AERONOMICA ACTA

Texte intégral

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i s s n 0 0 6 5 - 3 7 1 3

N S T I T U T D ' A E R O N O M I E S P A T I A L E D E B E L G I Q U E

3 - A v e n u e C i r c u l a i r e B - 1 1 8 0 B R U X E L L E S

A E R O N O M I C A ACTA

A - N° 334 - 1988

CHANGES IN STRATOSPHERIC OZONE : OBSERVATIONS AND THEORIES

by

Guy BRASSEUR and Paul C. SIMON

B E L G I S C H I N S T I T U U T V O O R R U I M T E - A E R O N O M I E 3 - Ringlaan

B • 1 1 8 0 B R U S S E L

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FOREWORD

This document is the text of two conferences presented before the

"Deutsche Gesellschaft für Chemisches Apparatewesem, Chemische Technik und Biotechnologie e.V." (DECHEMA) association in Frankfurt (FRG) in December 1987 and during the study-days on "Air pollution and its effects on the environment" in Padua (Italy) in April 1988. The text will be published in the proceedings of the meetings.

AVANT-PROPOS

Ce document constitue le texte de deux conférences présentées devant la société "Deutsche Gesellschaft für Chemisches Apparatewesem, Chemische Technik und Biotechnologie e.V." DECHEMA à Francfort (RFA) en décembre 1987 et aux journées d'étude sur "Air pollution and its effects on the environment" à Padoue (Italie) en avril 1988. Le texte sera publié dans les comptes-rendus des réunions.

VOORWOORD

Dit document vormt.de tekst van twee voordrachten die gegeven werden voor de vereniging "Deutsche Gesellschaft für Chemisches Apparatewesem, Chemische Technik und Biotechnologie e.V." (DECHEMA) te Frankfurt (BRD) in december 1987 en tijdens de studiedagen over "Air pollution and its effects on the environment" te Padua (Itali'é) in april 1988. De tekst zal gepubliceerd worden in de verslagen van de vergaderingen.

VORWORT

Dieses Dokument ist der Text von zwei Vorträgen präsentiert für die "Deutsche Gesellschaft für Chemisches Apparatewesem, Chemische Technik und Biotechnologie e.V." (DECHEMA) Vereinigung in Frankfurt

(BRD) im Dezember 1987 und während der Studientage über "Air pollution and its effects on the environment" in Padua (Italien) im April 1988.

Der Text wird publiziert werden in den Berichten der Tagungen.

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CHANGES IN STRATOSPHERIC OZONE : OBSERVATIONS AND THEORIES

by

Guy BRASSEUR and Paul C. SIMON

Abstract

The ozone abundance in the stratosphere is predicted to vary as a consequence of increasing emissions of trace gases such as the chlorofluorocarbons, methane, nitrous oxide, carbon dioxide, etc... The paper reviews available observations used to derive a possiDie ozone trend over the last decade. Model simulations of ozone changes are also considered. A particular attention is devoted to the spectacular ozone depletion observed over Antarctica especially during springtime. Several theories presented to explain this remarkable and unexpected phenomena are reviewed. Recent observations show unambiguously that elevated amounts of CIO are present over Antarctica in the lower» stratosphere during springtime. It is believed that the release of active chlorine is activated by the presence of polar stratospheric clouds.

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Résumé

La concentration de l'ozone dans la stratosphère est affectée par l'émission de constituants tels que les halocarbones, le méthane, l'hémioxyde d'azote, le dioxyde de carbone, etc. dans la troposphère. Ce travail présente et analyse les observations disponibles permettant de déterminer les changements de l'ozone stratosphérique pendant les dix dernières années. Les simulations à l'aide de modèles numériques sont également considérées. Une attention particulière est portée sur la diminution importante de l'ozone au-dessus du continent Antarctique au

printemps. Les théories présentées pour expliquer ce phénomène sont revues. Les observations récentes montrent une quantité élevée de CIO dans la stratosphère inférieure pendant le printemps en Antarctique. La présence des nuages polaires stratosphériques semble expliquer la présence de chlore chimiquement actif à des altitudes inférieures à 22 km.

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Samenvatting

De ozonconcentratie in de stratosfeer wordt getroffen door de emissie van minderheidsbestanddelen zoals chloorfluorkoolstoffen, methaan, distikstofoxide, koolstofdioxide enz. in de troposfeer. Dit werk onderzoekt de beschikbare waarnemingen die toelaten de veranderingen in het stratosferisch ozon te bepalen tijdens d e laatste tien jaar. Simulaties met behulp van numerieke modellen worden eveneens in beschouwing genomen. Bijzondere aandacht wordt geschonken aan de belangrijke ozonvermindering boven Antarctica tijdens de lente.

Verscheidene voorgestelde theorieën om dit opvallend en onverwacht verschijnsel uit te leggen, worden herzien. Recente waarnemingen tonen ondubbelzinnig een grote hoeveelheid CIO in de lage stratosfeer boven Antarctica tijdens de lente. De aanwezigheid van polaire stratosferische wolken schijnt de aanwezigheid van chemisch actieve chloor uit te leggen op minder dan 22 km hoogte.

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Zusammenfassung

Die Ozonkonzentration in der Stratosphäre wechselt durch die Emission von Minoritätsgasen wie Chlorfluorkohlstoffen, Methan, N20 , CC>2... in der Troposphäre. Diese Arbeit untersucht die verfügbare Beobachtungen die zulassen die Veränderungen im stratosphärischen Ozon zu bestimmen während des letzten Jahrzehntes. Simulationen mit der Hilfe von numerischen Modellen werden auch betrachtet. Besondere Aufmerksamkeit wird gewidmet an der wichtige Ozonverringerung .über Antarktis während des Frühlings. Verschiedene präsentierte Theorien zur Erklärung dieses auffallenden und unerwarteten Phänomenes werden revidiert. Rezente Beobachtungen zeigen unzweideutige grosse Quantitäten CIO in der geringen Stratosphäre über Antarktis während des Frühlings.

Die Anwesenheit von polarer stratosphärischer Wolken scheint die Anwesenheit von chemischem aktivem Chlor zu erklären auf geringerer Höhe als 22 km.

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1 . INTRODUCTION

Ozone, which protects the biosphere from harmful s o l a r u l t r a v i o l e t r a d i a t i o n and plays a key role in the r a d i a t i v e budget of the middle atmosphere, is present in the atmosphere from the s u r f a c e to as h i g h as 100 km a l t i t u d e . T h e peak density of t h i s chemical c o n s t i t u e n t is located near 25 km a l t i t u d e in the t r o p i c s and 18 km at high l a t i t u d e . The transmission of solar r a d i a t i o n in the 200-310 nm r e g i o n is determined in l a r g e part by the ozone column d e n s i t y , which corresponds,

for standard pressure and temperature c o n d i t i o n s , to a highly absorbing layer with a thickness of only 2 . 5 to 4 . 5 mm (250 to 450 D o b s o n ) .

