of the destruction of CHqCN by C1, as well as the parison between observations made in the Northern

GEOPHYSICAL RESEARCH LETTERS, VOL. 10, NO. 8, PAGES 725-728, AUGUST 1983

ACETONITRILE IN THE ATMOSPHERE

G. Brasseur(*), E. Arijs, A. De Rudder, D. Nevejans and J. Ingels Belgian Institute for Space Aeronomy, B-1180 Brussels, Belgium.

Abstract . The vertical distribution of aceto- to a bush and grass fire, where the CH3CN mixing

nitrile between 0 and 55 km altitude is ratio reached 35 ppbv. If these measurements

calculated assuming that CH3CN is released at the appear to be correct and to represent global

Earth's surface and that the main loss process in average conditions, a mixing ratio of 5 ñ 2 ppbv the stratosphere is the reaction with hydroxyl could be adopted as a background value at the radicals. The influence of different possible Earth's surface. Local peaks in relation to surface concentrations is investigated. The role man-made activity should also be considered. Com-

of the destruction of CHqCN by C1, as well as the parison between observations made in the Northern

use of different eddy •iffusion coefficients is and in the Southern Hemisphere should give an briefly discussed. The results are compared with indication on the anthropogenic emissions versus

recent measurements of acetonitrile. the natural sources of CH_CN.

According to the derivation of CH3CN concen-

Introduction tration from ion composition measurements (Arnold et al., 1981; Henschen and Arnold, 1981; Arijs et

Acetonitrile appears to play a considerable al., 1982, 1983a, 1983b), it appears that the

Henschen, 1982). Recent determinations of CH3CN (Becker and Ionescu, 1982). If the latter are concentrations in the middle atmosphere (Arijs et correct, a considerable sink for CH3CN must exist

al., 1983b) and at ground level (Becker and in the troposphere.

Ionescu, 1982) as well as new laboratory data In the stratosphere, an important loss process (Harris et al., 1981; Zetsch, 1981, 1982 and for acetonitrile is its reaction with OH:

Fritz et al., 1982) have stimulated us to develop

a model to calculate a CH3CN profile. The results (kl); CH3CN + OH + products. (1)

of such model calculations are presented here and

compared to CHqCN mixing ratios deduced from ion The rate constant as measured by Harris et al.

mass spectrome•ric data obtained between 20 and (1981) in the temperature range 298-424 K is

55 km altitude.

-13 3 -1

k = 5.86 x 10 exp(-750/T) cm s (2)

Acetonitrile budget 1

This constant is a factor of 2 higher than the

The origin of acetonitrile is not yet well value measured by Fritz et al. (1982) at 295 K

understood. According to recent measurements by and 7 torr and the •ne obtained by Zetsch (1981•

Becker and Ionescu (1982) however, large quanti- Destruction by O(-D) and C1 should also ties of CHACN are known to be released when considered but, since the rates of these

burning bu• and grass. This indicates that reactions are yet unknown, these processes will

appreciable amounts of this gas could be produced not be treated in detail here. The photo-

in the tropical regions in connection with agri- dissociation of CH•CN plays an insignificant role

cultural practices and partly injected in the below the stratapause (Figure 1) since the stratosphere through the equatorial Hadley cell. absorption cross section (Zetsch, 1982) becomes It is also well known that tobacco smoke contains very small at wavelengths larger than 180 nm, high nitrile concentrations (Schmetz and Hoffman, i.e. in the spectral region of the solar radia- l977). Moreover, it is likely that industrial tion penetrating into the stratosphere. The releases, synthetic rubber manufacturing and photodissociation frequencies J(CHqCN) which we turbine emissions contribute significantly to the have calculated for two extrapolations of the acetonitrile source (Graedel, 1978). absorption cross section above 185 nm as given by

The recent measurements of CH3CN by Becker and Zetsch (1982) are shown in Figure 1.

Ionescu (1982) have been carried out using a gas

chromatography mass spectrometric technique and Modelling of CH3CN

are based on a few samples collected near the

ground at different locations in Europe. The In order to simulate the average behavior of reported mixing ratios vary between 2 and 7 ppbv, CH_CN in the atmosphere, the vertical distri-

except for one sample taken in a rural area close bunion of this constituent has been calculated by

means of a 1-D model (Brasseur et al., 1982).

