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Analysis of mid-tropospheric space shuttle exhausted aluminum oxide particles
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VoI 21. No 5. pa I%?-11%. 1987.
ANALYSIS OF MID-TROPOSPHERIC SPACE SHUTTLE EXHAUSTED ALUMINUM OXIDE PARTICLES WESLEYR. COFER,III Atmospheric SciencesDivision, National Aeronautics and Space Administration, Langley Research Center, Hampton, VA 23665, U.S.A.
G. GARLAND
LALA
Atmospheric SciencesResearch Center, State University of New York at Albany, Albany, NY 12222, U.S.A.
and JAMES P. WIGHTMAN Department ofchemistry, Virginia Polytechnic Instituteand State University, Blacksburg, VA24061,U.S.A.
(First recdued 5 Septemberand ~n~nu~~orrn27 Oetolzer 1986) Abstract-Aluminum oxide (AllO,) particles from the exhaust of the space shuttle were collected from the shuttle column cloud immediately after the launch of STS-61A on 30 October 1985. The particulates were collected on T&on filters during a tight descending aircraft spiral maneuver over the altitude interval of 7.646 km. Scanning electron microscope (SEM) examination of the particles revealed that they were virtually all spherical and ranged in diameter from about 0.1 pm to 10 pm. Particles of < 0.1 pm in diameter were not readily visible in the SEM photomicrographs; however, such particles would not be captured efficiently on the Teflon filters used. Results from energy dispersive analysis by X-ray (EDAX) and electron spectroscopy for chemical analysis (ESCA) conlimd that the particles were pr~ominant~y composed of Al and 02. A particfe size distribution was determined from the AlsO, samples. The distribution was bimodal, with one observed peak centered near 2.0 pm. The data indicated the existenceof another mode centered at a diameter of < 0.3 pm, but could not be accurately located because our technique cut otTat diameters of < 0.1 pm. A mass median diameter of slightly c 2 pm was determined. The collection was evaluated for ice nucleation activity, using the fiIter techmque with a static vapor-diffusion chamber. Only a small fraction (about I : IO”)of active ice nuclei were determined among the Al,Os paruculates.
Key word index: Shuttle exhaust ehluents, aluminum oxide particles, ice nucleation.
INTRODUCTION
Particulate aluminum oxide (AlzOJ is a primary exhaust effluent of space shuttle’s solid-propellant rocket boosters. Approximately 3OQOOO kg of A&O, is exhausted by the two shuttle boosters during each launch (Potter, 1978). About 67% of the A1103 is released within the troposphere; the remainder is released into the stratosphere up to about 43 km in altitude, where the boosters burn out and separate (Space Shuttle Program, Environmental Impact Statement, 1978). Substantial interest exists concerning the role and impact of shuttle (or solid-propellant) exhausted A1,03 particles in the atmosphere since these particles could conceivably perturb the radiation budget of the Earth--atmosphere system (Turco cr ul., 1982). Interest has been particularly keen in the areas of nucleation of atmospheric water (Hindman et Qi., 1980. 1982; Radke et al., 1982) and ice (Parungo
and
Alee. 1978; Hindman and Lala. 1980; Hindman and Finnegan, 1982) in the troposphere and lower stratosphere.
Hofmann ef af. (1975) have suggested that the contribution of very small (r +.,0.01 @m) shuttle exhausted AlzOJ particulates to the stratosphere (above 25 km) may be significant. Brownlee er al. (1976), Zolensky and Mackinnon (1985) and Mackinnon and Mogk (1985) have shown with balloon and aircraft collections that substantial concentrations of spherical Al,03 particulates (presumably rocket produced) in the 2-8 pm size range exist in the lower stratosphere. Nevertheless, A&O, injections into the stratosphere from solid rocket motors have not been generally expected to result in discernable climatic impacts (Pollack et ai., 1976; Turco et Qf., 1980). The morphology and chemistry of solid-rocket propellant produced Al,03 particles have been studied extensively. The A&O1 has been shown to consist of both OL and y crystalline phases (Dobbins and Strand, 1970; Dawbarn and Kinslow, 1976). The overwhelming number of AI,O, particles appear to reside in the 0.05-0.3 pm diameter range (Varsi, 1977; Woods, 1978; Bosart er al., 1983) although many particles in the
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WESLEY R. COFER. III et al.
I-1Opm range are always present. The chemistry of soi~d-pro~ilant produced AI,O, particles has been investigated by Dawbarn and Kinslow (1976). Dillard et al. (1980) and Corer er 01. (1984). Prior work, however, has been done with simulated solid-rocket exhaust, with laboratory generated A120, samples, with samplings or collections from Titan III launches, from shuttle boundary layer exhaust clouds contaminated with surface debris (commonly referred to as ground clouds), or from aged collections made days to months after atmospheric injection. Thus, a need existed to study samples of shuttle exhausted AllO collected immediately after a launch at relevant altitudes. The Shuttle Exhaust Particle Experiment (SEPEX) was therefore conducted to acquire the first representative samples of shuttle exhausted AI,O, from an unaged mid-tropospheric shuttle column cloud. The physical and chemical properties determined from these samples would then closely resemble the initial state (as injected into the atmosphere) of shuttle exhausted A1203 particles.
Table I. A NASA Langley Research Center T-34C (single engine turbo-prop) aircraft was used as the particle collection platform. The flight plan consisted of the aircraft assuming a holding pattern at 7.6 km minutes before launch, then after launch and rangesafety clearance, proceeding into the resulting column cloud and sampling continuously while performing a tight descending spiral maneuver to about 4.6 km, then terminating sampling and returning to base with the collected sample of exhaust particulates. Pilot commentary and continuous onboard photography of the cloud were documented during the mission on VHS cassette. The nominal ascent trajectory for the space shuttle, the targeted region for exhaust cloud sampling, and estimated initial column cloud parameters (for I +2 min) arc shown in Fig. 1.
EXPERIMENTAL
Two identical high-vol filter collection systems were fabricated and fitted into pods which were suspended from pylons beneath the wings of the T-34C aircraft. Air was drawn through the system using a 28 VDC vacuum blower. All valving was pneumati~l~y or electrically actuated from the cockpit. Each pod was operated independently, and utilized a 23 cm by 16 cm Teflon filter for particle collection. The capture efficiency of the filters was a function of particle size. The
MISS1ONS
The SEPEX mission was conducted on 30 October, 1985 (STSdlA), a noon launch. Abbreviated meteorological parameters obtained from a Cape Canaveral rawinsonde release at launch time are presented in
Estimated Cloud Parameters
(t+2 min)
Cloud diameter -350 m -80 mg/m3 Al203 concentration Mean particle diameter - 1 pm HC1 concentration -70 ppmv I Hori~ofl~a~ range, Fig. 1.
I 6
km
NominaI ascent trajectory of a space shuttle launch.
