Combined mass spectrometric composition measurements of positive and negative ions in the lower ionosphere—I. Positive ions

Planet Space Sci., Vol. 30. No. 12, pp. 1295-1305, 1982 Printed in Great Britain.

0032-0633/X2/121295-llS03.00/0 Pergamon Press Ltd.

COMBINED MASS SPECTROMETRIC COMPOSITION MEASUREMENTS OF POSITIVE AND NEGATIVE IONS IN THE LOWER IONOSPHERE-I. POSITIVE IONS. F. ARNOLD and A. A. WGGIANO

Max-Pian~k-Institut

fur Kemphysik,

6900 Heidelberg, Germany

(Receiued 17 June 1982) Abstract-Recent improvements in rocket-borne mass spectrometer technology have made it possible to measure lower ionospheric ions with greater sensitivity and to extend the measurements to lower heights. The improvements made to the instrument and positive ion results from a flight of this instrument will be reported here. In addition to the previously known ions, such as NO’(H,O), and Hc(HzO),, new ion species were found. The total fractional count rate of these ions was found to be constant with height indicating an upper altitude source. Possible identifications of these ions are proposed along with possible production mechanisms.

INTRODUCTION

progress has been made during recent years in the study of the lower ionospheric ions by rocket-borne mass spectrometry. Improvements in the experimental technique, as well as numerous measurements made under a variety of geophysical conditions, have given us an understanding of many of the major features of the positive ion composition and the associated underlying physical and chemical processes. In contrast, however, our knowledge about lower ionospheric negative ions has remained in a rudimentary state. This is mainly due to experimental problems in detecting negative ions and a more complex negative ion chemistry. The former reason is related to sensitivity problems at heights lower than 75 km, where negative ions become abundant, and also to problems of sampling negative ions in an environment where free electrons are present. As a consequence, fewer measurements of lower ionospheric negative ions have been made than for positive ions. Since the first negative ion composition measurements of Arnold et al. (1971) and Narcisi et al. (1971), only a few measurements have been published (Arnold, 1980a) and the results are contradictory. Also, there appear to be substantial discrepancies between measurements and model predictions {Ferguson et al., 1979; Ferguson et al., 1979; Reid, 1976; Ferguson, 1974). The complexity of the lower ionospheric negative ion chemistry is mainly caused by the much longer lifetime of negative ions (about lo4 s) compared to positive ions (about 10” s) which recombine with electrons. Ion-ion recombination, the

Considerable

dominant loss process for negative ions, is slower than electron-ion recombination by up to two orders of magnitude. This implies trace gases with concentrations of only lo’-10” cm-” can convert a major fraction of the negative ion population. Consequently, lower ionospheric negative ions can be influenced by unknown ultra rare gases much more effectively than positive ions. While this fact complicates an understanding of the negative ion composition, it offers at the same time a sensitive tool for detecting any ultra rare trace gas that interacts with negative ions. Although, as mentioned above, the positive ion chemistry of the lower ionosphere is much better understood than that of negative ions, unresolved problems still remain. In particular, it is unciear which role meteoric metal compounds play in the chemistry of positive cluster ions. Furthermore, details of the growth processes of positive cluster ions are not well understood. In order to help resolve these problems, an improved rocket-borne quadrupole mass spectrometer capable of measuring both positive and negative ions was flown into the lower ionosphere. The positive ion data are reported here and the negative ion measurements are reported in an accompanying paper (Arnold et al., 1982). Numerous advantages resulted from using the improved instrument. The lower height range was extended to 5.5km, sensitivity was increased considerably and the mass range was extended to 420 a.m.u. Furthermore, break-up of complex ions during sampling was considerably reduced. These improvements helped reveal the presence of numerous ion species which were previously un1295

F. ARNOLD and

1296 detected. Some by

of these ions cannot be explained

current theoretical

methods.

EXPERIMENTAL METHOD

used in this study is similar to the one described by Arnold et al. (1977a). Several modifications were made in order to improve the sensitivity and increase the mass range. In the following the instrumental characteristics will be briefly reviewed and the major changes described in detail. The instrument consists of a quadrupole mass filter (MF) pumped by a high speed cryopump (CP), consisting of a cold surface (CS), cryogen dewar (CD) and a radiation shield (RS). The instrument is shown schematically in Fig. 1. Both the MF and CP are housed inside a stainless steel vacuum tank (VT) into which atmospheric ions and gases flow. The knife-edged inlet orifice is The rocket-borne mass spectrometer

FI

VT

IP

’

5cm

’

VA

FIG.

1. SCHEMATIC DRAWING OF

THE ROCKET-BORNE

QUADRUPOLEMASSSPECTROMETER.

