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GLOBUS 1983 campaign
Planer. Space Sci.. Vol. 35, No. 5, pp. 693-701,
1987
00324633/87 S3.00+0.00 Pergamon Journals Ltd.
Printedin Great Britain.
BALLOON-BORNE COMPOSITION MEASUREMENTS OF STRATOSPHERIC NEGATIVE IONS AND INFERRED SULFURIC ACID VAPOR ABUNDANCES DURING THE MAP/GLOBUS 1983 CAMPAIGN* Max-Planck-Institut
H. SCHLAGER and F. ARNOLD fiir Kemphysik, Postfach 10 39 80, D-6900 Heidelberg,
F.R.G.
(Received in final form 18 August 1986) Abstract-Balloon-bore composition measurements of stratospheric negative ions were carried out at altitudes between about 30 and 40 km using two improved mass spectrometer probes with high sensitivity and mass resolution mounted on a gondola carried by a 350,000 m3 balloon. In order to minimize the risk of contamination, data were taken only at float altitude and during balloon descent. Besides the major ion species, HSO; (H,SO,),(HNO,), and NO, (HNO& various minor ion species were detected including
also mixed clusters with H,O, SO, and possibly HOC1 ligands. It appears that the SO, ligands were formed by electric field-induced collisional cluster ion fragmentation during sampling of the ions into the mass spectrometer. Sulfuric acid vapor abundances inferred from the present negative ion composition data reveal the presence of a sulfuric acid vapor layer with a pronounced maximum around 37 km with a H,S04 vapor concentration of about 3 x lo6 cm-‘, which is in accord with a previous measurement by our group. The occurrence of a maximum suggests that sulfuric acid vapor is efficiently removed at heights above about 37 km. Potential removal processes include photolysis, OH-attack and eventually also reactions with meteor smoke particles.
1. INTRODUCTION
Stratospheric negative ions are mostly NO; (HNO,), and HSO; (H2S04),,, clusters as originally proposed by Arnold and Henschen (1978) building on their first in situ negative ion composition measurements using a balloon-borne quadrupole mass spectrometer which was pumped by a liquid-helium cooled cryostat. It was also suggested by these authors that the latter ions are formed by reactions of NO, (HNO& clusters with atmospheric sulfuric acid vapor which subsequently attained strong support from laboratory measurements of requisite reaction rate coefficients (Viggiano et al., 1980). Furthermore, it was recognized that the negative ion composition measurements can be employed as a powerful tool for the detection of ambient sulfuric acid vapor (Arnold and Fabian, 1980). This trace gas is of considerable interest as it is responsible for the formation of the stratospheric aerosol layer which, at least temporarily, has an influence on the Earth’s radiation budget and climate (Schneider and Mass, 1975 ; Pollack et al., 1976 ; Baldwin et al., 1976; Robock, 1981 ; Coakley, 1981; Porter, 1981 ; Turco et aI., 1982). To date, an alternative method for stratospheric sulfuric acid
*Joint campaign
publication of 1983.
of results
from
the MAP/GLOBUS
693
vapor measurements is not available. In the meantime, additional in situ negative ion composition measurements using balloon-borne mass spectrometers with improved sensitivity and mass resolution (Arnold et al., 1981, 1982; Viggiano and Arnold, 1981; McCrumb and Arnold, 1981 ; Arijs et al., 1981, 1982, 1983, 1985; Viggiano et al., 1983; Arnold and Qiu, 1984) not only confirmed the original findings but substantially extended the altitude range of the measurements (1545 km). It was found that the abundance ratios for HSO; (H2S04),,, and NO, (HNO,), clusters range within about 0.02-0.2 at altitudes between about 25 and 30 km and increase drastically above 30 km, reaching maximum values of about 15 around an altitude of about 37 km. Inferred sulfuric acid vapor concentrations were found to be mostly around 104-lo5 crre3 between 25 and 30 km and to increase steeply above this height, reaching maximum concentrations of about 3 x lo6 cmm3 around 3540 km. The steep rise above about 30 km is probably due to a corresponding rise of the equilibrium saturation pressure of sulfuric acid over stratospheric H,SOJH,O aerosols associated with the rise of stratospheric temperatures with altitude. The decrease of H,SO, vapor concentrations above 3540 km may possibly be due to photolysis, OH-attack or reactions with meteor smoke particles (Turco et al., 1981; Qiu and Arnold, 1984). However, only very
694
H.
SCHLAGER
few measurements are available for heights above about 33 km and therefore the maximum of the sulfuric acid layer around 37 km is not well established. The present paper reports on in situ negative ion composition measurements and inferred sulfuric acid vapor concentrations covering the height range between about 30 and 40 km using improved balloonborne mass spectrometers with high sensitivity and mass resolution. Particular emphasis is placed upon ion identification employing electric field-induced cluster ion fragmentation as a diagnostic tool for composition and structure analysis of complex cluster ions. Some results of positive fragment ion mass spectrometry studies, which were also performed during the present flight, have already been published (Schlager and Arnold, 1985). 2. EXPERIMENTAL
The balloon-borne ion mass spectrometer probe used for the present measurements has been described elsewhere (Arnold et al., 1978, 1981). The basic instrument is a cryogenically pumped quadrupole mass filter (QMF) equipped with a channel electron multiplier system operated in a single ion detection mode. The cryopump which is cooled by liquid neon includes an additional adsorption stage employing charcoal adsorption panels. The effective pumping speed for the major atmospheric gases, N2 and 02, is about 3000 1 s- ‘. Stratospheric ions are sampled by a small inlet hole (diameter 0.03 cm) located at the centre of an eletrically insulated circular electrode on-axis with the QMF-field axis. The optimum bias potential of this draw-in electrode can be chosen by remote control via telecommand. The sampled ions are injected into the QMF only by means of an extraction field established by the QMF-field axis potential. The QMF has an open front end geometry and there are no additional ion optical elements between the draw-in electrode and the QMF which, according to simulation experiments carried out in our laboratory, turned out to be a configuration well suited for high pressure ion sampling. Weakly bonded cluster ions may suffer from fragmentation during the sampling due to electric fieldinduced collisional activation (ECA) of the ions occurring in the early stage of the gas jet on the low pressure side of the sampling electrode. The degree of ECA-induced cluster ion break-up depends on the gas density within the gas jet and the strength of the extraction field. Thus, the ECA of sampled ions is determined by the size of the inlet hole, the ambient gas pressure and the QMF-field axis potential. The QMF-field axis potential as well as various
and
F.
ARNOLD
other operational parameters of the mass spectrometer can be varied in-flight by remote control for both instruments included in the present balloonborne payload. The instruments have several improvements compared to balloon-borne instruments previously used by our group. In order to achieve both high ion transmission and high mass resolution, they are equipped with larger quadrupole mass filters. The QMF-rods of the two ion mass spectrometers have a length of 20 cm and a diameter of 1.6 and 0.8 cm, respectively. The maximum mass ranges of these instruments are 250 and 1000 amu (atomic mass units) and the maximum mass resolutions are 0.5 and 1.O amu (mass peak width at 50% peak height), respectively. The data to be presented here were obtained during a stratospheric balloon flight on 5 October 1983 using a particularly large balloon of 350,000 m3. The flight took place in southern France at the CNES launching site at Aire sur 1’Adour (latitude: 44”N) as part of the MAP/GLOBUS campaign 1983. The balloon was launched at 12.15 U.T. and reached a float altitude of 39.1 km 2 h before sunset. After sunset, the balloon was slowly descending at a velocity of about 0.9 m S -' and thereby measurements were possible within an altitude range of 39.1-29.2 km. The altitudes to be given here rely on pressure measurements using a high precision manometer (Crouzet, type 44). In addition, temperatures were measured using thermistors. Atmospheric conditions were characterized by moderate winds and low temperatures in the lower stratosphere (216 K, 20 km; 215 K, 25 km; 238 K, 35 km; 246 K, 39 km). 3. RESULTS AND DISCUSSION
3.1. Negative ion composition measurements Figures 1 and 2 show examples of negative ion mass spectra obtained at 38.5 and 30 km, respectively. All spectra were taken while the instrument was operated in a constant peak width mode with either a peak width of 2 amu at 50% peak height (Fig. la) or 7 amu (Figs lb, c; 2). Figures lc and 2b are expanded versions of 1b and 2a. The spectra represent accumulations of 100 (Fig. 1) and 200 (Fig. 2) scans of 12 s each. The high resolution mode enables an unambiguous identification of ion mass numbers, but allows only the detection of the major ions due to a reduced dynamic range of about 100. When operated in the moderate resolution mode, the instrument is about five times more sensitive and allows the detection of minor ion species having fractional count rates as low as about 0.2%. The uncertainty for mass identification is f 1 amu for minor mass peaks.
