Environ. Sci. Technol. 1990, 24, 853-862
Relative Reactivity of Emissions from Methanol-Fueled and Gasoline-Fueled Vehicles in Forming Ozone Alan M. Dunker Environmental Science Department, General Motors Research Laboratories, Warren, Michigan 48090-9055 ~~~
w A trajectory model was used to estimate the ozoneforming potential of emissions from methanol- and gasoline-fueled vehicles. The composition of the emissions from the two types of vehicles was based on new measurements of the individual organic compounds emitted by vehicles operating on gasoline, M85 fuel, and MlOO fuel. Simulations were conducted using a range of atmospheric conditions and two different chemical mechanisms. The results suggest that replacing gasoline-fueled vehicles with methanol-fueledvehicles may not reduce ozone in all urban areas. Also, it was found that the experimental methanol-fueled vehicle tested provides, at best, small ozone reductions compared to prototype gasoline-fueled vehicles. Additional work is necessary to reduce formaldehyde emissions from methanol-fueled vehicles. Lastly, if both M85 and MlOO vehicles are assumed to have the same low-formaldehyde fraction in the emissions, the results suggest that M85 vehicles provide 8 0 4 5 % of the ozone reductions of MlOO vehicles.
Introduction Reducing urban ozone levels has proven to be a difficult task. One manifestation of this is that 101 US. urban areas presently fail to meet the National Ambient Air Quality Standard for ozone (I). A proposed strategy for controlling ozone that is receiving increasing attention is the replacement of gasoline- and diesel-fueled vehicles with alternative-fueled vehicles. Of particular interest are vehicles fueled on a mixture of 85% methanol and 15% gasoline (M85) or fueled on pure methanol (M100) (2-4). To estimate the effect of introducing methanol-fueled vehicles on urban ozone, several trajectory and grid modeling studies have been performed (5-9). These studies have been limited by a lack of detailed measurements of the composition of emissions from methanol-fueled vehicles. A less obvious but equally important limitation has been the lack of detailed emission measurements on current and possible future gasoline-fueled vehicles. The net ozone change upon introduction of M85 and MlOO vehicles depends, in addition to other factors, on the composition of the emissions from the vehicles they will replace. Since the composition of the emissions from gasoline-fueled vehicles has changed over time (IO),measurements on current or possible future gasoline-fueled vehicles are important. Recently, Williams et al. (11)measured the individual organic compounds emitted by an experimental vehicle operating on M85 and MlOO fuel and prototype gasoline-fueled vehicles. In the work described in the following sections, these new emission measurements are used along with a simple trajectory model to assess the relative reactivity or ozone-forming potential of the emissions from M85, M100, and gasoline-fueled vehicles. While ozone formation depends on both the amount and composition of organic emissions, this study examines the effect of changes only in the composition of organic emissions, not the amount. It is also important to recognize that only a limited number of vehicles were tested (11)and that the composition of the organic emissions from methanol- and 0013-936X/90/0924-0853$02.50/0
gasoline-fueled vehicles may change as vehicle technology evolves and fuel composition changes. Nevertheless, the work of Williams et al. provides detailed data, and apparently the only detailed data, on the composition of emissions from experimental methanol-fueled and prototype gasoline-fueled vehicles. The purpose of this study is to investigate the reactivity of the emissions over a wide range of urban conditions and model parameters and not to provide detailed simulations for a particular urban area. In particular, because previous studies (12-14) have shown that model results can depend strongly on the chemical mechanism employed, the reactivities are determined by use of the two most recent chemical mechanisms for the urban atmosphere (15,16) and the results are compared. The next section outlines the modeling approach, the reduction of the emission measurements to the organic compound classes in the two chemical mechanisms, and the conditions for the simulations. The third section presents results for ozone and describes how the reactivity of the emissions changes with vehicle type and simulation conditions. The fourth section compares the results to those found in previous studies and gives the limitations of this study. The final section gives the conclusions.
Description of the Calculations Twelve-hour simulations were done with the Ozone Isopleth Plotting with Optional Mechanisms/version 4 (OZPM4) trajectory model (17). This is a relatively simple trajectory model, but it does allow examination of a wide range of atmospheric conditions and it is intended for use in developing ozone control strategies. The general approach was first to simulate a base case with the composition of the non-methane organic compounds (NMOC) taken from ambient urban measurements. Then, additional cases were simulated in which the light-duty vehicle fraction of the NMOC was replaced with NMOC having the composition of the emissions from different vehicle types. Chemical Mechanisms. The Lurmann, Carter, Coyner (LCC) and the Carbon Bond 4 (CB4) chemical mechanisms for ozone formation were used in the calculations (15-18). These are the two most recent mechanisms developed for the urban atmosphere, and they are also the two mechanisms most extensively tested against smog chamber data. It is not clear at present which mechanism is more accurate. The LCC mechanism uses 13 classes to represent organic compounds. These are four or five carbon alkanes (ALK4), six or more carbon alkanes (ALK7), ethene (ETHE), terminal alkenes (PRPE), internal alkenes (TBUT), monoalkylbenzenes (TOLU), dialkylbenzenes (XYLE), tri- and multialkylbenzenes (TMBZ), formaldehyde (HCHO), acetaldehyde (ALD2),higher molecular weight aldehydes (RCHO), ketones (MEK), and nonreactive carbon other than methane (NR). The CB4 mechanism uses nine classes to represent carbon groups and organic compounds. The classes are paraffinic carbon (PAR), ethene (ETH), olefinic carbon (OLE), monoalkylbenzenes (TOL), multialkylbenzenes (XYL), formaldehyde (FORM), higher
0 1990 American Chemical Society
Environ. Sci. Technol., Vol. 24, No. 6, 1990 853
Table I. Allocation of Vehicle Emissions to the Organic Compound Classes in the Lurmann, Carter, Coyner Chemical Mechanism” compd classb
current MO fleetc
prototype MO vehiclesd
ALK4 ALK7 ETHE PRPE TBUT TOLU XYLE TMBZ HCHO ALD2 MEOH NR CH4
0.222 0.240 0.040 0.012 0.055 0.080 0.111 0.055 0.004 0.002
0.196 0.262 0.047 0.052 0.014 0.184 0.030 0.002 0.009 0.002 0.000 0.106 0.096
0.000
0.090 0.089
fraction of carbon in comoound class variable-fueled variable-fueled improved vehicle M85d vehicle MIOOd M85 vehicle‘ 0.091 0.063 0.027 0.017 0.004 0.040 0.010 0.002 0.076 0.001 0.585 0.023 0.061
0.007 0.007 0.003 0.003 0.001 0.005 0.001
0.091 0.063 0.027 0.017 0.004 0.040
0.000
0.002 0.040 0.001 0.621 0.023 0.061
0.045 0.000
0.914 0.008
0.006
0.010
ultimate M85 vehiclee
ultimate MlOO vehiclee
0.091 0.063 0.027 0.017 0.004 0.040 0.010 0.002 0.010 0.001 0.651 0.023 0.061
0.007 0.007 0.003 0.003 0.001 0.005 0.001 0.000
0.010 0.000
0.949 0.008 0.006
The results are for combined exhaust and evaporative emissions. *Defined in the text and ref 15. CLight-dutygasoline vehicles equipped with catalysts, refs 10 and 22. dDerived from the measurements in ref 11. ‘Hypothetical vehicles with a reduced formaldehyde fraction.
