Article pubs.acs.org/IECR
Detecting Fugitive Emissions of 1,3-Butadiene and Styrene from a Petrochemical Facility: An Application of a Mobile Laboratory and a Modified Proton Transfer Reaction Mass Spectrometer W. Berk Knighton,*,† Scott C. Herndon,‡ Ezra C. Wood,‡,§ Edward C. Fortner,‡ Timothy B. Onasch,‡ Joda Wormhoudt,‡ Charles E. Kolb,‡ Ben H. Lee,∥ Miguel Zavala,⊥ Luisa Molina,⊥ and Marvin Jones⊗ †
Department of Chemistry and Biochemistry, Montana State University, Bozeman, Montana 59717, United States Aerodyne Research Inc., Billerica, Massachusetts 01821, United States § Department of Public Health, University of Massachusetts, Amherst, Massachusetts 01003, United States ∥ School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, United States ⊥ Molina Center for Energy and the Environment, La Jolla, California 92037, United States ⊗ Texas Commission on Environmental Quality, Austin, Texas 78711, United States ‡
ABSTRACT: The petrochemical industry is a major source of 1,3-butadiene and styrene emissions within the HoustonGalveston area. Both compounds are listed as hazardous air pollutants by the Environmental Protection Agency (EPA), and the Texas Commission on Environmental Quality (TCEQ) lists 1,3-butadiene as a highly reactive volatile organic compound. The Aerodyne Mobile Laboratory (AML) was deployed in 2009 as part of the Study of Houston Atmospheric Radical Precursor (SHARP) project to survey the petrochemical complexes in the Houston ship channel area for air toxics releases. This paper describes how the AML, equipped with a modified proton transfer reaction mass spectrometer configured to operate with NO+ as the reagent ion, was used to characterize and quantify fugitive emissions. On April 26, 2009, the AML surveyed the Goodyear Tire and Rubber and Texas Petrochemical (GY-TPC) complex by circumnavigating the facility on public roads while making continuous measurements. The extensive suite of trace gas instrumentation onboard the AML was used to identify fugitive emissions of 1,3-butadiene and styrene from the industrial complex and to distinguish them from any interfering mobile sources. The mobile lab detected significantly enhanced concentrations of 1,3-butadiene (30 ppbv max) and styrene (15 ppbv max). These results are examined with respect to the prevailing winds and routine ambient air monitoring data from TCEQ’s Milby Park AutoGC, which is located adjacent to the GY-TPC complex. Simple Gaussian point source plume model calculations predict source emission rates that are consistent with reported emission inventories.
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area of concern with respect to 1,3-butadiene.3,4 In 2004, the Texas Commission on Environmental Quality (TCEQ) identified the need to reduce the levels of 1,3-butadiene in the Milby Park Area.4 Milby Park sits across the river to the west of the GY-TPC complex. Milby Park was added to the Air Pollution Watch List (APWL 1207) for 1,3-butadiene, and later styrene was proposed for addition to the AWPL.5 Improvements in the release of 1,3-butadiene from TPC have been made since 2005 through a voluntary emission reduction agreement, and in 2009, the average yearly concentrations of 1,3-butadiene decreased to levels where TCEQ removed it from the APWL.5 Styrene was not added to the APWL as a result of negotiations between GY and TCEQ through a voluntary emission reduction agreement.5 Significant releases of 1,3-butadiene and styrene, both of which are known or suspected carcinogens, still impact the local air quality and represent a health concern for local residents. TCEQ continues
INTRODUCTION Houston and the surrounding ship channel area are home to a major petrochemical processing complex. This extensive petrochemical complex emits significant amounts of hazardous air pollutants into the environment. These emissions affect human health directly through the release of toxic compounds and subsequently through photochemical reactions that produce ozone and formaldehyde. As part of the 2009 Formaldehyde and Olefins from Large Industrial Releases (FLAIR) that ran concurrently with the Houston Atmospheric Radical Precursors (SHARP), the Aerodyne Research, Inc. Mobile Laboratory (AML) was directed to survey areas within the Houston ship channel, Texas City, and Mount Belvieu petrochemical complexes for air toxics. The AML houses a comprehensive suite of sensitive real-time instrumentation for trace gas and particulate measurements.1 This study focuses on two large facilities, the Goodyear Tire and Rubber Company (GY) and Texas Petrochemicals (TPC), which are located adjacent to one another. TPC is a major producer of 1,3-butadiene, which is used by GY along with styrene to produce styrene-butadiene-rubber (SBR).2 As a result of these production facilities, the immediate area surrounding the GY-TPC complex has historically been an © 2012 American Chemical Society
