Environ. Sci. Technol. 1996, 30, 1092-1097
Removing CO2 from Atmospheric Samples for Radiocarbon Measurements of Volatile Organic Compounds REINHOLD A. RASMUSSEN,† C H A R L E S W . L E W I S , * ,‡ ROBERT K. STEVENS,‡ WILLIAM D. ELLENSON,§ AND STUART L. DATTNER| Oregon Graduate Institute of Science and Technology, P.O. Box 91000, Portland, Oregon 97291, National Exposure Research Laboratory, U.S. Environmental Protection Agency, MD-47, Research Triangle Park, North Carolina 27711, ManTech Environmental Technology, Inc., P.O. Box 12313, Research Triangle Park, North Carolina 27709, and Texas Natural Resources Conservation Commission, P.O. Box 13087, Austin, Texas 78711
Two methods employing LiOH to selectively remove CO2 from ambient whole air samples have been investigated. This is a necessary step in preparing an atmospheric sample for a measurement of the radiocarbon (14C) content of its non-methane volatile organic compounds (VOC) fraction, which in turn gives the fraction of VOC that is biogenic. A reliable estimate of the biogenic contribution to VOC is fundamental in deciding on a national tropospheric ozone abatement strategy. Both methods, using a LiOH-coated annular denuder or a LiOH-packed tube, were shown capable of decreasing CO2 in ambient air samples (nominal 360 ppm) by 4 orders of magnitude, while having only a modest effect on their VOC content (20% loss, on average). The samples were from a variety of urban and rural locations. Detailed VOC speciation showed that aldehydes generally experienced the greatest loss during CO2 removal, while hydrocarbons were virtually unaffected. The following paper in this issue gives VOC radiocarbon results for samples characterized in this article.
Introduction A fundamental uncertainty in deciding on a national tropospheric ozone abatement strategy (1) is the lack of reliable knowledge on the size of the biogenic contribution to precursor non-methane volatile organic compounds * To whom correspondence should be addressed; telephone: 919541-3154; fax: 919-541-0063; e-mail address: lewis.charlesw@epamail. epa.gov. † Oregon Graduate Institute of Science and Technology. ‡ U.S. Environmental Protection Agency. § ManTech Environmental Technology, Inc. | Texas Natural Resources Conservation Commission.
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(VOC). Biogenic VOC is known to be photochemically very active and is basically uncontrollable, so that if it constitutes an appreciable fraction of total VOC emissions, control of the remaining anthropogenic VOC may be less effective than NOx control in ozone abatement. In principle, biogenic VOC can be quantitatively distinguished by the isotopic 14C content of an VOC sample: a purely biogenic sample has a known small fraction of its carbon in the form of 14C, while a sample of fossil-fuel origin is devoid of 14C (2). One of the problems that must be solved in applying the 14C approach is separating the VOC component in a sample of ambient air from the far larger amount of CO2 (360 ppm) that also contains 14C (3). For a typical U.S. urban VOC concentration of 0.5 ppm of C (total), to decrease CO2 contamination of the VOC to 10% or less requires decreasing CO2 concentration by 4 orders of magnitude, with minimal effect on the VOC. The two other significant carboncontaining gases, methane and carbon monoxide, are potential interferents, but both can be eliminated by a simple cryogenic step (4). It is well known that LiOH is an aggressive scavenger of CO2 (5). Conversely, it was also anticipated that LiOH would have little affinity for most of the VOC species. Quantifying both of these desired features on ambient air samples was the main focus of the work presented here. Limited investigation of other CO2 scavengers [BaOH, NaOH (Ascarite), and CaOH (Sofnolime)] was performed. None were clearly superior to LiOH, so no results for those candidates have been included here. Two methods for exposing atmospheric samples to LiOH were investigated, in which the sample air flowed through either a LiOH-coated annular denuder or LiOH granules in a packed tube. For both methods, CO2 removal factors were measured as well as the impact on VOC for the total and for individual VOC species by GC/FID and GC/MS measurements. Table 1 gives a description of each of the ambient air samples that were used in these measurements, collected in a variety of urban and rural locations. Some were individual samples, while others were composites, as indicated. All were collected in 32-L Summa canisters at 20-28 psig. Results (6) and interpretation (7) of the VOC 14C content of some of the samples listed are given in two companion articles.
