Airborne, Continuous Measurement of Carbon Monoxide in the Lower

Environ. Sci. Technol. 1996, 30, 115-120

Airborne, Continuous Measurement of Carbon Monoxide in the Lower Troposphere OTTO KLEMM,* MICHAEL K. HAHN, AND HELMUTH GIEHL Fraunhofer-Institut fu ¨ r Atmospha¨rische Umweltforschung, IFU, Kreuzeckbahnstrasse 19, D-82467 Garmisch-Partenkirchen, Germany

Continuous measurements of CO on-board a fast moving research aircraft in the lower troposphere were made using a system operating on the principle of reduction of HgO and subsequent detection of Hg vapor by flameless atomic absorption (AA). The AA detector is a commercial unit with a modified HgO reaction bed. The system is operated at constant pressure to stabilize reaction conditions, system response, and background signal. The key analytical parameters are as follows: detection limit 4 ppb, precision (2.4 ppb, and 90% response time 110 s. The system was successfully deployed in a large number of research flights over central Europe. The response time is long compared to some other instruments (such as the NOy analyzer). However, the CO measurements can be interpreted in terms of differences between the lower free troposphere (LFT) and planetary boundary layer (PBL), in terms of quantitative analyses of vertical profiles within the PBL, and with respect to relative enhancement ratios of primary pollutants in plumes. During our 1992 and 1993 research flights, we found CO mixing ratios between 90 and 160 ppb to be typical for the LFT over Germany. Within the PBL, typical mixing ratios were between 150 and 320 ppb, by factors between 1.3 and 3 higher than those in the LFT. Higher mixing ratios were found over or downwind of urban agglomerations, where area sources, mainly traffic, lead to peak mixing ratios of several hundred ppb up to altitudes of about 150 m above ground.

Introduction Carbon monoxide (CO) is a key trace gas in the troposphere, whose general distribution, sources, and sinks have been extensively studied in the last 20 years (1-5). Its importance lies mainly in the reaction with hydroxy radicals (OH). This reaction is a major sink of OH and results in the formation * Author to whom correspondence should be addressed; fax: (08821)73573; e-mail address: [email protected].

0013-936X/96/0930-0115$12.00/0

 1995 American Chemical Society

of hydroperoxy radicals (HO2) as CO is oxidized to CO2. In clean tropospheric air (i.e., NO < 10 ppt), HO2 acts as a sink for O3, while in more polluted air masses, it oxidizes NO to NO2 and thus indirectly contributes to the formation of O3. Carbon monoxide has a mean atmospheric lifetime between 10 days and 2 months. It is ubiquitous in the troposphere with mixing ratios of at least 40 ppb. The distribution of CO is governed (1) by the spatial and temporal patterns of the oxidation of methane and higher hydrocarbons, (2) by the combustion of biomass and fossil fuels, especially under nonoptimum combustion conditions, (3) by emissions from the oceans, (4) by its reaction with OH, (5) by uptake by soil microorganisms, and (6) by diffusion into the stratosphere. Various techniques have been developed and successfully applied to measure CO on ground-based stations, from aircraft, and space (6-12). In this paper, we present recent developments of a system that is able to measure carbon monoxide with a time resolution high enough for aircraft measurements in the lower troposphere, where CO concentrations may vary very rapidly in vertical profiles through the boundary layer or in plumes. The instrument is small enough to be integrated into a small or medium-sized research aircraft together with a series of other chemical and meteorological instruments. We present some results from our 1992 and 1993 flight campaigns in central Europe when the instrument was in full operation for over 60 research flight hours.

