AERONOMICA ACTA

Texte intégral

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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, avenue Circulaire, U C C L E - BRUXELLES 1 8

A E R O N O M I C A A C T A

A - N° 34 - 1964

Nitrogen oxides in the chemosphere by M . NICOLET

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, UKKEL - BRÜSSEL 1 8

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FOREWORD

The preparation of space experiments as well as the recent observations at the Set Propulsion Laboratory, require a detailled analysis of the chemical processes in which nitrogen oxides are involved. "Nitrogen oxides in the chemosphere" is the result of a thoroughly research on that problem which has already been studied before. This paper will be published during 1965 in the Aeronomical and Planetary Sciences section of The Journal of Geophysical Research, USA.

M . Nicolet.

AVANT-PROPOS

La préparation d'expériences spatiales en même temps que les récentes observations du Jet Propulsion Laboratory, requièrent une analyse détaillée des processus chimiques où interviennent les oxydes d'azote. "Nitrogen oxides in the chemosphere" est le résultat d'une recherche très approfondie de ce problème qui avait déjà été l'objet d'études antérieures.

Ce travail paraîtra dans la section s Aéronomie et sciences planétaires du Journal of Geophysieal Research; USA, au cours de l'année 1965.

M . N'&eolet.

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De voorbereiding van ruimteproefnemingen, terzelftertijd als de jongste waarnemingen van het Jet Propulsion Laboratory ver- eisen een uitgediepte ontleding van de scheikundige reakties in dewelke de stikstofoxyden tussenkomen. "Nitrogen oxides in the Chemosphere" is het resultaat van een zeer diepe studie van dit probleem, dat reeds vroeger het voorwerp van studies was geweest.

Dit werk zal in de loop van het jaar 1965 verschijnen in de afde- ling : ABronomie en planetarische wetenschappen van de Journal of Geophysical Research, USA.

M. Nicolet.

VORWORT

Die Vorbereitung für Raumforschungexperimenten sowie die letzten Messungen im Jet Propulsion Laboratory erforden eine um- ständliche Analyse der chemischen Vorgange in welchen die Stick- stoffoxyde eintreten. "Nitrogen oxides in the chemosphere" ist das Ergebnis einer durchausgeführte Erforschung Uber dieses Problem, das schon vorher studiert war. Diese Arbeit wird in der Abteilung der Aeronomie und planetarischen Wissenschaften des Journal of Geophysical Research, USA, in 1965 herausgegeben werden.

M. Nicolet.

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NITROGEN OXIDES IN THE CHEMOSPHERE

by

Marcel NICOLET

ABSTRACT

A study is made of the various reactions in which nitrogen oxides are involved in the chemosphere. The hydrogen compounds do not play an important role, and it is found that the essential aeronomic reactions depend on ozone and atomic oxygen. Thus, the ratio nitrogen dioxide-nitric oxide can be determined. The absolute values of the NO and NO concentrations depend on the dissociation of molecular

2

nitrogen in the chemosphere. The chemical conditions cannot be applied in the mesosphere since the life-time of NO is relatively long, and a downward transport is involved. Very special assumptions concerning chemical reactions would be necessary to reconcile the photochemical

picture and the observational results. The introduction of ionic reactions, considered in an accompanying paper, w i l l lead to a correct interpretation.

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1. - INTRODUCTION

The subject of nitrogen oxides was introduced [Nicolet, 1945]

in aeronomic studies of nitric oxide in its consideration as an important ionic constituent of the ionospheric D region. The photochemistry of nitrogen was first studied by Bates [1952] and has been further studied by Bates [1954] and Nicolet [1954], This problem, which is much more complicated than that of the photochemistry of oxygen, was developed by Nicolet [1955] in relation with the airglow. Calculations are difficult because the chemical aeronomy of nitrogen in an oxygen atmosphere leads to a complex problem as recently discussed by Nicolet [i960] and Barth [l96l]. The presence of important concentrations of nitric oxide ions requires a special analysis of ionic reactions related to the presence or absence of the neutral molecule. Our experimental knowledge concerning rate coefficients is not yet complete, but has increased rapidly, however, in recent years and a systematic account can be found in several review papers : Three body reactions by Barth [1964], reactions involving nitrogen and oxygen by Schiff [1964] and aeronomic reactions involving hydrogen by Kaufman [1964].

