Airborne mass spectrometers: four decades of atmospheric and space research at the Air Force Research Laboratory

JOURNAL OF MASS SPECTROMETRYJ. Mass Spectrom. 34, 1107–1129 (1999)

SPECIAL FEATURE:PERSPECTIVE

Airborne Mass Spectrometers: Four Decades ofAtmospheric and Space Research at the AirForce Research Laboratory

A. A. Viggiano* and D. E. HuntonAir Force Research Laboratory, Space Vehicles Directorate, 29 Randolph Road, Hanscom AFB, Bedford, Massachusetts01731-3010, USA

Mass spectrometry is a versatile research tool that has proved to be extremely useful for exploring thefundamental nature of the earth’s atmosphere and ionosphere and in helping to solve operational problemsfacing the Air Force and the Department of Defense. In the past 40 years, our research group at the AirForce Research Laboratory has flown quadrupole mass spectrometers of many designs on nearly 100 soundingrockets, nine satellites, three Space Shuttles and many missions of high-altitude research aircraft and balloons.We have also used our instruments in ground-based investigations of rocket and jet engine exhaust, combustionchemistry and microwave breakdown chemistry. This paper is a review of the instrumentation and techniquesneeded for space research, a summary of the results from many of the experiments, and an introduction to thebroad field of atmospheric and space mass spectrometry in general. Copyright 1999 John Wiley & Sons,Ltd.

KEYWORDS: airborne mass spectrometers; atmospheric research; space research

INTRODUCTION

The Russian Sputnik satellite flights in the late 1950sshocked US funding agencies into accelerating theirefforts to explore and exploit space. Along with NASA,other agencies in the Department of Defense and theEuropean Space Agencies, the US Air Force became amajor contributor to Western space research. Together,these agencies created a remarkable climate of abun-dant funding, competition and innovation. Then, as now,the Air Force was concerned about the impact of thenatural environment on sensors, aircraft systems andweapons. For example, Air Force radio operators knewthat transmissions were sometimes clearer than at othertimes and that sunspots had something to do with theproblem. The details of how the ionosphere controlledradio wave propagation and the role of the coupledsun–magnetosphere–ionosphere system in governing theionospheric reflective layers were not understood at all.The Air Force invested heavily in research on the basicproperties of near-earth space. It used that knowledge topredict the performance of its equipment and operationsand to design new systems for the ‘Space Age.’

The first mass spectrometer measurements of the com-position, chemistry and structure of the earth’s atmosphere

* Correspondence to: A. A. Viggiano, Air Force Research Labora-tory, Space Vehicles Directorate, 29 Randolph Road, Hanscom AFB,Bedford, Massachusetts 01731-3010, USA.

and near-earth space took place in the late 1950s and early1960s.1–4 Dr Rocco Narcisi and his colleagues at the AirForce Cambridge Research Laboratories at Hanscom AirForce Base near Boston, MA, were among those early pio-neers in space and atmospheric mass spectrometry,5 real-izing the great potential of the newly described quadrupolemass spectrometer.6 Their early research was focused ondeveloping a compact and rugged mass spectrometer pay-load for sounding rockets. They used it to understand theion composition of the earth’s ionosphere and the role ofion chemistry in controlling the properties of the iono-sphere. In addition to learning the identities of the ionsin the various regions of the ionosphere, much researchwas directed toward understanding the production and lossmechanisms of the ions. Numerous surprises came fromthese studies. The space experiments helped to spawnsome pioneering measurements in laboratory ion kinetics,in particular the flowing afterglow technique.7–10

After many details of the positive ion composition ofthe ionosphere were understood, attention began to shift tounderstanding the negative ion composition of the lowerionosphere. To this day many details of the negative ionchemistry are not understood even though the researchstarted in the late 1960s.

When the knowledge base of the region explored bysounding rockets was sufficient to meet perceived mili-tary needs, attention was focused on mass spectrometryfrom balloons, airplanes and satellites. By the mid-1980smeasurements of atmospheric ion composition had beenmade from ground level to space, including planetaryatmospheres and comets. Our group was not involved in

CCC 1076–5174/99/111107–23 $17.50 Received 10 June 1999Copyright 1999 John Wiley & Sons, Ltd. Accepted 24 August 1999

1108 A. A. VIGGIANO AND D. E. HUNTON

all of these measurements but did participate in a widevariety. These measurements led to many new discover-ies and a fairly detailed picture of the ion chemistry ofthe atmosphere. For instance, it was found that the ionchemistry of the troposphere and stratosphere simplifiesinto acid–base chemistry approximately 1 ms after ioncreation.11 This holds for positive and negative ion chem-istry. Neutral mass spectrometry measurements were alsomade to understand some of the minor species in the upperatmosphere.

Mass spectrometers were flown on the Space Shuttlenot only to understand the neutral and ion composition inspace but also to monitor the outgassing around the shut-tle. Chemical releases were performed to create regions ofincreased or decreased ionization, and mass spectrometerswere used to follow the changes. A very active currentfield is in situ chemical ionization mass spectrometry(CIMS) to monitor trace compounds in the atmosphere.Variations of this technique include active CIMS wherereactant ions are created in an ion source and allowedto react with trace species, passive CIMS where ambientions are used as reactants, and titration of specific reactantneutrals.

Our group, now part of the Air Force Research Lab-oratory (AFRL) at Hanscom Air Force Base, has beeninvolved in mass spectrometry of the earth’s atmospheresince the earliest days of the field, i.e. for almost fourdecades. This paper reviews the program at our labora-tory. Changes in technology and application have led us topursue many aspects of the problems associated with thisfield. While we have almost exclusively used quadrupolemass spectrometers, the design of the mass spectrometerhas varied at least as often as our laboratory has changednames. Throughout this paper, we refer to our laboratoryby its current name, AFRL, even though it was variouslyknown as the Air Force Cambridge Research Laborato-ries, the Air Force Geophysics Laboratory and the AirForce Phillips Laboratory. We shall emphasize changes ininstrumentation that led to new discoveries by enablingnew regions of the atmosphere to be probed or previ-ously studied regions to be probed in new ways. Sinceour involvement in this field spans the entire history ofthis field, this paper also serves as an introduction to thefield in general.

POSITIVE ION MEASUREMENTS IN THELOWER IONOSPHERE: EARLY 1960S TOEARLY 1980S

Our field mass spectrometry program started at whatwas then the Air Force Cambridge Research Laboratoriesunder the direction of Rocco Narcisi in 1961. At that time,very few measurements had been made of the ion com-position of the upper atmosphere, and no measurementsof the ion composition below 90 km had been made.One of the principal requirements in making low-altitudemeasurements was the development of a lightweight, high-speed vacuum pump capable of rocket flight. Pumping wasneeded to separate the small concentration of ions fromthe neutral gas, and also to meet the need of all massspectrometers to run at low pressure. The ion concentra-tion at 90 km is about 1 ppb and the relative abundance

decreases with decreasing altitude, although the absoluteconcentrations of ions vary only by about an order ofmagnitude from 90 km to the ground.12

The importance of the pumping system is indicatedby the fact that the first paper in this field from ourgroup described such a pump.13 The choice was a liquidnitrogen-cooled zeolite adsorption pump. Figure 1 showsa schematic of the first mass spectrometer flown bythe AFRL group. The pump consisted of nested conicalsections coated with zeolite crumbs graded between sizes30 and 42 mesh. Another very important requirement wasto keep the weight and volume small while maintainingstructural rigidity, extremely important parameters forrocket-borne instruments that are usually neglected indesigns of laboratory instruments. This pump needed to betested on a shake table to ensure that no zeolite dust wouldcome off during flight. The Dewar was also of noveldesign. It was a triple-walled container where the twoouter walls acted as the insulating layer and the inner wallsserved as the liquid N2 reservoir which held 275 cm3.While this pump may seem crude by modern standards, itneeded to pump only 2 cm3 of air in 1 min and maintaina pressure of 10�3 Torr (1 TorrD 133.3 Pa). Laboratorytests indicated that the pump could handle 10 times therequired gas load.

AFRL’s use of the quadrupole system for remote mea-surements came only a few years after the original paperby Paul and colleagues describing the quadrupolar, r.f.electric field mass filter.6 Such fast adoption of the com-pact quadrupole design is testament to its suitability forrocket payloads. The quadrupole system was fabricated atthe Bell and Howell Research Center and was modified forspace use by AFRL.14,15 Table 1 lists the main operatingparameters of the flight quadrupole. Much of the instru-ment’s original performance had to be sacrificed in orderto make the electronics lightweight and power efficientand to ensure maximum sensitivity for the short rocketflight. The mass spectrum was scanned by varying ther.f. and d.c. voltages as an exponential function of time.This method caused the dwell time over a mass peak andmass resolution to be constant over the entire mass range.The quadrupole entrance was biased at�8 V relative tothe rocket skin to draw positive ions into the instrument.The electron multiplier amplified the current by a fac-tor of ¾2000. The current at the output of the multiplierwas read with a logarithmic electrometer with a rangeof 10�12–3ð 10�10 A. The response time of the elec-trometer was 20 ms. The short response time was neededbecause of the high speeds of the rocket. The electronicswere enclosed in a sealed can and filled with dry N2 toprevent breakdown.

For the first flights, a relatively complex sequence ofmass measurements was repeated every 2 s to insure thebest chance of detecting any signal. Total ions above mass38 were measured by turning off the d.c. component of thequadrupole field. Signal levels were highest in this high-pass filter mode. Next a mass scan from 46 to 10 amu wasdone, then the analyzer was stepped to masses 4, 2 and 1.Because the rocket traveled as fast as 2 km s�1, full massscans wasted valuable measurement time on unimportantregions of the spectrum. Mass stepping was required tomaximize the altitude resolution of the data.

