ESGT FEATURES
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concern about the potential environmental damage. How could scientists estimate the extent of the problem? The solution involved airborne measurements using research aircraft. With increasing frequency, scientists and policy makers are turning to research aircraft to provide critical data to address atmospheric issues such as urban and regional photochemical air pollution, visibility i n our national parks, stratospheric ozone depletion, and global climate change. Significant challenges exist in measuring atmospheric parameters and trace atmospheric constituents from aircraft. Space and electrical power are at a premium, mechanical vibration and heat often present problems for delicate instruments, and airsickness can snare the unwary flight scientist. However, in many circumstances, aircraft-based measurements provide the only cost-effective way to gather needed data over very large horizontal and vertical spatial scales. There are dozens of instrumented aircraft dedicated to atmospheric research that are operated by research groups around the world. These aircraft provide invaluable information on the composition and
412 A
dynamics of the atmosphere. Each aircraft has been instrumented to address certain types of atmospheric questions, and many are designed for flexibility so the instrument package can be reconfigured to meet changing needs. This article describes one such research aircraft and the challenges that were overcome to deploy stateof-the-art measurement technology in an aircraft environment. We also focus on the chemical instrumentation and the recent addition of tandem mass spectrometry to the capabilities available for atmospheric characterization. Recent work at the Max-Planck Institut fur Kemphysik in Germany has demonstrated the potential of this technique for airborne measurements ( I ) . This airborne research facility, our “laboratory in the sky,” was developed and used by Battelle and the U.S. Department of Energy’s (DOE’S)Pacific Northwest Laboratory over the past several years. The plane that we use to study atmospheric physical and chemical processes is a Grumman Gulfstream 1 (G-l), which is a twin-engine tur-
boprop, schematically shown in Figure 1. The G-l has a visual flight rule range exceeding 1500 nautical mi (endurance of about 6 h). It carries as much as 2800 lb of scientific payload with seats for four scientists and has a sampling speed range of 160-250 knots. At gross weight it can reach an altitude of 25,000 ft in one hour and has a maximum altitude of 30,000 ft. The aircraft has significant capabilities for measurements in atmospheric chemistry, winds and turbulence, cloud physics, and radiation. The data acquisition system on the G-1 contains special interfaces to log data from a Long-Range Navigation system, the Global Positioning System, and an inertial navigation system, as well a s particle measurement systems and other scientific probes. In addition to the special interfaces, the system accepts data through u p to 16 standard parallel ports and a %-channel analog-to-digital converter. The system provides continuous time series plots of measured variables. The aircraft position and sampling track are displayed on a map of the study area in real time.
CHESTER W. SPICER DONALD V, KENNY
Instrumentation The interior of the research aircraft is arranged for maximum flexibility so that instrumentation can be installed quickly for specific research missions (see photo, page 414A). A suite of standard instrumentation that can be included on research missions is listed in Table 1. This instrumentation monitors and records aircraft position, altitude, and motion: meteorological parameters: trace gas mixing ratios: particle com-
Environ. Sci. Technol., Vol. 28, No. 9, 1994
Battelle Columbus, OH43201
W I L L I A M J. S H A W KENNETH M. BUSNESS ELAINE G. CHAPMAN Pacific Northwest Laboratory Richland. WA 99352
0013-936)(/94/0927-412A$04.50/0 0 1994 American Chemical Societv
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Schematic diagram of the G-1 research aircraft
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414 A
Environ. Sci. Technol., Vol. 2& No. 9, 1994
