VERTICAL PROFILING
OF THE ATMOSPHERE U S I N G HIGH-TECH KITES tmospheric profiling using kites is a relatively inexpensive technique for measuring ozone concentrations in the lower atmosphere. Currently, single-location, quasicontinuous tropospheric measurements using the more conventional techniques of aircraft, balloons, towers, and lidar are limited by high costs, inability to make extended measurements over one fixed location, and difficulty in obtaining useful data at sufficiently high altitudes or during cloudy conditions. The use of kites for atmospheric profiling began in 1990 with profiling of the atmospheric electric field over Christmas Island in the Central Pacific ( 1 ) . That effort demonstrated the feasibility of continuously monitoring the atmosphere for extended periods at heights of up to 3 4 km. The primary limitations of this technique are the payload weight (which must be less than a few kilograms) and the power requirements (which must be less than a few watts). As part of the 1993 North Atlantic Regional Experiment (NARE), we initiated a program to obtain vertical profiles of temperature, pressure, humidity, and ozone concentration. To measure ozone, we employed a conventional balloonborne ozone sounding system, which was suspended below a device that travelled up and down the tether of a large kite. Given that the kite can carry only lightweight, lowpowered instrumentation and that we needed to measure ozone profiles rapidly (and relatively inexpensively) during NARE, we had to determine the feasibility of repeatedly transporting an ozone-profiling instrument up and down the
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tether, using a state-of-the-art kite as a ‘ skyho ok. Reasonably simple and lightweight instruments that measure ozone have been used extensively in atmospheric research for many years. Although these ozone sondes typically are carried aloft by untethered balloons, they could be modified easily to function for extended periods and could be raised and lowered repeatedly along the kite tether in order to obtain vertical profiles. It followed, therefore, that quasicontinuous profiling of ozone (and other chemical constituents) from kites could be practical. In the following sections we describe the technique, including the payload-transporting WindTRAM (Tethered Rover for Atmospheric Measurements) device used for the first time during this campaign, and present vertical profiles of temperature, humidity, and ozone concentration that were obtained during the August 1993 campaign. (See accompanying box for a description of
Environ. Sci. Technol., Vol. 28, No. 9, 1994
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B E N B, B A L S L E Y J O H N W, B l R K S MICHAEL L, JENSEN KARL GmKNAPP Cooperative Instifute for Research in the Environmental Sciences University of Colorado Boulder, CO 80309-0216
JOSEPH B mWILLIAMS Modelsym, Inc. Doylestown, P A 18901
G, WILLIAM TYRRELL Bill Tyrrell and Assoc. Doylestown, P A
the NARE launch site and local weather conditions during kite campaign.) Kite platform, monitoring systems Lessons from Christmas Island. The advent of lightweight, highstrength materials in the past 10 years has greatly improved the capabilities of kites to carry useful payloads to reasonable altitudes for extended periods of time. The Christmas Island tests demonstrated the relative simplicity of lofting stateof-the-art kites carrying scientific instrumentation to altitudes up to at least 3.5 km and maintaining them there for days. Theoretical extrapolations of these results suggest that prolonged flights at much higher altitudes are possible. The Christmas Island kites were especially designed for light winds a n d consisted of two 12.5-m2 vented parafoils attached in tandem near the upper end of a 6-km length of 430-kg-test Kevlar tether. Total system weight, including the kite array (8 kg), the Kevlar tether (18 kg), a n d seven data-collecting spheres (0.8 kg each), was approximately 32 kg. Maximum platform altitude was limited primarily by the length of the tether. Computer programs for correlating payload, altitude, mean wind speed, and kite area were developed as part of this demonstration project. Monitored parameters included pressure, temperature, humidity, and vertical electric field. As a result of these tests, we demonstrated that sonde payloads running up and down a kite tether can be re-used indefinitely. This results in a much lower cost per profile. In conventional free-balloon sounding, the payload typically is not recovered. Moreover, kite-borne mon-
001 3-936X/94/0927-422A$04.50/0 0 1994 American Chemical Society
The WindTRAMconnected to the kite tether. The ozonepayload can be seen suspended below the WindTRAM.
