On stratospheric acetonitrile detection by passive chemical ionization mass spectrometry

Planer. .Space Ser.. Vol. 35, No. 6, pp. 715-725, Pnnted in Great Britain.

1987

00324633/87 %3.oO+O.W Pergamon Journals Ltd.

ON STRATOSPHERIC PASSIVE CHEMICAL Max-Planck-lnstitut

ACETONITRILE DETECTION BY IONIZATION MASS SPECTROMETRY

H. SCHLAGER and fiir Kernphysik, Postfach

F.

ARNOLD

103 980, D-6900 Heidelberg,

F.R.G.

(Received in final form 24 October 1986) Abstract-Systematic ECA (Electric Field-induced Collisional Activation) studies of H+ (CH,CN),(H,O), cluster ions were carried out in the laboratory and in balloon-borne ion mass spectrometer experiments in the stratosphere. It was found that these ions, as a result of ECA, may become converted to H+(H,O), clusters. As a consequence, acetontrile (CH&N) abundances inferred from stratospheric ambient ion composition measurements by consideration of the measured abundance ratio for H.+(CH,CN),(H,O), and H+(H20), ions are too low by factors up to about 10 depending on altitude and experimental conditions. By contrast, data obtained above about 35 km are only little affected. Building on recent aircraft-borne acetonitrile measurements by our group, we have carried out one-

dimensional model calculations of the acetonitrile abundance which reproduce quite well those data not being severely affected by ECA if low atmospheric hydroxyl radical abundances are considered.

INTRODUCTION

Recent measurements of atmospheric acetonitrile in the tropopause region (Arnold and Hauck, 1985; Knop and Arnold, 1987) using aircraft-borne ACIMS (Active Chemical Ionization Mass Spectrometry) seem to be in conflict with previous balloon-borne PACIMS (Passive Chemical Ionization Mass Spectrometry) measurements carried out in the middle stratosphere. The ACIMS measurements gave volume mixing ratios ranging between about 9 and 36 pptv (parts per trillion by volume) with an average of 21 pptv, being similar to recent ground-level measurements by Snider and Dawson (1984) using a condensation collection technique (2&90 pptv ; average : 56 pptv). By contrast, middle stratospheric PACIMS measurements extending to a lowest altitude of 24 km gave volume mixing ratios which increase somewhat with decreasing height, reaching a maximum value of only about 2.5 pptv at 24 km (Henschen and Arnold, 1981). The strong decrease of the volume mixing ratio between the tropopause and the 24 km level by almost one order of magnitude can hardly be explained within the framework of our current understanding of the chemistry and transport of atmospheric acetonitrile (cf. Arnold and Hauck, 1985). It is thought that acetonitrile is released in the troposphere, probably at the ground, and destroyed in the atmosphere, preferably by hydroxyl radical attack (Henschen and Arnold, 1981 ; Harris et al., 1981 ; Brasseur et al., 1983), whereas removal by heterogeneous processes appears to be only of minor importance (Hamm et al., 1984). 715

In the present paper, we draw attention to a potential disturbance by ECA (Electric Field-induced Collisional Activation) of middle stratospheric PACIMS measurements, which leads to an underestimation of the acetonitrile abundance being sufficiently severe to account for the discrepancy with aircraft-borne ACIMS measurements. This disturbance increases markedly with decreasing height, leading to an underestimation of the acetonitrile abundance around 20 km altitude by factors between about 3 and 10 depending on the experimental conditions. By contrast, upper stratospheric PACIMS measurements of acetonitrile are not severely affected by ECA. Here, the CH,CN-abundance is underestimated by not more than about lO_20%. The present paper includes a critical discussion of the PACIMS method, laboratory ECA studies of requisite ion species used for the PACIMS detection of CH,CN, a compilation of as yet unpublished acetonitrile measurements by our group using valve-controlled descending balloons and theoretical model calculations of atmospheric acetonitrile abundances.

PACIMS

METHOD

Atmospheric acetonitrile was originally detected by balloon-borne PACIMS measurements (Arnold et al., 1978). In the meantime, various additional balloonborne PACIMS measurements were made using improved instruments with better sensitivity and mass resolution (Arijs et al., 1980, 1982, 1983; Arnold et al., 198 1 ; Arnold and Henschen, 1982). Very recently,

716

H. SCHLAGER and F. ARNOLD

our group has flown a PACIMS instrument equipped with an ECA-mode of operation (Schlager and Arnold, 1985) which largely improves ion identification and strongly supported the identification of atmospheric acetonitrile. The PACIMS method for acetonitrile detection relies on composition measurements of ambient positive ions using a cryogenically pumped mass spectrometer. It makes use of the fact that atmospheric CH,CN reacts with H+ (H20)n ions via processes H+ (H,O), + CHCN

+ H+ CH$N(H20),m.,

+ Hz0 (1)

whose reaction rate coefficients are known from laboratory measurements [4.6 x lo-’ cm3 s- ’ (n = 1), 4.0 x 10e9 cm3 s-’ (n = 2), 3.7 x lop9 cm3 SK’ (n = 3) and 3.5 x lop9 cm3 s-l (n = 4) (Smith et al., 1981)]. The product ions H+CH3CN(H20), may further react with CHCN leading to mixed cluster ions of the general form H+(CH,CN),,,(H,O),. Due to the large atmospheric abundance ratio for H,O and CH,CN back reactions displacing CH,CN-ligands by Hz0 come into play for m > I and therefore stratospheric H+(CH,CN),(H,O), cluster ions preferably contain only l-3 CH,CN-molecules. Backswitching-reactions for m = 1 are negligible with respect to ionic lifetimes. For example, the ratio for the effective time constant of the reverse reaction of process (1) and the ion-ion recombination lifetime is about 20 at an altitude of 30 km and 5 at 20 km, as can be calculated using the thermodynamic data from Lau et al. (1982) and MeotNer (1984). Hence the major sink for these ions is ionion recombination and their abundance relative to H+(H,O), ions is determined by a steady-state of process (1) and ionion recombination. The steady-state continuity equation ]H + W,OLlWH,CW