Methodology The principle of detection is the reduction of HgO to Hg as CO is oxidized to CO2 (13), and the subsequent measurement of gaseous Hg by flameless atomic absorption. The method was developed for applications in the atmosphere by Seiler and Junge (6). Atmospheric H2 also reduces HgO to Hg and is therefore detected in the signal together with CO. Two processes have been used to exclude H2 from the signal. First, a gas chromatograph is used to separate H2 and CO before detection. Commercial units working on this principle (Trace Analytical) are available with repetition times of the CO measurement on the order of a few minutes. Secondly, for continuously working instruments, which have been used on airborne platforms throughout the troposphere (7, 8), a catalytic converter has been used to selectively destroy the CO in ambient air for creating a zero signal (7). During our research flights in the lower troposphere (between ground level and 3000 m height), we found that under certain circumstances the catalytic converter easily became poisoned. Its ability to destroy CO changed with humidity as well as in the presence of high levels of organic air pollutants. Thus, in our setup, CO is not selectively destroyed for zeroing the detector. We operate the detector in a way that its sensitivity to H2 is kept low, while it still efficiently detects CO. Water vapor and hydrocarbons are removed from the sample air before it enters the HgO reaction chamber to avoid any potential interference from other gases such as H2O and hydrocarbons. Zero gas is made from synthetic air, which is cleaned with a molecular sieve and a catalytic converter that destroys CO.

VOL. 30, NO. 1, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Sensitivities of the detector versus CO and H2 as a function of HgO reaction bed temperature.

FIGURE 1. Schematic illustration of the instrumental setup. Centerpiece of the setup is a commercial (Trace Analytical) reduction gas analyzer, of which the HgO reaction bed has been redesigned [diameter ) 6.5 mm, height ) 25 mm, filled with a mixture of 50 mg of HgO powder (Merck 4461) and 25 mg of quartz sand (Merck 7536)]. Sample air is pulled into the instrument from a constant pressure (600 hPa) stainless steel manifold, which is common for a series of chemical instruments on-board the aircraft. Air enters the analyzer through a 1/16-in. sample line, is dried in a dry ice bath, and is passed over a molecular sieve column (quartz glass column, i.d. ) 9 mm, L ) 400 mm, filled with 1-nm molecular sieve, Merck 5703) before entering the reaction bed. For zeroing the instrument, the zero air valve is opened and synthetic air is cleaned on a calatyst (cartridge i.d. ) 26 mm, L ) 800 mm, filled with catalytic converter material Hopkalith, Dra1 ger CHI 8542) and offered to the analyzer in excess (with the overflow backflushing through the sample line into the main manifold). For flushing the system on the ground, during taxi, take off, and landing, the two three-way valves are switched, and synthetic air is injected directly into the reaction bed with the overflow bleeding into the aircraft cabin. The flow rates of sample air and zero air are regulated by electronic mass flow controllers (FC). All parts (except the molecular sieve cartridge) are made of stainless steel.

Figure 1 illustrates the experimental setup that we used in our airborne applications. The whole CO analyzer is kept under constant pressure (e.g., 600 hPa) in order to keep the slope and baseline of the detector constant. The water trap, the molecular sieve, and the HgO reaction bed are kept to the minimum possible volume and are physicly designed in shapes which keep delay times short.

Instrument Characterization We operated our instrument with a sample air flow rate of 100 standard cm3 min-1. As the detector sensitivity varies with pressure (the variation strictly follows Lambert-Beer’s law in the range 500-900 hPa), we run the sample and calibration gas inlet with a constant pressure manifold at

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FIGURE 3. Response of the detector to pulses of a 305 ppb calibrations gas. Pulse lengths varied from 20 to 600 s. From the response of the detector to the 600 s pulse, the 90% response times (t90) were determined to be 102 and 114 s for increasing and decreasing concentrations, respectively. Due to the slow response time, only about 30% of the maximum peak concentration was detected in case of the 20-s pulse. The measured peak areas (in units of ppb per s), however, were identical to the integrals of the pulse functions (deviations less than 2%).

600 hPa. At this operational pressure, measurements in the lower troposphere are feasible without compression of the sample gas before detection, and the sensitivity of the detector is sufficiently high for precise measurements. Figure 2 illustrates the detector sensitivities for CO and H2 as a function of reaction bed temperature. At temperatures above 180 °C, the sensitivity for CO decreases while the interference from H2 increases. These results indicate that a reaction bed temperature between 160 and 180 °C is ideal for the selective detection of CO (see also ref 6). On the other hand, it was found that low temperatures (below 200 °C) bear the risk of sublimation of Hg vapor in the atomic absorption detector. Sublimates of Hg blind the detector and require very thorough and time-consuming cleaning. We decided to operate the reaction bed at 215 °C, trading the advantage of a very low risk of detector poisoning for an enhanced cross-sensitivity against H2 of 4%. As the mixing ratio of H2 can be assumed to be constant at about 550 ppb (14), a correction factor of -22 ppb is applied to the data. The calibration is strictly linear with respect to CO mixing ratios of up to 500 ppb. For higher mixing ratios, a second-

