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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 - 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° 272 - 1983

Positive and negative ions in the stratosphère 0

by

«c-

E. ARIJS

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

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FOREWORD

The paper "Positive and negative ions in the stratosphere" has been presented at the Symposium SW 18 (Ions in the Middle Atmo- sphere) of the "European Geophysical Society Assembly" held in Leeds, A u g u s t 1982. This contribution will appear in Annales Geophysicae, 1983.

A V A N T - P R O P O S

L'article "Positivé and negative ions in the stratosphere" a été présenté au Symposium SW 18 (Ions in the Middle Atmosphere) du

"European Geophysical Society Assembly" tenu à Leeds en août 1982.

Cette contribution sera publiée dans Annales Geophysicae, 1983.

VOORWOORD

De tekst : "Positive and negative ions in the stratosphere" werd voorgedragen tijdens het Symposium SW 18 (Ions in the Middle Atmo- sphere) van de "European Geophysical Society Assembly" gehouden te Leeds, augustus 1982. Deze bijdrage zal gepubliceerd worden in Annales Geophysicae, 1983.

VORWORT

Die Arbeit : "Positive and negative ions in the stratosphere"

wurde zum Symposium SW 18 (Ions in the Middle Atmosphere) von der

"European Geophysical Society Assembly", Leeds, A u g u s t 1983, vor- gestellt. Dieser Text wird in Annales Geophysicae, 1983, heraus- gegeben werden.

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P O S I T I V E A N D N E G A T I V E I O N S IN T H E S T R A T O S P H E R E

by E. A R I J S

A B S T R A C T

Ion sources and s i n k s in the altitude region between 10 to 50 km are briefly reviewed. Up Lo 1977 only total ion density measurements and mobility data were available for the stratosphere.

Recent in situ data obtained with balloon borne mass spectrometers have revealed the nature of the most abundant ions in the stratosphere.

Proton h y d r a t e s , i.e. ions of the form H +(H2 ° )n a n d n o n Pr o t o n

h y d r a t e s , resulting from ion-molecule reactions of minor constituents with H +(H2 ° )n i o n s were found as positive ions. The major negative ions were NO^ and HSO^cluster ions.

The experimental method and the results of recent measurements will be discussed as well as the ion chemistry leading to the terminal cluster ions which were observed.

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RESUME

Après un bref aperçu des sources et pertes de l'ionisation atmo- sphérique dans le domaine d'altitude de 10 à 50 km, la nature des ions dans la stratosphère est discutée.

Avant 1977 les seules données expérimentales disponibles sur les ions stratosphériques étaient des mesures de densité et mobilité. Les observations récentes avec des spectromètres de masse-portés en ballon ont révélé l'identité des ions les plus abondants dans la stratosphère.

Les principaux ions positifs sont les protons hydratés, c'est-à-dire des ions de la forme H+( H20 )n, et les "non proton hydrates", pro- venant des réactions des gaz minoritaires avec les ions H+( H 2 Ô )n. Les ions négatifs sont surtout des agglomérats ayants comme noyau NO^" ou HSO4".

La méthode expérimentale et les résultats de mesures récentes sont revus, ainsi que la chimie qui transforment les ions primaires en ions observés.

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S A M E N V A T T I N G

Na een k o r t o v e r z i c h t v a n de b r o n - en verliestermen d e r atmo- s f e r i s c h e ionisatie in het hoogtegebied v a n 10 tot 50 km, w o r d t de a a r d v a n de ionen in de s t r a t o s f e e r b e h a n d e l d . Daar waar tot v o o r 1977 enkel metingen omtrent de totale ionendichtheden en ionenmobiliteiten v o o r h a n d e n w a r e n , h e b b e n recente g e g e v e n s , bekomen met b a l l o n - g e d r a g e n m a s s a s p e c t r o m e t e r s , de identiteit v a n de meest v o o r k o m e n d e ionen in de s t r a t o s f e e r o n t s l u i e r d .

De b e l a n g r i j k s t e positieve ionen zijn p r o t o n - h y d r a t e n , m . a . w . ionen v a n de gedaante H+( H?0 ) , en non proton h y d r a t e n , die o n t s t a a n

+

door reacties v a n H (H2 ° )n i °n e n m e t m i n d e r h e i d s g a s s e n . De negatieve ionen zijn voornamelijk N O ^ - en H S O ^ agglomeraten. Zowel de e x p e r i - mentele methode en de resultaten v a n recente metingen w o r d e n b e - s p r o k e n , als de ionenchemie die tot de waargenomen terminale ionen leidt.

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Z U S A M M E N F A S S U N G

Nach einen kurzen Überblick der Gewin- und Verlusttermen der Atmosphärischen lonization im Höhegebiet von 10 bis 50 km, wurde die Natur der stratosphärische Ionen behandelt. Vor 1977 waren n u r M e s - s u n g e n über die lonendichte und lonenbeweglichkeit v o r h a n d e n . Rezente Messungen mit Ballongetragen Massenspektrometer haben die Identität der wichtigste stratosphärische Ionen enthült.

Die positive Ionen sind vornehmlich "Proton H y d r a t e n " , d . h . Ionen der Form H+( H?0 ) und "Non Proton Hydraten" die d u r c h Reaktion von

+ t n

H ( H p O )n" l o n e n mit stratosphärischen Minderheitsbestandteile entstehen.

Die meist wichtigste negative Ionen sind NO^ - und H S O ^ - Agglomeraten. Die experimentäle Methode, E r g e b n i s s e n und die Ionen-Chemie werden kurz besprochen werden.

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1. I N T R O D U C T I O N

Until recently our knowledge of the charged particles in the strato- sphere had progressed rather slowly. The main reasons for this were the absence of any effect on radio wave propagation and the difficulties of performing in situ composition measurements, due to the high gas number density in this part of the atmosphere. Nevertheless a know- ledge of the number density and the nature of the charged species in the altitude region from 10 to 50 km is indispensable for an under- standing of atmospheric electricity problems. When, about five years ago the first stratospheric ion composition data became available, it became clear that the ion composition was strongly related to minor constituents and the possibility of detecting trace gases with v e r y low concentrations (such as sulfuric acid), so far not measured, was de- monstrated.

The important role that trace gases play in processes of general interest (such as the ozone layer problem) and the possible role of ions in aerosol formation (which in turn may influence the earth's radiation budget) renewed the interest in stratospheric ions.

Furthermore, it was recognized that an understanding of the ion chemistry in the stratosphere can contribute to enlarge our information on the thermochemistry of ion-molecule interactions in general.

It is the aim of the present paper to review our current knowledge of stratospheric ions. Although emphasis will be given to recent com- position data, a review will be presented of the so called buik properties of the stratospheric charged medium. T h i s was felt as a.

necessity in view of recent new data and the important role that these properties play in the interpretation and use of ion composition data as a diagnostic tool in trace gas analysis.

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2. B U L K P A R A M E T E R S OF T H E S T R A T O S P H E R I C C H A R G E D MEDIUM

2 . 1 . Ionization sources in the stratosphere

T h e ionization sources in the atmosphere have been the subject of many studies, especially with respect to their possible role in atmo- spheric electricity and dependent sun-weather relationships. A n excellent s u r v e y of most recent work in this field can be found in several review p a p e r s , which are brought together in the proceedings of the N A S A workshop on the "Middle Atmosphere Electrodynamics".

(Maynard, 1979).

For the purpose of the present discussion, however, we will restrict ourselves to those ionization s o u r c e s , which are active in the stratosphere or altitude region between 15 and 50 km.

T h e principal source of ionization under normal conditions in the altitude range of 15 to 50 km is provided by galactic cosmic r a y s . T h e nature of cosmic r a y s (being high energetic particles) results in a non selective ionization in contrast with the H Lyman-a ionization in the D - r e g i o n e . g . , which is only ionizing NO and certain metal atoms.