The mass spectrometer was part of a complete rocketpayload that also measured total positive ion, negative ion

Copyright 1999 John Wiley & Sons, Ltd. J. Mass Spectrom. 34, 1107–1129 (1999)

AIRBORNE MASS SPECTROMETERS 1109

Figure 1. Schematic of the first pumped rocket-borne mass spectrometer flown in the ionosphere.

Table 1. Operating parameters of the first posi-tive ion mass spectrometer flown in theupper atmosphere by AFCRL15

Rod radius 0.1905 cmField radius 0.165 cmLength of field 7.62 cmRadiofrequency 6.0 MHzPeak r.f. voltage 310 VMaximum ion injection voltage 128 VInjection port diameter UnrestrictedMaximum injection angle 6.3°

Mass range 1 46 amuMass resolution (M/M) 16(half-peak at half-maximum)

and electron number densities. Data were transmitted tothe ground by a 2 W FM–FM telemetry transmitter. Themass spectrometer was sealed by a nose tip and vacuumcap that were ejected at 60 km. This containment pre-vented the pump from saturating before the measurementswere started. The ejection opened the mass spectrome-ter to the atmosphere through a small orifice (diameter0.03–0.04 in) at the tip of a 100° cone.

As we reviewed these early papers and a history ofall sounding rocket programs at the Air Force CambridgeResearch Laboratories,16 it became clear that these mea-surements were taken in the wake of Sputnik when lit-tle was known about the properties of the ionosphereand the desire to learn and the sense of urgency weregreat. Numerous mass spectrometer payloads of the type

described above were built before even one of them hada successful flight. The first spectrometer was flown on9 July 1962 and failed because the nose tip did not release.The second one was flown on the same day and also failedfor the same reason. Two weeks later a third was flownbut suffered from low sensitivity. Another three failed on25 October 1962. Finally, after several time-of-flight massspectrometers had been launched17 to measure the neutralcomposition, the first successful flight of the quadrupolepositive ion mass spectrometer occurred on 31 October1963 at local noon from Eglin Air Force Base in Florida.The Nike–Cajun rocket reached an apogee of 112 km,and data were obtained from 64 to 112 km.14,15 This flightmarked the start of the exploration of the ion chemistryof the lower atmosphere.

Part of the raw data record from this first flight isshown in Fig. 2. Both the mass spectra and the r.f. monitorscan are shown. The magnetometer data shown wereneeded for determining the rocket attitude. The earlypapers in this field typically showed such raw data, inpart because the data were so new and unexpected. Thedata from all the AFRL ion mass spectrometry rocketflights have been archived in a public access World WideWeb site.18 Data such as those shown in Fig. 2 wereconverted to ion densities from measurements of the totalpositive ion density made on the same rocket and fromlaboratory and wind tunnel calibrations of the sensitivityof the mass spectrometer vs. altitude and mass. The iondensities derived from this flight between 64 and 90 kmare shown in Fig. 3. These results literally stunned theaeronomers at the time, who expected to see only ions



Figure 2. Raw data from the first successful flight of a rocket-borne mass spectrometer flown in the ionosphere on 31 October, 1963.

Figure 3. Height distribution of positive ions from the first successful flight of a rocket borne mass spectrometer flown in the ionosphereon 31 October, 1963.

such as NOC and O2C. While theseare the major ions

at higheraltitudes,the first flight experimentshowedthationswith masses19 and37 becomethedominantspeciesobservedbelow about80 km. (Later experimentsshowedthat ions with masseshigher than the rangeof the firstinstrumentarethemostabundantionsbelow75 km.) This

resultwasobviouslyunexpected,otherwisea wider massrangewould havebeenselectedfor the first flights. Datatakenon theascentanddescentof therocketagreedfairlywell. While in retrospectit may appearobviousthat theunexpectedions arethe waterclusterions,H3OC.H2O/n,now knownto beprevalentthroughoutmuchof the lower

Copyright 1999JohnWiley & Sons,Ltd. J. MassSpectrom. 34, 1107–1129(1999)


atmosphere,19 it took years of corroborating measurementsand laboratory work to understand the formation of theseions. The issue of contamination arose often in the earlydiscussions.

Above 82 km, the major ions were NOC and O2C,

although accurate densities for these species could notbe calculated since the signals were saturated owing tothe limited range of the electrometer in this first flight.Another set of ions appeared at masses 23, 24, 25, 26 and40 amu above 83 km. These ions formed a layer with apeak at 95 km and a vertical width of about 10 km. Asecondary maximum was observed above 105 km. Theratios of the intensities for several of these unknownspecies were the same as those for metal isotopes. On thisbasis, the ions were identified as NaC (23 amu), MgC (24,25 and 26 amu) and CaC (40 amu). These ions had beenfirst seen by Istomin earlier that year at higher altitudeswhere pumped mass spectrometers were not needed.20

The researchers speculated that the metallic ions were ofmeteoric origin, a fact since proven.

During the next several years, numerous improvementsto the instrument were made. Many of the improvementswere in the electronics. The r.f. frequency oscillator was apush-pull Hartley design able to run between 4 and 7 MHzwith a frequency tolerance of 2%. In addition, the massrange was extended to 86 amu by 1965, and the rangeof the electrometer was increased to five decades with anaccuracy of 10%. This improved the dynamic range by afactor of 300 and allowed ion densities between 1 and 105

ions cm�3 to be measured. A 10-stage secondary emissionmultiplier was used as the detector and the anode ran at200 V. The multiplier voltage was regulated to 0.3% toreduce variations in gain to less than 10%. The changepermitted more precise measurements of small variationsin ion density. Many of the original circuit schematics aregiven in a 1966 AFRCL report.21 The instrument was alsorepackaged so that it was suitable for satellite operation.An optional electron impact ion source, modeled on thedesign of Brubacker and Weins,22 was made as transpar-ent as possible to allow direct ion detection with the sameinstrument.21

During the rest of the 1960s, numerous rockets werelaunched with the emphasis on making measurementsof as many atmospheric conditions as possible. Theseincluded meteor showers, aurora, solar eclipses, sun-rise and sunset and latitudinal variations.23–28 The mea-surements clearly confirmed the observations of protonhydrates and, in fact, hydrates of NOC also. In order torule out contamination, the instruments had the cyrogenicpump redesigned to eliminate all cold surfaces from thesampling area. While many of the findings were extremelyimportant for ionospheric research, they are beyond thescope of the present review.

As discussed previously, the discovery that theH3OC.H2O/n series comprised the main ion speciesbelow 80–85 km was the most surprising result from theearly flights. Because that result was not consistent withthe then current understanding of the atmosphere’s ionchemistry, it prompted many laboratory studies. The mainion observed was then D 1 species, although smalleramounts ofn D 0 and 2 were routinely observed.13 Fouryears after the original data were obtained and after muchwork in both the laboratory and field, it still was notpossible to be certain that these ions did not result from

contamination. For this reason, most of the early papersincluded photographs of the original strip charts of themass spectra and mass scan monitor, such as shownin Fig. 2. By a process of elimination, it was finallyconcluded that the ions were ambient. Processes ruledout included direct solar ionization of the contaminationcloud (because the rocket travels too fast) and ionizationby the shock wave of the rocket. The facts that ascent anddescent data agreed and the results were independent ofion sampling and electric field geometry strongly favoredthe ambient explanation.

The dominance of these ions extended to all atmo-spheric conditions measured except during a solar cosmicray event, when NOC was found to be the primary iondown to 70 km and O2C was the second most abun-dant ion. Ionization in the upper mesosphere results fromextreme UV ionization of NO and O2, the former byLyman ˛ radiation. This is why O2C and NOC wereexpected to be dominant.19,29 Finally, in 1969 and 1970,two groups independently solved the problem of conver-sion of O2

C to H3OC.H2O/n.30–34 The reaction scheme is

O2C CO2CM . O2,N2/ ��! O4

C CM

O4C C H2O��! O2

C.H2O/CO2

O2C.H2O/C H2O��! H3OC COHCO2

��! H3OC.OH/CO2

H3OC.OH/C H2O��! H3OC.H2O/COH

H3OC.H2O/C H2OCM ��! H3OC.H2O/2 CM

An alternative route to H3OC.H2O/2 was also proposedinvolving clustering of O2 to H3OC.OH/ followed by twoswitching reactions with H2O. While much effort wentinto understanding the conversion to proton hydrates, thecomplexity and the fact that ion–molecule kinetics wasalso an emerging field prevented the above mechanismsfrom being discovered earlier. Model calculations showedthat O2

C could be converted to the water cluster ions inseconds at 80 km.28 The time-scale decreases substantiallywith decreasing altitude, and the reactions are completein less than 1 ms in the lower atmosphere. The steepchangeover in the ions is a result of decreasing water con-centration and the fact that three-body reactions depend onthe neutral number density squared.

Understanding the conversion of NOC to protonhydrates remained. The first important breakthrough camewhen the NOAA group found the following reaction to befast:30

NOC.H2O/3 C H2O��! H3OC.H2O/2 C HNO2

While this at least provided a route to proton hydratesfrom NOC, the clustering chemistry is too slow. In partic-ular, the rate-determining step for producing NOC.H2O/is slower than the ion–electron recombination rate indestroying NOC. The solution to the problem came fol-lowing realization that a stepwise process could addsuccesively more stable ligands to NOC. The reactionsequence is35

NOC C N2CM ��⇀↽�� NOC.N2/CM

NOC.N2/C CO2 ��! NOC.CO2/C N2

NOC.CO2/C H2O��! NOC.H2O/C CO2.



Even though N2 bonds only weakly to NOC, it is 106

times more abundant than H2O and the upper mesosphereis cold, facts that make the above mechanism fasterthan direct hydration. Higher hydrates cluster by similarsequences. Before settling on the above mechanisms,numerous other suggestions for the conversion to protonhydrates were postulated, including direct ionization ofneutral water clusters.

