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 B - 1180 BRUXELLES
A E R O N O M I C A A C T A
A - N° 2 6 9 - 1983
Acetonitrile in the atmosphere
1
by
G. BRASSEUR, E. ARIJS, A. DE RUDDER,
D. NEVEJANS and J. INGELS
FOREWORD
T h i s paper will appear in Geophysical Research L e t t e r s .
A V A N T - P R O P O S
Cet article sera publié dans Geophysical Research L e t t e r s .
VOORWOORD
Deze t e k s t zal verschijnen in Geophysical Research L e t t e r s .
VORWORT
Dieser T e x t w i r d herausgegeben Research L e t t e r s .
werden in Geophysical
A C E T O N I T R I LE IN T H E A T M O S P H E R E
by
G. B R A S S E U R ( * ) , E. A R I J S , A . DE R U D D E R , D. N E V E J A N S and J. I N G È L S
A b s t r a c t
The vertical distribution of acetonitrile between 0 and 55 km altitude is calculated assuming that C l l g C N is released at the Earth's surface and that the main loss process in the stratosphere is the reaction with h y d r o x y l radicals. The influence of different possible surface concentrations is investigated. The role of the destruction of C H g C N by C I , as well as the use of different eddy diffusion coefficients is briefly d i s c u s s e d . The results are compared with recent measurements of acetonitrile.
Résumé
La distribution verticale dans l'atmosphère de C H g C N entre les altitudes de 0 et 55 km est calculée en considérant que la molécule de C H _ C N est émise au niveau du sol et détruite dans la stratosphère par le radical h y d r o x y l e . Les effets des incertitudes s u r la . concentration de C H3C N au sol sont envisagés. La possibilité d'une destruction du C H3C N par l'atome de chlore est envisagée. De même, l'effet d'une variation des coefficients de transport vertical est discuté.
Les résultats du modèle sont comparés avec des observations récentes du C H _ C N .
Samenvatting
De v e r t i k a l e v e r d e l i n g van acetonitriel tussen 0 en 55 km w o r d t b e r e k e n d in de v e r o n d e r s t e l l i n g dat de emissie van CH^CN g e b e u r t vanaf de g r o n d , en dat het b e l a n g r i j k s t e verlies in de s t r a t o - sfeer de reaktie is met h y d r o x y l r a d i k a l e n . De invloed van v e r s c h i l l e n d e concentraties aan het o p p e r v l a k w o r d t o n d e r z o c h t . De v e r n i e t i g i n g v a n CHgCN door C l , alsook het g e b r u i k van verschillende t u r b u l e n t e diffusiecoëfficiënten wordt bondig b e s p r o k e n . De resultaten worden v e r g e l e k e n met recente C H - C N metingen.
Zusammenfassung
Die V e r t i k a l v e r t e i l u n g von der A z e t o n i t r i l zwischen 0 und 55 km w i r d berechnet wenn man v o r a u s s e t z t das die Emission von CHgCN gescheht von den Boden, und das der wichtigste V e r l u s t in d e r S t r a t o s p h ä r e die Reaktion ist mit H y d r o x y l r a d i k a l e n . Der Einfluss von verschiedenen Konzentrationen am Boden w i r d u n t e r s u c h t . Die V e r n i c h t u n g von C H3C N d u r c h Cl und die B e n u t z u n g von verschiedenen t u r b u l e n t e Diffusion koeffizienten w i r d bündig besprochen. Die Resultaten werden v e r g l e i c h t mit rezenten C H - C N Messungen.
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I N T R O D U C T I O N
A c e t o n i t r i l e appears to play a considerable role in the forma- tion of s t r a t o s p h e r i c ions ( A r n o l d et a l . , 1977; Arijs et a l . , 1978, 1980, 1983a; Henschen and A r n o l d , 1981; A r n o l d and Hensçhen, 1 9 8 2 ) . Recent determinations of CH^CN concentrations in t h e middle atmosphere ( A r i j s et a l . , 1983b) and at g r o u n d level ( B e c k e r and lonescu, 1982) as well as new laboratory data ( H a r r i s et a l . , 1981; Zetsch, 1981, 1982 and F r i t z et a l . , 1982) have stimulated us to develop a model to calculate a CHgCN p r o f i l e . T h e results of such model calculations are p r e s e n t e d here and compared to CH^CN mixing ratios deduced from ion mass spectrometric data obtained between 20 and 55 km a l t i t u d e .