FIGURE 4. Contours of interpolated CO mixing ratio data in vertical planes along sections of the flight path of September 17, 1992. Data were collected using the dolphin flight pattern in SE France and SW Germany between points A, B, and C. The upper limits of the vertical planes are the ceiling heights of the flight pattern; the lower limits are given by the topography of the terrain below the flight path. The border between France and Germany is between points A and B at 49.1° N. The map illustrates the location of points A-C and the flight path. The hatched area indicates the Vosges Mountains.

or third-order polynomial function is fitted to define a calibration function of the instrument. Detection and determination limits were analyzed from the stability of the calibration (15). The detection limit, which is the minimum mixing ratio that can be distinguished from zero within a preselected level of confidence (here 95%), was found to be 3.8 ppb. The determination limit, which is the minimum mixing ratio that can be quantitatively determined within a given limit of confidence (here again 95%), was found to be 5.7 ppb. The precision, determined as the variance of repeated (7×) measurements of a 200 ppb calibration gas, is 2.4 ppb (1.2%). In Figure 3, we present results of a laboratory experiment when pulses of calibration gas mixtures were detected with the CO instrument. It is important to note that the response of the detector is delayed due to the slow equilibration times in the HgO reaction bed. As a consequence, very sharp changes in the CO mixing ratio, as may occur during the flight of the aircraft through a plume, will be considerably smoothed. The integral of the measured peak, however, is identical to the area under the step function in Figure 3.

Airborne Application During the summers of 1992 and 1993, we employed the CO analyzer during our airborne activities of three research programs: TRACT [Transport of Air Pollutants over Complex Terrain, a subproject of EUROTRAC (16)] was a groundbased and airborne campaign in 1992 to study the dynamics of air pollutants in SW Germany, NE Switzerland, and SE France. SANA [Wissenschaftliches Begleitprogramm zur Sanierung der Atmospha¨re u ¨ber den Neuen Bundesla¨ndern (17)] is a program in eastern Germany, during which aircraft measurements were made from 1990 to 1994. The measurement of CO was included in summer of 1993. Photosmog in Sachsen-Anhalt was a summer campaign in 1993 to study the occurence and causes of photosmog in rural sections of eastern Germany (18). In all cases, we installed our air chemistry instrumentation into a twin-engine turboprop research aircraft (Dornier 228). Beside CO, we continuously measured SO2, NO, NO2, NOy, O3, H2O2, and (from 1993 on) CH2O. Further, we collected air samples in steel bottles for postflight analysis of volatile organic carbons (VOC, in our case C2-C7) in the laboratory.


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During the above-mentioned campaigns, the CO instrument was operated during 15 individual research flights with more than 60 flight hours. Since we often utilized the so-called dolphin or curtain flight pattern (19), we collected data from about 190 vertical profiles from ground level (about 150 m above ground) up to levels between 1500 and 3000 m above sea level. In most cases, these profiles reached well above the planetary boundary layer (PBL) into the lower free troposphere (LFT). The CO analyzer was calibrated on the ground on each day of flight activities, using dilutions of NIST-traceable calibration gases. The slopes of the daily calibrations varied by up to (6% within one campaign (typically 3 weeks). In flight, the zero signal, which is mainly caused by selfdissociation of HgO, was checked every 30 to 60 min for about 5 min each so that the drift of the base line could be corrected during postflight data analysis by interpolation. Special attention had to be paid to the risk of contamination of the instrument on the ground. High levels of hydrocarbons (kerosene) and/or aircraft exhaust can destroy the efficiency of the molecular sieve and thus make any CO measurement impossible before replacing the filter. Therefore, we backflushed (see Figure 1 for details) the CO sample line with synthetic air during any activity on the ground, especially taxi, and switched into the measurement mode a few minutes after takeoff. The instrument was thus protected and operated without further problem. During postflight data analysis the CO signal was timeshifted by 10 s in order to correct for instrument response time. By using this shift, we achieved a maximum possible agreement between the CO signal and the other chemical and meteorological measurements concerning locations of peaks (plumes) and structures of vertical profiles.