Ozone i s produced by the a c t i o n on molecular oxygen of s o l a r u l t r a v i o l e t r a d i a t i o n at wavelength shorter than 242 nm. I t s d e s t r u c t i o n by recombination with atomic oxygen is catalyzed by the presence of d i f f e r e n t r a d i c a l s belonging to the f a m i l i e s of hydrogen ( H , OH, H O g ) , nitrogen (NO, N 02) , c h l o r i n e ( C I , C 1 0 ) , bromine ( B r , B r O ) , e t c . . . T h e r e l a t i v e c o n t r i b u t i o n of these r a d i c a l s to the ozone l o s s v a r i e s with a l t i t u d e ; in the mesosphere (50-85 km), the most e f f i c i e n t processes are due to the hydroxyl r a d i c a l s while in the s t r a t o s p h e r e (15-50 km), the most important d e s t r u c t i o n agents for ozone are the nitrogen o x i d e s . T h e e f f e c t s of c h l o r i n e are the l a r g e s t near 40 km a l t i t u d e (under c o n d i t i o n s p r e v a i l i n g o u t s i d e the polar r e g i o n s ) . Recent measurements have shown that the c h l o r i n e chemistry is s u b s t a n t i a l l y modified in the cold stratosphere over A n t a r c t i c a in s p r i n g .

I n c r e a s e s in the atmospheric c o n c e n t r a t i o n of methane, nitrous oxide and the chlorofluorocarbons (CFCs) a r e expected to modify the density of the a c t i v e r a d i c a l s a f f e c t i n g the ozone amount. Some of these compounds a r e r e l e a s e d in the atmosphere as a r e s u l t of a g r i c u l t u r a l and i n d u s t r i a l a c t i v i t y . Because of the r e l a t i v e l y long l i f e t i m e of these species ( s e e T a b l e 1 ) , a large f r a c t i o n of these molecules penetrate in the s t r a t o s p h e r e , where their d e s t r u c t i o n leads to t h e formation of ozone destroying r a d i c a l s . A c t i v e c h l o r i n e i s for example released in

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o cr>

TABLE 1.- Main characteristics of chemical species released in the atmosphere 1980 conditions.

Approximate lifetime Spec ies

co2

n20 CHjCi C C I , C H3C C 13

F-11 (CFC1j) F-12 ( C F2C 12) F-113 ( C2F3C 13) F-22 (CHFgCl) Halon 1301 (CFjBr) Ha Ion 1211 (CF2CXBr)

Typical mixing ratio at the

surface

Total atmospheric mass

(10 Tonnes) (yrs)

Recent growth rate

(ï/yr)

Ozone depletion efficiency relative to CFC-11*

310 ppmv 2.1 x 10 0,2-0,5 -

1.6 ppmv 1 183 8 1-2 -

300 ppbv 2 170 170 0.25 -

700 pptv 5.27 1 0 -

100 pptv 2.DM 70 0-3 1.06

100 pptv 2.09 8 6-9 0.10

170 pptv 3-71 70 5-8 1.0

285 pptv 5.65 110 5-8 1.0

22 pptv 0.67 100 10-20 0.78

61 pptv 0.92 20 12 0.05

- 1 pptv 0.02 100 rapid 11.1

- 1.5 pptv 0.03 20 20 2.70

« according to LLNL one-dimensional model.

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J So Radi

lar y ^ ation 1

S T R A T O P A U S E (50 km)

i slow I CFC

Photodissociation

\ Photolysis of 02 of CFCs — CI. C I O S T R A T O S P H E R E

Production of 03 Dissociation of N A N O . N O , . .

| ^ Q Stratosphenc Ozone concentrations C A T A L Y T I C D E S T R U C T I O N 3 cooling

1 / Absorption of U V radiation \

, ^ ( 240nm—290nm )

Slow transport of 03 \ 2 9 0 n m - 3 2 0 n m (partial) J 25 km

transport 1 slow transport

s. N A C O , , and others. \ CI. CIO. N O . etc. T R O P O P A U S E (10-15 km)

r p r Tropospheric CFC

warming due to emissions T R O P O S P H E R E

_ 2 Greenhouse-effect

T r a c e g a s e s absorption of M & M : C O ,

CFC long-wave radiation • • • • • • N Ox

C 02 C H4 methane — e m i s s i o n s

Removal . - Photochemical r " y 1

U L

F i g . 1 .- S e l e c t e d p h y s i c a l and chemical p r o c e s s e s impacting on o z o n e d e n s i t i e s and c l i m a t i c p r o c e s s e s ( f r o m M i l l e r and M i n t z e r , 1 9 8 6 ) .

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the stratosphere from the p h o t o d i s s o c i a t i o n of the CFCs. These gases of anthropogenic o r i g i n as well as carbon d i o x i d e , a c o n s t i t u e n t partly produced by combustion processes, are a l s o r a d i a t i v e l y a c t i v e in the infrared and therefore c o n t r i b u t e to the "greenhouse" e f f e c t of the atmosphere. An i n c r e a s e in carbon d i o x i d e , a molecule without chemical a c t i o n on ozone, l e a d s to a warming of the troposphere (0-15 km) and of the E a r t h ' s s u r f a c e (greenhouse e f f e c t ) and to a cooling of the strato- sphere ( l a r g e r emission to space of r a d i a t i v e energy) a n d , through this temperature change, reduces the loss rate of ozone above 30 km and consequently enhances i t s concentration in the upper s t r a t o s p h e r e . A change in the ozone density a l s o m o d i f i e s the depth of p e n e t r a t i o n of solar u l t r a v i o l e t r a d i a t i o n and the r e l a t e d s t r a t o s p h e r i c h e a t i n g r a t e . Figure 1 i l l u s t r a t e s the most important processes a f f e c t i n g both the ozone d i s t r i b u t i o n and c l i m a t e .

The study of the ozone response to p e r t u r b a t i o n s of n a t u r a l and anthropogenic o r i g i n r e q u i r e s a d e t a i l e d understanding of chemical, r a d i a t i v e and dynamical processes occurring simultaneously in the atmo-

sphere. The most recent models account for the most important couplings between these processes. These models are the only tools presently a v a i l a b l e to p r e d i c t the e f f e c t s of perturbations in the f u t u r e . They are used to i d e n t i f y the r e l a t i v e importance of d i f f e r e n t atmospheric processes and to v a l i d a t e theory a g a i n s t a v a i l a b l e o b s e r v a t i o n s .