molecular weight aldehydes (ALD2),isoprene (ISOP), and nonreactive carbon other than methane (NR). For this study, two additional classes were added to each mechanism: methanol (MEOH) and methane (CH,). No reactions were added to the mechanism for CH,; it is only used to account for the varying amounts of methane emitted by the different types of vehicles, as detailed in the next section. The following reaction involving MEOH was added to each mechanism: MEOH
+ OH 2HCHO + H 0 2 + H 2 0
(1)
with the rate constant K , = 1.94 X 104e-805/T ppm-’ min-l from Atkinson (19). This is the only known gas-phase reaction of methanol that appears to be important in the atmosphere (6). Accurate coding of both mechanisms was verified through comparison with test cases. Representation of Vehicle Emissions. Williams et al. (11)measured exhaust and evaporative emission rates of over 100 organic compounds from methanol- and gasoline-fueled vehicles. This study employs the measurements for one variable-fueled vehicle (VFV) operating on M85 and on MlOO and the measurements for four gasoline-fueled (MO) vehicles. The former vehicle is one of the first experimental VFVs. The latter vehicles are gasoline-fueled prototypes having advanced exhaust emission control systems, principally revised catalyst formulations and improved air- and fuel-handling systems. Williams et al. made two sets of exhaust and evaporative emissions measurements for the VFV M85, and these were averaged for this work. For the VFV M100, one set of measurements was obtained in which the proposed carbon-based emissions standards (20)were achieved, and this set was used. One set of exhaust emission measurements was made on each of four different prototype MO vehicles, and these were averaged. While no evaporative emission measurements were made on the prototype MO vehicles, measurements were made on a 1987 production vehicle with evaporative control equipment similar to the prototype MO vehicles, and these evaporative emission measurements were used. In the tests selected for this study, all the vehicles met the proposed emissions standards (20). The M85 fuel was a mix by volume of 85% chemicalgrade methanol and 15% certification gasoline, and the MlOO fuel was chemical-grade methanol (11).The prototype MO vehicles were fueled with certification gasoline. All the certification gasoline used had a Reid vapor 854
Environ. Sci. Technol., Vol. 24, No. 6, 1990
pressure of 9 psi. The experimental measurements of Williams et al. (11)did not determine excess evaporative emissions or “running losses”. However, for gasolines with a Reid vapor pressure of 9 psi, these emissions should be minimized. The evaporative emissions of each compound were converted to an equivalent emission rate of milligrams per mile by using a formula based on MOBILE3 results (20) and then added to the exhaust emission rate of that compound. The combined exhaust and evaporative emission rates were allocated to the compound classes in the LCC and CB4 mechanisms by use of the procedures recommended by the authors of the mechanisms (15,17)and by Jeffries et al. (21). The carbon fractions in the compound classes are given in Tables I and I1 for the LCC and CB4 mechanisms, respectively. All fractions for RCHO and MEK in the LCC mechanism are 0. Apart from the three cases just described, four other cases are presented in Tables I and 11. Based on measurements by Sigsby et al. (IO),Shareef et al. (22)have recommended a detailed composition profile for the combined exhaust and evaporative emissions from the lightduty, catalyst-equipped, MO vehicles in the current fleet. The allocation of the organic compounds in this profile to the classes in the LCC and CB4 mechanisms is shown in Tables I and I1 as “current MO fleet”. It should be noted, however, that only vehicles through the 1982 model year were tested (lo),and therefore, the profile does not reflect the most recently produced vehicles. Also, the profile represents vehicles fueled on a commercial, regular-grade gasoline that differs in composition from the certification gasoline used for the tests on the prototype MO vehicles. The improved M85, ultimate M85, and ultimate MlOO cases represent hypothetical, not actual, methanol-fueled vehicles with formaldehyde emissions reduced by factors of 2-8 from the measured values. The carbon fractions for these cases were developed by simply removing carbon from the HCHO or FORM fraction in the VFV M85 or VFV MlOO cases and adding the carbon to the MEOH fraction. The ultimate M85 and ultimate MlOO cases have formaldehyde fractions essentially as low as the prototype MO vehicles. Two remarks on the results in Tables I and I1 are necessary. First, the carbon fractions for HCHO and FORM in Tables I and I1 are not identical, nor are the fractions for ALD2. The reason is that in the LCC mechanism only formaldehyde and acetaldehyde are allocated to the HCHO and ALDZ classes, respectively, whereas in the CB4
Table 11. Allocation of Vehicle Emissions to the Organic Compound Classes in the Carbon Bond 4 Chemical Mechanism4 compd classb
current MO fleetc
prototype MO vehiclesd
PAR ETH OLE TOL XYL FORM ALDB ISOP MEOH NR CH4
0.547 0.040 0.028 0.084 0.154 0.004 0.030 0.001 0.000 0.023 0.089
0.508 0.046 0.023 0.184 0.032 0.015 0.026 0.000 0.000
0.070 0.096
fraction of carbon in compound class variable-fueled improved variable-fueled M85 vehiclee vehicle MIOOd vehicle M85d 0.167 0.023 0.009 0.040 0.011
0.078 0.007 0.000 0.585 0.019 0.061
0.167 0.023 0.009
0.018 0.002 0.003 0.005 0.001 0.045 0.000 0.000 0.914 0.006 0.006
0.040
0.011 0.040 0.007 0.000 0.623 0.019 0.061
ultimate M85 vehiclee
u 1ti mate Ml00 vehiclee
0.167 0.023 0.009 0.040 0.011 0.010
0.018 0.002 0.003 0.005 0.001 0.010 0.000 0.000 0.949 0.006 0.006
0.007 0.000
0.653 0.019
0.061
OThe results are for combined exhaust and evaporative emissions. bDefined in the text and refs 16 and 17. CLight-dutygasoline vehicles equipped with catalysts, refs 10 and 22. dDerived from the measurements in ref 11. eHypothetical vehicles with a reduced formaldehyde fraction.