Analysis of mid-tropospheric space shuttle exhausted aluminum
oxide particles
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Table 1. Selected meteorological parameters for 1703 GMT 30 October Altitude
Ptw+.wn
0.004 km 1.525 3.050 4.570 6.095 7.620 9.150
1006mb 844 704 585 480 392 317
Temperature
289°C 15.1 7.1 - 1.2 -11.8 - 23.3 - 34.4
Teflon filters used were rated at 98.9% retention for 0.3 pm diameter DOP smoke particles. The capture efficiency for larger particles should approach 100%. Capture efficiency for 0.1 pm particles was estimated to be about 96 %. Particles with diameters of < 0.1 pm should progressively (with decreasing diameter) be less efficiently captured. The system was designed to be isokinetic at an aircraft speed of about 45 m se ‘. Flow through the systems at ambient laboratory pressures and temperatures was determined to be about 1.1 m3 min-‘. Both particle collection systems were evaluated in thermalvacuum chambers at temperatures and pressures approximating the planned mission. Volume fiow rates were measured in the chamber with a turbine flowmeter at 370 mb-2Y’C-‘, at 500 mb-lS”C-‘, and at 665 mb-O”C-‘. Data from these chamber tests indicated our collection systems would process about 0.6 standard m3 min - ’ over the sampled attitude interval (7.6 km-4.6 km). However, the measured volumetric flow at 370 mb-ZS”C- I (approximately equivalent to an altitude of 7.6 km) was only 10% lower (1.0 m3 min-‘) than at SIP. Since this represents about the same volume of air space sampled per unit of time, actual aircraft sampling should have remained close to isokinetic over the sampled altitudes.
ANALYSIS
Analysis of the particles consisted of scanning electron microscopy @EM) for morphology and size distribution, energy dispersive analysis of X-ray (EDAX) for elemental composition, and electron spectroscopy for chemical analysis (ESCA) of the surface composition. SEM anaIysis was performed on a Cambridge ISOR coupled with a EDAX model 9100 with windowless detection. Analysis was done on sections of Teflon filter approximately 1.3 cm by 1.3 cm. The sections were first photographed (SEM) and then EDAX analysis was performed. Samples for ESCA analysis were cut from regions of the Teflon filters where high particle densities were expected. Filter sections of about I cm by 1.9 cm were mounted on the sample probe using double stick tape. Samples were run on a Kratos XSAM-800 electron spectrometer. Both an ESCA survey (wide scan) and an ESCA multiplex (narrow scan) spectra were run on each sample. The C Is photopeak from the hydrocarbon
Humidity (r.h.)
Wind speed
Wind (dir)
70%
10 Kts
150”
78 25 35 35 22 26
17 28 23 41 36 32
212 216 207 206 213 211
background was assigned a binding energy of 285.0 electron volts (eV). A total of seven separate samples were run. RESULTS AND DISCUSSION
The aircraft penetrated the column cloud at approximately 7.6 km about 5 min after launch and sampled during descent for 5 15 min, exiting the exhaust plume at about 4.6 km. Total in-cloud sampling times of about I4 min and 12 min for pods one and two, respectively, were estimated from the cassette record. This translates into sample vol of 7.2 and 6.2 m3 of exhaust plume air processed through collectors 1 and 2, respectively. The column cloud appeared dull grey and thin, typical of a relatively dry shuttle exhaust plume (plumes with large amounts of water droplets appear brilliantly white). Meteorological parameters in Table 1 confirm low humidities over the sampled altitudes and support the dry-cloud observation. Filters from sampling pods 1 and 2 were weighed and found to have collected 26 f 3 and 2I+ 3 mg of material, respectively. Theshuttlecolumn cloud, therefore, averaged about 3.5 mg m - 3 of particulates over the integrated time interval. This result is reasonable when compared (see Fig. 1) with the calculated concentrations for the relatively undispersed column cloud at 2 min after launch. Particles were found to accumulate on the filters uniformly except near the perimeter (within w 0.5 cm of the filter retaining plates) where larger numbers of particles were observed to concentrate. This distribution is shown in Fig. 2 where representative scanning electron photomicrographs of sections of the filter from (a) midfilter and (b) a filter/perimeter edge section are shown. It can be readily seen in Fig. 2 that the collected particles were virtually all spherical and exhibited a considerable degree of agglomeration. Whether the A&O3 chtsters occurred before or during collection cannot be determined. None of the individual particles shown in Fig. 2 are larger than 10 pm in diameter. Infrequently, particles of diameter greater than lO#m were identified during this analysis. The largest single particle observed in the SEPEX experiment was slightly < 20 pm in diameter. The particles shown in Figs 2 and 3 are typical of the vast majority of particles collected during this mission. The microscopic smoothness of the surface of the particles typically observed
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WESLEY R. COFER, III ef al.
Fig. 2. Repnsentative
SEM photomicrographs
in these collections can be seen in Fig. 3 along with the large number of sub-pm particles collected. Almost no ‘frosted* or ‘snowlike’ particles were observed in this collection. as have been previously reported (Dawbarn and Kinslow, 1976, Dillard et a!., 1980; Gofer et al., 1984). In our prior work, where frosted particle surfaces were observed, the samples had been collected from ‘wet’ exhaust clouds. Dissolution of y A&O, in these HCI enriched droplets (Sebacher er al., 1984; Cofer et al., 1985) followed by evaporation and recrystallization may have ied to the surface frosting. Since the SEPEX samples were acquired from a ‘dry’ column cloud, these processes may have been negligible or minimized. Particles with several pronounced surface features (presumably the result of incomplete collisional coalescence of molten Al,O, in the exhaust plume) were observed in these analyses. The particles shown in Fig. 4a, b appear to be composed of several partially fused A&O, particles. This morphology when observed was almost always associated with the larger
of shuttle exhaust particles.
diameter particles (> 3 pm). Figure 4a is a particle from the SEPEX collection, while 4b was obtained from the wing surface of an aircraft that had penetrated a shuttle launch cloud from STS-SIB (29 April, 1985) at about 2.8 km. Such characteristic surface features have &en previously noted for large shuttle particles collected in the surface boundary layer and at the Iaunch site (Cofer et ol., 1984). However, a greater proportion of the >3 pm particles from the ground and boundary layer samples appeared to exhibit these features. That the SEPEX particles appeared to contain a smaller fraction of partially coalesced particles is difficult to explain. Two factors were significantly different in this collection. This collection did not have the bias toward large particles of the previous collections (Cofet et al., 1984) and this collection was made at much higher altitudes than the prior collections. A particle size distribution for the SEPEX collections was determined by counting - 1900 particles from four different sets of SEM photomicrographs at
several magnifications.
Even at the highest magnifi-
Analysis of mid-tropospheric space shuttle exhausted aluminum oxide particles
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WFSLEY R. COFES III et a[.
cation (13,000X), particles with diameters < 0 I pm were almost impossible to measure or count. Our data analysis, therefore, does not include particles below 0.1 pm in diameter. Likewise, particles with diameters > 8IOpm were not often encountered and are not included in our distribution. Nevertheless. we feel confident that very small particles likely dominated the particle number distribution, as reported by others (Woods, 1978; Bosart er al., 1983), and some particles with diameters > 10-20 pm as observed by others (Anderson and Keller, 1983) are most certainly exhausted by the shuttle boosters. The distribution is shown as a function of the number of particles accumulated cm-’ of Teflon filter in Fig. 5. It is obvious from Fig. 5 that the largest number of particles occur at the smallest diameters. The distributions based on number and mass cmw3 of sampled shuttle column cloud air (using 6.7 m3 as the total sample volume) are shown in Fig. 6 as a function of the change in particle number with respect to the change in the log of the particle diameter vs particle diameter, and in Fig 7 as the cumulative percentage of mass vs particle diameter. The distribution appears to be bimodal (an unobserved maximum in particle number must occur somewhere between 0.3 and 0.0 pm in particle diameter) with a mass median diameter of just below 2 pm. This result is consistent with previous studies (Dawbarn and Kinslow. 1976; Varsi, 1977; Woods, 1978: Bosart er al., 1983) that have indicated size modes for solid rocket produced Al,O, 1 x 18+Number
521085210’5210652Ids2IO45210352-
1x102-
I 0
Particle
’ 1
’ 2
’ 3
Diameter,
’
4
’
5 Dp,
’
6
’
7 (pm)
Fig. 5. Particles accumulated per cm2 on filler
J
8
105-
AN L\log I04 (cm-3) 103 500 :
200
0O
B
100 ” 50.0 20.0lO.O-
5.00 ‘.ool
\
1.00 .500 .200 .I00
0.1
I 0.2
I Ifilllll a4 0.6
Particle
1.0
Diameter,
Fig. 6. Number distribution
I 20
Ittllu) 4.0 60 Dp,
10
pm
in exhaust cloud.
