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5. Stratospheric Ozone: An Introduction to Its Study

Standard values of the solar constant and extraterrestrial solar spectrum are reviewed. In the visible and near UV, this listing of average irradiance over 100-A bandwidths at 50-A intervals was found to be inadequate for many applications. A more detailed spectrum obtained from solar scans with a Perkin-Elmer, Model 112 monochromator was found to give sufficient detail. A normalization program was developed to make the Perkin-Elmer curve agree with the standard curve. Values of extraterrestrial solar spectral irradiance at 1-A intervals in the range 3000-6100 A have been derived. The results are presented in tabular form and as spectral charts.

Extraterrestrial solar spectrum, 3000-6100 a at 1-a intervals.

Nitrogen dioxide and its related compounds react strongly with ozone, usually resulting in the destruction of the ozone, and life seems to be delicately adjusted to the radiation shield which the stratospheric ozone provides. We have found that atmospheric NO2 occurs in two layers, described here, which have very different properties.

Nitrogen Dioxide Concentrations in the Atmosphere

Rate constants for O(3P) recombination and association with N(4S)

The rates of the reactions of OH with CO, C2H4 and C2H2 have been determined between 210 K and 460 K using time-resolved resonance absorption to monitor the removal of OH radicals following their creation by flash photolysis of mixtures containing H2O or N2O + H2. At 300 K, the rate constant, k6, for OH + CO → CO2+ H (6) is 8.7 × 1010 cm3 mol–1 s–1; k6 shows a slight positive temperature dependence, but the Arrhenius plot appears to be slightly curved. The nature of the path of this reaction is discussed and the results of transition state calculations are shown to agree well with the experimental data and to predict marked curvature in the Arrhenius plot above 500 K. The rate constants, k9 and k10, for the primary reactions of OH with C2H4 and C2H2 also increase only very slowly with temperature but, in these cases, the experimental results for 210 K ⩽T⩽ 460 K do fit Arrhenius expressions (energies of activation in kJ mol–1): k9= 4.5 × 1012 exp[– 0.9/RT] cm3 mol–1 s–1. k10= 1.2 × 1012 exp[– 2.1/RT] cm3 mol–1 s–1.

Rate measurements of reactions of OH by resonance absorption. Part 2.—Reactions of OH with CO, C2H4 and C2H2

METHANE is produced at the Earth's surface by various processes, chiefly biological in origin. Koyama1 estimated an annual source of 3 × 1014 g, with rice paddy fields a major contributor, but did not consider the contribution from natural swamps, and his value for the total source may be low by as much as a factor 3. The data of Bainbridge and Heidt2 suggest that they may have observed substantial methane production from the coastal marshes of Louisiana. The important atmospheric sinks for methane are3
 These reactions constitute permanent destruction of CH4. Subsequent chemistry leads to formation of CO and CO2, and we shall argue that CO is more likely. The source of atmospheric CO due to CH4 is comparable with, and may be larger than, that associated with the internal combustion engine.

Natural sources of atmospheric CO.

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

Extraterrestrial solar spectrum, 3000-6100 a at 1-a intervals.

Nitrogen Dioxide Concentrations in the Atmosphere

Rate constants for O(3P) recombination and association with N(4S)

Rate measurements of reactions of OH by resonance absorption. Part 2.—Reactions of OH with CO, C2H4 and C2H2

Natural sources of atmospheric CO.

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