Oxygen in the Atmosphere
The content of oxygen in the atmosphere represents about ~1015 tons. In terms of the total oxygen content of the atmosphere - ocean - earth, the atmospheric pool represents the smallest quantity. However, it this atmospheric oxygen that is responsible for most of the chemical reactivity on earth and also the protective UV screening Ozone layer. |
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Oxygen concentration within the Atmosphere depend primarily on the atmospheric pressure. The higher up in elevation the lower the atmospheric pressure and the lower the [O2]. The oxygen concentration with elevation can be calculated from the Barometric formula:
where P0 = sea level Pressure (~1000 hPa), M = mass of 1 mol of air (~0.029 kg mol-1), g = gravity (9.8 ms-2), z = vertical height (m), R = gas constant (8.314 J K-1 mol-1) and T = temperature in kelvin. The pressure then of 1000 hPa is the same as 1 bar pressure and corresponds to 21% O2 and 0.1 bar is 2.1% oxygen, etc. The Barometric formula is a reliable estimate - but not perfectly accurate - measure of oxygen concentration. The figure below shows the oxygen concentration with elevation as well as the temperature. |
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In terms of human physiology reduced oxygen concentrations (<15% oxygen) start to become problematic for coordination and comprehension. This happens at elevations above 3,000 m (10,000 feet) and is the reason for airline pressurization and the carrying of oxygen when climbing the tallest mountains of the world. |
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Oxygen as an indicator of Global ChangeOver the last few decades very accurate measurements of the oxygen vs nitrogen concentration have been made. These ratios expressed as parts per million (per meg) have changed (relative to an air standard) and are indicative of seasonal and long term changes in the atmosphere. The experiment is possible based on the valid assumption that the N2 concentration is unchanged in the atmosphere. The figure below was data collected at Cape Grim Tasmania ( Australia). The same data collected in a northern latitude shows a 180 degree shift of phase. per meg = [ ( O2 / N2 ) / ( O2 / N2 )ref -1 } x 106 The strong diurnal cycle is in phase with the seasons depend on the sources and sinks of CO2 and O2. One factor contributing to this is the natural pumping in the ocean of CO2 and O2 from the northern hemisphere to south (thermohaline circulation). Another natural factor is the seasonal exchange of CO2 and O2 (photosynthesis) with the land biota and that of the surface ocean (see Geochemical Cycling). However, there is a striking decrease in the O2/N2 gradient and this is due to fossil fuel burning. With ~21% O2 in the atmosphere there no concern that the oxygen will run out, however, CO2 levels have doubled and this leads to significant changes. This may not sound much but CO2 concentrations are higher today than they have been in the last 120,000 years. |
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The above data reveal that a 100 meg decrease in the O2 / N2 ratio over an approximate 7 year period. The expression of O2/N2 is given in units of per meg and corresponds to an equivalent of 1 ppm O2 = 4.77 meg. This means the figure above translates into a 3 ppm decrease in the overall concentration of the oxygen in the atmosphere (0.0003%). This in turn equates to an equivalent of 3 x 109 tons of O2 (9.4 x 1013 mols ) every year that are consumed due to consumption of fossil fuels. |
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Ozone |
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Ozone chemistry is vital in protecting plant and animal life from the damaging effects of ultraviolet radiation. The ozone layer is formed at elevations 100-200 km due to a combination of temperature, [O2] and effects of ionising UV radiation. The ozone layer itself has strong electronic absorption 240-270 nm (Electronic). |
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Ozone chemistry also has a far greater specifics based on the excited states, see <Electronic Configuration> for details on the excited states, and kinetics and lifetimes of these states play an important role in the determination of the products. In industrial times pollutants such as CFC's and Br have been major players in the destruction of this fragile layer. |
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