The p r o t e c t i o n of the ozone l a y e r i s a major concern s i n c e t h i s layer s h i e l d s the b i o s p h e r e a g a i n s t harmful UV-B (290- 320 nm) and UV-C (200-290 run) r a d i a t i o n . A b i o t i c u l t r a v i o l e t r a d i a t i o n is strongly absorbed by the DNA molecules of l i v i n g c e l l s and a l t e r s reproductive processes of these c e l l s . A well-known example is the r e l a t i o n between UV-B exposure and human s k i n cancer although other b i o l o g i c a l e f f e c t s could l e a d to even more important consequences. The g l o b a l warming at the E a r t h ' s s u r f a c e expected from i n c r e a s i n g l e v e l s of chemical compounds in the atmosphere could produce major c l i m a t i c changes with s u b s t a n t i a l environmental consequences.

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The purpose of this paper is to review the most recent s t u d i e s d e a l i n g with p o s s i b l e trends in the s t r a t o s p h e r i c ozone d e n s i t y . These i n v e s t i g a t i o n s involve ground-based and s a t e l l i t e o b s e r v a t i o n s as well as model s i m u l a t i o n s . I t i s indeed important to estimate i f observed ozone and temperature v a r i a t i o n s over periods of the order of a decade can be explained by increasing l e v e l s of trace gases measured in the atmosphere. A p a r t i c u l a r a t t e n t i o n w i l l be given to the s p e c i f i c problem of the dramatic decrease in springtime ozone over the Antarctic c o n t i n e n t . T h i s phenomena, which was not predicted by any model, i s not yet e n t i r e l y understood, although several t h e o r i e s have been proposed.

These t h e o r i e s w i l l be r e v i e w e d , c r i t i c a l l y analyzed and confronted to recent atmospheric measurements.

2 . CLIMATOLOGY OF OZONE

O b s e r v a t i o n s of ozone reveal that the mean t o t a l ozone content increases with l a t i t u d e with the most pronounced l a t i t u d i n a l g r a d i e n t in l a t e winter and early spring ( F i g . 2 ) . T h e s p a t i a l d i s t r i b u t i o n i s s i g n i f i c a n t l y d i f f e r e n t in the two hemispheres. O z o n e , which i s produced by the a c t i o n of s o l a r r a d i a t i o n on molecular oxygen e s s e n t i a l l y in the upper stratosphere at mid- and low l a t i t u d e s , i s transported downwards and towards higher l a t i t u d e s by the s t r a t o s p h e r i c meridional c i r c u l a - t i o n , which is p a r t i c u l a r l y strong during winter when planetary waves propagate and d i s s i p a t e in the s t r a t o s p h e r e . Ozone t h e r e f o r e accumulates at h i g h l a t i t u d e during winter and the total column density reaches a maximum v a l u e in early s p r i n g . Because of hemispheric d i f f e r e n c e s in orography, the strength of the planetary waves (which a r e produced by the wind flow over l a r g e mountain ranges) is weaker in the Southern than in the Northern hemisphere and the meridional eddy f l u x of ozone and heat is s i g n i f i c a n t l y lower in the a u s t r a l than in the boreal r e g i o n s . The presence of a strong and undisturbed polar vortex over A n t a r c t i c a in winter e x p l a i n s the low ozone content and the r e l a t i v e l y cold temperature (about 10K lower than in the A r c t i c r e g i o n ) at the South Pole as well as the presence of a warm zonal b e l t near 60 degrees South

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LU Q 3 I-

CO

• 50

Si

J F M A M J J A

M O N T H S

F i g . 2 . - Ozone column abundance (Dobson u n i t s ) as a f u n c t i o n of l a t i t u d e and month (from London, 1 9 8 0 ) .

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with high ozone content. This morphology is important for explaining the observed ozone hole over Antarctica and shows how dynamics produces the conditions allowing for a rapid chemical ozone destruction in this region.

The temporal and geographical variations of ozone are also influenced by meteorological conditions near the tropopause, and by annual, semi-annual and quasi-biennal oscillations in wind and temperature in the stratosphere changes in the solar ultraviolet emission associated with the 27-day rotation period of the Sun and the 11-year cycle affect ozone above 30 km altitude.

3. OZONE MODELING

Atmospheric modeling of the chemical constituents has now reached the stage where it can reproduce with reasonable agreement the "natural"

atmosphere with some exceptions such as ozone in the upper stratosphere and mesosphere, where calculated densities are underestimated by 20 to 30 percent compared to observed values. The most realistic models handle more than 100 chemical and photochemical reactions and 40 active

species. Photochemistry is initiated by the penetration of solar radia- tion, which is governed by ozone and molecular absorption and multiple scattering. The transport of the species depends on the circulation which is driven essentially by wave drag associated for example with Rossby and gravity waves breaking. Dynamics is also closely related to deposition of radiative energy mainly controlled by the presence of molecules like ozone, molecular oxygen, carbon dioxide and water vapor.

Numerical models are currently used to predict the possible changes in stratospheric ozone density and temperature during the next decades. These models are based on plausible scenarios for the

increasing density of perturbing gases in the future. These scenarios are highly uncertain, so that a large number of plausible cases have to be considered. For example, in the model shown in Figure 3, the densities of C 02, CH^ and N20 are assumed to increase by 0.5, 1.0 and

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] 1 1 1 1 1 1 1 1 r 1 1 1 1 r

1950 2000 2050 2100

YEAR

F i g . 3.- Change in the ozone column abundance r e l a t i v e to 1940 predicted by the chemical-radiative-transport one- dimensional model of Brasseur ( 1 9 8 7 ) . For c o n d i t i o n s , see t e x t .

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0 . 2 5 percent per year respectively. Trends in CFC-11 ( C F C l ^ ) , CFC-12 ( C F2 c12^ ' a n d CFC-1 13 ( C ^ ^ C l ^ ) are based on h i s t o r i c a l emissions until 1985 and on prescribed scenarios for the future. In case 1, a constant emission at the 1985 level is assumed for CFC-11 and CFC-12. I n case 2 , a 3 percent growth, with a capacity cap of 1 .5 times the 1985 l e v e l , is specified for the production of CFC-11 and CFC-12. In both cases, the emission of CFC-113 is assumed to increase by 6 percent per year with an upper limit equal to the production of CFC-11. The change in the ozone column abundance calculated for these conditions by a one-dimensional chemical-radiative- transport model is characterized in both cases by a decrease until the second h a l f of the 21st century, essentially as a result of an increasing amount of chlorine in the stratosphere. The slow recovery predicted after the 2050-2080 time frame r e s u l t s from the assumed rapid growth in the methane concentration. The trend in the methane density is poorly understood and could be reduced in the future.