= tH+U-WNnW,QnI t<’ (2) relates the abundance ratio for product and precursor ions R = [H+(CH,CN),(H,O),]/[H+(H,O),] to the number density [CH,CN] of CH,CN-molecules. Here k and tR are the effective reaction rate coefficient for process (1) and the ion-ion recombination lifetime being equal to (CW- ’ where a is the ion-ion recombination coefficient and n is the total ion concentration (only ions of one sign). By measuring R, the acetonitrile concentration can be determined using expression (2). Typical uncertainties involved are +30%-+ 50% for k and about f 50% for tR. The uncertainty for R is not more than about f20% if systematic errors are neglected. Previously, systematic errors in the measurement of R, in fact, were not

considered and thus an overall error of inferred acetonitrile abundances of about ) 70% was quoted. In the present paper, we should like to draw attention to a potential systematic error in R-measurements arising from ECA. The ion mass spectrometer used for PACIMS measurements is usually cryogenically pumped in order to reduce the gas pressure from its ambient value (typically l-60 torr) to a value of 1 x lOa torr or less, allowing one to operate a quadrupole mass spectrometer. This is achieved by mounting the mass spectrometer inside a vacuum vessel into which atmospheric air and atmospheric ions flow through an inlet hole having a typical diameter D ranging between 0.1 and 0.4 mm. Behind the inlet hole, a gas jet is formed being directed towards the quadrupole system of the mass spectrometer, which is usually mounted on-axis with the inlet hole. In order to extract ions from the gas jet and inject them into the quadrupole system, electric fields are employed giving rise to energetic ion-molecule collisions within the gas jet. As a result, ions experience internal excitation (ECA), which may eventuahy lead to cluster ion fragmentation depending on the experimental conditions, especially the strength E of the electric field (cf. Arnold et al., 1981). For example, HC (H20),, cluster ions undergo fragmentation via (H+ (HzO),)* + H’ (HzO)n- , + Hz0

(3)

(here * denotes strong internal excitation). It has been recognized already at the beginning of balloon-borne ion composition measurements that substantial ECAinduced ion fragmentation of type (3) occurs since the measured H+(H,O), hydrate distribution was usually shifted to smaller hydration orders than the expected one. The latter was calculated by consideration of thermochemical data available from laboratory measurements and by taking atmospheric temperatures as measured on the balloon and by taking a typical volume mixing ratio for atmospheric water vapour. In view of the occurrence of process (3), it was assumed that H+(CH,CN),(H,O), cluster ions also undergo ECA-induced fragmentation (H + (CH,CN),(H,O)J* --, H+(CH,CN),(H,O),_ In particular, it was assumed cluster ions would decompose (H+ CH,CN(H,O),)*

, +HzO.

(4)

that H+CH,CN(H,O), only via

+ H+ CH3CN(H20),_,

+ H,O. (5a)

This was supposed since the proton affinity of CH,CN [PA(CH,CN) = 186 kcal mall ‘1 is much larger than the PA of H,O [PA(H,O) = 165.5 kcal mol- ‘1. How-

Stratospheric

acetonitrile

40 -

5 s

30 -

3 G!?CO Q

-

10 -

FIG.

I

I

I

I

I

I

1

2

3

4

5

6

CHAN:ES (AH’) FOR THE ENTHALPY FRAGMENTATION VIA PROCESSES (5a) AND (5b)vs THE NUMBER OF H,O-LIGANDS (n). 1.

H+(CH,CN)(H,O),

ever, recently it was recognized that even highly endothermic ion fragmentation processes, as for example, CHC -formation from H+CH,CN (Schlager and Arnold, 1985) and H+(CH,),CQ (Knop and Arnold, 1987), may be induced by ECA under conditions which are typical of mass spectrometric composition measure~nents of stratospheric ions. Therefore, we suspected that ECA-induced fragmentation of H +CH,CN(H20), cluster ions may proceed not only via the main channel (5a) but also via the energetically less favourable side channel (H’CH,CN(H,O),)*

+ H+(H,O),+CH,CN.

(5b)

If so, PACIMS measurements of atmospheric acetonitrile may be severely affected depending on the efficiency _aof process (5b) and the abundance ratio R for ambient ions. Since R is expected to increase with decreasing height, becoming about 15 at 20 km, already a relatively small effective efficiency for process (5b) would be suthcient to cause a severe disturbance of the PACIMS method at these heights. Qualitatively, E should increase with increasing internal excitation of H+CH,CN(H,O),, which, in turn, should increase with increasing ~?/~lp(E is the strength of the electric field and p is the gas density). ECA by multiple collisions with energies below the threshold for process (5b) should give rise to only a small E and strongly favour the energetically more favourable process (5a). By contrast, collisions with higher energies well above the threshold energy for process (5b) may lead to substantial c values. Enthalpy changes (AH”),, for processes (Sa) and (5b) are shown in Fig. 1. They rely on thermodynamical