Attention here will be confined to the photochemistry of nitrogen.

An attempt will be made in another paper to cover the subject of related ionic reactions which modify the photochemical picture and add greatly to the significance of the occurence of atomic nitrogen and its oxides in the ionospheres

2." NITROGEN DIOXIDE AND NITRIC OXIDE RATIOS BELOW 100 KM.

If nitrogen dissociation is operative at sufficiently low altitudes, the formation of nitric oxide may be due to a three-body process

N + 0 + M NO + M + 150 kcal (1)

for which the rate coefficient b is not yét fcmown with sufficient precision

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[Mavroyannis and Winkler, 1961as Krestchner and Petersen, 1963; Barth, 1964], Calculations will be made with the following recombination rate

-32 3 -1

b, = (1 + 0.5) x 10 n(M) cm sec . (2) la ~

Three-body recombination of nitrogen and oxygen atoms leads to the emission of various electronic bands of nitric oxide [Barth et al., 1959].

A pre-association process such as [Young and Sharpless, 19635 Callear and Smith» 1964]

N(4S) + 0(3p) - N0(4XI) - N0(C2n ) (3)

followed by

2 2 7 -1

N0(C n) N0(X n) + Y bands % A ^ = 1.5 x 10 sec (4a)

and

2 3 8 * 1

N0(c n) -» N( S) + 0( P) 5 a2 1 » 3.5 x 10 sec" (4b) results in nitric oxide formation with a rate coefficient, b., ,

lb

b_, = 1 x 10 ^cm^sec * (5) lb

15 -3 which is more effective than (2) where n(M) is less than 10 cm »

The chemiluminescent production of the (3, y a nd 6 bands which

was investigated by Young and Sharpless [1963] leads to the following absolute rates of the emission processes

®"3 1 P bands ; 2.4 x 10 n(M) n(0) n(N) cm sec

Y bands s 8.2 x 10~1 8 n(N) n(0) + 2.4 x 10~3 5 n(M) n(0) n(N) cm"3sec"1

measured at 4 mm Hg.

Before considering other nitric oxide formation processes, it is necessary to simplify the study of nitric oxide by analysing its reactions with atomic oxygen and ozone, which are the principal active constituents

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in the chemosphere. First, there is the following three-body process

0 + NO + M N 02 + M + 72 kcal (6)

The reaction has a relatively high rate coefficient [Kaufman, 1958{ Ogryzlo and Schiff, 1959; Clyne and Thrush, 1962c? Schiff, 1964? Reeves et al„, 1964] with a negative activation energy of about 1.8 + 0.4 kcal according to Clyne and Thrush [1962 c ] or 1.93 + 0.1 kcal according to Klein and Herron [1964]. The following rate coefficient for (6) is adopted

. -33 1000/T 3 -1

b_ 33 3 x 10 e n(M) cm sec (7)

2a

This £hree-body process leading to the formation of nitrogen dioxide must be compared with the radiative process [Bates, 1954j Nicolet, I960]

0 + NO -» N 02 + hv 2* 3975 A ) (8)

which should be a three-body mechanism according to various laboratory measurements. The measured rate coefficient 3875 A - 1.4 |i) is

[Fontijn et al., 1964]

b2 b - 6.4 x 1 0 °1 7 c m3s e c ™1 (± 30 % ) , (9)

For X < 7250 A, we consider the following value s

b.. • 3,2 x 1 0 "1 7 c m3s e c "1; (10) ZD

6 -1

Since the probability of emission is not less than 4 x 10 sec , w e adopt the rate coefficient given by (9) at all altitudes [Levitt, 1962} Schiff, 1964« Doherty and Jonathan, 1964; Reeves et al., 1964].