I t s effect is to enhance the ozone amount in the troposphere and consequently to p a r t i a l l y compensate the reduction of the ozone column resulting from the effect of chlorine compounds in the stratosphere near 40-45 km a l t i t u d e . At these h e i g h t s , the model predicts ozone reductions of 60 to 7 5 percent and temperature decreases of 25 to 30 degrees by year 2 0 4 0 . An increase of tropospheric ozone is expected but i t s magnitude w i l l dependent on the release rate of n i t r i c oxide and carbon monoxide at the surface, depending on the level of pollution at the E a r t h ' s surface. This ozone increase i s thus expected to be smaller in the Southern than in the Northern hemisphere. A warming of the surface is also predicted in the future with a magnitude which evolves as a function of the scenario for the release of the "greenhouse" gases and of the mean timescale for heat exchanges with the ocean.

Calculations based on two-dimensional models show that the response of the atmosphere is v a r i a b l e with latitude and season. The change in the ozone column resulting from a doubling of C 02 (to be achieved in the second half of the 21st Century) or from an increase in the chlorine concentration from 2 . 0 to 6 . 6 ppbv is shown as a function of latitude in Figure 4 . I n both cases, the change is largest at high

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LATITUDE (degrees)

Fig. 4.- Percentage change of the ozone column density as a function of latitude calculated for different perturbations. Upper curve : Effect of a doubling of CC>2. Lower curve : Effect of an

increase of 4 . 6 ppbv in the chlorine abundance. The change due to the latter effects as well as doubling in CH^ and a 20 percent increase in N O is shown by the third curve.

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latitude. The mean ozone depletion due to the effects of the CFCs is a factor 2-3 larger at the pole than in the tropics. The response to CC>2

is also more pronounced near the polar cap than near the equator. This situation results from a complex interplay between chemical, radiative and dynamical effects. For example, the efficiency of chlorine for destroying ozone in the upper stratosphere depends on the concentration of methane, which converts active chlorine (CI) into the reservoir.

Since methane is less abundant at high latitudes than in the tropics, where its injection into the stratosphere takes place, the chlorine catalytic cycle leading to the destruction of ozone is most efficient at high latitudes. Moreover, since the circulation is directed downwards over the pole in the winter hemisphere, air with depleted ozone will be transported from the upper to the lower stratosphere and produce a maxi- mum reduction in the column abundance at high latitude at the end of the winter season. When all perturbations are treated simultaneously, the latitudinal dependence of the change in the ozone column is reduced but the quantitative values are scenario dependent.

Finally, Figure 5 shows a simulation of the ozone and temperature variations calculated for the period during which operational satellite data are available. The densities are assumed to increase between 1979 and 1986 from 335.5 to 344 ppmv for CC>2, from 1.62 to 1.70 ppmv for CH^, from 305 to 310 ppbv for N20 , from 170 to 220 pptv for CFC-11 , from 290 to 418 pptv for CFC-12, and from 110 to 174 pptv for CH CClg. The solar irradiance is assumed to vary over the solar cycle (from peak to peak) by a factor 2 at Lyman a (121.6 ran), 15 percent in the Schumann-Runge bands region (180-200 nm), 9 percent at 205 run and 3 percent at 260 nm.

4. OZONE TRENDS

Total ozone content

Monitoring of total column ozone has been performed for more than 30 years by means of the ground-based Dobson network. The determination from these observations of a long-term trend is difficult to achieve

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MOOEL PERCENT DIFFERENCES 1986-1979

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10

-80 -60 -40 -20 0

LATITUOE (DEG)

20 40 60 80

Fig. 5.- Meridional distribution of the percent difference in ozone density between 1979 and 1986 (seasonal variation removed) in the upper stratosphere (about 30 to 50 km altitude).

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because of the poor geographical coverage of the ground-based instruments. For instance, observations are predominantly concentrated in the mid-latitudes of the Northern hemisphere leading to oversampling with respect to the equatorial zone and the Southern hemisphere. An analysis of the Dobson data shows that, after the effects of the dynamical oscillations and the solar cycle are removed, the residual (negative) trend in total ozone for the 1970-1986 time period is of the order of 2-3 percent at mid- and high latitudes in the Northern hemisphere.

Monitoring from space provides very good latitude coverage; it has been started in the 1970's with Nimbus 4 and Nimbus 7 satellites. The later, with the Solar Backscatter Ultraviolet (SBUV) spectrometer and the Total Ozone Mapping Spectrometer (TOMS), is providing continuous data since November 1978. The main problem and consequently the origin of controversies about the trends derived from these data is the degradation of the diffuser plate used when looking towards the Sun (and possible aging of both spectrometers). For the last 7 years of SBUV observations, Reinsel et al (1988) quote a linear trend of - 0.74 + 0.26 percent per year. This figure, when SBUV data are corrected for negative drift between SBUV and Dobson data, is reduced to - 0.35 + 0.28 percent per year.

Global trends in total ozone from TOMS, reported by Bowman (1988) from 1979 through 1986, show a linear decrease of 1.0 percent per year at mid-latitude with a strong latitude dependence. This instrument shares the same diffuser plate as SBUV. Consequently those trends cannot be supported because the model of the diffuser plate degradation, used to correct the data, is not unique and could introduce some artificial changes in the derived ozone abundance. If nevertheless the satellite data are normalized to the Dobson network, the trend between 65°N and 65°S is estimated to be only - 0.36 percent per year. Figure 6 shows the time-latitude distribution of the change in the ozone column between the late 70's and present, obtained after correction for instrument drift.

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. 6 . - Changes in t h e ozone column abundance between a r e f e r e n c e value (average of 1979 and 1980) and a f i n a l value (average of 1986 and 1987), as a f u n c t i o n of l a t i t u d e and time of t h e y e a r . The changes expressed in percent are deduced from TOMS d a t a c o r r e c t e d f o r d r i f t using the o b s e r v a t i o n s from the ground- based Dobson network. The d a t a of 2 consecutive y e a r s have been averaged t o avoid any s i g n a t u r e of the q u a s i - b i e n n a l o s c i l l a -

t i o n observed in t h e atmosphere ( S t o l a r s k i , personal communica- t i o n ) .

18

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The. ozone reduction is of the order generally increases with latitude, visible. The particular case of the November will be discussed in section

of 1-2 percent at the equator but A clear hemispheric asymmetry is

South Pole region in September to 5.