size distribution, and light I position, scattering properties: cloud physics information: and radiation. In addition to these standardized instruments, there are a number of specialized measurement capabilities that we have deployed on the G-1 for specific programs. These capabilities include continuous real-time and integrated tracer measurements, real-time measurements of peroxyacetyl nitrate (PAN), continuous monitoring of formaldehyde, continuous monitoring of hydrogen peroxide and organic peroxides, measurements of trace-level hydrocarbons and halogenated hydrocarbons, and identification a n d ultratrace monitoring of many organic and iuorganic chemicals by tandem mass spectrometry. Airborne tandem mass spectrometry Our use of tandem mass spectrometry illustrates how our scientific needs for fast, specific, sensitive measurements are being met. In 1987, we installed and began operation of an atmospheric pressure chemical ionization (APCI) tandem mass spectrometer (MSI MS) aboard the DC-3 aircraft that was the predecessor to Battelle’s G-l research aircraft. The potential of this technology for addressing many problems in the atmospheric sciences was so clear that a program was initiated in 1989, after acquisition of the GI, to integrate the tandem mass spectrometer aboard the new research aircraft. This effort was carried out under the auspices of DOE’S Atmospheric Chemistry Program. The goal was to modify the aircraft and the mass spectrometer to allow for rapid installation and removal, safe and reliable operation, and integration of the mass spectrometer data output with the aircraft data acquisition system. The tandem mass spectrometer that was modified for airborne monitoring is a Sciex Model 6000E.This instrument can sensitively, selectively, and in many cases simultaneously monitor numerous chemicals that a r e important i n atmospheric processes. (Data acquisition software allows collection of data for up to 128 ions at a time. At least two ions are monitored for each chemical species when the instrument is used in the selective MSlMS mode, so 64 species can be
tandard instrumentation that can be used on the 0-1 aircraft Instrument
Technique
Paflicles and gases
ace elements ze distnbutio (real time) ectrtcal aerosol analyzer xosol light scattering MRI 156011590 (bscat)
b
Filter pack
Integrating nephelometer
TECO 49
a),
eteofology(including turbulence) mperature Rosemount 102U2U/ 510BF
drtace/sky
temperature
Ran
monitored continuously i n this mode. In practice, we have rarely had occasion to monitor more than 5-10 chemicals at a time in any one ionization mode.) The mass spectrometer samples the atmosphere directly and has a response time of a fraction of a second. The instrument requires no external source of water or liquid cryogen, making it well suited for use on an airplane. It is an exceedingly versatile instrument; however, that same versatility often requires that considerable effort be devoted to optimizing the system for a desired set of target chemicals. The basic components of the airborne mass spectrometer are noted in Figure 2. These components include an inlet module (sampling prohe), an ionization source, transfer ion lenses, three quadrupole and a mass analyzers (Q1, (I, detector. The mass spectrometer samples air directly into its inlet at atmospheric pressure. Trace contaminants in the sampled airstream are ionized by a corona discharge at atmospheric pressure. Ionized molecules are electrically accelerated through a countercurrent flow of dry nitrogen toward a small orifice, where they are carried into the vacuum system by a small flow of nitrogen gas. The first mass analyzer (QJ is normally operated as a mass filter eliminating all but those ions of a specific mass of interest. The mass of interest is selected to correspond to a molecular ion of a particular contaminant that may also include molecular or fragment ions of interfering species. The high selectivity of the tandem mass spectrometer is achieved through the use of the second and third quadrupoles. Ions passing through the first quadrupole are accelerated into the second quadrupole where they are intercepted by a beam of neutral argon atoms. Collision with argon results in fragmentation of the ions in a predictable manner characteristic of their molecular structure. Fragments resulting from the molecular ion of interest are then sorted out by the third quadrupole mass analyzer (93). Independent computer control of the mass analyzers provides versatility. The instrument can monitor specific target compounds, or it can scan a mass range for compounds that have a common structural feature. Conventional scanning modes provide information for the identification of unknowns. The instru-
Pyrometer Instr. PRT-5
ewpoint temperature General Eastem kolute humidity
10HB AIR
ressure eator winds
Rosemount 1201F1 PNL Gust P m b
-
ondensation nuclei counter
TSI 3020,3025
supersaturation-
Environ. Sci. Technal., Val. 28, NO. 9, 1994 415A
FIGURE L
Schematic diagram of the main components of the airborne tandem mass spectrometer CAD (Collisionally activated dissociation) N~2~gas Detector ~~I~ ~
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416A Envimn. Sci. Technol., VoI. 28. No. 9. 1994
