Znd. Eng. Chem. Res. 1995,34, 619-625
619
GENERAL RESEARCH
Relationship between the Composition of Engine Particulate Emissions and Emission Control System Performance Mikio Zinbo,* Thomas J. Korniski, and James E. Weir Research Laboratory-MD 3061, Ford Motor Company, P.O. Box 2053, Dearborn, Michigan 48121-2053
This study was carried out to determine the effect of simulated engine and catalyst malfunction conditions on the mass and composition of tailpipe particulate emissions from a 1990 Ford Taurus, running on a 3.2% oxygenated gasoline blend. Particulate emissions ranged from 2.0 to 4.4 mg/mi under all conditions studied. These emission levels are far below the current U.S. EPA standard of 80 mg/mi. Soot was a major component of postcatalyst particulate emissions (6570%). Thermogravimetry demonstrated that only the low-to-medium boiling fraction of the engine oil was emitted from the cylinder walls and crevices. The catalyst aftertreatment significantly reduced the emissions of the low-to-medium boiling components, i.e., from 1.9 to 0.1 mg/mi. The formation of major organic compounds found in the postcatalyst particulate emissions has been explained by the thermal oxidation of aliphatic base-oil hydrocarbons. The level of ash emissions was relatively constant under all conditions (0.4-0.5 mg/mi>. The particulate composition data can be useful in helping to diagnose emission control system performance.
Introduction The applicable US. EPA standard for in-use particulate exhaust emissions from gasoline light-duty vehicles is 80 mg/mi for the intermediate useful life or 100 mg/ mi for the full useful life (U.S. EPA, 1993). It has been phased in beginning with the vehicles produced in model year 1994 and could be effective to the model year 2004. However, the average particle emission rate used in recent EPA calculations of the cancer-risk assessment from vehicle exhaust was 30 mglmi, determined for early 1980s vehicles running on unleaded gasoline (Carey, 1987). The average rate of particulate emissions for a 1989 Ford Taurus with an industry-average fuel (Siegl et al., 1994) has been reduced by a factor of 3 compared to those of particulate emissions for 14 1980-1983 catalystequipped gasoline vehicles, i.e., 5.8 from 17.6 mg/mi (Schuetzle, 1983; Hildemann et al., 1991). At the same time, the dichloromethane (DCM) extractable fraction of the particulate emissions collected from the 1989 vehicle is lowered by a factor of 5 compared to those of the particulate emissions collected from eight 19801981 vehicles, i.e., from 55% to 11%(Schuetzle, 1983). In fact, the particulate emission data clearly indicate not only an improved fuel combustion technology, but also improved current production catalyst systems, considering that a major source of soot is unburnt fuel and that of DCM extractables is base-oil hydrocarbons (BOHs) (Zinbo et al., 1993a). For the current model vehicles, the amount of unburnt-fuel hydrocarbons in the gasoline particulate emissions has been reduced significantly compared to that of soot, especially with recent improvements in engine designs and vehicle emission control systems. As the extent of postcatalyst particulate emissions is lowered in the range of 4-6 mg/mi (Salmeen et al., 1990; Zinbo et al., 1993b; Siegl et al., 19941, the malfunctioning effect of various vehicle emission control systems,
e.g., heated exhaust-gas oxygen (HEGO) sensor and platinudrhodium (Pt/Rh) three-way catalyst (TWC), would be more significant on the hydrocarbon particulate emissions, derived mainly from BOHs. Polytetrafluoroethylene (PTFE) bonded glass-fiber filters (Pallflex TX40 and T60.420) have been the filter media of choice for the collection of both diesel and gasoline engine particulate emissions (e.g., Alsberg et al., 1985; Salmeen et al., 1984; Zinbo et al., 1990;Watts et al., 1992). Recently, Hildeman et al. (1991) have reported the use of ultrathin PTFE-membrane filters (Gelman Teflo, 2-pm pore size) for the collection of various urban particulate matter, including particle emissions from automobiles. Also, the use of ultrathin PTFE-membrane filters has been recommended for the collection of low particulate emissions (53mdfilter) and the analysis of polycyclic aromatic hydrocarbons (PAHs) in the particulate samples (Zinbo et al., 1994). This paper presents experimental results of studies investigating the malfunctional effect of engine and emission-control systems on the emission rates of various particulate components (e.g., BOHs, soot, ash, PAHs, etc.), emitted from an oxygenated-gasoline fueled vehicle and determined by the recently developed compositional thermogravimetric analysis (TGA) and solvent extractiodgas chromatographic procedures (Zinbo et al., 1993a,b).