Vertical profile

The ground-based Umkehr technique provides the vertical distribution of ozone together with the Dobson instrument at a limited number of stations and consequently with a poorer geographical coverage than ground-based total column ozone network. This method is very sensitive to aerosol loading in the stratosphere which shows strong time dependence, as it did for example in 1982 with the El Chichon eruption.

The determination of an accurate trend in ozone as a function of height, based on this technique requires therefore a careful analysis.

Recent satellites, including Nimbus 7, the Stratospheric Aerosol and Gas Experiments (SAGE I and II), and the Solar Mesosphere Explorer (SME) have provided time series of observations by means of various instruments and different techniques. Ozone decreases of 20 percent near 50 km altitude from 1978 to 1985 have been reported on the basis of SBUV data for which, as indicated earlier, instrument drift is a major concern. This trend is not confirmed either by both SAGE I and II observations lasting respectively from 1979 to 1981 and from 1984 to present or by SME data from 1982 to 1986. These three satellite observa- tions are based on very different methods (solar occultation for SAGE and limb scattered light measurement from SME). Both data sets basically show no change near 50 km. SAGE observations suggest however a possible decrease of ozone around MO km at mid-latitudes with a maximum value of - 3 + 3 percent over the same time period. These numbers however need to be confirmed by further analyses.

Natural changes

Both the total column and the vertical distribution of ozone are affected by natural changes induced in the atmosphere by solar variation

19

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or related to the quasi-biennal oscillations (QBO). The later have a period of about 26 months. Solar irradiance variations affect ozone with a period of 27 days (rotation period of the Sun) and 11 years (solar activity cycle)

Long-term trend ozone analysis based on observations starting in late 1978 have to take into account the ozone response to solar activity during the declining phase of solar cycle 21 . According to model calculations, these changes ( - 0.20 +' 0.13 percent per year over the seven year period - Reinsel et al., 1988) are of the same magnitude as total ozone trends deduced from corrected satellite observations. It is therefore difficult to distinguish between anthropogenic and solar variability effects during this short period of time. Both effects are,

in addition, most pronounced in the upper stratosphere near 40 km altitude.

5. ANTARCTIC OZONE

Observations

In 1985, scientists of the British Antarctic Survey (Farman et al., 1985) reported that the ozone column measured in October over the scientific station of Halley Bay (76°S, 27°W) had gradually decreased by about 40 percent between 1979 to 1984 (Fig. 7). These results were based on ground-based observations obtained by means of a Dobson spectrometer. Farman suggested that this trend could have been produced by chlorine compounds of anthropogenic origin. Subsequently, satellite data available since 1979 were reanalyzed. They showed that the ozone hole is formed in early September and lasts until November, and that it extends over essentially the entire Antarctic continent. They also confirmed the Dobson measurements (Stolarski et al, 1986) as well as other ground-based data obtained for example from both the Japanese scientific station located at Syowa (69°S, 40°E) (Chubachi and Kajiwara, 1986) and the American station at the South Pole (Komhyr et al., 1986).

Finally, the satellite measurements showed that the decrease in ozone is

20

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^ 320

CO

'E

D

c o

to -Q O Û

CD C

o

N

O

280 240

|2 200

Halley Bay Station

t — i — r

— October

' ' L J L

I

I I

I960 1970

Year 1980

F i g . 7 . - Monthly mean t o t a l ozone observed at Halley Bay

(76°S,

27°W) in O c t o b e r , s i n c e

1957

(from Farman et a l . ,

1985).

21

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not entirely confined in the polar vortex but extends to latitudes near - 45°S but with smaller amplitudes. The most recent data analysis suggests that since 1979, Antarctic ozone has noticeably been perturbed all the year round (see Fig. 6).

A first campaign was organized by the United States to perform coordinated measurements at the U.S. station of McMurdo in spring 1986 and a second took place in August and September 1987 during which, in addition to the Mc Murdo's observations, measurements were taken from two airplanes flying from Punta Arenas (Chile) into the polar vortex at 12 and 18 km altitude respectively. During this campaign, the lowest value of ozone total column measured so far was recorded (about 150 Dobson).

The first campaign confirmed the recurrence of the "ozone hole".

Hofmann et al (1987) reported ozone profile measurements made from McMurdo in 1986 showing that the altitude of the ozone depletion was ranging from 12 to 22 km. Unusual chlorine and nitrogen concentrations were revealed by comparison with mid-latitude conditions. For example, observations showed very low abundances in N 02 (Mount et al, 1987;

Solomon et al, 1987 a) and large amounts of CIO in the lower stratosphere, near 20 km (De Zafra et al, 1987 ). For the first time, 0C10 molecules were detected (Solomon et al, 1987b), confirming the importance of chlorine chemistry in the polar vortex.

Measurements performed during the polar night at the South pole station (Komhyr et al, 1986) do not show such a dramatic ozone depletion, which seems to confirm that solar radiation is required to produce the ozone hole. Furthermore, measurements made by Dobson (1968) at Hal ley Bay using the moon as a light source confirm that the total ozone abundance at this station (280-300 Dobson) remains at similar levels from late January to late October. Actually, seasonal variations were always present with a minimum at springtime and a maximum value in December but with much higher springtime minima than those observed since the end of the 70's. Other atmospheric parameters such as the

22

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temperature also exhibit seasonal variations but there is no evidence that these affect ozone over Antarctica, even if recent observations have demonstrated a correlation between total ozone and the 50 mbar temperatures in October (Labitzke, 1987).

The second campaign confirmed the previous findings. An example of ozone profiles obtained from balloon soundings at Halley Bay is given in Figure 8 . The CIO abundances at the highest latitude, around 18.5 km were found 100-500 times greater than those observed at mid- latitudes, with maximum values between 0 5 and 1 ppbv at 18.5 km and a steep decrease towards lower altitudes. The stratospheric vortex was also found to be highly denitrified as well as dehydrated. T h e abundance of BrO of a few pptv was observed around 18 km altitude and was decreasing at lower levels. Very low values of CFC-11 and 12, CH CCl^

and were also measured.

An important feature is the apparently more frequent occurence of stratospheric clouds in both winter polar regions observed by the SAM II experiment on board Nimbus 7 satellite since late 1978 (McCormick et al, 1982). The presence of these clouds, illustrated by the optical depth measured at 1 micron, is noticeable in June-September 1979 with similar signatures repeated each year (Fig. 9). T h e same data also show a noticeable minimum in October, every year. Additional measurements are needed to understand the processes involved in the formation and dis- appearance of these clouds. Their importance is justified by their potential role in releasing gaseous active chlorine from chlorine reservoirs.