Another fascinating discovery was the observation thatall cluster ions completely disappeared and atomic metalions appeared in their place within about a 2 km range at85 km. An example of the metal layer taken during theLeonid meteor shower is shown in Fig. 4.24 All metal ionsare lumped together as a single species and plotted as apercentage of all ions. A very intense layer is observed at98 km with a half-width of only 1 km. In contrast to thesedense layers, metal ions are more commonly observed inmuch smaller abundances over a wider altitude range. This

experiment helped to prove that the metal ion layer wasdue to ablation of meteors in the upper atmosphere.

Normally, metal ions are observed between 82 and120 km and occasionally as high as 140 km.24,26,27 Inmost flights a layer of metal ions 5–10 km thick wasdetected near 93 km and was comprised of SiC, MgCand FeC with smaller amounts of NaC, AlC, CaC andNiC and occasionally also other ions. The relative abun-dances of the metals are approximately those found inchondrite meteorites. Having a sharp peak superimposedon the broader profile is typical. In Fig. 4, there is a verysharp peak at 98 km. The altitude profiles of the vari-ous metal ions are similar although the abundances varygreatly. The high-altitude layers are less regular and arecomposed mainly of SiC, MgC and FeC. Metal layerswere found to be enhanced during periods of high mete-orite flux and an ionospheric condition called sporadicE.24 The metal ions observed all have low recombination

Figure 4. An example of metal ion layers taken on 16 November, 1965. All metal ions are lumped into one parameter and shown as apercentage of total ions.



energies and therefore diatomic ions will charge transferrapidly to metal atoms.36,37 Because the metal ions aremonoatomic, they recombine very slowly with electrons.This occasionally causes the total charged particle con-centration in the metal layer(s) to be substantially greaterthan the regions just above and below.

The expected ions O2C and NOC were found to dom-inate higher altitude spectra, except in some cases whenmetal ions are the most abundant. The measurements alsoshowed that the lower ionospheric ledge (sharp change intotal concentration with altitude) was due to the appear-ance of NOC at that altitude. During daylight, O2C wassignificantly more abundant than at night. By 1971, therewere enough rocket-borne mass spectrometer measure-ments to show that ionospheric models worked fairly wellexcept for some of the lower altitude hydrates and themetal layers.28 While the details relating to these ions areimportant for a full understanding of the ionosphere, theyare beyond the scope of the present review. Subsequently,many of the observations discussed above were borne outby measurements made by other groups.38

Unintentional dissociation of some, but not all, of theweakly bound cluster ions occurred in the instrumentsdescribed above. The blunt front end of the earlier instru-ments created a detached shock wave between the sam-pling orifice and the undisturbed atmosphere. Weaklybound cluster ions broke apart when they passed throughthe shock. Therefore, near the end of the 1970s, the basicrocket instrument was redesigned in order to improve thesampling of cluster ions.39,40 Several major changes wereneeded and a schematic of the redesigned instrument isshown in Fig. 5. A conical sampling orifice was used toattach the shock wave to the sampling orifice. This, in turn,meant that the vacuum pump had to be redesigned. Thenew pump used liquid helium to cool the zeolite absorp-tion surfaces, resulting in a pumping speed 10 times higherthan the liquid nitrogen version.

The vacuum chamber was extended through addition ofa conical section. The cone had a 70° total angle and a

1 mm hole at the apex for sampling. Slicing and electri-cally insulating the cone 2.5 in from the apex allowed fora sampling bias to be added. For positive ions, the typicalbias was�4 or�10 V. For negative ions larger voltages,between 5.8 and 40 V, were found to maximize the totalsignal.

Circuits were also modified in a number of areas. Ther.f. generator was designed with the following in mind:higher output voltages, improved efficiency, improvedstability with the goal of better mass identification, betterr.f. shielding and smaller size. The detector electronicswere redesigned to permit operation over a wider dynamicrange. Digital processing was incorporated into the massprogramming capabilities, allowing user-defined programsfor the first time. The data were transmitted to the grounddigitally by pulse coding (PCM) telemetry.

Numerous changes to the rocket systems were alsomade. Attachment of the shock meant that the alignmentof the sampling cone with the direction of flight was alsoneeded. A three axis, gyro-referenced, gas-jet activatedorientation system was used. This required a new attitudecontrol system on the rocket. Complementary instrumenta-tion such as a Gerdien condenser and a retarding potentialanalyzer became part of the payload. The rocket was now‘despun’ by discarding weighted cables. A recovery sys-tem was also used so that a new instrument was not neededfor each flight.

New instruments incorporating these modifications wereused for only a few flight experiments before the untimelydeath of the founder of our group, Dr Rocco Narcisi, in1983. These flights did show that the redesign had beenat least partially successful. Compared with the flightsdiscussed above, the positive ion data showed heavierclusters. An example of the cluster ion data is shown inFig. 6. No new series of ions were observed. The sharpledge at the lower altitudes was due to a decrease ofthe instrument’s sensitivity caused by pumping limitationsrather than to actual ionospheric structure. Larger hydrate

Figure 5. Schematic of the redesigned instrument to measured cluster ions.



Figure 6. Height distribution of positive cluster ions taken with the redesigned rocket-borne mass spectrometer flown on 12 June, 1973.

distributionshave beenseenby other groups,in partic-ular massiveproton hydrateswith greaterthan 20 waterligandshavebeenseenin a very narrow layer during anoctilucentcloud event.41

NEGATIVE IONS IN THE IONOSPHERE: LATE1960STO EARLY 1970S

Although our group and othershad madegreatprogressin probingthe positiveion compositionof the ionosphereby the closeof the 1960s,measurementsof negativeioncompositionin the lower ionosphereproved to be morechallenging. Sampling negative ions is a problem fortwo reasons.First, the entire rocket floats at a slightlynegative potential becauseof its interactionswith theambient plasma, and the resulting electric field tends

to repel the negativeions. Second,if a positive draw-in potential is applied locally to the massspectrometer,electronsareattractedto the samplingorifice alongwiththe negativeions. In order to obtain negativeion data,both the samplinggeometryandthe detectionelectronicswere modified from thosedescribedabove.42 As shownin Fig. 7, the main changein the mechanicaldesignwasto place an electronmask over the ion samplingplate.By biasingthe plate positive and the masknegative,thegoal was to reducethe numberof electronshitting thesamplingplateandto minimizetheamountof charging ofthe rocket.Although this complicatedsamplingsituationwasneverfully understood,especiallywhenspacechargeeffects and collisions were considered,the addition ofthe mask did make the instrument more sensitive tonegativeions.

Becausethe negativecharge in the D-region of theionosphereis mainly in the form of electronsand the

Figure 7. Schematic of the mask used to increase the negative ion signal on the rocket-borne instrument.



total charge is zero, the negative ion concentration ismuch smaller than the positive ion concentration. A moresensitive detection system was required, and the analogsystem was replaced with pulse counting electronics. Theaccumulated counts from the multiplier were recordedevery 10 ms, an integration time still in use in our currentfield instruments. Three signal ranges were used, namely0–15, 2–27 and 16–1023 counts per 10 ms, or an overallrange of 0–102 300 counts s�1. The resolution of theinstrument was approximately 32 (M/M half-peak athalf-maximum).

The history of negative ion mass spectrometry in theionosphere starts as early as 1958 when a non-pumpedmass spectrometer was flown to high altitude on a sound-ing rocket.1 In reviewing our own archives, we foundthat a negative ion signal was detected for the first timeon 4 December 1967, during a flight from Ft Churchill,Canada, although in the total transmission mode only. Apeak in total negative ion density was detected at about120 km although these data had a low signal-to-noise ratioand it was difficult to be certain if the signals were real.

The first flight experiment with mass resolved negativeion spectra was performed on 13 August 1969, also fromFt Churchill. Although the count rates were generallylow, the results proved interesting since this was the firstmass spectrum of negative ions in the D-region of theionosphere. The mass identifications were O2

� (32) andCl� (35 and 37 amu). The ions originally thought to be atmasses 61 and 63 were more probably 60 and 62, namelyCO3

� and NO3�. In the higher height range between 90

and 114.5 km, the main ions observed were O�, Cl� andNO2

�. The most striking aspect of these data was thations with masses>78 accounted for the majority of ionsin certain regions, a point we shall return to later.

Based on these first results, the instrument was config-ured to mass analyze the heavier ions. During the nextflight, on 11 October 1969, the ion signal levels weremuch higher, and the major ions were a sequence of ionsseparated by 18 u, presumably H2O clusters around a core.Unfortunately, the mass resolution did not permit unam-biguous identification. The most likely candidates for thecore ion were CO3� or NO3

�. The hydration sequenceextended up to five ligands. A particularly important find-ing was a two orders of magnitude decrease in the negativeion concentration from 90 to 92 km. Decreases of thistype are explained below. The results from these first suc-cessful negative ion measurements were not reported until1971.42

The next flight experiments were performed during apolar cap absorption (PCA) event on 3 November 1969,where one of the more intriguing aspects of the negativeions measurements in the D-region was discovered.43 Ionsheavier than 150 u were sometimes the most abundantions, particularly in a layer from 82 to 92 km. This heavyion layer was fairly intense, at least 10 times larger thanthe signal level of the next most abundant ion (¾134 u,part of the series of ions at XO3�.H2O/n.X D C or N)) atthe peak of the layer. Remarkably, the exact identity andsource of the heavy negative ions are still unknown.

In the meantime, the first negative ion data from theHeidelberg group were taken on 22 March 1970.44 Theyfound many of the same ions and were able to determinethe mass numbers within a single mass unit. The ions seenat 60–62 u were actually three ions, CO3

�, HCO3� and

NO3�. They also saw CO4� at 76 amu and unidentified

ions at 111 and 125 amu, and confirmed the presence ofCl� ions at 35 and 37 amu.

