Environ. Sci. Technol. 1996, 30, 1098-1105
Radiocarbon Measurements of Atmospheric Volatile Organic Compounds: Quantifying the Biogenic Contribution† G E O R G E A . K L O U D A , * ,‡ CHARLES W. LEWIS,§ REINHOLD A. RASMUSSEN,| GEORGE C. RHODERICK,‡ ROBERT L. SAMS,‡ ROBERT K. STEVENS,§ LLOYD A. CURRIE,‡ DOUGLAS J. DONAHUE,⊥ A. J. TIMOTHY JULL,⊥ AND ROBERT L. SEILA§ National Institute of Standards and Technology, Gaithersburg, Maryland 20899, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina 27711, Oregon Graduate Institute, Beaverton, Oregon 97006, and NSF Facility for Radioisotope Analysis, University of Arizona, Tucson, Arizona 85721
The radiocarbon (14C) abundance of atmospheric volatile organic compounds (VOC) gives a quantitative estimate of contributions from biomass and fossilmass sources, important information for effective regulation of ozone precursors. We report here details of a methodology to perform such measurements and the first exploratory 14C results on VOC fractions separated from two composited urban tropospheric air samples, collected during the summer (1992) in Atlanta, GA. The upper limit of the percentage of VOC originating from biomass sources during the morning and evening hours in Atlanta were 9 and 17%, respectively, measurements reported at the 95% confidence level. However, due to the level of the process blank and its uncertainty, in both cases the percentage can be as low as zero. The results of these experiments, designed to (i) evaluate the entire [14C]VOC measurement process and (ii) obtain reliable estimates of biogenic contributions to atmospheric VOC, emphasize how important controls are throughout this multi-step chemical process to ensure quality data.
Introduction It is well known that tropospheric O3 arises from a series of atmospheric reactions involving volatile organic compounds (VOC), NOx (NO + NO2), and sunlight (1). As commonly defined, VOC are all gas phase organics except CH4, CO, and CO2. Many U.S. cities have been unable to comply with the National Ambient Air Quality Standard for O3: 0.12 µL L-1 daily maximum over a 1-h period. An effective national O3 abatement strategy will require a better understanding of the sources of O3 precursors and their contributions, particularly biogenic VOC (2). Since VOC from vegetative emissions (evaporation and combustion) contains 14C at levels comparable to atmospheric CO2, while similar emissions from fossil fuels are devoid of 14C, measurement of the [14C]VOC content of ambient samples provides the possibility of determining the fraction of sample VOC carbon (VOC-C), whose origin is biogenic. The present sensitivity of accelerator mass spectrometry (AMS) 14C/13C measurements allows quantitation of 10-µg modern carbon samples with a 1-µg modern carbon blank. A 1% precision is attainable on as little as 25 µg of modern carbon (3). The ability to obtain quality (interpretable) [14C]VOC measurements however relies on additional factors: (1) transferring individual VOCs to the measurement system in a manner that preserves their original relative concentrations in the atmosphere (recovery), (2) quantifying limitations in the VOC isolation process (bias), and 3) quantifying chemical and isotopic contamination (blanks). This paper describes the methods that have been developed to isolate the total VOC fraction in whole air samples, to oxidize this fraction to CO2, and to reduce the resulting CO2 to an AMS Fe-C target for 14C measurements. While the paper’s primary focus is methods description and evaluation, illustrative results from ambient samples collected in 1992 summertime Atlanta are also given.
Samples and Calibration Materials Ambient Air Samples. Air samples were collected in Atlanta, GA, during the late summer of 1992 on the campus of the Georgia Institute of Technology. The samples were collected in 32-L Summa canisters at 15-m elevation on a tower that had been constructed for the 1992 Southern Oxidants Study (SOS) Atlanta Intensive. Two-hour sampling durations were used, resulting in pressurized samples containing the equivalent of 96 L of air at 101 kPa (1 atmosphere ) 101.3 kPa) pressure. Because individual samples contained too little VOC-C for isotope measurements, several samples were combined into two composited samples, an “AM” composite (n ) 4) and a “PM” composite (n ) 6) by cryogenic means (4). For the AM composite, three samples were from 0800-1000 periods, and one was from a 0600-0800 period. For the PM composite, five * To whom correspondence should be addressed; e-mail address:
[email protected]; telephone: (301) 975-3931; fax: (301) 2161134. † Contribution of the National Institute of Standards and Technology. ‡ National Institute of Standards and Technology. § U.S. Environmental Protection Agency. | Oregon Graduate Institute. ⊥ University of Arizona.
