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Note: The concentration of HC1 in the troposphere is estimated to be around I ppb. Most of the gaseous chlorine is thought to be present as HCL The tropospheric level of gaseous bromine is much less than the values given in the table < — 0.01 ppb).

Note: The concentration of HC1 in the troposphere is estimated to be around I ppb. Most of the gaseous chlorine is thought to be present as HCL The tropospheric level of gaseous bromine is much less than the values given in the table < — 0.01 ppb).

19 % yr 1. The concentrations measured in clean air are practically constant with increasing altitude.

The growing interest for the atmospheric study of halocarbons is explained by the fact that these substances rise by diffusion into the stratosphere where halogen compounds play a certain role in photochemical reactions. For the estimation of the importance of these processes the determination of the tropospheric residence time is of crucial interest. According to Sze and Wu (1976) the tropospheric lifetime of F-11 is about 10 years while the corresponding value for F-12 is probably between 10 and 20 years. Jesson et al. (1977) argue that the most probable tropospheric residence time for CC13F lies between 15 and 20 yr. However, more recent data of Singh et al. (1979) make possible longer residence times: 65 to 70 years for fluorocarbon-12 and 40 to 45 years for fluorocarbon-11 but these estimates seem to be rather uncertain. It is obvious that much more investigation remains to be done in this important and interesting field.

3.3.4 The atmospheric carbon cycle

Figure 8 gives in a schematic way the atmospheric cycle of carbon compounds. The numerical values of the different budget terms are based on the results previously discussed in Sections 2.6 and 3.3. The cycle of hydrogen containing species taking part in the chemical processes illustrated is also given qualitatively. However, the atmospheric pathways of non-methane organic carbon vapours and particles are not plotted because of the uncertainties outlined in Subsection 3.3.3. The numbers at the arrows representing the chemical transformations are expressed as mass of the end products. The quantity of different compounds in the atmospheric reservoir is also illustrated.

It can be seen that the great majority of carbon is cycled in the atmosphere as carbon dioxide. Thus, although the oxidation of CO provides an important sink, the process does not supply an important C02 source. It follows from the data given that the residence times of methane, carbon monoxide and carbon dioxide are 5.0; 0.25 and 5.2 years, respectively.

Space

Space

Natural sources and sinks 1 expressed ,n

=> Chemical transformation J yr I I Atmospheric reservoirs in 10® t

Natural sources and sinks 1 expressed ,n

=> Chemical transformation J yr I I Atmospheric reservoirs in 10® t

Atmospheric pathways of carbon compounds. A/oii. CH4 released by paddy Fields is considered biogenic and not anthropogenic pollution

3.4 Ozone

3.4.1 Introduction

Ozone is an essential atmospheric trace substance. This gas plays an important role in the control of the radiation and heat balance of the stratosphere since it absorbs solar radiation with wavelengths shorter than about 0.3 pm. An important consequence of this absorption is that U V radiation lethal to living species does not reach the lower layers of the atmosphere. Because of the importance of atmospheric 03, its study started rather early. Junge (1963) mentions that Dobson and his associates measured total ozone (see later) beginning in the twenties by a European network consisting of six stations. Later, this network became world-wide and even the determination of the vertical distribution of 03 is now a routine measurement. Owing to these studies our knowledge of atmospheric ozone is rather substantial compared to that of other trace constituents.

A peculiarity of ozone is that 03 molecules are both formed and partly destroyed in the atmosphere. Some decades ago it was believed that, according to the classical concept of Chapman (1930), the 03 reaction chain involved only oxygen molecules and atoms. In the last two decades it has become clear that free radicals, nitrogen oxides and halogens also play an important role in atmospheric photochemical processes. These "modern" theories have received considerable attention recently since man-made sources contribute to the stratospheric budget of the species mentioned.

The aim of this section is to present, mainly on the basis of coherent reviews (e.g. Junge, 1963;Crutzen, 1971; Diitsch, 1973; Rowland, 1976; Johnston and Podolske, 1978), the elements of the chemical aspects of the problem. Details of the relation between the general circulation and ozone formation can be found in specialized textbooks (e.g. Khrguian, 1973).

However, before discussion of the formation and destruction of atmospheric 03 it seems to be necessary to define some basic notions of photochemistry and reaction kinetics for non-chemist readers. These definitions will also promote the understanding of other parts of this book.