A C E T O N T R I LE B U D G E T
T h e origin of acetonitrile is not y e t well u n d e r s t o o d . According to recent measurements b y Becker and lonescu ( 1 9 8 2 ) h o w e v e r , large quantities of CH^CN are known to be released when b u r n i n g bush and g r a s s . This indicates t h a t appreciable amounts of this gas could be produced in the tropical regions in connection with a g r i c u l t u r a l practices and p a r t l y injected in the s t r a t o s p h e r e t h r o u g h the equatorial Hadley cell. It is also well known t h a t tobacco smoke contains high n i t r i l e concentrations (Schmetz and Hoffman, 1977). M o r e o v e r , it is likely t h a t industrial releases, synthetic r u b b e r m a n u f a c t u r i n g and t u r b i n e emissions c o n t r i b u t e significantly to the acetonitrile source ( G r a e d e l , 1978).
T h e recent measurements of C H3C N b y Becker and lonescu ( 1 9 8 2 ) have been c a r r i e d out using a gas chromatography mass spectro- metric technique and are based on a few samples collected near t h e g r o u n d at d i f f e r e n t locations in Europe. T h e r e p o r t e d mixing ratios
v a r y between 2 and 7 p p b v , except for one sample taken in a rural area close to a b u s h and g r a s s fire, where the C H ^ C N mixing ratio reached 35 p p b v . If these measurements appear to be correct and to represent global average conditions, a mixing ratio of 5 ± 2 p p b v could be adopted as a b a c k g r o u n d value at the Earth's surface. Local peaks in relation to man-made activity should also be considered. Comparison between observations made in the Northern and in the Southern Hemi- sphere should g i v e an indication on the anthropogenic emissions v e r s u s the natural sources of C H ^ C N .
According to the derivation of C H g C N concentration from ion composition measurements ( A r n o l d et al., 1981; Henschen and A r n o l d , 1981; Arijs et al., 1982, 1983a, 1983b), it appears that the mixing ratio of acetonitrile above the tropopause is less than 10 p p t v , which is a factor of 500 lower than the value reported at g r o u n d level ( B e c k e r and lonescu, 1982). If the latter are correct, a considerable sink for C H g C N must exist in the troposphere.
In the stratosphere, an important loss process for acetonitrile is its reaction with O H :
( k j ) ; CH3CN + OH -» products. (1)
The rate constant as measured by H a r r i s et al. (1981) in the tempera- ture range 298-424 K is
kj = 5.86 x 10"1 3 exp(-750/T) cm3 s "1 (2)
T h i s constant is a factor of 2 higher than t h e value measured by F r i t z et al. ( 1 9 8 2 ) at 295 K and 7 t o r r and t h e one obtained b y Zetsch ( 1 9 8 1 ) . Destruction by 0 ( D ) and CI should also be considered b u t , since t h e rates of these reactions are y e t u n k n o w n , these processes will not be t r e a t e d in detail h e r e . T h e photodissociation of CH^CN plays an insignificant role below the stratopause ( f i g u r e 1 ) since the absorption cross section ( Z e t s c h , 1982) becomes v e r y small at wavelengths l a r g e r t h a n 180 nm, i . e . in t h e spectral region of t h e solar radiation p e n e t r a t i n g into the s t r a t o s p h e r e . T h e photodissociation frequencies J ( C H g C N ) which we have calculated f o r two extrapolations of the absorption cross section above 185 nm as g i v e n by Zetsch ( 1 9 8 2 ) a r e shown in f i g u r e 1.
M O D E L L I N G OF C H0C N
In o r d e r to simulate the average behavior of CH^CN in t h e atmosphere, the vertical d i s t r i b u t i o n of this constituent has been calculated b y means of a 1 - D model ( B r a s s e u r et a l . , 1982).