mechanism some reactive alkenes are allocated to the FORM and ALDB classes in addition to formaldehyde and acetaldehyde. Second, the carbon fractions for HCHO and FORM are lower for the VFV MlOO than for VFV M85 even though the formaldehyde emission rate in milligrams per mile was 10% higher for the VFV M100. The explanation is that the total exhaust emissions increased by a factor of 2 from the VFV M85 to the VFV MlOO, and thus the HCHO and FORM fractions are reduced for the VFV M100. The increase in total exhaust emissions is due primarily to the greater difficulty of cold starting the vehicle on M100. Base Case. A base case was defined with the composition of the NMOC chosen to represent the average, current, urban atmosphere. Jeffries et al. (21) allocated early morning hydrocarbon measurements in 41 cities from 1984 to 1986 (23,24)to the compound classes in the LCC and CB4 mechanisms and combined these results with estimates of aldehyde concentrations to give a NMOC composition for an average urban area. This composition along with other simulation conditions for the base case are given in Table 111. The surface and aloft concentrations transported from upwind areas, the location, and the day were chosen to emphasize, and thereby provide an upper limit to, the differences in reactivity of emissions from different vehicle types in the simulations. Emissions were added each hour after the start of the simulations. Variations of the standard conditions in Table I11 were also investigated and will be discussed later. Vehicle Replacement Scenarios. The following two assumptions were made in the design of the scenarios: 1. All vehicles emit the same total amount of volatile organic compounds (VOC) on a carbon basis. 2. All vehicles emit the same amount of NO,. The scenarios differ, therefore, only in the composition of the VOC from the light-duty vehicles. These assumptions were chosen to permit comparison of the reactivity of the VOC emissions alone, without introducing the added complexity of different total VOC or NO, emission rates from different vehicle types. The assumptions are reasonable on other grounds, however. An additional justification for the first assumption is that the proposed exhaust and evaporative emission standards for VOC from methanol-fueled vehicles are the same (on a carbon basis) as the standards for gasoline-fueled vehicles (20). An additional justification for the second assumption is that the after-catalyst NO, emissions from current light-duty M85 or MlOO vehicles are the same or higher than those from light-duty gasoline-fueled vehicles (1I,25). Since the VFV MlOO had twice the exhaust VOC emissions
Table 111. Simulation Conditions for the Base Case 1ocation day time period mixing height temperature surface and aloft NMOC, NO,, and Os4 NMOC and NO, emissns (fraction of init concn emitted/hIb NOz fraction of init and emitted NO,
Los Angeles June 21 0800-2000 PDT 510 m at 0800 rising to 630 m at 1500 303 K 0 PPm 0.15 at 0800-1800 0.05 at 1800-2000 0.25
Composition of Initial and Emitted NMOCc species
C fractn
species
C fractn
PAR ETH OLE TOL XYL
CB4 Mechanism 0.564 FORM 0.037 ALDZ 0.035 ISOP 0.089 MEOH 0.117 NR
0.021 0.052 0.000 0.000 0.085
ALK4 ALK7 ETHE PRPE TBUT TOLU XYLE
LCC Mechanism 0.214 TMBZ 0.280 HCHO 0.037 ALDP 0.048 RCHO 0.029 MEK 0.080 MEOH 0.079 NR
0.042 0.020 0.030 0.000 0.000 0.000 0.141
OTransported from upwind areas. CReference21.