Furthermore, v i r t u a l l y no absorption of the ionizing radiation o c c u r s , so that in the stratosphere the ionization rate merely depends on the total gas number d e n s i t y . Only at altitudes below 15 km a marked mass absorption of the cosmic r a y s o c c u r s , g i v i n g rise to de- crease in ionization with decreasing altitude. T h e latter effect is illustrated in F i g u r e 1, which is based on the balloon measurements of Neher (1967).

T h e galactic cosmic ray f l u x on the earths atmosphere is mainly affected by two factors, namely the geomagnetic field and the solar a c t i v i t y .

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Fig. 1.- Rate of production of ion pairs by cosmic rays as compiled from data of Neher (1967) for solar minimum at different latitudes.

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T h e e a r t h ' s magnetic field c a u s e s a deflection of the incoming particles of the cosmic ray s h o w e r a n d , t h e r e f o r e , an increase of about 60% of the ionizing radiation i n t e n s i t y is f o u n d at geomagnetic latitudes l a r g e r t h a n 70° ( S a n d s t r o m , 1965). A t lower latitudes o n l y particles with e n e r g i e s l a r g e r t h a n about 15 GeV are reaching the atmosphere.

A s a r e s u l t of the h i g h e r penetration depth of these h i g h e n e r g y particles the maximum in the ionization rate v e r s u s altitude c u r v e ( F i g u r e 1) s h i f t s to lower h e i g h t s for lower geomagnetic latitude.

Cosmic r a y intensity is also controlled b y solar activity c u r r e n t l y o b s e r v e d b y s u n s p o t number v a r i a t i o n . A n 1 1 - y e a r cycle is f o u n d in the ionization rate which is anticorrelated with the s u n s p o t n u m b e r v a r i a t i o n s . T h i s effect is clearly illustrated on F i g u r e 2, which is taken from Promerantz and D u g g a l (1974). T h e mechanism, however which is controlling the correlation between solar activity a n d cosmic r a y intensity is at p r e s e n t not fully u n d e r s t o o d .

B e s i d e s the long term v a r i a t i o n s of s t r a t o s p h e r i c ionization, s h o r t time fluctuations may o c c u r d u r i n g P C A (Polar C a p A b s o r p t i o n ) e v e n t s , which are due to the penetration of h i g h energetic particles ejected d u r i n g Solar Flares ( S o l a r cosmic r a y s ) into the e a r t h ' s atmosphere.

T h i s effect however is mostly limited to h i g h latitudes. It is, as can be seen from F i g u r e 3 (after G o l d b e r g , 1979) quite dramatic a n d can alter the ion p r o d u c t i o n b y a factor of 10000 at 40 km.

A p a r t from the non selective ionization p r o d u c e d b y cosmic r a y s the attention h a s been d r a w n v e r y recently ( A i k i n , 1981) to an ionization s o u r c e which s h o u l d be active in the u p p e r part of the s t r a t o - s p h e r e and the lower p a r t of the mesosphere (40 to 70 km) a n d which may result in ion production several times of that due to cosmic r a y s . It c o n s i s t s of photoionization of metal compounds o r i g i n a t i n g from meteoric d e b r i s . No experimental data however are available to demonstrate the effect of s u c h an ionization s o u r c e .

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1 1 I I 1 I I I I I I I I I I I I I I I I I I I I I I 1 I I I I I I I I I,

1940 1950 1960 1970 Y E A R

Fig. 2.- Ion-pair production at 27 km altitude normalized to the latitude of Thüle (Greenland) for the last solar cycles and Zurich sunspot number.

Note the inversion vertical axes to demonstrate the anticorrelation (after Pomerantz and Duggal, 1974).

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0 1

I ON-PAIR PRODUCTION (crrrV 1 )

Fig. 3.- Ion production rate during two PCA events compared to normal galactic cosmic ray production after Herman and Goldberg (1978).

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For t h e p u r p o s e o f t h e p r e s e n t d i s c u s s i o n we w i l l , t h e r e f o r e , a c c e p t t h a t s t r a t o s p h e r i c i o n i z a t i o n is n o n - s e l e c t i v e a n d r a t h e r c o n s t a n t at m i d - l a t i t u d e s .

R e c e n t l y a p a r a m e t r i z a t i o n o f t h e cosmic r a y i o n - p a i r p r o d u c t i o n r a t e has been r e p o r t e d b y Heaps ( 1 9 7 8 ) . A n e m p i r i c a l f o r m u l a was f o u n d f o r t h e i o n - p a i r p r o d u c t i o n r a t e w h i c h was f i t t i n g t o w i t h i n 10%

w i t h most o f t h e e x p e r i m e n t a l d a t a . F i g u r e 4 s h o w s a c o m p a r i s o n o f r e c e n t e x p e r i m e n t a l d a t a ( G r i n g e l e t a l . , 1978a) w i t h t h e f o r m u l a

Q = (A + B s i n 4A) Nq Y Nn (cm"3 s "1) ( 1 )

w h i c h is v a l i d t h r o u g h t h e 30-18 km a l t i t u d e r a n g e a n d A a n d B a r e c o n s t a n t s d e p e n d i n g on s o l a r a c t i v i t y as g i v e n b y Heaps (1978) a n d

N = 3 . 0 3 . 1 01 7 cm"3 ( 2 )

o

n = 0 . 6 + 0 . 8 cos A ( 3 ) and

V = 1 " n ( 4 )

For A t h e a p p r o p r i a t e v a l u e o f t h e g e o m a g n e t i c l a t t i t u d e o f Laramie ( W y o m i n g ) was i n s e r t e d .

A s i t can be seen in F i g u r e 4 , r e a s o n a b l e a g r e e m e n t is f o u n d b e t w e e n t h e m e a s u r e d a n d c a l c u l a t e d v a l u e s .

2 . 2 . Ion s i n k s in t h e s t r a t o s p h e r e

T h e main s i n k f o r ions in t h e s t r a t o s p h e r e is r e c o m b i n a t i o n . S i n c e t h e e l e c t r o n s f o r m e d i n t h e p r i m a r y i o n i z a t i o n p r o c e s s r a p i d l y a t t a c h t o e l e c t r o n e g a t i v e gases ( m a i n l y o x y g e n ) t h e r a r i f i e d plasma in t h e s t r a t o - s p h e r e c o n s i s t s o f p o s i t i v e a n d n e g a t i v e i o n s . T h e r e f o r e t h e ion loss

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Fig. 4.- Ionization rate as given by Heap's parametrization (Heaps, 1978) during solar maximum (1) and solar minimum (2) compared to experimental data of Gringel et al (1978a).

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process is ion-ion recombination. It has been customary to make a distinction between two body and three body recombination.

Above 30 km binary recombination is dominant. T h i s process has been studied rather extensively (Smith and C h u r c h , 1976. Smith and C h u r c h , 1977; Smith et al., 1976 and Smith et al., 1981) and values of the binary recombination rate coefficient a? have been determined

_ o 3

experimentally for various types of ions. Values between 5.8 x 10 cm - 1 - 8 3 - 1

s and 6.6 x 10 cm s were reported for ions which have the same nature as those actually found in the stratosphere (Smith et a I., 1981).

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The temperature dependence of o^ was found to be T (Smith and C h u r c h , 1977).

Much less is known about the three body recombination mechanism which becomes the most effective one in the lower part of the strato- sphere.

Experimental laboratory data are sparse but the existing ones ( F i s k , 1967; Mahan, 1973) s u g g e s t values for the three body recombi-

_ per c

nation coefficient a , of about 2 x 10 [M] cm s , [M] being the - 3

density of the third body in units of cm ) and a strong temperature dependence ( T ^ or T Relying upon their own measurements of a^

and the above mentionned data for a3 Smith and C h u r c h (1977) calculated the total recombination coefficient a ^ in the atmosphere up to 80 km altitude. Their results are represented in Figure 5.