695
Stratospheric negative ions and sulfuric acid vapor abundances
I
b
MASS
(A.M.U.1
FIG. ~.NEGATY~ION~A~SPECTRATA~N AT 38Skm USING ~IF~~~~~I,~~o~~oDES. The peak widths at 50% peak height are 2 amu (a) and 7 amu (b, c). Spectrum (c) is an expanded version of(b).
A compilation of observed major ion mass numbers is given in Table 1 along with ion identifications. All of the observed major ions have been detected previously (Arnold and Henschen, 1978 ; McCrumb and Arnold, 1981; Arijs et al., 1981; Arnotd et al., 1982) and belong either to the ion families NO;(HNO& or HSO; (H,SO.&HNO,),. Figure 3 shows the percent composition of cluster ions containing NO;- and HSO;-core ions. The transition from NO; to
HSO; cluster ions occurs around 33 km due to the sharp increase in the sulfuric acid vapor concentration at this altitude. Detailed height profiles for major negative ions are shown in Fig. 4. Fractional count rates are given instead of fractional abundances since fragmentation during ion sampling complicates the conversion to fractional abundances. However, a correction for mass discrimination of the QMF was made, building on careful laboratory calibrations. The
H. SCHLAGEK and F.
696
ARNOLD
800
I
600
“E tci i
400
FIG. 2. NEGATIVE ION MASSSPEC~UM OBTAINEDAT 30 km ALTITUDE. Spectrum (b) is an expanded version of (a).
ion species HSO;(H,SO& (293 amu) and HSOC (H$O& (391 amu) represent about 70% of the observed ions in the altitude range between about 35 and 40 km, whereas NO; (HNO& (188 amu) is by far the most prominent ion below this altitude. This finding is consistent with recent negative ion composition measurements performed around 40 km (Arnold and Qiu, 1984) and with height profiles of negative ions between 15 and 34 km reported by Viggiano et al. (1982). Recent mass spectrometric measurements at an altitude of 42.3 km (Arijs et al., 1983) revealed NO,-HN03 to be most prominent, suggesting a reverse changeover to NO;-core ions within an altitude range of only about 3 km. The percent distribution of NO;(HNO,),and HSO; (H~SO~)~~luster ions as observed in the present measurements are given in Figs 5 and 6. Here count rates are normalized to the total count rates of NO; and HSOl-cluster ions, respectively. To make an assessment of how severely ion fragmentation may have affected the measured ion abundances during sampling, we compare the observed ion ratio NO; (HNO~)~/NO~HNO~ (188/125) with a calculated ion ratio obtained by assuming an equilibrium with respect to HNO, clustering. Using laboratory ther-
TABLE 1. MA~ZSNUMBERSAND ION ID~TI~~A~ONS NEGATIVEION.9
Mass (amu) I25 160 188 195 223 293 391 489
OF MAJOR
Ion NO; HNO, HSO; HNOJ NO; (HNO,X HSCJ;H$Od HSO< @INO& HSO,_(H,SO& HSO; (HZ.=& HSO,_(KSO&
modynamic data from Davidson et al. (1977), the measured temperatures and average HNO:, concentrations (WMO Rep. 11, The stratosphere 1981), an ion ratio 188/125 at 39 km of 0.35 is obtained which is comparable to the measured ion ratio 0.25. This would imply that at this altitude the degree of fragmentation is small. However, since 125 is more abundant than 188 at this height, even 40% fragmentation of 188 would change the ion ratio only by a factor of about 1.5. A much better estimate of the degree of fragmentation is possible in a situation where 188 is the dominant ion, which is the case at 35
Stratospheric 4O
NEGATIVE IONS
negative
05 OCT
ions and sulfuric
83-g
c
40-
B
691
acid vapor abundances NEGATIVE IONS
05OCT83
$
5 _I
.
NO3 -cluster
. HSOZ -cluster
4:
1PERCENT COMPOSITION FIG. 3. PERCENT COMPOSITION OF NEGATIVE CLUSTER IONS CoNTAININGNO;-AND HSO,-COREIONS.
&0
NEGATIVE IONS
05OCT83
FIG. 5. PERCENT
PERCENT COMPOSITION DISTRIBUTION OF NO,(HNO,), IONS vs ALTITUDE.
: z
05OCT
40
30
FIG. 4. FRACTIONAL
COUNT
12 3 .. . . IIll 1 1 IllI 1 10 100 PERCENT COMPO%ilON
n:o IIll
RATES OF MAJOR NEGATIVE ION
SPECIES.
FIG. 6. PERCENT
km and below. The calculated ion ratio 188/125 at 35 km is 4.0, which is still comparable to the measured ion ratio 3.6. Below this altitude, the expected ion ratios increasingly exceed the measured ratios. This indicates that the amount of fragmentation is small at heights above about 35 km for ions having bond energies comparable to that of NO, (HNO&. Hence, the decrease of the fractional count rates for ions NO, (HNO& with n equal to 0 and 1 between about 40 and 35 km (see Fig. 5) reflects the natural decrease of their abundances, and the constant fractional count rates for these ions observed below 35 km are due to fragmentation. Considering NO; and NO, HNOS at 29 km to be fragment ions, we obtain an estimate of the maximum amount of dissociation for NO, (HNO,), at this lowest height of measurement of 20%, which is consistent with similar findings of Viggiano and et al. (1983). We will now turn to the discussion of the observed minor negative ion species. A compilation of detected mass numbers is given in Table 2 along with tentative
DISTRIBUTION OF HSO,(H,SO,), ALTITUDE.
IONS VS
ion identifications. Most of these ions have been observed in previous measurements (McCrumb and Arnold, 1981 ; Arnold et al., 1982; Arijs et al., 1982; Arnold and Qiu, 1984). In the following, possible ion identifications will be discussed in the light of measured fractional count rates, which are shown in Fig. 7. Particular attention will be given to the question of whether some of the observed minor negative ions may not be true ambient ions but fragments formed by decomposition of major negative ions during ion sampling. The most prominent detected minor mass peaks are located at mass numbers 178, 276 and 374. They accompany the major HSO, (HISO,),-cluster ions and fit the form HSO,(H,SO&* X with X having a mass of 81+ 1 amu. It has been hypothesized previously (Arnold et al., 1982; Qiu and Arnold, 1984) that these ions represent mixed cluster ions of the type HSO; (H2S04),* HSO,. Recent laboratory measure-
698
H.
S~HLAGER
TnsLE2.MAssNUMBERSANDTENTAn~lONID~FlCATIONS OFMINORNEGATIVEIONS
and
F. ARNOLD (HSO,- H2S04)* -+ HSO,- + H,SO, +HSO;SO,+H,O
Mass (amu)
+
1
Ion
CNCN-H,O, NO; NO; NO; H,O NO; HCl, HSO,(NO,SO,)-* NO; HNO, - H,O NO; HNO, - HOC1 HSO,-HNO, *H,O NO, (HNO& *Hz0 (NO$O>)-HNO,* NO; (HNO& *HOC1 HSO; (HNOj), *Hz0 HSO; H,S04 *HN03 NO; (HNO& - Hz0 HSO,-H,SO, *SOS* HSO,- (H2S0J2 *HNO, HSO,- (HZSO&*SO,* HSO,- (H2S0& *H,O
27 46 62 RO 97 143 178 205 239 241 258 269 276 356 374 409 *Indicates fragment ions. 1O
NEGATIVE IONS
05 OCT 83
: _i
*2?