LiOH-Coated Annular Denuder Measurements Apparatus and Preparation. Annular denuders are widely used to selectively remove a target gas from an airstream (8). As the air sample flows laminarly through an annular channel, gas molecules are transported by Brownian motion to the channel walls where the target gas is captured through chemical reaction with a specially chosen coating. This approach was selected for CO2 removal consideration, with the hope that it might also provide a “gentle” treatment of the VOC species, especially those of higher molecular weight that are less subject to Brownian diffusion. Multi-channel annular denuders (9), Model URG-2000-30X150-3CSS (URG Inc., Carrboro, NC), were used in the investigation. Developing a procedure for producing a LiOH-coated annular denuder that was both efficient for CO2 removal
0013-936X/96/0930-1092$12.00/0
1996 American Chemical Society
TABLE 1
Ambient Sample Description sample name
location
date
time
Houston AM1 Houston AM2 Houston PM (composite)
urban/Georgia Tech campus urban/Georgia Tech campus urban/Georgia Tech campus urban/Georgia Tech campus urban/Georgia Tech campus urban/Georgia Tech campus urban/Georgia Tech campus urban/Georgia Tech campus urban/Georgia Tech campus urban/Georgia Tech campus urban/Georgia Tech campus urban/Georgia Tech campus urban/Georgia Tech campus urban/Georgia Tech campus forest/27 km NW of Atlanta forest/27 km NW of Atlanta forest/27 km NW of Atlanta suburban/west Houston suburban/west Houston suburban/west Houston suburban/west Houston suburban/west Houston suburban/west Houston suburban/west Houston
Brazos State Park
forest/60 km W of Houston
8/19/92 8/27/92 9/8/92 9/8/92 8/18/92 8/19/92 9/8/92 9/9/92 9/9/92 9/10/92 9/9/92 9/9/92 9/9/92 9/9/92 8/19/92 8/19/92 8/27/92 9/2/93 9/2/93 9/3/93 9/3/93 9/2/93 9/3/93 9/2/93 9/3/93 9/2/93
0800-1000 0800-1000 0600-0800 0800-1000 2000-2200 2000-2200 2000-2200 2000-2200 2000-2200 0000-0200 0800-1000 0800-1000 1200-1400 1600-1800 0800-1000 1600-1800 0800-1000 0400-1100 1130-1830 0400-1100 1130-1830 0400-1100 0400-1100 1130-1830 1130-1830 1000-1015
Atlanta AM (composite)
Atlanta PM (composite)
Atlanta Region (composite)
Houston Day (composite)
and organically noncontaminating required a substantial effort. The usual coating solution recipes, developed for inorganic applications, include methanol or glycerine additives for enhancing the performance of the coatings. These additives cannot be used in the present application however as their residues grossly contaminate the VOC fraction of the sample airstream. Even nominally clean annular denuders previously used with glycerine were found to have a significant residue, requiring multiple boiling water washes and rinses to remove the glycerine from the porous etched-glass surfaces. An additional complication is that once the 10% LiOH coating solution is applied to the denuder surface, the coating must be heat-activated to produce the mixture of LiOH‚H2O monohydrate and anhydrous LiOH that seems to be required for effective CO2 removal (5). Details of the procedure developed to prepare the LiOH-coated annular denuders are given in the Appendix. CO2 Removal. Results for a typical whole air ambient sample before and after LiOH treatment are shown in Figure 1, panels a and b, respectively. A Carle Instrument CH4CO-CO2 analyzer, Model 211-M-FID, was used for the measurements. This instrument separates the three species chromatographically and uses a hot nickel catalyst to convert CO and CO2 to CH4 for quantititative measurement by FID. At a flow rate of 100 mL min-1, LiOH exposure decreased the concentration of CO2 from 369 ppm to 2030 ppb, the detection limit of the instrument. This is a removal factor of 12 000 or greater. The concentrations of CH4 and CO were unchanged. Varying the flow rate between 50 and 350 mL min-1 produced similar results. Figure 2 shows the system that was used for CO2 removal, starting with a canister sample of ambient whole air on the left and filling an initially evacuated canister on the right with CO2-depleted sample air. System components were connected with 0.16 cm (o.d.) stainless steel tubing. The system did not use a pump, but depended instead on the vacuum created in the liquid nitrogen-cooled receiving