717

detection

data as measured by Lau et uZ. (1982) for H+ (HzO), and Meot-Ner (1984) for ~+CH~CN(H~O)~. Usually, the ECA-scenario is characterized by a steep decrease of p along the axis of the expanding gas jet and by E-values around l-20 V cm- ‘. The gas density p decreases from its ambient atmospheric value (about 3 x 10’6-10’8 cm-’ in the height region of interest) to a background value of about 3 x 10” molecules cm-j, being typical for the region of the vacuum vessel containing the quadrupole mass spectrometer. The slope of the p-decrease along the jet axis decreases with increasing inlet hole diameter D. Beyond a certain distance from the inlet hole, usually being smaller than the distance between the QMS and the inlet hole, an ion moves into a collision-free region (p smaller than about 3 x IO” cm-‘). Since E/p increases steeply with increasing distance from the inlet hole, the average collision energy and thereby ECA do the same. Consequently, the most energetic collisions occur at the end of the gas jet where E/p is largest and ion-molecule collisions still occur. A collision length of 1 cm is reached at p equal to about 1 x lOI cm-3. Here, depending on E (l-20 V cm- ‘), the average energy for ion-molecule collisions (in the center of mass frame) obtained from the electric field during a collision interval is about 7-140 kcal mol-‘, being well above the threshold energies (see Fig. I) for processes (5a) and (5b) if E exceeds 7 V cm- ‘.

ECA STUDIES

IN THE LABORATORY

In order to study ECA-induced fragmentation of HfCH3CN(H,0)q cluster ions, we have carried out laboratory measurements using the same ion drift cell apparatus equipped with a high pressure ion source which was previously used by our group for studies of ECA and ion-molecule reactions. Since the apparatus has been described in detail elsewhere (Schlager el al., 1983 ; Glebe and Arnold, 1983 ; Schlager and Arnold, 1985, 1986, it will only briefly be reviewed here. It consists of a high pressure glow discharge ion source (IS), an ion drift cell (DC). and a quadrupole mass spectrometer (QMS) (Fig. 2) used for ion analysis and detection. For the present measurements H+(CH,CN),(H,O), cluster ions were prepared by adding laboratory air, water vapour and acetonitrile vapour to the IS chamber (total gas pressure: 10 torr). Upon effusion through an exit hole from the IS chamber, ions were injected into the DC, which was filled with argon and was used here as a collision chamber. By applying a nearly uniform electric field E across the DC, it was possible to collisionally activate the ions traversing the DC. The primary and ECA-product ions which effuse from the DC were

718

H.

SCHLAGEK

and

ARNOLD

F.

DI P

QMS CEM

I

TMP t

t

t 10 cm FIG. 2. DRIFT CELL/MASS SPECTROMETER APPARATUS.

: ion source ; DE : discharge electrodes ; SI : source gas inlet ; E, : ion source aperture plate ; E, : ion shutter ; DC : drift cell ; FP : front plate ; RP : rear plate ; DI : drift cell gas inlet ; P : pressure gauge port ; QMS : quadrupole mass spectrometer ; CEM : channel-electron multiplier; TMP : turbomolecular pump. IS

and detected by the QMS. An argon gas pressure of 0.02 torr was chosen in order to simulate the conditions met towards the end of the expanding gas jet in the balloon-borne experiment where only a few but the most energetic collisions occur. At this gas pressure, depending on the electric field strength across the DC (l-25 V cm- ‘), the average collision energy gained from the electric field during a collision interval is about 3-90 kcal mol-’ (in the centre of mass frame) and thus in the same range as the expected collision energies in the thin gas jet region of the flight experiment. Figure 3 shows measured fractional ion count rates vs E. To clearly arrange the experiment the H+(CH,CN),(H,O), cluster ions which effuse from the IS chamber were dissociated to a manageable number of ion species in the collision-controlled region between the IS and DC. Hence, the most abundant ion species, which were injected into the DC, are H+CH,CN (mass 42), HfCH,CN(H20) (60) and H+(CH,CN), (83). These ions may have already experienced some internal excitation, but the same applies for the ions in the balloon-borne experiment as they reach the end of the expanding gas jet. As E across the DC was raised, the fractions of 60 and 83 decreased whereas two new ion species, CH: (15) and H,O+ (19) appeared whose fractions quickly increased. The fraction of 42 increased only slightly and ultimately decreased again. Evidently, ECAanalysed

Pressure 0.02 TorrAr 42

IO-41 ” 0

”

”

5

’ I

““‘I

-

15 10 Field Strength (Volt/cm)

20

FIG.~. FRACTIONALION COUNTRATES ASAFUNCTIONOFTHE ELECTRIC FIELD STRENGTH IN THE DRIFT CELL FOR AN ARGON PRESSURE OF 0.02 TORR.

Mass numbers are given [15 : CH: ; 19: H,O+ ; 42: H+CH,CN; 60: H+CH,CN(H,O); 83: H+(CH,CN),].

induced fragmentation of 42,60 and 83 occurred leading to 15. The product ion HIOf, however, can be only due to H+CH,CN(H20) (60) and must have been formed via process (5b). The data imply that about 10% of the injected H+CH,CN(H,O) cluster ions

719

Stratospheric acetonitriie detection underwent process (5b) for an E of about 11 V cm-‘. At this relatively high E/p value, an ion while traversing the DC experiences only a few collisions (about 10) with an average collision energy (in the center of mass frame) of about 50 kcal mol-‘, exceeding the threshold energies (see Fig. 1) required for fragmentation of H+CH&N(H,O) into H+CH,CN [process (5a): 24.4 kcal mol- ‘1 and H,O+ Lprocess (5b): 45.5 kcal mol-‘1. It seems worth noting that both product ions CH: and H,O+ behave similarly, appearing at an E between about 3.5 and 6.9 V cm-]. Threshold energies for the formation of these product ions from their precursors H+CH$ZN and H~CH~CN~H~O) are 86.5 kcal mol- ’ and 45.5 kcal mol- ‘, respectively. The present measurements prove that process (5b), in fact, occurs, even under conditions being similar to those met in an atmospheric ion composition measurement. However, as mentioned above, the ECA scenario of an atmospheric ion composition experiment is more complex than that of the present laboratory experiment. Not only does E/p vary over a wide range, but also the ambient atmospheric H+(H,CN),(H,O), cluster ions are more complex than the ions used in the laboratory.