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0013-936X/96/0930-1098$12.00/0
1996 American Chemical Society
TABLE 1
Carbon Composition of Gravimetric Calibration Mixtures and Atlanta, GA, Air Samples (Uncertainties in Parentheses) before LiOH treatment VOC (uc) [µmol of C mol-1]
whole-gasoline vapor in N2
2.77 (0.01)a 2.44 (0.05)b 1.75 (0.01)a
Atlanta AM Atlanta PM
0.640 (0.013)c 0.400 (0.009)c
whole-gasoline vapor in air
after LiOH treatment
CO2 (uc) [µmol mol-1]
VOC (uc) [µmol of C mol-1]
Calibration Mixture 363.8 (0.5)
CO2 (uc) [µmol mol-1]
0.15(0.01)
0.080 (0.003) Ambient Sample 413 (1) 403 (1)
0.555 (0.013)c 0.341 (0.009)c
0.059 (0.009) 0.043 (0.009)
a Determined gravimetrically. b Determined by Method TO12. Comparison with the gravimetrically determined concentration suggests an apparent 14% bias assumed to be in Method TO12 (see text). c Determined by Method TO12 and corrected for an apparent negative bias (14%).
samples were from 2000-2200 periods, and one was from a 0000-0200 period. Additional details of the VOC sampling are given elsewhere (5). Total VOC concentrations in the AM and PM composites were determined by the U.S. EPA Method TO12 (6) using liquid oxygen (-183 °C) to preconcentrate non-methane VOC for direct injection into a flame ionization detector (FID) without the use of an analytical column (7). The system was calibrated by comparing detector responses of unknowns to the National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) 1665b, 2.87 ( 0.03 µmol mol-1 C3H8 in air. The Method TO12 results for total VOC are given in Table 1. Uncertainties reported for SRMs are 95% confidence intervals. Unless otherwise noted, all other uncertainties are combined standard uncertainties (uc, estimated standard deviation) obtained by combining individual standard uncertainties (u, standard errors) using the law of propagation of standard deviations (8) with nomenclature defined by Taylor and Kuyatt (9). The CO2 concentration of the composite samples was measured by diode laser infrared absorption using sweepaveraging techniques and second-derivative signal processing (10). No attempt was made to remove the H2O prior to the analysis; therefore, concentrations determined relative to the dry standards were likely to be lower by ∼0.25%. The following NIST SRMs and primary gravimetric standards were used for diode calibration: SRM 1672 (351.1 ( 0.4 µmol mol-1 CO2 in air); primary gravimetric standards X134470 (10.08 ( 0.05 µmol mol-1 CO2 in blended air) and X138333 (1.79 ( 0.02 µmol mol-1 CO2 in blended air); and Scott Specialty Gases ultra zero ambient monitoring (UZAM) N2 (0.171 ( 0.001 µmol mol-1 CO2, measured at NIST). Blended air refers to a mixture of pure N2 and O2 at a volume ratio equivalent to that of air. The CO2 results are given in Table 1. Organic Calibration Liquids. With the goal of acquiring “dead” carbon (14C ) 0) calibration materials from a source category that is a significant contributor to ambient VOC, British Petroleum (BP) and Exxon gasolines, octane rating 87, were collected from one gas station each in Research Triangle Park, NC. This occurred a few weeks prior to the mandatory addition of oxygenates to gasoline on November 1, 1993, for CO emissions reductions in this airshed. The oxygenate ethanol can contribute significant modern carbon (14.8%) to gasoline (11). The carbon contents of both gasolines were determined by combusting aliquots to CO2 for volumetric determina-
