Anal. Chem. 1997, 69, 4267-4270
Portable Cryogenic Collection of Atmospheric Nitrous Oxide and Carbon Monoxide for High-Precision Isotopic Analysis Amy K. Huff, Steven S. Cliff, and Mark H. Thiemens*
Department of Chemistry, University of CaliforniasSan Diego, La Jolla, California 92093-0356
A portable cryogenic collection system has been constructed for the isolation of atmospheric nitrous oxide (N2O) and carbon monoxide (CO). Ambient air enters the system at a rate of 10 L/min, and N2O condenses in a specially designed stainless steel ultrahigh-efficiency collection (UHEC) trap. The CO is next oxidized to CO2 and removed by a second UHEC trap. In conjunction with drying agents, the UHEC traps remove greater than 99.9989% of atmospheric carbon dioxide (CO2) that would otherwise interfere with CO-derived CO2 analysis. The oxygen in the N2O and CO-derived CO2 samples is subsequently separated for high-precision mass spectroscopic analysis. This system is unique in that it provides for in situ collection and permits simultaneous measurement of the 18O/16O and 17O/16O ratios in N2O and CO. Results are presented for a series of trials run to determine the ultimate collection efficiency of the UHEC cryogenic traps. Nitrous oxide and carbon monoxide are both important trace gases in Earth’s atmosphere. Each has a significant impact on the chemistry of the troposphere and stratosphere. N2O is a potent greenhouse gas with 180 times more warming potential than CO2 due to its long 150 year lifetime and its strong infrared absorption band at 7.9 µm.1 In the stratosphere, N2O photooxidation creates the dominant source of odd-nitrogen species. These nitrogen oxides are the main sink for stratospheric ozone at altitudes less than approximately 40 km.2 Carbon monoxide controls the oxidation state of the troposphere, however, since 75% of the hydroxyl radical (OH) sink is reaction with CO.3 In the presence of NO, carbon monoxide oxidation also creates tropospheric ozone, a serious environmental and health hazard.4 Furthermore, CO itself is a dangerous primary pollutant. Consequently, fluctuating N2O and CO concentrations will impact global warming, tropospheric pollution, and stratospheric ozone loss. Thus nitrous oxide and carbon monoxide levels should remain as low as possible, with contributions from anthropogenic sources minimized. Yet studies indicate that atmospheric N2O concentrations are increasing at a rate of (1) Lashof, D. A.; Ahuja, D. R. Nature 1990, 344, 529-531. (2) Wayne, R. P. Chemistry of Atmospheres; Clarendon Press: Oxford, U. K., 1991; pp 116-150. (3) Crutzen, P. J.; Zimmerman, P. H. Tellus 1991, 43B, 136-151. (4) Finlayson-Pitts, B. J.; Pitts, J. N. Atmospheric Chemistry; John Wiley and Sons: New York, 1986; pp 967-971. S0003-2700(97)00256-4 CCC: $14.00
© 1997 American Chemical Society
approximately 0.2-0.3% per year,5 presumably with a significant anthropogenic contribution. CO concentration trends are less straightforward; atmospheric levels were increasing steadily until approximately 1988, when they suddenly began to decrease significantly.6-8 The exact cause of the trend reversal is unknown, although many explanations have been suggested.7,8,9 The global budget of neither species is well defined despite decades of research. Current estimates suggest the existence of a missing N2O source10 and of underestimated CO sources.11 Effective emission regulation policies cannot be developed until the sources of these important molecules have been fully identified and quantified. Clearly a new approach is needed to resolve the budgets of N2O and CO. Stable isotope analysis is a well-established technique for obtaining information about the budgets and transformation mechanisms of atmospheric species.12 Currently, there are several limited studies of the 15N/14N (δ15N) and 18O/16O (δ18O) ratios in atmospheric N2O,13-16 as well as some Southern Hemisphere and free Northern Hemisphere troposphere CO δ18O measurements.17-20 To date, no measurements of the 17O/16O (δ17O) ratio in atmospheric