TABLE 2
Effect of CO2 Removal (Annular Denuder) on VOC (Method TO14) VOC fraction
before (µg m-3)
after (µg m-3)
loss (%)
Atlanta Region sample 77.4 ( 1.4 77.8 ( .5 -0.5 ( 1.9 total IDa 32.4 ( 4.1 31.0 ( .3 +4.3 ( 12.1 total UIDb c 109.8 ( 5.2 108.8 ( .5 +0.9 ( 4.7 total VOC a Identified hydrocarbons (58). Method TO14.
b
Unidentified organics. c U.S. EPA
canister. The process was monitored by the Carle analyzer, which periodically sampled the CO2 level downstream of the annular denuder. Experience showed that the CO2 capacity of the annular denuder is about 3 L of ambient air (nominal ambient CO2 concentration of 360 ppm), so that each annular denuder was routinely replaced by a fresh one before 3 L was reached. To minimize VOC contamination from system leaks, sample processing was stopped when the pressure in the original canister decreased to about 5 psig, with no further use made of the residual sample. Effect on VOC. The VOC impact of removing CO2 with an annular denuder was investigated on the Atlanta Region composite sample described in Table 1. The GC/FID measurements quantified 58 hydrocarbons from propane to n-decane (the C2 hydrocarbonssethane, ethylene, and acetyleneswere quantified in later tests along with many additional substituted hydrocarbons). Within measurement error, none showed any change in concentration before and after CO2 removal from the sample. Table 2 shows the before/after GC/FID results in terms of a sum of the 58 identified hydrocarbons (total ID), a sum of all remaining unidentified peaks (total UID), and total VOC (total ID + total UID). The uncertainties represent standard deviations based on three independent sets of measure-
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subsequent tables are referenced to 25 °C and 1 atm and refer to total mass (not just the carbon component). While the Table 2 results indicate no significant losses in the UID class overall, subsequent GC/MS measurements identified several compounds in this class that did show substantial losses, including benzaldehyde and the homologous series of aldehydes from hexanal through decanal. Interestingly, the presumed biogenic terpenes (isoprene, R- and β-pinene) were little affected. A more detailed investigation of VOC effects was undertaken in the LiOHpacked-tube measurements that follow. Annular Denuder Difficulties. While the LiOH-coated annular denuder investigation was generally successful, this approach has considerable disadvantages: (1) Preparation (applying and activating the coating) is laborious. (2) The etched-glass surfaces are vulnerable to organic contamination and release. (3) The CO2 collection capacity is only about 3 L of ambient air. Nonetheless, the annular denuder results provide a standard of performance against which any alternative CO2 removal method can be compared.
LiOH-Packed-Tube Measurements
FIGURE 1. CH4-CO-CO2 measurement on an ambient air sample before (a) and after (b) CO2 removal.
ments. This approach to estimating total VOC (a sum of all chromatographic peaks) is referred to as U.S. EPA Method TO14 (10). The concentration values given in Table 2 and
FIGURE 2. CO2 removal system.
