EPA-450/2-73-003
AN INTERIM REPORT
ON MOTOR VEHICLE EMISSION ESTIMATION
U.S. ENVIRONMENTAL PROTECTION AGENCY
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EPA-450/2-73-003
AN INTERIM REPORT
ON
MOTOR VEHICLE EMISSION ESTIMATION
David S. Kircher
Donald P. Armstrong
Environmental Protection Agency
Office of Air and Water Programs
Office of Air Quality Planning and Standards
Research Triangle Park, North Carolina 27711
October 1973
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This report is issued by the Environmental Protection Agency to report technical data of
interest to a limited number of readers. Copies are available free of charge to Federal
employees, current contractors and grantees, and nonprofit organizations - as supplies
permit - from the Air Pollution Technical Information Center, Environmental Protection
Agency, Research Triangle Park, North Carolina 27711, or from the National Technical
Information Service, 5285 Port Royal Road, Springfield, Virginia 22151.
Publication No. EPA-450/2-73-003
ii
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CONTENTS
LIST OF FIGURES v
LIST OF TABLES v
ABSTRACT vii
INTRODUCTION 1
1. EMISSION FACTORS FOR GASOLINE-POWERED MOTOR VEHICLES 3
Definitions 3
General Equations . 4
Light-Duty Gasoline-Powered Vehicle Emission Factors 5
Weighted Annual Mileage (m) 6
Weighted Speed Adjustment Factor (s) 7
Heavy-Duty Gasoline-Powered Vehicle Emission Factors. ............. 9
Test Cycle Emission Rate (c) 10
Deterioration Factors (d) 10
Weighted Annual Mileage (m) 12
Weighted Speed Adjustment Factor (s) 16
2. PREDICTION OF GASOLINE-POWERED MOTOR VEHICLE EMISSIONS 19
APPENDIX A. SAMPLE CALCULATION OF GASOLINE MOTOR VEHICLE
EMISSION FACTORS 29
APPENDIX B. GASOLINE-POWERED MOTOR VEHICLE EMISSION
CALCULATION SHEETS 33
REFERENCES 37
ill
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LIST OF FIGURES
Figure Page
1 Average Speed Correction Factor for All Model Years,
Carbon Monoxide 6
Average Speed Correction Factor for All Model Years,
Hydrocarbons . . . ;
3 Average Speed Correction Factor for All Model Years,
Nitrogen Oxides 8
4 Study Areas in Metropolitan Washington, D.C., for Auto
Emissions Model 22
5 Flow Chart of Auto Emissions Model 23
6 Flow Chart of the General Transportation-Air Pollution Model 24
7 Flow Chart of the Argonne Transportation-Air Pollution Model 24
8 Models for Evaluating Alternative Transportation-Related
Pollution Control Strategies 25
9 Sample Isopleth from a Computerized Diffusion Model 27
LIST OF TABLES
Table Page
1 Light-Duty Vehicle Exhaust Emission Factors for Low-Altitude
Cities 9
2 Light-Duty Vehicle Exhaust Emission Factors for California 10
3 Light-Duty Vehicle Exhaust Emission Factors for High-Altitude
Cities 11
4 Carbon Monoxide Deterioration Factors (d) for Light-Duty Gasoline-
Powered Vehicles in All Areas Except California 11
5 Exhaust Hydrocarbon Deterioration Factors (d) for Light-Duty
Gasoline-Powered Vehicles in All Areas Except California 12
6 Nitrogen Oxide Deterioration Factors (d) for Light-Duty Gasoline-
Powered Vehicles in All Areas Except California • 12
7 Carbon Monoxide Deterioration Factors (d) for Light-Duty Gasoline-
Powered Vehicles in California 13
8 Hydrocarbon Deterioration Factors (d) for Light-Duty Gasoline-
Powered Vehicles in California 13
9 Nitrogen Oxides Deterioration Factors for Light-Duty Gasoline-
Powered Vehicles in California 14
10 Sample Calculation of Weighted Annual Travel by Light-Duty
Vehicles 14
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Table Page
11 Light-Duty and Heavy-Duty Vehicle Crankcase and Evaporative
Hydrocarbon Emissions for All Areas Except California 15
12 Light-Duty and Heavy-Duty Vehicle Crankcase and Evaporative
Hydrocarbon Emissions for California '. 15
13 Heavy-Duty Gasoline-Powered Vehicle Exhaust Emission Factors
for All Areas Except High-Altitude and California 15
14 Heavy-Duty Gasoline-Powered Vehicle Exhaust Emission Factors
for High-Altitude Areas ' 16
15 Heavy-Duty Gasoline-Powered Vehicle Exhaust Emission Factors
for California . 16
16 Sample Calculation of Weighted Annual Travel by Heavy-Duty
Vehicles (m) 17
A-l Calculation Sheet for Gasoline Motor Vehicle Exhaust Emission
Factors , 30
A-2 Calculation Sheet for Weighted Speed Adjustment Factor for
Exhaust Emissions • • 31
A-3 Calculation Sheet for Gasoline Motor Vehicle Crankcase and
Evaporative Emission Factors • 31
B-l Calculation Sheet for Gasoline Motor Vehicle Exhaust
Emission Factors 34
B-2 Calculation Sheet for Weighted Speed Adjustment Factor for
Exhaust Emissions - 35
B-3 Calculation Sheet for Gasoline Motor Vehicle Crankcase and
Evaporative Emission Factors 36
Vi
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ABSTRACT
New gasoline-powered motor vehicle (light-duty and heavy-duty) emission factors,
which supersede those in EPA Publication AP-42 (Compilation of Air Pollutant Emission
Factors) for carbon monoxide, hydrocarbons, and nitrogen oxides, are presented based
on a recent nationwide study of over 1,000 automobiles. These factors account for such
variables as the model year, deterioration, and average speed differences. Sample
calculations are included to illustrate the method of obtaining emission factors that
are most representative of a particular region, vehicle mix (age and type), and average
speed.