ATMOSPHERIC

ECA-STUDIES

In order to directly investigate the influence of ECA on balloon-borne ambient ion composition measurements, we have carried out systematic in-flight ECAstudies. These included variations of the electric field strength E within the expanding gas jet region on the same balloon flight and the use of inlet holes with different diameters on different balloon flights. Figures 4a and 4b show mass spectra of positive ions as measured at 33 km altitude during the descent phase of the flight performed in October 1985 (for flight data see Table I). Here, the inlet hole diameter f) was 0.4 mm and the distance between the QMS (rod diameter 0.8 cm) and the planar sampling electrode (into which the iniet hole was drilled) was 4.2 cm. The QMS had an open front-end configuration (cf. Arnold et al., 1981) in order to minimize the scattering of gas jet molecules by the QMS which results in a gas density enhancement in front of the QMS. With this QMS configuration, the field-axis potential CJ,, of the QMS is used to extract ions from the gas jet and inject them into the QMS. In order to increase E in the gas jet region without using too large a U,:, (which lowers the mass resolution power of the QMS), we have used an additional ring electrode (RE), which was mounted just behind the sampling electrode (SE). The distance between the SE and RE was 2 mm, the outer diameter

30 -

O.0

50

100

150

MASS (A.M.~.) FIG.~.INSITUMEASURED~ITIVEIONMASSSPECTRAOBTAINED AT AN ALTITUUE OF 33 km FOR DIFFERENT EXPERIMENTAL CONDITIONS.

(a) Low, (b) high electric field strength used for the ion extraction from the expanding gas jet at the low pressure side of the inlet hole. Also given are the observed abundance ratios for H+ (CH,CN),(H,O),z and H +(H,O), ions (R). TABLE I.CHARACTERISTICUATAOFTHEBALLOONFLIGHTS

Location Aire (44”N, 0”E) Aire Gap (44“N, 6”E) Aire Aire Aire

Launch date

Descent velocity (m s- ‘)

21 Sept. 1981 17 Oct. 1981 10 June 1982 19 March 1983 5 Oct. 1983 21 Oct. 1985

1.4 I.2 1.7 1.2 0.9 1.8

Altitude range (km) ---33320 33320.5 32.5 27 31.5-25 39-29 33.5520

of the circular RE was 2 cm, and the diameter of the central hole of the RE was 4 mm. Figure 4a shows a low E situation (U,, = -3 V; UbA = - 100 V; Us, = 0 V). Here, the two most abundant ion families H*CH,CN(H,O), and H+(H,O), are represented mainly by H+CH,CN(H,O)j (96) and H+(H,O), (73), whereas the somewhat less abundant ion family H+(CH,CN),(H20),, is represented by H+(CH,CN)2 H,O (101) and H’(CH~CN)~(H~O)~ (119). For H+CH~CN(H~O)~ and H+(H,O), ions thermochemical data are available, allowing theoretical calculations of their size distributions. Taking an

720

H. SCHLAGER and F. ARNOLD

atmospheric temperature of 234 K as measured during the flight at 33 km altitude and assuming a typical water vapour volume mixing ratio of 4 ppmv (parts per million by volume), we calculated the size distributions of the above ions and obtained peaks of the distributions at n = 3 (H+CH,CN(H,O),) and n = 4 (H+(H,O),) being consistent with the observations. Therefore, the data shown in Fig. 4a appear not to be affected by severe ECA-induced cluster ion fragmentation. However, fragmentation is already sufficiently efficient to largely enhance smaller ions, in particular 37, which should not be measurable. Figure 4b shows a high E measurement (Us, = 0 V ; URE = - 80 V ; UFA= - 150 V) made shortly after the low E measurement (Fig. 4a). Here H+CH,CN(H,O)m ions are represented mainly by n = 0 and n = 1 while H+(H,O), ions are represented mainly by n = 1. Evidently, the high E-measurement is severely affected by ECA-induced cluster ion fragmentation. The electric field strength in the region between the SE and RE used in this high E measurement is much higher than fields usually used for in situ sampling of atmospheric ions but these extreme conditions were chosen to clearly demonstrate the effect of ECA on R. However, it should be kept in mind that the E/p-values found in this region where the gas number density is still high do not exceed the E/p-values which are found at the end of the gas jet region under usual ion sampling conditions. Compared to the expected value for the undisturbed distribution, the shift of the of hydration order Anman, where the maximum the H+CH,CN(H,O),, distribution occurs, is about 3 for spectrum 4b. In the following, Anmax for H+CH&N(H,O),, ions will be used as a crude measure of the degree of ECA. Although Anmax first of all is a measure of the efficiency of process (5a) under the present experimental conditions for ion sampling (free expanding gas jet, uniform electric field), it is also an indication of ECA in general. Another quantity to be compared is R (abundance ratio for H+(CH,CN),(H20), and H+(H,O), ions). The measured R is 2.7 for low E and 1.9 for high E. Therefore R, in fact, appears to be smaller for higher E as would be expected from the laboratory ECAstudies discussed in the preceding section. A comparison of measurements using inlet holes with different diameters D is shown in Figs 5a (D = 0.2 mm) and 5b (D = 0.4 mm) for an altitude of 31 km. The measurements were performed with the same instrument on different balloon flights (March 1983, October 1985, see Table I). The larger D implies a denser gas jet and thereby a larger average E/p and a larger number of collisions experienced by an incoming ion. Here, Anmax values were 0 (Fig. 5a)