2 2
_
w
x46 _ :E _ .143 0 97 * 178
935-
r205
t b
-
a
-
2::
& 251 A258 A269
f
_
-
and these authors suggested that the ions under consideration may be fragmentation products of the type HSO, (H,SOJ,* SOJ. To investigate this possibility, the measured fractional count rates of 178, 276 and 374 have to be considered with regard to their possible precursor ions 195, 293 and 391. As noticeable from Figs 4 and ‘7, masses 178,276 and 374 were only observed at heights where their possible precursor ions are also present. The ion abundance ratios (178/195), (276/293) and (374/391) range between about 0.02 and 0.5 and increase with decreasing altitude for (276/293) and (374/391) while (178/195) shows the opposite behavior. Thus, in view of the fact that the degree of fragmentation increases with decreasing height, it is conceivable that 276 and 374 represent fragment ions. However, it does not seem likely that 178 is mainly due to fra~entation of 195. Recently, first in situ fragment ion mass speo
trometry studies of negative ions were made in the stratosphere (Schlager and Arnold, 1986) and additional fragmentation channels, besides the dissociation of ligands, were observed also for HSO; (H,SO,),(HNO,),-mixed cluster ions : HSO,-(HNO,),
_
+ HSO; (HNO&_ , + HNOS
(2a)
_
-+ GODSON)-(HNO~)~_ , +H,O
(2b)
9276 _ 0374 _ o409
l3!r6
30 -
HSO,_ tH*SO~),(HNO~)~ + HSOi- (H,SO,),(HNO,),_ + HSO; (H2S04),-, + HSO; (H#Od),-
FIG.
7.
FRACTIONAL
(la) (lb)
COUNT RATES OF MINOR NEGATIVE ION SPECIES.
ments of Stockwell and Calvert (1983), however, revealed that HSO3 reacts very fast with O2 and should consequently reach no significant abundance in the stratosphere. Alternative possible identifications for these ions being hydrates of HSOL (H$O&(HNOJcluster ions seem unlikely for 276 and 374 since their possible unhydrated precursors are much less abundant. Very recently, Schlager and Arnold (1986) investigated the possibility that these ions might be due to fragmentation of sulfuric acid cluster ions. Their laboratory studies revealed that the fragmentation of excited HSO;; H,SO,-cluster ions involves two channels :
, + HN09
(HNO&, + HZSO.,
, (HN03)nS03
+ HZO.
(3a) (3b) (3~)
Detected minor negative ions which might be fragment ions of the type (2b) and (3~) are 143,205 and 239, 241, respectively. These ion species have previously been attributed to hydrates of NO; (HNO& and HSO; (HNO,),-cfuster ions. Again we consider fractional count rates of these ions with respect to their possible precursor ions. As far as 143 is concerned, the possible precursor ion [see equation (2)] is 160. In fact, 143 was only observed at altitudes where 160 is abundant. The alternative identification for 143 being a hydrate of NO;HN03 (125) is not likely at altitudes below 35 km, where the detected NO; HN03 is mainly due to fragmentation of NO; (HNO,),. However, NO; (HNO,)H,O may contribute to the mass peak 143 above 35 km. The ion
205 can be either a fragment of HSOy (HNO,), (223)
Stratospheric negative ions and sulfuric acid vapor abundances
or a hydrate of NO~~HNO~~~ (188). It was only observed below about 35 km which is also the case for 223, but the ion abundance ratio 205/223 becomes rather large around 30 km, being about one. This indicates that at these heights 205 is more likely a hydrate of 188. The ion 241 cannot be a fragment ion of HSO; (H,SO,)HNO, (258) since 2.58 is not abundant at altitudes where 241 was detected. An alternative identification for 241 is HSO, (HN0,)2Hz0. At float altitude, one of the most abundant detected minor ion mass peaks is located at mass number 239. This ion species seems to be neither a hydrate nor a fragment ion since its possible unhydrated precursor ion is not present as well as its possible precursor ion according to equation (3). An attractive identification of this ion might be NO;(HNO&* HOC1 (240) as already previously proposed by McCrumb and Arnold (1981). Notice that the observed ion 178 would also fit a HOC1 containing cluster ion (NOiWHN03- HOCI). No stratospheric measurements of hypochlorous acid have been reported so far, but model calculations predict a maximum HOC1 mixing ratio of about 0.1-3 ppbv to occur between about 35 and 40 km (Glasgow et al., 1978). Assuming the ion 239 to be in fact NO; (HNO&HOCl, a rough estimate of the HOC1 ~on~ntration can be made using the present ion composition data. Considering production of NO; (HNO& * HOC1 via three-body association at a gas collision rate and loss mainly due to displacement of HOC1 by HNO, molecules via ligand switching reactions, a steady-state treatment yields : [HOC11 = k;‘[M]-‘kl x F[NO, (HNO,), * HOC11 FtNO,
WNQM
WNQI. (4)
Here F[NO; (HNO&* HOCl]/~[NO~ (HNO,)J is the measured ion flux ratio and k, and k, are the rate coefficients for the association and switching reactions, respectively. Assuming k, [M] = k2 = lO-.9 cm3 s- ‘, HOC1 concentrations are obtained which are about 0.5 and 0.05 times that of HNO, at 39 and 35 km, respectively. These values are in reasonable agreement with abundance ratios [HOCl]/[HNO,] given by current model calculations. Some light minor ions with mass numbers 27, 46, 62, 80 and 97 were also detected. These ions have previously been attributed to CN-, CN-H,O, NO;, NO;, NO; H,O, NO; HCl and HSO; , respectively (McCrumb and Arnold, 1981 ; Arijs et al., 1982). The remaining minor negative ions : 251,258,269,3X and 409 amu can be readily identified as members of the major ion families as indicated in Table 2. Two
699
of them can be attributed to hydrates, namely 269 [NO, (HNO&H,O] and 409 [HSO,- (H,SO,),H,O]. Interestingly, 409 is the only hydrate of HSO; (H,SO,),,-cluster ions which was observed. This is in accordance with previous in situ measurements (Arnold et al., 1982) which revealed that the hydration of HSO;(H,SO,), with n equals 3 is much more efficient than for n equals 1 and 2. Additional support comes from laboratory hydration studies of sulfuric acid cluster ions (Glebe and Arnold, 1982), which showed that the hydration efficiency for HSOF (H,SO,),-cluster ions with n larger than 2 increases by a factor of about 30 compared to n equals 1 and 2. 3.2. Inferred ~~uric mid oapor ~b~ndan&~s The negative ion composition data can be used to derive gas phase sulfuric acid abundances. The method employed was described originally by Arnold and Fabian (1980) and considers the conversion of NO, -cluster ions to HSO, -cluster ions due to ionmolecule reactions involving H2S04. From a steadystate treatment the following equation is obtained : [H,SO,] = k- ‘tk “F(HSO,-)/F(NO;).
(5)
Here ~(HSO~)/~(NO~) is the flux ratio for all ion species containing HSO; - and NO, -core ions, k is an effective rate coefficient and tRis the ion-ion recombination lifetime. Values fork and tR were taken from Viggiano et al. (1980) and Bates (1982), respectively. In view of the discussion in Section 3.1, we believe that besides H,SO, other sulfur-bearing compounds, in particular HSO,, can be neglected with regard to the conversion of NO; - to HSO; -cluster ions. H,SO, number densities inferred from the present ion measurements are shown in Fig. 8 along with profiles recently obtained by our group (Viggiano and Arnold, 1983; Qiu and Arnold, 1984) and Arijs et al. (1983, 1985), which were formerly supposed to relate also to other sulfur-daring gases such as HSO,. The H,SO, measurements performed by Arijs ef aI. in September 1983 have also been part of the MAPjGLOBUS campaign. AI1 H,SO,-profiles shown in Fig. 8 were obtained in the fall at middle latitudes (over southern France). Here, only data taken during balloon descents were considered which can hardly be affected by contamination involving desorbing gases from the balloon or the payload. In addition, model profiles for H2S04 concentrations (Turco et al., 1981) are included in Fig. 8 as well as equilibrium saturation concentrations for HSO, vapor over H,SO,/H,O aerosols calculated for average summer and winter tem~ratures at middle latitudes. The recent H,SO, data of our group have been revised in two respects. A correction concerning alti-
700
H.
and F.