.14.2 Concepts of photochemistry and reaction kinetics

The principle of photochemical processes can be summarized as follows. A certain atmospheric gaseous component "A", absorbs a given band in the UV or visible solar spectrum. Due to the energy of the absorbed photon, hv, (where h is the Planck's constant and v is the frequency of the radiation) "A" is changed to an excited state. Because of this energy, the molecule decomposes or reacts with another compound "B" (disregarding quenching and fluorescence phenomena). The process may be symbolized in the following way (Leighton, 1961):

A*->D,+D2+... dissociation A* + B->D, 4-... direct reaction

The last two steps in this sequence are termed primary photochemical processes. If the atmospheric concentration of the species is denoted by square brackets then where ka is the absorption rate of photons. The rate of the primary photochemical process will be:

8 Asterisk denotes the excited state.

where $ is the so-called quantum yield. Its value is equal to the ratio of the number of "A" reacted to that of excited molecules.

The primary photochemical processes are generally followed by secondary thermal reactions the energy of which is provided by the thermal agitation of molecules. In the case of a bimolecular reaction9:

di dr

where k is the rate constant, which is equal to the quantity of D, reacted per unit time if the [D,] and [E] concentrations are also unity. It can be seen from the last equation that the loss rate of D, is equal to the formation rate of X in molar units.

The above bimolecular reaction is said to be second order, since its rate depends on the product of two concentrations. Generally, the order of a reaction is the sum of the exponents of concentrations on the right-hand side of kinetic equations. Thus, the primary photochemical reaction discussed above is a first order process since its rate depends only on the concentration of A. In the case of photochemical reactions the rate constant is given by the product of the absorption rate and quantum yield.

3.4.3 Formation and destruction of 03

The "classical" ideas on ozone formation and destruction in the stratosphere are discussed on the basis of Paetzold's work as summarized by Junge (1963). As Chapman (1930) demonstrated the formation of ozone is initiated by the following photochemical processes:

0J->0 + 0 dissociation which can be unified into one equation:

02 + hv-*0 + O, [O] formation rate = 2/, [02] [3.8]

where [02] is the oxygen concentration expressed in molecules x cm-3, while/, is the rate constant. This photodissociation is followed by a secondary thermal reaction leading to 03 formation:

9 In these equations D,, E, X and Y each denote an appropriate molecule.

where M is a third neutral body, generally oxygen or nitrogen (this means that strictly speaking the ozone formation also depends in some way on the concentration of nitrogen molecules). Molecular oxygen absorbs over the altitude range from 30 to 40 km in the band of 0.176-0.203 /xm of the solar spectrum (Schumann-Runge band), while below that altitude range solar radiation with a wavelength of 0.210 /xm also plays an important role in 02 photolysis.

The decay of 03, according to the classical theory is due to the following reactions:

Ozone absorbs in the band of0.200-0.320 /xm (Hartley band) of the U V part of the spectrum and also in the visible range between 0.4S0 /xm and 0.700 fim wavelengths (Chappuis band). A smaller absorption band can be identified in the infrared part of the spectrum. The strongest absorption is measured in the Hartley band.

It is to be noted that the possibility of other reaction« between oxygen species cannot be ruled out. One might demonstrate, however, that these processes can practically be neglected. Thus, e.g. the rate of the reaction

is insignificant because of the low concentration of atomic oxygen. Further, the thermal decomposition of 03 is also negligible at stratospheric temperatures.10

At equilibrium, equations [3.8]-[3.12] can be combined as follows:

=0 = 2;, [02] -k2 [03] [O] -fc, [02] [O] [M] +/2 [03] [3.14]

The combination of [3.13] and [3.14] yields:

10 For this reason the decomposition of 03 at night can be neglected.

4 Méuiros where k = kjk2. Since [M] is proportional to the sum of [N2] and [02] we can write that [M] = s[02], where s is a constant. The solution of equation [3.15] is:

From the fact that

/f «4/,/2fcs[02] the equilibrium ozone concentration will be:

By means of equation [3.17], the equilibrium vertical profile of the ozone concentration can be calculated. Thus, [02] on the right-hand side is known for various altitudes and k can be calculated for different temperatures." The greatest problem is the determination of f\ and f2 as a function of altitude. The values of these latter parameters depend on the absorption of radiation, which varies in a complex way as solar radiation penetrates into the atmosphere. Theoretically /, and f2 are calculated by the following two equations:

In these formulae /(/., h) is the radiation intensity with wavelength /. at altitude h. while a is the absorption coefficient of the gas specified by the subscript. Calculation using [3.18] is especially complicated by the fact that a0i also depends on the pressure (p). For these reasons the vertical profile of [03] cannot be calculated by analytical solution of the equations. Numerical integration, however, can be done stepwise down from the upper levels of the atmosphere.

Depending upon the assumptions employed, the equilibrium ozone concentrations, calculated by the classical theory, vary by a factor of no more than 2 or 3. Moreover the height of the maximum O, concentration is nearly independent of the assumptions used and lies around 23 km.