tion. This was accomplished by (1) weighing 1-3-mg aliquots in sealed capillaries, (2) placing each in individual 6-mm Vycor tubes with Ag (wire) and CuO and sealing the tubes under vacuum, (3) heating the tube to 900 °C for ∼3 h for quantitative conversion to CO2, (4) distilling the CO2 from H2O at -78 °C, and (5) measuring the pressure of CO2 recovered in a calibrated volume. The carbon contents of the whole-gasoline vapor calibration materials were 70.30 ( 0.16 (u, n ) 4) mmol of C g-1 for BP 87 and 69.04 ( 0.06 (n ) 1) mmol of C g-1 for Exxon 87 (Table 2). Both had a 14C abundance of essentially zero, from measurements on 0.2 -0.3-mg C aliquots of the product CO2. For quality control purposes, reagent benzene was similarly processed for its chemical and isotopic composition. The measured carbon content of the benzene was 5.938 ( 0.003(u, n ) 3) mol of C mol-1 and the measured 14C abundance was zero, both quantities being consistent with what was expected, i.e., nominally a C/C6H6 mole ratio of 6 and a 14C signal comparable to the AMS target blank (Table 2). Gaseous Calibration Mixtures. With the carbon content of the gasolines determined, a whole-gasoline vapor in air (gasoline/air) calibration mixture was prepared. The procedure required the following steps: (1) weighing 2.9199 ( 0.0007 mg of BP 87 (liquid) gasoline in a sealed capillary; (2) quantitatively transferring the liquid to an evacuated cylinder by expansion; (3) purging the capillary with the diluent air, Scott Specialty Gases UZAM (blended) air containing 363.8 ( 0.8 µmol mol-1 of fossil (dead carbon) CO2 from natural gas (12); and (4) filling the cylinder with 2144 ( 1 g (6.15 MPa) of the diluent air (13). Carbon dioxide in this calibration mixture is thus similar to atmospheric CO2 in concentration, but not in 14C content. The manufacturer’s impurity specifications for UZAM air were 100% recoveries calculated for the last four VOC separation experiments listed in Table 3, it is evident that VOC fM measurements require additional correction for an excess carbon blank (mX). Also, the VOC fM results for the calibration mixtures were substantially larger than the essentially null results for the original liquid gasoline (Table 2), which indicated that a significant amount of contemporary carbon was introduced during the sample processing. Therefore, final corrected fM′ values for samples were obtained using eq 2 again, where now
ΘBLK ≡
mX mREC - mCO2
(4)
For the remaining quantities in eq 2, fM is assumed to be corrected for CO2 contamination and fM(BLK) is assumed
TABLE 4
Cumulative Effects of Process Blanks on the Estimated Biogenic Fraction of Ambient VOC (Uncertainties in Parentheses) fM′ (uc) sample
fM (uc) uncorrected
AMS target correction
CO2 contamination correction
excess (X) carbon correction
VOC biogenic fraction (uc)a
VOC biogenic fraction, 95% ULb
Atlanta AM Atlanta PM
0.46 (0.02) 0.52 (0.02)
0.44 (0.02) 0.50 (0.03)
0.37 (0.03) 0.44 (0.04)
-0.05 (0.09) 0.04 (0.09)
-0.04 (0.08) 0.04 (0.08)
0.09 0.17
a [C4/1.14]; (C ) column). b [(C4 + (tu ))/1.14]; (C ) column) where t ) 1.645 assuming df ) ∞. The lower limit of zero is restricted by the physical c constraint that fM is nonnegative.
to be contemporary carbon, i.e., 1.14. The quantity mX in eq 4 is obtained by modeling the excess carbon blank, as described in the next section.
