Anal. Chem. 1998, 70, 3874-3879
Miniaturized Carbon Monoxide Sonde for Atmospheric Measurements John A. Bognar and John W. Birks*
Department of Chemistry and Biochemistry and Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, Colorado 80309-0216
The design and test results of a simple carbon monoxide detector based on the reducing gas detector principle are presented. This instrument has several features which distinguish it from similar carbon monoxide analyzers, all of which contribute to its usefulness on small airborne platforms such as kites, balloons, and light aircraft. Compact size, battery operation, low cost, and light weight are among the most important improvements made over to earlier systems. The instrument fits a package 10 × 20 × 25 cm, has a mass of 2.0 kg, and consumes an average of 20 W of power. This instrument will, therefore, offer a means of incorporating carbon monoxide measurements in recoverable or disposable balloon sondes. The instrument makes an independent measurement of carbon monoxide every 8 s, with a sensitivity of 3 ppbv carbon monoxide (limit of detection for S/N ) 3) and a precision of 4%. The measurement of atmospheric carbon monoxide is of great significance to atmospheric science. Carbon monoxide is a product of combustion processes, and its concentration in the atmosphere may be directly linked to anthropogenic sources and burning. Therefore, carbon monoxide can serve as a valuable tracer of anthropogenic emissions when studying the photochemistry of air masses. In the clean troposphere, carbon monoxide values typically range from 50 to 70 ppbv, while in highly polluted air several parts per million by volume may be observed. High concentrations will persist in the air mass as it is transported, since carbon monoxide is comparatively unreactive in the atmosphere. The lifetime of carbon monoxide with respect to reaction with hydroxyl radicals, which are the primary sink, is approximately 1 month.1 Instrumental techniques for the measurement of atmospheric levels of carbon monoxide include gas filter correlation or nondispersive infrared spectroscopy (NDIR),2-4 Fourier transform infrared spectrometry (FT-IR),5,6 tunable diode laser spectrometry (1) Birks, J. W. Oxidant Formation in the Troposphere. In Perspectives in Environmental Chemistry; Macalady, D., Ed.; Oxford University Press: Oxford, UK, 1998. (2) Chaney, L. W.; McClenny, W. A. Environ. Sci. Technol. 1977, 11, 11861190. (3) Dickerson, R. R.; Delany, A. C. J. Atmos. Ocean. Technol. 1988, 5, 424431. (4) Parrish, D. D.; Holloway, J. S.; Fehsenfeld, F. C. Environ. Sci. Technol. 1994, 28, 1615-1618. (5) Griffith, D. W. T. Appl. Spectrosc. 1996, 50, 59-70.
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(TDLS),7,8 spectrophotometry based on reduction of metal ions in solution,9 chromatography followed by conversion to methane for flame ionization detection (FID),10,11 and reducing gas detection with12 or without gas chromatography.13-19 The reducing gas detector system without gas chromatography was chosen for the development of a lightweight carbon monoxide sonde for several reasons. First, it has been successfully employed in the past for ambient carbon monoxide measurements.13-19 Furthermore, the instrument is relatively simple and amenable to miniaturization for use on low-payload airborne platforms. Previous carbon monoxide measurement systems, based on this and other techniques, have been employed either on the ground or in large aircraft. Significant limitations accompany instruments developed for those applications, in that ground- and aircraft-based instruments are typically heavy, require substantial power, and often cannot be flown on platforms such as kites and balloons. We have been employing high-performance tethered kites and balloons to obtain simultaneous vertical profiles of chemical species and meteorological parameters to altitudes as high as 7.5 km.20-23 For the study of photochemical air pollution, a real-time (6) Yokelson, R. J.; Susott, R.; Ward, D. E.; Reardon, J.; Griffith, D. W. T. J. Geophys. Res. 1997, 102, 18865-18877. (7) Sachse, G. W.; Hill, G. F.; Wade, L. O.; Perry, M. G. J. Geophys. Res. 1987, 92, 2071-2081. (8) Fried, A.; Henry, B.; Parrish, D. D.; Carpenter, J. R.; Buhr, M. P. Atmos. Environ. 1991, 25A, 2277-2284. (9) Selvapathy, P.; Pitchai, R.; Ramakrishna, T. V. Talanta 1990, 37, 539-544. (10) Porter, K.; Volman, D. H. Anal. Chem. 1962, 34, 748-749. (11) Swinnerton, J. W.; Linnenbom, V. J.; Cheek, C. H. Limnol. Oceanogr. 