Mass spectrometric measurements of fractional ion abundances in the stratosphere—Negative ions

Planet. SpaceSci.Vol. 29,~~.195-203 PergamonPress Ltd., 1981.prinred inNorthern ImImd

MASS SPECTROMETRIC MEASUREMENTS OF FRACTIONAL ION ABUNDANCES IN THE STRATOSPHERE-NEGATIVE IONS F. ARNOLD,

Max-Plaock-Institut

R. FAJDAN,

E. K FERGUSOW

and W. JOOS

fiir Kernphysik, 6900 Heidelberg, F.R. Germany

(Received 22 August 1980) Abs~~-Fractions abundances of stratospheric negative ions are for the first time explicitly reported. The measurements made by balloon-borne ion mass spectrometers also rely on recent studies of electric field induced collisional dissociation of negative cfuster ions conducted at our laboratory. These indicate that the negative ion composition measurements around 36 km conducted by OUTgroup do not suffer from any significant dissociation. The new composition data support ion identifications NO,-(HNO,), and HSO,-(H,SO,),(HNO,), and the underlying ion reactions proposed previously. Moreover, it is found that HSO,-(H,SO,),-ions appear to be particularly stable and that H,SO,-association is very fast. Implications of the ion composition data for ion processes are discussed.

INTRODUCIWN

Detailed information about the nature of stratospheric negative ions was first obtained by in situ mass spectrometer measurements using balloonborne ion mass spectrometers (Arnold and Henschen, 1978). Because of uncertainties relating to the possibility of ion dissociation occurring during ion sampling no fractional ion abundances, but only ion mass numbers, were reported. Based upon their mass numbers the observed ions were tentatively identified as acid cluster and ions of the types N03-@-INO& HSO,-(H,SO,)= (HNO& with b = c + d 5 2) (Amold and Henschen, 1978). Moreover, these authors proposed that the sulfate ion clusters are formed by ion-molecule reactions involving stratospheric sulfuric acid vapor. Recentfy, this hypothesis has attained strong support by laboratory measurements of ion-molecule reactions involving NO,-(IDJO,),-ions and sulfuric acid conducted at the NOAA-Aeronomy Laboratory at Boulder (Viggiano et al., 1980). In the meantime, detailed laboratory simulation experiments concerned with electric field-induced collisional dissociation of N03‘-(HNO&-ions occuring in mass spectrometric measurements were conducted at our laboratory. These measurements provide a safe basis for determining fractional abundances for stratospheric negative ions. * Alexander van Humboldt Foundation Awardee 1979-1980. Permanent address: Aeronomy Laboratory NOAA, Boulder, Colorado, U.S.A.

The present paper briefly describes the laboratory simulation experiments and summarizes their main results. Moreover, fractional abundance data for stratospheric negative ions obtained from the experiment B3 (Arnold and Henschen, 1978) are reported. In the light of these data the proposed ion identifications and the proposed ion-reaction scheme will be reconsidered. Attention will also be drawn to the possibility of inferring new information from the ion composition data on both ion processes and neutral stratospheric trace gases. EXPEltEWCNTALa lbfElX%OD AND

MEAStJRFMJl~

The experimental method and the circumstances of the experiment B3 have already been described (Arnold et at., 1978; Arnold and Henschen, 1978) and in the accomp~y~g paper (Arnold et al., 198Oa) and therefore will not be discussed here. Only points which are specific to the negative ion measurements will be briefly emphasized. The dark count rate of the channel electron multiplier used for ion detection was considerably larger in the negative ion mode (1 s) than in the positive ion mode (0.01 s) due to the larger high voltages applied. Therefore, in order to achieve a dynamic range for the negative ion measurements comparable to that of the positive ion measurements a much lower mass resolution setting [half width of a mass peak: 15 atomic mass units (amu)] was used which allowed for a correspondingly higher ion transmission factor for the mass filter and thus for a higher sensitivity. The value of the latter was about 0.05 counts s-l per ion cmm3allowing

196

F. ARNOLD, R. FABIAN, E. E. FERGUSON and W. Joos

for a dynamic range of about 100. Thus, the detection limit for negative ions was about 20 ions cmU3 corresponding to a fractional abundance of about 1% around 35 km altitude. The relatively large half width of the negative ion mass peaks implies that mass peaks adjacent to much larger mass peaks can easily become undetectable. Since the mass peaks have a somewhat asymmetric triangular form with a steeper flank at the high mass side of the peak, masking of smaller peaks should be more severe if they are located close to the low mass side of a large peak. Electric field-induced collisional dissociation in the gas beam which has been found to affect positive ion composition measurements (Arnold et al., 1980a) may also affect negative ion composition measurements. Therefore detailed laboratory simulation measurements using the same experimental method as described in the accompanying paper (Arnold et al., 1980a) have been conducted also for negative cluster ions. In these experiments ions of the type NO,-(HNO,), which were observed in the stratosphere (Arnold and Henschen, 1978) were produced by a discharge high pressure ion source at a pressure of a few torr using laboratory air. As a result of N,-dissociation in the discharge large amounts of nitric oxide are formed. The fractional abundance of nitric oxide was estimated from direct mass spectrometric measurements to be of the order of a few per cent. The nitric oxide was partly converted to nitrous acid (HNO,) possibly via NO~OH~~~HNO~+~.

