I 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 A C T A
A - N° 2 7 0 - 1 9 8 3
v
Coupling between the thermosphere and the stratosphere : the role of nitric oxide
by
G. BRASSEUR
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 1180 BRUSSEL
F O R E W O R D
T h i s paper has been presented at the International Symposium on G r o u n d - b a s e d Studies of the Middle Atmosphere held in Schwerin ( G D R ) from May 9 to May 13, 1983. It will be published in the Middle Atmo- sphere Program Handbook.
A V A N T - P R O P O S
Cet article résume la communication présentée au symposium inter- national intitulé " G r o u n d - b a s e d Studies of the Middle Atmosphere" qui s'est tenu à Schwerin ( R D A ) du 9 au 13 mai 1983. Il sera publié dans le "Middle Atmosphere Program Handbook".
V O O R W O O R D
Dit artikel werd voorgesteld tijdens het internationaal symposium over "Groünd^based Studies of the Middle Atmosphere", gehouden te Schwerin ( D D R ) van 9 tot 13 mei, 1983. Het zal verschijnen in het
"Middle Atmosphere Program Handbook".
V O R W O R T
Dieser Text wurde vorgestellt während der internationale T a g u n g über " G r o u n d - b a s e d Studies of the Middle Atmosphere", in Schwerin ( D D R ) von 9 zum 13 . Mai, 1983. Der Text wird in dem "Middle Atmo- sphere Program Handbook" herausgegben werden.
C O U P L I N G B E T W E E N T H E T H E R M O S P H E R E A N D T H E S T R A T O S P H E R E : T H E R O L E OF N I T R I C O X I D E
by
G. B R A S S E U R
Abstract
Two-dimensional model calculations reveal that the chemical condi- tions at the stratopause are related to the state of the thermosphere.
T h i s coupling mechanism can be partly explained by the downward transport of nitric oxide d u r i n g the winter season and consequently depends on the dynamical conditions in the mesosphere and in the lower thermosphere (mean circulation and waves). In summer, the photodisso- ciation of nitric oxide plays an important role and the thermospheric NO abundance modulates the radiation field reaching the upper strato- sphere. Perturbations in the nitric oxide concentration above the meso- pause could therefore have an impact in the vicinity of the stratopause.
Résumé
Des calculs effectués à l'aide d ' u n modèle bi-dimensionnel montrent que les conditions chimiques à la stratopause sont liées à l'état de la thermosphere. Ce mécanisme de couplage peut être expliqué en hiver' par le transport de l'oxyde d'azote vers le bas et dépend donc directe- ment des conditions dynamiques dans la mésosphère et la thermosphère inférieure (circulation générale moyenne et ondes). En été, la photodis- sociation de l'oxyde d'azote joue un rôle important et le NO thermo- sphérique absorbe la radiation qui produit cette photodissociation dans la mésosphère et la stratosphère supérieure. Les perturbations dans la concentration de l'oxyde d'azote a u - d e s s u s de la mésopause peuvent donc produire des effets jusqu'à proximité de la stratopause.
Samenvatting
Berekeningen die gedaan - werden met behulp van een twee- dimensionaal model tonen aan dat de chemische toestanden aan de strato- pauze nauw verbonden zijn met de staat van de thermosfeer. Dit koppe- lingsmechanisme kan gedeeltelijk verklaard worden door het beneden- waarts transport van stikstofoxyde tijdens de winter en is bijgevolg afhankelijk van de dynamische toestanden in de mesosfeer en de lagere thermosfeer (gemiddelde algemene circulatie en golven). Tijdens de zomer, speelt de fotodissociatie van de stikstofoxyde een belangrijke rol en de thermosferische NO absorbeert de straling die deze fotodissociatie veroorzaakt in de mesosfeer en de hogere stratosfeer. De storingen in de concentratie van de stikstofoxyde boven de mesopauze kunnen d u s hun invloed doen gelden in de nabijheid van de stratopauze.