Also shown in Figure 5 are some data on a-j. as available in the literature. It should be noted that most of these data were based on a combination of experimental and theoretical results existing at the time.

The only points of Figure 5 based on in situ experiments are those of Gringel et al. (1978b) and Rosen and Hofman (1981), the last ones being the most reliable ones since ion densities as well as ionization rate were measured simultaneously for their derivation.

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30 40 50

ALTITUDE (km)

Fig. 5.- Different values of total ion-ion recombination coefficient as published in the literature. It should be noted that the data of Cole and Pierce (1974) are less accurate due to an underestimation of the binary recombination rate coefficient.

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V e r y recently Smith a n d A d a m s (1982) h a v e f o u n d a p a r a - metrization for the total ion recombination CK-J. of the form :

aT = 1.63 x 1 0 "5 exp ( - h/7.38) + 5.25 x 10~8 cm3 s "1 (5)

where h is e x p r e s s e d in km.

T h i s e x p r e s s i o n which is valid in the altitude region 10 to 60 km is claimed to be accurate to better than ± 50%.

Recent theoretical p r o g r e s s ( B a t e s , 1982) has s h o w n that the total ion-ion recombination coefficient aT must be r e p r e s e n t e d b y

aT = a3 + A(X2 ^

where a^ is the three b o d y recombination coefficient a n d Aa^ is r e g a r d e d as an enhancement due to the b i n a r y c h a n n e l . T h i s e n h a n c e - ment is p r e s s u r e and temperature d e p e n d e n t .

Computer simulations have allowed a calculation of aT in the alti- tude region 0 to 40 km ( B a t e s , 1982) a n d these r e s u l t s are also s h o w n in F i g u r e 5.

In view of some approximations u s e d in the calculations of Bâtes ( s u c h as the derivation of an effective polarizability from a rather uncertain empirical relationship between ion mass and ion mobility) and the u n c e r t a i n t y of 50% on the parametrization of Smith and A d a m s , the agreement between both approximations for a ^ and the data of Rosen and Hofman (1981) is reasonable.

In general it can therefore be stated that the ion-ion recombination coefficient between 10 and 40 km is known within a factor of two.

A n o t h e r s i n k for c h a r g e d species in the s t r a t o s p h e r e may be ion annihilation b y aerosol particles ( Z i k m u n d a and M o h n e n , 1972), but

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v e r y little is known about the c o n t r i b u t i o n of t h i s mechanism to the total ion losses. T h e efficiency of ion annihilation t h r o u g h the at- tachment at aerosols is believed to be low in the s t r a t o s p h e r e , a l t h o u g h good estimations are not available because of the lack of knowledge of the size d i s t r i b u t i o n s of aerosol particles.

2 . 3 . Ion densities

Once the ionization rate is k n o w n as a f u n c t i o n of altitude a n d a s s u m i n g mutual recombination to be the o n l y ion s i n k , the ion d e n s i t y profile can be calculated from the simple s t e a d y state formula

Q = a n+n " (7)

Several s u c h calculations have been performed a l t h o u g h most of the early models related to h i g h e r altitude r e g i o n s a n d were more c o n c e r n e d with the p r o d u c t i o n of the electrons in the atmosphere a n d the resultant a b s o r p t i o n of radio w a v e s (Webber, 1962; Cole and Pierce, 1965).

F i g u r e 6 s h o w s a calculation performed b y Reid (1979). T h i s r e s u l t is compared with some of the many experimental data ( P a l t r i d g e , 1965;

B r a g i n , 1967; Rose et a l . , 1972 and Widdel et a l . , 1977). A s can be seen the general t r e n d of the experimental data, which were all obtained with Gerdian c o n d e n s e r s is in reasonable agreement with the modelling r e s u l t . However s t r o n g fluctuations and extreme deviations are o b s e r v e d . T h e s e were formerly a s c r i b e d to either imperfect modelling, due to an uncomplete knowledge of the ion c h e m i s t r y ( B r a g i n et al., 1966) or to effects of ion annihilation b y aerosols or attachment of ions on particles ( P a l t r i d g e , 1966; Morita et al., 1971).

R e c e n t l y , h o w e v e r , Rosen and Hofman ( 1 9 8 1 ) , have measured ion densities systematically with a Gerdian c o n d e n s e r operating in the saturation mode and based on the d e s i g n of K r o e n i g (1960). B y u s i n g a

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F i g . 6.- Ion densities versus altitude as measured b y different a u t h o r s . P64: m e a s u r e m e n t p e r f o r m e d on 13 F e b . 1964 b y Paltridge (1965), B 66 : B r a g i n (1967) on 19 A p r i l 1966; Ro 68 : R o s e et a l . (1972) on 15 M a y 1968; W 75 : W i d d e l et a l . (1977) on 7 June 1975. R 79 is a m o d e l calculation b y Reid (1979).

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lobe pump to d r a w air t h r o u g h the ion collector a c o n s t a n t and well k n o w n flow rate was obtained. T h e ion d e n s i t y profiles t h u s obtained were much smoother than those of F i g u r e 6 a n d were quite repeatable o v e r a 2 y e a r period.

It is, t h e r e f o r e , believed now that the s t r o n g fluctuations can be attributed to poorly known and variable air flow in the older G e r d i a n c o n d e n s e r s .

3. I O N C O M P O S I T I O N O F T H E S T R A T O S P H E R E

3 . 1 . E a r l y information and indirect data

A global picture of the ionized component of the s t r a t o s p h e r e does not only contain an o v e r v i e w of the ion s o u r c e s a n d s i n k s a n d ion d e n s i t i e s , but r e q u i r e s a full u n d e r s t a n d i n g of the ion c h e m i s t r y .

In o r d e r to comprehend the c h e m i s t r y of c h a r g e d particles it is of an absolute necessity to know the ion composition.

T h e most direct way to obtain this information is to perform ion mass spectrometry measurements in the same way as those made in the D - r e g i o n ( N a r c i s i and Bailey, 1965, Narcisi et a l . , 1972; A r n o l d a n d K r a n k o w s k y , 1977 and Z b i n d e n et a l . , 1975).

j

In view of the experimental difficulties, related to s t r a t o s p h e r i c ion mass s p e c t r o m e t r y , which will be d e s c r i b e d hereafter, no s u c h m e a s u r e - ments were reported until 1977.

T h e r e f o r e , the f i r s t data c o n c e r n i n g the ion composition of the s t r a t o s p h e r e were indirect data and were d e r i v e d from mobility m e a s u r e - ments or modelling efforts. Before g i v i n g an o v e r v i e w of the most recent mass spectrometry data, we will, therefore, briefly review these early indirect data.

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3 . 2 . Ion mobility data in the stratosphere

Mobility data were mostly obtained at the same time as ion d e n s i t y measurements. In fact both kind of informations (ion densities and ion mobilities) are d e r i v e d from conductivity measurements with a Gerdian condensor ( C o n l e y , 1974) or similar probe.

B y using a Gerdian condensor in the "ohmic" region, mobility and ion density can be d e r i v e d from v o l t a g e - c u r r e n t c h a r a c t e r i s t i c s .

Some ion mobility data as observed by d i f f e r e n t authors ( P a l t r i d g e , 1965; Morita et a l .y 1971; C o n l e y , 1974; Kocheev et a l . , 1976; Widdel et a l . , 1976; Mitchell et a l . , 1977 and Rosen and Hofman, 1981) in the altitude region 5 tot 50 km are summarized in Figure 7. In the polari- zation limit the mobility K of ions with mass m is given by the Langevin formula (McDaniel, 1973)

K = ( i + M )1/2 (cm2 v "1 s "1) (8) and

„1/2 „1/2 m

a M

K = K . Z60 . T ( 9 )

o p 273.16

where a is the polarizability of the gas in A , m the ion mass in 2

1 - 1

g.mole , M the mass of the neutrals (also in g.mole ) in which the ions are moving, p the p r e s s u r e in T o r r and T the temperature in K e l v i n .