particles, though locations of the modes have differed. Calculations based on our number distribution and an AlzO, density of 3.7 g cm _ 3 (approximate density for an equal mix of a and 7 A120, phases) indicate that about 43 mg of particles should have accumulated on each filter. Although our measured average accumulation (24 mg) appears to be low relative to the calculated accumulation, evidence has been presented suggesting that the density for rocket produced A1203 may range between 1.5 and 3.5 g cm-’ (Turco et al., 1982). An average density of about 2.1 g cme3 would bring our measured and calculated accumulations into good agreement. It has also been noted that our distribution was biased against small particles, although this bias would not be expected to account for much mass. EDAX analysis of the SEPEX particles always revealed strong AI and O2 peaks, as shown in Fig. 8a. A F peak was observed on one occasion, attributed to the Teflon filter. Cl was not detected in any of the EDAX analyses. A representative set of results of the ESCA analysis of the filter samples is shown in Table 2. The largest photopeaks were due to C and F. Indeed, the F to C ratio as calculated from the photopeak areas is about 2.0 as expected from Teflon. A small but persistent Cl signal was observed on most samples. It was not possible to determine whether the Cl was bonded to the particles or adsorbed (or chemisorbed) on the surfaces, or both. Of the seven samples run, the expected Al photopeak was prominent on only one sample, shown in Fig. 8b. The peak is broad and was curve fit to gwe the two photopeaks with corrected
Analysis of mid-tropospheric space shuttle exhausted aluminum oxide particles
(fim)
1193
432-
I0.8060.40.3-
Percentage
of mass
Fig. 7. Cumulative percentage of mass vs particle diameter.
Table 2. Typical ESCA analysis of Teflon filter samples Photopeak c Is F Is 0 1s Cl 2p
Binding energy (eV) 291.8 688.0 532.0 199.5
Atomic fraction 0.32 0.66 0.01 0.003
binding energies of 120.3 eV and 118.9 eV as shown in Fig. 8c. A full width at half maximum of 1.80 eV was used in the curve fit analysis. The higher binding energy peak at 120.3 eV is characteristic of AI in A120J, while the lower binding energy peak at 118.9 eV is typical of Al in hydrated A1203. The peak areas are proportional to concentration and suggest a slightly greater contribution of the hydrated A1,OJ. The fact that Al was easily detected by EDAX, but with great difficulty by ESCA, was unexpected. It was accounted for after surface- ionization mass spectrometry (SIMS) experiments revealed that the A1203 particles had been initially covered by a thin (several monolayers) Teflon film. After brief etching of individual particles with a gallium ion gun (lOkeVCa+), which initially yielded CF’ ions (characteristic of Teflon), AlzOJ composition was confirmed. We suggest that sputtering resulted from X-ray bombardment of the Teflon filter sections during the ESCA analysis, coating the surface of the AI,O, particles with Teflon. Since ESCA analysis (unlike the deeper penetrating EDAX analysis) is limited to about the top 5 nm of surface, the AlLO composition was masked. This hypothesis is currently under further investigation in our laboratory. Recent ESCA analysis
of shuttle samples collected in the surface boundary layer (using polycarbonate filters) produced strong Al photopeaks.
ICE NUCLEI Samples obtained during the SEPEX flight were analyzed for the presence of ice nuclei using the filter technique described by Zamurs et al. (1977). The principle of operation of this technique is to expose filter samples of aerosol to water-saturated environments at sub-freezing temperatures which cause the formation of ice crystals on the active ice nuclei. This is a~omplish~ by placing the sample into a lowtemperature thermal gradient chamber where the temperature and humidity are controlled independently. After exposure to water-saturated sub-freezing conditions for a period of an hour, any crystals nucleated will have grown to a size which can be detected using a low-power microscope. The number of crystals on a filter processed in this manner provides a direct measure of ice nucleus concentrations. A controversial aspect of this technique, however, is known as the ‘volume effect.’ The often discussed volume effect (Braham and Spyers-Duran, 1974) may occur as the result of the simultaneous collection of large amounts of hygroscopic aerosol and ice nuclei on the filter. A competition for water vapor may then occur between the hygroseopic aerosol and the ice nuclei during controlled chamber exposures, uitimately kading to depressed ice crystat counts. For the purposes of this analysis, 10 cm2 sections of the filters were selected for analysis at a temperature of
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WESLEY R. COFE~ III
et al
Bmdtng energy teV1 EDAX onolysrs sllmpte
of SEPEX Curve fit for ESCA At 2p photopeok
o3 135
130
t25
120
u5
Bmdtng energy teV1 Al~min~~m
2s photopenk
from SEPEX sample
Fig. 8. EDAX and ESCA analysis of SEPEX particles. - 20°C and water saturation. Comparable sections of unexpo~d fifters were similarly processed to provide a measure of blank filter concentrations which were typically 5-10 crystals per filter. Samples of the exposed filter processed under these conditions resulted in crystal counts of 65 and 45 for sample pods 1 and 2. Based on the estimated volumes of 0.21 and 0.18 m3, respectively, these counts correspond to concentrations of 310 m-’ and 260 mT3. The levels of ice nuclei determined by the SEPEX experiment are significantly lower than the values obtained in ground cloud samples from the third launch of the space shuttle (Anderson and Keller, 1983). Ground cloud co~entrations measured during the third launch were in the range of 2000 m- ‘-3OOOm- ’ during the first hour, and were about 2.5 times the concentration measured in clean air away from the cloud. Some of the differences between the SEPEX measurements and the ground cloud study may be related to the sampie volumes used ( - 0.2 m3 from SEPEX vs 0.02 m3 per filter section for the ground cloud study). However,
surface concentrations of particles on the SEPEX filters are similar to those encountered in routine ground based filter sampfings for natural ice nuclei. While no background collections were made at the Kennedy Space Center (KSC) before or after launch, natural concentrations of ice nuclei over S Florida are highly variable. Airborne measurements of ice nuclei (Radke er ai., 1978)made in the vicinity of KSC (during a period of launch inactivity) ranged from about 50m-3t08000m-3,withameanofabout 1000m-3. Based on the SEPEX measurements, it seems that the ice nucleus concentrations measured in the unaged coiumn cloud are well within the typical range of con~ntrations of natural ice nuclei over S Florida. However, because of the potentially large concentration of hygroscopic aerosol on the filters, these measurements could be an underestimate of actual ice nucleus concentration. In addition, the SEPEX collections were made within 20 min of the launch, and thus do not address the nature of this aerosol after undergoing aging processes in the atmosphere. Future work
Analysis of mid-tropospheric space shuttle exhausted aluminum oxide particles will address the influences of aging and sample volume on concentration exhaust
measurements
of ice nuclei in shuttle
clouds.