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Apparatus and Preparation. The practical difficulties encountered in using an annular denuder for CO2 removal provided strong motivation to investigate a simpler method, in which the denuder in Figure 2 was replaced by a LiOHpacked tube prepared as follows. Anhydrous LiOH of 4-12 mesh size (Cypress Foote Mineral Co.) was crushed and sieved to 10-18 mesh to increase surface area. Approximately 0.5 g of the resulting granules were packed in a 0.64 cm o.d. × 18 cm length of previously annealed (24 h at 50 °C) FEP Teflon tube and held in place with glass wool end plugs. Teflon tubing was used, rather than glass or stainless steel, because it was safer than glass (LiOH is caustic) and more convenient than stainless steel for verifying a lack of voids in the LiOH-filled tube. The tube was capped with Swagelok 0.64-cm nuts and flushed with heated (110 °C) zero air at 100 mL min-1 for 16 h, while the ambient temperature surrounding the tube was held at 50 °C to prevent overheating of the Teflon during flushing. Afterwards, the flushing continued as the tube cooled to room temperature and then was sealed until used. Gas chromatographic analysis of zero air passed through tubes
TABLE 3
Effect of CO2 Removal (Tube) on VOC (Method TO14) VOC fraction Atlanta AM sample total IDa total UIDb total VOCc Atlanta PM sample total IDa total UIDb total VOCc Houston Day sample total IDa total UIDb total VOCc
before (µg m-3)
after (µg m-3)
loss (%)
265 32 297
270 31 301
-1.9 +3.1 -1.3
160 27 187
162 23 185
-1.3 +14.8 +1.1
232 26 258
232 24 256
0 +7.7 +0.8
a Identified organics (134). b Unidentified organics. c U.S. EPA Method TO14.
FIGURE 3. Effect of CO2 removal on individual species: full range (a) and low range (b). 1, heptanal; 2, hexanal; 3, cyclohexane; 4, benzaldehyde; 5, undecane; 6, octanal; 7, 3-methylpentane.
prepared in this manner showed no measurable VOC contaminants introduced by the tubes. CO2 Removal. The effectiveness of LiOH-packed tubes for CO2 removal was measured with the same CH4-CO-CO2 analyzer used in the annular denuder work and at the same 100 mL min-1 flow rate. The results were virtually the same as those shown previously in Figure 1. The typical CO2 capacity of the tube however was at least 100 L of ambient air, a 30-fold improvement over the annular denuder’s capacity. Not surprisingly, the same degree of CO2 removal was more difficult to achieve for large volume samples than for small ones, for either system. For this reason, a two-pass procedure for CO2 removal was used for all “after” results given in Tables 2-5. In practice, a first pass through LiOH reduced the CO2 concentration from ∼360 000 to 140-400 ppbv. The second pass through fresh LiOH reduced the concentration to 85%, by mass) of the chromatographic peaks are identified in Table 3 than in Table 2. Otherwise the Table 2 and 3 results are similar: a greater percentage loss for the unidentified VOC fraction than for the identified fraction, but with an overall loss in total VOC that is less than a few percent. The small "negative loss" values in the two tables are attributed to an experimental inability to exactly match the sample sizes used in the before and after measurements. To gain some appreciation of the distribution by molecular weight of the unknown species, Table 4 lists the before/after sums of the mass concentrations of the unknown species that lie within successive retention time windows. For all three composite samples, the third window captures a large fraction of the mass concentration associated with the unknown species. In terms of identified species, this window is bounded by ethylbenzene and decanal. Finally, Table 5 provides an alternative view of the effect on VOC resulting from CO2 removal. In this case, total VOC was estimated by a cryogenic preconcentration/direct flame ionization detection (PD/FID) method using a Shimadzu FID, essentially as described by McElroy et al. (11). This is also referred to as U.S. EPA Method TO12 (10). More specifically, Method TO12 measures total nonmethane organic compounds (NMOC), which is regarded
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TABLE 4
Effect of CO2 Removal (Tube) on Unidentified VOC Mass (Method TO14) retention time (min) Atlanta AM sample 0-33 33-49 49-66 66-88 total Atlanta PM sample 0-33 33-49 49-66 66-88 total Houston Day sample 0-33 33-49 49-66 66-88 total