VOC Processing Blank (mX) Estimates The approaches used to estimate mX in eq 4 utilized both mass balance and 14C-related data. In the simplest approach, for each of the four experiments in Table 3 having recoveries >100%, a purely mass balance estimate of mX, given by (mREC - mCO2 - mVOC), can be obtained from the data given in the table for that experiment. The resulting estimates for mX are given in the last column (footnote e). From the experiments involving the gaseous calibration mixtures (GCM), independent estimates of mX can be obtained starting from an expression for fM(GCM)VOC, the measured (uncorrected) fM for the VOC fraction of a GCM:
fM(GCM)VOC ) ΘVOCfM(gas) + ΘCO2fM(CO2) + ΘXfM(X) (5) Each fraction Θi is the carbon mass mi associated with source i relative to the total recovered carbon, mREC, and fM(gas) refers to the liquid gasoline used to make the calibration mixtures. For the gasoline/N2 experiment, designed to evaluate just the separation/oxidation process blank, eq 5 (with GCM ) gas/N2) can be solved for mX to give
mX )
[fM(gas/N2)VOCmREC] fM(C)
- mCO2
(6)
using the approximations that fM(gas) is essentially zero (0.005), and fM(CO2) and fM(X) are approximately equal (1.16 vs 1.14), both quantities referred to in eq 6 as contemporary carbon, i.e., fM(C). The result, 23 ( 2 µg, assumed to be contemporary carbon, appears in the last column of Table 3 (footnote f). This is consistent with the previous duplicate mass balance-derived results of 22 ( 4 and 23 ( 4 µg of C for this experiment (Table 3). Since this experiment did not include LiOH treatment (the CO2 content of this mixture was small by design), the results indicate that significant carbon contamination was introduced in the VOC separation/oxidation step alone. An estimate of the entire process blank was similarly obtained from eq 5 for the gasoline/air experiment. However, in this case, (mVOC + mCO2 + mX) must be used for mREC since a reliable measure of the carbon recovered from the VOC fraction was not available, as discussed earlier. This is a valid substitution, if the 74% VOC recovery from the gasoline/air experiment was due only to incomplete distillation of the CO2 from the H2O, as discussed. Also, fM(CO2) in this case is estimated to be 0.52, derived from
FIGURE 2. Excess carbon blank vs processing time for separating VOC from CO, CH4, and air (N2/O2) matrix. (2) Blanks calculated from carbon mass balance. (1) Equivalent blanks from 14C measurements and recovered carbon of the gasoline/air calibration mixture (at 11 min) and the gasoline/N2 calibration mixture (at 100 min). Atlanta sample results are reported at 60 and 144 min. The linear regression line includes 95% confidence intervals (- -) and 95% prediction intervals (- - -). Error bars represent combined standard uncertainties.
54% dead residual-CO2 and 46% contemporary-CO2 contamination (footnote a, Table 3). Solving eq 5 (with GCM ) gas/air) for mX without approximations gives eq 7: mX ) [fM(gas/air)VOC(mVOC + mCO2)] - [fM(CO2)mCO2] - [fM(gas)mVOC] fM(X) - fM(gas/air)VOC
(7)
The following quantities from Tables 2 and 3 were used in eq 7 to calculate mX: (1) the measured fM(gas/air)VOC ) 0.34 ( 0.03; 2) the assumed fM(X) ) 1.14; 3) the measured mCO2 ) 2.6 ( 0.2 µg of C; (4) the measured mVOC ) 29.0 ( 0.4 µg of C; (5) the measured fM(gas) ) 0.005 ( 0.002; and (6) the estimated fM(CO2) ) 0.52. The result, mX ) 12.2 ( 2.0 µg of C for the gasoline/air experiment, appears in Table 3 (footnote d). The six estimates for mX in Table 3 span a range of 1234 µg of C. The disparity between these estimates suggested that the blank distribution may depend on some variable associated with the sample processing. Since the volume of gas processed, and therefore the processing time, spanned an order of magnitude for the five experiments presented in Table 3, the hypothesis was considered that the blank was a function of the time required to process the sample. For all the experiments, the time required to
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pass the VOC/H2O fraction through the high-temperature catalyst to oxidize the VOC fraction to CO2 and to cryogenically collect the CO2 was relatively constant. Figure 2 is a scatterplot of blanks and their combined standard uncertainties (error bars) vs the corresponding processing time. The plot indicates a trend of increasing blank with longer processing times. Linear regression analysis of the data reveals a slope of 0.169 ( 0.030(u) µg of C min-1 and an intercept of 7.1 ( 2.9(u) µg of C (R2 ) 0.887). A plot of the residuals and the lack-of-fit test gave no indication that the model was incorrect, although the intercept may be significantly different from zero. Other mechanisms may be possible, but not enough data exist to identify the contamination source and injection rate into the system. From the above regression equation, predicted blank values of 17.2 ( 1.5 µg of C for the Atlanta AM and 31.4 ( 2.2 µg of C for the Atlanta PM samples were obtained. These predicted blank values (mX), although not greatly different from the blanks estimated via mass balance considerations, were preferred over the latter estimates, since an analysis of all blanks (n ) 6) by linear regression would improve the certainty of the blank and would likely be more representative of the true blank values.