Theories

Different theories have been suggested to explain the increasingly stronger springtime ozone minimum over Antarctica. Some of these theories invoke natural fluctuations while other suggest a perturbation effect linked to industrial activity.

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INJ -tr X)

o

E on

Z>

V)

(/) ui a:

o.

100 -

1000

100

200

/ OCT 13

AUG 15

OZONE PARTIAL PRESSURE (nbor)

. ; .1 ...1 ( 1—I m i l l

0.01 0.10 1.00

OZONE MIXING RATIO (ppmv)

Fig. 8.- Vertical distribution of the ozone partial pressure (nbar) and volume mixing ratio observed at Halley Bay station on August 15, 1987 (high values) and October 1 3, 1987 (low values) respectively (Farman, personal communication, 1987).

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I

SAM II (1 pm) POLAR STRATOSPHERIC OPTICAL DEPTH

October 1978 - September 1986

F i g . 9.- SAM I I weekly-averaged Polar stratospheric o p t i c a l depth at 1 pm over the Arctic and Antarctic regions from October 1978 to September 1986 (from McCormick and T r e p t e , 1 9 8 7 ) .

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Among the f i r s t type of e x p l a n a t i o n s , a theory (Tung, 1 9 8 6 ) suggests that the strength of planetary wave a c t i v i t y could have been reduced over the l a s t decade eventually as a result of changes in the sea water t e m p e r a t u r e . T h i s e f f e c t would bring the Southern hemisphere closer to r a d i a t i v e e q u i l i b r i u m c o n d i t i o n s with reduced poleward and downward transport o f ozone and h e a t , the formation of u p w e l l i n g a f t e r

the return of the Sun at h i g h l a t i t u d e in spring and the l a t e r appearance of the f i n a l polar warming, Mahlman and F e l s ( 1 9 8 6 ) . The ozone d e p l e t i o n in the lower s t r a t o s p h e r e would then be produced by i n t r u s i o n of tropospheric a i r which i s known to be poor in ozone. T h i s mechanism, i f r e a l , should a l s o introduce r e l a t i v e l y high l e v e l s of gases such as N20 , CH^ or CFCs which are produced at the E a r t h ' s s u r f a c e and are t h e r e f o r e most abundant in the troposphere. T h e recent measurements made over A n t a r c t i c a c l e a r l y show that the d e n s i t y of these gases in the polar vortex is p a r t i c u l a r l y low. T h i s o b s e r v a t i o n suggests that the net v e r t i c a l transport over the polar r e g i o n i s d i r e c t e d downwards rather than upwards in winter and e a r l y s p r i n g . T h u s , dynamics cannot explain by i t s e l f the formation of an ozone hole but meteorology n e v e r t h e l e s s plays an important role by s e t t i n g up t h e s p e c i a l c o n d i t i o n s required for some chemical processes to happen. Meteorology

is a l s o c o n t r o l l i n g the termination phase of the ozone decrease in l a t e s p r i n g . It i s a l s o i n t e r e s t i n g to note that the August and September temperatures show l i t t l e changes over the 1979-1986 p e r i o d , suggesting l i t t l e dynamical v a r i a t i o n during the l a s t d e c a d e . The cooling observed s i n c e 1979 in the lower stratosphere in October and November ( a f t e r the occurence of the ozone d e p l e t i o n ) should be a t t r i b u t e d , at least in part, to a r e d u c t i o n in the absorption of solar r a d i a t i o n by ozone and in the r e l a t e d d i a b a t i c h e a t i n g .

Another theory invokes the e f f e c t of s o l a r a c t i v i t y ( C a l l i s and N a t a r a j a n , 1 9 8 6 ) . Nitrogen o x i d e s , which e f f i c i e n t l y destroy ozone in the stratosphere when s o l a r r a d i a t i o n is p r e s e n t , are produced in the thermosphere (above 85 km) by ionospheric p r o c e s s e s . The production of NO at these l e v e l s , which is c o n t r o l l e d by the i n t e n s i t y of extreme

x

u l t r a v i o l e t r a d i a t i o n , is h i g h l y dependent on s o l a r a c t i v i t y . Large

26

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amount of n i t r o g e n o x i d e s were produced d u r i n g t h e s o l a r maximum p e r i o d in t h e l a t e 7 0 ' s and e a r l y 8 0 ' s . If NO^ i s t r a n s p o r t e d downwards »in t h e w i n t e r p o l a r r e g i o n by t h e g e n e r a l c i r c u l a t i o n , and i f i t r e a c h e s t h e lower s t r a t o s p h e r e in f a i r l y l a r g e amount by t h e end of t h e w i n t e r , i t could e f f i c i e n t l y d e s t r o y ozone as t h e Sun r e t u r n s over A n t a r c t i c a . O b s e r v a t i o n s of d i f f e r e n t n i t r o g e n compounds i n d i c a t e however t h a t t h e lower s t r a t o s p h e r e i s h i g h l y d e n i t r i f i e d so t h a t t h i s t h e o r y h a s t o be r e j e c t e d . F u r t h e r m o r e , a r e c e n t a n a l y s i s of N0

2

d a t a i n d i c a t e s t h a t , i f t h e r e h a s been an i n c r e a s e in NC>

2

between 1979 and 1986, i t i s t o o small t o e x p l a i n t h e d r a m a t i c change in ozone observed over t h i s p e r i o d . F i n a l l y , t h e most s u b s t a n t i a l ozone change i n s p r i n g i s r e c o r d e d in a l a y e r below 22 km or s o , while t h e a c t i o n of n i t r o g e n o x i d e s would be n o t i c e a b l e a l s o above t h i s l e v e l , where t h e ozone d e c r e a s e , i f any, i s s i g n i f i c a n t l y s m a l l e r t h a n in t h e lower s t r a t o s p h e r e .