The abbreviations

are as follows:

inlet orifice (IO), cap (EC), filament (FI), cold surface (CS), cryogen dewar (CD), radiation shield (RS), vacuum tank (VT), quadrupole mass filter (MF), ion getter pump (IP), channeltron multiplier (CM) and pumping valve (VA).

sampling electrode (SE), ejectable

A. A. VIGGIANO

located on axis with the MF field axis at the tip of an electrically insulated sampling electrode (SE). The SE has a double cone in order to attach the shock wave which forms in front of the probe as it travels at supersonic speed (Mach 3). Ambient ions are accelerated towards the probe by a bias potential of k2.W applied to the SE, with the additional effect of preventing repulsion of ions due to electrical charge-up of the rocket. The potential is kept low in order to minimize collisional dissociation of weakly bonded clusters during sampling. Atmospheric ions contained in the gas beam, under the influence of an electric extraction field, decouple from the beam and are accelerated towards the entrance of the MF. After mass analysis the ions are counted by a channeltron multiplier (CM). Data are transmitted to a ground receiving station and stored on magnetic tape. In addition to the ion modes described above, neutral gas detection is also possible. This is accomplished by placing an electron filament ion source (FI) in the gas beam. The various modes of operation include positive ion, negative ion and neutral gas modes. For each of these modes, two mass resolution settings (A and B) can be used. For the neutral gas mode, the A and B submodes also have different electron bombardment energies (20eV and 70eV, respectively). This enables the sensitivity of the neutral mode to be increased at higher altitudes, where the ambient pressure is lower. Furthermore, the low electron energy keeps the production of 0’ from dissociative electron impact ionization of O2 low. This is particularly important at lower heights where the atmospheric abundance ratio, 0 : O2 is small (Arnold et al., 1977b). The instrumental changes made for the present studies are as follows: (a) liquid neon was used as the cryogen; (b) the size of the inlet hole was made variable; and (c) the mass range of the instrument was extended to 420 a.m.u. Modification (a) was made in order to increase the standing time of the CP. Since liquid neon has a much larger heat of vaporization than liquid helium, the previous coolant, it evaporates much slower at a given heat input. Thus, the standing time of the CP was increased from 15 min to 8 hr. This allowed more time for flight preparations and waiting for proper meteorological or geophysical conditions. The latter is particularly important if sporadically occurring short duration phenomena like aurorae or noctilucent clouds are to be investigated.

1297

Positive and negative ions in lower ionosphere-I Liquid neon has another important advantage over liquid helium. When a high rate of gas condensation to the CP occurs, as happens in the lower mesosphere and upper stratosphere, the flow of heat energy becomes so large that liquid helium rapidly evaporates. This causes a high vapor pressure inside the dewar which can push out liquid from the exhaust pipe resulting in a malfunction of the CP. The same problem does not exist for liquid neon under atmospheric conditions. This has the additional consequence that a larger inlet hole can be used at a given ambient pressure and thus a larger flux of ions into the MF and correspondingly a higher sensitivity. It turns out that for typical flight trajectories, a flux of neutral molecules through the inlet hole of lo** s-’ can easily be tolerated without decreasing the pumping efficiency. This implies that the lower altitude limit is not determined by the deterioration of the CP but only by the degeneration of the MF transmission by ions scattering on background molecules. Laboratory simulation experiments have shown that severe MF transmission losses occur above ambient pressures of approx. 1 torr if an inlet orifice of 0.5 cm in diameter is used. In the atmosphere, this corresponds to an altitude of 45 km. Thus ion composition studies should be possible throughout the mesosphere with the new instrument. A further increase in the inlet hole size does not appear to be worthwhile since 0.5 cm is already comparable to the size of the QMS inlet aperature. Modification (b) was made to ensure the instrument would work sensitively throughout the mesosphere. The diameter of the inlet hole was increased to 0.5 cm, as stated above, compared to 0.1 cm used previously. Since we had no in flight experience with the larger orifice, we began the measurements using a 0.1 cm inlet orifice. In order to use both orifices during flight, a mechanism that can change orifice size during flight had to be developed. The upper small cone of the sampling electrode containing the 0.1 cm orifice was designed to be able to be removed during flight. This electrode is pressed very tightly onto the large cone, which included the 0.5 cm knife edged orifice. The smaller cone is held in place by a tungsten wire and sealed with a silicon rubber O-ring. During flight, the tungsten wire is broken by a brief electric heating pulse at the desired altitude, causing the small cone to swing to the side under the action of a spring. Therefore, the 0.5 cm orifice is only exposed at lower pressures avoiding the risk