SCHLAGER
ARNOLD
ticles. Between 28 and 35 km, HzS04 is controlled by the equilibrium saturation pressure of H,S04 over aerosols which is very sensitive to temperature and relative humidity. Above about 35 km, measured H#O., concentrations increasingly deviate from the H,SO,-equilibrium curve, indicating that all H2S04 is in the gas phase. The low MPI-K values in the vapor controlled region (28-35 km) are probably associated with the low stratospheric temperatures prevailing at the time of our flight. The observation of peak H$O., concentrations of about 3 x IO6 cm-3 around 35 km confirms recent results of Arnold and Qiu (1984). Comparing the H,S04 data set given in Fig. 8 with 10s 10’ ld HzSOG NUMBER DENSITY kni3) the model caIculations of Turco et al. (1981) the measured H,SO., concentrations seem to agree reasonFIG. 8. COMPARISON OF PRESENT H2S04 CONCENTRATIONS able well with curve 3. Here heterogeneous removal of WITH PREVIOUS MEASUREMENTS OF OUR GROUP (MPI-K, H2S0., by “meteor smoke particles” and weak H,S04 TYPICAL ERROR BARS ARE GIVEN FOR TWO ALTITUDES) AND DATA OBTAINED BYAaus et al. (BISA ; 1983,1985). photolysis are assumed to be the H&SO4loss processes Model calculations (TWX ei al., 1981)are also given (curve at higher altitudes. However, as already pointed out 1 includes photochemical production of H2S04 vapor and heterogeneous removal without re-evaporation. Curve 2 by Arnold and Biihrke (1983) and Qiu and Arnold includes, in addition, re-evaporation from aerosols and weak (1984), this must not necessarily support the suggesH,SO, photolysis. Curve 3 includes also H2S09removal due tion that meteor smoke significantly reduces H$Q to meteoric smoke). (45W) and (45 S) are equilibrium satu- vapor in the upper stratosphere since consideration ration concentrations of H#& vapor over aerosols for aver- of stronger H*SO, photolysis comparable to HN03 age winter and summer temperatures at middle latitudes, photolysis would provide similar H2S04 profiles as respectively. given by curve 3. By contrast to the hypotheti~l H,SO, removal by meteor smoke, both H,SOI photudes was made for measurements above 35 km. In tolysis and OH-attack would not deplete gaseous sulthis region, previously given altitudes relied on presfur but ultimately convert H+SO, to SOZ. If so, curve 2 in Fig. 8 approximately representing total sulfur sure measurements using sensors which systematically gave values too low, as was recognized in the present (for heights above about 40 km) would give a rough measurements. In addition, an effective rate coefficient estimate of the SOZ concentration. of 1.75 x lWy cm3 s-’ was used instead of I x 10e9 cm3 s- ’ at higher altitudes taking into account the 4. SUMMARYAND CONCLUSIONS observed abundance ratio of NO;HNOl and NO; (HNO,),. Experimental errors for relative and absolThe present stratospheric negative ion composition ute H,SO, concentrations arising from un~rtainties measurements confirm previous findings that HSO; ions in k, ta and ion abundance measurements are esti(H,SO.+),(HNO,),- and NO, (HNO,),-cluster mated to be factors of about 1.5 and 2.5, respectively. represent the most abundant negative ion species in The limits for H,SO, concentrations given by Arijs et the stratosphere. They also show that NO; clusters al. for their measurements on 18 September 1983 refer dominate below about 33 km whereas HSO; clusters only to uncertainties in the ion abundance measuredominate above this altitude. Besides the above ions, various minor ion species were detected which seem ments whereas uncertainties in k and tR were not included as stated by these authors. The larger errors to be preferably of the type HSQ; (H,SO&* X (I = 1,2) with X having a mass of 81 rfr1 amu. The of the BISA data at lower heights are probably due consideration of observed height profiles of fractional to a low sensitivity. The measured H,S04 profiles are characterized by ion count rates supports the view that these ions may three major branches : roughly constant values below be formed by electric field-induced collisional fragabout 28 km, a steep rise between about 28 and 35 mentation of major sulfuric acid cluster ions as km and a decrease above about 35 km. At lower recently suggested by Schlager and Arnold (1986). In heights, H-SO4 concentrations are larger than the addition, fragment ions of the type (NO,SO,)equilibrium H,SO, vapor pressure over aerosols. (HNO,), also appear to be present. Other observed minor ions seem to be hydrates of the major negative Here H2S04 is controlled by local photochemical production and heterogeneous removal by aerosol parions. Interestingly, two of the detected minor ion i ? $
Stratospheric negative ions and sulfuric acid vapor abundances
species may contain HOC1 ligands [NO,(HNO,), HOCI, n = I, 21 which may offer the possibility to infer ambient HOC1 abundances. A preliminary estimation of HOC1 abundances building on our present data yields HOC1 concentratjons about 0.5 and 0.05 times that of HNOz at 39 and 35 km, respectively. These values are in reasonable agreement with current model calculations. Inferred sulfuric acid abundances are similar to the single previous height profile available for heights between about 30 and 40 km of Qiu and Arnold (1984) showing maximum H,SO, concentrations of about 3 x 10” cm-’ around 37 km. Thus, the present data support the view that there exists a pronounced sulfuric acid vapor maximum around 35-40 km and that the vapor is undersaturated with respect to the condensed H$O,/H,O phase for heights above about 37 km. If so, this height would precisely mark the upper boundary of the stratospheric aerosol layer. Above this height level, the H,SO$H,O aerosols become thermodynamically unstable and evaporate. By contrast, however, the present sulfuric acid abundances measured below about 33 km appear to be very close to equilibrium saturation values considering the measured ambient temperatures. Acknowledgements-The authors wish to express their thanks to the technical staff of the Max-Planck-institut fiir Kemphysik, Heidelberg and the balloon group of the Centre Nationale d’Etudes Spatiales at Aire sur I’Adour, France. This work was supported in part by the Bundesministe~um fiir Forschung und Technologie through the Gesellschaft fur Strahlen und Umweltforschung.
REFER~C~
Arijs, E., Nevejans, D., Frederick, P. and Ingels, J. (I981) Geophys. Res. Lett. 8, 121. Arijs, E., Nevejans, D., Frederick, P. and Ingels, J. (1982) J. atmos. terr. Phys. 44, 681. Arijs, E., Nevejans, D., Ingels, J. and Frederick, P. (1983) Planet. Space Sci. 31, 1459.