ment can be rapidly switched bet w e e n positive a n d negative ionization modes, allowing a wide variety of chemicals to be monitored. For specialized applications, a second ionization source that operates on the principle of Low Pressure Chemical Ionization (LPCI) can be installed in place of the APCI source. The LF'CI source ionizes hydrocarbon and halogenated hydrocarbon species and is complementary to the APCI source. The APCI ionizer provides highly efficient ionization, and consequently low detection limits, for a wide range of atmospherically important chemicals. With this ionizer, the instrument is relatively ins e n s i t i v e to hydrocarbons but exceedingly sensitive to most organic species containing oxygen, nitrogen, sulfur, and to many important inorganic gases. Table 2 lists the mass spectrometer's estimated detection limits for selected chemical compounds. An illustration of the operation of the tandem mass spectrometer is provided in Figure 3, using dimethyl sulfide [DMS) as an example. DMS has a molecular weight of 62 amu. By using the tandem mass spectrometric capability of the instrument, the first mass filter (QJ is tuned to pass only ions of mass 62. These ions are collisionally dissociated in (22, and the mass spectrum of the fragments, or product ions, is resolved by the third mass analyzer (Q3). The resulting product ion spectrum for DMS is shown in Figure 3(a). Both the parent ion at 62 amu and the product ion at 47 amu are evident. One important application of tandem mass spectrometry is identification of u n k n o w n chemicals through their product ion spectra. Once the principal product ions of a compound are identified, the tandem mass spectrometer can be tuned to monitor specific parent ion-product ion combinations. By setting Q1 to pass only mass 62 and Q1 to monitor mass 47, the mass spectrometer becomes a very specific and very rapid continuous monitor for DMS. Figure 3(b) shows the signal from the instrument operated in this mode during a multipoint calibration for DMS over the range 0-70 ppt. The resulting calibration curve is shown in Figure 3(c). Kelly and Kenny (2)have demonstrated linearity for DMS to at least 2500 ppt. Through rapid computer tuning of the quadrupole mass filters, numerous chemicals
stimated airborne mass spectrometer detection limits by Impound class I impound class
Example
sohols dehydes kaloids
Meth a n oI Benzaldehyde
nines
Nicotine Dimethylformamide Pyridine
~boxylicacids
Ammonia Acetic acid
nides
iters -.hers Halogens Inorganic acids
Ethyl butyrate
Ketones Nitriles Nitrosamines Organometallics Pesticides Sulfur compounds
Acetone Benzonitrile Dimethylnitrosamine Trimethylarsine Sulfotep Dimethyl sulfoxide Dimethyl sulfide Sulfurdioxide
(organic)
Sulfur compounds (inorganic)
?rpenes(terpene-like compounds)
Diethyl ether Bromine Sulfuric acid Hydrogen chloride
Linalool
Estimated detection limit (pptr) 500 10
can be monitored essentially simultaneously. Incorporating a mass spectrometer approximately 2.5 m long and weighing about 450 kg into a medium size aircraft with one small entrance door was a major challenge. Because of the size, weight, and power requirements of the mass spectrometer, modifications to both the aircraft and mass spectrometer systems were required. The tandem mass spectrometer requires substantial 22O-V, 60-Hz power to operate the cryogenic pump, and 115-V 60-Hz power to operate several electronic systems and the computer controlldata acquisition system. The electrical system on the G-1 provides ample 115 VAC power, but not the 220 VAC power required by the cryopump. For safety reasons, the existing generators on the aircraft engines could not provide power for research equipment during takeoff, and continuous 220 VAC power is essential to maintain the mass spectrometer high vacuum. Loss of cryopump power for even a few minutes results in loss of vacuum that terminates instrument operation for approximately 5 h. The need for continuous 220 VAC power during takeoff and normal flight operations was satisfied by adding an extra generator to the aircraft. A new DC generator was designed and built specifically for this application. The auxiliary generator provides an additional 300 amps of 28 VDC power that is totally independent of other aircraft power systems. A series-parallel arrangement of DCIAC inverters converts the 28 VDC power to 220 VAC. For ground operations, such as preflight testing and calibration, an external DC generator powers all inverters and equipment requiring either 115 V or 220 V power. Thus, readiness of onboard equipment is maintained before engine startup and after shutdown. Switchover from groundbased power to aircraft power can be made without interruption of research equipment operation. In its normal configuration, the mass spectrometer is too long and wide for installation through the G-1 cabin door. Therefore, the instrument was reconfigured into three interlocking units, each of which is small enough to fit through the aircraft door and cabinway. A platform was designed and built to mount the instrument to the G-1 aircraft frame, and provide both structural stability and vibration protection. Vibration measurements were Environ. Sci. Technol.. Vol. 28, No. 9.1994 417 A
made on the aircraft during flight, and the mass spectrometer's sensitivity to vibration frequencies was assessed in the laboratory. These measurements were used to design the mounting platform.