of early saturation of the CP during the ascent phase of the rocket trajectory. These improvements enable the instrument to be run at a higher sensitivity than previously used. According to laboratory simulation experiments the sensitivity is about 3 and 1 counts s-’ per ion per cm3 for the lower mass resolution mode (B) and for the higher mass resolution mode (A), respectively. Indications of very massive negative ions were seen previously (Arnold et al., 1971). In order to search for these ions, the mass range of the instrument was increased to 420 a.m.u., more than twice as large as previous instruments. The mass scan is divided into three segments in order to ensure adequate sensitivity over the entire mass scan is achieved. The first quarter of the scan is dead time to allow for stabilization of potentials after a mode switching operation. This is followed by a relatively slow scan of the masses between 0 and 105 a.m.u. Finally, in the last quarter of the sweep, the scan rate is increased in order to cover masses up to 420a.m.u. The extended mass range is made possible by increasing the amplitudes of RF and DC voltages applied to the MF-rods. An entire scan takes approx. 1 s. Usually, MF-transmission decreases as mass resolution is increased. This decrease is especially pronounced for high masses. Therefore, only relatively low mass resolution is used in the rapid high mass portion of the scan in order to keep the sensitivity high. A compilation of characteristic data of the mass spectrometer is given in Table 1. EXPERIMENTAL

AND GEOPHYSICAL

CONDITIONS

The improved mass spectrometer probe described above was flown on a Nike-Apache rocket. The rocket payload also contained several other instruments including an ionization gauge, gerdien TABLE

1. CHARACTERISTICSOFTHEINSTRUMENT

Total mass

Overall length Volume of Ne-Dewar Pumping capacity Pumping speed Quadrupole rod diameter Quadrupole rod length Mass filter high frequency Maximum count rate Mass range Peak width Scan time Altitude resolution MS detection limit ions

llkg 49.5 cm 0.5 litres - 10” N2-molecules 5000 1s-’ 0.48 cm IlScm 1.4 MHz 5 MHz l-420 a.m.u. 2 2 a.m.u. 1s 1km 2 0.1 cmm3

F.

1298

ARNOLD

and A. A. VIGGIANO

probe, Faraday antenna and magnetometer. The flight took place on 1 March 1978 at 01: 13 U.T. over Andoya, Norway (69”N, OS’E). The rocket was launched into a moderate aurora, characterized by a riometer absorption of 2 dB and a magnetometer reading of 50 y (Trane, private communication). At an altitude of 68 km, the vacuum sealed cap covering the sampling orifices was ejected and measurements commenced. The measuring sequence was as follows. The initial operation sequence was PI-A, NI-B and Ne-A where PI and NI stand for positive and negative ion modes and Ne for neutral mode. At 81.5 km the cycle was changed to PI-A, NI-B and Ne-A and the 0.5 cm orifice exposed. At 100 km the mode Ne-A replaced by Ne-B in order to increase sensitivity for neutral gas detection. At the apogee height of 108.7 km the cycle was changed again to NI-A, NI-B and PI-B and remained unchanged for the entire descent. All modes worked as designed and mass spectra of positive and negative ions as well as neutral gases were registered. In addition, the total gas pressure and total positive ion concentration were measured by the ionization gauge and spectra were recorded on the descent down to 55 km. The positive ion results will be reported here and the negative ion results in the adjoining paper. RESULTS AND

DISCUSSION

The total positive ion concentration profile as measured by the electrostatic probe is shown in Fig. 2. Absolute values were obtained by normalizing the relative ion concentrations to electron concentrations measured by Faraday rotation (Trane, 1982). These concentrations are also shown in Fig. 2 as well as the negative ion density. The normalization was made at heights above 85 km. The total positive ion concentration, II+, infrom more or less monotonously creases 10OO~~rn-~ at 50 km to 50,000 cmm3 at 100 km. Around 90 km a weak ledge can be observed. Absolute n, values are considerably higher than during undisturbed conditions due to ionization caused by energetic electrons from the aurora. The electron density profile is similar to the n, profile at heights above 81 km. Below this height, the electron density falls off much more rapidly then II+, indicating that the majority of the negative charge is in the form of negative ions. The low mass portion of a typical positive ion A-mode spectrum obtained at 81.4 km is shown in Fig. 3. It contains 5 major peaks and a number of

103 CHARGED

FIG.2.

TOTAL

PARTICLE

1Ob CONCENTRATION

ION AND ELECTRON ALTITUDE.

105 (cme3)

CONCENTRATIONS

vs

The positive and negative ion densities are represented by II, and n_ and the electron density by np.