Materials and Methods Test Vehicle. The test vehicle was a 1990 Ford Taurus with a 3.0-L engine, 0.4 g/mi NO, California calibration, and a production TWC. The odometer reading was 6273 mi a t the start of the test. A 3.2% oxygenated gasoline blend was used. The fuel meets the minimum oxygen content (3.1%) required by the federal oxygenated fuel programs (US. Congress, 1990). Some of the analysis data are: C = 83.1%, H = 13.7%, 0 = 3.2%, and S = 0.0029% in wlw; aromatics = 22.4% in vlv; and d20 = 0.7324.
0888-588519512634-0619$09.00/0 0 1995 American Chemical Society
620 Ind. Eng. Chem. Res., Vol. 34,No. 2, 1995
Particulate Collection. The particulate samples were collected from the test vehicle operated under four different conditions on a Ford Research dilution tube (d = 30.5 cm; 1 = 1005.8 cm); (a) normal mode of the system operation with a production TWC (to be called “normal mode”),(b)normal mode of the engine operation with a noncoated catalyst brick for the production TWC (to be called “engine-out”),(c) disabled heated exhaustgas oxygen sensor (HEGO) with a production TWC (to be called “no HEGO”), and (d) programmed ignition misfires with a production TWC (to be called “misfire”). For the collection of diluted particulate emissions (McKee, 19771, three equivalent sampling sites were utilized. At site 1, ultrathin PTFE-membrane filters (Gelman 2-pm Teflo: thickness, 25 pm; pore size, 2 pm) were used for compositional TGA. At sites 2 and 3,27% PTFE-bonded glass-fiber filters (Pallflex T60A20) were used for solvent extractiodchromatographic analysis of selected PAHs. The filters were mounted in Swagelok modified 47-mm in-line stainless steel holders (Gelman Sciences No. 2220). Two holders were connected in series for the primary and secondary particulate collection from diluted vehicle exhaust, while a single holder was used to collect the background particulates from the dilution tube after a particulate sample collection on the same day. The inlet diameter of the Teflon sampling probe tip was 19 mm. The pumping speed of diluted-exhaust emissions through the filters and the dilution tube flow rate were adjusted to provide an isokinetic sampling condition. The average flow rate through the sampling filters was 0.0461 m3/min, standardized a t 20 “C and 101.3 kPa. The duration of the sample collection was four consecutive US. EPA UDDS test procedures (average test miles driven 44.31 miles) (U.S. EPA, 1993),accumulated over 4 days. The aidexhaust dilution ratio was approximately 20:l during sample collection. The masses of the before- and after-sampling filters were weighed aRer equilibrating under controlled temperature and humidity by a Cahn C-33 microbalance (precision f 2 pg). SpectroscopicMeasurements of Gaseous Emissions. The regulated (HC, CO, and NO,) and some of the unregulated (e.g., COZ,SOz, NH3, and HCN) gaseous emissions from the vehicle were measured by Fourier transform infrared (FTIR) spectroscopy. A Mattson Instruments Nova-Cygni 120 equipped with a water cooled, glow bar source and a liquid nitrogen cooled, narrow band mercury/cadmium/telluride(MCT)detector was utilized. The spectrometer was operated at a resolution of 0.25 cm-l, and the interferograms were zero filled to an effective resolution of 0.125 cm-’. A variable path length, multipass gas cell with KBr windows was used in the 14th order, resulting in an effective path length of 21.75 m. The diluted exhaust sample was filtered through a 142-mm quartz fiber filter (Pallflex TISSUQUARTZ 2500QAO) at room temperature, and the filtered gaseous sample was pulled through the FTIR cell at a flow rate of 30 Umin under a constant pressure of 93.3 kPa. Each sample component concentration for 20 selected compounds was determined from a linear relationship between the concentration and spectral-line strength of standard reference spectra. The detailed fundamentals and applications in the analysis of diluted vehicle exhausts have been reported elsewhere (Gierczak et al., 1991). Thermogravimetric Analysis (TGA). A set of the blank, background, primary, and secondary ultrathin