R (a)

150

Altitude

: 31 km

Altitude

: 31 km

(b)

20 0 0

50

100

150

MASS (A.M.u.) FIG.5.POSITIVEION MASS SPECTRA MEASURE,, AT AN ALT,T”“E km USINGDIFFERENTIONSAMPL~NGHOLES:(~)D~AMETER 0.02cm,(b)0.04 cm.

OF31

and 2 (Fig. 5b), respectively. Again, the measured R decreased markedly with increasing Anmax. Figures 6a and 6b show measurements analogous to those in Figs 5a and 5b, but performed around 23 km altitude. Here, Anmax values are 0.5 (Fig. 6a) and 3 (Fig. 6b). Again, measured R values (4.3, Fig. 5a ; 1.9, Fig. 5b) decrease with increasing Anmax. A compilation of unpublished previous balloonborne PACIMS measurements of acetonitrile by our group is shown in Fig. 7. It includes only measurements performed on descending balloons (descent velocities l-2 m s- ‘) in order to minimize the risk of contamination by molecules desorbing from the balloon or the balloon gondola. All measurements took place over southern France (latitude: 44”N). The characteristic flight data are compiled in Table 1 and instrumental parameters of the used mass spectrometer probes relevant to ion sampling are listed in Table 2. The data given in Fig. 7 are classified according to Anmax. At heights above 31 km, most of our data are hardly affected by ECA (Anmax = 0) whereas at the lower heights ECA becomes quite efficient depending on the experimental conditions. Also presented are recent high-altitude measurements by Arijs et al. (1983) using a I ,OOO,OOO m3 balloon. We expect these high-altitude data to be hardly affected by ECA for reasons which will be discussed later on.

Stratospheric

acetonitrile

721

detection

(a) 60-

y30-?L :25= z

21 October 1985 Altl tude : 23 km

0

50

100

20-

150

MASS (A.M.u.) FIG. 6. Posrrw~ ION MASSSPECTRAMEASUREUAT AN ALTITUDE OF 23 km.

Diameters of the used ion sampling holes are (a) 0.02 cm, (b) 0.04 cm.

Figure 7 also shows the recent aircraft-borne AClMS measurements by our group (Arnold and Hauck, 1985 ; Knop and Arnold, 1987) and the ground-level measurements by Snider and Dawson (1984). ACETONITRILE

MODEL

In order the check the consistency of balloon-borne PACIMS and aircraft-borne ACTMS measurements, we have carried out theoretical model calculations. Our simplified one-dimensiona model includes acetonitrile emission at the ground, vertical eddy diffusion and loss of acetonitrile by OH-attack (rate coefficient .5.86x IO-“exp(-750/T)cm’s‘; Harris et al., 1981). Other CH,CN-sinks appear to be negligible, at least for the height region of interest (t&45 km). The steady-state continuity equation Jr/ i &” - k OH[OH]n(s)f(CH,CN)

(6)

(herej is the vertical flux for CH,CN, koH is the rate coefficient for the reaction of OH with CH,CN, n(z) is the gas number density and f‘ (CH,CN) is the volume mixing ratio for CH,CN) was solved by specifying f’(CH,CN) = 21 pptv (average of aircraft-borne

FIG. 7. VOLUME MIXING KATIOS OF CH,CN FROM THE PRESENT

BALLOON-BORNE

AS VEKIVEV

POSITIVE ION COMPOSITION

MEASUREMENTS.

The individual data points of each profile are classified according to the degree of fragmen~tion by which the measured ion spectra are affected. Also shown are CH,CNmeasurements performed at high altitude (Arijs et a/.. 1983) and ground level (Snider and Dawson, 1984) as well as an average CH,CN mixing ratio obtained by recent aircraftborne active CIMS measurements (Arnold and Hauck, 1985; Knop and Arnold, 1987). In addition, model calculations of the vertical distribution of CH,CN are shown (for details see text). Note that the error bar given for the aircraft ACIMS measurements applies only to a single data point and already includes the systematic error introduced by ECA.

data; Knop and Arnold, 1987) and j = 0 at 60 km (upper boundary). In principle, our model is similar to the model of Brasseur et al. (1983) but focusses on the investigation of the influences of the critical parameters Kz (vertical eddy diffusion coeficient) and [OH] (OHconcentration). Various values for Kfas used previously for model calculations are shown m Fig. 8. They vary by a factor of about 3 in the height range of greatest interest (tropopause to 45 km). Here we considered three cases, low Kz(Hunten type low), high I(, (Hunten type high), and average K_(arithmetic average). Various profiles for hydroxyl radical concentrations are shown in Fig. 9. Here it has to be kept in mind that measured values were obtained mostly at high solar elevations. For our purposes, however, ACIMS

722

W.

and F. ARNOLD

SCHLAGER

TABLE 2. CHARACTERISTIC

Flight

Diameter of sampling orifice (cm)