The observation of Cl� was unexpected at the time.The Heidelberg group proposed that the ions were due toelectron attachment to a Cl-containing compound with anabundance of about 105 molecules cm�3, a value muchlarger than the expected sources of chlorine in this part ofthe atmosphere. However, shortly after this discovery, theimportance of Cl-containing compounds in the catalyticdestruction of ozone was first proposed by Molina andRowland.45 Later modeling calculations agreed with thetotal chlorine concentrations in this altitude range calcu-lated by the Heidelberg group.38

The negative ion chemistry of the D-region has beendescribed in detail.35,46 Negative ions are formed by elec-tron attachment to O2. Low-energy electrons react to formO2� in a three-body process while high-energy electrons

form O� by dissociative attachment. In general, the for-mer process dominates. O� is also formed by dissociativeattachment to O3. Reactions with O3 and O2 convert theprimary ions to O3

� and O4�. These ions then react with

CO2 to form CO3� and CO4

�, respectively. The reactionof CO4

� with O forms CO3�. This chemistry is rather

fast. Associative detachment reactions of ions anywherealong this chain with atomic neutrals present in smallconcentrations destroys negative ions and is responsiblefor the ledge in the negative ion composition observedat 75–80 km. Two important examples of associativedetachment reactions are

CO3� CO��! O2 C CO2C e

andO2� CO��! O3 C e

Associative detachment reactions thus prevent large con-centrations of negative ions in the upper D-region. Fur-thermore, the production rate of negative ions, whichdepends on the square of the neutral density, falls offrapidly with altitude. CO3� is slowly converted to NO2�by reaction with NO. NO2� reacts with O3 to form thevery stable NO3�. The reaction to form NO2� has a smallrate constant and the NO concentration is low, producing abottleneck in the chemistry. Once formed, NO3

� is verystable and does not undergo any reaction in the upperatmosphere, except clustering to H2O and recombinationwith positive ions. HCO3� is formed in the followingmanner: NO2

� reacts with H to form OH�, which clus-ters to CO2 to form HCO3

�. Qualitatively, these reactionsexplain the chemistry although the uncertainties in therates, the scarcity ofin situ data and unknown minor neu-tral concentrations prevent a quantitative understanding.Interestingly, there was little impetus to study the chem-istry in more detail once the rates of ion–ion recombina-tion reactions were found to be approximately independentof the ion identity.47,48 A generic recombination rate wassufficient for further model development.

One of the most fascinating aspects of the negative iondata from this period is the observation of very heavyions of unknown mass and origin. A heavy ion layer hasbeen seen multiple times in the 80–90 km range38,43 usingquadrupole instruments in the ‘total ions’ mode. As earlyas 1969, it was determined that ions greater than 150 amuaccounted for a large fraction of all ions in this height



range. In 1982, Schulte and Arnold49 flew an instrumentthat determined that almost all ions were greater than400 amu in a narrow layer and that many ions were greaterthan 473 amu. One speculation is that the very heavynegative ions might be due to electron attachment to smallpieces of ablated meteorite called ‘meteoritic smoke’particles.50 If electron attachment to smoke particles isresponsible for these species, as few as 103 particles cm�3

are needed to explain the observations.51

E- AND F-REGION POSITIVE IONCOMPOSITION: LATE 1960S TO EARLY 1980S

Most of the research that we have described thus faraddressed the D-region. In this lowest region of theionosphere, below about 90 km, the ion chemistry andphotochemistry are complex. Ion composition is stronglyaffected by the relatively high pressure, by association orclustering reactions and by the presence of trace gases.In the E-region, above around 100 km, the ion chemistrysimplifies in part because three-body reactions becomeunimportant. The positive ions produced directly by pho-toionization of the major neutral species are N2

C, O2C,

NC, OC, HeC and HC. Subsequent reactions of these ionslead to NOC and O2

C as the terminal ions in the region.Interspersed with the D-region experiments, our group

conducted at least a dozen rocket flights at higher altitudesto probe the dynamics of the E- and F-regions. Rocket-borne mass spectrometer experiments are slightly easier toperform at these altitudes for several reasons. On-boardvacuum pumps are no longer required to maintain lowpressure for the quadrupole and multiplier. The instru-ments were mounted behind the deployable nose cone ofthe rocket, as in the lower altitude payloads, but the coni-cal sampling apertures designed to attach the shock waves,were no longer required. The front of the instrument typi-cally consisted of a flat aperture plate with a small orifice.The plate was biased at�8 to�10 V to attract the posi-tive ions into the mass filter. The current collected on theaperture plate was a measure of relative total ion density.A deployable vacuum cap sealed the instrument on theground and during the boost phase of the flight. The instru-ments were either evacuated and held at high vacuumduring launch or were backfilled with dry nitrogen. Thecap was held in place by structures within the nose coneand was pulled off at the same time the nose cone wasjettisoned, usually at an altitude above 75 km. The instru-ment was then given several seconds to reach low pressurebefore the mass filter and multiplier were turned on above110–120 km. As before, the self-contained payloads alsocarried batteries, command timers, telemetry transmittersand attitude control systems. Early payloads frequentlycontained only the mass spectrometers. As knowledge ofthe upper atmosphere increased, so did the sophisticationof the payloads. By the mid-1970s, the mass spectrome-ters were often part of large payloads that also measuredelectron densities and fluxes, electron energy distributions,atmospheric light emissions and other relevant parameters.Figure 8 shows a typical high-altitude instrument payload.

One extremely important aspect of ion compositionmeasurements of the atmosphere is the ability to deriveconcentrations of the trace neutrals involved in production

and loss chemistry of the ions—frequently the most sen-sitive method for determining neutral concentrations. Animportant example is extracting concentrations of nitricoxide from measured ratios of NOC and O2

C ion concen-trations. Our group devoted considerable attention to thederivation of NO concentrations from the ionospheric pos-itive ion composition measurements. The simplest modelfor deriving NO concentrations was based on two sourcereactions and two loss reactions.52 Photoionization ofnitric oxide by absorption of Lyman radiation is themost important source of ionization in the D-region. In theE-region, all ionization initially produces O2C, which thenis converted into NOC in a charge-transfer reaction withneutral NO. Both NOC and O2

C are lost by ion–electronrecombination, and all other chemistry is ignored. Aftersubstituting the rate constants for the above processes, thesteady-state assumption yields the NO concentration as52

[NO] D(

900[NOC]

[O2C]� 50

)[e]

where square brackets denote concentrations. Thus, theNO concentration is expressed in terms of the ion andelectron concentrations, all of which were measured onthe rockets, and reaction rate constants, which were deter-mined independently in laboratory experiments. Measur-ing the ambient NO concentration directly by electronimpact neutral mass spectrometry was difficult becauseion source reactions involving the major atmosphericspecies N2 and O2 also produced NOC.53 As a result,the method based on ion composition measurements gavethe best estimates at the time. With time, the modeling ofthe chemistry became more detailed and the altitude rangefor which the assumptions held increased.54–59 Ion mea-surements were also used to determine water outgassingrates from high-altitude sounding rocket flights.60 Neutralcomposition determinations from ion composition mea-surements have proven valuable in the lower atmosphereto determine, for example, acetonitrile and sulfuric acidconcentrations.11

One early study of auroral atmospheric compositionindicated that NO concentrations were enhanced in theauroral E-region.61 This result called into question thevalidity of the models of high-latitude atmospheric com-position that were in use at the time, such as theCOSPAR International Reference Atmosphere (CIRA)models. Between March 1963 and March 1985, our groupflew nine rocket payloads from Poker Flat, Alaska, FtChurchill, Manitoba and Sonderstromfjord, Greenland, tomeasure the positive ion composition in a variety ofauroras.59,62,63 The techniques described above were usedto extract nitric oxide concentration profiles from theNOC/O2

C density ratio. The conclusion from the earliestpaper was that the increased NOC/O2

C ratios in an IBCClass I aurora were consistent with an increase in the NOdensity to about 2ð 108 cm�3 at 120 km.56 An alternativeexplanation was increased atomic nitrogen to 109 cm�3 atthe same altitude. As more ion data were obtained and bet-ter laboratory reaction rate measurements were available,the agreement between the neutral atmosphere models,the ion composition models and the ion measurementsimproved. In the end, the CIRA 1972 model O, O2 and N2densities were found to be suitable for computing auroralE-region ion composition. No increase in the atomic oxy-gen density was required, as had been suggested earlier.



Figure 8. Schematic of the instrument and the support payload for high-altitude measurements. The mass spectrometer was mountedunder the deployable nose cone and the Langmuir probe booms were deployed slightly behind. The power, command and telemetryelectronics, the attitude control system and the separation section are also shown. The right side of the figure shows the trajectory ofthe rocket for the PLACES barium release experiment.

NeutralNO densitiesashigh as109 cm�3 werecalculatedasauroralactivity increased.54,55,58,59 Thesedensitiesareaboutan orderof magnitudehigherthanthe undisturbed,mid-latitude,daysideionosphere.

TheF-regionis themosthighly ionizedandthehighestregionof theionosphere.Plasmadensitiesreach106 cm�3

at altitudesfrom 200 to 600 km. This region is respon-sible for most of the major effects of the ionosphereonradiowavecommunication.Lower frequencyradiowavesin theHF bandandbelowarereflectedby the ionosphere,accountingfor theability of hamradiooperatorson oppo-site sidesof the earthto talk with eachother.The iono-sphereis transparentto higher frequencies,which allowsthe very-high frequency(VHF) and ultra-high frequency(UHF) bands to be used for satellite communications.Plasmairregularitiesor disturbancesin the F-regionhaveadverseeffectson both rangesof frequencies,leadingtosignalfading,dropout, scintillation and,ultimately,com-municationinterruption.