Special Issue: Industrial Flares Received: Revised: Accepted: Published: 12706
November 30, 2011 February 2, 2012 February 3, 2012 February 3, 2012 dx.doi.org/10.1021/ie202794j | Ind. Eng. Chem. Res. 2012, 51, 12706−12711
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Table 1. Description of Terms in Equations 1−4 and Variables Used to Compute Emission Release Rates term
units
C Q u H σy σz
g/m g/s m/s m m m
x
km
3
model variables12
definition down wind concentration mass emission rate wind speed release height horizontal standard deviation vertical standard deviation downwind distance
monoterpenes in the present study to impact the determination of toluene.7 NO+ was produced within the hollow cathode ion source by switching the source gas from water vapor to dry compressed air as described by Knighton et al.7 A small fraction of NO2+ impurity is produced within the hollow cathode, and its intensity depends strongly on the how the ion source was operated. During this study, an ion source extraction voltage of 100 V and a discharge current of 6 mA were used. The NO2+ signal was not measured during mobile operation but was observed to be 1.1% of the NO+ signal during the stationary measurement phases of the campaign. Similar levels are expected to have been present during the periods of mobile operation. The drift tube was held at 500 V, which translates to an electric field to number density ratio (E/N) of 108 Td (1 Td = 10−17 V cm2) at a drift pressure of 2.07 mbar and drift tube temperature of 308 K. The E/N was chosen as a compromise between maintaining maximum sensitivity to benzene while limiting fragmentation of 1,3-butadiene. Additional detail on the influence of E/N on response can be found in Knighton et al.7 Detection sensitivity of NO+ PTR-MS when it is operated at 1 Hz (100 ms integration time per mass) is approximately100 pptv for a single 1 s determination. Emission plumes often exceeded 1 ppbv and were easily distinguished from the ambient background. Improved detection limits, when required, can be achieved through signal averaging. Quantification was accomplished from instrument response factors determined from calibrated gas standards. Calibrations were conducted by dynamically diluting the gas standards in catalytically purified air9 and were performed periodically during the project. The instrument response factors varied by less than 10%, and the uncertainty in the reported concentrations is estimated as ±15%. Other Trace Gas Instrumentation. The combustion tracers, CO and CO2, were measured spectroscopically. CO2 was determined by nondispersive infrared absorbance using a LiCor 6262 instrument. CO was measured by tunable infrared differential absorption spectroscopy (TILDAS) using a pulsed quantum cascade laser system.10 Both instruments were calibrated using certified gas standards. Simple Gaussian Point Source Plume Model. Emission release rates were estimated using a simple Gaussian point source plume model.11 The equations employed are shown below as eqs 1−4, and a description of the parameters is provided in Table 1. Model inputs assume a neutrally stable air mass (Pasquill stability category D) with no crosswind component and a release height of 10 m. Down range distances were determined using Google Earth, and the wind speed measurement was taken from the TCEQ Milby Park monitoring station.
to monitor the area and maintains an air quality monitoring station at Milby Park, which records hourly data for a suite of 47 volatile organic compounds (VOCs) including 1,3-butadiene and styrene.6 On April 26, 2009, the AML was deployed to the GY-TPC complex where it conducted a survey of the emissions being released from the complex by driving downwind and upwind on public roads surrounding the facilities. During this survey the AML measured a large number of gas phase species, including CO2, CO, NOx, formaldehyde, ethene, 1,3-butadiene, styrene, benzene, toluene, C2-benzenes (sum of the xylenes and ethylbenzene), and C3-benzenes. This manuscript describes the results of this survey and focuses on the measurement of 1,3-butadiene and styrene, whose concentrations were observed to be significantly enhanced above the normal ambient background that day. These measurements are compared and discussed with complementary data recorded by the TCEQ Milby Park monitoring station. Emission release rates for 1,3-butadiene and styrene computed using a simple Gaussian point source plume model are compared to reported 2009 emissions inventories.