21 Sept. 1981 17 Oct. 1981 10 June 1982 19 March 1983 5 Oct. 1983 21 Oct. 1985

0.02 0.02 0.03 0.03 0.03 0.04

DATA

OF THE BALL~N-GREG

Diameter of quddrupok rods (cm) 0.48 0.48 0.8 0.8 1.6 0.8

INS~RU~NTS

Electric field between sampling orifice and rod system (V cm- ‘) ____ ..-.. 15 30 20 20 20 30

5c

5 4c --z 3 .s 3c

a

2c

Vertical Eddy Diffusion Coefficient (cm%) FIG. 8. VERTICAL EDDY DIFFUSIONCOEFFICIENTSUSEDFOR

m

CH,CN MODELCALCULATIONS (SOLID LINES). Also shown are eddy diffusion coefficients used in various aeronomic one-dimensional models (from Brasseur ef ul., 1983; Brasseur and Solomon, 1984; WMO Report No. 11,

1982). 24-h averages are needed. Again, three cases are considered, low, high and average. For the low and high cases, our OH-values are roughly half of the corresponding limits (12-h averages) used by Turco et al. (198 1). Unfortunately, direct measurements of OHabundances do not exist for heights between about 13 and 30 km. Our calculated acetonitrile volume mixing ratios are also shown in Fig. 7. Five cases (profile 1 : high OH/low K,; profile 2 : high OH/high K,; profile 3 : low OH/low K,; profile 4: low OH/high KZ,; profiie 5: low OH/average KZ) are given. Corresponding ground-level fluxes j, for these cases are about 1.8x IO’cm-” s--’ (case 1, 2) and 1.8x 106cm-* s-’ (case 3, 4, 5). Of these profiles, 3 and 5 show rough agreement with those balloon-borne PACIMS data

OH Concentration (cm-3) FIG. 9. OH CONCENTRATION PROFILESUSEDFOR THE CH,CN MODELCALCULAXONS (SOLID LINES). Low and high diurnally averaged OH concentrations are assumed. Also shown are previous OH measurements and model calculations.

which are not severely affected by ECA (An,,, = 0). Interestingly, profiles 3 and 5, which provide the best fits, are those with low OH. In fact, there is increasing evidence that OH-concentrations are on the low side for heights between about 10 and 30 km (Turco et al., 1981). Those PACIMS data which are affected by ECA are systematically lower than a reference profile as, for example, profile 5. Around 20 km altitude, the discrepancy may become as large as about a factor of IO. Summarizjng the comparison of model calculations and data, it can be concluded that those balloonborne PACIMS data which are not severely affected by ECA appear to be consistent with recent aircraft-

Stratospheric

acetonitrile

2 30

z

F

\\\’ -I 20 t $35

\

-

1=30 \‘i\ “\:

2010

0

FIG.

RATIOS FOR 10. EXPECTED ABUNDANCE H+(CH$ZN),(H,O), AND H-‘(H,O), CLUSTER IONS (R) VS ALTITUDE CALCULATED FOR THE MODEL VERTICAL DISTRIBUTIONOFCH~CN(CASE 5: LOWOH,AVERAGEK,). Also shown are R-values which refer to the same model

but include ECA-induced back reactions of the type H+CH,CN(H20), -+ H +(H,O),+ CH,CN (assumed effective efficiencies E,~: 10,20, 30%).

CH,CN

profile

to-t1 t 0

’

I

*

’

’

10

Qif

I

’

’

a

20

I

’

’

’

30

I

’

40

1

(percent)

OBSERVABLE ABUNDANCE RATIOS FOR AND H+(H,O), IONS (R) FOR VARIOUS INITIAL R-VALUES AND EFFECTIVE EFFICIENCIES(E& FOR THE! ECA-INDUCEDCONVERSIONTO H+(H,O), IONS. FIG.

11.

H+(CH,CN),(H,O),

borne ACIMS measurements. Taking acetonitrile abundances as given by profile 5 (Fig. 71, we have calculated R using expression (2). Resulting R-values are shown in Fig. 10. R increases with decreasing height from about 0.1 around 45 km to about 20 around 1.5 km. Figure 11 shows measur-

723

detection

able R-values (R,) to be expected for various initial R and effective E. Here, the effective E gives the fraction of H+CI$CN(H,O), cluster ions which become converted to HC(H20), ions via process (Sb) and not the probability for H+(H,O), formation in a single collision. For R = 10 (24 km) and E,~ = lo%, for example, R, becomes about 4.5. R,-curves corresponding to effective E-values of lo,20 and 30% are also shown in Fig. 10. Evidently, an effective c of only 30% would be sufficient to cause RM to be 7 times smaller than R at 20 km. Since acetonitrile concentrations measured by PACIMS are proportional to R, [expression (211, they become increasingly lower than the ambient CH,CNabundances as c,~ increases and height decreases. Finally, it will be attempted to deduce E,= for different experimental conditions (D, UFA, URE) for an altitude of about 30 km by using the observed Rvalues for Anmsx = 0 as a reference. Ecffcan be obtained from the relation R-RM Eeff = R(1-t R,)