Two bimolecular processes must be considered as important aeronomic reactions. One is a reaction with oxygen atoms

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5.«

0 + N02 '-» NO + 02 + 46 kcal. (11)

This is known to be rapid [Ford and Endow, 1957; Phillips and Schiff, 1962 as Schiff, 1964s Klein and Herron, 1964]. The molecules formed in (11) are vibrationally excited [Basco and Norrish, 1960s Phillips and Schiffj 1962 c] to v 08 ~ 8 (33.7 kcal) and perhaps electronically excited to - 2) with energies between 37.5 and 45.5 kcal

corresponding to 9 ^ v"" ^ 11. We adopt for (11) the rate coefficient

b3 - 1.5 x 10"12 T1 / 2 e -5 0 0 / T c m ^ e c "1. (12)

The other reaction of nitric oxide with ozone [Johnston and Crosby,,1954] leads to ground state NO^ molecules

03 + NO -» N02 + 02 + 48 kcal (13)

with a rate coefficient [Johnston and Crosby, 1954s Phillips and Schiff, 1962a]

c i ^1 4 1/2 -1200/T 3 - 1 „

b, = 5 x 10 T e cm sec . (14) 4

A chemiluminescent radiation [Greaves and Garvin, 1959] in the red and infrared (X # 5900 A) ^is—emitted during reaction (13). It corresponds to an electronically excited state with a rate coefficient [Clyne et al., 1964].

* ,„-14 „1/2 -2100/T 3 -1 ... .

b. a 5 x 10 T e cm sec (14a) 4a

In addition to these processes involving N02 > the following photodissociation process occurs in a sunlit atmosphere

N02 + hv (% < 3975) -» NO + 0 (15)

with the average photodissociation coefficient [Bates, 1954s Leighton» 1961]

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s 5 x 10 sec

NCL (16)

Under laboratory conditions the photolysis of nitrogen dioxide must involve an analysis [Ford„ I960] of N Q y N205 ° e £° ° H o w e v e r» i f a

reaction such as + NO ^ NO^ + 0^ has an activation energy of the order of 7 kcal„ N 02 will be unaffected during the night. For day time conditions, excluding NO« which has a dissociation energy of only 50 keal„ aeronomic

«J

conditions essentially correspond to the NO - NO^ system»

Neglecting reactions in which hydrogen and nitrogen atoms are involved„ the differential equation related to NO^ is written as

d n ( N 02)

dt b_ n + J,

NO. n(N0„ b2 n(0) + b^ n(03)"j n (NO) (17)

where b2 = b2 a n(M) + b ^ .

For daytime conditions„ an equilibrium exists since ^ 200 sec, and

n ( N 02) [ b2 a n(M) + b2 b] n(0) + b4 n ( 03)

n(N0) (18)

+ b„ n

Fig. 1 is an illustration of the vertical distribution of the ratio n(N02)/n(N0) which shows that,, above the stratopause0 nitrogen dioxi may be neglected in the analysis of nitric oxide reactions.

After twilight„ when atomic oxygen is rapidly removed in the stratosphere by its association with molecular oxygen0 the major process is„ according to

dn(N02

dt

dn (NO)

dt b ^ n(O^) n(N0) (19)

The life=time of NO is very short during the night in the stratosphere since

=4 =1

b. n(0„) > 3 x 10 sec (see Fig. 1). In other words„ nitric oxide 4 3

disappears during dark hours in atmospheric regions where ozone is present in sufficient abundance.

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n(N0

2

) J n (NO)

Pig> Vertical; distribution of; the nitrogen diöxidë aïïd tfifcric öxidë ratitf föfe nigïit and- daytime conditions^ Note thé ictión ó£ atomic oxygen^ t and ózóitë, b s ( 0 ' j .

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In the upper mesosphere and thermosphere where n(0) does not vary appreciably, the differential equation (17) with J = 0 and b ^ n(0)

> b n(0 ) indicates that the ratio n(NO-)/n(NO) increases after sunset as 4 3 ' follows

n ( N 02) b2 n(NO) b j t

-b n(0)tq

e J (20)

Night-time equilibrium conditions are reached very rapidly in fehe thermosphere but do not differ from daytime conditions (see F i g , 1).

The curves of F i g . 1 show that the ratio n ( N 02) / n ( N 0 ) shows the greatest change in the middle mesosphere where it depends on the rapidly varying concentrations of 0„ and 0 after sunset»

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Finally, in the stratosphere below 30 km where the photoaction plays a role the ratio n ( N 02) / n ( N 0 ) increases and m a y correspond to a greater N 02 concentration than that of N O , depending on the 0^ concen- tration.