Biogenic Fraction of Atlanta VOC Table 4 lists the measured fM results for the VOC fractions separated from the two Atlanta samples (column 1) and the cumulative corrections for various blanks (columns 2-4). The corresponding uncertainties, in parentheses, were estimated by conventional error propagation (8). An alternative estimate of the uncertainty in the final blankcorrected fM′ values (column 4) was obtained by a Monte Carlo simulation of replicate measurements of each variable of eq 2, using its actual measured value, standard deviation, and assumed normality. With 10 000 simulations per analysis, and each analysis repeated 10 times, the simulated fM′ value and uncertainty were virtually identical to those shown in column 4 for both samples. From the same simulations, estimated 95% upper limits of 0.0822 ( 0.0003 (u) and 0.1818 ( 0.0006 (u) were obtained for the final corrected fM′ value for the AM and PM samples, respectively. These simulated upper limits compared well with those from conventional error propagation for AM and PM periods, i.e., 0.098 and 0.1881, respectively, based on a onesided t-distribution assuming an infinite number of degrees of freedom (df ) ∞, t ) 1.645). Finally, columns 5 and 6 of Table 4 give the estimated biogenic fraction and the 95% upper limit of the biogenic fraction, respectively, for the AM and PM samples. The former was calculated according to
biogenic fraction )
fM′[VOC] 1.14
(8)
with a similar expression for the latter. Equation 8 assumes that the measured 14CO2 composition of the Atlanta PM composite sample (fM ) 1.14 ( 0.05) was also appropriate for the composition of the AM sample.
Discussion and Conclusions Table 4 results show 95% upper limits of 9 and 17% for the percentages of VOC biogenic carbon contributed to the AM and PM samples, respectively. (In the unlikely case that ∼15% of the original VOC lost in the CO2 (LiOH) removal step were entirely biogenic, then the upper limits would be
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23 and 29%.) However, due to the magnitude and uncertainty of the process blank, in both cases the biogenic fraction can be as low as zero. These uncertainty ranges indicate that blank levels must be reduced and that both blank and recovery be better controlled for more precise and accurate measures of the biogenic fraction. Canister collection of the ambient air samples raises two complicating issues: (1) storage stability and (2) differential VOC species loss. Stability is of concern, since the interval between collection and analysis was 18 months. Some reassurance is provided by results from seven additional 32-L canister samples collected contemporaneously with the 10 used to obtain the AM and PM composited samples. Measurements by GC were done within 2 weeks of sample collection and repeated 6 months later. Overall, the final/initial ratio, averaged over all samples and all GC peaks (≈150 compounds in the C2-C12 range, identified or not), was 1.03 ( 0.14 (mean ( sD). The ratio for most identified hydrocarbons was unity, within measurement error (∼5%). The same was true for the only two identified hydrocarbons clearly associated with biogenic emissions: isoprene and R-pinene. The issue of differential VOC species loss is more subtle and difficult to assess. During pressurized sampling of humid air, water will be condensed in the canisters. Since the canisters are not totally evacuated during subsequent sample processing, some residual water is left in the canisters. (During the canister-compositing/CO2-removal step, sample processing was stopped when the pressure in the original canister decreased to about 135 kPa; a residual pressure of about 30 kPa was left in the composited canister during the VOC separation step.) Consequently, the more soluble VOC species will be relatively more retained in the canister and be underrepresented at the point of 14C measurement. A recent review (25) suggests that primary emissions of poorly characterized oxygenated VOC such as methanol, formaldehyde, 3-hexen-1-ol, and others may be of major importance in biogenic emissions inventories. Such species are generally more soluble than ordinary hydrocarbons. In addition, because biogenic VOC is thought to be generally more reactive than anthropogenic VOC (2), relatively more of the biogenic VOC emissions may be converted to oxygenates in the atmosphere. Thus, whether the oxygenates arise from primary emissions or atmospheric reactions, solubility considerations suggest a relatively larger loss of biogenic-related VOCs. Recovering the VOC from the residual water, e.g., by sparging, could provide a quantitative assessment of the importance of such losses. With 14C measurements available for only two VOC ambient samples, the results should be regarded as strictly exploratory rather than definitive. Nonetheless, these results are qualitatively consistent (i.e., show only a modest biogenic fraction) with estimates obtained by two other methods: (1) the biogenic/anthropogenic classification of the individual VOC species identified and quantified in the ambient samples; (2) the biogenic/anthropogenic ratios present in an hourly, gridded, research-grade emissions inventory (26) for the greater Atlanta metropolitan region. These comparisons will be discussed in detail by Lewis et al. (5). Future work will include modifying the gas separation vacuum manifold to minimize the process blank. The existing flow controllers, containing elastomers and lubricants, will be replaced with all-metal units; glass-barrel
valves with Viton seats will be replaced with all-metal bellows valves. During the summer of 1994, canister samples were collected in Nashville, TN, and Houston, TX. The VOC characterization and processing of these samples for subsequent 14C measurements is underway.