0)

(M is a collision partner; mostly Nz and O,.) Positive ion reactions also produce I-J.NOz (Ferguson et al., 1979). The presence of HNOz was inferred from the occurrence of NO*-(~O~)~-ions. Some other part of the nitric oxide was converted to NO,. and ultimately to nitric acid HNO, possibly via NO,+OH+M-*HNO,+M.

(2)

The nitric acid gave rise to the formation of large amounts of N03-(HN03)b-ions. At a pressure of a few torr these ions were in fact the most abundant ion species sampled by the orifice probe. Electric field-induced collisional dissociation of NON-(~O~)~-ions was studied by changing the strength of the homogeneous electric field, E, existing between the planar sampling electrode and the planar extraction grid in a way analogous to that described in Arnold et al. (1980a). Figure 1 gives fractional abundances of NO,-(HNO&-ions observed at 3 torr, 323 K using

r

I 160

I M)

I

83

E, RG.

1.

&ACMONAL

STRENGTH STUDY

AS

I

I

30

v cm-’

ION ARUNDANCETS

MF!.ASURJZD

OF EL.ECl.RIC

I

40

IN A

VS ELJ3CTRIC

LABORATORY

FtFiLD-INDUCED

FIELD

%MtJLATION

COLLISIONAL

CLUSTER

ION DISSGUAWON.

a sampling orifice of 0.05 cm in diameter vs E. As can be noted significant dissociation occurs only at E larger than about 30Vcm-‘. Since this value is larger than the E-value used in the B3-experiment (20V cm-‘; Arnold et aZ., 1980a) and since the most massive ion species of the type NO,-(HNO,), to be expected under the conditions of the experiment B3 is as will be shown later on N03-(HNO& no significant dissociation should have occurred. Since the other ions observed in the experiment B3, namely HSO,-(H,SO,),(HNO,), should have bond energies comparable to those of NO,-#iNO,)*-ions, it may be concluded that by contrast to the positive ion measurements of B3 the negative ion measurements most likely were not severely affected by dissociation. This has been verified by laboratory observations in a similar way as described before. Here sulfuric acid vapor was formed in the high pressure ion source by adding SOz to an H&D-air mixture. The main reason for this difference is that the negative acid clusters are more stable than the positive hydrate ions. This, in turn, is due to three factors: Firstly, strong acids bond more strongly to negative ions than H,O-ligands of the same association order bond to H30i or Hx’. Secondly, H,Oligands bond less strongly to negative ions than to H,O’ or Hx*. Consequently, negative cluster ions under the conditions considered do not contain H,O-ligands as these are either replaced by acidmolecules or desorbed by thermal dissociation. Thirdly, the average number of ligands contained in the observed negative cluster ions (2) is smaller than that contained in the observed positive cluster ions (3-4). Hence, the negative ions are more stable. The main reason for the low ligand number of

Stratospheric fractional negative ion abundances

the negative ions is the low partial pressure of HNO, vapour in the stratosphere and as mentioned above thermal dissociation of H,O-ligands. RESUL’IS

AND DiSCUSSiON

At the float altitude of the balloon a total of 93 negative ion mass scans was obtained. Since the measured negative ion abundances did not change significantly during this period the individual mass spectra could be combined to obtain a timeaveraged spectrum of higher accuracy. The resulting spectrum contains 7 significant mass peaks whose masses and fractional abundances are given in Table 1. It is well possible that other major ions are present whose mass peaks are masked by larger adjacent peaks particularly in the mass range above about 145 amu. The existence of the mass peaks 253 f 3 and 289&3 identified on the basis of preliminary data (Arnold and Henschen, 1978) cannot be confirmed by the present more complete analysis which is based on a much broader data bank. The uncertainty of the fractional abundances given in Table 1 is typically less than l30%. No correction for collisional dissociation was applied to the measured fractional ion abundances. According to the laboratory simulation studies the ions NO,-(HNO,) and N03-(HN03)2 should not have been affected by dissociation provided that NO,JHNO,), was not present in large abundance. As will be discussed later the latter most likely can be excluded. Thus, it appears that the observed NO,-(HNO,),-ions were not affected by dissociation, For the observed HSO,--cluster ions this should also apply as their bond energies should be similar to those of the NON-(~O~)~-ions when ions of the same association order are compared. The reason of the inefficiency of dissociation are the relatively large bond energies of the observed negative ions NO,-(I-INO,), the most abundant negative ion species has a bond energy of about 0.8eV; (Davidson et al., 1977). When compared TABLE 1. MASSNUMBERS,TENTATIVE FFLACTIONAL