before (µg m-3)
after (µg m-3)
loss (%)
0.61 6.30 22.23 2.92 32.06
0.54 6.08 21.60 2.81 31.03
3.2
0.50 5.38 18.37 2.26 26.51
0.63 4.05 16.17 2.47 23.32
12.0
0.94 4.28 18.36 2.22 25.80
0.93 4.60 16.93 1.41 23.87
7.5
TABLE 5
Effect of CO2 Removal (Tube) on VOC (Method TO12) sample name
after (µg m-3)
loss (%)
Atlanta AM Atlanta PM Houston AM1 Houston AM2 Houston PM Brazos State Park
336 ( 8 210 ( 4 117 ( 2 132 ( 3 78 ( 3 61 ( 1
291 ( 7 179 ( 5 95 ( 1 115 ( 3 52 ( 3 45 ( 1 average
13 ( 3 15 ( 3 19 ( 2 13 ( 3 33 ( 5 26 ( 3 20
here the same as total non-methane VOC. The VOC losses in Table 5 range from 13 to 33%, averaging 20%. The VOC losses of Table 5, based on Method TO12, are clearly larger than those shown in Tables 2 and 3, based on Method TO14. Since Method TO12 does not use a chromatographic column, it is not subject to the loss, both before and after CO2 removal, of species that chromatograph poorly. The annular denuder glycerine experience noted previously provided a striking example of this. Air samples inadvertently contaminated with glycerine produced Method TO12 VOC values several times larger than expected, while GC/FID and GC/MS chromatograms of the same samples appeared normal and without a glycerine peak. Thus the Method TO12 is regarded as providing the more realistic estimate of VOC loss during CO2 reduction. Under humid conditions, a pressurized ambient air sample may produce water condensation in the sample canister. This in turn could lead to a partial fractionation of the VOC according to the differing solubilities of the individual species, an effect that may be amplified by the considerable amount of sample (5 psig) left in the canister during sample processing. A more subtle effect is that biogenic species may be more reactive in the atmosphere than petroleum hydrocarbons, so that biogenic species may be converted to oxygenated products to a greater extent. The generally greater solubility of oxygenates than hydrocarbons and the partial loss of some oxygenates in the LiOH step are both in the direction of a relatively larger decrease in the biogenic than in the non-biogenic portion of an
9
Conclusions Lithium hydroxide, either as an annular denuder coating or in granular form, is very effective in drastically decreasing CO2 in atmospheric samples, while having only a modest effect on their VOC content. In practice, however, the annular denuder method is far less convenient to implement. In both cases, CO2 could be routinely decreased from nominal ambient concentrations of 360 ppm to 2030 ppb, the detection limit of the CO2 instrument. The amount of VOC loss was a few percent or less when quantified by Method TO14 (summing over individual GC/ quantified species) but averaged 20% for Method TO12 (direct FID). Since Method TO12 does not use a chromatographic column, it is not subject to the loss of species that chromatograph poorly and thus gives a more realistic estimate of VOC loss. As a class, aldehydes exhibited the greatest loss during CO2 removal. Overall, the effects of the LiOH treatment and water condensation in the sample canister may lead to relatively more loss of biogenic than fossil-fuel VOC from a sample.
Acknowledgments
before (µg m-3)
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ambient sample. Such considerations have no direct impact on the results reported in this paper but are relevant to the interpretation of radiocarbon measurements performed on the samples (7).
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We are indebted to Will Ollison, American Petroleum Institute, for sharing his knowledge of the LiOH-CO2 literature. Dan Boryta, Cyprus Foote Mineral Co., provided helpful advice on activating LiOH for CO2 scavenging. David Stiles, ManTech Environmental Technology, Inc. (METI), contributed to the development of the LiOH coating technique and provided confirmatory measurements of CO2 removal efficiencies. The information in this document has been funded wholly or in part by the United States Environmental Protection Agency under Contract 68-D00106 to METI and by the Texas Natural Resources Conservation Commission under Contract 519-3-3609-18 to Biospherics Research Corporation. It has been subjected to Agency review and approved for publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.