Methods are given for obtaining estimates of area-wide (region or subregion) emissions
attributed to gasoline-powered vehicles. Local traffic survey data. Department of Trans-
portation studies, and other private surveys are used. The emissions contribution
from other mobile sources is calculated using the values in Compilation of Air Pollutant
Emission Factors. The new emission factors allow a more accurate computation of
air quality whether a proportional ("rollback") or a diffusion model is used. They also
permit calculation of the differences between alternative transportation systems without
determining the absolute values of air quality levels.
These methods have been developed specifically for use by state and local air pollu-
tion control agencies preparing transportation control measures and evaluating
alternatives.
V11
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AN INTERIM REPORT
ON MOTOR VEHICLE EMISSION ESTIMATION
INTRODUCTION
State Implementation Plans submitted on or before January 30, 1972, contained
plans for meeting national primary ambient air quality standards within 3 years after
the date of their approval by the EPA Administrator on May 31, 1972 (40 CFR Part 51).
A review of the plans indicated that many states would not meet national ambient air
quality standards even with expected emission reductions from emission control devices
on late model automobiles (90 percent reduction from 1970 allowable emissions of carbon
monoxide and hydrocarbons by 1975, and 90 percent reduction of 1971-model-year average
emissions of nitrogen oxides by the 1976 model year) . It was recognized that available
data were not sufficient to permit states to develop meaningful transportation control
schemes or to predict the impact of such schemes on air quality. Consequently, the
states were advised to supplement their general transportation control measures, where
required, by detailed strategies with a timetable for compliance and to submit these
plans on or before February 15, 1973.
The air quality levels resulting from the Federal Motor Vehicle Control Program
were approximated by many states that used the normalized pollutant emission rates
and reduction ratios presented in Appendix I to 40 CFR 51. Some states prepared trans-
portation-source emission inventories based on factors and methods from the National
Air Pollution Control Administration, McGraw and Duprey's Compilation of Emission
Factors,2 and more recently, EPA's Compilation of Air Pollutant Emission Factors.3
These factors were all nationwide averages and were used with nationwide estimates
of vehicle age mix and vehicle miles traveled (VMT) . A few large metropolitan areas
such as Washington, D.C. and Chicago were able to show substantially lower gasoline-
powered motor vehicle emissions because the local vehicle-age mix was biased more
toward late-model automobiles (with lower emissions) than was the nationwide age mix.
Such late-model vehicle emission factors developed above were extrapolations from early-
controlled vehicles, and it was believed their absolute values and variation with speed
were subject to error. Thus, a study of nearly 1,000 light-duty vehicles was undertaken
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covering six major U.S. cities.4 A somewhat more limited study is underway for heavy-
duty vehicles.5 Data from these projects form the basis of the revised emission factors
that will allow a more accurate prediction of the contribution of gasoline-powered vehicles
to transportation-related air pollution levels. The emission factors for other mobile
sources should be taken from the latest revision of EPA's Compilation of Emission Factors.3
Several approaches are presented for calculation of motor vehicle emissions that are
based on the availability of mobile source data, i.e., traffic, vehicle age mix, vehicle
miles traveled, average speeds, and miles of roadway types. The use of relatively simple
proportional models (rollback) does not require the detailed data of a sophisticated air
quality diffusion model. Gasoline motor vehicle emission calculation sheets with typical
sample calculations of motor .vehicle emission factors are included in the appendices of
this report for a better understanding of the technique.