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for computation of Θ, c = 12.5 and d = 1.0857 for x = 0.3−1.0 km, a = 32.093 and b = 0.810 66 for x = 1.01−3.0 km , a = 32.093 and b = 0.644 03
EXPERIMENTAL DESCRIPTION
Aerodyne Mobile Lab (AML). The AML is a fully functional laboratory containing state of the art trace gas and particulate measurement instrumentation, meteorological instrumentation, GPS, and a digital video recorder for making mobile air quality measurements.1 The AML was deployed to the GY-TPC complex on Sunday, April 26, 2009, where it was driven around the area on publically owned roads. It circumnavigated the complex making measurements on both the upwind and downwind sectors. The GPS continuously recorded the location of the AML, while an on-board anemometer recorded the wind direction and speed. AML staff utilized the real-time instrument readouts to detect and locate emission plumes emanating from the complex. Instrumentation. NO+ PTR-MS. A modified proton transfer reaction mass spectrometer operated using NO+ as the chemical ionization reagent ion7 was used to measure 1,3butadiene, styrene, benzene, toluene, C2-benzenes (sum of C8H10 isomers), and C3-benzenes (C9H12 isomers). NO+ is a selective reagent ion that reacts via charge transfer reaction with compounds having an ionization energy lower than that of NO, 9.26 eV. Previous study with this instrument suggested that there are no known or anticipated chemical interferences to the detection and quantification of the monitored compounds.7 The other isomeric C4H6 compounds except 1,2-butadiene all have ionization energies above that of NO8 and will not interfere with the quantification of 1,3-butadiene. Monoterpenes fragment to produce an ion at m/z 92, which is the mass monitored for toluene, but there are insufficient levels of 12707
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2 2 2 2 Q (e−(z − H ) /2σz + e−(z + H ) /2σz ) u2πσyσz
Article
(1)
σy = 465.116 28x tan Θ
(2)
Θ = 0.174 532 93(c − d ln x)
(3)
(σz = ax b)
(4)
RESULTS Figure 1 shows the time series plot of the measurements made while the AML was downwind of the GY-TPC complex. The
Figure 2. 1,3-Butadiene concentration data overlaid onto the GPS track of the AML. Marker size varies with concentration, and the marker color corresponds to the trace color code shown in Figure 1.
Figure 1. Concentration time series profiles measured during the downwind survey of the GY-TPC petrochemical complex. The trace colors for 1,3-butadiene and styrene are color coded as a function of time. Shaded areas indicate periods when the AML sampled air heavily influenced by vehicle exhaust emissions.
Figure 3. Styrene concentration data overlaid onto the GPS track of the AML. Marker size varies with concentration, and the marker color corresponds to the trace color shown in Figure 1.
upwind data set is not included since it showed no evidence of industrial emissions and was more heavily influenced by vehicle exhaust emissions. Vehicle exhaust emissions are easily distinguished from industrial sources since vehicle exhaust shows elevated levels of all of the measured species: hydrocarbons, CO, CO2. This is illustrated in Figure 1 where the gray areas on the time series trace indicate when the AML was being influenced by vehicle emissions. Inspection of this figure shows that the AML intercepted high concentration (>10 ppbv) plumes of 1,3-butadiene and styrene on numerous occasions, and these events are clearly recognizable from the intervening vehicle exhaust events. The trace colors of 1,3butadiene and styrene vary with time to aid in relating the concentration time series data to the position of the AML in the subsequent figures. By overlaying the concentration time series data onto the GPS track of the AML, Figures 2 and 3 show that these emissions were highly localized. In these figures, marker size reflects the magnitude of the concentration, while marker color
reflects time. For comparison, the 1,3-butadiene and styrene time series concentration traces in Figure 1 are similarly colored. The spatial resolution of the plumes indicates that the 1,3-butadiene and styrene sources are in close proximity to the AML and have different locations. The known wind direction, which was from the southeast during this time period, indicates that these emissions originate from within the GY-TPC complex. Conversely, there appears to be no significant emissions of benzene, toluene, C2-benzenes (not shown), C3-benzenes (not shown), or CO being released by GY or TPC. This conclusion is based on the observation that there was no concomitant increase in any of these compounds during the time periods when elevated levels of 1,3-butadiene and styrene were observed. We assume that when the levels of 1,3-butadiene and/or styrene were elevated, the AML was monitoring air that had been swept over the GY-TPC complex. Excepting the vehicle exhaust events and a single toluene plume (at 12:29 pm), none of the aforementioned compounds shows any 12708
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Figure 4. Concentration and wind direction data from the Milby Park monitoring station are shown in the left panel. The right panel shows polar plots of the concentrations as function of wind direction.