*Defined in ref 17.
of the VFV M85 and prototype MO vehicles, the first assumption is optimistic because it implies that the total VOC emissions from the VFV MlOO can be reduced in the future to the level of the other vehicles. The total VOC in the first assumption is the sum of the exhaust and evaporative emissions of NMOC and methane, VOC(tota1) = NMOC(exhaust) + CH,(exhaust) + NMOC(evap0rative). As Tables I and I1 show, the VFV MlOO had virtually no CH, emissions compared to the current MO fleet or the prototype MO vehicles. Hence, assuming equal VOC emissions is not equivalent to assuming equal NMOC emissions. Further, assuming equal VOC emissions implies that the MlOO vehicles have greater NMOC emissions than the MO or M85 vehicles. This is important because only NMOC is considered in urbanscale modeling due to the very low reactivity of CHI. Previous studies (5-9) have assumed equal NMOC emissions on a carbon or mass basis from the different types Environ. Sci. Technol., Vol. 24, No. 6, 1990 855
Table IV. Ozone Results for Different Scenarios Using the Lurmann, Carter, Coyner Chemical Mechanism” maximum hourly average ozone, ppm pointb NMOC/NO,’ scenario base case current MO fleet prototype MO vehicles variable-fueled vehicle M85 improved M85 vehicle ultimate M85 vehicle variable-fueled vehicle MlOO ultimate MlOO vehicle no vehicle NMOC
B 10
C 15
D
6
7
E 10
F 15
0.180 0.183 0.154 0.170 0.150 0.132 0.141 0.120 0.074
0.180 0.180 0.177 0.179 0.177 0.175 0.176 0.173 0.152
0.180 0.180 0.178 0.181 0.180 0.179 0.180 0.178 0.164
0.300 0.305 0.261 0.293 0.260 0.229 0.247 0.211 0.125
0.300 0.299 0.294 0.302 0.297 0.291 0.295 0.288 0.229
0.300 0.299 0.298 0.309 0.306 0.304 0.308 0.304 0.274
A
OFor the simulation conditions in Table I11 and the assumption that all vehicles emit the same total VOC on a carbon basis. bShown in Figure 1. Initial ratio for the base case in ppm C/ppm.
of vehicles. Assuming equal total VOC emissions on a carbon basis now is more consistent with the existing and proposed emission standards (20). For the scenario representing introduction of the prototype MO vehicles, 30% (carbon basis) of the NMOC in the base case is removed and replaced with an equal amount of NMOC having the composition of the emissions from those vehicles. This procedure approximates full replacement of the light-duty vehicle emissions since the average fraction of NMOC from light-duty vehicles in 20 US. cities is 30% (7). For another scenario representing introduction of another type of vehicle, call it vehicle A for simplicity, 30% of the NMOC in the base case is removed, and NMOC having the composition of the emissions from vehicle A is added. The amount of NMOC added for the vehicle A scenario is p X 30% of the NMOC in the base case. The factor p = (NMOC fraction of VOC for vehicle A)/(NMOC fraction of VOC for prototype MO vehicles) arises from the assumption of constant total VOC emissions and accounts for the varying CH4emission rates from the different types of vehicles. Denoting the maximum hourly average O3 in the base case and the vehicle A scenario as 03(basecase) and 03(vehicle A), respectively, then the change A03( vehicle A) is a measure of the reactivity of the emissions from vehicle A: A03(vehicle A ) = 03(vehicle A ) - 03(base case) (2) A negative value for A 0 3 corresponds to reduced reactivity of the emissions relative to the base case and a positive value corresponds to increased reactivity. Dodge (26) defined reactivity as the O3 change resulting from a small addition of an organic compound to a base case, and Carter and Atkinson (27) defined reactivity as the O3 change per amount of compound added or removed for vanishingly small changes in the base case. In the definition in eq 2, however, a large rather than small change is made in the composition of the base case, and substitution rather than addition or deletion is employed. The definition in eq 2 is more appropriate for a full replacement of the vehicle fleet whereas the other definitions (26,27) are more appropriate for introduction of small numbers of alternative-fueled vehicles. Results Standard Conditions. Calculations were performed over a range of initial NMOC and NO, concentrations, since these concentrations vary from one urban area to another and from one day to another in a given urban area. Figure 1presents contour plots of A 0 3 as a function of the initial concentrations for five scenarios. These results were 858
Environ. Sci. Technol., Vol. 24, No. 6, 1990