A given concentration at ground level and a

(*) Aspirant au Fonds National de la Recherche zero flux at 100 km are specified as boundary

Scientifique. conditions for CH^CN. In a first calculation

(curve 1 - Figure 3 2) the only loss process

Copyright 1985 by the American Geophysical Union. considered is the reaction with OH, for which the

Paper number $L0729. It is then found that in order to reproduce the

0094-8276/85/005L-0729505.00 CH3CN distribution, as observed in the strato-

725

726 Brasseur et al.' Acetonitrile in the Atmosphere

CHARACTERISTIC TIME

I00 yrs I0 yrs I yr I month

MAX

-Z •o .7 -

• • • • ' • . ( O• •1 O• •I•

•0

0 I I I I I IllJ I I I I Ill IJ I I I , ,•,•-- I I I Ill II•l ... '

10 '• 10-" t0 -•ø t• 9 10 -8 1• 7 10 6

LOSS COEFFICIENT (s -•)

•g. • •o• coe•c•ent• o• CH_C•, due to the reaction w•th OH, C• and the photod•oc•at•on. The •o•

coefficient o• CH_C• ver•u• C• • ba•ed on a rate con•tant a•ed to be •dent•ca• to that

ver•u• C•. An e•mp•e o• the •pec•ed tropospheric •o• (•) • a•o •ho•n. The •nver•e o• •he

characteristic t•me •or d•u•on • •nd•cated by the dashed dotted

sphere an acetonitrile mixing ratio at the tropospheric lifetime of CHACN, which can be Earth's surface of 10 pptv must be specified. adjusted through the proper choice of • .

This number is far below the observations by To reproduce consistently both ground level Becker and Ionescu (1982) which are of the order and stratospheric data by model calculations, a

of 5 ppbv. tropospheric lifetime for acetonitrile of about

To fit simultaneously these high values at 11 days must be adopted as illustrated by curve 2 ground level and the low stratospheric data in Figure 2. From the model calculations (see (< 10 pptv) it is therefore necessary to Table 1 - case 2) it then follows that these introduce a supplementary sink in the tropo- conditions imply a world emission of CH3CN of

sphere. To consider such a possibility an about 400 MT/yr., corresponding to an annual in- arbitrary loss coefficient of the form jection of nitrogen of nearly 140 MT. Such a

I (z•• I -1 value can hardly be accepted without

• - •o 1 - s (3) investigating the impact on the global atmo-

was introduced, were z is the altitude expressed spheric nitrogen cycle. This emission is indeed

injected in the stratosphere is not affected by NO from ammonia as determined by Levine (1982)

the detailed shape of the profile appearing in an• 8 to 32 times larger than the estimated NO

the adopted expression (3) but by the overall production by lightning (Dawson, 1980; Hill et

Fig. 2 Calculated distribution of acetonitril•

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 CHACN mixing ratios, from balloon ion mass spectra (Arijs et al., 1982, 1983a, 1983b;

Arnold and Henschen, 1981; Henschen and Arnold,

al., 1980). It exceeds the estimated anthro- pogenic production of NO by a factor of 10 and

is considerably larger t•an the natural release

of NO from soils (Ehhalt and Drummond, 1981) and than the NO emission by biomass burning (Crutzen

et al., 1•9; Ehhalt and Drummond, 1981). It

should be noted however that the injected aceto- nitrile will not interfere with the NO budget if

the main tropospheric loss would be d•rect wash-

out of C•CN molecules by rain. At present

however ø is difficult to evaluate the effectiveness of such a wash-out, since little is known about the CHACN vapor pressure above

aqueous solutions wi•h low acetonitrile con-

centrations, at low temperatures.

• •,o

D

_

4 20

3

VERTICAL EXCHANGE COEFFICIENT(cm2s -1)

1981). The area "ground data" represents the Fig. 3 Vertical distribution of the eddy

Brasseur et al.: Acetonitrile in the Atmosphere ?2?

TABLE 1. Characteristics and results of different model runs.

