5. Stratospheric Ozone: An Introduction to Its Study

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Summary

REACTIONS IN A HYDROGEN-OXYGEN ATMOSPHèRE

• 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 AFTER OXIDATION OF MéTHANE

• 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] .

ATMOSPHèRE

• 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.

and

• 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.

SOLAR RADIATION

• 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.

PHOTODISSOCIATION IN THE TROPOSPHèRE AND STRATOSPHèRE AND ITS EFFECTS

• 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.

FINAL INTRODUCTORY REMARKS

• 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.

CONCENTRATION Icm"'')

• 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.