In principle formulae ( 8 ) and ( 9 ) allow a derivation of m from K . It can be calculated however that a mass resolution of 10 amu requires a precision of about 1.5% on K . In view of the fact that already p and T are mostly not measured with this precision and in view of the spreading of the points on Figure 7 it is v e r y unlikely that mobility data can give sufficient information on the ion mass.

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1 — 1 | I 1111| 1 I I I II II] 1 I I I 1111 j 1 /I, I I 11111

/ # 7 *

/ /

//;<. *

t/t

/ • / > •

/> A PALTRIDGE (1965) - A s c e n t

JS I /*** A P A L T R I D G E ( 1 9 6 5 ) " R o a t

O MORITA et a l ( 1 9 7 1 ) y O « 7 / • + CONLEY (1974)

J f ^ / / • V KOCHEEV et al. (1976) / / • W I DDEL (1977) * MITCHELL et al. (1977)

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I i I I m i l I i I I i nil I 1 I I Mill 1 1 I I I Mil

io1 102 103 10

MOBILITY (cm

2

/ V.sec)

F i g . 7.- Ion m o b i l i t i e s v e r s u s a l t i t u d e from d i f f e r e n t

sources. Curves 1, 2 and 3 are calculated from K = K — ^ i - T T for K 0.92; 1.94 and 2.7

o p 273.16 o

respectively. Pressure p and temperature T versus altitude were taken from the U.S. standard atmo- sphere of 1967.

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T h e use o f o t h e r K v e r s u s m r e l a t i o n s h i p s leads t o t h e same c o n c l u s i o n .

T h e r e f o r e more d i r e c t m e a s u r e m e n t s s u c h as mass s p e c t r o m e t r y d a t a a r e n e e d e d .

3 . 3 . M o d e l l i n g e f f o r t s

In o r d e r t o u n d e r s t a n d t h e ion c h e m i s t r y o f t h e D - r e g i o n n u m e r o u s l a b o r a t o r y s t u d i e s o f i o n - m o l e c u l e r e a c t i o n s w e r e u n d e r t a k e n d u r i n g t h e last 15 y e a r s .

P r o g r e s s i n l a b o r a t o r y t e c h n i q u e s ( F e r g u s o n et a l . , 1969, McDaniel e t a l . , 1970, Adams a n d S m i t h , 1976) has allowed a m e a s u r e m e n t o f most c r i t i c a l r e a c t i o n r a t e c o n s t a n t . C o m p i l a t i o n s of t h e s e r e s u l t s may be f o u n d in F e r g u s o n (1973) a n d A l b r i t t o n ( 1 9 7 8 ) .

Based u p o n t h e s e l a b o r a t o r y m e a s u r e m e n t s s e v e r a l models o f t h e ion c h e m i s t r y i n t h e e a r t h ' s a t m o s p h e r e w e r e d e v e l o p p e d ( F e h s e n f e l d a n d F e r g u s o n , 1969; M o h n e n , 1971; T h o m a s , 1974; R e i d , 1 9 7 6 ) . Most o f t h e s e models h o w e v e r r e l a t e t o t h e D - r e g i o n . F e r g u s o n ( 1 9 7 4 ) h o w e v e r e x t r a p o l a t e d h i s model i n t o t h e s t r a t o s p h e r e . T h e r e a c t i o n schemes as p r o p o s e d b e f o r e 1977 a r e s h o w n i n f i g u r e s 8 a n d 9 .

i n t h e p o s i t i v e ion c h e m i s t r y t h e c o n v e r s i o n o f t h e p r i m a r y ions ( N2 + a n d 02 +) t o p r o t o n h y d r a t e s [ H+( H20 >n] ( h e r e a f t e r P H ) is v e r y e f f i c i e n t a n d t h e r e seems t o be no a l t e r n a t i v e t o t h e i r p r o d u c t i o n i n t h e s t r a t o s p h e r e ( D o t a n et a l . , 1 9 7 8 ) .

I t was r e a l i z e d h o w e v e r t h a t t h e p r e s e n c e of t r a c e gases w i t h a p r o t o n a f f i n i t y l a r g e r t h a n t h e p r o t o n a f f i n i t y o f w a t e r c o u l d g i v e r i s e t o a c o n t i n u a t i o n o f t h e i o n - m o l e c u l e r e a c t i o n c h a i n a n d lead t o c l u s t e r ions of t h e f o r m H+A ( Ho0 ) . For A molecules s u c h as N H - w e r e

n c m p r o p o s e d .

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T h e use of o t h e r K v e r s u s m relationships leads to t h e same conclusion.

T h e r e f o r e more d i r e c t measurements such as mass spectrometry data are needed.

3 . 3 . Modelling e f f o r t s

In o r d e r to u n d e r s t a n d t h e ion chemistry of the D - r e g i o n numerous laboratory studies of ion-molecule reactions were u n d e r t a k e n d u r i n g the last 15 y e a r s .

Progress in laboratory techniques ( F e r g u s o n et a l . , 1969, McDaniel et a l . , 1970, Adams and Smith, 1976) has allowed a measurement of most critical reaction rate constant. Compilations of these results may be found in Ferguson ( 1 9 7 3 ) and A l b r i t t o n ( 1 9 7 8 ) .

Based upon these laboratory measurements several models of t h e ion chemistry in t h e e a r t h ' s atmosphere were developped ( F e h s e n f e l d and Ferguson, 1969; Mohnen, 1971; Thomas, 1974; Reid, 1 9 7 6 ) . Most of these models however relate to t h e D - r e g i o n . Ferguson ( 1 9 7 4 ) however extrapolated his model into t h e s t r a t o s p h e r e . T h e reaction schemes as proposed before 1977 are shown in f i g u r e s 8 and 9 .

In t h e positive ion chemistry t h e conversion of t h e p r i m a r y ions ( N2 + and 02 +) to proton h y d r a t e s [ H+( H20 )n] ( h e r e a f t e r P H ) is v e r y e f f i c i e n t and t h e r e seems to be no a l t e r n a t i v e to t h e i r production in t h e stratosphere ( D o t a n et a l . , 1 9 7 8 ) .

It was realized however t h a t t h e presence of t r a c e gases with a proton a f f i n i t y larger than t h e proton a f f i n i t y of water could give rise to a continuation of t h e ion-molecule reaction chain and lead to cluster ions of the form H+A ( Ho0 ) . For A molecules such as NH„ were

n c. m J

proposed.

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T h e u s e of other K v e r s u s m r e l a t i o n s h i p s leads to the same c o n c l u s i o n .

T h e r e f o r e more direct measurements s u c h as mass s p e c t r o m e t r y data are needed.

3 . 3 . Modelling efforts

In o r d e r to u n d e r s t a n d the ion c h e m i s t r y of the D - r e g i o n n u m e r o u s l

laboratory s t u d i e s of ion-molecule reactions were u n d e r t a k e n d u r i n g the last 15 y e a r s .

P r o g r e s s in laboratory t e c h n i q u e s ( F e r g u s o n et a l . , 1969, McDaniel et a l . , 1970, A d a m s a n d S m i t h , 1976) has allowed a measurement of most critical reaction rate c o n s t a n t . Compilations of these r e s u l t s may be f o u n d in F e r g u s o n (1973) and A l b r i t t o n (1978).

B a s e d u p o n these laboratory measurements several models of the ion c h e m i s t r y in the e a r t h ' s atmosphere were developped ( F e h s e n f e l d a n d F e r g u s o n , 1969; M o h n e n , 1971; T h o m a s , 1974; R e i d , 1976). Most of these models however relate to the D - r e g i o n . F e r g u s o n (1974) however extrapolated h i s model into the s t r a t o s p h e r e . T h e reaction schemes as p r o p o s e d before 1977 are s h o w n in f i g u r e s 8 and 9.