ABUNDANCXS

OF NEGATIVE

IDENTIFICATION IONS OBSERVED

ANT3 AT

36.5 km ALmE Mass (amu)

Ion

Abundance

12szt2 161*2 188*2 197*3 224k3 26Ozt3 29413

NOJB’JO, HSO,-HNO, NO,-W’JO,), HSO,-H,SO, HSO,-WNO,k HSO,-H,SOJ-lNO, HSO.,-(H,SO,),

2.6 5.3 65.6 6.6 14.2 3.1 2.6

197

with those of the positive ions measured at the same time and location [the most abundant positive ion species observed which were H,O*(H,O), and H~O+(H~O)~ have bond energies of about 0.74 eV and 0.65 eV] the bond energies of the negative cluster ions are larger. This is as mentioned before mainly due to the fact that the positive ions are hydrates and that due to the large concentration of water vapour higher association order cluster ions are built up. On the contrary, the negative cluster ions do not contain H,O-ligands as these bond only weakly to negative ions and as these are rapidly replaced by acids, particularly the abundant nitric acid. In the following we will investigate whether or not the present ion abundance data support the ion ident~cations and the ion-reaction scheme proposed by Arnold and Henschen (1978). First of all, the NO,-(HNO,),-ion family will be discussed. Since nitric acid vapour reaches relatively large concentrations around 36 km (about 5 ’ 10’ cmT3; Lazarus and Gandrud, 1974) and since the rate coefficient for NO,-HNO,

+ FIN03 + M ---, N03-(HN0J2

+M

(3) should be larger than 10mz6cm3 s-’ (Fehsenfeld et al., 1975) corresponding to a phenomenological 2-body rate constant in excess of 10e9 cm3 s-i around 36 km (total gas density n(M) = 10” cm-“) process (3) is much faster (20 s) and also the reverse process (-3) is much faster (330 s) than ionion recombination (5000 s). Therefore, an equilibrium with respect to HNO,--clustering should be established, Considering the above n(HNO,), a measured temperature during the stratospheric ion-composition measurement of 227 K and the thermochemical data of Davidson et al. (1977), a theoretical ion abundance ratio n[NO,Jl-lBO,),]/n[NO,-(HNO,)l] of 14 is obtained. When compared to the measured ratio of 25, it is smaller by only a factor of 1.8 which strongly supports the identifications of the observed masses 125*2 and 188*2 as N03-HN03 and For the ion abundance ratio NO,-(HNO,),. n[N0,JHN0,),]/n[N0,-(HN0,)2] a theoretical value of 0.004 is obtained which is consistent with an upper limit of 0.017 set by the in situ measurement. For the process N03-+HN03+M~N0,-~N0,+~

(4)

an equilibrium constant is not available. Since NO,-HNO, has a higher bond energy than

F. ARNOLD, R. FABIAN, E. E. FERGUSON and W. Jaos

198

NO~-~HNO~)~ thermal dissociation of NO,-HNO, should be much less efficient. Probably it is less efficient than ion-ion recombination and therefore the ion abundance ratio becomes n(N03-HN03)

+ n(NO,-(HNO,), WO,-1

= kn(HN0,) an+

follows because a steady state treatment yields an equation equivalent to (5). It is conceivable that NO,--hydrates are removed also by reaction with molecules other than HN03. A likely candidate is HCl whose abundance in the middle stratosphere is comparable to that of HNO, (cf. Crutzen et al., 1978). Thus, ions of the general type NO,-(HNO,),(HCl), may be formed. Because the gas phase acidity of HNO?, exceeds that of HCl the heteromolecular clusters may undergo HNO,-switching reactions which result in NOB-(HNO~)~-ions (see Fig. 2). A steady state treatment yields an estimate of the ion abundance ratio ~[NO~-(HNO~)~(H~)~]/n~NO~-(H~O)~] of about 2 if n(HC1) is taken to be equal to n(HN0,). Thus, the total abundance of ions of the type NO,-(HNO,),(HCl), relative to the total negative ion abundance would be 0.008 which is alredy close to the detection limit of 0.01. The most prominent members of the NO,-(HNO,),(HCl),-family which contain HCl-ligands should have been NO,-HCl (mass 98) and N03-HNO,HCl (mass 161). While the first one was definitely not observed an ion mass 1611t2 was observed. As will be discussed later on, however, this ion most likely is not NON-HNOs~~. The absence of hydrated NO,-(HNO,),-ions in the measurement can be readily explained in terms of low bond energies. As already discussed above in the context of NO,--hydrates the abundance of N03-(H20)2 is very low. Since Hz0 probably does not bond to NO,-(HNO,) significantly