1968, 13, 193-195. (12) Novelli, P. C.; Elkins, J. W.; Steele, L. P. J. Geophys. Res. 1991, 96, 1310913121. (13) Seiler, W.; Junge, C. J. Geophys. Res. 1970, 75, 2217-2226. (14) Seiler, W.; Giehl, H.; Ellis, H. Air pollution measurement techniques; WMO Special Environment Report 10, WMO 460; World Meteorological Organization: Geneva, Switzerland, 1977; pp 29-39. (15) Seiler, W.; Giehl, H.; Roggendorf, P. Atmos. Technol. 1980, 12, 40-45. (16) Seiler, W.; Fishman, J. J. Geophys. Res. 1983, 88, 3662-3670. (17) McCullough, J. D.; Crane, R. A.; Beckman, A. O. Anal. Chem. 1947, 19, 999-1002. (18) Mu ¨ ller, R. H. Anal. Chem. 1954, 26, 39A-42A. (19) Robbins, R. C.; Borg, K. M.; Robinson, E. J. Air Pollut. Control Assoc. 1968, 18, 106-110. (20) Balsley, B. B.; Birks, J. W.; Jensen, M. L.; Knapp, K. G.; Williams, J. B.; Tyrrell, G. W. Nature 1994, 369, 23. (21) Balsley, B. B.; Birks, J. W.; Jensen, M. L.; Knapp, K. G.; Williams, J. B.; Tyrrell, G. W. Environ. Sci. Technol. 1994, 28, 422A-427A. (22) Knapp, K. G.; Balsley, B. B.; Jensen, M. L.; Hanson, H. H.; Birks, J. W. J. Geophys. Res. 1998, 103, 13339-13411. (23) Knapp, K. G.; Jensen, M. L.; Balsley, B. B.; Bognar, J. A.; Oltmans, S. J.; Smith, T. W.; Birks, J. W. J. Geophys. Res. 1998, 103, 13389-13397. S0003-2700(98)00209-1 CCC: $15.00
© 1998 American Chemical Society Published on Web 08/08/1998
Figure 1. Schematic diagram of the CO sonde.
ozone sensor is carried. The current ozone measurement system is a compact ultraviolet absorption instrument, described previously.24 The addition of a carbon monoxide instrument to the package allows a better real-time determination of the source of the air being studied. Humidity measurements and back-trajectory calculations are also helpful in such determinations. An example of the utility of the carbon monoxide instrument may be seen in data gathered by our system in Newfoundland in 1995.23 Here, a high ozone plume was observed at altitudes of a few kilometers. The low humidity observed in the air suggested a stratospheric source, but a carbon monoxide measurement would have provided far stronger proof of that hypothesis. The instrument described here is designed to fulfill the requirements of flying on small airborne platforms such as kites and balloons. These requirements include low weight, low power consumption as all power is provided by batteries, rapid measurement times, and low cost due to the risk of losing an instrument. This instrument design shares some common features with the previously reported ozone sonde, specifically the electronic and spectrometric sections. The characteristics of this instrument also make it suitable for flight on other small platforms, such as remotely piloted vehicles, drop sondes, and released balloon sondes. (24) Bognar, J. A.; Birks, J. W. Anal. Chem. 1996, 68, 3059-3062.
EXPERIMENTAL SECTION The theory of operation of this carbon monoxide sonde is that of a reducing gas detector. Air is brought into a heated cell, where it reacts with a bed of hot (210 °C) red mercuric oxide. The red oxide is chosen due to its greater stability compared to the yellow form in background mercury vapor release from thermal decomposition.17 However, it should be noted that the yellow form reacts much more rapidly with carbon monoxide.15 When the air contacts the bed, reducing gases are oxidized, and mercury vapor is released. An example reaction using carbon monoxide is shown:
CO(g) + HgO(s) f CO2(g) + Hg(g) This reaction occurs quantitatively for carbon monoxide. It will also occur to some extent for nearly every other reducing gas entering the detector, including aldehydes, hydrocarbons, and hydrogen.19 It is necessary to eliminate such interferences when making the measurement. It is simplest to make a zero measurement with carbon monoxide selectively and quantitatively removed from the air stream by a silver oxide scrubber bed19 and then make a second measurement without the scrubber. Silver oxide does not react to any measurable extent with any of the other reducing gases at room temperature.19 Water vapor must also be removed from the air stream, as it can alter the background Analytical Chemistry, Vol. 70, No. 18, September 15, 1998
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Figure 2. Diagram of the CO sonde detection cell.