PTFE-membrane filters with a diameter of 47 mm, including a 3-mm-widepolymethylpentene support ring, was analyzed by a Du Pont 951 thermogravimetric analyzer. The support rings were carefully cut out with a sharp blade and the rolled PTFE-membrane portion of each filter was weighed by a Mettler M3 microbalance (readability 1pg) prior to TGA. The blank, background, or sample PTFE-membrane filter without the support ring was heated from 25 to 400 “C in argon at a rate of 20 “C/min, and then the purge gas was switched to air while heating to 650 “C at the same heating rate with a flow rate of 50 mumin. Also, a fresh SAE 5W-30 engine oil used was analyzed under the same TGA conditions. The percent weight of the inorganic residues (ashes) in the background and primary/secondary PTFEmembrane samples were obtained by weighing the platinum sample pan before and after TGA using the Mettler microbalance since the PTFE membranes do not yield any ash under the analysis conditions. The TGA data reduction procedure for the compositional analysis of particulate evaporatives (i.e., particulate mass minus ash) has been reported elsewhere (Zinbo et al., 1993a, 1994). Solvent and Solid Phase Extractions. A set of three pairs of matrix-spiked PTFE-bonded glass-fiber filters (i.e., the primary and secondary filters for blank, background, and sample collected at site 2) and one pair of nonspiked PTFE-bonded glass-fiber filters (i.e., the primary and secondary filters for sample collected at site 3) for each particulate sample collection under the four conditions was extracted ultrasonically with dichloromethane (DCM; 4 x 2 mL) in four separate plain-top test tubes. To determine the recovery of selected particulatephase polycyclic aromatic hydrocarbons (PAHs)from the solvent-extraction procedure, 5 or 10 pL of an external standard solution containing pyrene (108.8 ng/pL), chrysene (113.2 ng(pL), benzo[alpyrene (BaP; 83.6 ng/ pL), and perylene (128.6 ng/pL) in a 9:l high-purity hexane/DCM mixture (Burdick & Jackson Laboratories, Inc., Muskegon, MI) was spiked on a pair of the blank filters placed in a plain-top test tube prior to the solvent extraction. For the relative recovery determination and gas-chromatographic retention-time marker, 5 or 10 pL of an internal standard solution containing perylene (89.9 ng/,pL)in a 9:l hexane/DCM mixture was spiked again on both background and site-2 sample (primary and secondary) filters. A 5-pL aliquot of the above standard solutions was used for “normal mode” and “no HEGO” samples, and a 10-pL aliquot of those was used for “engine-out” and “misfire” samples. The dried DCM extracts of each filter set were separated into hexane-solubleand -insoluble compounds by a series of hexane-dissolution steps (4 x 2 mL). The DCM-extractable/hexane-soluble compounds dissolved in 0.5-mL hexane were further fractionated into fraction 1 (3 x 2-mL hexane), fraction 2 (3 x 2-mL DCM), and fraction 3 (0.1 mL of methanol plus 3 x 2 mL of DCM) by nonbonded silica solid phase extraction (SPE). All of the dried fractions from SPE were redissolved into 250 pL each of hexane for the chemical identification and quantitative analysis of four selected particulatephase PAHs, i.e., pyrene (Mr = 202.31, chrysene ( M , = 228.31, benzo[elpyrene (BeP; M , = 252.31, and BaP (M, = 252.3). The detailed solvent and solid phase extraction procedures have been described elsewhere (Zinbo et al., 1992).
Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995 621 Table 1. Particulates Collected on Ultrathin “-Membrane Filters at Site 1 and -E-Bonded Glass-FiberFilters at Sites 2 and 3 over Four Consecutive U.S. EPA UDDS Test Procedures
sample ID “normal mode”” “engine-out”b “no HEGOnC “misfire”d
site
PMDP
1 2 3
263 250 256 261 242 249 265 257 257 257 239 251
1 2 3 1 2 3 1 2 3
lx)
particulates collected, mg prim sec cor filter filter totalf 0.400 0.430 0.413 0.805 0.924 0.857 0.523 0.524 0.509 0.683 0.700 0.634
-0.006 0.035 0.045 -0.003 0.087 0.069 -0.002 0.036 0.056 0.016 0.048 0.046
0.336 0.416 0.412 0.743 0.956 0.879 0.463 0.504 0.513 0.640 0.675 0.617
Under a fully functional, postcatalyst condition with a production TWC. Under an engine-out, precatalyst condition with a noncoated catalyst brick. Under a disabled HEGO sensor condition with a production TWC. Under a programmed ignitionmisfiring condition with a production TWC.e Particulate mass dilution factors (PMDF) with an average test distance of 44.3 mi. f Background corrected.
Analysis of Selected Particulate-Phase PAHs. Prior t o the quantitative gas-chromatographic (GC) analysis of the selected PAHs, each of the three SPE fractions were subjected to capillary GC analysis using a mass selective detector (MSD) for the chemical identification. The SPE fractions redissolved in 250 p L of hexane were subjected to quantitative GC analysis using a flame-ionization detector (FID) for determination of the four key PAHs. Both the chemical identification and quantitation procedures by capillary GC/MSD and GC/FID are described in detail elsewhere (Siegl et al., 1994; Zinbo et al., 1993b, 1994). Results and Discussion Particulate Sampling. The exhaust particulate emissions from a 1990 Ford Taurus (3.0 L), running on a 3.2% oxygenated gasoline blend, were collected on twostage ultrathin PTFE-membrane (Gelman 2-pm Teflo) and PTFE-bonded glass-fiber (Pallflex T60A.20) filters under four different vehicle operating conditions, i.e., (a) “normal mode”, (b) “engine-out”,(c) “no HEGO”, and (d) “misfire”. The “normal mode” was a fully functional, postcatalyst-emission condition. The “engine-out” was a precatalyst-emission condition with a noncoated brick, approximating the normal exhaust flow conditions of a production TWC. The pt/Rh three-way catalyst (TWC) operates most efficiently at a stoichiometric aidfuel ratio, which is controlled by a heated exhaust-gas oxygen (HEGO)sensor feedback. Under the “no HEGO”, the engine would operate in a slightly fuel-rich condition from the stoichiometric point because of the programmed default signals. The “misfire”was generated by programming a misfire after 13 normal ignitions. A summary of the particulate samplings is listed in Table 1. Each of the four diluted particulate emissions was collected at the three equivalent dilution sampling sites (Gelman 2-pm Teflo at site 1and Pallflex T60A20 at sites 2 and 3). The sample collection was made over the duration of four consecutive U S . EPA UDDS test procedures (US.EPA, 1993) to accumulate a sufficient amount of each diluted engine-exhaust particulate mass
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TEMPERATURE/OC Figure 1. Residue subtracted, membrane-mass adjusted, and scale expanded TGA curves (-1 of particulate samples collected under (a)“normal mode”, (b) “engine-out”, ( c ) “no HEGO”, and (d) “misfire” together with each scale expanded TGA curve (- - -1 of the blank PTFE-membrane.