obtained with the LCC mechanism. Superimposed on the A 0 3 contours are O3 contours for the base case to provide a reference to absolute O3 concentrations. The scenario labeled “no vehicle NMOC” represents removal of 30% of the NMOC in the base case with no change in NO, levels or, equivalently, replacement of 30% of the NMOC with completely unreactive compounds. Several points are clear from Figure 1. First, A 0 3 is very small for the current MO fleet scenario, IA031 I0.05 ppm. Second, O3 reductions occur for the other scenarios, and these reductions are greatest at NMOC/NO, ratios of 5-7 ppm C/ppm. Third, for the prototype MO, VFV M85, and VFV MlOO scenarios, the O3 reductions decrease rapidly to zero as the NMOC/NO, ratio increases beyond 7 ppm C/ppm. Last, the VFV M85 and VFV MlOO scenarios show O3 increases relative to the base case for NMOC/NO, > -12 ppm C/ppm. While the relative reactivity of the emissions from the different types of vehicles thus varies strongly as a function of NMOC/NO, ratio, some general statements are possible. The reactivity of the current MO fleet emissions is identical with and the reactivity of the VFV M85 emissions is very nearly the same as the reactivity of the ambient NMOC used in the base case. The emissions from the prototype MO vehicles and the VFV MlOO are less reactive than the ambient NMOC at low NMOC/NO,, with the VFV MlOO emissions being slightly less reactive than the prototype MO emissions. However, the emissions from these vehicles still react and contribute to O3 formation because in the scenario with completely unreactive (no vehicle) NMOC the O3 reductions were at least 3 times larger than for the prototype MO or VFV MlOO scenarios. A t NMOC/NO, ratios greater than 10 ppm C/ppm, the reactivity of the emissions from the current MO fleet, prototype MO vehicles, VFV M85, and VFV MlOO is very close to that of the ambient NMOC in the base case. These general statements on reactivity are illustrated further in Table IV, which compares O3concentrations for all scenarios at points A-F on Figure 1. The A 0 3 contours for the improved M85 and ultimate M85 scenarios show O3 reductions greater by 0.01-0.06 ppm than the VFV M85 scenario in the region of initial NMOC/NO, = 3-9 ppm C/ppm. Similarly, the A 0 3 contours for the ultimate MlOO scenario show O3 reductions greater by 0.01-0.04 ppm than the VFV MlOO scenario in this same NMOC/NO, region. This is illustrated in Figure 2, where O3progressively decreases as the formaldehyde fraction in the emissions decreases. Hence, the reactivity of the vehicle emissions is very sensitive to the formaldehyde fraction in the NMOC/NO, = 3-9 ppm C/ppm region. Outside the NMOC/NO, = 3-9 ppm C/
-
VFV M85
Current MO Fleet 0.07
0.07
0.06
0.06
-
0.05
-8
E
E
B1..-* C
0.05
0.04
0.04
0.03
B3.- 0.03 .-e E
-
-
0.02
0.02
0.01
0.01
O O O I L
I
0.0
01
I
I
0.2
I
I
I
I
I
I
0.5
0.4
0.3
_ _ _ _ _ _ _ _ _ _ _ _ _ _ 0.12
0.00 0.0
I
I
0.1
I
I
0.2
I
0.3
I
I
I
0.4
I 5
0.4
0.5
Initial NMOC (ppmC)
Initial NMOC (ppmC)
VFV M100
Prototvoe MO Vehicles
0.07 0.06 0.05
0.02 0.01
0.0
0.1
0.2
0.4
0.3
0.1
0.0
0.5
0.2
0.3
Initial NMOC (ppmC)
Initial NMOC (ppmC) No Vehicle NMOC 0.07
0.08 0.05
0.02 0.01
00
01
02 03 Initial NMOC (ppmC)
04
05
Flgure 1. Ozone changes obtained upon varying the composition (but not the total amount) of vehicle VOC emissions. The solid lines are contours of PO, (defined in eq 2), and the dashed lines are contours of maximum hourly average O3 in the base case. The units for A 0 3 and 0, are ppm. The simulations employed the Lurmann, Carter, Coyner chemical mechanism and the standard conditions in Table I1 I. Ozone concentrations at points A-F are given in Table I V for all scenarios.
ppm region, A 0 3 is similar and small for the three M85 and two MlOO scenarios, and the reactivity is therefore insensitive to the formaldehyde fraction there. This can be seen from the results in Table IV for points B, C, E, and F. The results for the CB4 mechanism are quite similar to those for the LCC mechanism. This can be seen by comparing the plots of A 0 3 in Figure 3, which were obtained
with the CB4 mechanism, with the corresponding LCC plots in Figure 1. One difference between the mechanisms is that the O3 reductions for the CB4 mechanism in the NMOC/NO, = 6-7 ppm C/ppm region are greater than those for the LCC mechanism by up to 0.01 ppm for the prototype MO, improved M85, and VFV MlOO scenarios and greater by up to 0.02 ppm for the ultimate M85, ultimate M100, and no vehicle NMOC scenarios. Also, for Environ. Sci. Technol., Vol. 24, No. 6,1990 857
Table V. Ozone Results for Different Scenarios Using the Carbon Bond 4 Chemical Mechanism" maximum hourly average ozone, ppm pointb NMOC/NOzc scenario base case current MO fleet prototype MO vehicles variable-fueled vehicle M85 improved M85 vehicle ultimate M85 vehicle variable-fueled vehicle MlOO ultimate MlOO vehicle no vehicle NMOC
6
B 10
C 15
D 7
E 10
F 15
0.180 0.179 0.147 0.172 0.144 0.121 0.137 0.111 0.068
0.180 0.179 0.175 0.180 0.178 0.176 0.178 0.176 0.159
0.180 0.179 0.176 0.184 0.182 0.181 0.184 0.183 0.170
0.300 0.300 0.250 0.297 0.249 0.209 0.240 0.193 0.110
0.300 0.299 0.290 0.305 0.299 0.293 0.300 0.292 0.235
0.300 0.298 0.293 0.316 0.313 0.311 0.319 0.316 0.290
A
a For the simulation conditions in Table I11 and the assumption that all vehicles emit the same total VOC on a carbon basis. Figure 3. 'Initial ratio for the base case in ppm C/ppm.
-k -
Prototype MO Vehicles
0.24
0.20
Shown in
1
LCC
NMOC/NOx = 6 ppmC/ppm
i
0.16
cn
2t
-a
e s .$
0.12
a
3
0.08
1 0.03
0.04
C
0.02
0 Base Case
Formaldehyde Percentage in Vehicle Emissions
VFV Improved Ultimate VFV M85 M100 M85 M85 76
40
10
45
Ultimate M100
No Vehicle NMOC
10
Flgure 2. Effect of reducing formaldehyde in the emissions from methanol-fueled vehicles at a low initial NMOC/NO, ratio, point A of Figure 1. The percentages are on a carbon besls, and all vehicles are assumed to emit the same total amount of VOC. See text for a discussion of the formaldehyde fraction for the VFV M100.