N2O or CO have been published, despite the fact that δ17O analysis may provide insights not possible using δ18O alone.21 Stable isotope measurements give unique insight into the properties of atmospheric species because of the subtle effects arising from differences in atomic mass between the isotopes of (5) WMO. Scientific Assessment of Ozone Depletion: 1994; Ennis, C., Ed.; World Meterological Organization, Global Ozone Research and Monitoring ProjectsReport No. 37; WMO: Geneva, 1995; pp 2.20-2.22. (6) Khalil, M. A. K.; Rasmussen, R. A. Nature 1988, 322, 242-245. (7) Khalil, M. A. K.; Rasmussen, R. A. Nature 1994, 370, 639-641. (8) Novelli, P. C.; Masarie, K. A.; Tans, P. P.; Lang, P. M. Science 1994, 263, 1587-1590. (9) Bekki, S.; Law, K. S.; Pyle, J. A. Nature 1994, 371, 595-597. (10) Watson, R. T.; Meira Filho, L. G.; Sanhueza, E.; Janetos, A. In Climate ChangesThe Supplementary Report to the IPCC Scientific Assessment; Houghton, J. T., Callander, B. A., Varney, S. K., Eds.; Cambridge University Press: New York, 1992; pp 37-38. (11) Logan, J. A.; Prather, M. A.; Wofsy, S. C.; McElroy, M. B. J. Geophys. Res. 1981, 86, 7210-7254. (12) Kaye, J. A. Rev. Geophys. 1987, 25, 1609-1658. (13) Kim, K.-R.; Craig, H. Nature 1990, 347, 58-61. (14) Kim, K.-R.; Craig, H. Nature 1993, 262, 1855-1857. (15) Wahlen, M.; Yoshinari, T. Nature 1985, 313, 780-782. (16) Yoshida, N.; Matuso, S. Geochem. J. 1983, 17, 231-239. (17) Brenninkmeijer, C. A. M. J. Geophys. Res. 1993, 98, 10595-10614. (18) Stevens, C. M.; Krout, L.; Walling, D.; Venters, A.; Engelkemeir, A.; Ross, L. E. Earth Planet. Sci. Lett. 1972, 16, 147-165. (19) Brennikmeijer, C. A. M.; Lowe, D. C.; Manning, M. R.; Sparks, R. J.; van Velthoven, P. F. J. J. Geophys. Res. 1995, 100, 26163-26172. (20) Mak, J. E. Ph.D. Thesis, Scripps Instiyution of Oceanography, University of CaliforniasSan Diego, April 1992; Chapter 3. (21) Thiemens, M. H.; Jackson, T.; Mauersberger, K.; Shueler, B.; Morton, J. Geophys. Res. Lett. 1991, 18, 699-672.
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Figure 1. Portable cryogenic N2O/CO collection system.
a single element. Stable isotopes are fractionated mainly by isotope exchange reactions and during kinetic processes, which depend on differences in the reaction rates of the isotopic molecules. Most isotopic fractionation processes are dependent on the mass of the species involved.22 In a small number of cases, however, the products of certain chemical reactions can be produced in a mass-independent manner. Only simultaneous measurements of δ18O and δ17O can distinguish between massindependent and mass-dependent processes.23 In the present case, preliminary results indicate that both atmospheric N2O and CO are mass independently fractionated. This isotopic anomaly provides new information about the chemistry and origins of nitrous oxide and carbon monoxide in the atmosphere.24,25 Here we present the collection system used to isolate N2O and CO from an ambient air stream. The nitrous oxide and carbon monoxide samples are then returned to the laboratory for high-precision δ17O and δ18O analysis. This system is unique in that N2O/CO collection is in situ, eliminating concerns over sample contamination from air storage in cylinders or from a compressor.26 Furthermore, this system is lightweight, robust, and easily transported to remote field locations. EXPERIMENTAL SECTION The portable cryogenic N2O/CO collection system is shown in Figure 1. The vacuum manifold is made of stainless steel and joined by Kwik-Flange connections (MDC Vacuum Products (22) Hoefs, J. Stable Isotope Geochemistry; Springer-Verlag: Berlin, 1997; pp 3-15. (23) Thiemens, M. H. In Isotope Effects in Gas Phase Chemistry; Kaye, J. A., Ed.; ACS Symposium Series 502; American Chemical Society: Washington, DC, 1992; pp 138-154. (24) Cliff, S. C.; Thiemens, M. H. Eos Trans. AGU 1996, 77, F121. (25) Huff, A. K.; Thiemens, M. H. Eos Trans. AGU 1996, 77, F124. (26) Mak, J. E.; Brenninkmeijer, C. A. M. J. Atmos. Ocean. Technol. 1994, 11, 425-431.