(7)

where R, is the measured ion abundance ratio for H+~CH~CN)~(H~O)~ and H+(H,O), cluster ions for different ECA conditions indicated by An,,,. For fixed U,, = -2 V and lJ,, = - 100 V the effective E is about 10% for D equals 0.03 cm (Aflmax = OS) and about 20% for D equals 0.04 cm (An,,, = 1). However, for U,, = -80 V and UpA = - 150 V the effective E is only about 10% although An,,, = 2. Here, E is smaller in the region between the RE and the QMS whereas it is much larger in the region between the SE and RE. Therefore, it seems that the lower E,~ is due to a lower average collision energy (lower E/p) in the region of low p (end of the gas jet). For several reasons, no attempt has been made to deduce initial R-values from observed R,-values for altitudes below 30 km using the E,~ values estimated From ECA measurements performed at an altitude of 30 km. Since Anmax is not a direct measure for E,~, any change in the ECA scenario may result in a different E,* for equal observed Anmax. Moreover, a change in the distribution of the ambient H+(CH,CN),,,(H,O), cluster ions at lower heights would lead to different ecR even for equal ion sampling conditions. Furthermore, the different rod systems used in the various measurements (see Table 2) will have different acceptances for fragment ions formed by high energetic ion-molecule collisions. Finally, it is obvious from Fig. 11 that in the case of large initial R-values small un~rtainties for E,~ and RiLlwould introduce enormous errors for the deduced R.

124

H. SCHLAGER and SUMMARY AND CONCLUSION

The present ECA-studies of H+(CH&N),(H,O), cluster ions carried out in the laboratory and the stratosphere provide convincing evidence for the occurrence of ECA-induced reconversion to H+(H,O), ions. It is found that up to about 30% of the ambient H+(CH,CN),(H,O), clusters may become reconverted under experimental conditions met in balloon-borne ion composition measurements using pumped mass spectrometers. As a consequence, stratospheric acetonitrile detection by PACIMS may suffer from severe disturbances induced by ECA leading to an underestimation of ambient acetonitrile abundances. It turns out that the disturbance increases with decreasing height (increasing ambient gas pressure) for otherwise fixed experimental conditions (diameter of inlet hole, strength of electric field) and may become as large as a factor of about 10 around 15 km altitude. By contrast, PACIMS measurements above about 35 km are not severely affected by ECA. Also, ACIMS measurements are not severely affected by ECA simply as the relative abundances of product ions like H+(CH,CN),(H,O), usually remain very low (about a few percent) in these experiments (cf. Arnold and Hauck, 1985 ; Knop and Arnold, 1987). One-dimensional model calculations show that those balloon-borne PACIMS data not being severely affected by ECA are consistent with data from aircraft-borne ACIMS measurements carried out at the tropopause if OH-abundances are low. Best fits are obtained for average and low eddy diffusion coefficients. An interesting aspect of acetonitrile is its use as a tracer for OH. It appears that the present paper provides additional support for low OHabundances. However, it should be kept in mind that the model calculations of the vertical distribution of CH,CN at present include considerable uncertainties. Future balloon-borne PACIMS measurements may be improved by using smaller inlet holes and/or weaker electric fields for ion extraction and injection, particularly in the region where the largest E/p values are found. This region extends from the RE to the QMS. Another step towards improved acetonitrile measurements would be balloon-borne ACIMS measurements. The development of a ballon-borne ACIMS-instrument is presently under way at our laboratory. Acknowledgements-We are grateful to E. E. Ferguson for helpful discussions. Support by the technical teams of the Max-Planck-Institut fiir Kernphysik, particularly W. Thron and D. Darflinger, and of the Centre National des Etudes

F. ARNOLD

Spatiales (Division Ballons), especially by P. Prigent and P. Vincent, is gratefully appreciated. This work was supported in part by the Bundesministerium fur Forschung und Technologie through the Gesellschaft fur Strahlenund Umweltforschung.

REFERENCES Anderson, J. G. (1971) Rocket measurement of OH in the mesosphere. J. geophys. Res. X,7820. Anderson, J. G. (1976) The absolute concentration of OH in the Earth’s stratosphere. Geophys. Res. Letf. 3, 165. Arijs, E. and Brasseur, G. (1986) J. geophys. Res. 91, 4003. Arijs, E., Nevejans, D. and Ingels, J. (1980) Unambiguous determination of major stratospheric positive ions. Nature 288,684. Arijs, E., Nevejans, D. and Ingels, J. (1982) Stratospheric positive ion composition measurements, ion abundances and related trace gas detection. J. atmos. terr. Phys. 44, 43. Arijs, E., Nevejans, D. and Ingels, J. (1983) Positive ion composition measurements and acetonitrile in the upper stratosphere. Nature 303, 314. Arnold, F., Biihringer, H. and Henschen, G. (1978) Composition measurements of stratospheric positive ions. Geophys. Res. Lett. 5, 653. Arnold, F. and Hauck, G. (1985) Lower stratosphere trace gas detection using aircraft-borne active chemical ionization mass spectrometry. Nature 315, 307. Arnold, F. and Henschen, G. (1982) Positive ion composition measurements in the upper stratosphere. Evidence for an unknown aerosol component. Planet. Space Sci. 30, 101. Arnold, F., Henschen, G. and Ferguson, E. E. (1981) Mass spectrometric measurements of fractional ion abundances in the stratosphere-positive ions. Planet. Space Sci. 29, 185. Brasseur, G., Arijs, E., De Rudder, A., Nevejans, D. and Ingels, J. (1983) Acetonitrile in the atmosphere. Geophys. R& Lett. ‘IO, 725. Brasseur, G. and Solomon, S. (1984) Aeronomy of the Middle Atmosphere. D. Reidel, Dordrecht, Holland. Campbell, W. (1980) Tropospheric hydroxyl radical concentrations. Proc. Conf. Methyl Chloroform and other Halocarbon Pollutants, Washington D.C. Off. Res. Dev. EPA 600/9-80-003. U.S. E.P.A. Davis, D. D., Heaps, W. and McGee, T. (1976) Direct measurements of natural tropospheric levels of OH via an air-borne tunable dye laser. Geophys. Res. Lett. 3, 331. Glebe, W. and Arnold, F. (1983) Negative acid cluster ion hydration studies of atmospheric interest. Proc. 3rd Intern. Swarm Seminar, Innsbruck, Austria, 132-134. Hamm, S., Hahn, J., Helas, G. and Warneck, P. (1984) Acetonitrile in the troposphere: residence time due to rainout and uptake by the ocean. Geophys. Res. Letf. 11, 1207. Harris, G. W., Kleindienst, T. E. and Pitts, J. N., Jr. (1981) Rate constants of reactions of OH radicals with CH,CN, C$H,CN and CH,--CH-CN in the temperature range 298424 K. Chem. Phys. Lett. 80,419. Heaps, W. S. and McGee, J. J. (1983) Balloon-borne Lidar measurements of stratospheric hydroxyl radicals. J. geophys. Res. 86,528 1. _ Henschen, G. and Arnold, F. (1981) Extended positive ion composition measurements in the stratosphere, impli-