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1. EMISSION FACTORS FOR GASOLINE-POWERED MOTOR VEHICLES
Emission factors published by the Federal Government in the past have, as a
general rule, been "average" values based on nationwide statistics. Compilation of Air
Pollutant Emission Factors^ contains emission factor information by calendar year up to and
including 1975. These data were generated by using results of vehicle emission tests on
the 1972 Federal Certification Test Procedure (Federal Register, November 10, 1970) and
by using nationwide statistics for variables such as vehicle distribution by model year
and the fraction of total vehicle miles attributed to heavy-duty vehicles (>6,000 pounds
gross vehicle weight). As the motor vehicle population becomes more highly influenced
by controlled vehicles, the model year mix becomes increasingly important in emission
factor calculation. The general concept of "city specific" emission factors is essential
for accurate assessment of gasoline-motor vehicle emission. For this reason, the technique
outlined below is recommended in lieu of that used to obtain average nationwide emission
factor values.
DEFINITIONS
The following definitions apply to terms that are used in this document:
Light-duty vehicle — any motor vehicle designated primarily for transportation of
property and rated at 6,000 pounds gross vehicle weight (CVW) or less, or
designated primarily for transportation of persons and having a capacity of 12
persons or less.
Heavy-duty vehicle — any motor vehicle designated primarily for transportation of pro-
perty and rated at more than 6,000 pounds GVW or designated primarily for
transportation of persons and having a capacity of more than 12 persons.
1975 Federal Test Procedure — the Federal motor vehicle emission test as described in
the Federal Register, July 2, 1971.
Deterioration factor — the ratio of the pollutant (p) exhaust factor at x miles to the p
exhaust emission factor at 4,000 miles.
Model year mix — the distribution of vehicles registered by model year expressed as a
fraction of the total.
Speed adjustment factor — the ratio of the p exhaust emission factor at speed x to the p
exhaust emission factor as determined by the 1975 Federal Test Procedure (19.6
miles per hour).
Fuel evaporative emissions — vaporized fuel emitted into the atmosphere from the fuel
system of a motor vehicle.
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High-altitude emission rates — substantial changes in emission rates occur in most
gasoline-powered vehicles due to increases in altitude. These changes are
caused by fuel metering enrichment due to atmospheric pressure variations with
altitude. To date, no relationship between mass emissions and altitude has been
developed. Tests have been conducted, however, at near sea level and at
approximately 5,000 feet above sea level. Since the majority of U.S. urban areas
at high altitude are close to 5,000 feet, an arbitrary value at 3,500 feet and above
is used to define high-altitude cities.
Calendar year — a cycle of 365 or 366 days divided into 12 months beginning with January
and ending in December.
Model year — a motor vehicle manufacturer's annual production period. If a manufacturer
has no annual production period the term "model year" means "calendar year."
Crankcase emissions — airborne substance emitted to the atmosphere from any portion of
the crankcase ventilation or lubrication systems of a motor vehicle.
Oxides of nitrogen — the sum of the nitric oxide and nitrogen dioxide contaminants in a
gas sample if the nitric oxide is in the form of nitrogen dioxide.
1972 Federal Test Procedure — the Federal motor vehicle emission test described in the
Federal Register, November 10, 1970.
GENERAL EQUATIONS
The calculation of emission factors for carbon monoxide, hydrocarbons, and
oxides of nitrogen from light- and heavy-duty vehicle exhaust can be expressed mathe-
matically as:
n+1
e = Y^ c. d. m. s.
np i=n-12 lp lpn m lp
where:
Cip -
ipn =
in =
emission factor in grams per vehicle mile for calendar
year n and pollutant p
the 1975 Federal Test Procedure emission rate for pollutant
p (grams/mile) for the i**1 model year, at low mileage
the controlled vehicle pollutant p emission deterioration
factor for the i**1 model year at calendar year n
the weighted annual travel of the i**1 model year during
calendar year n (the determination of this variable
involves the use of the vehicle model year distribution)
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Sjp = the weighted speed adjustment factor for exhaust emission
for pollutant p for i"1 model year vehicles
In addition to exhaust emission factors, the calculation of hydrocarbon gasoline
motor vehicle emissions involves evaporative and crankcase hydrocarbon emission rates.
Evaporation and crankcase emissions can be determined using:
n+1
f = T"1 h. m.
i = £l2 1 m
where:
ifi — the combined evaporative and crankcase hydrocarbon
emission factor for calendar year n
h^ = the combined evaporative and crankcase emission rate
for the ith model year
mjn = the weighted annual travel of the i"1 model year
during calendar year n
LIGHT-DUTY GASOLINE-POWERED VEHICLE EMISSION FACTORS
Figures 1 through 3 and Tables 1 through 12 contain the necessary input data for
the calculation of light-duty, gasoline-powered motor vehicle emission factors.