all five methanol-fueled vehicle scenarios in the NMOC/NO, > 12 ppm C/ppm region, the CB4 mechanism yields O3 increases greater by -0.01 ppm than the LCC mechanism. These statements refer to differences in PO3 between the mechanisms at the same initial NMOC and NO, for both mechanisms. Since the same initial NMOC and NO, concentrations give different base-case O3 concentrations with the two mechanisms, another comparison of interest is the difference in PO3 at the same base-case O3concentration and NMOC/NO, ratio for both mechanisms. For this comparison, the CB4 results in Table V may be compared with the LCC results in Table IV. The differences between these two sets of results are approximately the same as the differences found at the same initial NMOC and NO, for both mechanisms. Hence, while differences exist between the results of the two mechanisms, the differences are small enough that both mechanisms give the same overall picture of the relative reactivity of the emissions. Sensitivity Tests. Beyond examining the sensitivity of the results to the formaldehyde fraction in the vehicle emissions and to the chemical mechanism employed, other sensitivity tests were conducted. To examine the sensitivity to the amount of emissions added after the start of the simulations, simulations were conducted with no emissions added after the start; only the initial concentrations were present. These results are shown in Figure 4. In the absence of added emissions, higher initial NMOC and NO, are needed to obtain the same base-case 858
Environ. Sci. Technoi., Vol. 24, No. 6, 1990
0.001
I
0.0
I
1
0.1
1
I
I
I
0.2 0.3 Initial NMOC (ppmC)
I
I
I
0.5
0.4
VFV M100
------012 ------_ _ _ _ _ _ _ 1. 0.00 00
I
I
01
I
I
I
I
0.2 0.3 Initial NMOC (ppmC)
I
I
04
I
05
Flgure 3. Ozone changes obtained upon varying the compositlon of vehicle VOC emissions. Same as Figure 1 except that the Carbon Bond 4 chemical mechanism was used in the simulations. Ozone concentrations at points A-F are given in Table V for ail scenarios.
O3 concentration. However, at the same base-case O3 concentrations, the PO3 results in Figure 1 (with added emissions) and Figure 4 (without added emissions) are very similar. The major difference is that the maximum O3 reductions occur at slightly lower initial NMOC/NO, ratios in Figure 4 than in Figure 1. Similar statements hold for the other scenarios not shown in Figure 4. Separate sets of calculations were conducted using September 1rather than June 21 as the day and using 1530
0.14
-E,
0.12
i
Prototype MO Vehicles
0.24 NMOC/NOx = 6 ppmClppm
LCC
n
;0.20 0.16
-E
0
0.16
8
2 -=.
a 'gi
.s 0.08
ff5
-
.-C
-
0.12
I 0.08 0.06
0.04
I
0.04
0 Prototype Current MO Fleet MO Vehicles
Base Case
0.02
0 Low OH+MEOH Rate Constant
0.00 0.0
0.2
0.4 0.6 Initial NMOC (ppmC)
0.8
1.o
h9 Standard OH+MEOH
No Vehicle NMOC High OH+MEOH Rate Constant
Rate Constant
+
=g
0.24 NMOC/NOx = 6 ppmclppm
LCC
P
8
-
VFV MlOO
Figure 5. Effect of varying the OH methanol rate constant by f30%, its uncertainty, at point A of Figure 1. The other simulation conditions are unchanged from those in Table 111.
-E
V N M100 0.14
VFV M85
0.20
0.16
9
2* -5 8
0.12
0.08
.: 5
0.04
I 0 Prototype Current MO Fleet MQ Vehicles
Base Case 0.12
- _ --- - - -
a Low HCHO Photolysis Rate
0.00 0.0
0.2
0.4 0.6 Initial NMOC (ppmC)
08
1.o
Flgure 4. Ozone changes obtained upon varying the composition of vehicle VOC emissions. Same as Figure 1 except that no emissions are added after the start of the simulatlons. Note that the axes extend to higher initial concentrations.
m rather than 630 m as the mixing height at 1500 PDT. The A 0 3 contours for these two sets of calculations are for all scenarios again similar to those in Figure 1if the comparisons are made for base-case O3 concentrations comparable to those in Figure 1. Important rate constants for the simulations in this study are the OH + MEOH rate constant, eq 1,and the formaldehyde photolysis rates, eq 3. The OH + MEOH HCHO + hw HCHO + h~
+
2H02 CO Hz + CO
+
(34 (3b)
rate constant is uncertain to *30% (19),and the effect of varying this rate constant is shown in Figure 5 for the low initial NMOC/NO, region. In this region, the 0,for all the methanol-fueled vehicle scenarios increases or decreases by 0.01-0.02 ppm as the rate constant is varied to its upper or lower limit, respectively. For NMOC/NO, > 12 ppm C/ppm, O3 varies by 10.01 ppm as the rate constant is varied to its limits. The uncertainty in this rate constant hence is a concern in accurately determining AO, for the methanol-fueled vehicle scenarios a t low NMOC/NO,. However, when the upper limit for this rate constant is used, the ultimate M85 and ultimate MlOO scenarios still show greater O3 reductions than the prototype MO scenario by 0.01-0.02 ppm for low NMOC/NO,.
VFV M85
Standard HCHO Photolysis Rate
VFV
M100
No Vehicle NMOC
I High HCHO Photolysis Rate
Figure 6. Effect of varying the formaMehyde photolysis rates by f35 %, their uncertainty, at point A of Figure 1. The other simulation conditions are unchanged from those in Table 111.
The formaldehyde photolysis rates are uncertain to *35% (28). Varying these photolysis rates over their uncertainty range has little effect on O3 and A 0 3 for initial NMOC/NO, I 15 ppm C/ppm. For low NMOC/NO,, varying these photolysis rates to their upper and lower limits varies O3 by up to 0.05 ppm for the base case and the scenarios, as illustrated in Figure 6. The formaldehyde photolysis rates are clearly important in accurately predicting absolute 0,concentrations. However, the effect on A 0 3 is not as great because the O3 changes for the base case and the scenarios are in the same direction and tend to cancel in calculating A03, and the overall picture of the relative reactivity of the emissions from different vehicles is not changed significantly. Reactivity Ratio. A reactivity ratio can be defined that measures the ozone-forming potential of the emissions from vehicle A relative to the ozone-forming potential of the ambient NMOC used for the base case: R(vehic1e A) = 1 - AO,(vehicle A)/A03(no vehicle NMOC) (4) With this definition, R = 0 corresponds to vehicle emissions with no ozone-forming potential, and R = 1 corresponds to vehicle emissions with the same ozone-forming potential as the base-case NMOC. Values of R are given in Table VI for points A, D, and F of Figures 1 and 3. At low initial NMOC/NO,, the results for points A and D agree well, and the results for the two mechanisms agree Environ. Sci. Technol., Vol. 24, No. 6 , 1990 859
Table VI. Reactivity Ratio for Different Scenariosn reactivity ratiob pointc NMOC/NO,d base-case O3 scenario current MO fleet prototype MO vehicles variable-fueled vehicle M85 improved M85 vehicle ultimate M85 vehicle variable-fueled vehicle MlOO ultimate MlOO vehicle
A 6 0.18
F 15 0.30
D 7 0.30
LCC
CB4
LCC
CB4
LCC
CB4
1.03 0.75 0.91 0.72 0.55 0.63 0.44
0.99 0.71 0.93 0.68 0.48 0.62 0.39
1.03
0.78 0.96 0.77 0.59 0.70 0.49
1.00 0.73 0.98 0.73 0.52 0.68 0.44
0.93 0.89 1.30 1.19 1.09 1.24
0.75 0.31 2.24 2.03 1.82 2.52 2.25
1.11
OFor the simulation conditions in Table I11 and the assumption that all vehicles emit the same total VOC on a carbon basis. bSee eq 4. CShownin Figures 1 and 3. dInitial ratio for the base case in ppm C/ppm.