Results We present some of our airborne CO measurements to demonstrate the instrument’s capabilities and limitations and to give a short summary of CO concentrations measured in the campaigns in the lower troposphere during the European summer. Figure 4 is a contour plot of interpolated CO mixing ratio data that we collected during the TRACT campaign on September 17, 1992. The section between points A and B runs from south to north along the ridges of the Vosges Mountains in SE France, through Germany at 49.14° N and reaches the more industrialized region of the Saarland. The section between points B and C (from west to east) crosses the Rhine River and extends another 80 km east. The measurements were taken during a short period of clear weather conditions, with low winds (wind speeds below 7 m s-1, within the PBL below 3.5 m s-1) from varying directions (easterly to southerly winds in the French part of the flight leg, westerly winds east of the Rhine river). The mixing ratios of CO ranged from below 100 ppb to over 150 ppb in the lower free troposphere (here above 700-900 m above sea level), with a tendency toward higher values in the east and in the very south of the flight legs. PBL mixing ratios of CO mostly are of 150-250 ppb in the rural areas of the Vosges Mountains. In the more densly populated and industrialized sections of the flight path, near-surface mixing ratios were always above 300 ppb. Enrichments were found in the industrialized urban regions of the Saarland. Within the Rhine Valley, we found a boundary layer that was stable and exhibited peak CO mixing ratios of about 480 ppb.

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TABLE 1

Averages and Standard Deviations of CO Mixing Ratios in Lower Free Troposphere (LFT) and in Planetary Boundary Layer (PBL) as Found during Research Flights in the TRACT Investigation Area (September 16 and 17, 92) and in Eastern Germany (June 19-July 9, 93)a date

investigation area

LFT

PBL

9-16-92 A 9-16-92 B 9-17-92 6-19-93 A 6-19-93 B 6-30-93 A 6-30-93 B 7-1-93 A 7-1-93 B 7-2-93 A 7-2-93 B 7-9-93 A 7-9-93 B

SW Germany SW Germany SW Germany HLB HLB HLB HLB E Germany E Germany E Germany E Germany HLB HLB

153 ( 9 125 ( 16 109 ( 24 102 ( 14 93 ( 7 106 ( 9 121 ( 9 125 ( 3 124 ( 3 109 ( 6 110 ( 10 98 ( 4 107 ( 4

314 ( 94 283 ( 60 320 ( 121 229 ( 37 208 ( 20 176 ( 70 162 ( 21 164 ( 28 147 ( 17 154 ( 30 152 ( 16 187 ( 16 172 ( 11

a HLB stands for the heavily industrialized urban area around the cities of Halle, Leipzig, and Bitterfeld in the southern part of eastern Germany. A and B refer to the first or second flight if two flights were made during one single day.

In Table 1, average CO data for the LFT and PBL are listed for all those flights where an appreciable amount of flight time was spent in the LFT for data collection. Mixing ratios in the LFT were in all cases between 93 and 153 ppb. In most cases, the variability of the CO mixing ratios in the LFT was low, with standard deviations being on the order of 1-9% of the average. Only when larger distances were covered during individual flights (such as on September 17, 1992, see also Figure 4), a spatial gradient and consequently a greater statistical variability (in this case 22%) within the LFT data was observed. The PBL mixing ratios were in all cases considerably higher than those in the LFT (average factors between 1.3 and 3). Near-surface emissions of CO had an appreciable influence on the mixing ratios within the PBL during all our flights. To illustrate an example of the vertical variation of CO, we present a single vertical profile in Figure 5. The data were collected during a warm, sunny midsummer day, in the afternoon hours of July 1, 1993, just outside the city of Magdeburg during the onset of a photochemical episode. The potential temperature (Θ) indicates that the PBL was instable and capped by an inversion above about 1300 m above sea level. In the LFT (above 1450 m), CO ranged between 115 and 125 ppb, and the other pollutant gases exhibited backgound mixing ratios (e.g., NOy and SO2 below 1 ppb). The PBL winds were from easterly directions and with low speeds (3-6 m s-1). The O3 mixing ratios were around 60 ppb throughout the entire profile, indicating well-mixed conditions within the PBL. The primary pollutant gases, however, exhibited layers with enhanced mixing ratios. These layers become apparent from the CO, NO, NO2, NOy, SO2, and CH2O measurements, whereas the best correlations were found between CO and the nitrogen oxides. The simultaneous occurence of NOy and CO indicates that common sources are responsible for peak concentrations of these compounds in plumes. In this particular case, the pollutants were relatively fresh (indicated by the anticorrelation between O3 and nitrogen oxides at 940 m above sea level) and probably originated from the city of Magdeburg.