It is evident that the equilibrium ozone concentration also depends on the temperature profile and on the zenith distance of the Sun. For this reason the equilibrium concentration at a given height decreases with increasing altitude. It

1' According to a reaction rate table recently published by Vupputuri (1977) kt = 1.07 x 10 3VM" " and k2 = 1.9 x 10" where Tis the absolute temperature.

will be shown in the next section that atmospheric observations do not agree with this theoretical finding. This discrepancy can be explained by assuming that in the control of the ozone concentration atmospheric dynamics also plays an important part.

Theoretical investigations done in this field also show (Junge, 1963) that the time necessary to regain an equilibrium state again after it is disturbed varies as a function of altitude. This time can be expressed by the following equation:

where t, is the time for the deviation from the equilibrium to decrease to \/e of its initial value. According to the results of calculations obtained using equation [3.20], t, is equal to 0.6 day at 40 km, while the respective figure at an altitude of 20 km is 10J days. This means that above 30 km equilibrium conditions are practically satisfied, while in lower layers of the stratosphere the ozone is a conservative property of the air.

In the sixties it became clear that the hydrogen containing free radicals formed from water (see equations [2.1] and [3.1]) also play a role in stratospheric photochemistry since these free radicals directly react with 03 and O (see Table 10). Recent studies of Johnston and Podolske (1978) suggest that these processes account for about 5 10 "„ of O, destruction. Further, it was also found that the oxides of nitrogen12 are involved in determining the stratospheric ozone budget in the following way (Crutzen. 1971):

If the effect of these reactions is included the decreases of the ozone and atomic oxygen concentrations are proportional to the NOA. concentration. Theoretical considerations also show that the effect of NOv is particularly important in the stratosphere below 40 km. It is believed at present (Johnston and Podolske, 1978) that about 60 of ozone molecules are removed by these processes (the corresponding figure for the "classical" removal is only 10%). In the mid and lower stratosphere the equilibrium ozone concentration is approximately given by the following equation (Dutsch. 1973):

Their cycle in the atmosphere will be discussed in Section 3.5.

where C is a constant, while [NOx]mix is the mixing ratio of nitrogen oxides (/, and f2 are as above). It can be seen that [3.23] differs from equation [3.17], in addition to NOAmix, in the exponents of oxygen concentration and /,//2 ratio.

The indirect role of free radicals consists of decreasing the NO and N02 levels. The end products of these reactions are nitric acid and nitrous acid vapours:

Thus, free radicals reduce the ozone loss due to the presence of nitrogen oxides.

It follows from this discussion that the activities of mankind can modify the natural photochemical processes in the stratosphere by adding nitrogen oxides and water vapour to stratospheric air. This possibility was first stressed by Crutzen (1971) and Johnston (1971). They stated that supersonic aircraft flying in the stratosphere could emit these species in a quantity sufficient to alter the chemical composition. Figure 9 gives numerical examples for these modifications (Diitsch,

The vertical profile of ozone expressed in nanobars as & function of atmospheric pressure (ordinate* millibar) for different NO,(ppb)and HjO(ppm) mixing ratios (Diitsch, 1973). (By courtesy of Birkhauser

Verlag and the author)

The vertical profile of ozone expressed in nanobars as & function of atmospheric pressure (ordinate* millibar) for different NO,(ppb)and HjO(ppm) mixing ratios (Diitsch, 1973). (By courtesy of Birkhauser

Verlag and the author)

1973). On the left side of the figure the curve a represents the equilibrium ozone profile in case of "normal" NO* vertical distribution (curve a-d in the middle) and H20 concentrations (curve a-c on right). The ozone profiles b and c refer to a certain increase of the NOx distribution (curves b and c for nitrogen oxides) without any variation in the H20 profile. On the other hand the 03 vertical distribution labelled d gives the equilibrium ozone concentration as a function of the altitude (pressure) when H20 level at different heights increases from a-c to d while NO, concentrations remain "normal". One can see that the concentration increase of NO, results in a rather significant decrease in the ozone concentration at some altitudes. Curves plotted in Fig. 9 also suggest that the effect of an increase in water content in the stratosphere is less important (see also Chapter 6) than that of nitrogen dioxides.

It thus appears from theory that nitrogen oxides may constitute a threat to ozone. However, more recent work has led to a total revision of the theory. While the details are more complex than it is possible to describe here, the key reactions are understandable. One possible sequence is given in Table 9. The result is that NO, forms ozone by this sequence while destroying it by the previous one (Johnston and Podolske, 1978).

Table 9

a possible reaction sequence in the stratosphere involving no, (Johnston and Podolske, 1978)

Table 9

a possible reaction sequence in the stratosphere involving no, (Johnston and Podolske, 1978)

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