minor ones. The major peaks have masses 30, 37, 48,55 and 74. Minor peaks are found at masses 14, 19, 64, 66 and 81. At higher masses there is significant signal although individual masses cannot be determined. The mass resolution used was not sufficient to distinguish minor peaks separated by only 1 a.m.u. from the large peaks and therefore we may have missed some minor mass peaks. For B-mode spectra this is even more severe as a larger peak width was used. Since mode PI-A was only applied at heights between 68 and 81 km, masking of minor peaks may be more severe at other altitudes. This had to be tolerated in favor of the use of both NI-A and NI-B in the mode sequence, clearly indicating a priority for negative ion measurements. A compilation of the observed positive ion masses and tentative identifications is given in Table 2. The fractional count rate range of each ion is also indicated. Many of these ions are well known lower ionospheric ions, which have been previously observed. However, there are also ion species, particularly massive ones, which have, as yet, not been reported. The mass uncertainties are 1 or 2 a.m.u. for ions lighter than 1OOa.m.u. and 10 a.m.u. for heavy ions. Height profiles of the fractional ion count rates measured during the ascent phase of the flight are shown in Figs. 4 and 5. The data have been divided for clarity. We prefer to give fractional ion count rates rather than concentrations because the data is complicated by collisional dissociation at lower

Positive and negative ions in lower ionosphere--I 1000

I

I

I

I

I

I

I

-iE

800

is 5

800

r’

3 5 400 F a iii = 200

:

r-k \i

50

10

MASS FIG.~. THELOWMASSPORTIONOFATYPICAL

A-MODESPECTRATAKENAT

81.4km.

TABLE 2. OBSERVEDPOSITIVEIONSFRACTIONALABUNDANCE

Mass

0.1

0.01

0.001 +

14 :49

Ion possible identification

21 30

+

CH;, N+ H+H*O Mg+, Na+ Al+ H+HCN Si’ NO’ ’

;:

+ +

0; H+M%

+ + +

+ +

45 48

AI’H,O; H+HCN.H*O, SiOH+ NO+H*O H+(H,O),

+ :z 64

+ +

66 68 70 73 74 80

+ +

85 2 99 103

:;; Al+(H,O),; H+HCN(H,O& SiOH’(H,O) NO+(H,O)z +

+ + +

H+(HzO)d NO+C02 AI+(H,O),; H+HCN(H,O),; SiOH+(HzO),

+ + + + +

H+(CH,CN)(H,O), H+(CH,CN)2H20

1300

F. ARNOLD and A. A. VIGGIANO

10-3

10-2 FRACTIONAL

10-l COUNT RATE

FIG. 4. FRACTIONAL COUNT RATES vs ASCENCE(PART 1).

100

ALTITUDE ON

85

65 I, 10-S

10-2

FRACTIONAL COUNT

10-l RATE

FIG. S. FRACTIONALCOUNTRATESvs ALTITUDEON ASCENT (PART 2).

altitudes. Dissociation occurs when weakly bonded cluster ions undergo energetic collisions with the background gas molecules. Such collisions may be induced by the shock wave or the electric fields applied for ion sampling. This problem will be discussed later in the paper. At 81-84 km, the ion composition shown in Fig. 4 shows a pronounced change. Above this region,

light ions, masses 30(NO’) and 32(0;), and metal ions, mass 24 (Mg’) dominate. Below this height, more massive cluster ions, mostly 55 (H+(H20&) and 73 (H+(H20).J are prominent. The ions 19 (H,O+), 37 (H+(H*O)& 48 (NO+(H,O)), 64 (possibly 0;) 66 (NO’ (H,O)J and 74 (NO’(C0,)) reach their largest fractional count rates in the transition region (80-90 km). All these features have been observed previously (see e.g. Arnold, 1980a), and can be explained within the framework of current positive ion reaction schemes. According to these schemes N’, N;, 0’ and 0; are the most important primary ions under the conditions of energetic particle ionization. N: is rapidly converted to 0; via charge exchange. 0: then reacts with NO to produce NO’. This last process is accelerated in aurorae since energetic particle radiation represents an efficient source for NO resulting in large enhancements in the NO concentration. At heights above the transition region, NO’ is mainly lost by recombination with electrons. Then by assuming a steady state, NO concentrations can be derived (c.f. Arnold and Krankowsky, 1979). Both 0: and NO’ charge transfer with gaseous metallic species deposited in the atmosphere around 90-100 km by meteor ablation. The resulting ions, mostly Fe’ and Mg’, form a layer usually peaking between 90 and 100 km, which is seen in this work. Besides undergoing recombination, the light ions can also be converted to cluster ions by attaching atmospheric molecules. Generally, this conversion proceeds by 3-body processes and therefore becomes more efficient as pressure increases. As a consequence, the fractional abundance of cluster ions increases with decreasing altitude. In detail, however, this conversion can be rather complex. The most important reactions of the conversion scheme are given in Table 3. The clustering of 0: can be short circuited by reaction (2). This is reflected by the steep drop of the observed 0: concentration above 81 km. For this flight it seems that the bottom of the atomic oxygen layer was located around 81 km. This finding is also supported by the negative ion composition data measured in this flight. Atomic oxygen had another more indirect impact on the positive ion composition. Oxygen atoms destroy negative ions by associative electron detachment and therefore negative ions only became abundant at heights below about 81 km. Since positive cluster ions recombine about 100