Zusammenfassung
Berechnungen gemacht mit der Hilfe von einem zweidimensionalen Modell zeigen dass die Zustände an der Stratopause eng v e r b u n d e n sind mit dem Zustand der Thermosphäre. Dieser Kupplungsmechanismus kann teilweise erklärt werden d u r c h das hinuntertransportieren vom Stikstoff- o x y d im Winter und ist folglich abhängig von den dynamischen Z u - ständen in der Mesosphäre und der niedriger Thermosphäre (mittlere allgemeine Zirkulation und Wellen). Im Sommer, ist die Photodissoziation vom Stikstoffoxyd sehr wichtig und das thermosphärische NO absorbiert die Strahlung die die Photodissoziation v e r u r s a c h t in der Mesosphäre und der höhern Stratosphäre. Die S t ö r u n g e n in der Konzentration vom Stikstoffoxyd über der Mesopause können also Effekt haben in der Nähe der Stratopause.
A s indicated for example b y Danilov a n d T a u b e n h e i m ( 1 9 8 3 ) , the b e h a v i o r of the D - r e g i o n is s i g n i f i c a n t l y different in summer and in winter. D u r i n g the f i r s t of these s e a s o n s , the electron d e n s i t y seems f a i r l y d e p e n d e n t on the solar zenith angle while, d u r i n g the w i n t e r , considerable d a y to d a y v a r i a t i o n s completely mask a n y control of the i o n o s p h e r e b y solar or g e o p h y s i c a l parameters. F u r t h e r m o r e , as r e - ported a l r e a d y b y A p p l e t o n in 1937, anomalous i n c r e a s e s in a b s o r p t i o n of h i g h f r e q u e n c y radio waves o c c u r d u r i n g certain g r o u p s of winter d a y s . Even outside these i r r e g u l a r winter a n o m a l y e v e n t s , the a b s o r p t i o n a n d c o n s e q u e n t l y the electron concentration a p p e a r s to be c o n s i d e r a b l y h i g h e r in winter than, in summer. S i n c e the quiet time ionization in the D - r e g i o n is due primarily to the action of the solar Lyman a radiation on nitric oxide molecules, the u n d e r s t a n d i n g of the lower ionosphere a n d its probable control b y dynamical p r o c e s s e s r e q u i r e s a detailed u n d e r s t a n d i n g of the N O d i s t r i b u t i o n in the meso- s p h e r e .
In o r d e r to achieve s u c h a s t u d y , a two-dimensional model with coupled chemical and dynamical p r o c e s s e s has been c o n s t r u c t e d . T h i s model r a n g e s from 40 to 100 km altitude and from the N o r t h to the S o u t h pole. Dynamical parameters s u c h as the meridional circulation a n d the e d d y d i f f u s i o n t e n s o r are p r e s c r i b e d . In p a r t i c u l a r , the two- dimensional e d d y components are taken from Ebel (1980) b u t , as
indicated hereafter, some c h a n g e s h a v e been i n t r o d u c e d in several model r u n s in o r d e r to estimate the s e n s i t i v i t y of the dynamical activity in the lower t h e r m o s p h e r e on the calculated concentration v a l u e s . T h i s model which is in many aspects similar to the model developped b y Solomon et al. (1982) and which is d e s c r i b e d in detail b y B r a s s e u r a n d De Baets (1983), c o n s i d e r s the most important chemical a n d p h o t o - chemical p r o c e s s e s related to the odd o x y g e n , o d d n i t r o g e n a n d o d d h y d r o g e n species as well as the positive and negative ions and the electrons. T h i s paper will deal essentially with the b e h a v i o r of nitric oxide below 100 km.