NITRIC OXIDE AND ATOMIC NITROGEN,

In order to determine the behavior of nitric oxide, it is necessary to examine aeronomic reactions in which atomic nitrogen is effectively

involvedo In addition to (1) and (3), nitrogen atoms can react with nitrogen dioxide [Kistiakowsky and V o l p i , 1957s Harteck and D o n d e s , 1958; Kaufman and K e l s o , 1959j Verbeke and W i n k l e r , I960; Clyne and T h r u s h , 1961d]

N + NO ~> 0 + N O + 34 kcal (21a) -» NO + NO + 78 kcal (21b)

-» N2 + 02 + 121 kcal (21c)

for which a rate coefficient, b » m a y be of the same order as for the other

J

bimolecular reactions„ However, while the reaction of atomic oxygen with nitrogen dioxide is more important in the aeronomic ratio n ( N 02) / n ( N 0 ) ,

the reactions of atomic nitrogen with nitric oxide and molecular oxygen

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9.- are certainly the principal reactions in the ehemosphere.

The following reaction [Kistiakowsky and V o l p i , 19575 Verbeke and W i a k l e r , 1960s Herron, 1961°, Clyne and T h r u s h , 1961a? Phillips and Schiffs 1962b]

N + NO -» N2 + 0 + 75 kcal (22)

has a very small activation energy- W e adopt the rate coefficient

b£ = (1.5 + 0.5) x 10~U T1 / 2 c m3s e c "1 (23) 6 —

which varies by about a factor of 2 between 200®K and 700°K. The fact that vibrational excifeation [Phillips and Schiff, 1962b] of N2 (v ^ 12) occurs in reaction (22) and may lead to a decomposition of ozone„ is of no practical aeronomic consequence. It can be shown that reaction (22) is not important in the stratosphere and lower mesosphere where atomic nitrogen does not exist. Such a reaction is, however, the most important nitrogen recombination process in the thermosphere.

T h e reaction of atomic nitrogen with molecular oxygen produces nitric oxide

N + 02 -» NO + 0 + 32 kcal (24)

with a maximum vibrational N0(v ® 6) excitation, and requires a relatively high activation energy [Kistiakowsky and V o l p i8 1957s Kaufman and Decker,

1959S Clyne amd T h r u s h , 1961a, Mavroyannis and .Winkler, 1961b] between 6 and 7 k c a l . Laboratory determinations of the rate coefficient b^ of (24) are fitted betwee® 400®K and 1500®K by the expression

-13 _l/2 -3QQ0/T 3 =1

b_ - 2 x 10 T e c m sec (25) 7

which is adopted for the whole ehemosphere. Such a rate coefficient is small at low mesospherie temperatures % nevertheless, it leads to an

e>5 =1

aeronomic production rate of NO molecules of about 10 sec per nitrogen

=3 "=1

atom at the mesopause level and to about 10 sec in the thermosphere.

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Production of nitric oxide in the laboratory m a y also result from the reaction of nitrogen atoms with ozone [Chen and T a y l o r , 1961*

Phillips and Schiff, 1962a],

N + 0- -* NO + 0 . + 126 k c a l . (26) 3 Z

W e adopt the following rate coefficient

=12 1/2 -1200/T 3 -1

b 83 2 x 10 T e cm sec . (27)

8

This reaction would have an aeronomic role (only in the mesosphere) if nitrogen atoms were present in sufficient numbers. In fact, this

reaction can be neglected since the concentration of atomic nitrogen is small compared to that of atomic oxygen.

In addition to the loss process (22) of nitric oxide occuring under day and night-time conditions„ it is necessary to add the following photo-dissociation process

*

NO + hv (X ~ 1900 A) -» N + 0 (28)

for which Bates [1954] has adopted a rate coefficient at zero optical depth

. =7 -1 Jf ® 10 sec

NO

Furthermorej the photoionisation of NO by Lyman-alpha followed by dissociative recombination is also a loss process

NO + hv (X - 1216 A) N 0+ + e -» N + 0 . (29)

= 2 = 1

An average value of about 4 ergs cm sec for solar Ly-a leads to a rate coefficient at zero optical depth of

- 5 x •10."7sec'1. (30)

NO

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11.- Under equilibrium conditions between nitrogen dioxide and nitric o x i d e , the rate of change of n(NO) is given by the following equation

dn(NO)

d t + n

<

N

°) C^Q

+ J N 0 + b 6 n ( N )