Positive and negative ions in lower ionosphere-I TABLE3. HYDRATION REACTIONS OF 0: AND NO’

O;+O,+M + O:+M o:+o 0; + Hz0 O:(H,O) + Hz0 H,O+OH + H20 NO++N,+M NO+N, + CO2 NO+CO> + Hz0 NO+H,O + Hz0 + M NO+(H,O), + Hz0 + M NO+(H,O), + Hz0

(1) +

o:+o,

-+ + + + -+ + + -+ -+

O:H,O+O, H,O+ + OH+ + O2 HjO+OH + O2 H+(H,O)* + OH NO+N,+M NO+CO, + N2 NO+H20 + CO2 NO+(H*O)* + M NO+(H20)s + M H+(H,O), + HN02

+

(2) (3) (4) (5) (6) (7) (8) (9) (10) (II) (12)

times more slowly with negative ions than with electrons, the positive cluster ion lifetime increased steeply below about 81 km. Consequently, the fractional abundance of cluster ions increased steeply below about 81 km as one would have expected. In the transition region three intermediate product ions of the O&cluster chain, O:, HXO+ and H’(H*O) were observed. Only O:(H,O) was not observed, probably due to the poor mass resolution which did not allow separation of a minor O:(H*O) (50) peak from NOf(H20) (48). Now, a quantitative comparison between the present data and model predictions will be attempted. According to the model, the ions O;, H30+ and H+(H20J are formed via reactions (3)(6). Assuming that the dominant product, H+(H*O) is formed from 0; and lost via electron-ion recombination, an ratio abundance for H‘(H,O)JO: of 30 is estimated at 85 km. A rate coefficient, k, = 2.2 x 10m9cm’ s-’ (Fehsenfeld et al., 1981), a recombination coefficient, (Y= 3 X 10mhcm” ss’ (Leu et al., 1973) and a water vapor volume mixing ratio of 4 x 10m6(Arnold and Krankowsky, 1977b) were used. The observed value of 17 is in good agreement with this simple estimate. At 80 km, the abundance ratio, H’(H,O)JO;, where H’(H,O), includes all proton hydrates formed from O:, should be much larger because the abundance of electrons is much lower and therefore the recombination lifetime much larger. The estimated ion abundance ratio of 300 is much larger than the observed value of 40. Therefore, it seems that the ion at mass 64 may not be mainly 0: at this height. This argument holds even more strongly at lower heights. An alternative explanation of this ion will be discussed later.

1301

Further hydration of H+(H,O), does not appear to be efficient above 81 km. This is due to the fact that the 3-body hydration rate is too slow compared to the recombination rate. The recombination lifetime is approx. 20s. A hydration time of 660 s is estimated assuming a rate constant of 2 x 10-27(300/T)5 cm6 s-’ (Smith, 1981). Considering these time scales, the H’(H,O), to H’(H,O), ratio should be about 30 which is close to the measured value of 15. The clustering of NO’ is thought to proceed via reactions The intermediate ions, (7~(12). NO’(CO,), NO’(H,O) and NO+(H,0)2 were observed. In contrast to the 0; clustering chain, several slow ?-body processes are involved in forming proton hydrates from NO’. At heights around 85 km, where the recombination lifetime is short, the abundance of cluster ions formed from NO’ is small. The dominant product ion is NO’(H,O) whose formation involves only one 3-body process. The conversion of NO’ to NO+(H*O) around 85 km is sensitively controlled by temperature (Arnold et al., 1980). Thus, the atmospheric temperature can be inferred from the ion abundance ratio, NO’H,O/NO’. Using this method, a temperature of 215 K is obtained around 85 km, in agreement with the average temperature for the relevant season and geographic latitude (CIRA 1972 reference atmosphere). Another observed feature of the NO’ clustering chain which can be compared with model predictions is the ratio NO’(H20)/NO’(C02). Its ratio should be about 2 at 85 km also in good agreement with the observations. Therefore, from the preceeding calculations it appears that the observed positive ion composition above the cluster ion region (above 81 km) is consistent with current D-region models. Since the positive ion recombination lifetimes are longer below 81 km, slower reactions can also be important at these heights. Therefore, proton hydrates grow until they reach a size where larger clusters are no longer stable with respect to thermal decomposition. For typical winter temperatures and water concentrations, Ht(HzO), ions become unstable for n larger than 4. Again, this is consistent with the fact that mass 73 is the dominant ion at these heights. At heights below 76 km, cluster ion fragmentation becomes important. This is suggested by the increase in the H’(HZOh fractional count rate and the decrease in the 73/55 ratio. Although qualitatively these effects are expected due to the rise in temperature, they