In the positive ion chemistry the c o n v e r s i o n of the p r i m a r y ions ( N2 + and 02 +) to proton h y d r a t e s [ H+( H20 )n] (hereafter P H ) is v e r y efficient a n d there seems to be no alternative to their p r o d u c t i o n in the s t r a t o s p h e r e ( D o t a n et a l . , 1978).

It was realized however that the p r e s e n c e of trace g a s e s with a proton affinity larger than the proton affinity of water could g i v e rise to a continuation of the ion-molecule reaction chain a n d lead to c l u s t e r

ions of the form H+A ( Hn c. m00 ) . For A molecules s u c h as N H „ were J p r o p o s e d .

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Fig. 8.- Reaction scheme leading to positive ions in the stratosphere as proposed by models before 1977 (adapted from Ferguson, 1974).

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R E C O M B I N A T I O N W I T H

P O S I T I V E I O N S

Fig. 9.- Stratospheric negative reaction scheme from model before 1977 after Ferguson (1974).

(28)

F o r the n e g a t i v e ion c h e m i s t r y , c l u s t e r ions with N O ^ c o r e s were e x p e c t e d . A l t h o u g h o r i g i n a l l y h y d r a t e s of N O ^ were p r e d i c t e d , it was s h o w n later on b y l a b o r a t o r y m e a s u r e m e n t s ( F e h s e n f e l d et a l . , 1975) that HNC>3 d i s p l a c e s H ^ O in the NC>2 h y d r a t e s . T h e p r e s e n c e of HNC>3

b e i n g s h o w n in the s t r a t o s p h e r e ( A c k e r m a n , 1975), ions of the t y p e N O g ( H N O g ^ were t h e r e f o r e e x p e c t e d .

3 . 4 . In situ ion composition m e a s u r e m e n t s 3 . 4 . 1 . Positive ion composition

T h e f i r s t s t r a t o s p h e r i c mass s p e c t r o m e t r i c m e a s u r e m e n t s of p o s i t i v e ions were r e p o r t e d b y A r n o l d et al. ( 1 9 / / ) . T h e data, w h i c h were obtained on the d o w n l e g portion of t h r e e r o c k e t f l i g h t s e x t e n d e d from 55 d o w n to 35 km a n d revealed the e x i s t e n c e of ions with mass n u m b e r s i 19, 29, 37, 42, 55, 60, 73 a n d 80. all with ± 2 amu, u n c e r t a i n t y . R e l y i n g on h i g h resolution measurements at h i g h e r altitude the ions with mass n u m b e r s 19, 37, 55 a n d 73 were identified as P H , w h i c h was in g o o d agreement with p r e v i o u s models. Below 40 km h o w e v e r the dominant ions ( 2 9 , 42, 60 a n d 80 ± 2 a m u ) had mass n u m b e r s , w h i c h d i d not c o r r e s p o n d to PH a n d c o n s e q u e n t l y t h e y were called non £i~oton h y d r a t e s ( N P H ) . T o e x p l a i n their p r e s e n c e a simple model was p r o - p o s e d , w h i c h was b a s e d on neutral model calculations ( S t e w a r t a n d H o f f e r t , 1975) a n d l a b o r a t o r y m e a s u r e m e n t s ( K a r p a s a n d K l e i n , 1975) a n d w h i c h i n v o l v e d the i r r e v e r s i b l e reactions of PH with f o r m a l d e h y d e ( C H2O ) .

A verification of t h i s h y p o t h e s i s b y s u b s e q u e n t l a b o r a t o r y measurements ( F e h s e n f e l d et a l . , 1978) s h o w e d that a l t h o u g h the reaction

H+(H2O) + CH2O CH2OH+ + H2O (9)

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-9 3 -1

was fast (a reaction rate coefficient of 2.2 x 10 cm s was reported) hydrates of protonated formaldehyde rapidly converted to PH. The thermochemical data obtained for the reaction of Ch^O with PH indicated that formaldehyde could not be the source of the NPH ions observed in the stratosphere.

A danger inherent to the first observations of Arnold et al. (1977) is the possibility of ion fragmentation, due to the use of rockets (shock-wave). Furthermore, only a short measuring time is available during a rocket flight. Therefore, in order to obtain a reasonable height resolution, only limited mass resolutions, resulting in comfortable signals, can be used with a rocket-borne mass spectrometer. The main problem with rocket-borne instruments is the decrease ot sensitivity with increasing air density due to collisional ion loss in the instrument.

It was, therefore, obvious to t r y to repeat the measurements with a balloon borne instrument.

The development of such an instrument, however, posés some serious problems among which the design of a good pumping system with a long standing time is one of the most critical ones.

Balloon borne ion mass spectrometers have been developped in the mid seventies at several places (Arijs et a l . , 1975; Arnold et a l . , 1978;

Olsen et a l . , 1978). T h e y consist of a high speed cryopump in which a quadrupole mass filter is built. The stratospheric ions are sampled through a small orifice, and focussed by an electrostatic lens into the mass filter. After mass filtering they reach a high gain electron multiplier, the signals of which are treated by pulse counting. A more detailed description of these instruments in beyond the scope of this work and is given in detail elsewhere ( A r i j s et a l . , 1975; Arijs and Nevejans, 1975 and Ingels et a l . , 1978; Olsen et a l . , 1978).

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T h e f i r s t positive ion mass spectra obtained with a balloon b o r n e i n s t r u m e n t at an altitude of 35 km c o v e r e d the mass r a n g e 0-109 amu ( A r i j s et a l . , 1978) a n d revealed the existence of PH with mass 19, 37, 55, 73, 91 and 109 amu and N P H with mass 42, 60, 78 and 96 amu. T h e uncertainties on the reported mass n u m b e r s was ± 2 or ± 3 amu. Some minor mass p e a k s , ( T a b l e 1) a p p a r e n t l y not b e l o n g i n g to the p r e v i o u s l y mentionned series were also detected, b u t o n l y a n a l y s e d later on. T h e existence of N P H in the s t r a t o s p h e r e was confirmed later on b y data obtained with a similar i n s t r u m e n t at 37 km altitude ( A r n o l d et a l . , 1978), b u t , due to the h i g h e r mass r a n g e , ions up to mass 140 amu were measured.

T h e mass n u m b e r s as o b s e r v e d in the f i r s t rocket and balloon f l i g h t s are summarized in Table 1. From the above data it was c o n - cluded ( A r n o l d et al., 1978) that the major ions at 35-37 km altitude were PH and N P H , which could be r e p r e s e n t e d b y H + X£ (H2 ° )m w h e r e x

s h o u l d have

a mass number of 41 ± 1 amu

5 - 3

an atmospheric number d e n s i t y of about 10 cm at 35 km and a proton affinity larger than 175 kcal.mole - 1

O n the base of these c o n c l u s i o n s , F e r g u s o n (1978) p r o p o s e d N a O H as a candidate for X . T h i s h y p o t h e s i s , which was r e l y i n g on the expected existence of N a O H , r e s u l t i n g from a reaction chain s t a r t i n g at the atmospheric sodium layer ( R i c h t e r and S e c h r i s t , 1979), was backed up b y calculations of Liu and Reid (1979) and laboratory measurements of M a r c k et al. (1980). A l t h o u g h the d i f f u s i v e chemical model of Liu and Reid (1979) did not take into account loss p r o c e s s e s for N a O H , which would g i v e rise to other sodium compounds s u c h as NaCI ( M u r a d and S w i d e r , 1979), it was s h o w n b y laboratory measurements ( P e r r y et a l . , 1980) that practically all sodium compounds e x i s t i n g in the s t r a t o s p h e r e would g i v e rise to N a+( H?0 ) ions.