9 (.v

where k is a phenomenological 2-body rate constant of about 10m9cm3 s-*. Taking n(HNO,), and n, as before one obtains a theoretical ratio of about 250. By comparison the absence of NO,- in the measurement implies a lower obse~ational limit to this ratio of the order of 100 which is consistent with the theoretical value. Before associating HNO, the ion N03- may undergo hydration and thus NO,-(HNO,) may be formed rather by HNO,-switching with N03-HzO. Considering laboratory studies of NO,--hydration equilibria (Keesee and Castleman, 1980), a temperature of 227 K and a water vapour volume mixing ratio of 3 +10m6 (this was inferred from the positive ion hydration equilibria measured at the same time and location; Arnold et al., 1980a) the following fractional abundances within the NO,-(H,O),family are obtained: NOst- (0.14), NO,-HZ0 (0.72), NO,JH,O), (0.14). Since none of these ions was observed, their abundances relative to the total negative ion abundance (2000 cme3) must have been lower than about 0.01 which is consistent with a theoretical upper limit of 0.004. This Cosmic

rays

I

1 NO; (HN03)e(HCl)f

’ FIG. 2. GE~R~I~D~~ONSC~~FORS~TOSP~RIC~~A~IONS. M

is N2 or 0,.

Stratospheric

fractional negative ion abundances

stronger

than it bonds to NO,-H,O, the abundance of N03-HNO&O should be very low. For NO,-(HN03)&0 this argument applies even more. Even if NO,-(HNO,),-hydrates would resist thermal dissociation, they would rapidly be removed by HNO,- and even HCl-switching. In the following the fractional abundances of the observed ions having masses 161 f 2, 197 f 3,224 f 3, 260*3, and 294&t, which were identified as members of the HSO~-(~~SO~)=(~O~)~-family (Arnold and Henschen, 1978) will be discussed. It was proposed by these authors that the ions are formed by the sulfuric acid vapour reactions:

NO,-(HNO,),

+ HZSO, (7)

+ HSO,-(HNO,)b -t HN03 HSO,-(HNO,),

+ H,SO,

+HSO,-H,SO,(HNO,),_, HS04-I-IZS04(HN0,)j

+ HN03

(8)

+ H,SO,

+ HSO~-(H~SO~)~(HNO~)j_~ + HNO,.

(9)

For simplicity in the above expressions the number of ligands is kept constant. In reality once a new ion is formed a new HNO,-cluster equilibrium will rapidly be established. Thus, the observed HS04-(H2S04),(HN03fd-ions may be grouped into the subfamilies: A: HSO,-(HNO& ; B: HSO,-H,SO&JNO~)i ; C: HSO,-(H,SO#INO,),. A steady state treatment for the formation and 10s~ of these ions yields the following ion abundance ratios: n(B)

+ n(C) + n(o)

4?W-(HNO,)J n(B)

= WI-WU

(10)

an+ + n(C) = WH,SO,)

n(A) 40 -= n(B)

(11)

an+ kM-W&) Cm+

*

(121

Here it is assumed that H,SO,-reactions which may be switching-or association reactions all have a common rate constant k. The measured total fractional abundances of the subfamilies A, B and C are 0.195, 0.097 and 0.026. Thus, the ratios (lo), (11) and (12) become 0.47, 0.63 and 0.27. In fact, these are all of the same order of magnitude which is consistent with the proposed scheme. The somewhat smaller experimental value for ratio (12) may that indicate existed also an ion there HSO,-(H,SO,), (mass 391) which due to the limited mass range of the instrument (328 atomic mass units} remained undetectable. It is also conceivable that thermal dissociation contributes to increase the loss of HSO,-(H$O&.

