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5. Stratospheric Ozone: An Introduction to Its Study

Ozonesonde ascents have shown that frequently the vertical distribution of the ozone in the stratosphere is very complicated with strong maxima and minima. Such a complicated distribution was not expected. In an attempt to find the cause of this highly laminated structure of the ozone, the variations in the intensity of the lamination with both season and latitude have been studied. It is found that a highly laminated structure is most frequent in spring and in the higher latitudes. It is also shown that well marked minima in the vertical distribution of the ozone are found at a height of about 15 km. The frequency of occurrence of these minima is also greatest in spring and in the higher latitudes. It is suggested that the ozone-weak air in these minima has entered the stratosphere at the sub-tropical tropopause break and has been carried polewards. A region of minimum ozone is also sometimes found at heights of around 23 km but less frequently than at 15 km. These high level minima are only found in late summer or autumn and in very high latitudes.

The laminated structure of the ozone in the atmosphere

A large number of measurements of atmospheric N2O have been made by a gas chromatic method. Over a period of 2.5 years, almost daily samples were taken at Mainz. There is a weak annual variation, higher values occurring during the warmer season, and there is no indication of anthropogenic sources. Measurements at mountain stations show a general decrease in mixing ratio with altitude, as do measurements from commercial aircraft along the polar route within the tropopause region. A few stratospheric balloon samples show a pronounced decrease in the stratosphere, in agreement with calculations of Bates and Hayes. These data suggest that the atmospheric lifetime is about 70 years, and that the limiting factor is photochemical destruction. Estimates from seasonal variations and other observations seem to indicate shorter atmospheric lifetimes, however, suggesting additional destruction processes.

Studies of atmospheric N2O

Spectroscopic measurements have been made of the nitric oxide and water vapor concentrations in the stratosphere at an altitude of 28 kilometers. The measurements, carried out in situ with the use of a spin flip Raman laser, represent the first accurate determination of nitric oxide as a function of time (as the sun rose) from about 6: 30 to 14: 00 C. D. T.

https://science.sciencemag.org/content/sci/184/4142/1173.full.pdf

Spectroscopic Measurements of Stratospheric Nitric Oxide and Water Vapor

Freons are a potential source of stratospheric chlorine and may indirectly cause serious reductions in the concentration of ozone. The reduction could be as large as 3 percent by 1980, or 16 percent by 2000, if Freon consumption were to grow at 10 percent per year. Even if Freon use were terminated as early as 1990, it could leave a significant effect which might endure for several hundred years.

https://science.sciencemag.org/content/sci/187/4176/535.full.pdf

Freon Consumption: Implications for Atmospheric Ozone

The photochemistry and transport of N and NO are studied by means of a program incorporating molecular and eddy diffusion. The sources considered are ionospheric, and metastable reactants are included in order to match the large observed NO concentrations. With (O2(¹Δ) a satisfactory result can be obtained, but only with a rate coefficient 2 orders of magnitude larger than suggested by Norton found in the laboratory. The N(²D) is also satisfactory, and the required rate coefficients have some laboratory support. The N(²D) is taken to be produced by dissociative recombination of NO+, and the yield is required to be between 0.5 and 0.6; the NO concentrations are very sensitive to the exact value in this range. An important new source of N (assumed to be ground state) is introduced: dissociation of N2 by photoelectrons. The yield of N(²D) in NO+ recombination must then be near unity. A possible consequence is an inverse dependence of D-region electron density on solar activity. It is pointed out that NO is determined by quite different factors during the polar night, and that much larger concentrations are plausible.

Production and diffusion of nitric oxide

When an analysis of the various reaction rates is made, a certain number of them can be ignored, and for several years it was assumed [e.g.. Fig. 2 . Observed and calculated ozone profiles.

Hunt. 1966; Leovy. 1969] that the reactions of OH and HO2 radicals with O and Oj were the essential reactions explaining the aeronomic behavior of stratospheric ozone.

The normal photodissociation process (a,3) H2O + hv  neS) + OH{Xm) (30) which can still occur in the stratosphère, is less important than the reaction process (29).

This reaction may be introduced into the aeronomic chemistry of molecular hydrogen.

The rate coefficient «20 should be of the same order of magnitude as 13,5, but no acceptable value has been found.

REACTIONS IN A HYDROGEN-OXYGEN ATMOSPHèRE

Methyl radicals, which are produced by oxidation processes of CH,, may react rapidly with atomic oxygen (c) CH, + O -H + HjCO + 67 Iccal (63û) with u rate coefficient [Slagle et al..

According to Levy [1972] , CH,0,H either reacts with OH or is subject to photodissociation (c") CH,0,H + hv ^ CH,0 + OH (78) Finally, if methylperoxynitrite and methylperoxynitrate are formed, the photodissociation should be considered to be and (Oo) EQUATION Reactions of CH,, CH3O, and CHjO, with ozone have also been considered [Simonaitis and Heicklen, 1975a] .