Acknowledgments The authors would like to thank W. E. Ellenson for his role in the field sampling component of the study and W. A. Lonneman for his assistance with the GC analysis. The information in this document has been funded wholly or in part by the U.S. Environmental Protection Agency under Interagency Agreement DW13935098 to the National Institute of Standards and Technology. It has been subjected to Agency review and approved for publication. Certain commercial equipment, instruments, or materials are identified in this paper to specify adequately the experimental procedure. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology or the U.S. Environmental Protection Agency, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.
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(9) Taylor, B. N.; Kuyatt, C. E. Natl. Inst. Stand. Technol. 1993, Technical Note 1297. (10) Fried, A.; Sams, R. Tunable diode laser absorption spectrometry for ultra-trace measurement and calibration of atmospheric constituents; Special Technical Publication 957; American Society for Testing and Materials: Philadelphia, 1987; 121-131. (11) Klouda, G. A.; Connolly, M. V. Radiocarbon (14C) measurements to quantify sources of atmospheric carbon monoxide in urban air. Atmos. Environ. 1995, 29 (22), 3309-3318. (12) Gittler, W. E. Scott Specialty Gases, personal communication, 1994. (13) Rhoderick, G. C.; Zielinski, W. L. Anal. Chem. 1988, 7 (11), 24542460. (14) Mattingly, G. E.; Baumgarten, G. P. National Institute of Standards and Technology, personal communication, 1992. (15) Brenninkmeijer, C. A. M. Anal. Chem. 1991, 63, 1182-1184. (16) 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. In Proceedings of the 1993 U.S. EPA/A&WMA International Symposium Measurement of Toxic and Related Air Pollutants; Air Waste Management Association: Pittsburgh, 1993; pp 585-603. (17) Verkouteren, R. M.; Klouda, G. A.; Currie, L. A.; Donahue, D. J.; Jull, A. J. T.; Linick, T. W. Nucl. Instrum. Methods Phys. Res. 1987 B29, 41-44. (18) Linick, T. W.; Jull, A. J. T.; Toolin, L. J.; Donahue, D. J. Radiocarbon 1986, 28 (2A), 522-533. (19) Stuiver, M. Radiocarbon 1983, 25 (2), 793-795. (20) Klouda, G. A.; Currie, L. A.; Donahue, D. J.; Jull, A. J. T.; Naylor, M. H. Radiocarbon 1986, 28 (2A), 625-633. (21) Levin, I.; Graul, R.; Trivett, N. B. A. Tellus 1995, 47B, 23-34. (22) Dorr, H.; Mu ¨ nnich, K. O. Radiocarbon 1980, 22 (3), 909-918. (23) Levin, I.; Mu ¨ nnich, K. O.; Weiss, W. Radiocarbon 1980, 22 (2), 379-391. (24) MG Industries Specialty Gas Division, personal communication, 1995. (25) Guenther, A.; Hewitt, C. N.; Erickson, D.; Fall, R.; Geron, C.; Graedel, T.; Harley, P.; Klinger, L.; Lerdau, M.; McKay, W. A.; Pierce, T.; Scholes, B.; Steinbrecher, R.; Tallamraju, R.; Taylor, J.; Zimmerman, P. J. Geophys. Res. 1995, 100 (D5), 8873-8892. (26) Cardelino, C.; Chang, W.-L.; Chang, M. E. The use of traffic counters in the estimation of day-specific mobile emissions. In Proceedings of the A&WMA International Conference, Regional Photochemical Measurement and Modeling Studies; Air Waste Management Association: Pittsburgh, 1993; Paper M3-I.8.
Received for review March 23, 1995. Revised manuscript received November 1, 1995. Accepted December 5, 1995.X ES9501981 X
Abstract published in Advance ACS Abstracts, February 15, 1996.
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