199

Interesting implications of the above data are: (a) association of H,SO, to HS04-H2S04 appears to have a large phenomenological 2-body rate constant of the order of 10m9cm3 s-* around 36 km; (b) thermal dissociation of HSO,-(H,SO& appears to have a very small rate constant at 227 K. An upper limit of 2 . lO-‘l cm3 s-l can be inferred. An implication of (a) is that the 3-body rate constant for H,S04-association to HS04-H2S0., is at least lO-26 cm6 s-l at 227 K. Thus, it is similarly large as the rate constant for HNO,-association to N03-. Implication (b) follows from the fact that thermal dissociation of HS0,-(H2S0J2 appears to be slower than ion-ion recombination. An implication of (b) is that thermal dissociation of HS04-(H2S0,), appears to be significantly slower than thermal dissociation of NO~-(~O~)~ (rate constant at 227 K larger than 3 * 10-20 cm3 s-l; estimated from equilibrium constants), This most likely reflects that the bond energy of HSO,-(I&SO& exceeds that of NO,-(HNO&. Although there exist no quantitative laboratory data on sulfuric acid clustering, implication (b) would at least qualitatively be expected. The reason is that the bond energy of HSO,-(HzSO,), should significantly exceed 20.15 kcal mol-’ which is the heat of evaporation (L) for liquid sulfuric acid. Laboratory measurements of cluster ion equilibria indicate that cluster ion bond energies are larger than the L-vaIue of the bulk ligand material (cf. Arnold et al., 1980a). Generally, bond energies are much larger than L for low association orders and approach L asymptotically with increasing association order: for acidligand clusters such as for example NON-(HNO~)~ and Cl-(HCOOH), the bond energies are much larger than L at least up to association orders 3 and 4 correspondingly. For N03-(HNO& the bond energy is 18.3 kcal mol-’ (Davidson et ul., 1977) whereas the L-value for bulk HNO, is only about 8.5 kcal mol-‘. Therefore it appears quite likely that HSO,-(H,SO& has a bond energy much larger than 20.15 kcalmol-‘. Taking the above upper limit to the thermal dissociation rate constant of 2 . 1O”m21 cm3 s-’ and a lower limit to the rate constant for H,SO,-association to HSO,-H,SO, of 10-26cm6s-1 (see above) a lower limit to the equilibrium constant of about 10*4atm-’ is obtained. Assuming an entropy difference of 30 cal mol-’ a lower limit to the bond energy for HSO,-(H,SO,), of about 21.4 kcal mol-l may be estimated. Consequently, it is conceivable that the bond energy of HSO,-(H,SO& in fact exceeds that of NO,-(HNO,),. It may turn out that even the

F. ARNOLD, R. FABIAN,E. E. FERGUSON and W. Joos

200

bond energy of HS04-(H$O& is sufficiently large not to allow thermal dissociation to compete with ion-ion recombination. If this would be the case and if the equivalent 2-body rate constant for I&SO,-association to HSO,-(H,SO,), would also be 10v9 cm3 s-” the ion abundance ratio n[HS04-(H2S04)3]/n[HS0,-(H,S0,),] would be the same as ratios (7) and (8) which are around 0.5. Considering an additional loss of HSO,-(H,SO,) by I&SO,-association and no thermal dissociation of HSO,-(H,SO.,), the ion abundance ratio n[HSO,-(H,SO,)&n(B) becomes

n[HSO,-UWUI= n(B) (13) Taking n+lkn(H$O.,) equal to the experimental ratios (10) and (11) (around 2) ratio (13) becomes 0.33 which is close to the experimental value 0.27. Thus it appears that there may have existed HSO.+-(H,SO,),-ions in considerable abundance. If this was the case, the thermal dissociation rate constant for HSO,-fH$O& may in fact also be smaller than about 2 * 1O-21 cm’ s-l as suspected already above. So far, the discussion of the present data has provided strong support in favour of an identification of the observed ions 161*2, 197rt3, 224*3 and 294 f 3 as members of an ion family having the general form R-(I-IR),(HNO,),. Whether or not HR can in fact be identified, as proposed, as sulfuric acid, will be discussed in the following. Sulfuric acid vapour so far has not independently been observed in the stratosphere. Its presence is, however, inferred from analyses of stratospheric aerosol particles which revealed that these are mainly composed of sulfate (Cadle, 1972). It is generally assumed that the sulfate particles are formed by nucleation of sulfuric acid vapour onto condensation nuclei (cf. Hamill et al., 1977). It is thought that sulfur compounds such as SOz, CSO and possibly also CSz, which are transported from the troposphere into the stratosphere, ultimately lead to sulfuric acid in a way, not yet fully understood (cf. Turco et al., 1979). The main loss processes for H,S04-vapour are thought to be heterogeneous removal and photolysis. Thus, the HzS04-vapour concentration depends on the rate of injection of sulfur compounds, the photodissociation coefficient, vertical turbulent transport, the total aerosol surface area density and on the supersaturation. All these factors are not very well

known and hence the sulfuric acid vapour concentration is rather uncertain. An upper limit to n(H2S04) may be obtained from one-dimensional phot~hemic~ diffusive models such as that of Harker (1975) being about 2 0 10’ cm-’ around 36km. If, however, H,SO,-vapour is supersaturated around 36 km at 227 K and the total aerosol surface area density is sufliciently large, H,SO,vapour may be removed primarily by heterogeneous processes and consequen~y n(HzS04) would be lower than the photochemical value. Unfo~nately, however, neither the total aerosol surface area density nor the equilibrium of H,SO,-vapour are well known. Therefore a lower limit for n(H,SO,) cannot be safely estimated at present. Considering n(H,SO,) = 2 - 10’ crne3 as predicted by phot~hemic~ diffusive models and a rate constant, k, of 4 - lo-‘* cm3 s-* (Viggiano et al., 1980) for NO,-(HNO,),