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Corp.), enabling easy shipping and assembly. The system is mounted on a rectangular, four-sided steel frame measuring approximately 1.5 m high and 0.6 m square. Casters on the bottom of the steel frame provide mobility. Fully assembled, the vacuum line is attached to the outside of the steel frame, while two rotary pumps sit inside, making for a fully self-contained sampling unit. The collection system consists of an inlet, a UHEC trap for nitrous oxide/carbon dioxide condensation, a reaction cylinder containing Schu¨tze reagent (I2O5 on acidified silica gel; Leco Corp.) for oxidation of carbon monoxide to carbon dioxide, and a UHEC trap for condensation of CO-derived CO2. The UHEC traps are the most critical aspect of the system. Ambient air enters the line and first flows through the initial UHEC trap (no. 1), which quantitatively removes N2O. It is essential that this first UHEC trap also remove nearly all atmospheric carbon dioxide, since CO in the air stream is subsequently converted to CO2 and is collected in the second UHEC trap (no. 2). Atmospheric CO2 is 3 orders of magnitude more abundant than CO, so if the UHEC traps were not extremely efficient, atmospheric carbon dioxide could remain in the air stream and be collected with the CO-derived CO2, thus contaminating the sample. The collection efficiency of the UHEC traps is further discussed in the Results and Discussion. The UHEC trap is a set of two stainless steel minitraps welded together in series; see Figure 2. Each minitrap is a hollow cylinder 40 cm long and 6 cm in diameter. The minitraps are filled with 6 mm hollow glass beads (Fisher Scientific), which increase the surface area available for condensation. Five liter metal 5-LD Dewar flasks (Taylor-Wharton) slip over the outside of the minitraps and are filled with liquid nitrogen during sample collection. Each UHEC trap set is also fitted with a platinum resistance thermometer (PRT; Omega Engineering), which moni-
Figure 2. Ultrahigh-efficiency collection trap.