The ion chemistry of the F-region is even simplerthan in the E-region and is dominatedby atomic oxy-gen ions, OC. As a result, rocket-borne mass spec-trometric investigationsof the F-region in our grouphave focusedon plasmadynamics,transport,and irreg-ularity formation rather than the chemistry.The PlumeExperiment(PLUMEX) rocket flights probed ion com-position in mature plasma depletionsin the equatorialionosphere.64–66 Occurring in the post-sunsetequatorialionosphere,spread-Fdepletionsare very large, unstableregionsof reducedplasmadensityextendingfrom thebot-tom to the top of the F-region.Becauseof the sharpden-sity gradientsandregionsof reducedreflectivity, spread-F causessignificant problemsfor many communicationsystems.67

In July 1979, two high-altitude rockets (apogeenear600 km) were launched from Kwajalein Atoll in theMarshall Islands as part of the PLUMEX campaign.The launch times and directionswere determinedfrom



real-time incoherent scatter radar measurements of spread-F plumes above the launch site. The instrumented rocketswere launched when the radar indicated that large spread-F plumes were accessible. The data from PLUMEX-1showed large ‘bite-outs’ in the OC altitude profile dueto the spread-F plasma depletions. The total density andthe OC concentration dropped by at least 90% comparedwith the undisturbed profiles in the parts of the trajectorythat crossed the plumes. No evidence for upwelling ofthe E-region molecular ions NOC and O2

C was observed,although such transport was expected based on mod-els of spread-F plume transport. Instead, the NC/OCratio decreased in the plumes compared with undisturbedregions at the same altitude. The smaller atomic ion ratiowas taken as the signature of the bottomside origin of therising plasma bubble.64–66

Other F-region mass spectrometry experiments wereconcerned with intentional perturbations to the ionosphere.In the PLACES (Position, Location, and CommunicationsEffects Simulations) experiments, a pair of sounding rock-ets was used to investigate releases of barium vapor intothe ionosphere. The first rocket released 48 kg of bariummetal, vaporized by a high-temperature thermite reaction,into the lower F-region at 184 km altitude.68 Approx-imately 30 min later, a second rocket carrying an ionmass spectrometer and a Langmuir probe instrument waslaunched through the earlier chemical release. The bar-ium metal vapor was rapidly photoionized by sunlightand created a layer of dense ionization around the releasepoint. The mass spectrometer detected a peak BaC num-ber density of 5ð 106 cm�3 at 162 km. It also detectedsmaller concentrations of Ba2C and CaC from the releaseand showed that the concentrations of ambient E-regionions such as NOC and O2

C were significantly depleted inthe volume affected by the release. The Langmuir Probedata showed that the edges of the release volume werehighly structured. Our group has also pursued other chem-ical release experiments.69,70

SATELLITE EXPERIMENTS: 1960S TOEARLY 1990S

Sounding rockets provide relatively easy and compara-tively inexpensive access to space. Payloads can be builtup and tested rapidly, multiple rockets can be launched ona single day, and data can be analyzed soon after the flight.The drawback to rocket flights is their short duration ofonly a few minutes. Satellite experiments are more com-plex and significantly more expensive. They require largerlaunch vehicles and more robust electronics to survivelong exposure to the harsh radiation and thermal environ-ment of space. Their advantage is the huge amount of datathat can be collected over much longer periods of time.Satellite missions are seldom planned for less than severalmonths’ duration. The Air Force Research Laboratory andits predecessor organizations have flown more than 350satellite experiments beginning with Explorer I in 1958and continuing to the present.71 The vehicles and pay-loads that included AFRL mass spectrometers are listedchronologically in Table 2.

The Air Force has the responsibility to track morethan 8000 satellites and space debris objects in orbitaround the earth. NORAD, at Cheyenne Mountain, CO,provides orbital predictions and collision probabilities tothe Air Force, NASA and other space agencies. Currentalgorithms use computer models of atmospheric density tocalculate the frictional drag on the satellites. The accuracyof the neutral density models has always been a limitationin this calculation.

In response to this need, several of the neutral massspectrometers on low-orbiting satellites measured theabsolute density of the atmosphere. The measured den-sities were correlated with position, time and solar andgeomagnetic activity and were then used to validate andimprove the models.72 The instruments on OV1-15, OV1-21, S3-1, S3-2 and ADS were combined with accelerome-ters, which measure atmospheric drag directly, to provide

Table 2. Satellite mass spectrometry payloads

Launch date Satellite name Experiments Comments

28 June 63 GRS Aerospace composition Success31 Jan. 67 OV3-5 Mass spectrometers(2) Failed to orbit

Ion density gage(3)Impedence probe

5 Dec. 67 OV3-6 Latitudinal variations in neutral andion species

Success

11 July 68 OV1-15 SPADES mass spectrometer andaccelerometer

Success

7 Aug. 71 OV1-21 Velocity mass spectrometer SuccessMagnetic mass spectrometer

1974 S3-1 Single Axis and piezoelectricaccelerometers

Success

Ionization gageNeutral Ion mass spectrometers

19 Nov. 75 S3-2 Neutral mass spectrometer SuccessIonization gagePiezoelectric Accelerometer

25 July 90 CRRES Ion mass spectrometer for LASSII Data obtained forlow-altitude chemicalreleases

27 June 94 ADS Atmospheric density massspectrometer and accelerometers

Pegasus XL launchfailure



accurate neutral density determinations.73,74 The Atmo-spheric Density Mass Spectrometer (ADMS) that was tohave flown on the ADS satellite in 1994 incorporatedseveral design improvements. These included an accom-modation chamber in front of the ion source to allow allatomic oxygen in the atmosphere to recombine on surfacesinto molecular oxygen. A beam flag was placed within theaccommodation chamber to block or admit the direct beamof atmospheric species. An ion source mode that discrim-inated against thermalized species in the accommodationsphere was also developed. The ADMS instrument wascalibrated extensively in static pressure calibration cellsand in dynamic, atomic oxygen beam facilities. Althoughthe instrument was lost along with the rest of the pay-load when the Pegasus XL launch vehicle failed to reachorbit, the program demonstrated that high-accuracy cal-ibration of mass spectrometers was a promising methodfor measuring absolute neutral density to within 5%. TheAir Force is planning a new ADS mission in the earlypart of the next decade.

The Small Scientific Satellites (S3) were a series offour satellites sponsored by the Air Force in the mid- tolate-1970s. The polar-orbiting satellites carried a varietyof instruments to measure plasma density, composition,electric fields and currents and other parameters to probethe electrodynamics of the earth’s high-latitude and auro-ral regions. The Air Force Research Laboratory built manyof the instruments, including the ion–neutral mass spec-trometers on S3-1 and S3-2. In one study of the resultsfrom S3-1, the ion composition obtained from the massspectrometer was shown to be in good agreement withapproximate indications of composition derived from animpedance probe and a retarding potential analyzer on adifferent satellite, AEROS-B.75

The most recent of the satellite programs that we havepursued was a joint Air Force–NASA research missioncalled the Combined Release and Radiation Effects Satel-lite (CRRES).76 The quadrupole ion mass spectrometer(QIMS) was similar to instruments that had been designedfor Space Shuttle use. The most interesting data that camefrom the mass spectrometer were obtained during uniquebarium and strontium chemical releases at orbital velocityin the upper ionosphere.77–81

SPACE SHUTTLE: EARLY 1980S TO EARLY1990S

Beginning in the early 1980s, the Space Shuttle offered anew avenue to space for a host of government, academicand industry payloads. No-one at the time doubted that theShuttle could deploy and retrieve satellites successfully.However, the complexity of the Shuttle raised questionsabout the suitability of the vehicle for scientific experi-ments that required a clean or undisturbed environment.Among its many components, the orbiter carried life sup-port systems, several kinds of rocket engines and powergeneration systems. It was also covered with porous, heat-absorbing tiles. All of these had the potential to releasegases or other contaminants into the environment near theShuttle and its attached payloads.

To investigate the environment induced near the Shuttleby surface outgassing and system operations, NASA flew

the Induced Environment Contamination Monitor (IECM)on the first four flights of the Shuttle.82 This group ofinstruments included a neutral mass spectrometer, pressuregauges and plasma instruments.

At the same time, the Air Force was planning to fly alarge infrared telescope in the Shuttle’s payload bay. TheCryogenic Infrared Radiance Instrumentation for Shut-tle (CIRRIS) experiment team was particularly concernedabout water vapor near the telescope. Water adsorbed onthe cryogenically cooled primary mirror would first inval-idate the careful calibrations of the focal plane detectors.If the water layer became thick enough, it would reducethe sensitivity of the instrument significantly. Further,infrared-active molecules, such as water, in the near-fieldof the telescope could cause spurious emissions or absorp-tion of radiation that might not be separable from the farfield data of interest.

The Air Force Research Laboratory assembled the Sec-ondary Experiments Test Set (SETS) pallet to address thecontamination concerns for CIRRIS. Our group built anion–neutral mass spectrometer that was very similar tothe high-altitude rocket instruments. As for the other F-region instruments, no vacuum pumps were required. Thespectrometer was sealed with an O-ring vacuum cap thatwas retracted slowly with motorized arms once the Shuttlewas in orbit and the payload bay doors had been opened.

