Envlron. Scl. Technol. 1994, 28, 1615- 1618
Routine, Continuous Measurement of Carbon Monoxide with Parts per Billion Precision Davld D. Parrish,' John S. Holloway,t and Fred C. Fehsenfeidt
Aeronomy Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado 80303
A system including a commerical instrument (Thermo Electron Corp., Model 48) operating on the principle of gas filter correlation, nondispersive infrared absorption is utilized to approach 1ppbv precision with a 1-h averaging period in the measurement of slowly varying ambient CO levels in rural and remote regions. The instrument modifications, setup, and signal averaging processes include gold-plated mirrors, selected detector, 1-s signal resonse time (all available as options from the manufacturer), installation of a lens to more fully focus the infrared beam on the detector, standard addition calibration, catalytic zeroing, rapid switching between equal time periods in the measurement and zero modes, and longterm averaging. Introduction Measurement of carbon monoxide at rural and remote locations is a valuable tool for interpreting the history of emissions and photochemistry in sampled air masses (14 ) . A technique to make these measurements routinely and continuously using commercial instruments has been developed (5,3).The accuracy of this method has been established by intercomparison (6, 7) with the more selective and sensitive technique of tunable diode laser absorption spectrometry, and the precision has been quantified in field measurements (3). Here we review the implementation of this technique to make accurate measurements of the CO levels found in rural and remote regions and describe in detail the optimization of the sensitivity through signal enhancement and noise reduction that can be made to achieve parts per billion precision. This precision is roughly a factor of 10 improvement over that reported previously for this instrumentation (3, 6).
Instrumentation Figure 1 illustrates the instrument setup. Briefly, the commercial CO instrument (Thermo Electron Corp., Franklin, MA, Model 48) operates on the principle of gas filter correlation, nondispersive infrared absorption. It is preceded by an all-Teflon inlet line including a particulate filter, which minimizes the accumulation of dirt on the optics. Immediately upstream of the instrument, the sample flow can be diverted through a zero trap by means of a three-way valve. Calibration of instrument response to CO is obtained routinely by the addition of a small flow of calibration gas through a second three-way valve located upstream of the particulate filter and as close to the inlet as possible. The sample flow is quantitatively maintained by a mass flow controller located downstream of the instrument. The ratio of the two measured flows gives the dilution of the calibration standard. + Also at Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO. 0013-936X/94/0928-1615$04.50/0
0 1994 American Chemical Society
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water vapor level, and perhaps other variables. The zero level of some instruments have a positive water vapor interference, and others have a negative interference. Third, a significant pressure drop across the zero trap causes the pressure to be different in the absorption cell between the zero and measurement modes, with a resultant offset in the measured level. For example, in one test a difference of approximately 1 mmHg between the two modes resulted in a 2-3 ppbv offset in the measured levels. Fourth, the catalyst in the zero trap has a significant capacity for water absorption. Thus, when the ambient humidity changes,the water vapor level may differ between the measure and zero modes, which would introduce an error into the measurement until the catalyst comes into equilibrium with the new humidity level. This error is minimized by using a small volume of catalyst; 50 cm3 is adequate for quantitative conversion of CO with minimal water absorption capacity. Finally, the instrument calibration remains quite constant. In one instrument, standard additions were performed twice daily for more than 15 months of continuous operation. No systematic variations in the sensitivity could be discerned, and the derived values exhibited a 1-0precision of 2.7 % ,which is consistent with the precision of the standard addition determination.
