raised concerns that anthropogenically produced N2O could decrease stratospheric ozone concentrations (McEIroy et al., 1976 Soderlund and Svensson, 1976 Weiss, 1981). 

The polymer lifetime is also influenced indirectly by stratospheric ozone and its photochemistry. Terrestrial fluxes of the solar UV radiation are changing with the stratospheric ozone concentration. Stratospheric ozone depletion accounting for global ozone losses of 2.7 1.4% per decade may have increased to about... 

The NO produced through reaction (41) allows the chain of reactions leading to catalytic destruction of stratospheric ozone to be initiated. N2O is the main source of NO in the stratosphere and is, therefore, the important natural regulator of stratospheric ozone. An increase in N2O most probably affects stratospheric ozone concentrations. At present, there is too much uncertainty to predict the extent of this destruction but in any case, the role of the Asian countries in magnification of this process is significant. 

The relationship between UV-B radiation and skin cancer is by now well documented and accepted. The US EPA has estimated that each 1 % decrease in stratospheric ozone concentration will result world wide in a 2% rise in cutaneous malignant melanoma. Among the Asian and Pacific region, people of many countries, such as Australia, New Zealand, South Pacific Islands, Indonesia, Malaysia, Philippines, Thailand, etc, are the most vulnerable to the exposure of UV-B radiation. For example, between 1980 and 1991, when the growth of CFCs production was the maximum, melanoma cancer registration rates in New Zealand increased by 22% (ESC AP, 1995). 

Stratospheric ozone concentrations are maintained naturally by a simple sequence of reactions ... 

Quantities of airborne particles in industrialized regions of the Northern Hemisphere have increased markedly since the Industrial Revolution. Atmospheric particles (aerosols) arise both from direct emissions and from gas-to-particle conversion of vapor precursors. Aerosols can affect climate and stratospheric ozone concentrations and have been implicated in human morbidity and mortality in urban areas. The climatic role of atmospheric aerosols arises from their ability to reflect solar radiation back to space and... 

FIGURE 5.5 Comparison of stratospheric ozone concentrations as a function of altitude as predicted by the Chapman mechanism and as observed over Panama (9°N) on November 13, 1970. 

If stratospheric ozone concentrations remained constant, the 10% increase in tropospheric ozone would increase the total column abundance of ozone by about 1%. Thus the additional tropo.spheric ozone is believed to have counteracted only a small fraction of the stratospheric loss, even if the trends observed over Europe are representative of the entire northern midlatitude region. 

Since most (over 99%) of the odd oxygen is in the form of O3, the Chapman mechanism predicts that local stratospheric ozone concentrations are proportional to the square root of the O2 photolysis rate. At night, reactions 4.1 and 4.3 cease, but reactions 4.2 and 4.4 persist. Atomic oxygen concentrations fall rapidly, with the net effect that reactions 4.2 and 4.4 more or less balance each other so that diurnal variations in stratospheric O3 are small. 

In addition, stratospheric ozone concentrations appear to be declining, due to photolysis by shortwave UV of volatile chlorine-containing compounds such as CFCI3 and its relatives (Reaction 1.48) the Cl atoms produced in this reaction scavenge O atoms (Reaction 1.49) and... 

These compounds reach the stratosphere because they are so unreactive in all tropospheric processes, including reactions with HO- to which they are virtually inert. Measurements of atmospheric ozone in Antarctica (Stolarski et al., 1986), in the Arctic (Zurer, 1990), and even in the temperate latitudes (Watson et al., 1988) all point to a decrease in stratospheric ozone concentrations. Recent attempts to diminish the use of ozone-depleting compounds on a global basis appear to have been successful however, only minor effects on the rate of ozone destruction are likely to be observed for many years to come. 

This day had the lowest stratospheric ozone concentration yet recorded. One Dobson unit corresponds to 2.69 X 10 ozone molecules in a 1 cm column of atmosphere. 

Scientists, who had no inkling of what they were about to discover, began measuring stratospheric ozone concentrations over 80 years ago using relatively unsophisticated instrumentation. Eventually, as a credit to G. M. B. Dobson, one of the scientists who invented an early instrument used for these measurements, the unit of ozone concentration in the atmosphere became... 

Dobson Unit A measure of stratospheric ozone concentration 1 Dobson unit is 1 ppb of ozone in air. 

---