CONCLUSIONS
The first successful collection of A1,03 particles from an unaged mid-tropospheric space shuttle exhaust cloud was made after the launch of STS-61A on 30 October, 1985. The collection was accomplished on Teflon filters with a > 9Sn/:, capture efficiency for particles of diameters 5 0.1 pm at altitudes between 7.6 and 4.6 km. The particles were essentially spherical and composed of Al and 02, Both Al,O, and hydrated Al,O3 were identified using ESCA. A particle size distribution was determined that revealed a large particle mode centered around 2 pm in diameter aid the existence of an accumulation mode peaking at a diameter of less than 0.3 pm. The distribution indicated a mass median diameter of about 2 pm. Accumulations of Al,Os on the filters were reasonably close to anticipated levels. The majority of the results from
these space shuttle particle
collections
largely
support previous conclusions drawn from other solid rocket exhaust studies (e.g. Titan). The SEPEX
results
that the fraction (_ 1: 106) and concentration ( - 285 m - 3, of active ice nuclei in a relatively unaged
suggest shuttle
column
cloud
are within
the ranges found
naturally in the atmosphere above S Florida. However, ice nuclei determinations made by our filter technique could seriously underestimate the numbers of active ice nuclei since our collections most Iikely contained significant amounts of hygroscopic aerosol. Acknowledgments-We wish to acknowledne the maior contributtons made to the SEPEX experim& by our pilot, Mr Phtlip W. Brown, and the T-34 crew chief. Mr Paul R ‘Pfelfer. We also wish to acknowledge the vital support furnished to this project by the mission manager, Mrs Shirley S. Grice,and the instrument manager, Mr Gerald C. Purgold. Spectal thanks are also offered to Mr Wendell G. Avers for his help in planning and executing the SEPEX mission, to Dr G. L. Pellett for his helpful discussions,to Dr W. Van Ooij for his help with the Sl MS analysis, and to Mr Paul F. Wetzel for his continuing support.
REFERENCES Anderson 3. J. and Keller V. W. (1983) Space shuttle exhaust cloud properties. NASA TP-2258, Marshall Space Center. (Available as NTIS-84N-I4~ from the Nat]. Tech. Inf. Serv., Springfield. VA.) Bosart L., Juisto J., Lala G.. Mohnen V., Schaefer V. and Bolay E. (1983) A study of potential inadvertent weather modification resulting from space shuttle rocket exhaust clouds. National Aeronautics and Space Administration Contract Rep. CR-166082. Braham R. R., Jr. and Spyers-Duran P. (1974) Ice nucleus measurements in an urban atmosphere. J. app!. Met. 13, 940-945. Brownlee D. E., Ferry G. V. and Tomandl D. (19761 Stratospheric aluminum oxide. Science 191, 1270-1271.
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Gofer W. R., III. Benduza R. 1.. Sebacher D. I., Pellett G. L., Gregory G. L. and Maddrea G. L. (1985) Airborne mcasurcments of spaae shuttle exhaust constituents. AlAA J. 23, 283-287. Gofer W. R., Ill, Pellett G. L., Sebacher D. I. and Wakelyn N. T. (1984) Surface chloride formation on space shuttle exhaust alumina. J. geopbp. Rcs. 89,2535-2540. Dawbarn R. and Kinslow M. (1976) Studies of the exhaust products from solid propellant rocket motors. AEDC-TR76-49, U. S. Air Force. (Available as NTIS-77N-18210 from the Nat]. Tech. Inf. Serv.) Dullard J. G., Seals R. D. and Wightman J. P. (1980) Electron spectroscopy for chemical analysis (ESCA) study of aluminum~ontaining atmospheric particles. Afmosp~eric Enuironmenr 14, 129-135. Dobbins R. A. and Strand L. D. (1970) A comparison of two
methods of measuring particle size of AI,O, produced by a small rocket motor. AIAA J. 8, 15441550. Hindman E. E. and Finnegan W. G. (1982)Will space shuttle launch clouds be an important source of ice nuclei? J. Weather Modif. 14, 75-77. Hindman E. E., Garvey D. M., Langer G., Odencrantz F. K. and Gregory G. L. (1980) Laboratory investigations of cloud nuclei from combustion of space shuttle propellant. J appl. Met. 19, 175-184.
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L.~fl.12, 93-96. Parungo F. P. and Alee P. A. (1978) Rdcket et&rent: its ice nucleation activity and related properties. J. appf. Ma. 17, 1856-1863. Pollack J. It., Toon 0. B., Summers A., Van Camp W. and Baldwin B. (1976) Estimates of the climatic impact of aerosols produced by space shuttle, SSTs, and other high flying aircraft. J. appl. MIX IS, 247-258. Potter A. E. (1978) Environmentaf elTectsof the spaceshuttle. J. Encir. Sci. 2I, 15-21. Radke L. F., Hobbs P. V. and Hegg 13.A. (1982) Aerosols and trace gasesin the elguents produced by the launch of large liquid- and solid-fueled rockets. J. appl. Met. 21, l332- 1345. Radke L. F., Langer G. and Hindman E. E. (1978) Airborne measurements of cloud forming nuclei and aerosol particles at Kennedy Space Center, Florida. CR-160359, National Aeronauttcs and Space Administration. (Available as NTIS-80N-t 1716 from the Nat]. Tech. Inf. Serv., Springfield, VA.) Sebacher D. I., Cofer W. R. III, Woods D. C. and Maddrea G. L. (1984) Hydrogen chloride and aerosol ground cloud characteristtcs resultmg from space shuttle launches. Atmospheric E~~i~onrnenf 18, 763-770. Space Shuttle Program, Environmental Impact Statement, final. (1978) NASA Tech. Memo., TM-82278. 1978. (Available as NTIS-N81-16624 from the Nat]. Tech. Inf. Serv.. Springfield, VA.) Turco R. P.,Toon 0. B.. Poilack J. 8.. Whitten R. C., Poppoff I C. and Hamtll P. (1980) Stratospheric aerosol moditication by supersonic transport and space shuttle operations-climate implications. 1. appl. Met. 19, 78-89. Turco R. P., Toon 0. B., Whitten R. C. and Cicerone R. J. (1982) Space shuttle ice nuclei. Nature 298, 830-832. Varsi C (1977) Appendix E-particulate measurements.
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of the Workshop
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Envi?a~rneRtul Eflecrs, Potter,
A. E. Compiler, National Aeronautics and Space Admrnistration, TMX-58198. (Available as NTIS-77N18602 from the Natl. Tech. Inf. Serv.) Woods D. C. (1978) Rocket effluent size distributron made with a cascade quartz microbalance. P~oceedj~gs afthe 4th Joinr Conjivence on Sensing of Enrironmentaf
Pollutants,
American
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DC,
pp
716-718.
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00044981,%7 f3.00+0.00 Fqamon Jownals Lid.
VoI 21. No 5. pa I%?-11%. 1987.