^

= n

(°> + V

n ( N ) ( 3 1 ) Ilence, writing dn(NO)/dt « 0 in (31), the ratio n(NO)/n(N) is given by

n(NO) bx n(0) + b7 n ( 02)

" XN 0 + JN 0 + b6 n ( N ) ( 3 2 >

In this formula, b , n(N) is the important term in the denominator if 4 -3

n(N) > 5 x 10 cm . Ignoring and J ^ , the equilibrium value of nitric oxide n (NO) is then

n*(NO) - 1 0 - V3 0 0 0 / T n ( 02) + 5 x 10~7 n ( 0 ) . (33) The second term on the right is small compared to the first in the

mesosphere as well as where the temperature is high in the thermosphere.

It becomes important at the atomic oxygen density peak which occurs in the lower thermosphere. A concentration of oxygen atoms of the order of

12 -3

2 x 10 cm leads to

* 6 "=3

n (NO) = 10 cm . (34) At the mesopause level with T = 190°K, equation (33) leads to

n* (NO) - 6 x 1 05 c m "3 (35)

i . e . about 3 x 10 of the total concentration. -9

The time to reach an equilibrium according to (31) depends on the term n(N) a n d , if n(N) > n ( N 0 ) , all conditions are required to reach a perfect chemical equilibrium. At the mesopause level, an atomic nitrogen

6 »3

concentration of at least of 10 c m is required to apply strictly equili- brium conditions, which can be attained in less than one d a y . Consequently,

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the production of nitrogen atoms must be studied near the mesopause level before deducing the agronomic behaviour of nitric oxide.

If we write + > b n(N)„ so that (32) becomes NO NO ©

n(NO) b n(0 )

— - 1 1 - » (36)

+ JN 0

which is applicable to mesospheric conditions» we obtain a ratio n(NO)/n(N) greater than unity and increasing downwards.

4.° ATOMIC NITROGEN IN THE CHEMOSPHERE

Using the set of reactions written aboves the equation governing the rate of change of atomic nitrogen is

dn (N)

+ n(N)[bx n(0) + b6 n(N0) + b n(©2)] - n(N0) [ JN 0 + i j + 2P(N) (37)

in which the last term on the right takes into account the various possibi- lities of atomic nitrogen production.

In the chemosphere5 only the following predissociation process, considered by Herzberg [l948]s can be introduced

N2 + hv (XX ~ 1200 to 1250 k ) N2 ( a ^ ) -» M + N (38)

in which Lyman=Birge°Hopfield bands are involved5 namely (7-0) at 1250 A, (8-0) at 1226 A and (9-0) at 1205 A. The most penetrating radiation corres- ponds to 1226 A for which the absorption cross-section should be of the

4 9 2 r

order of 5 x 10 cm . According to Bates [1954J an approximate value of the dissociation rate coefficient at zero optical depth should be about

J„ 10 sec (39) 2

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1 3 . -

By adding (31) and (37)» w e obtain the proper expression for photochemical changes

~ d[n(NO) + n(N)]/dt + bfi n(NO) n(N) - n ( N£) JN (40)

or for equilibrium conditions

b6 n(N0) n(N) » n ( N2) JN . (41)

6 =3 -1

For example, a round figure of n(N0) being 10 cm sec near 100 k m 6 -3

leads to n(N) ^ 10 cm .

If w e adopt the relations b n(0) + b n(0 )

n (NO) = — — (42) 6

and

* P<N>

* < V " bx n(0) + b? n ( 02) ( 4 3>

for the solutions of (41), the actual value of n(N0) is

n(N0) - n*(NO) / [l + 1 05/ n * ( N ) ] ^ (44)

-5 -1 At the mesopause level, the term b n(0.) is about 10 sec and any

-3 =1

production of nitrogen atoms greater than 1 atom cm sec leads to a stationary value for n ( N 0 ) , which is practically the equilibrium value n* (NO) „

In the mesosphere, the loss of nitrogen atoms increases rapidly with decreasing height and equilibrium conditions can always be applied to atomic nitrogen. T h u s , w e h a v e , for the change of n(N0) in the m e s o s p h e r e , assuming that only chemical changes o c c u r ,