FIGURE 6. CO and NOy mixing ratios along a meandering horizontal flight leg during the afternoon hours of July 2, 1993, in and around the industrial area of the cities of Halle, Leipzig, and Bitterfeld in eastern Germany. Individual plumes are identified by the base lines, which were used for integration of the CO and NOy peaks. Numbers indicate the relative enhancement ratios ∆NOy/∆CO (on molecular basis) of the plumes. TABLE 2

Characterization of very pronounced CO peaks as found during the 1992 and 1993 flightsa

FIGURE 5. Vertical profile of meteorological and chemical parameters as measured over the city of Magdeburg, eastern Germany, on July 1, 1993, at 15:30 local time. Wind measurements (direction: scatter plot) failed between 870 and 1300 m above sea level. Θ is the potential temperature and Td is the dew point temperature. All chemical measurements were associated with a maximum imprecision of about 15-20%.

Similar features were also found during our horizontal flight legs at low altitudes (about 150 m above ground). An example is illustrated in Figure 6. A number of plumes of widths between 2 and 10 km (along the flight path) were observed. The peaks of the CO mixing ratio were in most cases correlated to NOy peaks. To compare the enhancements of CO and NOy, we integrated the individual peaks with the mixing ratios before and after the peaks defining the base lines for integration (see Figure 6). Also given in Figure 6 are the relative enhancement ratios ∆NOy/∆CO, as peak area ratios calculated on a molecular basis. Most of the enhancement ratios range between 0.043 and 0.120, which are typical values for area emission sources in the area of investigation (20). Only one peak, which was measured just before 16:05 local time, had a ∆NOy/∆CO enhancement ratio of 0.253. The plume associated with this high enhancement ratio probably originates from a point source not far from the flight track. This hypothesis is supported by the fact that the shape of this NOy plume is very sharp compared to the shapes of the other plumes. Although the plumes in Figure 6 have high concentrations of CO, they are not representative of the highest measured CO mixing ratios. Larger mixing ratios of CO

date

UTC

location

9-16-92 9-16-92 9-16-92 9-16-92 9-17-92 6-19-93 6-19-93 7-2-93 7-2-93 6-30-93 6-30-93 6-30-93 7-1-93 7-1-93 7-9-93 7-9-93

07:21 07:52 14:44 16:06 10:51 07:42 08:28 08:00 14:08 07:00 07:26 11:15 06:55 11:57 07:16 14:32

Saarbru¨ cken Ludwigshafen/Mannheim Saarbru¨ cken Ulm-Stuttgart Basel Leipzig NW of Halle W of Dresden Leipzig Magdeburg SE of Braunschweig Halle W of Magdeburg W of Naumburg S of Leipzig Leipzig

max CO max NOy 1280 1200 470 485 1230 430 478 382 223 513 730 257 291 235 255 206

84 25 12 17 74 27 7 38 15 64 28 4 21 25 21 9

a Listed are all those peaks with CO mixing ratios larger than the mean plus 3 SD of the PBL data of the respective flight [for example peaks with CO > 314 ppb + (3 × 94) ppb ) 596 ppb for the morning flight of September 16, 1992).

were found in the vicinity of urban agglomerations. In Table 2, we present extraordinarily high values of CO that we measured during our 1992 and 1993 flights. The highest mixing ratios that we encountered were in the range of 1.2-1.3 ppm. During single flights, the peak values which were directly influenced by the emissions from cities were higher than typical PBL values (see Table 1) by factors between 1.2 and 4.1. Therefore, the cities had a strong influence on the concentrations of CO in the downwind measurements. All plumes with high CO were associated with pronounced nitrogen oxides peaks. Both CO and NOy are emitted from large area sources. Traffic is thought to be responsible for more than 60% of the total emissions in Germany (21). Our data show that CO and NOy components have the tendency of simultaneous occurence, at least in identifiable pollutant plumes.