ANALYSIS OF MID-TROPOSPHERIC SPACE SHUTTLE EXHAUSTED ALUMINUM OXIDE PARTICLES WESLEYR. COFER,III Atmospheric SciencesDivision, National Aeronautics and Space Administration, Langley Research Center, Hampton, VA 23665, U.S.A.
G. GARLAND
LALA
Atmospheric SciencesResearch Center, State University of New York at Albany, Albany, NY 12222, U.S.A.
and JAMES P. WIGHTMAN Department ofchemistry, Virginia Polytechnic Instituteand State University, Blacksburg, VA24061,U.S.A.
(First recdued 5 Septemberand ~n~nu~~orrn27 Oetolzer 1986) Abstract-Aluminum oxide (AllO,) particles from the exhaust of the space shuttle were collected from the shuttle column cloud immediately after the launch of STS-61A on 30 October 1985. The particulates were collected on T&on filters during a tight descending aircraft spiral maneuver over the altitude interval of 7.646 km. Scanning electron microscope (SEM) examination of the particles revealed that they were virtually all spherical and ranged in diameter from about 0.1 pm to 10 pm. Particles of < 0.1 pm in diameter were not readily visible in the SEM photomicrographs; however, such particles would not be captured efficiently on the Teflon filters used. Results from energy dispersive analysis by X-ray (EDAX) and electron spectroscopy for chemical analysis (ESCA) conlimd that the particles were pr~ominant~y composed of Al and 02. A particfe size distribution was determined from the AlsO, samples. The distribution was bimodal, with one observed peak centered near 2.0 pm. The data indicated the existenceof another mode centered at a diameter of < 0.3 pm, but could not be accurately located because our technique cut otTat diameters of < 0.1 pm. A mass median diameter of slightly c 2 pm was determined. The collection was evaluated for ice nucleation activity, using the fiIter techmque with a static vapor-diffusion chamber. Only a small fraction (about I : IO”)of active ice nuclei were determined among the Al,Os paruculates.
Key word index: Shuttle exhaust ehluents, aluminum oxide particles, ice nucleation.
INTRODUCTION
Particulate aluminum oxide (AlzOJ is a primary exhaust effluent of space shuttle’s solid-propellant rocket boosters. Approximately 3OQOOO kg of A&O, is exhausted by the two shuttle boosters during each launch (Potter, 1978). About 67% of the A1103 is released within the troposphere; the remainder is released into the stratosphere up to about 43 km in altitude, where the boosters burn out and separate (Space Shuttle Program, Environmental Impact Statement, 1978). Substantial interest exists concerning the role and impact of shuttle (or solid-propellant) exhausted A1,03 particles in the atmosphere since these particles could conceivably perturb the radiation budget of the Earth--atmosphere system (Turco cr ul., 1982). Interest has been particularly keen in the areas of nucleation of atmospheric water (Hindman et Qi., 1980. 1982; Radke et al., 1982) and ice (Parungo
and
Alee. 1978; Hindman and Lala. 1980; Hindman and Finnegan, 1982) in the troposphere and lower stratosphere.
Hofmann ef af. (1975) have suggested that the contribution of very small (r +.,0.01 @m) shuttle exhausted AlzOJ particulates to the stratosphere (above 25 km) may be significant. Brownlee er al. (1976), Zolensky and Mackinnon (1985) and Mackinnon and Mogk (1985) have shown with balloon and aircraft collections that substantial concentrations of spherical Al,03 particulates (presumably rocket produced) in the 2-8 pm size range exist in the lower stratosphere. Nevertheless, A&O, injections into the stratosphere from solid rocket motors have not been generally expected to result in discernable climatic impacts (Pollack et ai., 1976; Turco et Qf., 1980). The morphology and chemistry of solid-rocket propellant produced Al,03 particles have been studied extensively. The A&O1 has been shown to consist of both OL and y crystalline phases (Dobbins and Strand, 1970; Dawbarn and Kinslow, 1976). The overwhelming number of AI,O, particles appear to reside in the 0.05-0.3 pm diameter range (Varsi, 1977; Woods, 1978; Bosart er al., 1983) although many particles in the
1187
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WESLEY R. COFER. III et al.
I-1Opm range are always present. The chemistry of soi~d-pro~ilant produced AI,O, particles has been investigated by Dawbarn and Kinslow (1976). Dillard et al. (1980) and Corer er 01. (1984). Prior work, however, has been done with simulated solid-rocket exhaust, with laboratory generated A120, samples, with samplings or collections from Titan III launches, from shuttle boundary layer exhaust clouds contaminated with surface debris (commonly referred to as ground clouds), or from aged collections made days to months after atmospheric injection. Thus, a need existed to study samples of shuttle exhausted AllO collected immediately after a launch at relevant altitudes. The Shuttle Exhaust Particle Experiment (SEPEX) was therefore conducted to acquire the first representative samples of shuttle exhausted AI,O, from an unaged mid-tropospheric shuttle column cloud. The physical and chemical properties determined from these samples would then closely resemble the initial state (as injected into the atmosphere) of shuttle exhausted A1203 particles.
Table I. A NASA Langley Research Center T-34C (single engine turbo-prop) aircraft was used as the particle collection platform. The flight plan consisted of the aircraft assuming a holding pattern at 7.6 km minutes before launch, then after launch and rangesafety clearance, proceeding into the resulting column cloud and sampling continuously while performing a tight descending spiral maneuver to about 4.6 km, then terminating sampling and returning to base with the collected sample of exhaust particulates. Pilot commentary and continuous onboard photography of the cloud were documented during the mission on VHS cassette. The nominal ascent trajectory for the space shuttle, the targeted region for exhaust cloud sampling, and estimated initial column cloud parameters (for I +2 min) arc shown in Fig. 1.
EXPERIMENTAL
Two identical high-vol filter collection systems were fabricated and fitted into pods which were suspended from pylons beneath the wings of the T-34C aircraft. Air was drawn through the system using a 28 VDC vacuum blower. All valving was pneumati~l~y or electrically actuated from the cockpit. Each pod was operated independently, and utilized a 23 cm by 16 cm Teflon filter for particle collection. The capture efficiency of the filters was a function of particle size. The
MISS1ONS
The SEPEX mission was conducted on 30 October, 1985 (STSdlA), a noon launch. Abbreviated meteorological parameters obtained from a Cape Canaveral rawinsonde release at launch time are presented in
Estimated Cloud Parameters
(t+2 min)
Cloud diameter -350 m -80 mg/m3 Al203 concentration Mean particle diameter - 1 pm HC1 concentration -70 ppmv I Hori~ofl~a~ range, Fig. 1.
I 6
km
NominaI ascent trajectory of a space shuttle launch.