REACTIONS AFTER OXIDATION OF MéTHANE

The présence of nitrogen oxides in the upper atmosphère requires the production of atomic nitrogen [Nicolei.

From this analysis of the various reactions of nitrogen trioxide, it is not clear if NO, can play a major rôle in stratospheric aeronomy.

Among the various dissociation processes, the authors may consider the following: Cl, + hv{\ < 483 nm) -2CI (159a) is photodissociated in the stratosphère and troposphère by radiation of X > 300 nm [Seery and Britton.

ATMOSPHèRE

Thus the addition of nitrogen oxides NO and NO,, which destroy odd oxygen by various reactions involving ozone and atomic oxygen, must be considered with its counterpart, the photodissociation of NO, NO,, and NO, and the N formation, as production processes in addition to the photodissociation of molecular oxygen.

Furthermore, the differential équation for nitric oxide must be written as foilows:.

Thus the nitrogen oxide concentrations, and particularly those of HNOs, NO, and NO2, must dépend on atmospheric conditions in the lower stratosphère  [Brasseur and Nicolet, 1973] , and their behavior will be related to the variation of the tropopause.

The authors knowledge of solar radiation in the ultraviolet which plays a rôle in the photodissociation of molecular oxygen is due to rocket and balloon data.

The percentages are given for the spectral range AK = 500 cm"'; standard conditions prevail.

There is therefore no doubt that the stratospheric ozone below its concentration peak is essentially due to a downward transport from the production régions Figure 20 is another illustration of this distribution of the ozone formation resulting from the atomic oxygen production with a peak in the upper stratosphère between 40 and 50 km even for overhead sun conditions.

The température-altitude profiles indicate that the important différences occur between 10 and 20 km; they are related to the hcight of the tropopause and have therefore an effect on the rate coefficients in the lower stratosphère.

SOLAR RADIATION

The photodissociation in the lower régions of the terrestrial atmosphère is of particular interest, since it is the necessary process to start various chemical reactions.

The ozone photolysis (see for example, Welge [1974] for a récent analysis of the photolysis of O», Hd, COj, and SO, compounds) occurs in the visible région in the spectral range of the Chappuis bands with production of oxygen molécules and atoms in their normal States.

Its concentration dépends on the exchange processes between the stratosphère and troposphère.

The absorption cross section of nitrous oxide varies within very low values less than 10"" cm^ between 310 and 250 nm, and its photodissociation coefficient  is not greater than 10"' s"' at the stratopause and reaches only values less than 10"' S"' in the low stratosphère.

The photodissociation rates are relatively small, and departures from photochemical equilibrium conditions are aiways the rule.

PHOTODISSOCIATION IN THE TROPOSPHèRE AND STRATOSPHèRE AND ITS EFFECTS

The authors have seen that it is aiways possible to résolve the theoretical problem of stratospheric ozone with the introduc-PHOTOOI SSOCIAT ION COEFFICIENT I sec"'l Fig. 37 . 1972] . tion of correct aeronomic équations and with the adoption oi the principal atmospheric parameters.

The boundary conditions, which are used in stratospheric models, are not aiways adopted to varying atmospheric conditions.

Seiler, 1974; Seiler and Schmidi, 1974] il may be pointed out that the CO concentration must be known with précision in the lower stratosphère in order to détermine the ratio n(OH)/«(H02).

In the same way, the tropospheric ozone problem requires more attention, since a photochemical theory has been proposed by Chameides and Walker [1973.

FINAL INTRODUCTORY REMARKS

Récent measurements by Schmidt [1974] and Seller and Schmidt [1974] lead to an almost constant mixing ratio ofO.55 ppm for tropospheric molecular hydrogen which can be taken as the normal mixing ratio above the tropopause level.

Sampling [Ehhall, 1974] in the stratosphère at various latitudes is required in order to obtain enough vertical profiles to compare with the calculated vertical distributions of méthane and molecular hydrogen.

It is not yet clear how the vertical and horizontal transports play their rôle [Wofsy et ai. 1967] , and even spécial sources [Deuser et al., 1973] .

Récent measurements at ground level of HNO2 by Nash [1974] lead to mixing ratios from 1 to 10 ppb which must be explained by its various reactions with nitrogen oxides and hydroxyl and hydroperoxyl radicals.

5. Stratospheric Ozone: An Introduction to Its Study

References

The laminated structure of the ozone in the atmosphere

Studies of atmospheric N2O

Spectroscopic Measurements of Stratospheric Nitric Oxide and Water Vapor

Freon Consumption: Implications for Atmospheric Ozone

Production and diffusion of nitric oxide

Related Papers (5)

Stratospheric ozone recovery

Scientific assessment of stratospheric ozone, 1989

Destruction of stratospheric ozone

Destruction of Stratospheric Ozone and Its Effects

OZONE DEPLETION AND RELATED TOPICS | Stratospheric Ozone Recovery