tors the internal trap temperature. A strict protocol is followed in order to minimize errors associated with routine sampling by multiple technicians. Prior to sample collection, the entire system is evacuated to a pressure below 1 × 10-3 Torr by a Welch duo-seal vacuum pump, model 1400. In all cases, the vacuum for the collection system is provided by the model 1400 pump. A model 1402 Welch duoseal vacuum pump is used during sample collection, in order to process the large volume of air flowing through the system. Two pumps are used so that the sample transfer/measurement section can be evacuated during sample collection. Any pump oil backstreaming from either pump is collected in a trap at liquid nitrogen temperature, which is located in front of the inlet to each pump. The vacuum system is checked for leaks before beginning a collection. The pressure in the line is monitored by a Baratron pressure gauge (MKS Instruments) and a Granville-Phillips 275 Series Convectron gauge. Before sampling, the UHEC traps are wrapped with heating tape and warmed to approximately 80 °C. The entire system is heated, degassed, and pumped overnight. The ultrahigh collection efficiency required for quantitative N2O/CO sampling is only obtained when the internal temperature of the UHEC traps is approximately -140 °C. Since the traps have been heated overnight, they must be “prechilled” before N2O/CO collection can begin. The 5-LD Dewar flasks are placed on the UHEC no. 1 trap and filled with liquid nitrogen. “Prechill” is accomplished by allowing dry nitrogen gas to flow through the system at a rate of 5 L/min. The nitrogen gas acts as a heat conductor. Internal trap temperature is monitored by the PRT, and gas flow rate is controlled by a mass flow controller (MKS Instruments). After 20-22 min of nitrogen gas flow, the UHEC no. 1 trap internal temperature is approximately -135 °C. 5-LD Dewar flasks are next placed on the UHEC no. 2 trap and filled with liquid nitrogen. Dry nitrogen gas flow is continued for another 25-30 min. After 45-50 min of “prechilling”, both the UHEC no. 1 and UHEC no. 2 traps are at -140 °C and at optimum efficiency. Once the UHEC traps are chilled, N2O/CO sample collection begins. Ambient air enters the system at a rate of 10 L/min. The air stream first passes through a 180 cm long Perma Pure Nafion Dryer (Perma Pure Inc.) and a 90 cm long by 3 cm diameter cylinder filled with Drierite (CaSO4; W. A. Hammond Drierite Co.). These drying agents remove water from the air stream to a dew
point of approximately -70 °C. Flow continues to the UHEC no. 1 trap, which removes N2O, atmospheric CO2, and any remaining water. The air stream next passes through the Schu¨tze reagent, quantitatively oxidizing CO to CO2. The CO-derived CO2 then condenses in the UHEC no. 2 trap. Sampling continues for 180 min, during which 1800 L of air is processed. Depending on ambient conditions, typical corresponding sample sizes range from 23 µmols for N2O to 30-80 µmol for CO. After sampling is terminated, the system is evacuated below 10-3 Torr and each UHEC trap is isolated. The frozen traps are wrapped with heating tape and warmed to approximately 120 °C over 1 1/2 h. During that time, the CO sample tube is placed on the sample inlet port and evacuated. The CO sample tube is a small glass tube, 5 cm3 in volume, which is equipped with an O-ring stopcock. When the gases are completely expanded, the CO-derived CO2 is condensed into the “distillation volume”, a hollow, stainless steel cylindrical trap 35 cm long and 5 cm in diameter. In all cases, liquid nitrogen is the cryogen used for sample transfer. Any noncondensables are pumped away, the distillation volume is isolated, and the CO-derived CO2 from UHEC no. 2 is distilled using an ethanol/liquid nitrogen mixture at -78 °C. Water and iodine, byproducts of the Schu¨tze oxidation, remain in the distillation volume, while CO-derived CO2 is frozen into the CO sample tube. This is a critical step, since water and carbon dioxide readily undergo isotope exchange; any water stored in the sample tube will compromise the CO-derived CO2 sample. CO-derived CO2 sample transfer is monitored using the Convectron gauge. Once transfer is complete, the distillation volume is heated with a heat gun and residual waste gases are pumped away. The N2O sample tube is placed on the sample inlet port and evacuated. The sample section is then isolated, and the contents of the UHEC no. 1 trap are frozen into the distillation volume. Noncondensables are pumped away, the distillation volume is isolated, and N2O and atmospheric CO2 are distilled using an ethanol/liquid nitrogen mixture at -78 °C. Water remains in the distillation volume, while the N2O sample and atmospheric CO2 are transferred into the N2O sample tube. N2O and atmospheric CO2 transfer is monitored using the Baratron and Convectron pressure gauges. There is no concern regarding isotope exchange between N2O and CO2 in this step, as reported by Cliff and Thiemens.27 The N2O sample tube is a 2.5 cm by 30 cm long stainless steel tube, containing approximately 125 g of Ascarite (NaOH on a nonfibrous silicate carrier; Thomas Scientific). Ascarite is utilized since it quantitatively reacts with CO2, leaving N2O for analysis. This sample tube is unique in that it allows both transport and purification of the N2O sample. Once isolated from the atmosphere, N2O and CO-derived CO2 samples are returned to the laboratory and prepared for isotopic analysis. In order to simultaneously measure both δ17O and δ18O, it is necessary to perform mass spectroscopic analysis on O2, rather than N2O or CO-derived CO2, so as to avoid isobaric interferences from 13C and 15N. The methods for isolating pure O2 from the N2O and CO2 samples and the accuracy with which the oxygen isotopes can be measured are described in detail elsewhere.27,28 (27) Cliff, S. C.; Thiemens, M. H. Anal. Chem. 1994, 66, 2791-2793. (28) Bhattacharya, S. K.; Thiemens, M. H. Z. Naturforsch. 1989, 44A, 435444.