A given concentration at g r o u n d level and a zero f l u x at 100 km are specified as boundary conditions for C H ^ C N . In a f i r s t calculation ( c u r v e 1 - f i g . 2 ) the only loss process considered is t h e reaction with O H , f o r which t h e rate constant of H a r r i s et al. ( 1 9 8 1 ) is adopted. It is then found t h a t in o r d e r to reproduce the CHgCN d i s t r i b u t i o n , as observed in t h e stratosphere an acetonitrile mixing ratio at the Earth's surface of 10 p p t v must be specified. T h i s number is f a r below the observations by Becker and lonescu ( 1 9 8 2 ) which are of the o r d e r of 5 p p b v .
To f i t simultaneously these high values at g r o u n d level and the low stratospheric data (< 10 p p t v ) it is t h e r e f o r e necessary to
CHARACTERISTIC TIME
LOSS COEFFICIENT (s"
1)
Fig. 1.- Loss coefficients of CI^CN, due to the reaction with OH, CI and the photodissociation. The loss coefficient of CH^CN versus CI is based on a rate constant assumed to be identical to that of CH^Cl versus CI. An example of the specified tropospheric loss (P) is also shown. The inverse of. the characteristic time for diffusion is indicated by the dashed dotted lines.
introduce a supplementary sink in the troposphere. To consider such a possibility an a r b i t r a r y loss coefficient of the form
" •".['-(¥ JV 1
was introduced, were z is the altitude expressed in km. In fact the flux of acetonitrile, which is injected in the stratosphere is not affected by the detailed shape of the profile appearing in the adopted expression ( 3 ) but by the overall tropospheric lifetime of C H ^ C N , which can be adjusted through the proper choice of 0q.
To reproduce consistently both ground level and stratospheric data by model calculations, a tropospheric lifetime for acetonitrile of about 11 days must be adopted as illustrated by c u r v e 2 in figure 2.
From the model calculations (see table 1 - case 2) it then follows that these conditions imply a world emission of C H ^ C N of about 400 M T / y r . , corresponding to an annual injection of nitrogen of nearly 140 MT. S u c h a value can hardly be accepted without investigating the impact on the global atmospheric nitrogen cycle. T h i s emission is indeed about 14 times larger than the source strength of NOx from ammonia as determined by Levine (1982) and 8 to 32 times larger than the estimated NOx production by lightning (Dawson, 1980; Hill et a l . , 1980). It exceeds the estimated anthropogenic production of NO^ by a factor of 10 and is considerably larger than the natural release of NO from soils (Ehhalt and Drummond, 1981) and than the N Ox emission by biomass burning ( C r u t z e n et a l . , 1979; Ehhalt and Drummond, 1981). It should be noted however that the injected acetonitrile will not interfere with the NO^ budget if the main tropospheric loss would be direct wash-out of C H3C N molecules by rain. At present however it is difficult to evaluate the effectiveness of such a wash-out, since little is known
Fig. 2.- Calculated distribution of acetonitrile mixing ratios for the different cases of table 1 compared ' to experimental results. The area labelled "Rocket
data" covers the data deduced from rocket ion mass spectrometry measurements (Arnold et_al., 1978, and Henschen and Arnold 1981). The area "balloon data" summarizes the range of C H3C N mixing ratios, from balloon ion mass spectra (Arijs et al. , 1982, 1983a, 1983b; Arnold and Henschen, 1981; Henschen and Arnold, 1981). The area "ground data" re- presents the observations of Becker and Ionescu,
1982.
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about the CH^CN vapor p r e s s u r e above aqueous solutions with low acetonitrile concentrations, at low t e m p e r a t u r e s .
Keeping in mind t h a t t h e g r o u n d level concentrations r e p o r t e d by Becker and lonescu ( 1 9 8 2 ) may be more r e p r e s e n t a t i v e for polluted areas and could be f a r removed from global a v e r a g e conditions, we have also investigated cases with lower surface b o u n d a r y conditions in our model calculations. T a b l e 1 shows t h e d i f f e r e n t model cases considered together with t h e p r e s c r i b e d tropospheric lifetime of CH^CN and t h e global emission which is r e q u i r e d to balance the total atmospheric s i n k . T h e results of the calculations are also shown in f i g u r e 2.
A n y f u r t h e r progress in our u n d e r s t a n d i n g of t h e acetonitrile b u d g e t r e q u i r e s new independent observations of CH^CN in t h e t r o p o - s p h e r e , studies of individual source s t r e n g t h s and sinks of this molecule and investigations of a possible w a s h - o u t .