F. ARNOLD and A. A. VIGGIANO

1302

are much larger than temperature alone can explain, invoking one to use collisional dissociation to explain the data. This becomes even more evident from the descent data (Figs. 6 and 7). In these data, H+(H20)4 is heavily depleted while H+(H*O)* becomes more abundant than Hf(HzO), at lower heights. However, the decrease of H’(H,0)2 below 77 is seen in both ascent and descent implying that this feature is real. It may reflect a further increase in the recombination lifetime. The more pronounced fragmentation observed during descent is probably due to a less favorable attitude of the rocket. In contrast to ascent, during descent the sampling cone is hit sideways by the supersonic flow resulting in a shock wave that is not attached to the sampling orifice. Therefore, on descent, the ambient ions must traverse the high density, high temperature region of the shock wave before being sampled. Consequently, more dissociation occurs on descent. In addition to the normal ions discussed above, many other ions were observed below approx. 81 km. We shall lump the ions into one group and term them non-proton hydrates. Their total fractional abundance is approx. 0.1-0.2 up to 81 km and much less above this height (Figs. 5 and 7). Total fractional count rates on ascent and descent are comparable.

85

80

F.48 DESCENT 60

55

. 2‘ 0 30 .a2 0 57 P 48

55 A 56 . 70 0 13 . 7‘ n

I 10-a

I I I ,cltll

10-Z FRACTIONAL

I I I111111

10 -1 COUNT RATE

I

100

FIG. 6. FRACTIONAL COUNT RATES vs ALTITUDE ON DESCENT(PART 1).

85

80

60

55 10-3

lo-'

10-Z

FRACTIONAL

COUNT

RATE

FIG. 7. FRACTIONAL COUNT RATES vs ALTITUDE ON DESCENTfPART 2).

Above 76 km, the ion at mass 45 is the most abundant member of this group. More massive ions (masses 64, 80, 95, and 103) also become abundant at lower heights. Other NPH’s were observed in addition to these. Particularly interesting among these are very massive ions which are most abundant between 75 and 80 km. However, accurate determination of the masses is impossible. Most of the NPH’s masses observed here have not been previously reported and their nature is unknown. Due to the limited mass resolution applied it is difficult to accurately identify these ions. However, by applying ion chemical and aeronomic arguments, some insight may be reached. First of all, it appears that the NPH’s are ambient ions, since their count rates are very similar on ascent and descent. One, also, expects that these ions are hydrated since hydration is an efficient process below 81 km. The number of Hz0 ligands usually decreases as ion size increases and therefore the PH’s (PH defined as H’(H,O).) contain an exceptionally high number of Hz0 hgands. Therefore, other ions should contain at least one less Hz0 molecule under similar conditions. Since we expect the NPH’s to be cluster ions, sampling fragmentation should be important for these ions, also. If the NPH’s are indeed hydrated ions, it may be possible to group them in individual hydrate ion

Positive and negative ions in lower ionosphere-I families containing various cores, A’. For the PH’s one or two ion species are usually dominant at each height and a similar behavior may be expected for NPH’s as well. An inspection of Table 2 suggests that the ions 27, 45, 64 and 80 may belong to one hydrate family considering a mass uncertainty of 2 2. As mentioned before, the ion at mass 64 does not appear to be 0; at heights below 80 km. More likely, this ion is a hydrate of the ion at mass 45. Another feature worth mentioning is the shift towards lighter NPH’s on descent, analogous to the behavior of the PH’s, due to stronger ion fragmentation. This conclusion is supported by the observation of very light NPH’s at the lowest heights where data was obtained during ascent. Now we shall examine whether the height variation of the NPH’s contains information on their origin. Since primary ions are rapidly converted to PH’s below 81 km, it is unlikely that the NPH’s represent intermediates in this chain. Rather, it is more probable that the NPH’s are formed from PH’s via reactions with trace gases. The major loss of NPH’s would then be recombination with negative charges. This implies that the trace gases must have proton affinities larger than that of H20 (170 kcal/mole-‘) or alternatively, ionization potentials lower than the neutralization energy of HsO+ (6.2 eV). Assuming the above hypothesis is correct, a simple steady state model can be used to derive concentrations of the trace gases involved. Using a simple steady state treatment analogous to that discussed by Arnold et al. (1978) one derives the concentrations given in Fig. 8. A common rate constant of 2 X 10m9cm3 s-’ and a recombination coefficient of 7 x lo-* cm3 s-’ (Smith and Church, 1977) were used in the calculations. The trace gas densities derived are generally around l-4 x 10“ cmm3 throughout the height region. This implies that the total volume mixing ratio decreases exponentially with decreasing altitude. This behavior suggests that the reactant trace gases do not originate in the lower atmosphere but are rather mixed downwards from the region above 81 km. Additionally, it implies that these gases are removed while they mix downwards. In summary, the general conclusions regarding the nature of the newly detected NPH’s are: (a) The NPH’s are cluster ions which undergo partial fragmentation during sampling. (b) The NPH’s are probably formed from PH’s by trace gas reactions. (c) The reactant trace gases must have proton

60 -

:70-

x

2

-

F

A

;1

0

60 -

t 50

0 [A]

ASCENT

0 [Al

DESCENT

A [Bl

DESCENT

I 103

104

NUMBER

FIG. 8.

d

I 1 I ,,,,,I

I I / llll,l

DENSITY

,I

106

105

(~rn-~)

CONCENTRATION OF TRACE GASES THAT NON PROTON HYDRATESVS ALTITUDE.