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Table 1. Results of the UHEC Trap Collection Efficiency Trials trial no.
µmol of CO2 in UHEC no. 2
1 2 3 4 5 6
0.19 0.23 0.21 0.27 0.40 0.23
trial no.
µmol of CO2 in UHEC no. 2
7 8 9 10
0.18 0.44 0.32 0.37
av
0.28 ( 0.09
RESULTS AND DISCUSSION In order to determine the ultimate collection efficiency of the UHEC traps, a series of trials were performed; results are given in Table 1. Normal sampling collections were run with the exception that the air stream did not flow through the Schu¨tze reagent. As a result, CO was not oxidized to CO2 and subsequently trapped in the UHEC no. 2 trap. In these trials, the only gas condensed in the UHEC no. 2 trap was atmospheric CO2 not trapped in the UHEC no. 1 trap with N2O. In this manner, it was straightforward to determine how effectively the UHEC no. 1 trap removed atmospheric CO2. The atmospheric CO2 concentration is approximately 360 parts per million (ppm), while atmospheric CO concentrations average 100 parts per billion (ppb) up to a few ppm in polluted areas. A reasonable limit for this application is that the CO-derived CO2 sample contain less than 1% atmospheric CO2. Thus, any effective trap must have greater than 99.9989% efficiency for removing atmospheric CO2 from the ambient air stream. Results from Table 1 indicate that the average amount of atmospheric CO2 left in the air stream and collected in the UHEC no. 2 trap during a routine collection is 0.28 ( 0.09 µmol. The 0.09 µmol uncertainty is the calculated standard deviation. At an
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atmospheric CO2 concentration of 360 ppm, there are 27 435 µmols of CO2 in 1800 L of air, corresponding to a UHEC trap collection efficiency of 99.99898%. Since typical CO-derived CO2 sample sizes are rarely less than 30 µmol, the UHEC traps do reduce the amount of ambient CO2 to less than 1% of the sample. At this level, ambient CO2 will have a minimal effect on the isotopic analysis of CO-derived CO2. It is important to note that use of the Perma-Pure Dryer and Drierite drying agents is essential. Control experiments with air that had not been rigorously dried (approximately -20 °C dew point) resulted in a 10-fold increase in the amount of atmospheric CO2 not trapped by UHEC no. 1 and recovered in UHEC no. 2. The UHEC traps are superior for use in a portable, in situ system for several reasons. Most importantly, the UHEC traps are extremely robust, as they are made completely of stainless steel. The benefits from ease of shipping and assembly are manifold in comparison to traps made from glass components. Also, in conjunction with drying agents, the UHEC traps are highly efficient at removing CO2 and water from ambient air. The UHEC traps make possible the in situ collection and high-precision isotopic analysis of atmospheric N2O and CO. ACKNOWLEDGMENT We thank the National Science Foundation for financial support of the CO project under Grant No. ATM-9510665. For financial support of the N2O research, we thank the National Science Foundation under Grant No. CHE 9632311 and the Environmental Protection Agency under Grant No. R822264-01-0.
Received for review March 6, 1997. Accepted August 4, 1997.X AC970256W X
Abstract published in Advance ACS Abstracts, September 15, 1997.