Analysis of mid-tropospheric space shuttle exhausted aluminum
oxide particles
1189
Table 1. Selected meteorological parameters for 1703 GMT 30 October Altitude
Ptw+.wn
0.004 km 1.525 3.050 4.570 6.095 7.620 9.150
1006mb 844 704 585 480 392 317
Temperature
289°C 15.1 7.1 - 1.2 -11.8 - 23.3 - 34.4
Teflon filters used were rated at 98.9% retention for 0.3 pm diameter DOP smoke particles. The capture efficiency for larger particles should approach 100%. Capture efficiency for 0.1 pm particles was estimated to be about 96 %. Particles with diameters of < 0.1 pm should progressively (with decreasing diameter) be less efficiently captured. The system was designed to be isokinetic at an aircraft speed of about 45 m se ‘. Flow through the systems at ambient laboratory pressures and temperatures was determined to be about 1.1 m3 min-‘. Both particle collection systems were evaluated in thermalvacuum chambers at temperatures and pressures approximating the planned mission. Volume fiow rates were measured in the chamber with a turbine flowmeter at 370 mb-2Y’C-‘, at 500 mb-lS”C-‘, and at 665 mb-O”C-‘. Data from these chamber tests indicated our collection systems would process about 0.6 standard m3 min - ’ over the sampled attitude interval (7.6 km-4.6 km). However, the measured volumetric flow at 370 mb-ZS”C- I (approximately equivalent to an altitude of 7.6 km) was only 10% lower (1.0 m3 min-‘) than at SIP. Since this represents about the same volume of air space sampled per unit of time, actual aircraft sampling should have remained close to isokinetic over the sampled altitudes.
ANALYSIS
Analysis of the particles consisted of scanning electron microscopy @EM) for morphology and size distribution, energy dispersive analysis of X-ray (EDAX) for elemental composition, and electron spectroscopy for chemical analysis (ESCA) of the surface composition. SEM anaIysis was performed on a Cambridge ISOR coupled with a EDAX model 9100 with windowless detection. Analysis was done on sections of Teflon filter approximately 1.3 cm by 1.3 cm. The sections were first photographed (SEM) and then EDAX analysis was performed. Samples for ESCA analysis were cut from regions of the Teflon filters where high particle densities were expected. Filter sections of about I cm by 1.9 cm were mounted on the sample probe using double stick tape. Samples were run on a Kratos XSAM-800 electron spectrometer. Both an ESCA survey (wide scan) and an ESCA multiplex (narrow scan) spectra were run on each sample. The C Is photopeak from the hydrocarbon
Humidity (r.h.)
Wind speed
Wind (dir)
70%
10 Kts
150”
78 25 35 35 22 26
17 28 23 41 36 32
212 216 207 206 213 211
background was assigned a binding energy of 285.0 electron volts (eV). A total of seven separate samples were run. RESULTS AND DISCUSSION
The aircraft penetrated the column cloud at approximately 7.6 km about 5 min after launch and sampled during descent for 5 15 min, exiting the exhaust plume at about 4.6 km. Total in-cloud sampling times of about I4 min and 12 min for pods one and two, respectively, were estimated from the cassette record. This translates into sample vol of 7.2 and 6.2 m3 of exhaust plume air processed through collectors 1 and 2, respectively. The column cloud appeared dull grey and thin, typical of a relatively dry shuttle exhaust plume (plumes with large amounts of water droplets appear brilliantly white). Meteorological parameters in Table 1 confirm low humidities over the sampled altitudes and support the dry-cloud observation. Filters from sampling pods 1 and 2 were weighed and found to have collected 26 f 3 and 2I+ 3 mg of material, respectively. Theshuttlecolumn cloud, therefore, averaged about 3.5 mg m - 3 of particulates over the integrated time interval. This result is reasonable when compared (see Fig. 1) with the calculated concentrations for the relatively undispersed column cloud at 2 min after launch. Particles were found to accumulate on the filters uniformly except near the perimeter (within w 0.5 cm of the filter retaining plates) where larger numbers of particles were observed to concentrate. This distribution is shown in Fig. 2 where representative scanning electron photomicrographs of sections of the filter from (a) midfilter and (b) a filter/perimeter edge section are shown. It can be readily seen in Fig. 2 that the collected particles were virtually all spherical and exhibited a considerable degree of agglomeration. Whether the A&O3 chtsters occurred before or during collection cannot be determined. None of the individual particles shown in Fig. 2 are larger than 10 pm in diameter. Infrequently, particles of diameter greater than lO#m were identified during this analysis. The largest single particle observed in the SEPEX experiment was slightly < 20 pm in diameter. The particles shown in Figs 2 and 3 are typical of the vast majority of particles collected during this mission. The microscopic smoothness of the surface of the particles typically observed
1190
WESLEY R. COFER, III ef al.
Fig. 2. Repnsentative
SEM photomicrographs
in these collections can be seen in Fig. 3 along with the large number of sub-pm particles collected. Almost no ‘frosted* or ‘snowlike’ particles were observed in this collection. as have been previously reported (Dawbarn and Kinslow, 1976, Dillard et a!., 1980; Gofer et al., 1984). In our prior work, where frosted particle surfaces were observed, the samples had been collected from ‘wet’ exhaust clouds. Dissolution of y A&O, in these HCI enriched droplets (Sebacher er al., 1984; Cofer et al., 1985) followed by evaporation and recrystallization may have ied to the surface frosting. Since the SEPEX samples were acquired from a ‘dry’ column cloud, these processes may have been negligible or minimized. Particles with several pronounced surface features (presumably the result of incomplete collisional coalescence of molten Al,O, in the exhaust plume) were observed in these analyses. The particles shown in Fig. 4a, b appear to be composed of several partially fused A&O, particles. This morphology when observed was almost always associated with the larger
of shuttle exhaust particles.
diameter particles (> 3 pm). Figure 4a is a particle from the SEPEX collection, while 4b was obtained from the wing surface of an aircraft that had penetrated a shuttle launch cloud from STS-SIB (29 April, 1985) at about 2.8 km. Such characteristic surface features have &en previously noted for large shuttle particles collected in the surface boundary layer and at the Iaunch site (Cofer et ol., 1984). However, a greater proportion of the >3 pm particles from the ground and boundary layer samples appeared to exhibit these features. That the SEPEX particles appeared to contain a smaller fraction of partially coalesced particles is difficult to explain. Two factors were significantly different in this collection. This collection did not have the bias toward large particles of the previous collections (Cofet et al., 1984) and this collection was made at much higher altitudes than the prior collections. A particle size distribution for the SEPEX collections was determined by counting - 1900 particles from four different sets of SEM photomicrographs at
several magnifications.
Even at the highest magnifi-
Analysis of mid-tropospheric space shuttle exhausted aluminum oxide particles
1191
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WFSLEY R. COFES III et a[.
cation (13,000X), particles with diameters < 0 I pm were almost impossible to measure or count. Our data analysis, therefore, does not include particles below 0.1 pm in diameter. Likewise, particles with diameters > 8IOpm were not often encountered and are not included in our distribution. Nevertheless. we feel confident that very small particles likely dominated the particle number distribution, as reported by others (Woods, 1978; Bosart er al., 1983), and some particles with diameters > 10-20 pm as observed by others (Anderson and Keller, 1983) are most certainly exhausted by the shuttle boosters. The distribution is shown as a function of the number of particles accumulated cm-’ of Teflon filter in Fig. 5. It is obvious from Fig. 5 that the largest number of particles occur at the smallest diameters. The distributions based on number and mass cmw3 of sampled shuttle column cloud air (using 6.7 m3 as the total sample volume) are shown in Fig. 6 as a function of the change in particle number with respect to the change in the log of the particle diameter vs particle diameter, and in Fig 7 as the cumulative percentage of mass vs particle diameter. The distribution appears to be bimodal (an unobserved maximum in particle number must occur somewhere between 0.3 and 0.0 pm in particle diameter) with a mass median diameter of just below 2 pm. This result is consistent with previous studies (Dawbarn and Kinslow. 1976; Varsi, 1977; Woods, 1978: Bosart er al., 1983) that have indicated size modes for solid rocket produced Al,O, 1 x 18+Number
521085210’5210652Ids2IO45210352-
1x102-
I 0
Particle
’ 1
’ 2
’ 3
Diameter,
’
4
’
5 Dp,
’
6
’
7 (pm)
Fig. 5. Particles accumulated per cm2 on filler
J
8
105-
AN L\log I04 (cm-3) 103 500 :
200
0O
B
100 ” 50.0 20.0lO.O-
5.00 ‘.ool
\
1.00 .500 .200 .I00
0.1
I 0.2
I Ifilllll a4 0.6
Particle
1.0
Diameter,
Fig. 6. Number distribution
I 20
Ittllu) 4.0 60 Dp,
10
pm
in exhaust cloud.