in a weight range of 0.5-1.0 mg. Each primary PTFEmembrane filter usually collects more than 97% of total diluted particulate mass, compared to ca. 91% of total diluted particulate mass collected on each primary PTFE-bonded glass-fiber filter as reported previously (Zinbo et al., 1993a). The particulate-emission rates obtained under the conditions (a)-(d) were 2.2, 4.8, 2.9, and 3.7 mg/mi, respectively. It is noteworthy that the average particulate-emission rate of this 1990 Ford Taurus with the oxygenated gasoline at 6273 odometer miles is lowered by a factor of ca. 2.5 compared to that of the same vehicle (5.4 mg/mi) with unleaded gasoline at 765 odometer miles (Salmeen et al., 19901, both tested under a normal, fully functional mode of operation, i.e., “normal mode”. Particulate Composition. A compositional TGA procedure has been developed for the characterization of gasoline-fueled, low-particulate emissions collected on ultrathin PTFE-membrane filters (Zinbo et al., 1993a). The residue subtracted and scale expanded TGA curves of the particulate samples collected under conditions (a)-(d) are shown in Figure 1together with each TGA curve of the blank PTFE membranes for the analysis of the particulate evaporative components, i.e., particulate mass minus ash. As reported previously, the blank PTFE membrane decomposes cleanly over a temperature range of 520-620 “C under an air atmosphere (Zinbo et al., 1993a). The results of the compositional analysis together with those of the inorganic residues by microgravimetry are summarized in Table 2. As the extent of gasoline engine particulate emissions is reduced to the current-model vehicles of 2-3 mg/mi with a production TWC and oxygenated-fuel blend, the soot particles have become a major component of particulate emissions, ranging from 65% to about 70%
622 Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995 Table 2. Compositional Thermogravimetric Analysis of Various Gasoline Engine Particulate Emissions Collected on Ultrathin PTFE-MembraneFilters at Site 1
sample IDa “normal mode” “engine-out” “no HEGO” “misfire”
ashb 20.1 9.4 17.2 12.6
composition, wt 9i evaporative components LTEc BOHd HTEe sootplud 4.0 5.3 1.2 69.4 2.2 42.3 5.1 41.0 3.0 12.5 2.1 65.2 3.4 16.0 2.6 65.4
See Table 1 for the explanation of sampling conditions. The percentage of each ash content was determined by microgravimetry. Low-temperature evaporatives (LTE) in the range of 40140 “C. d Base-oil hydrocarbons (BOH) in the range of 140-300 “C. e High-temperature evaporatives (HTE) in the range of 300440 “C. fNot likely, but it could contain other than soot, Le., oxidizable andor thermally degradable salts, etc. Q
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TEMPE RAT URE / O C Figure 2. TGA curves of a fresh factory-fill SAE 5W-30 oil (-1 and normalized 40-400 “C portion of engine-out, precatalyst particulates collected on an ultrathin membrane filter under “engine-out” (- - -) with their corresponding differential curves.
(w/w). The soot particles emitted were basically formed due to pyrolysis of the unburnt fuel at the oxygen-poor locations of combustion chambers. The TGA curves of a fresh factory-fill SAE 5W-30 oil and normalized portion (40-400 “C) of the “engine-out” particulates collected on a primary ultrathin PTFEmembrane filter (cf. Figure l b j are shown in Figure 2. The SAE 5W-30 oil was in use when the vehicle particulate emissions were collected. A comparison of the weight-loss profiles between the fresh oil (Cla-Cd and probable engine BOH components in the “engineout” particulates indicates that only a low-to-medium boiling fraction of the BOHs is emitted from the cylinder walls and crevices to the diluted exhaust particulate collection system. Since the unburnt-fuel hydrocarbons are a very minor constituent of the particulate emissions, the weight losses occurring in the 140-300 “C temperature range under an argon atmosphere have been attributed to the volatilization of largely unoxidized low-to-medium boiling BOHs as discussed in the following section. For the PAH analysis of the particulate emissions collected on dual PTFE-bonded glass-fiber filters at sites 2 and 3, an extractiodseparation procedure reported previously was used to determine the concentrations of pyrene, chrysene, BeP, and BaP (Zinbo et al., 1993b, 1994). The recovery analysis of the solvent/SPE extraction process for selected PAHs was performed by the
Table 3. Capillary GC Analysis of Selected PAHs in the Particulate Emissions Collected on PTFE-Bonded Glass-Fiber Filters (Pallflex W A ! 2 0 )at Sites 2 and 3 concentration: &sample collection sampleIDa site pyrene chrysene BeP BaP “normal mode” 2 57 123 (22) (19) 3 57 108 (20) (18) 123 113 “engine-out” 2 168 308 136 127 3 170 293 44 41 “no H E G O 2 74 185 60 198 41 38 3 “misfire” 2 52 190 (28) (26) 3 38 175 (28) (26) See Table 1for the explanation of sampling conditions. The estimated values in parentheses would probably contain larger errors than others.