The SETS pallet flew along with the CIRRIS telescopeon the fourth flight of the Space Shuttle in July 1982.The mass spectrometer worked well, collecting more than50 h of data throughout the 94 orbits of the mission. Theinitial report on the flight83 highlighted many new aspectsof the Shuttle environment. Water was the most concen-trated neutral contaminant in the payload bay, and itsconcentration was directly related to the temperature ofthe outgassing surfaces near the bay. Surprisingly, heliumwas also a major contaminant, perhaps because of a leakof CIRRIS cryogen. The rocket engines used to controlattitude and orbital maneuvers caused the largest per-turbations to the gaseous and plasma environment, butthese changes were short-lived. Water dumps also seemedto cause longer term increases in the water concentra-tion. Decreases in the ambient atomic oxygen densitywere observed during thruster firings, often accompa-nied by increases in molecular ion signals. NOC, H2OCand other ions were produced by ion chemical reactionsbetween ambient OC and the species released by thethruster engines. Despite the contamination, depletions ofthe ambient ionosphere plasma due to equatorial spread-Fwere detected from the Shuttle bay with the mass spec-trometer. This result confirmed that the Shuttle could bean effective platform for space research. In later papers,we re-examined many of these results in more detail.84–89

When CIRRIS was reflown on STS-39 in 1991,90 ourgroup again provided the Quadrupole Ion–Neutral MassSpectrometer (QINMS), as we called the instrument then,for the CIRRIS-1A pallet as a contamination monitor.Unfortunately, we did not obtain a full set of data fromthat flight because the tape recorder for the secondaryexperiments failed. The astronauts read small amounts ofdata over the air-to-ground voice radio, an extremely inef-ficient method to transfer data to the ground. Nevertheless,the few readings of water concentrations that we did getwere consistent with the STS-4 results.



Our third Shuttle flight experiment was a collabora-tion with the materials research group at NASA’s JohnsonSpace Center under the direction of Lubert Leger. Spaceflight hardware returned to earth from early Shuttle mis-sions showed unexpected signs of surface degradationand mass loss. For example, thermal control paints hadbeen eroded and some non-metallic materials had beenreduced in thickness. The effects were more pronouncedwhen the materials had faced into the oncoming wind ofatmospheric gas, suggesting that oxidative reactions withneutral atomic oxygen in the atmosphere were responsible.Leger’s group quickly assembled two passive exposuretrays to quantify the effects on a wide variety of spacehardware materials and coatings. These trays, called Eval-uation of Oxygen Interactions with Materials (EOIM) 1and 2, flew on STS-5 and STS-8 in 1982 and 1983.91

The rates of the surface reactions between atomic oxy-gen and the spacecraft materials were needed to design thestructural trusses and other components for the large spacestation that NASA was then planning. Early designs hadcalled for graphite–epoxy composite truss tubes in themain structure. However, the Shuttle experiments indi-cated that these tubes would have eroded away substan-tially by the end of the planned station lifetime. Accuraterates of material loss were obviously critical, and werecalculated from the measured loss of material and theintegrated flux of atomic oxygen striking the surface.The largest unknown in the calculation was the oxygenfluence, which was estimated from thermospheric den-sity models. These were the same models of atmosphericdensity described above and were subject to the sameuncertainties.

The third experiment in the EOIM series added therefurbished STS-4 mass spectrometer from our group anda large carousel of material samples to the complement ofpassive trays. The main objective for the mass spectrom-eter was to measure the flux of atomic oxygen strikingthe samples on the pallet throughout the flight. To accom-plish this, the mass spectrometer was carefully calibratedbefore and after flight at an atomic oxygen beam appa-ratus at Los Alamos National Laboratory.92 The secondobjective was to measure the products from the reac-tions of atmospheric atomic oxygen with the materialssamples.

EOIM-3 flew on STS-46 in August 1992.93,94 Themass spectrometer measurements of atomic oxygen flu-ence were in good agreement with the fluences calculatedfrom neutral density models. The reaction rates betweenthe surface materials and oxygen were calculated from themeasured material loss and oxygen fluence with greaterprecision and confidence than earlier. The mass spec-trometer also detected reaction products from the testsurfaces that were similar to those observed in ground-based simulation studies.95 CO2 was found to be one ofthe major gas phase reaction products, indicating that oxy-gen atoms oxidized the organic material completely bychemical reaction rather than physically sputtering unre-acted polymer material from the surface. This importantresult showed that ground simulations of surface reactionmechanisms, which were significantly cheaper and fasterthan flight experiments, were valid indicators of on-orbitphenomena.

ELECTRONS BY TITRATION: MID-1980S

Electrons control atmospheric conductivity and transmis-sion of radio waves because electrons have much greatermobility than ions. Even when electrons are present inminor concentrations compared with negative ions, as lowas 10 cm�3, the conductivity is still dominated by theelectrons. Normal methods of measuring electron con-centrations are not capable of making measurements ofsuch small concentrations. When this became an issue forthe Defense Nuclear Agency in the 1980s, we devised amass spectrometric method for measuring the low con-centrations of electrons. The low electron concentrationswere of interest because of their impact on ELF and VLF(extremely low frequency and very low frequency) radiocommunication.

The method relied on the fact that electrons attachextremely rapidly to SF6 and that the product ions, SF6

�and SF5�, are non-reactive. The chemistry involved issummarized in Table 3 along with estimated atmosphericreaction times. SF6� is formed from both electrons andO2�. However, electrons react much faster. Therefore, it

was possible to make a controlled release of SF6 in front ofthe mass spectrometer that converted ‘all’ of the ambientelectrons to SF6� while allowing ‘none’ of the O2

� ions toreact with SF6. The ratio of the concentrations of electronsto ions was simply the SF6� plus SF5� signals divided bythe sum of all other ion signals. Knowing the total positiveion concentration as measured by a Gerdien condenserallowed the measurements to be put on an absolute basis.

A low-altitude, pumped rocket instrument was modifiedto include a gold-plated coil several inches in front ofthe mass spectrometer orifice. SF6 was released througha series of small pinholes in order to obtain a uniformdistribution of SF6 in as short a time as possible. Therewas only one flight of this instrument, on 14 August1987, before the funding stopped. The mass spectrometerworked for only a short time before failing. However,enough data were taken to prove the concept. At 85 km,the SF6� signal increased from 0 to 17 counts in a10 ms period upon first opening of the SF6 valve. Theion spectra indicated that 20% of the negative ions wereSF6� and the Gerdien measured positive 3000 ions cm�3,

resulting in a derived electron density of 600 cm�3, avalue consistent with other measurements.96 Based onthese data, we estimated the ability to detect on the order

Table 3. Rate constants and time constants relevant to elec-tron determinations by SF6 releases

Rate constant ReactionReaction (cm3 s�1) time (s)

e� C SF6 ! SF6� (99.5%) 3.7ð 10�7 1.4ð 10�7

! SF5� C F (0.5%)

O2� C SF6 ! SF6

� CO2 3.3ð 10�11 1.3ð 10�3

O� C SF6 ! SF6� CO Endothermic

Cl� C SF6 ! products EndothermicNO3

� C SF6 ! products EndothermicHSO4

� C SF6 ! products EndothermicSF6

� CO2 ! products EndothermicSF6

� CO3 ! products EndothermicSF6

� CO! products EndothermicSF6

� CO2.1/! SF6� CO2 ? 5ð 10�2 max.



of 0.1 electron cm�3, a value easily small enough toindicate this is a viable technique should interest in thisproblem arise again.

CHEMICAL IONIZATION MASSSPECTROMETRY: 1990S

The research that we have discussed so far has focused onmeasurements of the ion and neutral composition of theupper atmosphere, both for their own sake and for derivingthe concentrations of trace neutral species. More recently,we have extended our interests to the lower atmosphereand have participated in several campaigns to measuretrace gases from sea level to the troposphere–stratosphereboundary. The technique used for these measurements,chemical ionization mass spectrometry (CIMS), is anactive area of current research involving several inter-national research groups. A review of the entire field isbeyond the scope of this paper, and a summary of the workthrough 1993 is available elsewhere.11 Field measure-ments now mainly use active ion sources for the chemicalionization, although there is still some work using naturalions to measure sulfuric acid.97

Most of the atmospheric work on CIMS detectionof trace neutrals, including our effort, is now done onairplanes. Our instrument development effort has focusedon flexibility, small size and low power. We have usedthe instrument on a variety of platforms. A schematic ofthe current version of our CIMS instrument is shown inFig. 9. Most parts of this instrument are designed andfabricated at AFRL, the exceptions being componentssuch as pumps and pressure and temperature gauges.Neutrals and ions enter the mass spectrometer througha laser-drilled orifice (A). Depending on the altitude ofinterest, the orifice diameter varies from 50 to 200µm. Asupersonic expansion ensues and the core of the expansionis sampled by a skimmer (S). The gas–ion mixture passesthrough an electron impact ion source (I), which allowsionization of the neutral beam. Next, the ions enter a lensregion before entering the quadrupole mass spectrometerthrough a 1/4 in orifice. The quadrupoles were originally1/4 in rods, although they have been replaced by 3/8in rods recently. The rods are aligned to 1/10 000 in.Brubaker lenses, additional sets of rods at the entranceand exit of the quadrupole that carry the r.f. field only,have recently been added. The ions are detected by adiscrete dynode electron multiplier (M), chosen becauseit has a longer lifetime than the channel multipliers usedpreviously.

There are three differentially pumped regions in thesystem: (1) the high-pressure area of the expansion istypically at 10�3 Torr; (2) the quadrupole section operatesat 10�5 Torr; and (3) the multiplier region operates at10�6 Torr. One of the great technical advances that allowsthe system to be relatively small and easy to use isthe ceramic bearing turbomolecular pump. This is mucheasier to use in the field than cryopumps, the standard forfield instruments for many years. We use four 300 l s�1

turbomolecular pumps, two on the expansion region andone each on the other two regions. The two low-pressurepumps are backed by the intermediate stage of the two

high-pressure pumps which are in turn backed by an oil-free diaphragm pump. The pumps are very rugged, canbe mounted in any position, and tolerate even the largeamount of turbulence when the jet aircraft carrying theinstrument is directly in the exhaust of another jet.