particles, though locations of the modes have differed. Calculations based on our number distribution and an AlzO, density of 3.7 g cm _ 3 (approximate density for an equal mix of a and 7 A120, phases) indicate that about 43 mg of particles should have accumulated on each filter. Although our measured average accumulation (24 mg) appears to be low relative to the calculated accumulation, evidence has been presented suggesting that the density for rocket produced A1203 may range between 1.5 and 3.5 g cm-’ (Turco et al., 1982). An average density of about 2.1 g cme3 would bring our measured and calculated accumulations into good agreement. It has also been noted that our distribution was biased against small particles, although this bias would not be expected to account for much mass. EDAX analysis of the SEPEX particles always revealed strong AI and O2 peaks, as shown in Fig. 8a. A F peak was observed on one occasion, attributed to the Teflon filter. Cl was not detected in any of the EDAX analyses. A representative set of results of the ESCA analysis of the filter samples is shown in Table 2. The largest photopeaks were due to C and F. Indeed, the F to C ratio as calculated from the photopeak areas is about 2.0 as expected from Teflon. A small but persistent Cl signal was observed on most samples. It was not possible to determine whether the Cl was bonded to the particles or adsorbed (or chemisorbed) on the surfaces, or both. Of the seven samples run, the expected Al photopeak was prominent on only one sample, shown in Fig. 8b. The peak is broad and was curve fit to gwe the two photopeaks with corrected
Analysis of mid-tropospheric space shuttle exhausted aluminum oxide particles
(fim)
1193
432-
I0.8060.40.3-
Percentage
of mass
Fig. 7. Cumulative percentage of mass vs particle diameter.
Table 2. Typical ESCA analysis of Teflon filter samples Photopeak c Is F Is 0 1s Cl 2p
Binding energy (eV) 291.8 688.0 532.0 199.5
Atomic fraction 0.32 0.66 0.01 0.003
binding energies of 120.3 eV and 118.9 eV as shown in Fig. 8c. A full width at half maximum of 1.80 eV was used in the curve fit analysis. The higher binding energy peak at 120.3 eV is characteristic of AI in A120J, while the lower binding energy peak at 118.9 eV is typical of Al in hydrated A1203. The peak areas are proportional to concentration and suggest a slightly greater contribution of the hydrated A1,OJ. The fact that Al was easily detected by EDAX, but with great difficulty by ESCA, was unexpected. It was accounted for after surface- ionization mass spectrometry (SIMS) experiments revealed that the A1203 particles had been initially covered by a thin (several monolayers) Teflon film. After brief etching of individual particles with a gallium ion gun (lOkeVCa+), which initially yielded CF’ ions (characteristic of Teflon), AlzOJ composition was confirmed. We suggest that sputtering resulted from X-ray bombardment of the Teflon filter sections during the ESCA analysis, coating the surface of the AI,O, particles with Teflon. Since ESCA analysis (unlike the deeper penetrating EDAX analysis) is limited to about the top 5 nm of surface, the AlLO composition was masked. This hypothesis is currently under further investigation in our laboratory. Recent ESCA analysis
of shuttle samples collected in the surface boundary layer (using polycarbonate filters) produced strong Al photopeaks.
ICE NUCLEI Samples obtained during the SEPEX flight were analyzed for the presence of ice nuclei using the filter technique described by Zamurs et al. (1977). The principle of operation of this technique is to expose filter samples of aerosol to water-saturated environments at sub-freezing temperatures which cause the formation of ice crystals on the active ice nuclei. This is a~omplish~ by placing the sample into a lowtemperature thermal gradient chamber where the temperature and humidity are controlled independently. After exposure to water-saturated sub-freezing conditions for a period of an hour, any crystals nucleated will have grown to a size which can be detected using a low-power microscope. The number of crystals on a filter processed in this manner provides a direct measure of ice nucleus concentrations. A controversial aspect of this technique, however, is known as the ‘volume effect.’ The often discussed volume effect (Braham and Spyers-Duran, 1974) may occur as the result of the simultaneous collection of large amounts of hygroscopic aerosol and ice nuclei on the filter. A competition for water vapor may then occur between the hygroseopic aerosol and the ice nuclei during controlled chamber exposures, uitimately kading to depressed ice crystat counts. For the purposes of this analysis, 10 cm2 sections of the filters were selected for analysis at a temperature of
1194
WESLEY R. COFE~ III
et al
Bmdtng energy teV1 EDAX onolysrs sllmpte
of SEPEX Curve fit for ESCA At 2p photopeok
o3 135
130
t25
120
u5
Bmdtng energy teV1 Al~min~~m
2s photopenk
from SEPEX sample
Fig. 8. EDAX and ESCA analysis of SEPEX particles. - 20°C and water saturation. Comparable sections of unexpo~d fifters were similarly processed to provide a measure of blank filter concentrations which were typically 5-10 crystals per filter. Samples of the exposed filter processed under these conditions resulted in crystal counts of 65 and 45 for sample pods 1 and 2. Based on the estimated volumes of 0.21 and 0.18 m3, respectively, these counts correspond to concentrations of 310 m-’ and 260 mT3. The levels of ice nuclei determined by the SEPEX experiment are significantly lower than the values obtained in ground cloud samples from the third launch of the space shuttle (Anderson and Keller, 1983). Ground cloud co~entrations measured during the third launch were in the range of 2000 m- ‘-3OOOm- ’ during the first hour, and were about 2.5 times the concentration measured in clean air away from the cloud. Some of the differences between the SEPEX measurements and the ground cloud study may be related to the sampie volumes used ( - 0.2 m3 from SEPEX vs 0.02 m3 per filter section for the ground cloud study). However,
surface concentrations of particles on the SEPEX filters are similar to those encountered in routine ground based filter sampfings for natural ice nuclei. While no background collections were made at the Kennedy Space Center (KSC) before or after launch, natural concentrations of ice nuclei over S Florida are highly variable. Airborne measurements of ice nuclei (Radke er ai., 1978)made in the vicinity of KSC (during a period of launch inactivity) ranged from about 50m-3t08000m-3,withameanofabout 1000m-3. Based on the SEPEX measurements, it seems that the ice nucleus concentrations measured in the unaged coiumn cloud are well within the typical range of con~ntrations of natural ice nuclei over S Florida. However, because of the potentially large concentration of hygroscopic aerosol on the filters, these measurements could be an underestimate of actual ice nucleus concentration. In addition, the SEPEX collections were made within 20 min of the launch, and thus do not address the nature of this aerosol after undergoing aging processes in the atmosphere. Future work
Analysis of mid-tropospheric space shuttle exhausted aluminum oxide particles will address the influences of aging and sample volume on concentration exhaust
measurements
of ice nuclei in shuttle
clouds.