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Acknowledgments We gratefully acknowledge the cooperation with the DLR (German Aerospace Establishment) in Oberpfaffenhofen, Germany, from whom we rented the aircraft. The pilots were greatly helpful in making the experiments a success. We are indebted to F. Marchfelder, K. I. Klemm, and P. Ko¨mp for their help in the field and with the data. H.-E. Scheel performed checks of our calibration gases, F. Fiedler provided topography data (Figure 4). We further thank the Bundesministerium fu ¨ r Bildung, Wissenschaft, Forschung und Technologie (BMBF) for funding the flights within the TRACT and SANA campaigns and the Bundesland SachsenAnhalt for funding within their Photosmog experiment.

Literature Cited (1) Seiler, W. Tellus 1974, 24, 116-135. (2) Seiler, W.; Conrad, R. In The Geophysiology of Amazonia; Dickinson, R. E., Ed.; John Wiley and Sons Ltd.: New York, 1987; pp 133-160. (3) Logan, J. A.; Prather, M. J.; Wofsy, S. C.; McElroy, M. B. J. Geophys. Res. 1981, 8, 7210-7254. (4) Khalil, M. A. K.; Rasmussen, R. A. Chemosphere 1990, 20, 227242. (5) Reichle, H. G.; Connors, V. S.; Holland, J. A.; Sherril, R. T.; Wallio, H. A.; Casas, J. C.; Condon, E. P.; Gormsen, B. B.; Seiler, W. J. Geophys. Res. 1990, 95, 9845-9856. (6) Seiler, W.; Junge, C. Meteorol. Rundsch. 1967, 40, 175-176. (7) Seiler, W.; Zankl, H. Umschau 1975, 75, 735-736.

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(8) Seiler, W.; Giehl, H.; Roggendorf, P. Atmos. Technol. 1980, 12, 40-45. (9) Sachse, G. W.; Hill, G. F.; Wade, L. O.; Perry, M. G. J. Geophys. Res. 1987, 92, 2071-2081. (10) Connors, V. S.; Cahoon, D. R.; Reichle, H. G.; Scheel, H. E. Can. J. Phys. 1991, 69, 1128-1137. (11) Novelli, P. C.; Steele, L. P.; Tans, P. P. J. Geophys. Res. 1992, 97, 20731-20750. (12) Parrish, D. D.; Holloway, J. S.; Fehsenfeld, F. C. Environ. Sci. Technol. 1994, 28, 1615-1618. (13) Moser, L.; Schmidt, O. Anal. Chem. 1914, 53, 217-233. (14) Brimblecome, P. Air Composition and Chemistry; Cambridge University Press: Cambridge, 1986; 224 pp. (15) Funk, W.; Damman, V.; Vonderheid, C.; Oehlmann, G. Statistische Methoden in der Wasseranalytik; VCH: Weinheim, 1985; 225 pp. (16) Fiedler, F. Atmos. Environ., submitted for publication. (17) Seiler, W.; Kramm, G. Meteorol. Zeitschr., submitted for publication. (18) Mo¨ller, D.; et al. Staub-Reinh. Luft, submitted for publication. (19) Klemm, O.; Schaller, E. Atmos. Environ. 1994, 28, 2847-2860. (20) Klemm, O.; Werhahn, J.; Schaller, S.; Schlager, H.; Krautstrunk, M.; Geophys. Res. Lett. 1995, 22, 2021-2024. (21) Umweltbundesamt, Ed. Daten zur Umwelt 1992/93; Erich Schmidt Verlag: Berlin, 1993; 689 pp.

Received for review March 3, 1995. Revised manuscript received July 31, 1995. Accepted August 7, 1995.X ES950145J X

Abstract published in Advance ACS Abstracts, November 1, 1995.

Airborne, Continuous Measurement of Carbon Monoxide in the Lower

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