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itoring is the only technique that can provide an essentially continuous profile over a fixed location to altitudes possibly extending into the lower stratosphere. (Tethered ballons are useful only u p to 1-2 km at best.) Finally, the position of the monitor along the tether can be remotely controlled from the ground, providing excellent temporal and height resolution. However, in addition to payload and power-consumption limits, there are several other disadvantages to kite-borne monitoring. To keep the kite aloft for extended periods, sustained winds in excess of 5 mls are needed, depending on the payload and the desired height range. On the other hand, long-term observations require that the kites be flown in regions lacking strong gusty winds or heavy turbulence. Finally, the kite and tether pose a potential aircraft hazard. The first two of these limitations require that kites be launched where winds are constant and relatively strong and where convective activity is minimal. The last limitation is clearly more problematical, but it can be circumvented by flying
in so-called “restricted” areas or by obtaining temporary restrictions to prevent aircraft from overflying the designated region. Such restrictions are disseminated by the Federal Aviation Administration or its overseas equivalents in the form of Notices to Airmen and are issued for specific areas for limited periods. Kite systems for NARE. Three separate kite systems were required for NARE. The primary system was a 15-mZ (160-ft ) cross-section kite designed specifically for the nominal wind conditions over Sable Island. In addition to a spare 15-m‘ kite, we also had available a smaller 10-mZkite for heavier wind conditions. Two large reels holding 7500 m of woven Kevlar with breaking strengths of 280 kg and 386 kg were acquired for the larger and smaller kites, respectively. (See photograph of the 15-mZkite during launch.) All kites were constructed of Kevlar-strengthened Mylar for optimum strengthlweight ratios. Another advantage of the Mylar over the nylon used in our initial Christmas Island tests is that Mylar is waterproof: nylon readily absorbs moisture, which adds weight and
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reduces available payload capability. A gasoline-powered, continuously controllable, hydraulic winch provided power during launching and retrieving operations. The winch and the tether were coupled by a variable-speed, reversible capstan. -Transport of the ozone sonde payload up and down the tether was provided by a specially designed WindTRAM. Briefly, the WindTRAM comprised a triangular, kitelike crosssection of 1 m’ that was attached to the tether via a * 2-cm diameter, 2-m long tube. During ascent, the TRAM was able to lift 5-kg payloads rapidly from the ground to the kite. Telemetry was provided to bring the TRAM down, based on a “command’ from the ground station. A manual release was also provided to bring the TRAM down automatically if it approached too close to the kite itself (see photograph of the WindTRAM connected to the tether). A single ozone-measuring payload was used during most of the campaign. The entire sounding package comprised a conventional Vaisala “rawinsonde” (a device that measures and transmits tempera-
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ture, humidity, and pressure to the ground station) coupled to an ozone concentration measuring system via digital sampling circuitry. With this system, some 40 profiles were ohtained between August 6 and 29, 1993. Ozone was measured using a conventional electrochemical ozone sonde manufacturedby Science Pump Corporation (Camden, NJ).In this instrument, originally developed by Komhyr (21, air contacts the cathode solution of a n I-11,- concentration cell. Oxidation of iodide to triiodide by ozone produces an imbalance in the cell potential, which drives a c m n t through an external circuit. T h e measured current i s related to the ozone concentration by the gas flow rate and Faraday’s law. Calibration of the instrument consists of measuring t h e flow rate produced hy t h e air pump and correcting the flow rate at every altitude for the measuredtemperature and pressure. The ozone sonde was twice intercalibrated with an ozone photometer [made by Thermal Electron Instrument Company (Franklin, MA)] during t h e field campaign. This was done by dynamic dilution of ozone generated by a n electrical discharge of air in order to simulate sampling of six or seven different ozone concentrations in the range 0-225 pphv. Linear regression of plots of mixing ratios obtained from the ozone sonde versus those obtained from the photometer produced slopes of 1.01 and 1.00, intercepts of 1.1 and 2.6 pphv, and correlation coefficients (rz) of 0.999 and 1.000, respectively. Within experimental error, we found no significant difference between the two measurements. To obtain profiles, the ozone sonde was attached either to the WindTRAM (to travel up and down the tether) or directly to t h e tether (during periods w h e n w e were working on the TRAM). To obtain profiles inthe latter situation, i t was necessary to move the kite platform itself up and down using the motorized winch. This proved to be a satisfactory, albeit lengthy and laborious, procedure.
