Structure of the atmosphere

One can see from Table 1 that in the Earth's atmosphere molecular nitrogen has the greatest relative concentration near the Earth's surface. The effective molecular weight of air is thus rather close to that of this constituent (the two values are 28.973 and 28.022, respectively). However, the composition of the air and consequently its molecular weight are constant only in the lower 80-100 km layer of the atmosphere which is termed the hoinosphere. Above this layer the so-called heterosphere can be found in which the molecular weight is a function of the altitude. The constant chemical composition in the homosphere is controlled by atmospheric mixing, while in the heterosphere the variations in the composition are due to the photodissociation of air molecules produced by solar radiation and to their sheer mass differences in the gravitational field. These effects lower the molecular weight of the air. In the heterosphere, on the other hand, the concentration of the free electrons and positive ions is rather high (the electron density has a maximum near 300 km) which justifies the name ionosphere which is frequently applied in atmospheric physics to this layer.

In all atmospheric layers the density and the pressure of the air decrease with increasing altitude. Under normal conditions near the Earth's surface when the temperature and pressure are 0 °C and 1013.25 mb, respectively, the air density is equal to 1.2923 x 10~3 g/cm3.1 Around 100 km the density is about 106 times smaller.

According to the character of the thermal structure, the homosphere is divided into three further layers. The lowest one is termed the troposphere. The altitude of the troposphere is approximately 18 km above the equator and 8 km above the poles. In this layer the temperature generally decreases with increasing altitude2 (Fig. 1). The average temperature gradient is equal to 6.5 °C/km.

The troposphere receives the thermal energy from the Earth's surface which absorbs the Sun's radiation. Because of the heating of the air by the infrared radiation emitted by the surface, intensive vertical (convective) motions can be generated. This convection transports heat, water vapour and other trace constituents of surface origin to the higher levels of the troposphere. In such an

1 The number of molecules in 1 cm3 of air is 2.687 x 10" under these conditions.

2 On occasion there are thin lavers in the troposphere in which the temperature is constant or increases with height. These are called isothermal and inversion layers, respectively.

updraft the air cools down leading to the condensation of the water vapour. Thus, the formation of clouds and precipitation, and, generally speaking, the atmospheric cycle of the water, essentially takes place in the troposphere. Since during the formation of clouds and precipitation a large part of the atmospheric aerosol particles and soluble gases is ad- and absorbed by the cloud and precipitation

Atmosphere Structure Precipitation

The structure of the atmosphere according to Nicolet (1964). The plotted curve gives the temperature profile, whi le AY, and g are t he molecular weight and the gravitational constant, respectively. (By courtesy of Publishing House Mir)

The structure of the atmosphere according to Nicolet (1964). The plotted curve gives the temperature profile, whi le AY, and g are t he molecular weight and the gravitational constant, respectively. (By courtesy of Publishing House Mir)

elements (wet removal), the cycle of the water significantly controls the tropospheric pathway of many other trace constituents. In the troposphere the speed of the horizontal motions generally increases with increasing altitude. Both the horizontal and vertical motions have a turbulent character which promotes the mixing of atmospheric constituents as well as the dry removal of aerosol particles and such gaseous components as are adsorbed by the soil and vegetation (see Chapter 5). The top of the troposphere is called the tropopause. Between the high and low tropopauses of equatorial and polar regions there is frequently a tropopause gap which is a very important factor in the control of the vertical exchange.

The second layer in the homosphere is termed the stratosphere. In this part of the atmosphere the temperature generally increases with altitude due to the interaction of the short wave solar radiation and different oxygen molecules (02,03). Thus the tropopause is defined as the point at which temperature stops decreasing with increasing altitude and begins to increase. This termal pattern hinders the formation of strong convective currents. If we disregard the relatively slow diffusion processes, the bottom of the stratosphere is closed by the tropopause, except in the regions of tropopause gaps.

On the other hand, the effect of the wet removal can be practically neglected here.3 It is thus understandable that the residence time of trace constituents is greater in the stratosphere than in the troposphere. Above the tropopause the horizontal wind speed first decreases then increases with height. Consequently, a secondary maximum in the wind speed can be observed in this atmospheric layer. The increase of the temperature ends approximately at an altitude of 50 km (stratopause), where the temperature is around 0 °C (see Fig. 1). Above this level, in the mesosphere, the temperature again decreases (third layer in the homosphere). For this reason the stratopause can be considered as an active heat-supplying surface similar to the Earth's surface. In this atmospheric region the distribution of the temperature makes possible the convection which, in favourable cases, results in a formation of so-called noctilucent clouds at an altitude of about 80 km (mesopause) where the temperature is only around —80 °C. This is the coldest level of our atmosphere.

Above the mesosphere a rather hot atmospheric layer can be found, the thermosphere. Since its chemical composition changes with altitude, this thermal layer is the same as the heterosphere, where air molecules (mainly 02) dissociate under the effect of absorbed external radiation.

In this book air chemistry is defined as a branch of atmospheric science dealing with the atmospheric part of the biogeochemical cycle of different constituents. In other words this means that we will deal mainly with the atmospheric pathways of those components that are involved in the mass flow between the atmosphere and biosphere, as well as in chemical interactions between the air and the other media of our environment (soils, oceans etc.). It follows from this definition that, on the one hand, our discussion will be restricted to the troposphere and the stratosphere4 and, on the other hand, the photochemistry of the upper layers, the subject matter of the aeronomy (e.g. Nicolet, 1964), will be omitted. This separation of the (photo) chemistry of the lower (troposphere and stratosphere) and upper atmosphere makes it possible to give a more compact treatment of our problem, including the global anthropogenic effects due to the increase of air pollution.

3 In the case of tall thunder clouds which penetrate into the lower part of the stratosphere there is a possibility for stratospheric wet removal.

4 As in Junge's classical book (Junge, 1963).

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