Appendix: Procedure for Preparing LiOH-Coated Annular Denuders for CO2 Collection (1) Prepare a 10% LiOH solution by adding 10 g of LiOH crystals to 100 mL of degassed distilled water. Heat to 40 °C. Handle LiOH with care; it is extremely destructive to tissue of the mucous membranes and upper respiratory tract, eyes, and skin. (2) Add 10 mL of LiOH solution to the denuder capped off at one end and cap the other end. Shake a few times and decant. Allow the denuder to dry-drain for several minutes (capped), preventing ambient air from entering the denuder. Repeat three times. (3) Replace with valved end caps (Nupro SS4H bellows valves) and blow out excess water at 100 mL min-1 at 5 psig for 15 min with zero air (no CO2, etc.) at room temperature. (4) Dry at 45 °C at 100 mL min-1 for 12 h. Evacuate to 10-20 mTorr at 70 °C for 2-4 h.
Literature Cited (1) National Research Council. Rethinking the ozone problem in urban and regional air pollution; National Academy Press: Washington, DC, 1991.
(2) Clayton, G. D.; Arnold, J. R.; Patty, F. A. Science 1955, 122, 751753. (3) Klouda, G. A.; Norris, J. E.; Currie, L. A.; Rhoderick, G. C.; Sams, R. L.; Dorko, W. D.; Lewis, C. W.; Lonneman, W. A.; Seila, R. L.; Stevens, R. K. A method for separating volatile organic carbon from 0.1 m3 of air to identify sources of ozone precursors via isotope (14C) measurements. Proceedings of the EPA/A&WMA Symposium Measurement of Toxic and Related Air Pollutants; Durham, NC, 1993; Air Waste Management Association: Pittsburgh, 1993; pp 585-603. (4) Brenninkmeijer, C. A. M. Anal. Chem. 1991, 63, 1182-1184. (5) Boryta, D. A.; Maas, A. J. Ind. Eng. Chem. Process Des. Dev. 1971, 10 (4), 489-494. (6) Klouda, G. A.; Lewis, C. W.; Rasmussen, R. A.; Rhoderick, G. C.; Sams, R. L.; Stevens, R. K.; Currie, L. A.; Donahue, D. J.; Jull, T. A.; Seila, R. L. Environ. Sci. Technol. 1996, 30, 1098-1105. (7) Lewis, C. W.; et al. Biogenic fraction of ambient VOC using 14C measurements: Atlanta GA results. Manuscript in preparation. (8) Conner, T. L.; Stevens, R. K.; Paur, R. J.; Baumgardner, R. E.; Bell, J. P. Atmos. Environ. 1988, 22 (8), 1729-1736.
(9) Stevens, R. K.; Purdue, L. J.; Barnes, H. M.; Ward, R. P.; Baugh, J. O.; Bell, J. P.; Sauern, H.; Sickles, J. E.; Hodson, L. L. Annular Denuders and Visibility Studies. In Visibility and Fine Particles; AWMA Transactions Series 17; ISSN 1040-8177; AWMA: Pittsburgh, PA, 1990; pp 122-130. (10) Winberry, W. T.; Murphy, N. T.; Riggan, R. M. U.S. Environmental Protection Agency Report EPA/600/4-89-017. U.S. EPA: Research Triangle Park, NC, 1989. (11) McElroy, F. F.; Thompson, V. L.; Holland, D. M.; Lonneman, W. A.; Seila, R. L. J. Air Pollut. Control Assoc. 1986, 36 (6), 710-714.
Received for review March 23, 1995. Revised manuscript received September 1, 1995. Accepted September 13, 1995.X ES9501979 X
Abstract published in Advance ACS Abstracts, February 15, 1996.
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