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Nitric oxide is p r o d u c e d in the s t r a t o s p h e r e b y the oxidation of n i t r o u s oxide ( N „ 0 ) in the p r e s e n c e of the electronically excited atomic
1
o x y g e n 0 ( D ) . A n additional s o u r c e , essentially at h i g h latitude, is due to the action of the cosmic r a y s . In the t h e r m o s p h e r e , ions which are p r o d u c e d b y solar E U V and X r a y s as well as b y particle precipitation, especially in the auroral belts (relativistic electron precipitation, solar proton e v e n t s , . . . ) lead to the formation of N O molecules. Direct dissociation or dissociative ionization of is another s o u r c e of N O . Calculations made b y R u s c h et al. (1981) indicate that each ion pair formation p r o d u c e s 1.3 nitric oxide molecules. C o n s e q u e n t l y the thermo- s p h e r i c N O p r o d u c t i o n rate will be controlled b y the solar a n d g e o - magnetic a c t i v i t y . In the p r e s e n t p a p e r , only quiet conditions will be c o n s i d e r e d . D o w n w a r d t r a n s p o r t of nitric oxide from the t h e r m o s p h e r e will d e p e n d on the s t r e n g t h of the vertical e x c h a n g e s and of the chemical stability of N O in the m e s o s p h e r e . A s indicated b y f i g u r e 1, the nitric oxide f l u x is directed d o w n w a r d s in the whoje m e s o s p h e r e d u r i n g the winter when the lifetime of N O is long a n d the Kz z v a l u e s are large, indicating that t h e r m o s p h e r i c nitric oxide could reach the s t r a t o s p h e r e and interact with the ozone layer. C o m p a r i s o n s of calculated and o b s e r v e d 03 d e n s i t y for different solar activity levels (Solomon and Garcia, 1983) g i v e indirect evidence for s u c h a N Ox
t r a n s p o r t above 60° N d u r i n g the winter.
D u r i n g the summer s e a s o n , the d o w n w a r d t r a n s p o r t b y e d d y d i f f u s i o n is weak and is even slowed down b y the u p w a r d meridional circulation. T h e loss of NO b y photodissociation and recombination is intense a n d is even enhanced b y the fact that the lower temperature in
4
the v i c i n i t y of the summer mesopause r e d u c e s the rate of the N ( S ) + 02 reaction (reformation of NO after its photodissociation) a n d t h u s f a v o r s the N (4S ) + N O reaction ( d e s t r u c t i o n of o d d n i t r o g e n ) . T h e r e - fore no dynamical coupling between the t h e r m o s p h e r e and s t r a t o s p h e r e a p p e a r s in summer and the nitric oxide f l u x is directed u p w a r d s d u r i n g this period of the y e a r . H o w e v e r , as pointed out b y F r e d e r i c k et al.
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V E R T I C A L O D D N I T R O G E N F L U X ( c m "2s ~1)
WINTER LATITUDE (degrees) SUMMER Fig. 1. Meridional distribution of the vertical flux component of nitric oxide
calculated with the exchange coefficients suggested by Ebel (1980).
(1983), the absorption of the UV radiation by variable thermospheric NO could modulate the radiation field reaching the lower mesosphere and the upper stratosphere and consequently modify the dissociation rate of nitric oxide in the 6 bands at these levels. T h e magnitude of this effect appears however to be probably smaller than the 11 year variability of the solar irradiance. F i g u r e 2 shows the calculated distribution of the nitric oxide concentration for winter and summer conditions. It can be seen that more nitric oxide is present in winter and that the concentra- tion minimum at the mesopause level is v e r y weak d u r i n g this season.
In order to estimate the sensitivity of the NO distribution on the strength of the vertical t r a n s p o r t , the Kz z values have been decreased in the thermosphere by a factor which is uniform with latitude. T h r e e cases have been considered : case 1 refers to the Ebel's values, case 2 to a v e r y slow diffusion coefficient K . and case 3 to an intermediate ' min value. F i g u r e 3 shows the 3 corresponding profiles for summer and winter mid-latitude. T h e Kz profile suggested b y Allen et al. (1981) to explain observed atomic o x y g e n distributions by their 1 - D model is also indicated.
T h e nitric oxide mixing ratio and flux at the stratopause for the two extreme cases (1 and 2) are shown in f i g u r e 4a and b. T h e s e f i g u r e s indicate again that a coupling between the thermosphere and the stratosphere is possible essentially d u r i n g the winter. T h e s t r e n g t h of the coupling as well as the latitude of the border between the downward and the upward NO exchange regions varies with the adopted profile and with the downward f l u x imposed at the upper boundary (which reflects the integrated NO production above this level and consequently the solar and geomagnetic a c t i v i t y ) . In order to estimate this last effect, the nitric oxide flux at 100 km has been uniformly doubled. T h e corresponding impact on the stratopause NO is also in- dicated in f i g u r e s 4a and b.