LEAD TO

affinities sufficiently larger than 170 kcal mole-’ or ionization potentials lower than 6.2 eV. (d) The reactant trace gases have a total concentration of approx. l-4 x lo’4 cmm3 throughout the 57-81 km height range. (e) The total volume mixing ratio of these gases decreases exponentially with decreasing height, suggesting that the gases originate above 81 km and that there is a removal process for these gases in the 57-81 km height range. The characteristic properties of these gases described above suggests that they are meteoric metal or silicon compounds. These have recently been suggested by Ferguson and colleagues (Ferguson, 1978; Ferguson et al., 1981; Viggiano et al., 1982) as reaction partners for D-region ions. In particular, it was pointed out that certain silicon and metal compounds have large proton affinities and therefore should react irreversibly with the PH’s. The abundances of these compounds are not known, and no quantitative estimation of their impact on the ion composition can be made. However, an upper limit to their concentration can be made by using a one-dimensional diffusion model and neglecting heterogeneous effects. Liu and Reid (1978) used such a model for sodium concentrations. Assuming that the other metals behave in the same way, one can scale their cal-

1304

F. ARNOLDand A. A. V~GGIANO

culation upwards by 60, the ratio of the total abundance of meteoric metals (including silicon) to the meteoric abundance of sodium. Doing this calculation, one finds a total abundance of these compounds to be 6 x 10’ cmM3at 80 km. The value is much higher #an our inferred value of reactant trace gases. This at least does not rule out the possibility that these trace gases may be meteoric metals. The much lower abundance inferred from the ion composition may be due to an efficient depletion of these compounds from the gas phase. The depletion may be caused by reaction or absorption of the gas phase meteoric compounds with meteoric “smoke particles”. According to the model of Hunten et al. (1980) the lifetime of a gaseous species with respect to collision with a “smoke particle” is of the order of 10’s or one day. This is less than the typical time scale for vertical turbulent mixing through such a layer. Consequently, meteoric compounds may be heavily depleted from the gas phase and reach the lower mesosphere mostly in the form of particulate matter. An alternative removal process for gaseous compounds of meteoric origin was recently suggested by Arnold (198Ob) and discussed in some detail by Arnold and Henschen (1981). It involves reactions of these species with ions followed by polyion formation. The lifetime of a gaseous species against collision with positive ions is of the order of 10” s or 10 days around 70 km. Since this is comparable to or less than the time scale for vertical turbulent mixing, removal of meteoric compounds via ion processes may also occur. The relative importance of the “smoke particle” and ion depletion processes depends on the details of “smoke particle” and polyion formation which are not well known. Among the major meteoric compounds with high proton affinities only aluminum or silicon and their corresponding oxides and hydroxides fit the mass pattern for the one distinct group of ions observed (masses 27, 45, 64, and 80a.m.u.). An alternative to these compounds would be H’HCN but one would expect a low altitude source of HCN not a high altitude source. Accurate identification of the high mass NPH’s is impossible due to the larger mass uncertainty of these ions and the larger number of possible core ass~nments. The very massive ions observed in the 75-80 km region must have massive cores A’. They may arise froq ion reactions with neutral clusters composed of meteoric material which may A similar be very small “smoke particles”.

mechanism has been discussed in detail for massive negative ion formation (Viggiano et al., 1982). Alternatively, these massive ions may be polyions. If NPH’s indeed contain metal compounds having large proton affinities or low ionization potentials they may be stable with respect to neutralization with negative ions. They may, upon recombination, give rise to ion pairs which may then attach a third ion giving rise to polyions which may be detected by mass spectrometry (Arnold, 1980b).