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dn (NO)

+ 2b, n 6

2P(N)

^ O + JN 0

b

7

n ( 0

2

} + b

6

n ( N 0 )

_ b? n ( 02) = b6 n(NO)

b? n ( 02) + bfi n(NO) (45)

Since b n(0_) must be greater than b n(NO), according to (42) and 7 2 o

(44), the following approximation

dn(NO) dt

NO NO 2 . x

= , /r> v 1» (NO) 6 b n(0 )

(46)

leads» by integrationv to an approximate life-time of nitric oxide in the mesosphere and lower atmospheric regions. The time tn q necessary to reduce

the concentration n(N0) to 50 percent of its initial value nQ( N 0 ) is

NO

, 6 n (NO) 10 / sec

n (NO) o

(47)

which is at least 10 days for any concentration less than the photochemical

*

value defined by (42)„ Since n (NO) is certainly greater than the actual value n(N0) in the lower mesosphere„ the possibility of maintaining nitric oxide in the mesosphere is real«

=>9

A photochemical value of n(N0) = 3 x 10 n(M) at the mesopause with its extrapolation into the mesosphere,, stratosphere and troposphere

leads to an acceptable concentration of nitrogen oxide at ground level [Hutchinsonp 1954]. Considering the various ratios of n(N02)/n(N0)„ as obtained by (18) and (20) and shown in Fig» 1„ it is possible to illustrate the vertical distributions of n(N0) and n ( N 02) . Fig« 2 and Fig. 3 show the variation in the stratosphere and mesosphere» respectively. Nitrogen dioxide

and nitric oxide are of equal importance in the middle stratosphere around 30 km and0 during the night, NO disappears by its transformation into N02» In the mesosphere there is a large variation between day and night.

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50

2 4 0

^

LU O 3

< 3 0

20

N 0 2 D A Y

. n ( N O ) + n ( N O 2 ) = 3 x l 0 n ( M )

— i 1 1 1 r

STRATOSPHERE

10'

NO

2

NIGHT

10 10° 10

s

C O N C E N T R A T I O N ( c m "3)

10 ii

F:ig.-2.- Vertical distribution of nitric oxide and nitrogen dioxide in the stratosphere for day and night-time conditions assuming a ratio [n(N0> + n(NO-)]/n(M) = 3 x 10 ~ .

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Fig. 3.- Vertical distribution of nitric oxide and nitrogen dioxide in the mesosphere for chemica^

conditions without introducing the effect of ionospheric reactions. n(NO)/n(M) =» 3 x 10 . »

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1 7 . -

There is, furthermore, a transition zone corresponding to the region where atomic oxygen recombines after sunset. Above the mesopause, NO again

6 "3 increases and reaches a concentration in the E layer of about 10 cm . Nevertheless, ionic reactions must be introduced in the ionosphere and the numerical values given cannot be considered as real in the ionospheric regions. Such conditions will be discussed after an analysis of the

ionization processes.

5.- NITROGEN TRIOXIDE, TETROXIDE AND PENTOXIDE

In the ozonosphere, the following reaction

°3 + N 02 N°3 + °2 + 2 6 k c a l ^8*

may occur. It has been studied in the laboratory [Johnston and Yost, 1949}

Ford et al., 1957s Husain and Norrish, 1963]. However, its activation

energy (about 7 kcal) leads to a low rate coefficient, in the stratosphere,

b , = 5 * 10"1 3 T1 / 2 e -3 6 0 0 / T (49)

Also the three-body reaction

0 + N 02 + M N03 + M + 50 kcal (50)

may occur in the region where atomic oxygen is present * Its rate coefficient, b1 Q, is high«

may be about

b1 Q, is higher [Ford and Endow, 1957] than ordinary three-body reactions and

~31 6 —1

b,^ =• 5 x 10 cm sec (51) 10

For day-time conditions such reactions are followed by the rapid reaction

NO + N03 -» 2N02 + 22 kcal (52)

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which has a very small activation energy and may have a rate coefficient, b ,^of about

11

, _ ,„-13 _% -1000/T ....

b = 5 x 10 T e (53) 11

The presence of NO^ may, therefore, be neglected in a sunlit

atmosphere due to the presence of NO and also to a photodissociation process since there is an absorption spectrum in the visible region [Jones and