well. The ratio is smallest for the ultimate MlOO scenario, the ozone-forming potential being 40-50% that of the base-case NMOC. The ratio increases rapidly as the initial NMOC/NO, increases, until at point F the ratio is greater than 1 for all the methanol-fueled vehicle scenarios.
Discussion Comparison of the results of this study with the results of previous modeling studies is complicated by differences in the assumptions made. In particular, constant total VOC (on a carbon basis) was assumed for the scenarios in this study because the focus was on the reactivity of the emissions, whereas both the composition and the total VOC were varied for some scenarios in previous studies. However, an examination of scenarios in previous work with assumptions similar to those in this study indicates that there are several important results from this study. One important result is that the reactivity of the emissions from the prototype MO vehicles is -25% less than that of the emissions from the current MO fleet or the ambient NMOC in the base case at low NMOC/NO, (Table VI). If the prototype MO vehicles are representative of the future gasoline-fueled fleet, then the O3 benefit of introducing methanol-fueled vehicles will be less than suggested by a comparison of methanol-fueled vehicles with the current MO fleet or current ambient NMOC. In previous studies, the comparison has always been between methanol-fueled vehicles and the current MO fleet or ambient NMOC, though the compositions used for the emissions from the current MO fleet or ambient NMOC are not the same as those used in this study (Tables I and 11). Clearly, the O3benefit of introducing methanol-fueled vehicles depends strongly on what composition is assumed for the emissions from the gasoline-fueled vehicles that will be replaced. The reason that the prototype MO vehicles have less reactive emissions than the current MO fleet is unclear. The emission control systems on the prototype MO vehicles, particularly the catalysts, may remove more of the very reactive organic compounds. The difference in reactivity may also reflect differences in composition between the commercial fuel used for the tests on the current MO fleet and the certification fuel used for the tests on the prototype MO vehicles. Another important result of this study is that the O3 reductions (from the base case) for the ultimate M85 scenario approach those of the ultimate MlOO scenario at low NMOC/NO,. A very good approximation is A 0 3 (ultimate M85 vehicle) = 0.80 AO,(ultimate MlOO vehicle) for the LCC mechanism and A03( ultimate M85 vehicle) = 0.85 A03(ultimate MlOO vehicle) for the CB4 mecha860
Environ. Sci. Technol., Vol. 24, No. 6 , 1990
nism. This is somewhat better than expected for the ultimate M85 scenario considering that 30% of the NMOC for the ultimate M85 vehicle is hydrocarbons. Similar AO, relationships hold for the improved M85 scenario versus the VFV MlOO scenario. Hence, if the formaldehyde fractions are low and similar for the M85 and MlOO vehicles, the M85 vehicle gives much of the 0, benefit of the MlOO vehicle a t low NMOC/NO, ratios. This is in contrast to the results of Russell and Harris (9),which give A03(M85 vehicle) = 0.45 AO,(M100 vehicle) for their Pasadena trajectory. The reason for the difference appears to be that Russell and Harris assumed a much larger fraction (50% on a carbon basis) of the NMOC from M85 vehicles is hydrocarbons. A third notable result from this study is that the simulations show 0, increases for the methanol-fuel scenarios relative to the base case at high initial NMOC/NO, ratios. This is particularly true when the CB4 mechanism is used and is true even for the scenarios with very low formaldehyde fractions (1%) in the vehicle emissions. Earlier modeling studies (6-9) and one experimental study (29) give no evidence of O3 increases for methanol-fuel scenarios. Another experimental study (30) that used a 10% /90% formaldehyde/methanol mix to represent methanol-fueled vehicle emissions gives limited evidence for O3 increases a t high NMOC/NO, ratios. The reason for the O3increases in simulations with the LCC and CB4 mechanisms is, a t least in part, the following. At high NMOC/NO, ratios, O3production is limited by the availability of NO,. The degradation of methanol (eq 1)removes no NO,., but the degradation of some other compounds does remove NO, in products such as organic nitrates. Hence, at high NMOC/NO,, methanol utilizes the available NO, more efficiently in forming O3 than the NMOC mix in the base case. While the O3increases are not large, they do suggest the need to examine the effect of introducing methanol-fueled vehicles on areas downwind of cities and on rural areas. Such areas generally have high NMOC/NO, ratios, and they would presumably receive significant amounts of methanol exported from the cities. To date, no studies of the impact of methanol-fueled vehicles on regional O3 have been conducted. The fact that the magnitude of the O3 increases varies with the chemical mechanism employed suggests the need to determine the accuracy of the LCC and CB4 mechanisms for high NMOC/NO, ratios. The results of this study agree with the results of previous studies (7-9, 29-31) that the O3 reductions for methanol-fueled vehicles are highly sensitive to the NMOC/NO, ratio in the urban atmosphere. This study also shows at low NMOC/NO, a strong sensitivity of the