SUMMARY AND CONCLUSIONS

A recent in-situ positive ion composition measurement has revealed the presence of various heretofore undetected ion species in addition to the well-known D-region positive ions. The abundance of the latter ions appear to be consistent with the present understanding of the D-region positive ion chemistry. The nature of the newly detected ions is still uncertain. The new ions became abundant at altitudes below the iedge in electron density. Since the conversion of electrons to negative ions greatly increases the lifetime of positive ions with respect to neturalization it appears likely that the newly observed ions are formed from proton hydrates reacting with trace gases having large proton affinities. The concentration of these gases is approx. constant (l-4 x lo4 cm31 in the height range 57-81 km. This implies that their volume mixing ratio decreases exponentially with decreasing altitude. This, in turn, implies the trace gases originate above 81 km and are removed while being mixed downwards. Arguments are put forward that may suggest a meteoric origin of these trace gases, building on the ideas of Ferguson and colleagues (Ferguson, 1978; Ferguson et al., 1981; and Viggiano et al., 1982). The most likely candidates are the meteoric metals (including silicon) and their oxides and hydroxides. Alternatively, it is hypothesized that positive cluster ions containing metal and silicon compounds may be stable against neutralization by negative ions. Thus, they may lead to stable ion pairs and ultimately polyions. Indeed, it is suggested that the very massive newly discovered ions may be polyions with a net positive charge. It was also mentioned that the removal of the reactant trace gases involved may occur via a gas to particle conversion. Two suggestions for a gas to particle conversion process were presented. The

Positive and negative ions in lower ionosphere-I first involved the heterogeneous meteoric “smoke particles”. (Hunten

removal by et al., 1980).

Alternatively, it was suggested that metal and silicon compounds may be removed from the gas phase via reactions following polyion formation. If these newly discovered ions are indeed formed from meteoric trace gases, one should expect that their total fractional abundance decreases below 50 km. One reason is that the meteoric compounds are further depleted from the gas phase. Lower in the stratosphere depletion becomes even more efficient since these metal compounds can adhere to the stratospheric sulfuric acid aerosol layer. Another reason for a lower fractional abundance of these ions is that the lifetime of positive ions decreases. This is caused by the increase in total ion concentration as well as an enhanced ion-ion recombination co-efficient due to 3-body effects. This makes the importance of such minor trace gases less important. Therefore it may not be surprising that no significant quantities of similar ions have been found in the middle or lower stratosphere. Further insight into the nature of mesospheric non proton hydrate ions requires in situ ion composition measurements of even higher sensitivity and mass resolution. Furthermore, laboratory measurements of ion-molecule reactions involving meteoric compounds may add to our knowledge about these ions. Acknowledgements-We are grateful to E. E. Ferguson for stimulating discussions and to E. Thrane, M. Friedrich and K. M. Torkar for supplying unpublished material. Part of this project was funded by the Bundesministerium fur Forschung und Technologie through DFVLR.

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(1980) Strong temperature control of the ionospheric D-region: Evidence from in situ ion composition measurements. J. atmos. terr. Phys. 42, 249. Arnold, F. and Krankowsky, D. (1979) Mid latitude lower ionosphere structure and composition measurements during winter. J. atmos. terr. Phys. 41, 1127. Arnold, F., Krankowsky, D., Marien, K. H. and Joos, W. (1977a) A mass spectrometer probe for composition and structure analysis of the middle atmosphere plasma and neutral gas. J. Geophys. 44, 125. Anrold, F. and Krankowsky, D. (1977b) Ion composition and electron- and ion loss processes in the Earth’s atmosphere, in Dynamical and Chemical Coupling of the Ionized and Neutral Atmosohere (Edited bv Gnandal, B. and Holtet, J.), p. 93. keidel,‘Dordrecht. Arnold, F., Kissel, J., Wieder, H. and Zahringer, J. (1971) Negative ions in the lower ionosphere: A mass spectrometric measurement. J. atmos. terr. Phys. 33, 1669. CIRA (1972) COSPAR Int. Ref. Atmosphere (North Holland, Amsterdam). Fehsenfeld, F. C., Moseman, M. and Ferguson, E. E. (1971) Ion molecule reactions in the O&H20 system. J. them. Phys. 55, 2115. Ferguson, E. E., Fahey, D. W., Fehsenfeld, F. C. and Albritton, D. L. (1981) Silicon ion chemistry in the ionosphere. Planet. Space Sci. 29, 307. Ferguson, E. E. (1979) Kinetics of Zon Molecule Reactions (Edited bv Ausloos. P.). Plenum Press. Ferguson, E. E., Pehsenfeld, F. C. and Albritton, D. L. (1979) Gas Phase Zon Chemistry (Edited by Bowers, M.), Vol. 1. Academic Press. Ferguson, E. E. (1978) Sodium hydroxide ions in the stratosphere. Geophys. Res. Lett. 5, 1035. Ferguson, E. E. (1974) Laboratory measurements of ionospheric ion-molecule reaction rates. Reo. Geophys. 12, 703. Hunten, D. M., Turco, R. P. and Toon, 0. B. (1980) Smoke and dust particles of meteoric origin in the mesopause and stratosphere. J. atmos. SciI37, 1342. Leu, M. T., Biondi, M. A. and Johnson. R. (1973) Measurements of the recombination of electrons with H,O+. (H,O),-series ions. Phys. Reo. A. 7, 292. Liu, S. and Reid, G. C. (1979) Sodium and other minor constituents of meteoric origin in the atmosphere. Geophys. Res. Lett. 6, 282.

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