1
I
50
70
Applications Many important problems in the field of atmospheric science have been addressed using measurements made by research aircraft. In
90
the past five years, the G-l has been used in a variety of field measurement programs. The Frontal Boundary Study, carried out in OctoberNovember 1989,was part of a series of studies conducted for DOE and coordinated by the National Acid Precipitation Assessment Program. The Acid Model Operational Diagnostic Evaluation Study (MODES) campaigns of 1988 and 1990 were components of a larger EPA effort to assemble a database for evaluation of regional acid deposition models. In 1990 the G-1was used in a study in the Grand Canyon to assess the Navajo Generating Station's effects on visibility. More recently, the aircraft has been used to investigate the chemical and physical properties of the Kuwait oil fire plumes (19911 and to investigate pollutant layering and transport over the northwestern Atlantic Ocean as part of the North Atlantic Regional Experiment (NARE, 1992 and 1993). A few examples of data gathered during recent programs illustrate the potential of this versatile airborne monitoring facility.
FIGURE 6
Pollutant layerlng over Sable Island (North Atlantic Ocean) on August 31,1992 Temperature (solid),dewpoini (dolted),and potenhal temperature 2.51,
0,(solid)and SO, x 100
2. Y 1.5 U
a E 1.0
a
0.51
418 A Ennmn. Sci. Technol.. Val. 28. No. 9, 1534
Chester W.Spicer (11 i s a senior research leoder ot Bottelle in Columbus, OH. His B.A. degree in chemistry is from Rutgers University and his Ph.D. (analytical chemistry] is from Pennsylvonia State University. He conducts research in otmospheric chemical processes, indoor air quality/chemistry, development and evaluation of measurement methods for tmce atmospheric constitutents, and toxic air pollutant issues. Donald V.Kenny (r)i s a research scientist in the Atmospheric Science and Applied Technology Department at BatteI1e (Columbus). He received a B.S. degree in chemistry from the University of New England. He wos worked in environmentol onalysis for 11 years, especiallyin utilizingreal-time tandem mass spectrometry. He has co-authored sevem1 papers on mass spectrometry and holds two patents with other Batelle employees on inlet devices foro moss spectrometer. While working at Bottelle h e is continuing his education and is now a Ph.D. candidote in onalytical chemistry at the Ohio State University.
William J. S h a w (I] is a senior research scientist at the Department of Energy's Pacific Northwest Laboratory. His Ph.D. in atmospheric sciences is from the University of Washington. His research interests include atmospheric boundary layer processes and atmospheric measurement techniques.
One of the primary objectives of reeional acid deoosition models has been accurate projections of acidic sulfur and nitrogen species and their precursors. Figure 4 shows modeled sulfur and nitrogen species distributions [from the Regional Acid Deposition Model) (3) compared with measurements of these species obtained using the G-1
-
aircraft. The comparison is for an 800-km cross-section of the Ohio Valley at the back side of a high pressure weather system on September 2, 1988. The measured and modeled concentrations of total sulfur and total nitrogen agree quite well, and even the distributions among the chemical species are in fair agreement. The model predicts
Elaine G. Chapman (middle) i s a senior development engineer at DOEs Pacific Northwest Laboratory. Her M.Eng. degree i s from Rensselaer Polytechnic Znstitute. Her reseorch interests include the statistical analysis of environmental data and computer modeling of otmospheric processes. Kenneth M. Busness (r) is o senior development engineer with DOEs Pacific Northwest Labomtoly. His M.S. degree in electrical engineering is from Washington Stote University.
Environ. Sci. Technol., Vol. 28,NO. 9, 1994 419 A
7 perspectives and recommendations from the Chemrawn VI1 conference, Chemical Research Applied to World Needs, and addresses the problem of protecting the earth’s atmosphere from the damage caused by human activity. It addresses the problem from the many economic, political, and technical perspectives that must be considered by the policy makers involved today. Viewpoints of science and industry, as well as developing countries, are presented, and the theme of international cooperation is emphasized. The book serves as an introduction to the subject for members of the general public and is a useful resource for environmental scientists, workers in industry, and government and international organizations involved with public policy decisions. Johiz I$’ Bzrks, Unztersity of Colorado, Edztor Jack C Caloert, National Centerfor Atmospheric Research, Editor Robed E . Sieaevs, University of Colorado, Editor 180 pages (1993) Clothbound: ISBN 0-8412-2532-X-$34.95 Paperbound: 1SB.U 0-8412-2533-8-$24.95
Order from: American Chemical Society Distribution Office, Dept. 74 1155 Sixteenth Street, N W Kashington, DC 20036
Or CALL TOLL FREE AT 1-800-227-5558(in Washington, DC, 202-872-4363)and use your credit card!