Wulf, 1937? Husain and Norrish, 1963]. The night-time conditions are different in the stratosphere due to the absence of nitric oxide. The .following processes [Ford, I960? Benson, 1960; Leighton, 196l] may be

considered i

NO, + NO. N„0_ + 2 1 kcal (54)

N03 + N02 -» NO + N 02 + 02 - 4 kcal (55)

N03 + N 03 - » 2 N02 + 02 + 18 kcal (56)

for which the following, very approximate, rate coefficients are adopted, respectively :

b1 2 = 5 x 10"1 2 T* e -1 0 0 0 / T (57)

b u = 3 x K f1 4 I* e~2000/T ( 5 8 )

bM = 1.5 x 10"1 3 T% e"3 6 0 0 / T. (59)

Reactions (48) and (54) will result in an N02 loss process for night-time conditions. Since the rate coefficient of (48) decreases from

3 »1 -18 about 1.7 x 10 cm sec at the stratopause to 1.7 x 10 near 30 km,

the production of nitrogen trioxide is very small during a night of twelve hours. As a result the nitrogen dioxide is not affected and the production of nitrogen pentoxide is relatively small for normal night-time

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1 9 . - conditions. Nevertheless, the problem of nitrogen trioxide and pentoxide cannot be neglected during long nights«, The three-body association leading to nitrogen tetroxide should be considered for night-time conditions

NO. + NQ„ + M N„0. + M 4- 15 kcal (60)

2 2 2 4

for which the rate coefficients b „ is small [Clyne and Thrush, 1962a]

13

b1 e a 8 ac 10 ^ cm^ sec (61)

15

The effect of (60) must be considered at low altitudes for night-time conditions.

Thus9 the reactions involving nitrogen dioxide (and ozone), (48) and (60), which occur during night-time conditions may contribute somewhat to a temporary disappearance of nitrogen oxide in the stratosphere and below.

6.- NITROXYLn NITROUS ACID AND NITRIC ACID.

The formation of nitroxyl and its loss are due to a catalytic action of nitric oxide or atomic hydrogen. Such a process may be considered in the mesosphere and lower thetoosphere where H and NO are present.

The three-body process

H + NO + M -o HNO + M + 49 kcal (62)

has been the subject of recent laboratory studies since an infrared spectrum was detected [Cashion and Polanyi, 1959]. Absorption and emission,, bands [Dalby,

1958$ Clement and Ramsay, 1961j Bancroft et al., 1962] have been analyzed and detailed investigations of process (62) were carried out by various authors [Clyne and Thrush, 1961b and 1962bj Strausz and Grunning, 1964;

Bulewicz and Sugden, 1964]. Considering that (62) has a negative activation

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energy [CXyne and Thrush, 1961b], the following rate coefficient, b ^ , is used

, ~32 300/T 3 =1

b a 1 x 10 e n(M) cm sec „ (63) 16

The chemiluminescent reaction involving H + NO, namely

H + NO -» HNO + hv (XX 7625 - 6915 - 7965 - 6 2 7 2 ) , (64)

is slow [Clyne and Thrush, 1962b], The rate coefficient is about 3

i '16a ' A V T

b „ . . 7 x 1 0 "1 9f ¥ ) cm3 s e c "1 (65)

and process (64) i s , therefore, unimportant in aeronomy.

The catalytic action of nitric oxide occurs in the recombination of hydrogen atomsg the bimolecular process [Clyne and Thrush, 1962b, Bulewicz and Sugden, 1964]

H + HNO -* NO + H2 + 55 kcal (66)

having a small activation energy, has a rate coefficient, b ^ , of the order of

b1 7 - 5 x 10-13 T% e -1 2 0 0 / T. (67) A a

*

In a hydrogen-oxygen atmosphere, the action of OH and HO^ must also be considered5 the catalytic action ©f NO leads to

OH + HNO -» NO + H20 + 69 kcal (68)

which has a larger rate ©oefficient [Bulewicz and Sugden, 1964] at flame temperature than reaction (66)„ We adopt the following value

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2 1 . - -12 k -1200/T

b = 5 x 10 T e (69) 18

which is about 10 times b ^ <

The reaction

H 02 + HNO -+ NO + H202 + 40 kcal (70)

should, also occur, but its rate coefficient, b ^ , is not known. W e consider that

b1 7 ' "20 ' V ( 7 1>

The net result of reactions (62) to (70) is that nitric oxide is not affected. Atomic hydrogen is subject to other more important loss processes and its chemiluminescent reaction in the formation of nitroxyl is not important.