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N I T R I C O X I D E C O N C E N T R A T I O N ( c m "3)
0 30 L A T I T U D E ( d e g r e e s )
Fig- 2. Meridional distribution of the nitric oxide concentration calculated with the exchange coefficients suggested by Ebel (1980).
VERTICAL E X C H A N G E COEFFICIENT ( c m2^ )
Fig. 3. Different vertical exchange coefficients K ^ adopted in the model calculations. Vertical distributions represented at 30° latitude for winter and summer conditions. The profile used by Allen et al. (1981) is also indicated.
W I N T E R SUMMER
Fig. 4a. Latitudinal and seasonal distribution of ,the nitric oxide mixing ratio calculated at the stratopause (50 km altitude) assuming different conditions : refers to the exchange coefficients suggested by Ebel, K„. to the smallest vertical eddy diffusion depicted in fig. 3;
' M m
<|)(N0) refers to an upper boundary flux condition corresponding to quite solar conditions (Solomon, private communication, 1981) and <|)(N0) x 2 to the imposed flux which has been multiplied by 2.
LATITUDE(DEGREES)
W I N T E R SUMMER
Fig. 4b. Same as in fig. 4a but for the* nitric oxide vertical flux.
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C o m p a r i s o n s between o b s e r v e d a n d calculated nitric oxide profiles are not s t r a i g h t f o r w a r d since the measured c o n c e n t r a t i o n s exhibit large v a r i a t i o n s . T h i s v a r i a b i l i t y might p a r t l y be attributed to instrumental e r r o r s b u t it also reflects the large c h a n g e s o c c u r i n g in the real w o r l d . It seems however that most o b s e r v a t i o n s show a concentration minimum
6 7 - 3
near 85 km altitude (10 to 10 cm ) b u t that this minimum is c o n - 6 7 - 3
s i d e r a b l y weaker d u r i n g the winter (5 x 10 to 5 x 10 cm ) . O b s e r - vations made d u r i n g winter anomaly e v e n t s ( B e r a n a n d B a n g e r t , 1979) show large N O densities a n d a vertical profile indicating almost perfect mixing conditions ( a n d c o n s e q u e n t l y s t r o n g vertical e x c h a n g e s between 50 and 80 k m ) .
T h e comparison between available data a n d the calculated profiles obtained with the 3 different t r a n s p o r t coefficients s u g g e s t s that case 3 (intermediate Kz z) is somewhat more r e p r e s e n t a t i v e of most nitric oxide o b s e r v a t i o n s than the other e d d y d i f f u s i o n profiles. T h e c o r r e s p o n d i n g meridional d i s t r i b u t i o n of N O is s h o w n in f i g u r e 5 a n d s h o u l d be com- p a r e d with the r e s u l t s depicted in f i g u r e 2. It s h o u l d be remembered that these model r e s u l t s refer to a v e r a g e seasonal a n d d i u r n a l c o n - ditions. T h e magnitude of the diurnal variation of N O at selected alti- tude a n d at 30 d e g r e e s latitude can be estimated from f i g u r e 6.
Finally, the electronic concentration which is d e r i v e d from the N O d i s t r i b u t i o n shown in f i g u r e 5 a n d which is obtained from a detailed ionic model is r e p r e s e n t e d in f i g u r e 7. It can be seen that the c o n c e n - tration of electrons is c o n s i d e r a b l y h i g h e r in winter owing to the fact that the nitric oxide d e n s i t y is l a r g e r d u r i n g this season and that the temperature a n d c o n s e q u e n t l y the effective electron loss are h i g h e r in the winter hemisphere. T h e model e x p l a i n s t h u s satisfactorily the o b s e r v e d h i g h e r radio wave a b s o r p t i o n d u r i n g wintertime ( w h i c h is sometimes called the r e g u l a r component of the winter anomaly) but cannot explain the c a u s e s of the i r r e g u l a r components of s u c h anomalous e v e n t s since the calculations are performed with seasonal a v e r a g e s of temperature, d i f f u s i o n coefficients and wind components. Satellite data
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N I T R I C OXIDE C O N C E N T R A T I O N (cm~J) K „ ( n r 3)
60 30 W I N T E R
LATITUDE ( d e g r e e s )
30 60 S U M M E R
RUN 38
Fig. 5. Meridional distribution of the nitric oxide concentration calculated with the intermediate values of (case 3) and the Ebel's values for K and K .
