Note: / is ihe wavelength of radiation absorbed.
Note: / is ihe wavelength of radiation absorbed.
According to the above concept of ozone destruction, the troposphere is an inert medium concerning ozone chemistry. However, as Crutzen (1974) pointed out, there are several possible reaction steps for tropospheric 03. Thus ozone can be removed chemically from the air by transformation processes tabulated in Table 10. One reaction chain starts with the photolysis of 03, which is caused by radiations in the Hartley and Chappuis bands. The excited oxygen atoms, formed by Rl, are partly transformed to ground state atomic oxygen by R4. However, they also react with water vapour to give OH radicals (R5). The sum of reactions 1-5 can be written in the following way:
Furthermore, 03 combines with free radicals to form molecular oxygen and OH. An important consequence of these reduction processes is that OH radicals are formed which are very reactive species. Ozone is also removed from the troposphere by nitrogen oxides (R8 and R9). Under clean air conditions nitric oxide is of biological origin (see Section 3.5) while NOz comes from the oxidation of NO in the atmosphere. Fishman and Crutzen (1977) showed that the global reaction [3.31] could destroy about half of the stratospheric ozone before it reached the ground. However, we cannot rule out that an important amount of 03 is also formed in the troposphere by the photolysis of N02
followed by reaction [3.9]. Thus, Fishman and Crutzen (1978) recently argue than over the Northern Hemisphere three times more 03 is destroyed than over the
Southern Hemisphere due to the asymmetry in land area (over lands the downward ozone flux is much larger than over oceans, see Aldaz, 1969) and observed surface air ozone concentrations. Taking into account that this higher removal rate cannot be explained by differences in mass exchange between stratosphere and troposphere over the two hemispheres Fishman and Crutzen (1978) speculate that the higher ozone destruction rate over the Northern Hemisphere is probably balanced by tropospheric ozone formation17 which might be very significant. Thus, they hypothesize that, in spite of the evidences of stratospheric origin discussed above, a part of 03 in the troposphere is an in situ product. This tropospheric ozone formation is connected to the cycle of carbon monoxide, methane and other hydrocarbons since NO may be oxidized by H02 radicals formed during atmospheric reactions of some species (see, e.g. reaction [3.6]):
The concentration of CO (see Fig. 7), hydrocarbons and NO are higher in the air over the Northern Hemisphere than over the Southern Hemisphere, which leads to a higher tropospheric 03 formation rate in these areas. Considering the complexity of the problem and our insufficient knowledge of some chemical processes we are not able at present to draw a final conclusion on the tropospheric ozone cycle. It is to be stressed, however, that there are severe! arguments in the literature (besides Fishman and Crutzen (1978) see, e.g. Ripperton et aL 1971) supporting the hypothesis that tropospheric air also acts as an O , source.
The tropospheric processes discussed above are all homogeneous gas phase reactions. However, we cannot exclude the possibility that ozone is also removed from the air by chemical transformations taking place in liquid water (cloud and precipitation drops). Thus, Penkett (1972) and Penkett and Garland (1974) showed in laboratory experiments that ozone rather rapidly oxidized sulfur and nitrogen dioxides absorbed in artificial fog droplets. It was demonstrated that the S02 oxidation rate was proportional to the mixing ratio of 03 in the air. If this latter is 0.05 ppm and the S02 concentration in the air is 0.1 ppm (this is too high for clean tropospheric air, Subsection 3.6.4), then in 1 minute 0.88 mg/l SO4" is formed in the fog water (temperature: 11 °C; pH = 5). This laboratory finding seems to be supported by atmospheric observations. In the annual variations of sulfate and nitrate concentrations of precipitation samples collected over different European territories (see Subsection 5.4.5) a spring maximum can be identified. This phenomenon may be explained (E. Meszaros, 1974a) by the effect of 03 on S02 and N02 transformation. This follows from the fact (Fig. 13) that the concentration of tropospheric ozone has a maximum at this time of year.
11 It has long been known that in locally polluted air ozone is formed in so-called photochemical smogs (see, e.g. Cadle, 1966). However, Fishman and Crutzen (1978) discuss the ozone formation in unpolluted tropospheric air.
The upper limit of the ozone reduced in this way can easily be estimated. According to Kellogg et al. (1972) the global yearly wet sulfate deposition is 475 x 106 t. For the formation of such a sulfate mass, 79 x 106 t of atomic oxygen is necessary, taking into account that sulfur dioxide transforms in water to HS03 and S03 ~ ions (see Subsection 5.3.2). This atomic oxygen quantity is equal to 237 x 106 t of 03 if we assume that each S02 molecule absorbed is oxidized by one 03 molecule. This value is about one third of the estimate of Junge (1962) for total tropospheric ozone destruction. It follows from this high proportion that this process is probably a non-negligible 03 sink even if other species take part in S02 oxidation, and if a part of the sulfate ions in atmospheric precipitation comes from sulfur containing aerosol particles.
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