external and internal standard matrix-spiking experiments. The identification and distribution of the selected PAHs in the SPE fractions (fractions 1-3) were made by a GC/MSD technique prior to the quantitative GCFID analysis. These analyses indicated that pyrene was eluted in both fractions 1 (hexane) and 2 (DCM), and chrysene, BaP, and perylene were eluted only in fraction 2. None of the spiked PAHs were detected in fraction 3 (MeOWDCM). The average-percent recoveries of each PAH in fraction 2 for the particulate emissions collected on PTFE-bonded glass-fiber filters were 54,86,75, and 78% for pyrene, chrysene, BaP, and perylene, respectively,in the quadruplicate experiments with relative standard deviations of ca. 10%. The results of the quantitative GC analysis for the selected particulate-phase PAHs are summarized in Table 3, including some of the borderline concentrations estimated for BeP and BaP under the “normal mode” and “misfire” conditions. Thermooxidative Reaction of Base-Oil Hydrocarbons (BOHs). Studies of the liquid-phase autoxidation of hexadecane carried out at 100-110 kPa of oxygen and 120-180 “C have led to the identification of a,y and a,d intramolecular hydrogen abstraction reactions of alkylperoxy radicals (R02’) (Jensen et al., 1979) and the cleavage reaction of a,y-hydroperoxysubstituted ketones (Jensen et al., 1981). In the early stages of hydrocarbon autoxidation with pure oxygen at 120-180 “C, 50-60% of the total alkanoic acids (RCOOH) would be formed from the thermal molecular decomposition of difunctional a,y-hydroperoxy-substituted ketones (a,y-HOOR=O) together with an equal amount of methyl ketones (CH3COR) and 30-40% of them would be formed from the /?-scissionof secondary alkoxy radicals (’RO’) followed by free radical chain oxidation of aldehydes to acids (Jensen et al., 1981). Also, at low oxygen pressures (4-75 kPa), isomerization and cyclization reactions of a,d-hydroperoxyalkyl radicals (a$-HOOR’) become important (Jensen et al., 1992). The cyclization reaction of a,d-HOOR’ would form more volatile cyclic ethers (i.e., 2,5-dialkyloxolanes or 2-alkyloxolane) (Zinbo and Jensen, 1985) than the corresponding a,d-alkyl dihydroperoxides (a,d-R(OOH)d and a,d-hydroperoxy ketones (a$-HOOR=O) via a,& hydroperoxyperoxy radicals (a,d-HOOR02’) under sufficiently high partial pressures of oxygen (Po2> 75 kPa) to transform all R’to RO2’. However, this latter reaction would become reversible above ca. 250 “C and the predominant oxidation process for R’ 0 2 would be the production of olefin and hydrogen peroxide (e.g., Benson and Nangia, 1979; Benson, 1981). A radical reaction scheme for the autoxidation of longchain n-alkanes (C ? 5) below ca. 250 “C is shown in
+
Ind. Eng. Chem. Res., Vol. 34, No. 2,1995 623
.
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OOH
OOH
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INITIATION
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Figure 3. Reaction scheme for autoxidation of long-chain nalkanes (C I5)at elevated temperatures (ca.120-220 “C). Table 4. Average Pre- and PostcatalystTemperatures Measured at the Three EPA UDDS Test Phases (U.S. EPA, 1993) average temperature: “C sample ID UDDS phase precatalyst postcatalyst 412 378 “normalmode” la 454 408 2b 424 407 3c 331 396 “engine-out” la 366 396 2b 393 345 3c 347 394 “noHEGO” la 395 413 26 386 393 3c 439 399 “misfire” la 528 2b 400 487 400 3c a Phase 1 is the “transient”phase of the cold-starttest. Phase 2 is the “stabilized”phase of the cold-start test. Phase 3 is the “transient”phase of the hot start. Averages of four measurements.
Figure 3. On the basis of our kinetic and mechanistic knowledge of cleavage product formation in the oxidation of n-alkanes, a major hydrocarbon component of the precatalyst particulate emissions (e.g., “engine-out”) would be a series of unoxidized aliphatic BOHs (Zinbo et al., 1993a) with a minor amount of less volatile alkanoic acids, which were formed from a,y-HOOR=O cleavage and >RO’ /3-scission reactions. The rate of intramolecular hydrogen abstraction reactions of RO2’ to a,y- and a,d-HOOR’ is significantly faster than that of an irreversible intermolecular hydrogen abstraction reaction to ROOH under a low gas-phase hydrocarbon concentration, [RHI, a t elevated temperatures, i.e., k3[RH]