701
Arijs, E., Nevejans, D., Ingels, J. and Frederick, P. (1985) J. geophys. Res. 90,5891. Arnold, F., Biihrlnger, H. and Henschen, G. (1978) Geophys. Res. Lett. 5, 653. Arnold, F. and Biihrke, T. (1983) Nuiure 301, 293. Arnold, F. and Fabian, R. (1980) Nature 283,55. Arnold, F. and Henschen, G. (1978) Nature 275, 521, Arnold, F., Henschen, G. and Ferguson, E. E. (1981) Pluner. Space Sci. 29, 185. Arnold, F. and Qiu, S. (1984) Planet.Space Sci. 32, 169. Arnold, F., Viggiano, A. A. and Schlager, H. (1982) Nature 297,371. Baldwin, B., Pollack, J. B., Summers, A. and Toon, 0. B. ( 1976) Nature 263, 55 1. Bates, D. R. (1982) Planet. Space Sci. 30, 1275. Coakley, J. A., Jr. (1981) f. geophys. Res. 86,976l. Davidson, J. A., Fehsenfeld, F. C. and Howard, C. J. (1977) Itrt. f. Chem. Kinetics 9, 17. Glasgow, L. C., Jesson, J. P., Filkin, D. L. and Miller, C. (1978) Planet. Space Sci. 27, 1047. Glebe, W. and Arnold, F. (1983) Proc. 3rd lnt. Swarm Seminar, Innsbruck, Austria 132. McCrumb, J. L. and Arnold, F. (1981) Nature 294, 136. Pollack, J. B., Toon, 0. B., Sagan, C., Summers, A. and Van Camp, N. (1976) J. geophyi Res. 81, 1071. Porter, S. C. (1981) Nature 291. 136. Qiu, S. and Arnold, F. (1984) !&me& Space Sci. 32, 87. Robock, A. (1981) Science 212, 1383. Schlager, H. and Arnold, F. (1985) Planet. Space Sci. 33, 1363. Schlager, H. and Arnold, F. (1986) Pianet. Space Sci. 34, 245. Schneider, S. H. and Mass, C. (1975) Science 190,741. Stockwell, W. R. and Calvert, J. G. (1983) Atmus. Environ. 17, 2331. Turco, R. P., Toon,O. B., Hamill, P. and Whitten, R. (1981) J. geuphys. Rex 86, 1113. Turco, R. P., Whitten, R. C. and Toon, 0. B. (1982) Rev. Geophys. Space Phys. 20, 233. Viggiano, A. A. and Arnold, F. (1981) Planet. Space Sci. 29, 895. Viggiano, A. A. and Arnold, F. (1983) J. geophys. Res. 88, 1457. Viggiano, A. A., Perry, R. A., Albritton, D. L., Fergnson, E. E. and Fehsenfeld, F. C. (1980) J. geophys. Res. 85, 4551. Viggiano, A. A., Schlager, H. and Arnold, F. (1983) Planet. Space Sci. 8, 8 13. WMO Report (1981) The stratosphere 1981. WMO global ozone research Rep. 11,Geneva.
1987
00324633/87 S3.00+0.00 Pergamon Journals Ltd.
Printedin Great Britain.
BALLOON-BORNE COMPOSITION MEASUREMENTS OF STRATOSPHERIC NEGATIVE IONS AND INFERRED SULFURIC ACID VAPOR ABUNDANCES DURING THE MAP/GLOBUS 1983 CAMPAIGN* Max-Planck-Institut
H. SCHLAGER and F. ARNOLD fiir Kemphysik, Postfach 10 39 80, D-6900 Heidelberg,
F.R.G.
(Received in final form 18 August 1986) Abstract-Balloon-bore composition measurements of stratospheric negative ions were carried out at altitudes between about 30 and 40 km using two improved mass spectrometer probes with high sensitivity and mass resolution mounted on a gondola carried by a 350,000 m3 balloon. In order to minimize the risk of contamination, data were taken only at float altitude and during balloon descent. Besides the major ion species, HSO; (H,SO,),(HNO,), and NO, (HNO& various minor ion species were detected including
also mixed clusters with H,O, SO, and possibly HOC1 ligands. It appears that the SO, ligands were formed by electric field-induced collisional cluster ion fragmentation during sampling of the ions into the mass spectrometer. Sulfuric acid vapor abundances inferred from the present negative ion composition data reveal the presence of a sulfuric acid vapor layer with a pronounced maximum around 37 km with a H,S04 vapor concentration of about 3 x lo6 cm-‘, which is in accord with a previous measurement by our group. The occurrence of a maximum suggests that sulfuric acid vapor is efficiently removed at heights above about 37 km. Potential removal processes include photolysis, OH-attack and eventually also reactions with meteor smoke particles.
1. INTRODUCTION
Stratospheric negative ions are mostly NO; (HNO,), and HSO; (H2S04),,, clusters as originally proposed by Arnold and Henschen (1978) building on their first in situ negative ion composition measurements using a balloon-borne quadrupole mass spectrometer which was pumped by a liquid-helium cooled cryostat. It was also suggested by these authors that the latter ions are formed by reactions of NO, (HNO& clusters with atmospheric sulfuric acid vapor which subsequently attained strong support from laboratory measurements of requisite reaction rate coefficients (Viggiano et al., 1980). Furthermore, it was recognized that the negative ion composition measurements can be employed as a powerful tool for the detection of ambient sulfuric acid vapor (Arnold and Fabian, 1980). This trace gas is of considerable interest as it is responsible for the formation of the stratospheric aerosol layer which, at least temporarily, has an influence on the Earth’s radiation budget and climate (Schneider and Mass, 1975 ; Pollack et al., 1976 ; Baldwin et al., 1976; Robock, 1981 ; Coakley, 1981; Porter, 1981 ; Turco et aI., 1982). To date, an alternative method for stratospheric sulfuric acid
*Joint campaign
publication of 1983.
of results
from
the MAP/GLOBUS
693
vapor measurements is not available. In the meantime, additional in situ negative ion composition measurements using balloon-borne mass spectrometers with improved sensitivity and mass resolution (Arnold et al., 1981, 1982; Viggiano and Arnold, 1981; McCrumb and Arnold, 1981 ; Arijs et al., 1981, 1982, 1983, 1985; Viggiano et al., 1983; Arnold and Qiu, 1984) not only confirmed the original findings but substantially extended the altitude range of the measurements (1545 km). It was found that the abundance ratios for HSO; (H2S04),,, and NO, (HNO,), clusters range within about 0.02-0.2 at altitudes between about 25 and 30 km and increase drastically above 30 km, reaching maximum values of about 15 around an altitude of about 37 km. Inferred sulfuric acid vapor concentrations were found to be mostly around 104-lo5 crre3 between 25 and 30 km and to increase steeply above this height, reaching maximum concentrations of about 3 x lo6 cmm3 around 3540 km. The steep rise above about 30 km is probably due to a corresponding rise of the equilibrium saturation pressure of sulfuric acid over stratospheric H,SOJH,O aerosols associated with the rise of stratospheric temperatures with altitude. The decrease of H,SO, vapor concentrations above 3540 km may possibly be due to photolysis, OH-attack or reactions with meteor smoke particles (Turco et al., 1981; Qiu and Arnold, 1984). However, only very
694
H.
SCHLAGER
few measurements are available for heights above about 33 km and therefore the maximum of the sulfuric acid layer around 37 km is not well established. The present paper reports on in situ negative ion composition measurements and inferred sulfuric acid vapor concentrations covering the height range between about 30 and 40 km using improved balloonborne mass spectrometers with high sensitivity and mass resolution. Particular emphasis is placed upon ion identification employing electric field-induced cluster ion fragmentation as a diagnostic tool for composition and structure analysis of complex cluster ions. Some results of positive fragment ion mass spectrometry studies, which were also performed during the present flight, have already been published (Schlager and Arnold, 1985). 2. EXPERIMENTAL
The balloon-borne ion mass spectrometer probe used for the present measurements has been described elsewhere (Arnold et al., 1978, 1981). The basic instrument is a cryogenically pumped quadrupole mass filter (QMF) equipped with a channel electron multiplier system operated in a single ion detection mode. The cryopump which is cooled by liquid neon includes an additional adsorption stage employing charcoal adsorption panels. The effective pumping speed for the major atmospheric gases, N2 and 02, is about 3000 1 s- ‘. Stratospheric ions are sampled by a small inlet hole (diameter 0.03 cm) located at the centre of an eletrically insulated circular electrode on-axis with the QMF-field axis. The optimum bias potential of this draw-in electrode can be chosen by remote control via telecommand. The sampled ions are injected into the QMF only by means of an extraction field established by the QMF-field axis potential. The QMF has an open front end geometry and there are no additional ion optical elements between the draw-in electrode and the QMF which, according to simulation experiments carried out in our laboratory, turned out to be a configuration well suited for high pressure ion sampling. Weakly bonded cluster ions may suffer from fragmentation during the sampling due to electric fieldinduced collisional activation (ECA) of the ions occurring in the early stage of the gas jet on the low pressure side of the sampling electrode. The degree of ECA-induced cluster ion break-up depends on the gas density within the gas jet and the strength of the extraction field. Thus, the ECA of sampled ions is determined by the size of the inlet hole, the ambient gas pressure and the QMF-field axis potential. The QMF-field axis potential as well as various
and
F.