exceptional increase above background during the measurement period. The 1,3-butadiene plumes appear to be fugitive emissions, as opposed to gas flare, since there is no evidence of elevated concentrations of the major combustion related products, CO2 or CO, associated with these events. This observation is consistent with the reported emission inventory. For 1,3butadiene, the 2009 emission inventory reports that 86% (18.3 tons/year) of TPC’s and 58% (4.3 tons/year) of GY’s emissions are fugitive with the remaining being stack releases.13 Characterization of the styrene emissions is less definitive. The 2009 emission inventory for GY reports that 62% (55.6 tons/year) of the styrene is from stack emissions,13 which we presume means that the release is from process flares. There is some evidence that the styrene plumes observed in Figure 1 may represent process flare emissions as there does appear to be an associated elevated CO2 signature. There is, however, no detectable corresponding increase in the measured CO. The lack of any appreciable CO emission suggests that if the styrene and CO2 are from a process flare, that this flare is highly efficient. On the basis of the data in Figure 1, we can estimate the combustion efficiency (CE) of the flare using eq 5. The concentration terms in eq 5 represent the increases in the concentrations above their ambient background. The styrene concentration is multiplied by 8 to express its concentration on a per carbon basis.
CE =
Δ[CO2 ] Δ[CO2 ] + Δ[CO] + 8Δ[styrene]
(5)
Using the plume data from 12:05−12:10, the plume concentrations of CO2 and styrene are estimated at 40 ppmv and 10 ppbv, respectively, and leads to a CE of 0.998. Addition of a CO contribution of 200 ppbv (a change well above the detection threshold of the CO instrument) would only decrease the CE to 0.993. The absence of any visible flare that day combined with what appears to be an unusually high CE is taken as evidence that these are not flare emissions. This simple analysis suggests that the styrene emissions are most likely fugitive and are collocated with a separate source of CO2. TCEQ maintains a monitoring station in Milby Park which records 1 h average concentrations of 47 different volatile organic compounds, including 1,3-butadiene and styrene.6 Direct comparison of these measurements with those made with the AML is not useful because the monitoring station was located outside of the plumes for most of the time period. It is useful, however, to examine the Milby Park data over a longer time period to get a sense for whether the April 26th represented a typical day or a day when there was a reportable release. According to TCEQ’s air emission event reporting database, there were no reportable events on this day or at any time during the months of March−May.14 Figure 4 shows plots of the 1 h averaged 1,3-butadiene and styrene concentrations reported at Milby Park for the period from March 1−June 1, 12709
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2009 along with the resultant wind direction. As expected the concentrations of 1,3-butadiene and styrene are strongly dependent on wind direction, and this is demonstrated in the accompanying polar plots where the concentration data is plotted as a function of wind direction. The polar plots include data for the entire year (2009). Overall the magnitude of the 1,3-butadiene and styrene concentrations observed with AML (Figure 1) are similar to those reported by the Milby Park AutoGC (Figure 4) for the last week of April. This is encouraging but not definitive. Comparing instantaneous (1 s) measurements with longer-term averages is not straightforward, and the comparison is challenged by the strong temporal and spatial variation of the 1,3-butadiene and styrene emissions. Short of collocating the AML adjacent to the AutoGC is not possible to make any direct comparison between the respective results. Notwithstanding, we consider the agreement between AML and AutoGC results to be reasonable. Quantification of emission rates from fugitive sources using measured concentrations alone is not possible. There is no method, like that for combustion emissions, which can be scaled via CO2 or CO to obtain emission ratios that then can be related to total emissions through fuel usage. For fugitive emissions, the observed concentrations depend on the source strength, its displacement distance vertically and horizontally, and the local meteorological conditions. Sophisticated models are capable of computing fugitive emission rates, but these require good spatial and temporal information on both the pollutant concentrations and meteorological measurements. Data acquisition of sufficient quality for this type of analysis requires coordinating and funding a dedicated study. Survey studies such as this one are generally not amenable to detailed modeling efforts. It is still of interest and valuable to use simple models to gain insight between the measured concentrations and the computed emission release rates. The modeling effort employed here is very basic. It computes downwind concentrations as a function of emission release rate assuming that the wind direction is aligned directly with the emission source and requires that we know the location of the emission source. Detailed information on the location of the emission point sources is available2 and shows that the 1,3, butadiene sources are widely distributed throughout the GYTPC complex. The styrene sources are less distributed and are concentrated within a small section of the GY facility. In the present study, we use this detailed emission source information2 in conjunction with the concentration versus wind plots in Figure 4 to deduce the locations of the most persistent emission sources. This approach assumes that the most persistent sources will also be most likely sources to be active during our study. It also assumes that April 26th was a typical day. On the basis of what has been reported publicly,14 it is concluded that April 26th appears to be a typical day with respect to emission releases of 1,3-butadiene and styrene. The polar plots indicate that the majority of 1,3-butadiene emissions originate from a source located 135° from the Milby Park monitoring station, while the styrene source lies along a 113° vector. An aerial view showing the location of the AutoGC along with these emission vectors is illustrated in Figure 5. The circles drawn fall along these vectors and correspond to the known locations of large emission point sources. Simple models provide at best coarse estimates and are applied here to only to provide some sense as to whether the observed concentrations can reasonably be explained by the
Figure 5. Aerial view showing the location and the Milby Park monitoring station and the location of the major 1,3-butadiene and styrene sources.
