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Fig. 11

Vertical ozone distribution at different latitudes in the Northern Hemisphere: (a) January; (b) April; (c) July; (d) October (Dutsch, 1978). (By courtesy of Birkhauser Verlag and the author)

The distribution patterns shown in Fig. 11 can briefly be explained as follows. Stratospheric ozone formed by photochemical processes is transported in poleward direction by atmospheric motions. This circulation is particularly strong in winter and spring months when stratospheric air moves downward over polar regions. At the same time the lower stratosphere over the tropics is characterized by a slow updraft (Brewer, 1949). Thus, stratospheric dynamics lead to the accumulation of ozone rich air in the lower polar stratosphere. It should be recalled here that at this altitude 03 is a conservative property of the air. During the late spring and summer, especially, the stratospheric 03 reaches the troposphere first of all through the tropopause gaps. In the troposphere this species is removed from the air by various sinks, as this will be shown in the next section.

14.5 Tropospheric ozone

The ozone molecules formed in the stratosphere reach the troposphere where they move downward by turbulent diffusion. The stratospheric origin of 03 in the surface air is formally proved by Fig. 12. In this figure, on the basis of data gained by different workers, the mean tropospheric ozone concentration is plotted as a

Fig. 12

The concentration of tropospheric ozone as a function of latitude (Pruchniewicz, 1973). (By courtesy of

Birkhauser Verlag and the author)

Fig. 12

The concentration of tropospheric ozone as a function of latitude (Pruchniewicz, 1973). (By courtesy of

Birkhauser Verlag and the author)

function of geographical latitude (Pruchniewicz, 1973). It can be seen that in the distribution three maximums are observed. The latitude of the three maximums might be related to the territory of frequent tropopause discontinuit├ęs. Thus, in the vicinity of 30 air masses of tropical origin usually meet the colder air of mid-latitudinal regions. At 60\ polar air is frequently in contact with warmer air of mid-latitudes. The contact of two air masses with different thermal structure produces a tropopause gap through which the mass exchange between the stratosphere and troposphere becomes very intensive. Furthermore, over latitudes of 42 -45 the same phenomenon can take place because of the tropopause fold between polar and tropical air masses. It follows from the observations represented by Fig. 12 that the most pronounced ozone maximum is found below the tropopause gap nearest to polar regions. This finding is explained by the fact that in the lower stratosphere the air with great ozone concentrations moves from the pole in the direction of the equator. One can also see that the tropospheric ozone concentration is very low over equatorial territories. This is not surprising, since tropical latitudes are characterized by a strong updraft in the troposphere.

The annual variation of the total ozone (see Fig. 10) can be approximated at the mid-latitudes by a sine curve. It should be noted that the tropospheric 03 level shows similar annual changes. The only difference is that maximum concentrations are observed in the troposphere 1-2 months later (see Fig. 13).- On the basis of this time lag Junge (1962) estimated that the residence time of 03 in the troposphere is 2 months. In contrast the atmospheric residence time of 03 is calculated to be 1-2 years (Junge, 1963) that is, the lifetime of this species is very different in the stratospheric and tropospheric reservoirs.

According to the classical concept, the Earth's surface is a sink for stratospheric ozone. This is proved by the fact that a net downward ozone flux was measured in the lower tropospheric layers (e.g. Aldaz, 1969). By assuming a first-order process for ozone destruction and using the above residence time, Junge (1962) estimated that the ozone loss rate is 780 x106 t yr~1 at the surface. To calculate this rate he

Fig. 13

Annual variation of total ozone and tropospheric ozone concentration in Arosa (Switzerland) between April, 1950 and March, 1951 (Junge, 1963). (By courtesy of Academic Press and the author)

Fig. 13

Annual variation of total ozone and tropospheric ozone concentration in Arosa (Switzerland) between April, 1950 and March, 1951 (Junge, 1963). (By courtesy of Academic Press and the author)

also took into account the tropospheric ozone mass, which was found to be 130 x 1061. It should be noted that this latter figure is about one order of magnitude smaller than the global 03 burden in the total atmospheric reservoir (see Subsection 3.4.4).

Table 10

Reaction chain leading to the destruction of ozone in the troposphere (Crutzen, 1974)
0 0

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