+ H$O,

+ HSO,-(HNO,),

+ HNOS, (14)

the ion abundance ratio (7) becomes 40 which is much larger than the observed ratio being 0.47. Therefore photochemical diffusive models appear to predict much larger sulfuric acid vapour concentrations around 36 km than are needed to account for the observed sulfate ion clusters. As discussed by Arnold and Fabian (1980) the ion composition data can be used to determine the concentration of sulfuric acid vapour which turns out to be about 10’ cms3 at 36.5 km during our measurement. This value is lower than that predicted by phot~hemic~ diffusive models by at least a factor of 100. Therefore it is likely that there may have existed an additional efficient loss process for sulfuric acid vapour. Among the various possibilities discussed by Arnold and Fabian (1980b) heterogeneous removal seems most likely. It would imply that sulfuric acid vapour was in fact su~rsat~ated at 36.5 km which, in turn, implies that the equilibrium vapour pressure of sulfuric acid over the aerosol must have been considerably lower than currently thought. Another implication is that the total surface area density of the aerosol must have been appreciable (cf. Arnold and Fabian, 198Ob). Alternatively, as proposed by these authors SUIfuric acid vapour may have been removed by reacting with N03-(HNO&-ions or by attachment to HSO,--cluster ions, the latter of which may have been incorporated into the so-called “multi-ion complexes” proposed recently (Arnold, 198Oa).

Stratospheric fractional negative ion abundances It is conceivable that the sulfuric acid vapour removal is due to the same mechanism which is responsible for the removal of gaseous metal compounds (Arnold et al., 1980a). Thus it appears that there exist already two independent pieces of evidence for an efficient gas to particle conversion mechanism being operative in the upper stratosphere. As a consequence there must exist a considerable amount of aerosol in the upper stratosphere and possibly also in the lower mesosphere. Although this aerosol appears to be chemically very important its abundance may not be large enough to allow for a detection by optimal methods. Therefore species such as sulfuric acid or gaseous metal compounds whose concentrations now can be inferred from ion composition measurements may in fact be used as tracers for this aerosol. Now the composition of the subfamilies A, B and C will be discussed. As can be noted from Table 1 for both the N03JHN0&and the Afamily the cluster containing 2 NHO,-ligands is most abundant. However, the ratio of clusters containing 2 and 1 HNO,-ligand is much larger for the N03-(HNO&-family indicating that the corresponding equilibrium constant is larger. Probably this indicates that N03-(HN0& is more stable than HS04-(HNO&. Qualitatively, this may be explained in terms of a larger basicity of the core NO,- when compared to the core HSO,-. For the subfamilies B and C whose core ions HSO,-H,SO, and HSO,-(H,SO,), are much larger the most abundant members contain no HNO,ligand. A cluster with one HNO,-ligand was observed for B but not for C. Quantitatively, this behaviour can be explained in terms of a decrease of the bond energy for an HNO,-ligand with increasing size of the ion core. For all ion families the most abundant members contain 2 or 1 ligands which are either HNO, or H,SO,. Thus, also the observed distributions of HN03cluster ions appear to be consistent with the proposed ion identifications. From the foregoing discussion the following picture of the negative ion chemistry in the middle stratosphere evolves (Fig. 2). Ions of the type NO,-(H,O), which are rapidly formed from primary negative ions being mainly 02- through various reaction channels involving trace gases such as OS, CO*, NO,, H20, and others (cf. Ferguson et al., 1979) are rapidly converted to NO,-(HNO,),-ions by reactions involving HN03. Alternatively, HCl may attach to NO,- or replace H,O-ligands by switching reactions, thus giving rise to the forma-