Other t h e o r i e s s u g g e s t t h a t t h e f o r m a t i o n of t h e ozone h o l e i s r e l a t e d to t h e r e l e a s e i n t h e atmosphere of i n c r e a s i n g amounts of c h l o r o f l u o r o c a r b o n s . The l i n k between t h e r a p i d d e s t r u c t i o n of ozone below 20 km and t h e e m i s s i o n s of t h e CFCs i s not s t r a i g h t f o r w a r d s i n c e a t t h e s e h e i g h t s , a c t i v e c h l o r i n e i s , in p r i n c i p l e , r a p i d l y t r a n s f o r m e d i n t o c h l o r i n e r e s e r v o i r s such a s HC1 and C10N0

2

, w i t h o u t e f f e c t on ozone. I f under s p e c i a l c o n d i t i o n s p r e v a i l i n g in t h e lower s t r a t o s p h e r e over A n t a r c t i c a , t h e s e r e s e r v o i r s could be d e s t r o y e d by some mechanisms t o be i d e n t i f i e d (and d i s c u s s e d h e r e a f t e r ) , ozone could be removed i n a couple of weeks by s e v e r a l c a t a l y t i c c y c l e s , provided t h a t t h e l e v e l of CIO would r e a c h about 1 ppbv. P o s s i b l e c y c l e s a r e

CI + o

3

+ CIO + o

2

OH + 0

3

-»• H0

2

+ 0

2

H0

2

+ CIO + H0C1 + 0

2

H0C1 + hv OH + CI

Net : 20 ^ + hv • 30

2

(

1

)

CI + o

3

C10 + o

2

Br

+

°

3 B r 0 +

°

2

C10 + BrO + CI + Br + 0

2

Net : 20

3

- 30

2 2 ?

(2)

(30)

2 ( C I + O3 - CIO + 02) CIO + CIO + M -»• C 1202 + M

3

C 1 0 + h v ClOO + CI ClOO + M -»• CI + 0 _ + M Net : 2 0 . + hv 30

2 ( 3 )

2 ( C I + O3 CIO +

CIO + CIO + M + C 12 ° 2 + M

C 1202 + M C l2 + 02 + M C l2 + hv 2C1

3

2

Net : 2 0 . + hv -»• 30

2 ( 4 )

The second of these c y c l e s , to be e f f i c i e n t over A n t a r c t i c a , r e q u i r e s BrO to be present in the polar vortex. O b s e r v a t i o n s made in September 1987 in the r e g i o n of the ozone hole i n d i c a t e that the amount of BrO is not l a r g e r in the vortex than o u t s i d e the vortex. However observations of 0C10 over the s t a t i o n of McMurdo, A n t a r c t i c a , in September 1986 and 1 9 8 7 , with l e v e l s 50 times larger than expected under mid-latitude c o n d i t i o n s , suggest that Bromine monoxide could play a c e r t a i n r o l e . I n d e e d , i f our understanding of the c h l o r i n e chemistry is c o r r e c t , 0C10 i s produced by the f o l l o w i n g r e a c t i o n

CIO + BrO + 0C10 + Br ( 5 )

and accumulates during n i g h t t i m e . During daytime, 0C10 is photo- d i s s o c i a t e d in the v i s i b l e and i t s concentration d e c r e a s e s . The photo- chemistry of 0 C 1 0 i s e n t i r e l y d i f f e r e n t from that of ClOO. T h i s form of c h l o r i n e o x i d e is b e l i e v e d to be produced by p h o t o d i s s o c i a t i o n of the CI 02 dimer. Anyhow, the o b s e r v a t i o n of elevated l e v e l s of 0C10 over Antarctica provides an important i n d i c a t i o n that the amount of CIO is extremely high in the polar vortex at the end of the winter and that the i n a c t i v e c h l o r i n e r e s e r v o i r s a r e probably destroyed in l a t e August and early September. I n d e e d , observations made in l a t e September 1987 from the NASA ER-2 a i r c r a f t i n d i c a t e that the mixing r a t i o of c h l o r i n e monoxide at 1 8 . 5 km a l t i t u d e i s a factor o f 100 l a r g e r w i t h i n the r e g i o n

28

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of very low ozone than at mid-latitudes. The measured density of CIO at 18.5 km (about 1 ppbv) is sufficiently large to explain the destruction of ozone at this height, if our current understanding of the chlorine dimer catalytical cycles (3) and (4) is correct. The abundance of CIO seems to decrease rapidly below 18 km, suggesting that other processes (including a vertical downward transport) could be involved.

As indicated earlier, the chlorine theory requires significant amounts of active chlorine to be liberated from the reservoirs (HC1 and C10N0o). It also requires low levels of NO to avoid the transformation

2 x of CIO into C10N02. Several ways to destroy the chlorine reservoirs have

been suggested. These explanations are constrained by the fact that the proposed mechanism should operate only in early spring, primarily in the lower stratosphere over Antarctica as opposed to other altitudes, latitudes and time of the year, i.e., in a stable region with temperatures lower than about 200 K. Crutzen and Arnold (1986) have noted that the condensation of nitric acid (HNO^) could condense into small particles, as soon as the temperature of the lower stratosphere decreases below about 205 K. Ice crystals can start forming only below 191 K. A polar stratospheric haze with small H N O ^ - H ^ and H N O ^ ' S H ^ particles should thus be formed in the lower stratosphere at temperatures between 205 and 191 K, while, below this latter temperature, together with HNO^, H20 and HC1 would also freeze. The largest particles, which form the so-called polar stratospheric clouds (PSCs), are expected to be removed from the stratosphere by gravita- tional sedimentation. This mechanism may contribute to remove N0x and H20 from the lower stratosphere (if N ^ is converted into HNO^ by heterogeneous reactions on the surface of ice particles). As nitric acid

is removed from the gas phase, the abundance of the OH radicals, which are destroyed essentially by HNO^ in the lower stratosphere, is expected to be significantly enhanced. OH is very efficient in destroying HC1 and producing active chlorine. When HNO^ is again released to the gas phase by evaporation at higher temperature, active chlorine is transformed back into HC1 and C10N02 and the ozone decay slows down. Figure 10 shows a model simulation of density of the chemical species at 18 km from

29

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>

jQ Q. CL

200 210 220 230 240 250 260 270 280

DAY

Fig.10.- Chemical developments at 18 km from day 185 (July, 5) to day OH concentrations volume 285 (October, 15) in 1986.

in units of 10^ molecules Left hand scale

-3 - right hand scales

mixing ratios of 0^ (ppmv) and for CIO, HNO^» C10N0 cm

2

, CIOH and HC1 (ppbv) for the temperature sequence measured by Hofmann et al (1987 ) at McMurdo ' (7 8°S) in Antarctica. The small bumps in the curves indicate the times when daylight hours change discontinuously in the program (from Crutzen et al, 1988).

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July, 5 to October, 15, 1986 based on the theory expressed by Crutzen and Arnold (Crutzen et al., 1988). The significant increase in OH to a density as high as 10^ cm"^ and the resulting decrease in HC1 and ozone are clearly visible. Also, the enhancement in the CIO and H0C1 abundance can be noticed. A detailed quantitative study of this mechanism is required to establish if it can entirely explain the observed springtime polar ozone trend and if it would account for the rather sudden appearance of the ozone hole after 1979.