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TABLE 1.- Observed major positive ions in the stratosphere

Mass in amu

(1) (2) (3) (4

19 ± 2 20 + 3

29 ± 2 29 + 3

37 ± 2 37 + 3

42 ± 2 43 + 3

50 + 3

55 ± 2 55 + 3 55

60 ± 2 60 + 2

73 73 + 2 73 ±

80 ± 2 78

±

2 78 ±

82

±

2

91 91

±

2 91 ±

96

±

2 96 ± 99

±

2 100 ± 109 109

±

2

114 ± 2 1Î8 ± 1 136 ± 2 140 ± 2

(1) Arnold et a l . (1977) (2) Olsen et a l . (1978)

(3) Arijs et a l . (1978, 1979) (4) Arnold et al. (1978).

Proposed ion clusters

H+( H2O )

? H+( H20 )2

H+X

1

H+( H20 )3

H+X ( H20 ) H+( H20 )4

H+ +X ( H 0 ) H X2

H+( H2O )5

H+X ( H2O )3

H+X2( H2O ) H+( H2O )6

H+X ( H2O )4

H+X2( H20 )2

H+X2( H20 )3

H+X3( H20 )

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A n o t h e r p o s s i b l e c a n d i d a t e w h i c h was p u t f o r w a r d f o r X was MgOH ( M u r a d a n d S w i d e r , 1979) a l t h o u g h i t was e x c l u d e d l a t e r on b y l a b o r a t o r y m e a s u r e m e n t s ( F e r g u s o n et a l . , 1 9 8 1 ) .

T h e r e f o r e NaOH r e m a i n e d t h e most l i k e l y c a n d i d a t e f o r X u n t i l 1980. In t h a t y e a r t h e f i r s t h i g h r e s o l u t i o n mass s p e c t r a o f p o s i t i v e ions became a v a i l a b l e ( A r i j s et a l , 1 9 8 0 ) . A t y p i c a l s p e c t r u m f r o m t h i s f l i g h t is s h o w n in F i g u r e 10. From t h e s e d a t a i t was s h o w n u n - a m b i g u o u s l y t h a t t h e mass o f X was 41 a m u , a n d , s i n c e no Mg i s o t o p e s w e r e d e t e c t e d , t h e NaOH a n d MgOH h y p o t h e s e s w e r e r e j e c t e d a n d , C H g C N , t h e c a n d i d a t e p r o p o s e d f o r X b y A r n o l d et a l . (1978) became t o p i c a l a g a i n .

T h i s s u g g e s t i o n was s t r e n g t h e n e d b y a c a r e f u l i n s p e c t i o n of ion f r a c t i o n a l a b u n d a n c e s ( A r n o l d et a l . , 1981a), l a b o r a t o r y m e a s u r e m e n t s

( S m i t h et a l . , 1981; B ô h r i n g e r a n d A r n o l d , 1981) and an i n s p e c t i o n of mass peaks d u e t o h e a v y isotopes ( A r i j s et a l . , 1982a).

T h e major o b s e r v e d s t r a t o s p h e r i c p o s i t i v e ions can t h e r e f o r e be r e p r e s e n t e d b y H+( H20 )n and H+X£( H20 )m, X b e i n g most p r o b a b l y C H g C N . A t p r e s e n t t h e s o u r c e of C H3C N in t h e s t r a t o s p h e r e is n o t y e t c l e a r .

Recent s t u d i e s of t h e f r a c t i o n a l ion a b u n d a n c e s o f PH a n d NPH h a v e led t o an e s t i m a t i o n of t h e m i x i n g r a t i o of C H ^ C N in t h e s t r a t o - s p h e r e at t y p i c a l balloon f l o a t a l t i t u d e s as well as at lower a l t i t u d e s ( A r n o l d et a l . , 1981a, H e n s c h e n a n d A r n o l d , 1981a, A r i j s et a l . , 1982a, A r i j s et a l . , 1982c).

A t y p i c a l r e s u l t o f s u c h e s t i m a t i o n s is shown in F i g u r e 11.

A l t h o u g h in s i t u p r o d u c t i o n mechanism f o r C H3C N was p r o p o s e d ( M u r a d and S w i d e r , 1982) t h e CHOISI p r o f i l e s of f i g . 11 seem t o s u g g e s t a s o u r c e t e r m of C H3C N in t h e t r o p o s p h e r e f o l l o w e d b y an u p w a r d d i f f u s i o n a n d photochemical d e s t r u c t i o n at h i g h e r a l t i t u d e s . In t h e

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Fig. 10a.- High resolution positive ion mass spectrum obtained at 34 km altitude in the mass domain 50-106 amu (Arijs et al, 1980). The peak width is constant over the mass range and equals 0.9 amu.

b.- In flight calibration obtained in the same mass domain, showing clearly the krypton isotopes. This spectrum was taken just before the spectrum of fig. 10a by putting on an ion source and injecting krypton into the instrument.

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SJJ

Q

35

<

30

25

20

JJLJLL

A

0 A

a

o V

B

©

B 3 NOV 77 B 7 JUN 79 B 9 APR 80 B 10 JUN 80 B 11 SEP 80 B 12 SEP 80 B U SEP 80

« » » I I m l « I I I i I n 10"

.14 J 3

10" 10

VOLUME M I X I N G RATIO

J 2

o o

' » ' » i 1111

10

Fig. 11.- Mixing ratio of CH^CN versus altitude as deduced from different balloon and rocket flights (after Henschen and Arnold, 1981).

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derivation of the C H ^ C N mixing ratios, it was assumed that the main loss process for NPH is recombination. It has been shown, however, that reverse switching reactions

H+X„(H 0) + X

JT 2 ym (10)

or thermal dissociation

H+X£+1 2 m 0 (H.O) + M ^ H+X.(H 0) + X + M JT 2 'm (11)

may be occuring ( B ö h r i n g e r and A r n o l d , 1981; Arnold et al., 1981a, Arijs et al, 1981a).

If such reactions are possible for SL = 0, a higher concentration of C H g C N may be found. Reconversion of NPH to PH, however, seems rather unlikely as an effective loss process for NPH ( A r i j s et al., 1982a).

Recent studies also revealed that collisional induced cluster break up in the mass spectrometer ( A r n o l d et al., 1981a; Arijs et al., 1982a) is also falsifying the results.

A detailed study of fractional ion abundances and a comparison with models has, therefore, to take into account this effect and has to await for extensive laboratory measurements of collisional induced d i s - sociation.

A p a r t from the major positive ions, discussed herefore, many other positive ions have been detected in the stratosphere (Henschen and A r n o l d , 1981b; Arijs et al., 1982a). The minor mass peaks as reported so far in the open literature are summarized in Table 2, together with their tentative identification. It can be seen from this table that ion compositions offer the possibility of trace g a s detection of species so far not measured. It should be noted, however, that signal instabilities,

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TABLE 2.- Minor positive Mass in amu

(1) (2)

ions detected in the stratosphere Tentative identification

(1) (2)

19 + 3 2 8 ± 2 33 ± 2

37 42 + 2

51 + 2

60

69 + 2 77

8 2 ± 1 87 ± 2

99 + 1

37 + 1 42 ± 1 45 ± 1 49 ± 1

58 + 1 60 i 1 63 ± 1

67 + 1

81 ± 1

83 + 1 86 i 1

104 ± 1

110 + 1

H+( H20 ) H+. H C N H+CII3OH H+( H2O )2 H+C H3C N

HJH3OH.. H20

H+C H3C N ( H20 )2

H+C H3O H ( H2O )2

N a+( H20 )2

H+Y ( H20 )

H+C H3O H ( H2O )3

H - Y ( H20 )

117 117 + 1

122 ± 1 125 ± 1

H Y ( H20 )j + 2

1 2 8 + 1

(1) Arijs et al. , (1982a) • ' ! = CHgCOII ? (2) Henschen and Arnold (1981).