201

tion of NO,-(HNO,),(HCl),-ions. These, in turn, are rapidly converted to NO,JHNO,),-ions by HNO,-switching reactions. The NOX-(HN03)r,-ions are converted to HSO,JHNO&-ions by reaction with sulfuric acid vapour. Subsequent replacement of HNO,-ligands by HzS04 via H2S04switching gives rise to the formation of HSO,-(H,SO,), (HNO,), -ions. It is also conceivable that HSO,-(H2S0&-ions ultimately formed via HzS04 switching may further grow by HzS04attachment. The HSO,-(H,SOJ, (HNO,), -ions and a large fraction of the N03-(HN03)b-ions, namely those which do not react with H2S04 undergo recombination with positive ions. Depending on the nature of the positive ions involved this process may give rise to spontaneous neutralisation or to the formation of ion pairs which, in turn, may be converted to larger multi-ion complexes (Arnold, 1980a). The total fractional abundances of the cluster ion families included in Fig. 2 sensitively depend on the concentrations of the trace gases HNO-,, HCl and H,SO, which undergo marked spatial and temporal variations. Consequently, corresponding changes of the ion composition should occur. In the middle stratosphere the volume mixing ratio of HN03 decreases above about 30 km whereas those of H,O and HCl remain about constant with height. Therefore the fraction of HCl among the ligands attached to N03- should increase with increasing height above 30 km. The concentration of H2S04vapour increases steeply with height in the region where the removal of H,SO,-vapour is controlled by heterogeneous processes and it remains almost constant above. In the lower region the H,SO,vapour concentration is mainly controlled by the H,SO,-equilibrium vapour pressure over the aerosol which, in turn, increases steeply with increasing temperature. Generally, the temperature increases with increasing height throughout the middle and upper stratosphere. The upper boundary of the region where HzSO,-vapour removal is controlled by heterogeneous processes depends mainly on two factors, namely the H2S04equilibrium vapour pressure and the total aerosol surface area density. Both are not well known and therefore the location of the boundary is uncertain. According to current models the boundary may on the average be located around 3540 km. Thus, the fractional abundance of HSO,--cluster ions should increase throughout the middle stratosphere up to the boundary and remain about constant above. Strong temporal variations of the sulfuric acid vapour concentration should occur as a result of

202

F. ARNOLD, R. FABIAN, E. E. FERGUSON and W.

temperature variations which, in turn, may be systematic or irregular. For high temperatures higher sulfuric acid vapour concentrations and consequently higher total fractional abundances of HSO,--cluster ions should be expected. As far as the association orders of the negative cluster ions are concerned these depend mainly on the temperature and on the concentrations of nitric acidand sulfuric For acid-vapour. NO,-(HNO&-ions higher association orders should be expected at lower altitudes where temperatures are lower and therefore thermal dissociation is less efficient and where the nitric concentration acid vapour is larger. For HSO,JH,SO,),-ions depending on their bond energies their average association order may be controlled by recombination with positive ions rather than by thermal dissociation. If this would in fact be the case, their average association order would even increase with increasing temperature because the sulfuric acid vapor concentration increases. However, the bond energy, as already discussed above, should not be much larger than I_. and therefore thermal dissociation should become efficient even at the lowest stratospheric temperatures for sufficiently large g. SUMMARY

AND CONCLUSIONS

Fractional abundance data for stratospheric negative ions presented provide considerable support for the ion identifications NO,-(HNO& and HSO,-(H,SO,),(HNO& previously proposed by Arnold and Henschen (1978) on the basis of ion mass determinations. Moreover, the present data allow to obtain new information on cluster ion equilibria and on cluster ion bond energies. It is inferred that the ion HSO,-(H,SOJ, and possibly also the ion HSO,-(H,SO,), have thermal dissociation rate which are than about constants smaller 2 * lO-*l cm” s-l at a temperature of 227 K corresponding to a lower limit to the bond energy of about 21.4 kcal mol-‘. Concerning HNO,clustering to different core ions a decrease of bond energies is found for the sequence: NOs-, HS04-, HS04-H$Od, HSQ-(H,SO,)z. The present data provide a basis for determining nitric acid and sulfuric acid concentrations. A detailed discussion of trace gas analysis from ions has been given by Arnold and Fabian (1980), Arnold et al. (1980a), and Arnold (1980b). Here attention will be drawn to the possibility of determining HCl-concentrations by measuring ions of the type NO,-(HNO,),(HCl), whose total frac-

Joos

tional abundance around 36 km was estimated to be of the order of a few tenths of a per cent. This value was below the detection limit of the B3 instrument. Detection limits of future balloonborne negative ion mass spectrometers may well be significantly below the 0.001 level. Thus, ions of the type NO,-(HNO,),(HCl), may become measurable already in the near future. Finally, conceivable spatial and temporal changes of the negative ion composition have been discussed. Among these variations of the total fractional abundance of HSO,--cluster ions appear to be most pronounced. The average association order of HS04-(H$O,J,-ions may behave quite curiously by increasing with increasing stratospheric temperature. This finding and the large bond energies of the HSO,-(H$O,),-ions raise an interesting question which relates to the possibility of multi-ioncomplex formation. The large bond energies for HS04-(H$O,),-ions inferred from the present data imply that the effective electron affinities EA of HSO,-(H$OJ,-ions may be even larger than previously thought (cf. Arnold, 1980a). Since EAvalues for HS04-(H$O,),-ions appear to be significantly larger than those for N03JHN0&-ions, formation of ion pairs by recombination of the former with positive cluster ions may be much more efficient. For example, recombination with proton hydrates H+(H,O), may give rise to ion pair formation already for relatively small n-values. If a lower limit to the electron affinity of HSO, of 4.5 eV (Viggiano et al., 1980) and a lower limit to the bond of H,SO,-ligands being equal to the bonds for HNO,-ligands of the same association order in NO,-(HNO,),-ions are considered, the following lower limits to the effective electron affinities for HS04-(H$O&,-ions may be estimated: h = 0: 103.5 kcal mol-‘; h = 1: 130.5 kcal mol-‘; h = 2: 148.35 kcal mol-‘; h = 3: 161.0 kcal mol-‘; h = 4: 174.0 kcal mol-‘. Considering the effective neutralization energies for H’(H,O),and Hxfx,(HzO)M-ions as given by Arnold et al. (1980a) and considering a chemical energy of 4.5 eV (Arnold, 1980a) it turns out that even some of the observed ion species may give rise to stable ion pair formation. The observed negative ion having the largest effective electron affinity namely those containing an HSO,- core and two

