Composition of the atmosphere

In a mixture of gases like the troposphere, Dalton s law of partial pressure (Box 3.1) is obeyed. This means that individual gases in the atmosphere will decline in pressure at the same rate as the total pressure. This can all be conveniently represented by the barometric equation  

The total pressure of a mixture of gases is equal to the sum of the pressures of the individual components. The pressure-volume relationship of an ideal gas (i.e. a gas composed of atoms with negligible volume and which undergo perfectly elastic collisions with one another) is defined as  

R the gas constant and T the absolute temperature. Real gases behave like ideal gases at low pressure and we denote a mixture of gases (1, 2, 3) through the use of subscripts  

The relationship between partial pressure and gas-phase concentration explains why concentrations in the atmosphere are frequently expressed in parts per million (ppm) or parts per billion (ppb) (see Table 3.1). This is done on a volume basis so that 1 ppm means 1 cm3 of a substance is present in 106cm3 of air. It also requires that there is one molecule of the substance present for every million molecules of air, or one mole of the substance present for every million moles of air. This ppm unit is thus a kind of mole ratio. It can be directly related to pressure through the law of partial pressure, so at one atmosphere (1 atm) pressure a gas present at a concentration of 1 ppm will have a pressure of 10 6atm. 

It is well known that the atmosphere consists mostly of nitrogen (N2) and 02, with a small percentage of argon (Ar). The concentrations of the major gases are listed in Table 3.1. Water (H20) is also an important gas, but its abundance varies a great deal. In the atmosphere as a whole, the concentration of water is dependent on temperature. Carbon dioxide (C02) has a much lower concentration, as do many other relatively inert (i.e. unreactive) trace gases. Apart from water, and to a lesser extent C02, most of these gases remain at fairly constant concentrations in the atmosphere. 

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Viggiano A A and Arnoid F 1995 ion chemistry and composition of the atmosphere Afmosp/reric Electrodynamics ed H Voiiand (Boca Raton, FL CRC Press)... 

In a sense this subdivision of the composition of the atmosphere is arbitrary since some of the so-called contaminants are derived partly or wholly from natural sources. However, in that their concentrations vary appreciably within very narrow geographical limits, they may be distinguished from the contents of Table 2.8 (with the possible exception of water vapour). Table 2.6 lists those contaminants which are important from a corrosion standpoint. Excluded are contaminants found only in very specific locations, e.g. in the vicinity of a chemical works. The concentrations given are intended only to indicate general levels in the usual classification of environments and not to define a particular environment. 

The composition of the atmosphere to which components at high temperature may be exposed varies very widely, and most work on these aspects has accordingly been carried out in clean air. The aggressiveness of air is considerably enhanced by the presence of trace amounts of other reactive gases such as steam, carbon dioxide and sulphur dioxide thus the figures subsequently quoted may in fact be appreciably lower than those encountered in specific atmospheres. The data presented should, however, prove an adequate guide to the order of the effect to be expected. 

Changes in the composition of the atmosphere are perhaps our most valid indicators of the influence of human activities on global systems. The next best barometer may very well be loss of biodiversity due to habitat destruction. 

Debates over the validity of the Gaia hypothesis have focused on two very different issues. One concerns the degree to which the composition of the atmosphere and, hence, Earth s... 

Just as in the case for the hydrosphere, the atmosphere participates in all of the major biogeochemical cycles (except for phosphorus). In turn, the chemical composition of the atmosphere dictates its physical and optical properties, the latter being of great importance for the heat balance of Earth and its climate. Both major constituents (O2, H2O) and minor ones (CO2, sulfur, nitrogen, and other carbon compounds) are involved in mediating the amounts and characteristics of both incoming solar and outgoing infrared radiation. 

Role of composition in atmospheric physical process. The composition of the atmosphere plays a distinct set of roles in controlling and affecting certain physical processes of the atmosphere, most notably the thermal structure. 

It is now widely accepted that the compositions of the atmosphere and world ocean are dynamically controlled. The atmosphere and the ocean are nearly homogeneous with respect to most major chemical constituents. Each can be viewed as a reservoir for which processes add material, remove material, and alter the compositions of substances internally. The history of the relative rates of these processes determines the concentrations of substances within a reservoir and the rate at which concentrations change. Commonly, only a few processes predominate in determining the flux of a substance between reservoirs. In turn, particular features of a predominant process are often critical in controlling the flux of a phase through that process. These are rate-controlling steps. 

All deep waters of the ocean were once in contact with the atmosphere. Since over 95% of the total of all gases (except radon) reside in the atmosphere, the atmosphere dictates the ocean s gas contents. CO2 is also a special case because the ocean has high total CO2. As discussed in Chapter 7, the composition of the atmosphere is nearly constant horizontally. 

While there would seem to be a very large number of possible biogeochemical feedbacks, only a few have been identified and even these are not quantified. Two main classes of feedbacks can be defined those in which biota influence albedo, and those that involve changes in the composition of the atmosphere. 

The characteristics of the changes in several species discussed in this chapter are shown in Fig. 19-3. This figure depicts the changing composition of the atmosphere on three time scales. Figures 19-3a-c show the simultaneous variation of CO2, CH4, temperature, and SO from the Vostok ice core (see also Fig. 1-2). These records also clearly demonstrate that the Earth functions as a coupled system. Temperature, CO2 and CH4 are positively correlated with one another, but each is negatively correlated with (for reasons that are not yet known). This time period covers 160000 years including the... 

All the models of the chemical composition of the atmosphere of primeval Earth are hypothetical. Samples from this period of development of the Earth are not available And the oldest rocks give us only a limited amount of information. 

Summary of Data on the Probable Chemical Composition of the Atmosphere During Stages I, II and IIP... 

Observe the chemical composition of the atmospheres and surfaces of comets, planets and satellites. 

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