H+( H2O )2 H+C H3C N

A 1 ( H20 ) or H+H C N . H20 H+C H30 H . H20 or

H+c h3N H2H2O N a+( H20 )2 H+C H3C N ( H2O )2

A 1 ( H20 )2 or H+H C N ( H2O )2 H+C H3O H ( H2O )2

A 1+( H20 )3 or H+H C N ( H2O )3

H+( C M3 C N )2

H+C H30 H ( H20 )3 or H+C H3N H2( H2O )3

A 1 C H3C N . ( H20 )2 or H+H C N . C H3C N ( H2O )2

H+C2H3N S ( H20 ) ,

1I+CH3-CN.CH30H(H20)2 or H+C H3C N . C H3N H2( H20 )2

H+C H3N 02( H20 )3

I L+C H3N 02. C H3. C N . H20 H+C H202C H3C N . ( H20 )2, H+C2H60 . C H3C N . ( H20 )2 or H+C H4S . C H3C N . ( H20 )2 1 I+C2H3N S ( H20 )3

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w h i c h a r e v e r y p r o n o u n c e d at low ion c o u n t r a t e s , a n d p o s s i b l e c o n - t a m i n a t i o n f r o m t h e b a l l o o n h a m p e r t h e u s e f u l n e s s o f t h e m e t h o d as a f u l l g r o w n a n a l y t i c a l t o o l .

In t h e c o n t e x t of t h e d e t e c t i o n o f m i n o r ions i t is w o r t h w h i l e t o r e t u r n t o t h e NaOH h y p o t h e s i s . S i n c e no major ions d u e t o N a+- c l u s t e r ions h a v e been o b s e r v e d , t h e p r o b l e m r i s e s w h a t h a p p e n s t o t h e s o d i u m o r i g i n a t i n g f r o m m e t e o r i t e a b l a t i o n . A p o s s i b l e loss mechanism f o r m e t e o r i c s o d i u m is a b s o r p t i o n i n a t m o s p h e r i c d u s t a n d aerosols ( A r n o l d et a l . , 1981; A r n o l d a n d H e n s c h e n , 1 9 8 2 ) . R e c e n t model c a l c u l a t i o n s ( T u r c o et a l . , 1981) t a k i n g i n t o a c c o u n t s u c h removal p r o c e s s e s i n d i c a t e i n d e e d much l o w e r metal v a p o r c o n c e n t r a t i o n s t h a n t h o s e f o u n d b y L i u a n d Reid ( 1 9 7 9 ) . T h e r o l e o f s t r a t o s p h e r i c ions i n aerosol f o r m a t i o n has been t h e s u b j e c t o f many s t u d i e s ( C a s t l e m a n , 1979, 1982; Mohnen a n d K i a n g , 1976; C h a n a n d M o h n e n , 1 9 8 0 ) . H o w e v e r , t h e n a t u r e a n d o r i g i n of s t r a t o s p h e r i c c o n d e n s a t i o n n u c l e i is n o t k n o w n . A p o s s i b l e mechanism f o r t h e f o r m a t i o n o f s u c h c o n d e n s a t i o n n u c l e i may i n v o l v e m u l t i - i o n com- p l e x e s ( F e r g u s o n , 1979; A r n o l d , 1980; A r n o l d , 1 9 8 2 ) . R e c e n t l y posi t i v e l y c h a r g e d p a r t i c l e s h a v i n g masses l a r g e r t h a n 327 amu w e r e d e t e c t e d w i t h a balloon b o r n e i o n - m a s s s p e c t r o m e t e r ( A r n o l d a n d H e n s c h e n , 1 9 8 1 ) . T h e s e p a r t i c l e s a p p e a r t o be a r r a n g e d i n a l a y e r p e a k i n g a r o u n d .30 km. A r g u m e n t s w e r e p u t f o r w a r d t o i n t e r p r e t t h e s e p a r t i c l e s as m u l t i - i o n complexes a n d p o s s i b l e c o n d e n s a t i o n n u c l e i .

As w i l l be p o i n t e d o u t in a c o n t r i b u t e d p a p e r h e r e a f t e r ( A r i j s et a l . , 1983a) h o w e v e r c a r e s h o u l d be t a k e n w i t h t h e s e i n t e r p r e t a t i o n s in v i e w of p o s s i b l e c o n t a m i n a t i o n p r o b l e m s .

3 . 4 . 2 . S t r a t o s p h e r i c n e g a t i v e ion c o m p o s i t i o n m e a s u r e m e n t s

Due t o t h e somewhat l a r g e r t e c h n i c a l d i f f i c u l t i e s associated w i t h t h e d e t e c t i o n o f n e g a t i v e i o n s , w h i c h a r e c a u s e d b y t h e use of h i g h e r h i g h v o l t a g e s on t h e d e t e c t o r a n d associated h i g h e r noise l e v e l s , t h e f i r s t r e p o r t s on t h e n e g a t i v e ion c o m p o s i t i o n in t h e s t r a t o s p h e r e w e r e

- 3 3 -

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p u b l i s h e d s o m e w h a t l a t e r t h a n t h o s e f o r p o s i t i v e i o n s . T h e f i r s t mass a n a l y s i s o f s t r a t o s p h e r i c n e g a t i v e i o n s ( A r n o l d a n d H e n s c h e n , 1978) r e v e a l e d t h e e x i s t e n c e o f NO^ ( H N O ^ ^ c l u s t e r i o n s , w h i c h was e x p e c t e d a c c o r d i n g t o p r e v i o u s models as e x p l a i n e d b e f o r e .

A d d i t i o n a l l y , h o w e v e r , i o n s o f t h e t y p e R ( H R )n( H N C >3)m w e r e f o u n d . A l t h o u g h l a c k o f r e s o l u t i o n made i t d i f f i c u l t t o i d e n t i f y R , A r n o l d a n d H e n s c h e n ( 1 9 7 8 ) p r o p o s e d h ^ S O ^ as a p o s s i b l e c a n d i d a t e f o r H R .

S u b s e q u e n t l a b o r a t o r y m e a s u r e m e n t s ( V i g g i a n o e t a l . , 1980) s h o w e d t h a t s u l f u r i c a c i d r e a c t s r a p i d l y w i t h NOg ( H N O ^ ) - i o n s a n d r e a c t i o n r a t e c o n s t a n t s w e r e r e p o r t e d f o r

NO ~ + H-SO. -» HSO. ~ + HNO (12) 3 2 4 4 3

N O3" ( H N O3) + H2S O4 HSO4~ HNO3 + HNO3 (13)

N O3" ( H N O3)2 + H2S O4 + H S O4" ( H N O3)2 + HNO3 (14)

- 1 0 q . i n Q - i n 3 - 1

o f 9 . 7 x 10 I U cm s 8 . 6 x 10 , u cm-3 s a n d 4 x 10 cm s r e s p e c t i v e l y .

T h e d e r i v a t i o n s o f t h e s e r e a c t i o n r a t e c o n s t a n t s u s e d a d i p o l e moment o f I ^ S C ^ o f 0 . 5 5 D e b y e . R e c e n t l y h o w e v e r t h i s d i p o l e moment was m e a s u r e d a n d f o u n d t o b e 2 . 7 2 5 D e b y e ( K u c z k o w s k i e t a l . , 1 9 8 1 ) . T h i s i n c r e a s e s t h e r a t e c o n s t a n t s o f r e a c t i o n s ( 1 2 ) , ( 1 3 ) a n d ( 1 4 ) b y a f a c t o r o f 2 . 7 ( V i g g i a n o e t a l . , 1 9 8 2 ) .