The NARE campaign
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Data presentation Two examples of ozone concentration profiles are shown in Figure 1. These profiles, which were measured on August 25,1993, were obtained by attaching t h e ozone sonde directly to the kite tether 15-20 m below the kite harness. Concurrent profiles of potential temperature and humidity obtained from t h e at-
The North Atlantic Regional Experiment is a multiyear program whose goal is to study the chemical and transport properties of tropospheric ozone from, roughly, Maine to the Azores. During the first major NARE campaign in August 1993, separate research teams established sites on the Maine coast and in southern Nova Scotia to monitor ozone and other environmentally important atmospheric constituents. The site selected for the preliminary kite tests was Cape Sable Island, near the southern tip of Nova Scotia. The site was chosen for its low population density, the extended (and almost deserted) beach aligned with the prevailing WSW surface wind, and its proximity to other NARE experiments. A 30-year-averaged profile of winds alofl over Shelburne, Nova Scotiasome 25 km up the coast from Cape Sable Island-appears as the longdashed curve in the graph. The two sets of short-dashed curves on either side of this average profile depict the one- and two-sigma departures from the mean (i.e., wind magnitude ranges expected 68.27% and 95.45% of the time, respectively). The two profiles shown by the solid curves depict typical wind conditions during the NARE Campaign, prior to and following August 24. These profiles were obtained from a 915-MHz wind profiler temporarily located near Chebogue Point, NS. The solid-line profiles illustrate that the August winds were relatively light until August 24. The wind during this period was blowing either from the north or east. During the period following August 24, however, winds exceeded two-sigma magnitudes. During this latter period, the wind direction was more typical, that is, from the southwest. The weak winds during most of August precluded attaining kite heights greater than about 700-800 m (because our kites were designed for higher winds over Sable Island some 450 km east of our site). However, the wind direction assured that the air over Nova Scotia was coming from relatively benign, ozone-free regions. Following August 24, however, when the wii freshened sufficiently to enable the kites to reach 2-3 km, they blew from WSW, thereby ensuring reasonably contaminated ozone profiles.
4m
tached rawinsonde are also plotted for both figures. The figure shows the excellent height resolution available from the kite-borne technique; note the narrow ( - 2 0 m) enhanced layer at about 650 m in the first profile. Note also the associated “kink” in the po-
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tential temperature p r o f i l e and structure in the humidity profile at the same height. A strong temporal variability of the ozone profile can be inferred by comparing these two profiles. The enhanced ozone concentration lying between roughly 0.5 km and
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to August 23, and the corresponding higher profiles show typically hieher ozone concentrations that wire obtained after the wind intensity increased on August 23.
Profiles of temperature, humidity, and ozone density obtained on 25 August by connecting the ozone sonde directly to the tether and reeling the kite out (ascent profiles) and back in [descent profile) Cape Sable Island, NS
Economics of kites Although the 39 profiles plotted in Figure 2 were obtained using two separate ozone sondes for comparison, the data could have been obtained using a single instrument. This point highlights the potential economy of kite-borne measurements. In the present example, the cost of a balloon-borne ozone profile ( - $800 for the expendable ozone sonde) is reduced by the total number of profiles per (reusable) instrument available using a kite to about $20. Similar statements can be made for any type of sounding instrument that is light enough to be borne aloft by the WindTRAM-kite technique. The costs for a typical kite campaign are difficult to assess because many of the incurred costs are developmental or one-time-only expenditures. The initial kite design, the design and construction of the tether system, and telemetry systems design and purchase are good examples of such costs. On the other hand, if one assumes that such costs have been covered by previous campaigns, then the primary expenses are those incurred for the two- to four-person field team for a specific campaign and shipping costs for a few hundred kilograms of equipment. There are few expendibles, and most of the systems can be used for many campaigns.
25 August, 1993 n^/^.:..^ L..-:.:...
Ascent (12:35:17-15:54:21 A D
[%I IO 100
J
0
Potential temperature, degrees C
60 80 100 0, mixing ratio, ppbv
120
14"
Profiles took roughly three hours each to measure and are separated in time by about the same amount. Approximate directions from which the winds were blowing appear to the right 01 both figures.
1.5 km has all but disappeared by the time the sonde has retraced through the region some three hours later. Because the time to make each profile is comparable to the time separation between profiles, it is apparent that neither profile represents the true profile at any given instant. A much higher time resolution would be needed to accurately study the time evolution of the profiles. The WindTRAM, which has a measured rise rate at least an order of magnitude larger than that used in the current examples, has this capability. During the experimental period a number of ozone profiles were obtained by other NARE groups using aircraft. Without going into detailed
comparisons in this preliminary report, it is important to point out that the kite-borne profiles were very similar to profiles obtained by aircraft in the region (M. Trainer, personal communication). A composite of the entire set of kite-borne profiles gathered during the NARE campaign appears in Figure 2. The ozone concentrations on individual profiles are indicated by the width of the plots. Note that the abscissa is not linear in time, but rather the individual profiles have been more or less evenly spaced across the plot for convenience. The height distribution of the profiles illustrates clearly the light-wind (Le., low-altitude), low ozone concentration results that were obtained prior
Conclusions A number of ozone concentration profiles were obtained in southern Nova Scotia during NARE using an innovative kite-b-orne t e c h n h u e and conventional reusable ozone sondes. Good preliminary agreement was found with concurrent profiles obtained by nearby aircraft when they were available. The profiles exhibited a strong variability in structure and intensity throughout the campaign period. The ozone concentration aloft depended largely on the direction of the prevailing wind, exhibiting the largest values when the wind was coming from the northeastern United States. This layering effect appears to be caused by warm, polluted air from the continent rising and flowing over the colder, more stable marine boundary layer. This meteorological phenomenon was not an-
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A summary plot of all of the 39 ozone concentration profiles obtained during the NARE campaign 3 0)
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, ............