All calculations presented up to now have been performed with a r a t h e r large eddy diffusion coefficient K^ ( B r a s s e u r et a l . , 1982) assuming strong vertical mixing (see f i g . 3 ) . A computation with con- ditions similar to t h a t of case 3 was made with t h e K^ profile suggested by Massie and Hunten ( 1 9 8 1 ) . T h e l a t t e r , also r e p r e s e n t e d in f i g u r e 3, leads to an increase by 40 percent of the characteristic diffusion time in the s t r a t o s p h e r e . Although below 30 km t h e stratospheric acetonitrile d i s t r i b u t i o n obtained with the K,, profile of Massie and Hunten ( f i g . 4 ) is almost identical to these d i s t r i b u t i o n s d e r i v e d using the K^ p r o f i l e , deviations appear above 30 km, leading to a factor of 4 d i f f e r e n c e at t h e stratopause. T h e d i s t r i b u t i o n s based on the K2 profile seem to f i t quite well t h e observations obtained from rocket f l i g h t s ( A r n o l d et a l . , 1977; Henschen and A r n o l d , 1981) at high a l t i t u d e .
Finally, the possibility of a supplementary d e s t r u c t i o n of CH_CN by chlorine atoms has been considered. To our knowledge, the
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Fig. 4.- Vertical distribution of acetonitrile calculated for different eddy diffusion coefficients (see figure 3) assuming destruction by OH only
rate of this reaction has never been measured but an order of magnitude can be inferred from similar reactions. We have adopted a rate constant k? equal to that of C H - C I + C I , i.e. 3.4 x 10~1 1
3 -1
e x p ( - 1 2 6 0 / T ) cm s as recommended b y WMO (1982). The c o r r e s p o n - ding distribution of acetonitrile computed with this loss coefficient and the original large eddy diffusion coefficient K1 is shown in fig. 4. The effect of chlorine could be significant, particularly in the upper strato- sphere and could even be the dominant loss if the reaction rate was similar to that of C H g O H + C I . Laboratory work dealing with this question is urgently required.
C O N C L U S I O N
Model calculations show that the available observations of acetoni- trile can be consistently reproduced by making different assumptions about the processes affecting this molecule. A more detailed u n d e r - standing of the C H g C N b u d g e t in the atmosphere requires a better knowledge of the s t r e n g t h s of all plausible sources and of the exact loss rate in the troposphere and in the stratosphere. Supplementary observations of acetonitrile at g r o u n d level and in the whole tropo- sphere (particularly in the vicinity of the tropopause) would g i v e useful information about the various source s t r e n g t h s of C H ^ C N , its tropo- spheric lifetime and its injection rate in the stratosphere. Moreover, measurements at different locations on the Earth's surface are required to confirm the recent observations reported by Becker and lonescu (1982).
A C K N O W L E D G E M E N T S
We would like to thank Prof. D r . C . Zetsch for putting at our disposal the valuable information on the absorption cross sections of
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CHgCN and other non published material. We are also indebted to D r s . R. Z e l l n e r , K . H . Becker and A . lonescu f o r p r o v i d i n g p r e p r i n t s , and to D r s . O . Klais, P. C r u t z e n and P. Warneck f o r useful discussions and suggestions. T h i s work , was p a r t l y supported by t h e Chemical M a n u f a c t u r e r s Association ( c o n t r a c t 8 2 - 3 9 6 ) .
TABLE 1.- Characteristics and results of different model runs.
Case Mixing ratio at ground level
Loss coefficient
V s " 1 )
Global lifetime
Required flux at ground level (cm ^s
Required World .on of CI
(MT/yr) emission of CH^CN
1 2 3 4 5
1 X 10 5 X 10 1 X 10 1 X 10 5 X 10
- 1 1
-9 -9
- 1 0
-9
2 X 10 1 X 10
- 6 - 6
0.5 X 10 1 X 10~6
- 6
3 yrs 11 days 20 days 36 days 20 days
2.0 X 10 3.6 X 10 5.1 X 10S
3.4 X 10 2.5 X 10 10
8 10
0.023 400 55 3-6 280
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