ARNOLD
other operational parameters of the mass spectrometer can be varied in-flight by remote control for both instruments included in the present balloonborne payload. The instruments have several improvements compared to balloon-borne instruments previously used by our group. In order to achieve both high ion transmission and high mass resolution, they are equipped with larger quadrupole mass filters. The QMF-rods of the two ion mass spectrometers have a length of 20 cm and a diameter of 1.6 and 0.8 cm, respectively. The maximum mass ranges of these instruments are 250 and 1000 amu (atomic mass units) and the maximum mass resolutions are 0.5 and 1.O amu (mass peak width at 50% peak height), respectively. The data to be presented here were obtained during a stratospheric balloon flight on 5 October 1983 using a particularly large balloon of 350,000 m3. The flight took place in southern France at the CNES launching site at Aire sur 1’Adour (latitude: 44”N) as part of the MAP/GLOBUS campaign 1983. The balloon was launched at 12.15 U.T. and reached a float altitude of 39.1 km 2 h before sunset. After sunset, the balloon was slowly descending at a velocity of about 0.9 m S -' and thereby measurements were possible within an altitude range of 39.1-29.2 km. The altitudes to be given here rely on pressure measurements using a high precision manometer (Crouzet, type 44). In addition, temperatures were measured using thermistors. Atmospheric conditions were characterized by moderate winds and low temperatures in the lower stratosphere (216 K, 20 km; 215 K, 25 km; 238 K, 35 km; 246 K, 39 km). 3. RESULTS AND DISCUSSION
3.1. Negative ion composition measurements Figures 1 and 2 show examples of negative ion mass spectra obtained at 38.5 and 30 km, respectively. All spectra were taken while the instrument was operated in a constant peak width mode with either a peak width of 2 amu at 50% peak height (Fig. la) or 7 amu (Figs lb, c; 2). Figures lc and 2b are expanded versions of 1b and 2a. The spectra represent accumulations of 100 (Fig. 1) and 200 (Fig. 2) scans of 12 s each. The high resolution mode enables an unambiguous identification of ion mass numbers, but allows only the detection of the major ions due to a reduced dynamic range of about 100. When operated in the moderate resolution mode, the instrument is about five times more sensitive and allows the detection of minor ion species having fractional count rates as low as about 0.2%. The uncertainty for mass identification is f 1 amu for minor mass peaks.
695
Stratospheric negative ions and sulfuric acid vapor abundances
I
b
MASS
(A.M.U.1
FIG. ~.NEGATY~ION~A~SPECTRATA~N AT 38Skm USING ~IF~~~~~I,~~o~~oDES. The peak widths at 50% peak height are 2 amu (a) and 7 amu (b, c). Spectrum (c) is an expanded version of(b).
A compilation of observed major ion mass numbers is given in Table 1 along with ion identifications. All of the observed major ions have been detected previously (Arnold and Henschen, 1978 ; McCrumb and Arnold, 1981; Arijs et al., 1981; Arnotd et al., 1982) and belong either to the ion families NO;(HNO& or HSO; (H,SO.&HNO,),. Figure 3 shows the percent composition of cluster ions containing NO;- and HSO;-core ions. The transition from NO; to
HSO; cluster ions occurs around 33 km due to the sharp increase in the sulfuric acid vapor concentration at this altitude. Detailed height profiles for major negative ions are shown in Fig. 4. Fractional count rates are given instead of fractional abundances since fragmentation during ion sampling complicates the conversion to fractional abundances. However, a correction for mass discrimination of the QMF was made, building on careful laboratory calibrations. The
H. SCHLAGEK and F.
696
ARNOLD
800
I
600
“E tci i
400
FIG. 2. NEGATIVE ION MASSSPEC~UM OBTAINEDAT 30 km ALTITUDE. Spectrum (b) is an expanded version of (a).
ion species HSO;(H,SO& (293 amu) and HSOC (H$O& (391 amu) represent about 70% of the observed ions in the altitude range between about 35 and 40 km, whereas NO; (HNO& (188 amu) is by far the most prominent ion below this altitude. This finding is consistent with recent negative ion composition measurements performed around 40 km (Arnold and Qiu, 1984) and with height profiles of negative ions between 15 and 34 km reported by Viggiano et al. (1982). Recent mass spectrometric measurements at an altitude of 42.3 km (Arijs et al., 1983) revealed NO,-HN03 to be most prominent, suggesting a reverse changeover to NO;-core ions within an altitude range of only about 3 km. The percent distribution of NO;(HNO,),and HSO; (H~SO~)~~luster ions as observed in the present measurements are given in Figs 5 and 6. Here count rates are normalized to the total count rates of NO; and HSOl-cluster ions, respectively. To make an assessment of how severely ion fragmentation may have affected the measured ion abundances during sampling, we compare the observed ion ratio NO; (HNO~)~/NO~HNO~ (188/125) with a calculated ion ratio obtained by assuming an equilibrium with respect to HNO, clustering. Using laboratory ther-
TABLE 1. MA~ZSNUMBERSAND ION ID~TI~~A~ONS NEGATIVEION.9
Mass (amu) I25 160 188 195 223 293 391 489
OF MAJOR
Ion NO; HNO, HSO; HNOJ NO; (HNO,X HSCJ;H$Od HSO< @INO& HSO,_(H,SO& HSO; (HZ.=& HSO,_(KSO&
modynamic data from Davidson et al. (1977), the measured temperatures and average HNO:, concentrations (WMO Rep. 11, The stratosphere 1981), an ion ratio 188/125 at 39 km of 0.35 is obtained which is comparable to the measured ion ratio 0.25. This would imply that at this altitude the degree of fragmentation is small. However, since 125 is more abundant than 188 at this height, even 40% fragmentation of 188 would change the ion ratio only by a factor of about 1.5. A much better estimate of the degree of fragmentation is possible in a situation where 188 is the dominant ion, which is the case at 35
Stratospheric 4O
NEGATIVE IONS
negative
05 OCT
ions and sulfuric
83-g
c
40-
B
691
acid vapor abundances NEGATIVE IONS
05OCT83
$
5 _I
.
NO3 -cluster
. HSOZ -cluster
4:
1PERCENT COMPOSITION FIG. 3. PERCENT COMPOSITION OF NEGATIVE CLUSTER IONS CoNTAININGNO;-AND HSO,-COREIONS.
&0
NEGATIVE IONS
05OCT83
FIG. 5. PERCENT
PERCENT COMPOSITION DISTRIBUTION OF NO,(HNO,), IONS vs ALTITUDE.
: z
05OCT
40
30
FIG. 4. FRACTIONAL
COUNT
12 3 .. . . IIll 1 1 IllI 1 10 100 PERCENT COMPO%ilON
n:o IIll
RATES OF MAJOR NEGATIVE ION
SPECIES.
FIG. 6. PERCENT
km and below. The calculated ion ratio 188/125 at 35 km is 4.0, which is still comparable to the measured ion ratio 3.6. Below this altitude, the expected ion ratios increasingly exceed the measured ratios. This indicates that the amount of fragmentation is small at heights above about 35 km for ions having bond energies comparable to that of NO, (HNO&. Hence, the decrease of the fractional count rates for ions NO, (HNO& with n equal to 0 and 1 between about 40 and 35 km (see Fig. 5) reflects the natural decrease of their abundances, and the constant fractional count rates for these ions observed below 35 km are due to fragmentation. Considering NO; and NO, HNOS at 29 km to be fragment ions, we obtain an estimate of the maximum amount of dissociation for NO, (HNO,), at this lowest height of measurement of 20%, which is consistent with similar findings of Viggiano and et al. (1983). We will now turn to the discussion of the observed minor negative ion species. A compilation of detected mass numbers is given in Table 2 along with tentative
DISTRIBUTION OF HSO,(H,SO,), ALTITUDE.