Stratospheric fractional negative ion abundances

ligands for example may give rise to a stable ion pair upon recombination with a proton hydrate whose n is equal to or larger than 7 or with an Hx+x=(H~O)~-ion whose total number of Iigands is equal to or Iarger than 4. If it is taken into account that the effective electron affinity used for HS04-H2S04 represents only a lower limit it is conceivable that the critical rr may be as small as 6. It therefore appears that the observed PH are not efficient in forming stable ion pairs, whereas one among the observed NPH-ion species, namely Hx‘x(H,O), is. Taking the measured fractional abundances a and b of positive and negative ion species which have the potential to form stable ion pairs it turns out that the rate of formation of stable ion pairs was only about one per cent of the total recombination rate which equals the ionisation rate of 0.4 cmm3s-’ around 36 km. According to Arnold (1980a) the concentrations of ion pairs (ion pair = two ion complex; 2-1~) formed by the association of oppositely charged ions and positive or negative 3-ion complexes (3 - Ic) formed by attachment of an ion to a two ion complex and positive or negative three ion complexes may be obtained from a steady state treatment. Neglecting ion pair interaction with aerosols and n(3-Icrt) = n(2-Ic) = arab/2k abnJ2 where k is the rate coefficient for ion pair attachment to free ions. ~nsidering the measured a- and b-values the estimates n(2 - Ic) = 900 cmm3 and n(3 - Ic *) = 9 crnm3 are obtained. The present data stress the potential importance of HSO,-(H,SO,),-ions in “multi-ion complex formation” because of their large stability. Therefore the investigation of these ions needs particular attention. Here both further in situ composition measurements and laboratory measurements of the kinetics and thermochemistry of these ions are needed. The stratospheric in situ measurements should include measurements under different temperature conditions and height dependences of the negative ion composition. Moreover, Iarger mass ranges should be used in order to allow for a detection of possibly existing higher order members of the HSO,-(H2S0,),-ion family. Ac~~owZedgemen~s~upport provided by the technical staffs of the ~smochemis~ department of the MaxPlanck-Institut fiir Kernphysik and of the CNES is acknowledged. The authors also wish to thank D. Ehhalt and R. Fabian for supporting this project. Part of the project was funded by the Bundesministerium fiir Forschung und

203

Technologie through the Gesellschaft fiir Strahlen- und Umweltforschung under grant KBF 47. REFERENCES Arnold, F. (1980a). Multi-ion complexes in the stratosphere-Implications for trace gases and aerosol. Nature, Lend. 284, 610. Arnold, F. (1980b). The middle atmosphere ionized component. Proc. of the ESA-Symposium on Rocket and Balloon Experiments. Bournemouth, April 1980. Arnold, F., Bohringer, H. and Henschen, G. (1978), Composition measurements of stratospheric positive ions. Geopkys. Res. Lett. 5, 653. Arnold, F. and Fabian, R. (1980). First measurements of sulfuric acid vapor concentrations in the stratosphere. Nature, Lond. 283, 105. Arnold, F., Fabian, R., Henschen, G. and Joos, W. (198Ob). Stratospheric trace gas analysis from ions: H,O and HNO,. P~a~e~. Space Sci. 28, 681. Arnold, F. and Henschen, G. (1978), First mass analysis of stratospheric negative ions. Nature, Lond. 257,521. Arnold, F., Henschen, G. and Ferguson, E. E. (1980a). Mass spectrometric measurements of fractional ion abundances in the stratosphere-positive ions Planet. Space Sci. 29, 185.

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Viggiano, A. A., Perry, R. A., Albritton, D. L., Ferguson, E. E. and Fehsenfeld, F. C. (1980). The role of H,SO, in stratospheric negative ion chemistry. To be published in J. Geopkys. Res.