O3 reductions to the formaldehyde fraction in the methanol-fueled vehicle emissions, in agreement with the work of Nichols and Norbeck (7) and Carter et al. (29). Other studies (6, 8, 9) show a lower sensitivity to the formaldehyde fraction. However, the sensitivity to the formaldehyde fraction varies strongly with the NMOC/NO, ratio (Tables IV and V), and these other studies may have been conducted at NMOC/NO, ratios where the sensitivity to the formaldehyde fraction is diminished. A recent study by Chang et al. (31) also examined the ozone-reduction potential of M85- and M100-fueled vehicles. Relatively small ozone reductions were found. The average ozone reduction for 20 cities for full penetration of M85 and MlOO light-duty vehicles was 1.3% and 2.6%, respectively, under the assumption of equal NMOC emissions from methanol- and gasoline-fueled vehicles. It should be noted that these results were obtained with low-formaldehyde fractions in the emissions from M85 and MlOO vehicles, corresponding approximately to the improved M85 and ultimate MlOO cases in Table I. It should also be noted that the hydrocarbons from methanol- and gasoline-fueled vehicles were assumed to have the same composition as the current urban atmospheres. Hence, the comparisons Chang et al. made are between the reactivity of emissions from methanol-fueled vehicles and the reactivity of current urban atmospheres rather than a comparison between the reactivity of emissions from methanol-fueled vehicles and the reactivity of emissions from gasoline-fueled vehicles, as has been done in this study. One limitation of this study is that only 12-h simulations were conducted. For many urban areas, however, such simulations can be used to describe O3 formation (32). A second limitation is that, because this study addresses the relative reactivity of the emissions under a wide range of conditions, the simulations are not specific to a particular urban area and cannot be used to provide precise estimates of O3 changes upon introduction of methanol-fueled vehicles to a particular urban area. In fact, because the O3 changes are such a strong function of the NMOC/NO, ratio and because the region of maximum O3 reduction shifts somewhat as the model input varies, the O3 changes for a given urban area may vary strongly depending on the day simulated. Hence, assessing in detail the impact of methanol-fueled vehicles on an urban area will require simulations for more than one O3 episode. Another limitation is that the in-use emissions from actual future production vehicles may have compositions different from the experimental and prototype vehicles used in this study due to changes in the engines, the emission-control equipment, the purity of the methanol, or the composition of the gasoline. Furthermore, future methanol-fueled and gasoline-fueled production vehicles may not have the same total VOC emissions and the same NO, emissions in use, as has been assumed in this study. Conclusions Within the limitations of this study (given in the Discussion), the following specific conclusions may be drawn: 1. The relative amounts of ozone formed from the emissions of gasoline- and methanol-fueled vehicles are very sensitive t o the morning NMOC/NO, ratio in the urban atmosphere and the formaldehyde fraction in the emissions; they are sensitive, but to a lesser extent, to the OH + methanol reaction rate constant. 2. For morning NMOC/NO, ratios less than approximately 10 ppm C/ppm, emissions from the experimental VFV using M85 form more ozone and emissions from the VF'V using MlOO form somewhat less ozone than emissions from the prototype gasoline-fueled vehicles.
3. To obtain ozone reductions from the experimental VFV significantly greater than from the prototype gasoline-fueled vehicles, the formaldehydefraction in emissions from the VFV must approach that in emissions from the prototype gasoline-fueled vehicles. 4. If the formaldehyde fraction in the emissions from the VFV using M85 is assumed to be the same as the formaldehydefraction in the emiasions when MlOO is used, and the fraction is in the range 1-4%, then M85 fuel provides 8045% of the ozone reductions of MlOO fuel at low morning NMOC/NO, ratios. 5. For morning NMOC/NO, ratios greater than approximately 10 ppm C/ppm, emissions from the experimental VFV using M85 or MlOO form as much or more ozone as emissions from the prototype gasoline-fueled vehicles, and this is true regardless of the formaldehyde fraction in the VFV emissions. These specific conclusions suggest three general conclusions. First, replacing gasoline-fueled vehicles with methanol-fueledvehicles may not provide ozone reductions in all urban areas and therefore may be appropriate only as a local, not national, strategy for controlling ozone. The simulations predict that urban areas with high ambient NMOC/NO, ratios may not realize any ozone reduction by introducing methanol-fueled vehicles. Second, current experimental methanol-fueled vehicles provide, at best, small ozone reductions compared to prototype gasolinefueled vehicles. Additional work is necessary to reduce formaldehyde emissions from methanol-fueled vehicles. Third, careful consideration should be given to M85 vehicles and not just to MlOO vehicles. At the same formaldehyde fraction in the emissions, M85 vehicles appear to give most of the ozone reductions of MlOO vehicles. Coupled with the technical difficulties in cold-starting MlOO vehicles, this suggests that M85 vehicles may be a more attractive strategy for reducing urban ozone than MlOO vehicles. It must be reiterated that these conclusions are derived under the assumption that all vehicles emit the same total amount of VOC on a carbon basis and only the composition of the emissions varies. This is an optimistic assumption for the MlOO vehicles, since the experimental VFV MlOO had total VOC emissions twice that of the VFV M85 and prototype gasoline-fueled vehicles. Acknowledgments I thank Jerome P. Ortmann and Julie C. Smith for assistance with the calculations. Registry NO.ETHE, 74-85-1; HCHO, 50-0-0; ALDZ,, 7507-0; CHI, 74-82-8; NMOC/NO,, 11104-93-1; ozone, 10028-15-6; methanol, 67-56-1.
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Received for review May 22,1989. Revised manuscript received January 16, 1990. Accepted January 23, 1990.