Our program still emphasizes small, lightweight instru-mentation even though many aircraft platforms couldcarry much larger instruments. Several of our programsare on high-altitude aircraft without pressurized cabins. Inaddition, the basic instrument can be repackaged for useon satellites. These diverse requirements put unique con-straints on the electronics, resulting in in-house design ofthe control boards for the mass spectrometer and modi-fications of several other components such as the turbo-molecular pump controllers. Several aspects of the elec-tronics designs are worthy of discussion. The multiplierhigh-voltage power supplies, pulse amplifier, and counterare mounted on two small circuit boards (E) which areinstalled in the multiplier region of the instrument. Thisdesign serves two purposes. For situations in which theinstrument is operated below ambient pressure but abovespace pressure, e.g. in an unpressurized airplane com-partment, the high-voltage supplies needed for the mul-tiplier will not suffer arc breakdown. Incorporation of thepulse-counting electronics in the vacuum reduces noiseproblems and allows gains as small as 104–105 to beused. This extends the life of the multipliers, preventingthe need to replace them in the middle of a field cam-paign.

We initially used d.c. square waves to create the r.f. fieldin the quadrupole mass filter. Using two such sources onopposite sets of rods with a slight time shift created the r.f.These electronics worked well for low masses. However,for high masses the resolution was not sufficient for 1 amupeak separation. We have just completed a redesign ofthe control electronics that uses normal r.f. supplies. Thisredesign supplies r.f. (2000 V, peak to peak) and d.c.(180 V peak to peak) voltages to the 0.375 in diameterrods, which again have the potential for arcing in reducedpressure environments. The solution is the same: the r.f.oscillator is installed inside the quadrupole chamber. Inaddition, much attention had to be placed on temperaturestability of the electronics since the instrument runs from190 to over 300 K. Even small changes in voltage resultin shifts in mass peak position. The temperature stabilityresults in mass shifts less than 0.1 u over the entiretemperature range of operation. The resolution of theinstrument has also been improved. Adjacent mass peakscan be separated even when they have a difference inabundance of 105 at 112 amu. Recently in the laboratorywe have managed to separate N2

C and COC.Several ion source positions and orientations were tried

before settling on a final design. The ion source wasoriginally placed 1 m upstream from the sampling orificeas in CIMS systems used at the Max Planck Institutein Heidelberg.98 However, we later found that locatingthe corona discharge (C) directly opposite the inlet tothe mass spectrometer reduced the saturation of the ionchemistry without reducing sensitivity. We have used thissource geometry successfully in all of our recent flightexperiments.

The interaction of the source ions with the wet airleads to hydrated ions. In order partly to dehydrate theions a drying region (D) was added. Just before the



Figure 9. Schematic of the current CIMS instrument, mass spectrometer section only. (A) Sampling aperture; (D) dryer; (C) corona;(S) skimmer; (F) flow tube; (I) electron impact ion source; (T) turbomolecular pump; (Q) quadrupole; (M) discrete dynode electronmultiplier; (E) multiplier electronics. Large arrows indicate pumping direction.

sampling orifice of the mass spectrometer,a dry gasflows into an isolatedregion that is also warmed.Upontransversingthis region, ions are partially dehydrated.The drying allows the instrument to be run at loweraltitudes and greatly decongeststhe massspectra.Thiswas especiallyimportant before we improved the massresolution.

Figure10 showsthe completeinstrumentas flown ona DC-8 during the NASA SONEX (SASS ozone andnitrogen oxide experiment)campaign.Air entersa raminlet outside the boundary layer of the airplane. Thesampledair is carriedundiluteddown a 3.5 cm i.d. tubeby the ram effect. The air velocity is ¾40 m s�1 andcanbe adjustedby a valve on the exhaustportion of the flow



Figure 10. Schematic of the current CIMS instrument in flight configuration.

tube.The air exits the airplanethrougha secondhole intheairplaneskin.TheReynoldsnumberin theflow tubeis¾30000 andthe flow is well into the turbulentregime.99

Bendsin the flow tubehavea radiusof curvaturegreaterthan the flow tube radius to minimally disturb the flowdynamics.

Up to now, we used CO3�.H2O/n ions to detect

mainly HNO3 andSO2.100–102 The CO3� ions aremade

using reactionsthat are similar to those that occur inthe atmosphere(see earlier). O2 is added to the ionsourceand mostly O3

� is made.A traceof CO2 is alsoadded,convertingthe ions to CO3

�. Hydrationoccursbyclusteringto atmosphericwatervapor.Thereactionsusedto detectHNO3 andSO2 are

CO3�.H2O/n C HNO3 ��! CO3

�.HNO3/C n H2O

and

CO3�.H2O/n C SO2 ��! SO3

� C n H2OC CO2

followed by

SO3� CO2CM ��! SO5

� CM

All productshydratemuch lessreadily than the primaryion and are shownwithout hydration.100,103 In very wetsituationssomehydrationdoesoccur.For conveniencewealsousethe O2 sourcegasasthe drying gas.

With the sourceposition upstream,neutralconcentra-tions were derivedfrom known rate constantsand mea-suredreactiontimes.Moving the ion sourceoppositethemassspectrometerentrancecreatesan ill-definedreactiontime, makingcalibrationnecessary.Two typesof calibra-tion sourcesareused.For gasessuchasSO2, a mixture ismadeanda knownamountof SO2 is introducedthroughaflow controlleror a calibratedcritical orifice. For speciessuch as HNO3, which are sticky and have low vaporpressures,a permeationtube is used.A carriergasflows

througha temperature-controlledovencontainingtheper-meationtube. The gas mixture entersthe systemeitherupstreamfor calibrationor downstreamof the massspec-trometerso that the reservoiris continually flushed.Theinlet into theflow tubemustbeheatedsinceHNO3 freezesat the temperaturesof the uppertroposphere/lowerstrato-sphere.A diffusershapedlike a stationaryfan is insertedto mix the calibrantasquickly aspossible.The total gasflow in the tubeis determinedfrom a pitot tubemeasure-mentof theflow velocity.Thepitot tubemeasuresonly thecenter-lineflow velocity andlaboratorycalibrationsindi-catea 4% correctionis neededto accountfor thevelocityprofile.Thecalibrationsaredoneperiodicallyandproducea square-wavesignal on top of the atmosphericsignal.The sensitivity of the instrumentis determinedfrom theincreasein the ion signalandthe known concentrationofthe addedgas.Figure11 showsa plot of the SO5

� sig-nal vs. addedSO2 during a flight on 26 June,1997.Thisshowsthe instrumentis linear to at least100 ppbv.

Since the early rocket instrumentswere first devel-oped,muchprogresshasbeenmadewith respectto dataacquisition.Computershaveallowedfor greatlyimprovedflexibility and easeof use.The currentinstrumentshaveprogrammablememory on the main control board andare capableof running themselvesfor flights where anoperatoris not on-board.An RS-232computerconnec-tion allows the user to adjust the instrumentduring aflight. Dataarestill storedevery10 ms.Massesareeitherpeak steppedfor maximum time resolution or scannedto ensureall massesthat have signal are collected.Thepeak stepping requiresvery good mass stability whenhigh resolutionis needed.Dataarestoredon a harddiskalong with relevantdata such as pressureand tempera-ture.

Figure12 showsmassspectratakenduring the SNIF-3(SASSNearField InteractionFlight experiment)flight inJune,1997, while measuringairplaneengineexhaust.104

Four spectraareshownin two panels.The bottompanelshowsa spectrumtakenduring a calibrationas bold and



Figure 11. SO5� signal vs added SO2 during a flight on 26 June, 1997 during a test of the linearity of the instrument.

Figure 12. Mass spectra taken during an F-16 jet engine plume crossing and during a calibration during the SNIF-3 campaign, shownin bold. Reference spectra are shown as thin lines.

a referencespectrumas a thin line. In both spectra,themain ions are CO3

�.H2O/n with n D 0 and 1 being themost abundantand n D 2 down by about an order ofmagnitude.N2 clustersof CO3

� are also observed.Theonly substantialdifferencebetweenthe two spectrais

the signal at mass112, SO5�, from the addition of the

SO2 calibrant.Note that our presentmassresolutionandsensitivity are considerablybetter than shown here butour first test flights with the improved instrumentationhavejust beenmade.The top panelshowsa background



spectrum and a spectrum taken during a plume cross-ing. Several differences between these spectra are evi-dent. The SO5� peak is clearly visible, an indicationof SO2 in the plume. More hydration is found due tothe main engine exhaust product, H2O. Nitrogen oxides,NO and NO2, react with CO3

�.H2O/n ions to formNO2

�.H2O/n and NO3�.H2O/n ions. At the high-mass

end of the spectra, a small increase in the mass 123 sig-nal .CO3

�.HNO3// shows the presence of HNO3 in jetengine exhaust.

During our latest campaign (SONEX),105 the instru-ment was sensitive enough to produce 1 count per pptvof HNO3 with a 1 s integration time. However, tur-bulence and background HNO3 (from wall outgassing)prevented measurements below 50 pptv. The instrumentresponse time also suffered from turbulence, resultingin noisy data. We estimated that we could observechanges of the order of 10 pptv in 10 s. We are cur-rently working on ways to circumvent the turbulenceproblem. For instance, high-speed fans are being testedto drive the flow. This has the added advantage ofallowing a rear-facing inlet to prevent sampling largeaerosols since situations arise where evaporating aerosolsmay contain enough material to affect the gas-phasemeasurements of the neutrals of interest. For the out-gassing problem, we plan to heat the entire flow tubeand to use coatings better suited for HNO3, such asTeflon. Some of the background appears to come fromHNO3 produced in the ion source. This is reduced bylowering the power to the discharge and decreasing

the CO2 flow. An example of the HNO3 data fromSONEX is shown in Fig. 13. For comparison, the totalNOy .NOC NO2C HNO3,CHNO2C 2N2O5C HNO4CNO3 C PAN/ is shown. The large peaks in the concen-tration are excursions into stratospheric air. Many of thewiggles in the spectra are real, as seen in the comparisonbetween the sets of data.