IONS VS
ion identifications. Most of these ions have been observed in previous measurements (McCrumb and Arnold, 1981 ; Arnold et al., 1982; Arijs et al., 1982; Arnold and Qiu, 1984). In the following, possible ion identifications will be discussed in the light of measured fractional count rates, which are shown in Fig. 7. Particular attention will be given to the question of whether some of the observed minor negative ions may not be true ambient ions but fragments formed by decomposition of major negative ions during ion sampling. The most prominent detected minor mass peaks are located at mass numbers 178, 276 and 374. They accompany the major HSO, (HISO,),-cluster ions and fit the form HSO,(H,SO&* X with X having a mass of 81+ 1 amu. It has been hypothesized previously (Arnold et al., 1982; Qiu and Arnold, 1984) that these ions represent mixed cluster ions of the type HSO; (H2S04),* HSO,. Recent laboratory measure-
698
H.
S~HLAGER
TnsLE2.MAssNUMBERSANDTENTAn~lONID~FlCATIONS OFMINORNEGATIVEIONS
and
F. ARNOLD (HSO,- H2S04)* -+ HSO,- + H,SO, +HSO;SO,+H,O
Mass (amu)
+
1
Ion
CNCN-H,O, NO; NO; NO; H,O NO; HCl, HSO,(NO,SO,)-* NO; HNO, - H,O NO; HNO, - HOC1 HSO,-HNO, *H,O NO, (HNO& *Hz0 (NO$O>)-HNO,* NO; (HNO& *HOC1 HSO; (HNOj), *Hz0 HSO; H,S04 *HN03 NO; (HNO& - Hz0 HSO,-H,SO, *SOS* HSO,- (H2S0J2 *HNO, HSO,- (HZSO&*SO,* HSO,- (H2S0& *H,O
27 46 62 RO 97 143 178 205 239 241 258 269 276 356 374 409 *Indicates fragment ions. 1O
NEGATIVE IONS
05 OCT 83
: _i
*2?
2 2
_
w
x46 _ :E _ .143 0 97 * 178
935-
r205
t b
-
a
-
2::
& 251 A258 A269
f
_
-
and these authors suggested that the ions under consideration may be fragmentation products of the type HSO, (H,SOJ,* SOJ. To investigate this possibility, the measured fractional count rates of 178, 276 and 374 have to be considered with regard to their possible precursor ions 195, 293 and 391. As noticeable from Figs 4 and ‘7, masses 178,276 and 374 were only observed at heights where their possible precursor ions are also present. The ion abundance ratios (178/195), (276/293) and (374/391) range between about 0.02 and 0.5 and increase with decreasing altitude for (276/293) and (374/391) while (178/195) shows the opposite behavior. Thus, in view of the fact that the degree of fragmentation increases with decreasing height, it is conceivable that 276 and 374 represent fragment ions. However, it does not seem likely that 178 is mainly due to fra~entation of 195. Recently, first in situ fragment ion mass speo
trometry studies of negative ions were made in the stratosphere (Schlager and Arnold, 1986) and additional fragmentation channels, besides the dissociation of ligands, were observed also for HSO; (H,SO,),(HNO,),-mixed cluster ions : HSO,-(HNO,),
_
+ HSO; (HNO&_ , + HNOS
(2a)
_
-+ GODSON)-(HNO~)~_ , +H,O
(2b)
9276 _ 0374 _ o409
l3!r6
30 -
HSO,_ tH*SO~),(HNO~)~ + HSOi- (H,SO,),(HNO,),_ + HSO; (H2S04),-, + HSO; (H#Od),-
FIG.
7.
FRACTIONAL
(la) (lb)
COUNT RATES OF MINOR NEGATIVE ION SPECIES.
ments of Stockwell and Calvert (1983), however, revealed that HSO3 reacts very fast with O2 and should consequently reach no significant abundance in the stratosphere. Alternative possible identifications for these ions being hydrates of HSOL (H$O&(HNOJcluster ions seem unlikely for 276 and 374 since their possible unhydrated precursors are much less abundant. Very recently, Schlager and Arnold (1986) investigated the possibility that these ions might be due to fragmentation of sulfuric acid cluster ions. Their laboratory studies revealed that the fragmentation of excited HSO;; H,SO,-cluster ions involves two channels :
, + HN09
(HNO&, + HZSO.,
, (HN03)nS03
+ HZO.
(3a) (3b) (3~)
Detected minor negative ions which might be fragment ions of the type (2b) and (3~) are 143,205 and 239, 241, respectively. These ion species have previously been attributed to hydrates of NO; (HNO& and HSO; (HNO,),-cfuster ions. Again we consider fractional count rates of these ions with respect to their possible precursor ions. As far as 143 is concerned, the possible precursor ion [see equation (2)] is 160. In fact, 143 was only observed at altitudes where 160 is abundant. The alternative identification for 143 being a hydrate of NO;HN03 (125) is not likely at altitudes below 35 km, where the detected NO; HN03 is mainly due to fragmentation of NO; (HNO,),. However, NO; (HNO,)H,O may contribute to the mass peak 143 above 35 km. The ion
205 can be either a fragment of HSOy (HNO,), (223)
Stratospheric negative ions and sulfuric acid vapor abundances
or a hydrate of NO~~HNO~~~ (188). It was only observed below about 35 km which is also the case for 223, but the ion abundance ratio 205/223 becomes rather large around 30 km, being about one. This indicates that at these heights 205 is more likely a hydrate of 188. The ion 241 cannot be a fragment ion of HSO; (H,SO,)HNO, (258) since 2.58 is not abundant at altitudes where 241 was detected. An alternative identification for 241 is HSO, (HN0,)2Hz0. At float altitude, one of the most abundant detected minor ion mass peaks is located at mass number 239. This ion species seems to be neither a hydrate nor a fragment ion since its possible unhydrated precursor ion is not present as well as its possible precursor ion according to equation (3). An attractive identification of this ion might be NO;(HNO&* HOC1 (240) as already previously proposed by McCrumb and Arnold (1981). Notice that the observed ion 178 would also fit a HOC1 containing cluster ion (NOiWHN03- HOCI). No stratospheric measurements of hypochlorous acid have been reported so far, but model calculations predict a maximum HOC1 mixing ratio of about 0.1-3 ppbv to occur between about 35 and 40 km (Glasgow et al., 1978). Assuming the ion 239 to be in fact NO; (HNO&HOCl, a rough estimate of the HOC1 ~on~ntration can be made using the present ion composition data. Considering production of NO; (HNO& * HOC1 via three-body association at a gas collision rate and loss mainly due to displacement of HOC1 by HNO, molecules via ligand switching reactions, a steady-state treatment yields : [HOC11 = k;‘[M]-‘kl x F[NO, (HNO,), * HOC11 FtNO,
WNQM
WNQI. (4)
Here F[NO; (HNO&* HOCl]/~[NO~ (HNO,)J is the measured ion flux ratio and k, and k, are the rate coefficients for the association and switching reactions, respectively. Assuming k, [M] = k2 = lO-.9 cm3 s- ‘, HOC1 concentrations are obtained which are about 0.5 and 0.05 times that of HNO, at 39 and 35 km, respectively. These values are in reasonable agreement with abundance ratios [HOCl]/[HNO,] given by current model calculations. Some light minor ions with mass numbers 27, 46, 62, 80 and 97 were also detected. These ions have previously been attributed to CN-, CN-H,O, NO;, NO;, NO; H,O, NO; HCl and HSO; , respectively (McCrumb and Arnold, 1981 ; Arijs et al., 1982). The remaining minor negative ions : 251,258,269,3X and 409 amu can be readily identified as members of the major ion families as indicated in Table 2. Two
699
of them can be attributed to hydrates, namely 269 [NO, (HNO&H,O] and 409 [HSO,- (H,SO,),H,O]. Interestingly, 409 is the only hydrate of HSO; (H,SO,),,-cluster ions which was observed. This is in accordance with previous in situ measurements (Arnold et al., 1982) which revealed that the hydration of HSO;(H,SO,), with n equals 3 is much more efficient than for n equals 1 and 2. Additional support comes from laboratory hydration studies of sulfuric acid cluster ions (Glebe and Arnold, 1982), which showed that the hydration efficiency for HSOF (H,SO,),-cluster ions with n larger than 2 increases by a factor of about 30 compared to n equals 1 and 2. 3.2. Inferred ~~uric mid oapor ~b~ndan&~s The negative ion composition data can be used to derive gas phase sulfuric acid abundances. The method employed was described originally by Arnold and Fabian (1980) and considers the conversion of NO, -cluster ions to HSO, -cluster ions due to ionmolecule reactions involving H2S04. From a steadystate treatment the following equation is obtained : [H,SO,] = k- ‘tk “F(HSO,-)/F(NO;).