level of mercury vapor resulting from thermal decomposition of the mercuric oxide. A schematic diagram of the carbon monoxide sonde is shown in Figure 1. A two-way solenoid valve (Lee Co., LFDX0501950BA) selects whether fresh air or a standard CO sample in air is admitted into the instrument. Standards are conveniently stored for periods of up to 1 day in 20-L, 2-mil Tedlar bags (Cole-Parmer, E-01409-14). Currently, a 2-min standard run is followed by a 7-min ambient analysis. This frequency is comparable to those of commercial NDIR analyzers. Each independent measurement takes approximately 8 s to run, so approximately 50 data points are taken during each ambient run, and 15 data points are taken of the standard for later calibration adjustments. A pump from an electrochemical ozone sonde (EN-SCI Corp., model 1Z) is then used to drive the air through the system at a flow rate of approximately 150 cm3/min. This ensures positive pressure throughout the system to minimize interferences from any possible leaks. The air next passes through a filter to remove water vapor, which is a two-stage design where calcium chloride is followed by phosphorus pentoxide. A tee then splits the air stream to two solenoid valves (LDI Pneutronics Corp., series 11), which are used to select whether the air passes straight into the cell or is first exposed to the silver oxide bed. A second tee rejoins these two paths, and the air then enters the reaction cell. A diagram of the reaction cell is shown in Figure 2. The cell was machined from titanium and has a mass of 280 g, with dimensions of 3.8 × 3.8 × 5.1 cm. The cell is fabricated in two pieces, which are bolted together, allowing access to the reaction bed chamber. A seal is maintained by a Teflon O-ring which is seated between the two halves, surrounding the bed chamber. The cell is heated with a 30-W, 12-V dc cartridge heater taken from a portable soldering iron (Radio Shack, 64-2105). This is 3876 Analytical Chemistry, Vol. 70, No. 18, September 15, 1998
switched on and off by a controller board to maintain the cell temperature at the optimum value of 210 °C. A glass bead thermistor (Watlow Gordon, S20DPM4C012A) provides temperature feedback to the controller. Air enters at the base of the block and is preheated as it passes to the reaction bed. The reaction bed consists of powdered reagent grade red mercuric oxide, which is coated onto an extracoarse glass frit. This frit is held in place against the inlet of the bed chamber by a plug of quartz wool, which also serves to catch any oxide particles escaping from the bed. At this point, the reactant gases create the mercury vapor, which passes into the optical cell. The cell is 6.5 mm in diameter with a length of 3.8 cm. Each end is sealed with a silica window (Melles Griot, 02 WLQ 105) and a Teflon O-ring. Torlon mounts, which are bolted onto the cell, are used to hold the windows in place. It is also necessary to ensure that the temperatures of the cell and windows are maintained at approximately 210 °C (a condition which is assured due to how the cell is constructed) to avoid condensation of the mercury vapor. The air is exhausted out the top of the cell via a wide, 6.5-mm-o.d. tube. A tee fitting allows a pressure sensor (Motorola, MPX5100A) for cell pressure measurement to be placed here. This pressure reading reflects the cell pressure due to the wide bore of the tube and relatively low flow rate. The other side of the tee connects to a sulfur scrubber, which removes the mercury vapor from the exhaust and helps isolate the cell from external pressure fluctuations. The optical layout of the system is a simple single-beam ultraviolet photometer with a reference lamp intensity channel. The source is a low-pressure mercury lamp (Sankyo Denki, type G4T5), which is powered by the driver circuit included with the lamp (Ultraviolet Products Inc., model UVG-4). The driver circuit is powered in turn by a regulated 6-V dc supply. Light is brought to and from the cell via a wide-bore fiber optic (Oriel, 77513) to keep the lamp and especially the sensitive photodiodes away from the hot cell. Another piece of fiber optic carries light directly from the lamp to the reference photodiode. The photodiodes (Hamamatsu, type S1226-18BQ) are located directly behind an interference filter (Melles Griot, 03 FIM 016; center wavelength 253.7 nm, fwhm 10 nm) to isolate the mercury line of the lamp and to provide shielding from ambient light. The signal from the cell photodiode is continuously divided in real time by the signal from the reference photodiode, using an analog division chip, to correct for any lamp intensity fluctuations. A schematic of the photodiode circuit through the A/D converter is shown in Figure 3. Each photodiode is connected to a current-to-voltage amplifier with a gain of 109, and the outputs are fed to the analog divider (Analog Devices, AD734BN). The OP177GP op amp is included for proper operation of the analog divider chip.25 The divider ratios the two photodiode signals, to yield a normalized cell signal corrected for lamp fluctuations. This signal is converted to digital form by a 16-bit A/D converter (Analog Devices, AD7701AN), with a full-scale value set by a precision 2.5-V reference source. The operation of the instrument is controlled by a Microchip PIC 16F84 microcontroller, which is programmed using a BASIC compiler (MicroEngineering Labs). The microcontroller controls the operation of all the solenoid valves via a driver chip (Allegro, UCN5841A) and uses a 12-bit A/D converter (Linear Technology (25) AD734 Data Sheet; Analogue Devices: Norwood, MA.