ACS IIIPUBLICA”0NS h e n i z a ! Reioavcei fofo. the Chemical Sciencex
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a relatively greater contribution of the secondary products PAN and HNO,,compared to our field observations. Such findings are being used in a diagnostic sense to improve the model’s treatment of oxidized nitrogen compounds. Understanding pollutant transport and transboundary air pollution is important to atmospheric scientists and policy makers. The G-1 and the tandem mass spectrometer participated in a 1992 study of the chemical and physical aspects of pollutant transport over the North Atlantic Ocean. Under certain conditions, a “continental plume” was observed several hundred kilometers downwind from the primary anthropogenic source region of the northeastern United States. The maximum concentrations of many species were found in layers aloft and could not have been detected from surface measurements. Figure 5 shows this layering for ozone from a flight carried out on August 3 1 , 1992. The flight track altitude is superimposed on the plot. The pollutant layering is also evident for other species from the vertical profile flown over Sable Island during this mission. Figure 6 shows the vertical distributions of O,,SO,, formic acid, acetic acid, particles, and light scattering, along with the temperature profile. All of the chemical species show a pronounced maximum at the top of the marine boundary layer. The elevated SO, concentration associated with this layer suggests an anthropogenic source. Two additional layers are observed in the formic and acetic acid profiles and, to a lesser extent, in the particle trace. The continuous SO,, formic acid, and acetic acid measurements from this flight were made by the tandem mass spectrometer. On other flights over the North Atlantic, this instrument was used to map DMS concentrations, measure the vertical and horizontal distribution of nitric acid and ammonia, and search for gas phase sulfuric acid from DMS oxidation. As noted earlier, the G-1 flew a number of missions in the Middle East in the summer of 1991 to characterize the composition, properties, dimensions, and transport of plumes resulting from the Kuwait oil fires. Cross-sections of t h e plume at 2 8 O 09’ north latitude on August 2,1991, are shown in Figure 7 . The upper portion of the figure shows the NO, profile at an altitude of 800 m, with the plume evident
Environ. SCI.Technol., Vol. 28,No. 9, 1994
between 48.5” and 49’ longitude. The 0, deficit in the plume is also illustrated in this figure. The lower portion of Figure 7 shows the particle n u m b e r d e n s i t y for p l u m e transects at four different altitudes. Again, the plume is very apparent between 48.5’ and 49’, and the data show that it extends to at least 1700 m, with evidence for elevated particle concentrations at 2300 m. These few examples illustrate the capability of research aircraft, especially when instrumented with sophisticated monitoring tools such as tandem mass spectrometry, to address atmospheric questions that often cannot be answered by surface measurements alone. The combination of complementary surface and airborne measurements has often been found to be the best approach for addressing complex atmospheric issues. Continuing efforts to enhance this airborne measurement capability include development of inflight calibration procedures for the mass spectrometer to enhance data quality, and evaluation of the possibility of combining data from the G-1’s turbulence instrumentation with fast response data from the mass spectrometer to directly measure turbulent fluxes of trace chemical species by the eddy correlation technique. The current and evolving capabilities of the G-1 make the aircraft one of the nation’s important tools for addressing fundamental environmental problems that include the exchange of chemicals between the atmosphere and the ocean, the transport and fate of pollution in the atmosphere, and the nature of global climate change. Acknowledgments The authors wish to acknowledge the leadership, advice, and support of Dr. Jeremy Hales in developing the research aircraft facility and the assistance of Robert Hannigan, chief pilot of the G-1 aircraft. The research reported in this paper was supported in part by Battelle’s Corporate Technical Development Program and by DOE at Pacific Northwest Laboratory under the auspices of the Atmospheric Chemistry Program, Pacific Northwest Laboratory is operated for DOE by Battelle Memorial Institute under contract DE-AC0676RLO 1830.
References Mohler, 0 . ;Reiner, T.; Arnold, F. Rev. Sci. Instrum. 1993, 64, 1199-1207. ( 2 ) Kelly, T. J.; Kenny, D. V. Atmos. Environ. 1991,25A, 215.5-60. (3) Chang, J. S. et al. J. Geophys. Res. (1)
1987,92, 14,681-14,700.