I n 1980 t h e f i r s t h i g h r e s o l u t i o n s p e c t r a o f s t r a t o s p h e r i c n e g a t i v e i o n s ( A r i j s e t a l . , 1981) a l l o w e d a n u n a m b i g u o u s mass d e t e r m i n a t i o n o f R a n d c o n f i r m e d t h e H5S 0 . h y p o t h e s i s .

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A t y p i c a l n e g a t i v e i o n s p e c t r u m o b t a i n e d a t 35 km a l t i t u d e is s h o w n i n F i g u r e 1 2 .

B y u s i n g t h e q u a d r u p o l e i n a " l o w r e s o l u t i o n " mode a n d r e l y i n g u p o n r i s i n g mass e d g e d e t e c t i o n , masses as h i g h as 391

[ H S 04~ ( H2S 04)3] w e r e d e t e c t e d ( A r i j s e t a l . , 1 9 8 1 ) , w h i c h w e r e c o n - f i r m e d l a t e r b y m o d e r a t e r e s o l u t i o n d a t a ( A r n o l d e t a l . , 1 9 8 2 ) . A s a c o n s e q u e n c e a m o r e d e t a i l e d r e a c t i o n s c h e m e f o r t h e c o n v e r s i o n o f NOg - i o n s t o H S O ^ - i o n s w a s p r o p o s e d ( A r i j s e t a l . , 1981) w h i c h is s h o w n i n F i g u r e 13.

T h e o r i g i n o f h ^ S O ^ i n t h e s t r a t o s p h e r e i s , t h o u g h n o t c o m p l e t e l y , f a r b e t t e r u n d e r s t o o d t h a n t h a t o f C H ^ C N . S u l f u r i c a c i d is s u p p o s e d t o o r i g i n a t e f r o m t h e p h o t o c h e m i c a l o x y d a t i o n o f S C ^ , O C S a n d o t h e r s u l f u r g a s e s t h a t r e a c h t h e s t r a t o s p h e r e ( T u r c o e t a l . , 1 9 7 9 ) . E v e n w i t h i t s low n u m b e r d e n s i t y , s u l f u r i c a c i d is a v e r y i m p o r t a n t m i n o r c o n s t i t u e n t a n d p l a y s a n i m p o r t a n t r o l e i n t h e f o r m a t i o n o f s t r a t o s p h e r i c a e r o s o l s . ( H a m i l l e t a l . , 1977; T u r c o e t a l . , 1 9 8 2 ) . T h e a p p l i c a t i o n o f t h e s t e a d y s t a t e c o n t i n u i t y e q u a t i o n t o t h e f o r m a t i o n a n d loss m e c h a n i s m o f H S O ^ i o n s a l l o w s t h e c a l c u l a t i o n o f t h e s u l f u r i c a c i d n u m b e r d e n s i t y

[ H ^ S O ^ ] , t h r o u g h t h e f o r m u l a

+ n(HSO~)

" ^ • n(NO^) ( 1 5 )

w h e r e a ^ is t h e i o n - i o n r e c o m b i n a t i o n c o e f f i c i e n t , n+ t h e p o s i t i v e i o n n u m b e r d e n s i t y , n ( H S Q4" ) a n d n(NC>3 ) t h e t o t a l n u m b e r d e n s i t y o f a l l s u l f a t e a n d n i t r i t e - i o n s r e s p e c t i v e l y a n d k t h e r e a c t i o n r a t e c o n s t a n t f o r r e a c t i o n s o f t h e t y p e ( 1 2 ) , ( 1 3 ) a n d ( 1 4 ) . S e v e r a l d e r i v a t i o n s o f [ H2S 04] h a v e b e e n p u b l i s h e d i n t h e p a s t ( A r n o l d a n d F a b i a n , 1980;

A r i j s e t a l . , 1981; A r n o l d e t a l , 1981 a n d V i g g i a n o a n d A r n o l d , 1981) a n d a l t h o u g h t h e e a r l y m e a s u r e m e n t s w e r e low c o m p a r e d t o m o d e l c a l c u l a t i o n s ( T u r c o e t a l . , 1979) m o r e r e c e n t d a t a ( V i g g i a n o a n d A r n o l d , 1982; A r i j s e t a l . , 1 9 8 3 b ) seem t o b e i n r a t h e r g p o d a g r e e m e n t , as s h o w n i n F i g u r e 1 4 . C a r e s h o u l d b e t a k e n , h o w e v e r , i n c o m p a r i n g

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Fig. 12.- Typical nighttime high resolution spectrum for negative ions obtained near 35 km altitude (Resolution m/Am = 100) (Arijs et al. , 1981).

(41)

F i g . 13.- Proposed reaction scheme (Arijs et a l . , 1981) for negative ions in the s t r a t o s p h e r e . N e u t r a l reaction partners are indicated along the a r r o w s , M means a collision p a r t n e r . I , II and III are the only reactions studied in the l a b o r a t o r y .

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SULFURIC ACID NUMBER DENSITY ( c m "

3

)

Fig. 14.- I^SO^ + HSOy concentrations versus height compared to model curves (MPI : Viggiano et al, 1982; BIRA : Arijs et al, 1982a). The model curves are the H2S 04 vapour pressure over a 75% H2S04/25% H20 mixture (for fall temperatures). The other curves are from Turco et al. (1981) and Hamill et al,

(1982) (RA : radical agglomeration) (after Viggiano and Arnold, 1982).

- 3 8 -

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theory and experiment since recent investigations (Arnold et a l . , 1982) have revealed the existence of H S 0

3

in the stratosphere and sulfuric acid vapour concentrations derived from ion mass spectra include HSO^, whereas models calculating [H

2

SC>

4

] do not.

By using laboratory data of N 0

3

" ( H N 0

3

)

n

clustering (Davidson et a l . , 1977; Wlodeck et a l . , 1980) and the fractional abundances of N 0

3

" . ( H N 0

3

)

n

~ i o n s in the stratopshere one can also deduce nitric acid concentrations. The results are shown in Figure 15 ( A r i j s et a l . , 1982b). Unfortunately the measured N 0

3

~ ( H N 0

3

)

n

" i o n abundances are strongly influenced by cluster break up at altitudes below 30 km, which hampers the use of this method at lower altitudes.

Recently minor mass peaks in negative stratospheric ion mass spectra have been investigated. Several ions not belonging to the N0

3

~ - or H S 0

4

" -ion families were observed (Arijs et a l . , 1982b;

McCrumb and Arnold, 1981). The power of the use of ion mass spectrometry in detecting trace gases (such as HCI, HOCI a . s . o ) is illustrated by the list of those ions in Table 3, although the same remarks concerning signal instabilities and contamination are valid. "

C O N C L U S I O N S

Our knowledge on positive and negative ions in the stratosphere has considerably increased during the last few years. The source of ions is fairly well understood and the information on ion-ion recom- bination has improved through recent experimental and theoretical work (Smith and Adams, 1982; Bates, 1982).

Due to the in situ composition measurements we have obtained a better insight into the ion chemistry in the altitude region from 20 to 35 km altitude. Modelling of stratospheric ion chemistry however is still tentative due to the lack of knowledge of reliable ion-molecule reaction rate constants. Nevertheless it has been possible to use in situ ion

-39-

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AO

E

LU

Q 35

3

30

— H h

I I I I I III

I I I I M i l

h — V '

• Q - H

tO

J P

—O

1

O

i — i i 11 i n

LAZRUS & GANDRUD, 1974 EVANS et al. , 1 9 7 7

ARNOLD et a l . , 1980 McCRUMB & ARNOLD, 1981 _ VIGGIANO & ARNOLD, 1981a ARIJ5 et a l . , 1982 b

: MODELS

I i i i i i

m l

\rvm

I I I I I I I N

0.1 1.0 10 100

H N 0

3

- V O L U M E M I X I N G RATIO ( P P B V )

F i g . 15-- HN°3 concentrations versus altitude (above 30 km) as measured by different techniques and calculated b y current m o d e l s .

-40-

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