Figure 3. Electronic circuit of the spectrometer section.
Corp., LTC1298CN8) to read the pressure and temperature sensors. It also interfaces to the 16-bit A/D converter and uses those data to compute CO concentrations. The microcontroller outputs the CO value in both analog form (via a LTC1452CN8 12-bit D/A converter) and digital form (RS-232). The power supply for the microcontroller and detector circuitry is isolated from that of the solenoid valves and heater, to avoid noise spikes. One custom-printed circuit board was constructed for mounting most of the electronics, with separate boards for the heater controller and detectors. The instrument requires a 24-V dc supply for operation of the electronics, with a separate supply for the heater, valves, and pump. Power sources used with this instrument have included gel-cell batteries, lithium batteries, and 28-V dc aircraft bus power. The overall power consumption is 20 W averaged over time, with a peak of 40 W with the heater on and approximately 10 W with the heater off. The 10 W is budgeted as follows: electronics, 2 W; lamp, 4 W; valves, 1 W each; and pump, 1.5 W. An independent CO measurement is made once every 8 s, with a sensitivity (detection limit) of 3 ppbv CO for a S/N ratio of 3, and a precision of 4% at typical atmospheric concentrations. The instrument analyzes ambient air for periods of 7 min, separated by 2-min standard runs. The overall instrument, minus batteries, fits into a box 10 × 20 × 25 cm and has a mass of 2.0 kg. Therefore, this system compares very favorably with similar systems previously employed. For example, Seiler et al. have employed a system of this design for aircraft measurements.15 That system had a mass of 30 kg (excluding a freezer for drying
incoming air), with dimensions of 50 × 50 × 23 cm. The instrument described here has a unit construction cost for materials of approximately US$600, which is far less than the cost of purchasing any commercial system. The only drawback to this system is a comparatively short sampling time before some of the traps, especially that for water, need to be changed. The lifetimes of the water trap and CO converter are highly variable and depend on factors such as the humidity and CO content of the incoming air stream, as well as trap size and geometry. In this system, both the water trap and CO converter are typically replaced once every 8 h of operation. The mercuric oxide bed, in contrast, requires replacement far less frequently. Some improvements could be made to this system if it were to be employed on the surface, or if greater power consumption would be permissible. One anticipated improvement is the addition of thermoelectric coolers to the inlet for removal of water vapor, replacing the current trap which (in its present dimensions) requires frequent replacement. Another improvement for enhancing sensitivity would be to modulate or chop the light source and use a lock-in amplifier technique for recovering the signal. Finally, this instrument could be adapted to the measurement of selected other gases, particularly hydrogen, by simply changing the filter system.15 RESULTS AND DISCUSSION The CO value is computed by using a standard calibration curve. A two-point calibration curve for the instrument, specifically a zero and a signal for the standard CO value, is automatically Analytical Chemistry, Vol. 70, No. 18, September 15, 1998
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redetermined in a periodic manner. Presently, the cycle consists of a 2-min calibration period, followed by 7 min of ambient monitoring. It would be possible to add more points to the curve by adding more standards and valves, but that has not been found to be necessary. Typically, a 1.02 ppmv CO standard in air is used with this instrument. When only one standard is used, it should be kept in the expected range of CO values to be observed, as the calibration curve may become slightly nonlinear at concentrations over several hundred parts per billion, as was observed in our laboratory and elsewhere in the past.15 The microcontroller software is configured to output 0-2.5 ppmv as 0-5 V dc based on a rough, fixed internal calibration, so the standard output voltage is always approximately 2 V. The exact voltage is then used in a spreadsheet program to correct the ambient measurements to more precise numbers. While the Beer-Lambert law could also be used to determine CO values on the basis of the absorption data, it was not