reported emissions inventory. Using a simple Gaussian point source model emission rates are computed to match the observed concentrations reported by AML. The source locations for 1,3-butadiene and styrene are those indicated in Figure 5. The wind speed is taken as 7.2 m/s, which was the average wind speed reported6 from the Milby Park monitoring site for this time period. The release height is assumed to be 10 m. Distances are computed using Google Earth. The parameters dependent on downwind distance were computed using eqs 2−4 and the information contained within Table 1. The wind direction was always assumed to pass directly over the source, and the computed concentration was taken on the plume centerline. For 1,3-butadiene, the distance between the emission source (Figure 5) and the closest road on which the AML made the east−west transect is approximately 1 km. This is also the approximate distance when the AML was in Milby Park. At its northernmost point, the AML was about 1.6 km from the source. The concentrations of 1,3-butadiene recorded by the AML at 1 and 1.6 km were on the order of 20 and 8 ppbv, respectively. These concentrations can be replicated using the model and a 2.3 g/s emission release rate, which predicts concentrations of 20 and 10 ppbv for the 1 and 1.6 km distances. An emission rate of 2.3 g/s translates to release rate of 18.2 lbs/h or 80 tons/year. The predicted emission rate is approximately 3 times greater than the total emissions of 1,3butadiene for TPC (21.4 tons/year) and GY (7.4 tons/year) in the 2009 toxic release inventory.13 A comparable analysis of the styrene data yields a predicted emission release rate of 1 g/s. This calculation uses AML concentrations of styrene of 12 and 4 ppbv at distances of 0.55 and 1.1 km. At an emission rate of 1 g/s, the predicted downwind concentrations are 12 and 4 ppbv, respectively. An emission rate of 1 g/s translates to a release rate of 7.9 lbs/h or 35 tons/year. For comparison, the 2009 emission inventory reported by GY for styrene was 33 tons/year fugitive and 56 tons/year as stack emissions.13 In summary, all of the information presented here appears to be self-consistent. The concentration measurements of 1,3butadiene and styrene made using the NO+ PTR-MS are in reasonable agreement with those reported by traditional GC 12710
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(6) TCEQ. AutoGC Data by Month-Houston Milby Park A169 [K]. http://www.tceq.state.tx.us/cgi-bin/compliance/monops/agc_ monthly_summary.pl (accessed October 20, 2011). (7) Knighton, W. B.; Fortner, E. C.; Herndon, S. C.; Wood, E. C.; Miake-Lye, R. C. Adaptation of a proton transfer reaction mass spectrometer instrument to employ NO+ as reagent ion for the detection of 1,3-butadiene in the ambient atmosphere. Rapid Commun. Mass Spectrom. 2009, 23 (20), 3301−3308. (8) Lias, S. G.; Bartmess, J. E.; Liebmann, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. Ion Energetics Data; National Institute of Standards and Technology: Gaithersburg, MD, 2009. (9) Rogers, T. M.; Grimsrud, E. R.; Herndon, S. C.; Jayne, J. T.; Kolb, C. E.; Allwine, E.; Westberg, H.; Lamb, B. K.; Zavala, M.; Molina, L. T.; Molina, M. J.; Knighton, W. B. On-road measurements of volatile organic compounds in the Mexico City metropolitan area using proton transfer reaction mass spectrometry. Int. J. Mass Spectrom. 2006, 252 (1), 26−37. (10) McManus, J. B.; Zahniser, M. S.; Nelson, D. D.; Shorter, J. H.; Herndon, S.; Wood, E.; Wehr, R. Application of quantum cascade lasers to high-precision atmospheric trace gas measurements. Opt. Eng. 2010, 49, 111124. (11) Turner, D. B. Workbook of Atmospheric Dispersion Estimates: An Introduction to Dispersion Modeling, 2nd ed.; CRC Press: Boca Raton, FL, 1994. (12) U.S.E.P.A. User’s Guide For The Industrial Source Complex (ISC3) Dispersion Modules Volume II - Description of Model Algorithms. http://www.epa.gov/scram001/userg/regmod/isc3v2.pdf (accessed January 30, 2012). (13) U.S.E.P.A. 2009 Toxics Release Inventory. http://www.epa.gov/ TRI/index.htm (accessed October 20, 2011). (14) TCEQ. Air Emission Event Reporting Database. http://www11. tceq.state.tx.us/oce/eer/index.cfm?fuseaction=main.searchForm (accessed October 20, 2011).