sample
The response of the instrument of CO can be increased in several ways. The manufacturer offers two options: gold-plated mirrors in the multipass White cell and a selected detector. The gold plating of the mirrors increases their reflectivity in the infrared region as well as providing increased corrosion resistance, which may be advantageous for sampling in environments such as the marine boundary layer. The selected detector option includes the selection of a detector with particularly high sensitivity and low noise and the matching of it to an optimal preamplifier. Dickerson and Delany (5) have described an additional retrofit, the insertion of a lens between the absorption cell and the detector to focus the infrared beam more fully on the detector. The resulting increase in signal level requires that the gain of the preamplifier be reduced to prevent overload of the signal conditioning circuitry of the instrument. In addition, care must be taken so that the increase in optical intensity incident on the detector does not saturate its output. We have incorporated each of these three signal enhancement modifications into our instruments. While we have not quantitatively evaluated their efficacy, indications are that each gives roughly a factor of 2 gain in the overall signal to noise ratio. One additional technique to increase the signal has been reported (5); installation of a pump upstream of the absorption cell to increase the pressure and, hence, the absorption. We have not utilized this technique since it involves an additional alteration of the sampled airstream. It is difficult to establish under all conceivable conditions that this process does not affect the mixing ratio of the CO. However, Dickerson and Delany (5) have reported no problems in practice.
Noise Reduction For a given signal level, the imprecision in the measurement can be reduced by three means: stabilization of the zero level of the instrument, frequent monitoring of the zero level to follow the remaining variation and to allow 1616
Envlron. Scl. Technol.. Vol. 28. No. 9. 1994
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Figure 2. 32 min of example data including a 16-min standard addition period. Each data point representsa 1-s average. Switching between the measurement and zero modes occurs on the even minute. The signal is in volts, with the equivalent of 200 ppbv indicated by the bar.
the zero catalyst to remain close to equilibrium with changing ambient humidity levels, and integration over longer measurement periods. The zero level is most strongly affected by the temperature of the detector and the level of water vapor in the sample. To stabilize the zero level, we use a temperature controller (Omega Engineering Inc., Stamford, CT, Model 9100) to maintain the absorption cell in the instrument, and thus the detector that is mounted on the absorption cell, a t a constant temperature. The controller drives the heating elements that are built into the instrument to warm the absorption cell to prevent condensation of water vapor from the ambient air sample. Other workers have controlled the zero level further by removing water vapor from the ambient sample stream by an absorbent or cryogenic trap (5) or a dryer utilizing a hygroscopic, ionexchange membrane (8). We have not employed this procedure, again to avoid any possibility of altering the ambient CO level before measurement. Frequent zeroing of the instrument reduces the uncertainty due to the remaining variations in the zero level. Since the ambient CO level is determined from the difference between the signal levels in the measurement and zero modes, the most accurate and precise determination is achieved when equal time is spent in each of the two modes with transitions as often as is consistent with the response of the instrument. The instrument as supplied by the manufacturer has a 10-s signal averaging period; optional memory chips can be purchased from the manufacturer that reduce the averaging time to 1-s. With this option, the instrument time response is determined by the time for the sample flow to sweep out the absorption cell, which has a volume of approximately 600 cm3. With a flow of 2.5 slpm, the effective response time is less than 15s. Figure 2 shows 1-s average data with the instrument
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Flgure 3. Data from the first 60 h after instaliation of the equipment at a site in the Azores. The points are 1-min averages (excluding 10 safter each modeswitch)of the data in the respectivemodes. Switching between the measurementand zero modes occurred every 2 min. The signal is in volts, with the equivalent of 100 ppbv indicated by the bar.