Case Mixing ratio Wash-out

at ground level coefficient

o(S--)

Global Required

lifetime flux at gr•un•

level (cm- s- )

Required World

emission of CH3CN

(MT/yr)

1 1 x 10 -11 0 -6 3 yrs 2.0 x 10610 0.023 2 5 x 10 -9 2 x 10 6 11 days 3 6 x 10 400

3 1 x 10 -9 -

4 1 x 10 10 1 x 10 20 days 5 1 x 109 _ 55 5 5 x 10 -9 0.5

x !• -6 36

days 3 4 x 10•0

3.6

Keeping in mind that the ground level almost identical to these distributions derived concentrations reported by Becker and Ionescu using the K_ profile, deviations appear above

(1982) may be more representative for polluted 30 km, leading to a factor of 4 difference at the areas and could be far removed from global stratopause. The distributions based on the K 2

average conditions, we have also investigated profile seem to fit quite well the observations

cases with lower surface boundary conditions in obtained from rocket flights (Arnold et al., our model calculations. Table 1 shows the 1977; Henschen and Arnold, 1981) at high alti-

the prescribed tropospheric lifetime of CH^CN and Finally, the possibility of a supplementary

the global emission which is required to •alance destruction of CH_CN by chlorine atoms has been

the total atmospheric sink. The results of the considered. To our knowledge, the rate of this

Any further progress in our understanding of magnitude ca n be inferred from similar reactions.

the acetonitrile budget requires new independent We have adopted a rate constant k 2 equal to tha•

observations of CH3CN in the troposphere, studies O•lCH3C1 + C1, i.e. 3.4 x 10 exp(-1260/T) cm

of individual source strengths and sinks of this s as recommended by WMO (1982). The correspon-

out. this loss coefficient and the original large eddy

All calculations presented up to now have been diffusion coefficient K 1 is shown in Figure 4.

performed with a rather large eddy diffusion The effect of chlorine could be significant, par-

coefficient •a• (Brasseur et al., 1982) assuming ticularly in the upper stratosphere and could

strong verti mixing (see Figure 3). A computa- even be the dominant loss if the reaction rate

tion with conditions similar to that of case 3 was similar to that of CH30H + C1. Laboratory

was made with the K? profile suggested by Massie work dealing with this question is urgently

in Figure 3, leads to an increase by 40 percent

of the characteristic diffusion time in the Conclusion

stratosphere. Although below 30 km the strato-

spheric acetonitrile distribution obtained with Model calculations show that the available

the K 2 profile of Massie and Hunten (Figure 4) is observations of acetonitrile can be consistently

reproduced by making different assumptions about

• STRATOSPHERE

30 - CH3CN o -•* _{ß % .}

, , , , , ,,,I , , , , , ,,,J , m , ,

10 10 •0 10 -•

VOLUME MIXING RATIO

Fig. 4 Vertical distribution of acetonitrile

the processes affecting this molecule. A more

detailed understanding of the CHqCN budget in the

atmosphere requires a better Enowledge of the strengths of all plausible sources and of the exact loss rate in the troposphere and in the stratosphere. Supplementary observations of acetonitrile at ground level and in the whole troposphere (particularly in the vicinity of the tropopause) would give useful information about the various source strengths of CH^CN, 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 Ionescu (1982).

Acknowledgements

We would like to thank Prof. Dr. C. Zetsch for

calculated for different eddy diffusion coef- putting at our disposal the valuable information

ficients (see Figure 3) assuming destruction by on the absorption cross sections of CH3CN and

OH only and both OH and C1. The measurements are other non published material. We are also again shown for comparison. Open symbols are data indebted to Drs. R. Zellner, K.H. Becker and from Henschen and Arnold (1981) and full symbols A. Ionescu for providing preprints, and to Drs.

those of Arijs et al. (1983a, 1983b). O. Klais, P. Crutzen and P. Warneck for useful

728 Brasseur et al.' Acetonitrile in the Atmosphere

discussions and suggestions. This work was partly supported by the Chemical Manufacturers Association (contract 82-396).

source of atmospheric gases, CO, H , N O, NO,

CH3C1 and COS, Nature, 282, 253-•56• 1979.

Dawson, G.A., Nitro•-• •ix•-•on by lightning,

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spectrometric measurements of the positive ion Fritz, B., K. Lorenz, W. Steinert and R. Zellner, composition in the stratosphere, Nature, 271, Laboratory kinetic investigations of the

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(Received March 29, 1985;

revised March 30, 1983;

accepted April 18, 1983.)