(5)
Here ~(HSO~)/~(NO~) is the flux ratio for all ion species containing HSO; - and NO, -core ions, k is an effective rate coefficient and tRis the ion-ion recombination lifetime. Values fork and tR were taken from Viggiano et al. (1980) and Bates (1982), respectively. In view of the discussion in Section 3.1, we believe that besides H,SO, other sulfur-bearing compounds, in particular HSO,, can be neglected with regard to the conversion of NO; - to HSO; -cluster ions. H,SO, number densities inferred from the present ion measurements are shown in Fig. 8 along with profiles recently obtained by our group (Viggiano and Arnold, 1983; Qiu and Arnold, 1984) and Arijs et al. (1983, 1985), which were formerly supposed to relate also to other sulfur-daring gases such as HSO,. The H,SO, measurements performed by Arijs ef aI. in September 1983 have also been part of the MAPjGLOBUS campaign. AI1 H,SO,-profiles shown in Fig. 8 were obtained in the fall at middle latitudes (over southern France). Here, only data taken during balloon descents were considered which can hardly be affected by contamination involving desorbing gases from the balloon or the payload. In addition, model profiles for H2S04 concentrations (Turco et al., 1981) are included in Fig. 8 as well as equilibrium saturation concentrations for HSO, vapor over H,SO,/H,O aerosols calculated for average summer and winter tem~ratures at middle latitudes. The recent H,SO, data of our group have been revised in two respects. A correction concerning alti-
700
H.
and F.
SCHLAGER
ARNOLD
ticles. Between 28 and 35 km, HzS04 is controlled by the equilibrium saturation pressure of H,S04 over aerosols which is very sensitive to temperature and relative humidity. Above about 35 km, measured H#O., concentrations increasingly deviate from the H,SO,-equilibrium curve, indicating that all H2S04 is in the gas phase. The low MPI-K values in the vapor controlled region (28-35 km) are probably associated with the low stratospheric temperatures prevailing at the time of our flight. The observation of peak H$O., concentrations of about 3 x IO6 cm-3 around 35 km confirms recent results of Arnold and Qiu (1984). Comparing the H,S04 data set given in Fig. 8 with 10s 10’ ld HzSOG NUMBER DENSITY kni3) the model caIculations of Turco et al. (1981) the measured H,SO., concentrations seem to agree reasonFIG. 8. COMPARISON OF PRESENT H2S04 CONCENTRATIONS able well with curve 3. Here heterogeneous removal of WITH PREVIOUS MEASUREMENTS OF OUR GROUP (MPI-K, H2S0., by “meteor smoke particles” and weak H,S04 TYPICAL ERROR BARS ARE GIVEN FOR TWO ALTITUDES) AND DATA OBTAINED BYAaus et al. (BISA ; 1983,1985). photolysis are assumed to be the H&SO4loss processes Model calculations (TWX ei al., 1981)are also given (curve at higher altitudes. However, as already pointed out 1 includes photochemical production of H2S04 vapor and heterogeneous removal without re-evaporation. Curve 2 by Arnold and Biihrke (1983) and Qiu and Arnold includes, in addition, re-evaporation from aerosols and weak (1984), this must not necessarily support the suggesH,SO, photolysis. Curve 3 includes also H2S09removal due tion that meteor smoke significantly reduces H$Q to meteoric smoke). (45W) and (45 S) are equilibrium satu- vapor in the upper stratosphere since consideration ration concentrations of H#& vapor over aerosols for aver- of stronger H*SO, photolysis comparable to HN03 age winter and summer temperatures at middle latitudes, photolysis would provide similar H2S04 profiles as respectively. given by curve 3. By contrast to the hypotheti~l H,SO, removal by meteor smoke, both H,SOI photudes was made for measurements above 35 km. In tolysis and OH-attack would not deplete gaseous sulthis region, previously given altitudes relied on presfur but ultimately convert H+SO, to SOZ. If so, curve 2 in Fig. 8 approximately representing total sulfur sure measurements using sensors which systematically gave values too low, as was recognized in the present (for heights above about 40 km) would give a rough measurements. In addition, an effective rate coefficient estimate of the SOZ concentration. of 1.75 x lWy cm3 s-’ was used instead of I x 10e9 cm3 s- ’ at higher altitudes taking into account the 4. SUMMARYAND CONCLUSIONS observed abundance ratio of NO;HNOl and NO; (HNO,),. Experimental errors for relative and absolThe present stratospheric negative ion composition ute H,SO, concentrations arising from un~rtainties measurements confirm previous findings that HSO; ions in k, ta and ion abundance measurements are esti(H,SO.+),(HNO,),- and NO, (HNO,),-cluster mated to be factors of about 1.5 and 2.5, respectively. represent the most abundant negative ion species in The limits for H,SO, concentrations given by Arijs et the stratosphere. They also show that NO; clusters al. for their measurements on 18 September 1983 refer dominate below about 33 km whereas HSO; clusters only to uncertainties in the ion abundance measuredominate above this altitude. Besides the above ions, various minor ion species were detected which seem ments whereas uncertainties in k and tR were not included as stated by these authors. The larger errors to be preferably of the type HSQ; (H,SO&* X (I = 1,2) with X having a mass of 81 rfr1 amu. The of the BISA data at lower heights are probably due consideration of observed height profiles of fractional to a low sensitivity. The measured H,S04 profiles are characterized by ion count rates supports the view that these ions may three major branches : roughly constant values below be formed by electric field-induced collisional fragabout 28 km, a steep rise between about 28 and 35 mentation of major sulfuric acid cluster ions as km and a decrease above about 35 km. At lower recently suggested by Schlager and Arnold (1986). In heights, H-SO4 concentrations are larger than the addition, fragment ions of the type (NO,SO,)equilibrium H,SO, vapor pressure over aerosols. (HNO,), also appear to be present. Other observed minor ions seem to be hydrates of the major negative Here H2S04 is controlled by local photochemical production and heterogeneous removal by aerosol parions. Interestingly, two of the detected minor ion i ? $
Stratospheric negative ions and sulfuric acid vapor abundances
species may contain HOC1 ligands [NO,(HNO,), HOCI, n = I, 21 which may offer the possibility to infer ambient HOC1 abundances. A preliminary estimation of HOC1 abundances building on our present data yields HOC1 concentratjons about 0.5 and 0.05 times that of HNOz at 39 and 35 km, respectively. These values are in reasonable agreement with current model calculations. Inferred sulfuric acid abundances are similar to the single previous height profile available for heights between about 30 and 40 km of Qiu and Arnold (1984) showing maximum H,SO, concentrations of about 3 x 10” cm-’ around 37 km. Thus, the present data support the view that there exists a pronounced sulfuric acid vapor maximum around 35-40 km and that the vapor is undersaturated with respect to the condensed H$O,/H,O phase for heights above about 37 km. If so, this height would precisely mark the upper boundary of the stratospheric aerosol layer. Above this height level, the H,SO$H,O aerosols become thermodynamically unstable and evaporate. By contrast, however, the present sulfuric acid abundances measured below about 33 km appear to be very close to equilibrium saturation values considering the measured ambient temperatures. Acknowledgements-The authors wish to express their thanks to the technical staff of the Max-Planck-institut fiir Kemphysik, Heidelberg and the balloon group of the Centre Nationale d’Etudes Spatiales at Aire sur I’Adour, France. This work was supported in part by the Bundesministe~um fiir Forschung und Technologie through the Gesellschaft fur Strahlen und Umweltforschung.
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