Solomon et al. (1986) were the first to point out that the formation of the ozone hole in spring could be associated with the presence of polar stratospheric clouds which are produced and observed

in winter (McCormick, 1987). They suggested that the following reactions occur heterogeneously on the surfaces of ice particles :

HCI + CIONO2 H N O3 + c i2 (6)

H20 + C10N02 H N 03 + H0C1 . (7 )

The efficiency of these reactions however are expected to be small since the probability for two gaseous phase molecules to collide on an aerosol surface is low. Molina et al. (1987 ) as well as Wofsy et al. (1988) have shown that the reaction of atmospheric HCI with C10N02 is considerably enhanced in the presence of ice particles. In fact, at low temperature, HCI molecules dissolve in ice crystals and the probability for a reaction of gas phase C10N02 with dissolved HCI to occur at the surface of the solid particles is of the order of 5 to 10 percent at about 200 K

- 2

for an HCI mole fraction between 3.5 x 10 and 1 x 10 . Nitric acid which is produced by this reaction, remains in the condensed phase (so that this process contribute to the denitrification of the stratosphere), while C l2 is released in the gas phase on a time scale of a few milliseconds. The chlorine molecule is then photodissociated into 2 atoms which react immediately with ozone molecules. The reaction of C10N02 with ice (HgO) has a collision efficiency of about 2 percent at 200 K and produces H0C1 in the gas phase on a time scale of minutes.

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Wofsy et al. (1988) have shown that solid solutions of HC1 in H20 ice with a mole fraction of (2.0 - 3-5) 10 2 will form in the stratosphere between 193 and 190 K and that condensation of small quantities of water vapor leads to nearly complete removal of HC1 (and HNO^) from the gas phase. The following cycle will transform HC1 into C10NC>2 and NC>2 (in the gas phase) into HNO^ (in the condensed phase).

C10N0'2 (gas) + HC1 (solid) + C l2 (gas) + HNO^ (solid) (8.a)

C l2 + hv 2C1 (8 . b)

2(CI + 03 - CIO + 02) (8.c)

2(CIO + N 02 C 1 0 N 02) (8.d)

Net : HC1(solid) + 2N02'(gas) + 2 03 + hv

+ C10N02(gas) + H N 03 (solid) (8)

So far, this mechanism has little effect on ozone. However, if the initial HC1 number density (before onset of condensation) is equal or larger than half of the N 0x density (N0x being the sum of nitrogen oxides readily converted to N 02) , the last reaction in this cycle (reaction 8.d) will no more be possible due to exhaustion of NO^. At this stage, CIO accumulates and ozone is being efficiently destroyed by cycle's (1) to (4). Thus, during polar night, when stratospheric clouds are present, preexisting HC1 condenses into ice particles and reaction (8.a) depletes gas phase C 1 0 N 02, producing gaseous C l2 > As the Sun returns over Antarctica, C l2 is photolyzed and N 0x depleted, producing first C10N02 molecules and later CIO radicals. It is interesting to note that the initial density of HC1 is a key factor in this scheme.

According to Wofsy et al., the sudden onset of the Antarctic ozone depletion in the late 1970's could reflect the growth of HC1 density beyond a threshold equal to half the N 0x abundance.

In conclusion, the formation of an ozone hole in spring over Antarctica seems to be closely linked to the presence of polar strato-

spheric clouds. These are observed in the altitude range where a large fraction (eventually more than 90 percent) of ozone is depleted in

32

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September. The details of the clouds formation are not yet fully under- stood and an extensive program in the area of heterogeneous chemistry is urgently needed. Dynamics creates the conditions for the winter temperature to become sufficiently low for clouds to be produced. The polar vortex is quasi isolated from the rest of the stratosphere.

Although the ozone hole seems to be particular to conditions prevailing near the South pole, it is important to determine if the processes involved could play a role in other regions of the world and if large ozone reductions could be produced at present or in the future, • for example, in the Arctic region. It is also necessary to determine if the region of large ozone reduction could become wider or deeper and to what extend the air with low ozone could be diluted towards lower latitudes and affect the entire Southern hemisphere. If the theory in which the ozone hole is linked to increasing abundances of chlorine is proved to be correct, it can be stated that the springtime hole over Antarctica should be persistent over several decades, even if the release in the atmosphere of chlorofluorocarbons was 'dramatically reduced. This is a direct consequence of the long atmospheric lifetime (about 100 years) of

the CFCs. On the other hand, the ozone column abundance in the hole is not expected to further decrease in a substantial way because in the region, where the depletion mechanisms are important, almost all of the ozone is already destroyed each spring. It is nevertheless crucial to determine if the extend in altitude and latitude of the PSCs remains unchanged from year to year.

6. CONCLUSIONS

The ozone abundance in the atmosphere is predicted to change as a result of increasing concentrations of trace gas such as methane, nitrous oxide, carbon dioxide and the chlorofluorocarbons. Model calculations show that stratospheric ozone and temperature should be significantly reduced in the future if present growth in the abundance of these source gases, mainly as a result of anthropogenic activity, is extrapolated to the next century. Ozone and temperature in the tropo- sphere are expected to increase. Analyses of data provided by ground-

33

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based instruments indicate that total ozone has decreased by as much as 2 percent since 1970. During the 1979-1986 period, during which satellite measurements were made on a continuous basis, the observed ozone reduction in the upper stratosphere by anthropogenic effect is believed to be comparable to the ozone decrease expected from the reduction in solar activity over this period. Quantitative values are subject to controversy because of instrument degradation, leading to large uncertainties in current observations.

Over the Antarctic continent, the springtime ozone column has been reduced by about 50 percent since 1979. The processes leading to this ozone hole are not yet fully understood. Recent measurements however suggest that, in the presence of polar stratospheric clouds, active chlorine is released in the gas phase and destroys efficiently ozone in the lower stratosphere.

The protection of the ozone layer requires limitations in the emission of the chlorofluorocarbons. Even the recent protocol on the protection of the ozone layer, signed in Montreal, Canada, in September 1987 is applied by most countries, the amount of active chlorine in the stratosphere should reach 6 to 7 ppbv in year 2050 as opposed to 2.5 to 3.0 ppbv at present. Therefore despite regulations recently adopted, ozone and temperature are expected to be modified in the future, as a result of human activities.

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R E F E R E N C E S

BOWMAN, K.P., Global trends in total ozone, Science, 239, 48-50, 1988.

Brasseur, G., The endangered ozone layer environment, 29, 6 - 1 1 , 39-45, 1987.

CALLIS, L . B . and M . NATARAJAN, The Antarctic ozone minimum : relation- ship to odd nitrogen, odd chlorine, the final warming, and the 11-year solar cycle, J . Geophys. Res., 91., 10771-10796, 1986.

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