spending 2 min in each mode, both for an ambient sample and for a standard addition period. Within 15 s of the switch, the signal has nearly equilibrated to the new mode. When the instrument is switched every 2 min, the zero and measurement signals can be averaged for arbitrarily long times, and the average ambient CO level is proportional to the difference of the twoaverages. This procedure is analogous to the standard physical technique of lock-in detection (see, for example, ref 91, where the signal of interest is modulated (here at 0.0083 Hz), and only the signal with the frequency and phase of the modulation is amplified. The length of the averaging period and the concomitant reduction in the imprecision of the average are limited only by the time resolution required to follow interesting variations in the ambient CO levels. In rural and remote regions, which exhibit the lowest CO levels and require the longest averagingperiods, these variations are generally slow, so little is lost by averaging for periods as long as 1 h. If contamination of the “background” CO level by occasional local sources occurs, the contaminated periods can be eliminated from the average by objective means such as a limit on the standard deviation of shorter averaging periods that can then be combined for a longer period average. Sample Results
Figure 3 shows 1-min averages for the two modes during the 2.5 days following the installation of the instrument at a site in the Azores. The variations in the zero mode are typical for an instrument installed in a facility with large temperature variations. Figure 4 shows results obtained for three different averaging periods. As ex-
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Flgure 4. Carbon monoxide mixing ratios extracted from the data of Figure 3. The dots indicate 1-min averages, the lines connect successive 10-min averages, and the circles indicate 1-h averages. For the 1-min averages, interpolations of the 1-min zero averages were subtracted from the respective measurement averages.
pected, the scatter in the measurements decreases as the averaging period is increased. Fried et al. (6)showed that the 1-a precision of a similar instrument for 5-min averages was about 10 ppbv. The present setup has an improved signal to noise ratio from gold-plated mirros (about a factor of 2), a longer averaging time (giving an expected factor of the square root of 12 or 3.5 for 1-h averages), and more frequent and more precise determination of the zero. Thus, an improvement of the precision to near 1ppbv is expected. Evidence that this precision is approached comes from the performance of the system at the Azores site. During one 27-h period of nearly constant CO levels, the consecutive 1-h averages exhibited a standard deviation of 2.2 ppbv. This value represents an upper limit for the 1-0precision, since some of the variability in the averages represent real variability in the ambient levels. During this period, the 810 1-min averages that were collected had a standard deviation of 7.8 ppbv, and the average of the standard deviations of the 1-min averages over the 1-h periods was 7.6 ppbv. If the 1-min averages are independent and if their variances arise only from the limits of instrument precision, then the 1-h precision would be expected to be 1.4 ppbv (7.6 ppbvdivided by the square root of 30). This value probably represents a lower limit for the 1-a precision of the 1-h averages.
Literature Cited (1) Fishman, J.; Seiler, W. J. Geophys. Res. 1983, 88, 36623670. (2) Chameides, W. L.; Davis, D. D.; Rogers, M. 0.;Bradshaw, J.; Sandholm, S.; Sachse, G.; Hill, G.; Gregory, G.; Rasmussen, R. J . Geophys. Res. 1987, 92, 2131-2152. (3) Parrish, D. D.; Trainer, M.;Buhr, M. P.; Watkins, B. A.; Fehsenfeld, F. C. J. Geophys. Res. 1991, 96, 9309-9320. (4) Parrish, D. D.; Holloway, J. S.; Trainer, M.;Murphy, P. C.; Forbes, G. L.; Fehsenfeld, F. C. Science 1993,259, 14361439. (5) Dickerson, R . R.; Delany, A. C. J . Atmos. Oceanic Technol. 1988,5,424-431. Environ. Scl. Technol., Vol. 28, No. 9, 1994 1617
(6) Fried, A.; Henry, B.; Parrish, D. D.; Carpenter, J. R.; Buhr, M. P. Atmos. Environ. 1991,25A, 2277-2284. (7) Poulida, 0.; Dickerson, R. R.; Doddridge, B. G.; Holland, J. z.; Wardell, R*G*; Watkins, J. G*J*Geophys. Res* l9g19 96, 22,461-22,475. (8) Doddridge, R. G.; Dickerson, R. R.; Spain, T. G. Oltmans, S. J. In Ozone in the Troposphere and Stratosphere;NASA Conference Publication 3266; Hudson, R. D., Ed.; NASA: 1994.
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(9) Horowitz, P.; Hill, W. The Art of Electronics, 2nd ed.; Cambridge University Press: Cambridge, 1989, p 1031. Received for review hlovember 29, 1993, ~ received 15, lgg4.Accepted 27J
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Abstract published in Adoance ACS Abstracts, June 1, 1994.
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