The instrument was also flown to measure Cl�-containing species in rocket plumes on a WB-57 aircraftat 65 000 ft.106–108 For these experiments the electronimpact source was used and HCl, Cl2 and ClO weremeasured. Kr in the atmosphere is used as a calibrationsignal along with laboratory calibrations of relativedetection efficiencies. Figure 14 shows the signals ofmasses 70 and 72 vs time. The large spikes are rocketplume crossings. The ratio of mass 70 to 72 indicatesthat these two peaks are due to Cl2. HCl is monitored atmass 35 since atmospheric36Ar and 38Ar interfere withmonitoring of the parent ion. Corrections to the mass 35signal are made for the Cl2 observed. A major findingof this work is the observation of Cl2 long after launch,which is not understood since the lifetime with respect tophotolysis is short.

In contrast to the early campaigns for which themass spectrometer data were important by themselves,the CIMS instrument is usually used now in campaignsinvolving numerous instruments. The present knowledgebase is larger and more coordinated measurements areneeded, i.e. measurement of a suite of gases is needed.

Figure 13. Comparison between the AFRL CIMS HNO3 measurements for 10 s averages (points) and NOy data scaled by 0.55 (solidline) obtained on a flight on 5 November 1997. Calibration events are the cause of the gaps in the data. The scaling is done since HNO3

is only a fraction of the NOy signal.



Figure 14. Data collected at masses 70 and 72 following the launch of the Space Shuttle STS-85. The first three plume penetrations areshown. The mass 72 data are offset for clarity.

The value is then determinedby comparisonwith com-plicatedmodels.In addition,accuracyandprecisionoftenneedto be high. Errors of more than 20% are often nottolerable.Of course,exceptionssuchasthefirst measure-mentsin rocket plumesdo show somevalue in a singlemeasurement.107

THE FUTURE

For almost four decades,our researchgroup at the AirForce ResearchLaboratoryhas usedmassspectrometryas a versatile researchtool to explore the fundamentalnatureof the earth’s atmosphereand ionosphereand tohelp solveoperationalproblemsfacing the Air Forceandthe Departmentof Defense.We have flown massspec-trometersof manydesignsonnearly100soundingrockets,ninesatellites,threeSpaceShuttlesandmanymissionsofhigh-altituderesearchaircraftandballoons.We havealsoused our instrumentsin ground-basedinvestigationsofrocketand jet engineexhaust,combustionchemistryandmicrowavebreakdownchemistry.Each of theseexperi-mentshasraiseduniquechallengesin the instrumentationandhasled to manyimprovementsandinnovationsin thehardware.As with mostlong-termresearchprograms,wehavehad our shareof vehicle failures, electronicsprob-lems,vacuumcapsthatdid not deployandtaperecordersthatdid not record.Luckily, thesuccessesgreatlyoutnum-ber the failuresandincludeamongthemsomeuniqueandmarvelousdatasets.

In the future, we hope to continuethe long march togreatersensitivity,selectivity,ruggedness,versatility and

usefulnessof our instruments.We arecurrentlyexploringusing our instrumentationin a numberof new environ-ments.Theseinclude:

ž detectingtraceneutralgasesin thepolarstratosphereaspartof aNASA campaignto understandtheforma-tion of a northernarctic ozonedepletion(SOLVE);ž detectingtraceemissionsof explosivecompoundsas

a way of finding and identifying buried mines andordnance;ž detectingtraceexplosivesin securityportals;ž detectingchemical weaponsin battlefield environ-

ments and during treaty verification site inspec-tions; andž detectingtracergasesreleasedin the atmosphereto

monitor dispersionanddiffusion rates.

GLOSSARY

Accelerometer An instrumenton a satelliteusedto measurethe decelerationforcesdueto atmosphericdrag.Neutraldensitiesandwinds arederivedfrom the data

Bottomside The transitionbetweentheE-regionandthe F-regionofthe ionospherewheretheelectronconcentrationincreasesrapidly with increasingaltitude



Brubaker rods Small sections of quadrupole rodsattached to either end of aquadrupole. R.f.-only fields areapplied to reduce stray d.c.fields at the ends resulting ingreater resolution in the wingsof a peak by narrowing the ionenergy spread

Cluster An aggregate species composedof a central ion and weaklybonded molecules or ligands

Core ion The central ion to which a weaklybonded molecule or ligand isattached, e.g. NO3� inNO3

�.H2O/

D-Region See Ionosphere

E-Region See Ionosphere

F-Region See Ionosphere

Gerdiencondenser

An instrument used for derivingcharge concentration, composedof either parallel plates orcylinders. The applied voltageis enough to collect all thecharge in between the plates orcylinders

Ion massspectrometer

A mass spectrometer without anion source used to measure theambient ion composition of theatmosphere, or a massspectrometer in which the ionsource is external to thevacuum system such as thechemical ionization massspectrometers used to detecttrace gases in the atmosphere

Ionosphere The region of the upperatmosphere that has highenough electron concentrationto interact with radio waves.The ionosphere is composed ofa series of overlapping layersor regions that aredifferentiated by the types ofionizing radiation that form theelectrons and ions and thetypes of chemical reactions thatcontrol the ion population. Forhistorical reasons, these layersare referred to as the D-region(70–90 km), E-region(95–140 km) and F-region(140–1000 km). The highestelectron concentrations,typically ¾106 cm�3, occur inthe F-region at altitudesbetween 300 and 500 km

Langmuir probe An instrument used to measureelectron or ion densities. Asmall wire, cylinder or sphereis placed in the plasma, avoltage is applied and the ion

or electron current collected bythe probe is measured. Thechanges in the current as thevoltage is swept are then usedto derive the concentrations andthe ion and electrontemperatures

Ledge A region of the ionosphere wherethe electron or ion density ischanging rapidly with altitude

Ligand A molecule that is weaklyattached to a central ion (alsoreferred to as a core ion) e.g.the water molecule inNO3

�.H2O/ is a ligand. Theforces holding ligands to thecore ion are weaker thanchemical bonds

Magnetometer A sounding rocket or satelliteinstrument used to measure thestrength and direction of theearth’s magnetic field

Neutral massspectrometer

A mass spectrometer in which theion source is internal to thevacuum system for measuringthe neutral composition of theatmosphere

Plasma A high-energy state ofmatter in which some or all ofthe molecules or atoms in a gashave electrons stripped away.The plasma remains electricallyneutral overall because the freeelectrons and the positive ionshave equal concentrations. Theionosphere and the gas in a neonlight are examples of plasmas

Plasmairregularities

Regions in a plasma where thetotal concentration of ions andelectrons changes chaotically asa function of position or time.Such regions interfere with thereflection or passage of radiowaves and can cause loss ofcommunication

Polar cap ab-sorption event(PCA)

An increase in the D-regionelectron and ion concentrationcaused by the penetration ofhigh-energy protons into theearth’s atmosphere. The protonsare produced by large flares onthe sun and are channeled intothe polar regions by the earth’smagnetosphere. Also calledsolar proton event or solarcosmic ray event

Solar cosmic rayevent

See Polar cap absorption event

Sounding rocket A small rocket used to transportpayloads on sub-orbitaltrajectories



Sporadic E A thin layer of ionization atE-region altitudes where theelectron concentration is higherthan normal E-region values.Sporadic E is thought to formwhen a change in the neutralwind speed with altitude, awind shear, works togetherwith the earth’s magnetic fieldto compress the ionization

Total ions mode A mode of operating a quadrupolemass spectrometer. When thed.c. voltage to a quadrupole isturned off and r.f.-only isapplied, the quadrupolebecomes a high-pass filterrather than a bandbass filterand transmits all ions greaterthan a selected value. The totalions mode is more sensitivethan the bandpass mode and isuseful when signals are smallin order to obtain generalinformation about the massdistribution. Also called thetotal transmission mode

Acknowledgements

It is a pleasure to acknowledge the dedication of the many scientists,engineers, technicians and data analysts who, for nearly 40 years, havebuilt, tested, flown and understood the mass spectrometry experiments.Many of these people predate our own tenures at AFRL and deservethe credit for the early work. In particular we want to mentionRocco S. Narcisi, the founder of the group. His untimely death in1983 left a great void to fill and deprived us of a wonderful mentor.We also give special thanks to John Ballenthin, the current designer ofthe hardware and much of the software. We also thank Alan D. Bailey,Gennaro S. Federico, Lois D. Wlodyka, Ray Wlodyka, ChristopherSherman, C. Russell Philbrick, David M. Thomas, William Swider,D. Baker, Edmund Trzcinski, Margaret Gardner, Jose M. Calo,Patricia Bench, Edward P. Szuszczewicz, R. Rossi, John Paulson,John F. Borghetti, John Dorian, Lubert J. Leger, Steven L. Koontz,Charles L. Hakes, Jon B. Cross, J. Vince Eccles, Russell Armstrong,Thomas M. Miller, W. Burke Knighton, Roger Gosselin, GeoffreyKirpa, Roger Meads and Willard F. Thorn for their contributions tothe research over the years. We further thank the many other peoplewho helped to integrate the instruments into the payloads and to fly thevehicles whom we do not have room to list individually. We gratefullyacknowledge the funding agencies that have supported the massspectrometry work through the many years of productive research:the United States Air Force Science and Technology Program;the Air Force Office of Scientific Research; The Air Force SpaceTechnology Center; the Defense Nuclear Agency (formerly DASA,the Defense Atomic Support Agency); the National Aeronautics andSpace Administration; and the Strategic Environmental Researchand Development Program. We especially thank Bill Swider andJohn Paulson for their helpful and constructive reviews of themanuscript.

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Airborne mass spectrometers: four decades of atmospheric and space research at the Air Force Research Laboratory