Carbon dioxide. Chapter atmospheric concentration

Figure 1.1 shows the increase in the concentration of carbon dioxide in the atmosphere since the start of fossil fuel use. During the summer in the northern hemisphere, the carbon dioxide concentration decreases because of plant photosynthesis. The decrease is in the range of 5 to 15 parts per million. In the fall, this carbon dioxide is largely returned to the atmosphere. The figure shows the average for the year. Values beyond the year 2000 are projected as a simple continuation of current trends. The Kyoto Treaty line will be addressed later in this chapter. 

We have to emphasize that the correct prediction of the future COz concentrations is one of most important tasks of atmospheric science at present. This is explained by the fact that the C02 content of our atmosphere regulates, among other things, the radiation balance of the Earth-atmosphere system by absorbing infrared radiation emitted by the surface. Thus, we cannot exclude the possibility that the increase of the carbon dioxide concentration may cause inadvertent climatic variations in the future (see Chapter 6). 

Moist tropical forests account for a substantial amount of global plant productivity. And several lines of evidence suggest that they may be sequestering signiheant amounts of anthropogenically released carbon at the present time. But there are also indications that the productivity of many of these forests is limited by low phosphorus availability. This has led to suggestions that moist tropical forests may be constrained in their ability to increase their growth rates in response to increases in atmospheric carbon dioxide concentrations. This notion is examined in this chapter. 

The low solubility of methane in water limits its diffusive transport in the flooded soil, and most methane is oxidized to carbon dioxide. The aerenchyma of plants mediates the transport of air (oxygen) to the roots and methane from the anaerobic soil to the atmosphere. The flux of gases in the aerenchyma depends on concentration and total pressure gradients and internal structure, including openings of the aerenchyma (see Chapter 7 for details). 

As noted in Chapter 2, the minimum allowable oxygen concentration in an oxygen-deficient atmosphere depends on the nature of the diluting gas. Thus, because of the peculiar role of carbon dioxide in the respiratory functions of the body, the oxygen concentration in the atmosphere must not drop below 19 vol. % this corresponds to a carbon dioxide content of 9 vol. % accordingly, air-line respirators and SCBA units must be used with relatively small carbon dioxide concentrations in the air. 

Carbon dioxide is not considered a pollutant in the same sense that nitrogen oxides, sulfur oxides, ozone, carbon monoxide, and other gaseous pollutants are. However, there is much concern that the increasing concentration of carbon dioxide in our lower atmosphere is having a major impact on our climate. In this chapter we will consider the following issues ... 

We depend on burning fossil fuels (coal, natural gas, or petroleum) for energy. We burn coal and natural gas to produce electricity, we burn gasoline in the internal combustion engine, and we burn natural gas, oil, wood, and coal to heat homes. In addition, industrial processes burn fuel to produce heat. As a result of all this burning of fossil fuels, the carbon dioxide level in the atmosphere has risen from 318 parts-per-million (ppm) in 1960 to 362 parts-per-million (ppm) in 1998. (For a discussion of the concentration unit ppm, see Chapter 11.) The excess carbon dioxide has led to an increase of about a half-degree in the average temperature of the atmosphere. 

In Chapter 5 we saw that, in terms of the Br0nsted-Lowry theory, acid-base reactions involve proton transfer. Another large and important group of chemical reactions, particularly in aqueous solutions, involves electron transfer these are referred to as oxidation-reduction (or redox) reactions. Redox reactions are involved (1) in photosynthesis, which releases oxygen into the Earth s atmosphere (2) in the combustion of fuels, which is responsible for rising concentrations of atmospheric carbon dioxide (3) in the formation of acid precipitation and (4) in many chemical reactions in Earth sediments. 

An important solute in water that is associated with both acidity and alkalinity is dissolved carbon dioxide, CO2, which is almost always present in natural water from contact with atmospheric air or as a product of microbial biodegradation of organic matter. Present in the atmosphere at a level of about 390 ppm of dry air (and increasing due to release from the anthrosphere at a rate of almost 2 ppm per year), atmospheric CO2 gas makes rainwater from even a totally unpolluted atmosphere slightly acidic. At 25 C, in water in equilibrium with unpolluted air containing 390 ppm carbon dioxide, the concentration of dissolved C02(aq) is 1.276 x 10" mol/L (M), a value that is used for subsequent calculations in this chapter. 

The possible effects of a decrease in stratospheric ozone concentrations include (1) an increase in skin cancers, (2) an increase in damage to crops and trees, (3) stratospheric cooling leading to large-scale climate changes, and (4) additional production of tropospheric ozone as more and more uv radiation is able to penetrate into the atmosphere closer to Earth. In addition, CFCs themselves, like carbon dioxide and other compounds mentioned in Chapter 11, are greenhouse gases. 

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