CONCLUSIONS
The first successful collection of A1,03 particles from an unaged mid-tropospheric space shuttle exhaust cloud was made after the launch of STS-61A on 30 October, 1985. The collection was accomplished on Teflon filters with a > 9Sn/:, capture efficiency for particles of diameters 5 0.1 pm at altitudes between 7.6 and 4.6 km. The particles were essentially spherical and composed of Al and 02, Both Al,O, and hydrated Al,O3 were identified using ESCA. A particle size distribution was determined that revealed a large particle mode centered around 2 pm in diameter aid the existence of an accumulation mode peaking at a diameter of less than 0.3 pm. The distribution indicated a mass median diameter of about 2 pm. Accumulations of Al,Os on the filters were reasonably close to anticipated levels. The majority of the results from
these space shuttle particle
collections
largely
support previous conclusions drawn from other solid rocket exhaust studies (e.g. Titan). The SEPEX
results
that the fraction (_ 1: 106) and concentration ( - 285 m - 3, of active ice nuclei in a relatively unaged
suggest shuttle
column
cloud
are within
the ranges found
naturally in the atmosphere above S Florida. However, ice nuclei determinations made by our filter technique could seriously underestimate the numbers of active ice nuclei since our collections most Iikely contained significant amounts of hygroscopic aerosol. Acknowledgments-We wish to acknowledne the maior contributtons made to the SEPEX experim& by our pilot, Mr Phtlip W. Brown, and the T-34 crew chief. Mr Paul R ‘Pfelfer. We also wish to acknowledge the vital support furnished to this project by the mission manager, Mrs Shirley S. Grice,and the instrument manager, Mr Gerald C. Purgold. Spectal thanks are also offered to Mr Wendell G. Avers for his help in planning and executing the SEPEX mission, to Dr G. L. Pellett for his helpful discussions,to Dr W. Van Ooij for his help with the Sl MS analysis, and to Mr Paul F. Wetzel for his continuing support.
REFERENCES Anderson 3. J. and Keller V. W. (1983) Space shuttle exhaust cloud properties. NASA TP-2258, Marshall Space Center. (Available as NTIS-84N-I4~ from the Nat]. Tech. Inf. Serv., Springfield. VA.) Bosart L., Juisto J., Lala G.. Mohnen V., Schaefer V. and Bolay E. (1983) A study of potential inadvertent weather modification resulting from space shuttle rocket exhaust clouds. National Aeronautics and Space Administration Contract Rep. CR-166082. Braham R. R., Jr. and Spyers-Duran P. (1974) Ice nucleus measurements in an urban atmosphere. J. app!. Met. 13, 940-945. Brownlee D. E., Ferry G. V. and Tomandl D. (19761 Stratospheric aluminum oxide. Science 191, 1270-1271.
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Gofer W. R., III. Benduza R. 1.. Sebacher D. I., Pellett G. L., Gregory G. L. and Maddrea G. L. (1985) Airborne mcasurcments of spaae shuttle exhaust constituents. AlAA J. 23, 283-287. Gofer W. R., Ill, Pellett G. L., Sebacher D. I. and Wakelyn N. T. (1984) Surface chloride formation on space shuttle exhaust alumina. J. geopbp. Rcs. 89,2535-2540. Dawbarn R. and Kinslow M. (1976) Studies of the exhaust products from solid propellant rocket motors. AEDC-TR76-49, U. S. Air Force. (Available as NTIS-77N-18210 from the Nat]. Tech. Inf. Serv.) Dullard J. G., Seals R. D. and Wightman J. P. (1980) Electron spectroscopy for chemical analysis (ESCA) study of aluminum~ontaining atmospheric particles. Afmosp~eric Enuironmenr 14, 129-135. Dobbins R. A. and Strand L. D. (1970) A comparison of two
methods of measuring particle size of AI,O, produced by a small rocket motor. AIAA J. 8, 15441550. Hindman E. E. and Finnegan W. G. (1982)Will space shuttle launch clouds be an important source of ice nuclei? J. Weather Modif. 14, 75-77. Hindman E. E., Garvey D. M., Langer G., Odencrantz F. K. and Gregory G. L. (1980) Laboratory investigations of cloud nuclei from combustion of space shuttle propellant. J appl. Met. 19, 175-184.
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L.~fl.12, 93-96. Parungo F. P. and Alee P. A. (1978) Rdcket et&rent: its ice nucleation activity and related properties. J. appf. Ma. 17, 1856-1863. Pollack J. It., Toon 0. B., Summers A., Van Camp W. and Baldwin B. (1976) Estimates of the climatic impact of aerosols produced by space shuttle, SSTs, and other high flying aircraft. J. appl. MIX IS, 247-258. Potter A. E. (1978) Environmentaf elTectsof the spaceshuttle. J. Encir. Sci. 2I, 15-21. Radke L. F., Hobbs P. V. and Hegg 13.A. (1982) Aerosols and trace gasesin the elguents produced by the launch of large liquid- and solid-fueled rockets. J. appl. Met. 21, l332- 1345. Radke L. F., Langer G. and Hindman E. E. (1978) Airborne measurements of cloud forming nuclei and aerosol particles at Kennedy Space Center, Florida. CR-160359, National Aeronauttcs and Space Administration. (Available as NTIS-80N-t 1716 from the Nat]. Tech. Inf. Serv., Springfield, VA.) Sebacher D. I., Cofer W. R. III, Woods D. C. and Maddrea G. L. (1984) Hydrogen chloride and aerosol ground cloud characteristtcs resultmg from space shuttle launches. Atmospheric E~~i~onrnenf 18, 763-770. Space Shuttle Program, Environmental Impact Statement, final. (1978) NASA Tech. Memo., TM-82278. 1978. (Available as NTIS-N81-16624 from the Nat]. Tech. Inf. Serv.. Springfield, VA.) Turco R. P.,Toon 0. B.. Poilack J. 8.. Whitten R. C., Poppoff I C. and Hamtll P. (1980) Stratospheric aerosol moditication by supersonic transport and space shuttle operations-climate implications. 1. appl. Met. 19, 78-89. Turco R. P., Toon 0. B., Whitten R. C. and Cicerone R. J. (1982) Space shuttle ice nuclei. Nature 298, 830-832. Varsi C (1977) Appendix E-particulate measurements.
WESLEY R. COFER III et d.
1196 Proceedings Assessment
of the Workshop
Space Shuttle on Stratospheric
Envi?a~rneRtul Eflecrs, Potter,
A. E. Compiler, National Aeronautics and Space Admrnistration, TMX-58198. (Available as NTIS-77N18602 from the Natl. Tech. Inf. Serv.) Woods D. C. (1978) Rocket effluent size distributron made with a cascade quartz microbalance. P~oceedj~gs afthe 4th Joinr Conjivence on Sensing of Enrironmentaf
Pollutants,
American
Chemical
Society,
Washmgton.
DC,
pp
716-718.
Zamurs J., Lala C. G. and Jmsto J. E. (1977) Factors afTecectmg ice nucleus concentrations measurements with a stattc vapor-diflitsion chamber. J. appI. Me: 16, 419-424. Zolensky M. E. and Ma&Anon D. R. (1985) Accurate stratospheric particle srze djstributions from a flat-plate collection surface. J. geophys Rex 90, 5801-5808.