8
10
. ....
11
15
16
21 23
25
26 28
Day of month, August 1993 The dark profiles were obtained during the kite ascsm, while the lighter ones were obtained durin descent. Each set of profiles correspondsto a singl u down lravene of the ozone sonde, either by the kite going UP and dawn or by the TRAM travetng up and down the tether.The concentration is S!&n by the width of the shaded area, in accordancewith the scale shown at the top of the fi ure Note that the higher flights were made toward the end of the campaign. when the winds were stronger and the ozone concentrations consider*& Iaige,.
ticipated in advance of the NARE campaign, but its discovery strongly reinforces the necessity of vertical profile measurements in assessing the kansport of air pollutants. This preliminary experiment underscores the practicality of multiple reuse of conventional ozone sondes using a kite-borne technology. In particular, the use of a specially designed WindTRAM to carry the ozone sonde u p and down the kite tether enables measurements to be made rapidly and relatively continuously. Possibly more important, however, is our demonstration of the feasibility of inexpensive (and reasonably continuous) profiling of any number of atmospheric constituents using reusable sondes carried aloft by this technology. The primary requirements for such measurements are that the specific ins t r u m e n t weigh little (a few kilograms) and consume minimal
power [a few watts). Furthermore, based on aerodynamic calculations, we expect that such measurements c a n be made to much greater heights (Le., to the upper troposphere and possibly to the lower stratosphere). Acknowledgments We are ha py to acknowledgethe coo eration ofthe government of Cana& during our portion of the NARE Campaign. Thanks go to George Dewar of the Department of Transportation for obtaining permission for us to fly kites over Cape Sable Island and to Jim Thiessen of the Department of Communication for arranging radio frequency permits. Appreciation is also extended to F. Fehsenfeld and D. Parrish of NOAA's Aeronomy Lab for their efforts and suppori in coordinating our portion of this program and to W. Angevine for providing the profiler wind profiles. S. Oltmans of NOAAs Climate Research provided valuable su estions on the ozone measurement tecgology. P. Garcia of
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the University of Colorado (CIRES) contributed heavily to the design and testing of an alternate TRAM device, and we sincere1 thank him. We also thank R. Skaggs 0PCU.s Department of Chemistry and Biochemistry for her help during the field o erations on Cape Sable Island. Kevlar &r the kite tether was donated by the DuPont Corporation through the efforts of R. Pariser and A. Koralek. The warm interest and able assistance of citizens of the Cape Sable Island Community is gratefully acknowledged. JWB thanks the Council on Research and Creative Work of the University of Colorado for a faculty fellowship and travel stipend and the National Center for Atmospheric Research for a visitor's grant. This project was partially supported by the U S . Department of Commerce (NOAA) Global Change Program. References (11 (21
Balsley, B. B. et al. Bull. Am. Meteo-
rol. SOC. 1992,73, 17-29. Komhyr, W.D. Ann. Geophys. 1969, 25.203-210.
I Ben B. Balsley 111 is o research professor i n the Deportment of Electricnl ond Computer Engineering (ECE] and o Fellow of the Cooperative Institute for Research in the Environmentol Sciences IClRES1 at the University of ColoradoBoulder. His interests include atmospheric and ionospheric dynamics and the development and use of groundbased systems for remote sensing.
John W.Birks lr1 is pmfessor of chemistry and biochemistryand a Fellow of the Cooperative Institute for Research in the Environmental Sciences (CIRES) at the University of ColoradwBoulder. His research interests include loborotorystudies of chemic01 reactions important in the atmosphere and the development and field testing of new, miniaturized instruments for measurements of atmospheric species.
Michael L. Jensen is n grnduate student at the University of ColorodeBoulder. He i s working toward o doctorate i n oerospoce engineering in the field of control systems.
Karl G. Knapp is o graduate student in the Department of Chemistryot the Universify of Colorado-Boulder. He is working toword o doctorate in chemistry.
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