chosen, since the mercury vapor line absorption coefficient is highly dependent on cell conditions. Laboratory testing of the instrument included runs with a primary standard (1.02 ppmv CO in air) as well as dilutions prepared from the primary standard. These tests were used to determine the instrument’s sensitivity and precision, as well as the approximate calibration curve used internally by the instrument, as referred to above. Tests were also made with various potential interfering gases to ensure that the instrument was, indeed, blind to them (i.e., they would not react with the silver oxide bed). For these tests, the standard was spiked with an appropriate amount of the interference to represent expected atmospheric levels or greater. Compounds tested included hydrogen (0.5 ppmv), methane (2 ppmv), formaldehyde (20 ppbv), and water (saturated). No significant interference was observed from any of these. The instrument has been field tested on two different aircraft. First, it has been integrated as part of an atmospheric sampling package which fits on the rear seat of a Cessna 182 light airplane. This package also measures ozone, condensation nuclei, pressure, temperature, humidity, and aircraft position via GPS. Several flights have been conducted from the Boulder, CO, airport (1V5) to obtain real-time CO data in both clean and polluted air using this package. A series of intercomparison flights was made in cooperation with the National Center for Atmospheric Research (NCAR), in Boulder, CO. The aircraft employed was a specially configured Lockheed Electra operated by NCAR. This instrument and a nondispersive infrared CO analyzer supplied by NCAR (Thermo Electron Corp., model 48) were mounted in the aircraft and connected to the same inlet via a pump which brought outside air up to cabin pressure. The instruments also shared a common standard (570 ppbv CO in air) supplied by NCAR. Data were collected at 10-s intervals by the aircraft data system. A plot of CO values versus time from one of the flights is shown in Figure 4. In this flight, the aircraft climbed while on a northbound course from Jefferson County Airport, Broomfield, CO, to a maximum altitude of 19 000 feet. This was followed by a slow descent over southeastern Wyoming, and then by a low-altitude circuit around Cheyenne, WY, and finally a return along Interstate 25 to the original airport. In this flight, a fairly wide range of CO values 3878 Analytical Chemistry, Vol. 70, No. 18, September 15, 1998
Figure 4. CO versus time data obtained 30 September 1997 onboard the NCAR Electra.
Figure 5. Comparison of CU and NCAR CO sensor responses: data obtained 30 September 1997 onboard the NCAR Electra.
were encountered, 70-250 ppbv. The spike at 18:51 is due to the plume from the city of Cheyenne, as indicated by wind data from the aircraft. The later spike around 19:11 is due to pollution in the metro area surrounding Denver, CO, specifically Commerce City, where a large amount of heavy industry is located. This figure shows the excellent agreement between the two CO measurement systems. The periodic gaps in data from each instrument correspond to the times that the standard was being run to calibrate the instruments. Figure 5 plots the response of this instrument versus that of the NCAR instrument for the same flight and again illustrates the good agreement of the two systems.
CONCLUSIONS A simple, compact, low-power, real-time instrument for atmospheric measurements of carbon monoxide has been developed and tested. It is based on the simple and reliable technique of reducing gas detection, without the high weight and power consumption of previous instruments which utilized this technique. This instrument is now employed routinely on our kite, balloon, and light aircraft atmospheric measurement packages. The instrument is sufficiently light and inexpensive that it could become part of expendable balloon packages, making it possible
to measure vertical profiles of carbon monoxide from the ground to 35-40 km. ACKNOWLEDGMENT This work was supported by the National Oceanic and Atmospheric Administration Global Change Program (Grant No. NA56GP0238) and the Environmental Protection Agency (Grant No. R821252-01-0). We thank Bill Ingino for the machining of the
titanium cell and Dr. Greg Kok of NCAR for coordinating and assisting with the NCAR intercomparisons.
Received for review February 23, 1998. Accepted July 8, 1998. AC980209J
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