Stratospheric

acetonitrile

cations for neutrai trace gases. Geophys. Res. IAt. 8,999. Knop, G. and Arnold. F. (1987) Atmospheric a~etonit~le measurements in the trepopause region using aircraftborne active chemical ionization mass spectrometry. Pliinel. Space Sci. 35, 259. Kurylo, M. and Knable, J. (1984) J. phyx Chem. 88, 3305. Lau, Y. K., Ikuta, S. and Kebarle, P. (1982) Thermodynamics and kinetics of the gas-phase reactions H,O+(HZO), , + H,O = H,O+(H,O),. J. Am. them. Sot. 104, 1462. Logan, J. A., Prather, M. J., Wofsy, S. C. and Meelroy, M. D. (1981) Tropospheric chemistry : a global perspective. J. geophys. Rex. 86, 7210. Meot-Ner (Mautner), M. (I984) The ionic hydrogen bond and ion salvation. 2. Solvation of onium ions by one to seven H,O molecules. Relation between monomolecular, specific and bulk hyd~tion. 1. Am. c/renr. Sac. 106, 1265. Perner. D., Ehhalt, D. H., Patz, H. W., Platt. V.. Roth, E. P. and Volz, A. (1976) OH-Radicals in the lower troposphere. Geophys. Rex. Letf. 3, 466. Philen, D., Heaps, W. and Davis, D. D. (1978) Boundary layer and free tropospheric OH measurements at tropical and subtropical latitudes in Northern and Southern Hemisphere EOS Trans. AGU 59, 1079. Schlager, H. and Arnold, F. (1985) Balloon-borne fragment ion mass spectrometry studies of stratospheric positive ions : unambiguous detection of H ’ (CH,CN),(H?O),,,clusters. Plunei. Space Sci. 33, 1363. Schlagcr, H. and Arnold, F. (1986) Implications for atmospheric negative ion composition measurements of laboratory ECA studies of sulfuric acid cluster ions. P&et. .Qu(P Ski. 34. 245. Schlager, H., Fabian. R. and Arnold. F. (1983) A new cluster ion source/ion drift cell apparatus for atmospheric ion studies --first mobility and reaction rate coefficient measurements. Proc. 3rd intern. Swarm Seminar, Innsbruck, Austria, 2577262. Singh, H. 6. (1977) Atmospheric halocarbons : evidence in

detection

725

favor of reduced average hydroxyl radical con~ntrations in the troposphere. Ge0phy.s. Res. Left. 4, 101. Smith, D., Adams, N. G. and Alge, E. (I981) Ion-ion mutual neutralization and ion-neutral switching reactions of some stratospheric ions. Planet. Space Sci. 29,449. Snider, J. R. and Dawson, G. A. (1984) Surface acetonitrile near Tucson, Arizona. Geophys. Res. Lett. 11, 241. Turco, R. P., Whitten, R. C., Toon, 0. B., Inn, E. C. Y. and Hamill, P. (1981) Stratospheric hydroxyl radical concentrations : new limitations suggested by observation of gaseous and particulate sulfur. J. geoph_ys. Res. 86, 1129. Volz, A., Ehhalt, D. H. and Denvent, R. G. (1981) Seasonal and latitudinal variation of “‘CO and the tropospheric concentration of OH radicals. J. ~e~phy.~. Rex 86, 5 163. Wang, C. C., Davis, L. I., Wu, C. H., Japan, S., Niki, H. and Weinstock, B. (1975) Hydroxyl radical concent~dtion measured in ambient air. Science 189, 797. Watanabe, T., Yoshida, M., Fujiwara, S., Abe, K., Onoe, A., Hirota, M. and igarashi, S. (1982) Spin trapping of hydroxyl radical in the troposphere for determination by electron spin resonance and GC/MS. Ana&. Chem. 54, 2470. WMO (1982) World Meteorological Organization, The stratosphere 1981 : theory and measurements. Rep. I I, WMO Global Ozone Rcs. Monit. Proj., Geneva 1982.

Note added in proof: recent model calculations of the atmospheric acetonitrile vertical distribution carried out by Arijs and Brasseur (1986) reveal that in addition to KZand the OHabundance the rate coefficient for the reaction of a~etonitriie with OH is also a critical parameter in the model. The use of the rate coefficient given by Kurylo and Knable (1984) instead of the rate coellicient reported by Harris et ui. (I 983) in the present model calculations would lead to CH,CN volume mixing ratios which are about a factor of three higher at an altitude of 40 km.