The possible three-body associations leading to nitrous and nitric acids :

N 02 + H + M H N 02 + M + 80 kcal (72)

NO + OH + M -» H N 02 + M + 60 keal (73)

N 02 + OH + M -» HN03 + M + 53 kcal (74)

and NO + H 02+ M -» H N 03 + M + 63 kcal (75)

with the, respective, rate coefficients b ^ , b ^ , b ^2 and b2^ (which are not known) should be compared with the reactions

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HN02 + H H2 + N 02 + 23 kcal (76)

H N 02 + OH H20 + N 02 + 38 kcal (77)

H N 03 + H -» H20 + N 02 + 66 kcal (78)

and H N 03 + H -> ^ 0 + N 03 + 15 kcal (79)

which are exothermic reactions for which the activation energies likewise are not known. Their, respectives rate coefficients b ^ , b ^ , b ^ and b should be very small if HNCL and HNO have some importance. In any

27 2 3

case, N 02 has a catalytic action and the nitrogen oxides are not affected by reactions (72) to (79). The loss processes of H, OH and H 02 are not

important compared with other processes in which 0, 02 and 0^ are involved.

Finally,, three other reactions in which nitrogen oxides are involved must be considered. The presence of N 02 leads to a very rapid process [Rosser and Wise,, 19615 Clyne and Thrush, 1961c; Ashmore and Tyler, 19625 Phillips and Schiff, 1962dj Kaufman„ 1964] with practically no activation energy

H + N 02 -> NO + OH + 30 kcal (80)

with

b „0 - 2 x 1 0 ~1 2T1 / 2 (81)

Zo

which corresponds to laboratory measurements [Phillips and Schiff, 1962d].

Such a reaction is, however, less important than the reaction H + 0^ -»

OH + 02 since n ( 03) > n(N02). The role of (80) as an M >2 loss process is also limited since 0 + N 02 -* NO + 02 is the principal reaction.

The effect of NO on H 02 and H202 is given by the following reaction

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23.-

NO + H 02 -* OH + N 02 + 9 kcal (82)

which is fast relative to other reactions of H 02 in similar circumstances [Tylers 1962]» If an upper limit of about 2.5 kcal is assumed for the activation energy of a reaction such as 0 + NO -» 0- + NO with a rather

-11 3 -1 high pre-experimental factor, a rate coefficient,, b ^ , of about 10 cm sec at 500°K leads to

b2 9 = 5 , 1 0 -1 2T1 / 2e "1 2 0 0 / I. (83)

Under aeronomic conditions, such a reaction must be compared with the rapid reaction H 02 + 0 -» OH + 02 or with NO + 03 and NO + 0. Its aeronomic role is, therefore, not important. The reaction

NO + H202 -» H N 02 + OH + 11 kcal, (84)

for which the rate coefficient b ^ is not known may also be neglected since 0 + H2©2 -» H20 + 02 has the leading role as a loss process of hydrogen peroxide under aeronomic conditions.

7." SUMMARY

The essential reactions in which nitrogen oxides are involved in the chemosphere depends on atomic oxygen and ozone. The hydrogen compounds do n«t play an important role, and all their reactions with NO and N 02 may be neglected. The processes affecting the ratio n(N02)/n(N0) are essentially described by equations (17) and (18) in which-only three reactions involving atomic oxygen,-.and^»zotie witlf .the photodissociation of N 02, describe the aeronomic conditions.

The absolute values of NO and N 02 depend on the dissociation of N0 in the lower thermosphere. Since the life-time of nitric oxide in the

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mesosphere is relatively long, any downward transport leads to a vertical distribution of NO which follows the hydrostatic distribution. In view of the fact that the chemical reactions occur in the ionosphere and that a chemospheric hyp©thesis for nitric oxide does not hold there as will be

shown in another paper, it is necessary to introduce other indirect processes for the dissociation of molecular nitrogen.

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