methods. The emission rates computed from the measured concentrations compare favorably to the reported emission inventories. Agreement within a factor of 3 for the modeled emissions and the emissions inventory is considered favorable given the variable nature of industrial emissions and that we are comparing a single 1 h determination to a yearly average.
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SUMMARY This manuscript demonstrates the utility of a mobile laboratory equipped with sensitive real-time instruments for characterizing and quantifying industrial emissions. Using an extensive suite of trace gas instrumentation allows one to easily distinguish industrial emissions from any interfering vehicle exhaust emissions. Furthermore, fugitive emissions can be distinguished from flare emissions. Simple atmospheric dispersion models can be employed to provide emission release estimates. This paper shows that a mobile laboratory equipped with sensitive realtime instruments can be a valuable tool for characterizing emissions in challenging environments. Regulatory agencies could benefit from such a laboratory to augment existing stationary monitoring efforts for surveillance and enforcement.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The 2009 field work in Houston was funded by the Air Quality Research Program (AQRP) of the Texas Commission on Environmental Quality (TCEQ) as administered by the Houston Advanced Research Center (HARC). The analysis was funded by AQRP/TCEQ via the University of Texas at Austin. B.H.L. was supported by the National Science Foundation under Awards AGS-0813617 and AGS-0814202. M.Z. was supported by the Molina Center. We are extremely grateful for contributions from the following colleagues: Alex Cuclis and Eduardo Olaguer (Houston Advanced Research Center), David Allen (U. of Texas at Austin), Barry Lefer and James Flynn (U. of Houston), and John Jolly (TCEQ).
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NOTE ADDED AFTER ASAP PUBLICATION This paper was published on the Web on February 17, 2012, with minor errors in equation 1. The corrected version was reposted on April 18, 2012.
REFERENCES
(1) Herndon, S. C.; Jayne, J. T.; Zahniser, M. S.; Worsnop, D. R.; Knighton, B.; Alwine, E.; Lamb, B. K.; Zavala, M.; Nelson, D. D.; McManus, J. B.; Shorter, J. H.; Canagaratna, M. R.; Onasch, T. B.; Kolb, C. E. Characterization of urban pollutant emission fluxes and ambient concentration distributions using a mobile laboratory with rapid response instrumentation. Faraday Discuss. 2005, 130, 327−339. (2) Wang, W. Y. Mobile Laboratory to Measure Air Toxics in the Houston Ship Channel Area. http://epa.gov/ttnamti1/files/ 20052006csatam/COHMobileLabFinalReport.pdf (accessed October 20, 2011). (3) Hendler, A. H.; Bunch, A. T. G. Long-Term Trends in Ambient Air 1,3-Butadiene Levels in Houston, Texas. Environ. Sci. Technol. 2010, 44 (19), 7383−7390. (4) TCEQ. Focus on Air Quality in Houston. http://www.tceq.texas. gov/publications/pd/020/10-02/focus-on-houston (accessed October 20, 2011). (5) TCEQ. 2009 Annual Report on the Air Pollutant Watch List Areas in Texas. http://tceq.com/assets/public/implementation/tox/ apwl/annual_report/2009.pdf (accessed October 20, 2011). 12711
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