yy yz
co I
f> I
E o
<
cr
LU
u z
O (J
12 18 1U
T I M E (HOURS)
6- Diurnal variation of nitric oxide and nitrogen dioxide calculated at 70 and 85 km altitude for spring conditions and 30 degrees latitude. The total N0x concentration is assumed to be 1.2 x 107 and 1.2 x 106 cm'3
at 70 and 85 km respectively.
ELECTRONIC CONCENTRATION (cm~3)
i
30 0 30 LATITUDE (degrees)
RUN 53
Fig^_7. Meridional distribution of the 24 hours averaged concentration of electrons calculated with the NO distribution shown in figure 5. These values correspond to solar quite conditions.
might provide indications on the relative role played by nitric oxide and by the temperature in the appearance of sudden anomalous absorption events.
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R E F E R E N C E S
- A L L E N , M . , Y . L . Y U N G and J.W. W A T E R S , Vertical t r a n s p o r t a n d p h o t o h e m i s t r y in the terrestrial m e s o s p h e r e a n d lower t h e r m o s p h e r e (50-120 k m ) , J. G e o p h y s . R e s . , 86, 3617, 1981.
- A P P L E T O N , E . , Regularities and i r r e g u l a r i t i e s in the i o n o s p h e r e - I, P r o c . R o y . S o c . , A 162, 451, 1937.
- B E R A N , D . a n d W. B A N G E R T , T r a c e c o n s t i t u e n t s in the m e s o s p h e r e and lower t h e r m o s p h e r e d u r i n g winter anomaly e v e n t s , J. Atm.
T e r r . P h y s . , 1091, 1979.
- B R A S S E U R , G. and P. D E B A E T S , Minor c o n s t i t u e n t s in the meso- s p h e r e and lower t h e r m o s p h e r e , in p r e p a r a t i o n ( 1 9 8 3 ) .
- D A N I L O V , A . D . a n d J. T A U B E N H E l M , N O and temperature control of the D - r e g i o n , S p a c e S c i . R e v . , 34, 413, 1983.
- E B E L , A . , E d d y d i f f u s i o n models for the m e s o s p h e r e and lower t h e r m o s p h e r e , J. Atm. T e r r . P h y s . , 42, 617, 1980.
- F R E D E R I C K , J . E . , R . B . A B R A M S and P . J . C R U T Z E N , T h e delta- b a n d dissociation of nitric oxide : A potential mechanism for c o u p l i n g t h e r m o s p h e r i c v a r i a t i o n s to the mesosphere and s t r a t o - s p h e r e , submitted to J. G e o p h y s . R e s . , 1983.
- R U S H , D . W . , J . C . G E R A R D , S . S O L O M O N , P . J . C R U T Z E N , a n d G . C . R E I D , T h e effect of particle precipitation e v e n t s on the neutral a n d ion chemistry of the middle atmosphere, I O d d n i t r o g e n , Planet. S p a c e S c i . , 29, 767, 1981.
- S O L O M O N , S . , P . J . C R U T Z E N and R . G . R O B L E , Photochemical c o u p l i n g between the t h e r m o s p h e r e a n d the lower atmosphere, I O d d n i t r o g e n from 50 to 120 km, J. G e o p h y s . R e s . , 87, 7206, 1982.
- S O L O M O N , S . and R . R . G A R C I A , T r a n s p o r t of t h e r m o s p h e r i c N O to the u p p e r s t r a t o s p h e r e ? , p r e p r i n t , 1983.
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