Test Cycle Emission Rates (c and h)
A recent study of light-duty vehicle exhaust emission rates in six cities resulted in
the data presented in Tables 1 through 3. "* The choice of the six cities was based on
the atmospheric areas of the United States. 6 Statistical analysis of the results of these
tests in six different cities leads to the conclusion that emission rates should be averaged
to cover vehicles in all areas of the United States except those in high-altitude areas
(Denver test results) and 1966-1967 models sold only in California. -The values
presented in Tables 1 through 3 are emission rates for low-mileage (4,000 mile), nondete-
riorated vehicles. The evaporative and crankcase hydrocarbon emission values are
shown in Tables 11 and 12.
Deterioration Factors (d)
Exhaust deterioration factors for emission-controlled vehicles by model year
and pollutant are presented in Tables 4 though 9. Deterioration factors enable the
modification of low-mileage emission rates to account for the aging or deterioration of
exhaust-emission control devices.
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O
LU
CC
O
O
AVERAGE SPEED, mph
Figure 1. Average speed correction factor for all model years, carbon monoxide.7
Weighted Annual Mileage (m)
The determination of m is best illustrated by the example in Table 10. In this
example the model year distribution (in this case nationwide) as of December 31 is com-
bined with annual travel by model year. In the calculation of city-specific emission
factors, the model year distribution for the area under consideration should be obtained
from registration statistics and combined with the annual mileages contained in Table 10,
unless localized annual mileage data are available.
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1
15
30
AVERAGE SPEED, mph
45
60
Figure 2. Average speed correction factor for all model years, hydrocarbons.7
Weighted Speed Adjustment Factor(s)
The weighted speed adjustment factor enables the calculation of a region-wide
emission factor that takes into account variation in average route speed. This variable
is calculated using:
where:
= the weighted speed adjustment factor for exhaust
emission of pollutant p for the i model year during
calendar year m
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cc.
o
u
LU
o:
o
o
60
AVERAGE SPEED, mph
Figure 3. Average speed correction factor for all model years, nitrogen oxides.4
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Table 1. LIGHT-DUTY VEHICLE EXHAUST EMISSION
FACTORS FOR LOW-ALTITUDE CITIES*
Model year
Pre-1968
1968
1969
1970
1971
1972C
1973-74C
1975d
1976 and latere
Exhaust emission factors at low mileage'3
Carbon monoxide
g/mi
87
46
39
36
34
19
19
12.5
1.8
g/km
54
29
24
22
21
12
12
7.8
1.1
Exhaust
hydrocarbons
g/mi
8.8
4.5
4.4
3.6
2.9
2.7
2.7
1.3
0.23
g/km
5.5
2.8
2.7
2.2
1.8
1.7
1.7
0.81
0.14
Nitrogen oxides
g/mi
3.6
4.3
5.5
5.1
4.8
4.8 .
2.3
2.2
0.31
g/km
2.2
2.7
3.4
3.2
3.0
3.0
1.4
1.4
0.19
Excluding California.
References 4, 8-11. It should be noted that pre-1968 results are not
at low mileage but are arithmetic means from tests of a random sample
of vehicles. There is no reason to present low-mileage emission rates
for these vehicles since they are not subject to exhaust control device
deterioration.
Estimates based on the relationship of low-mileage emissions to stand-
ards for 1971 and earlier controlled vehicles.
Based on EPA low-mileage estimates for the 1975 interim standards.
eBased on estimates in Reference 8.
Jm
Vi =
the fraction of the total annual vehicle miles traveled
at speed j during calendar year m
the vehicular average speed correction factor for
average speed j
The carbon monoxide and hydrocarbon speed correction factors are based on
tests of uncontrolled vehicles but are assumed to apply to controlled vehicles also.
The speed correction factors for nitrogen oxides emissions are based on tests of both
controlled and uncontrolled vehicles and are assumed to be a linear relation from the
two test points.
HEAVY-DUTY GASOLINE-POWERED VEHICLE EMISSION FACTORS
Calculation of heavy-duty gasoline-vehicle exhaust and evaporative and crankcase
emission factors is accomplished using the same equations as those for light-duty vehicles.
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Table 2. LIGHT-DUTY VEHICLE EXHAUST EMISSION FACTORS FOR CALIFORNIA
Model year
Pre-1966
1966
1967
1968
1969
1970
1971
1972&
1973-74b
1975C
1976 and laterd
Exhaust emission factors at low mileage3
Carbon monoxide
g/mi
87
51
50
46
39
36
34
19
19
2.8
1.8
g/km
54
32
31
29
24
22
21
12
12
1.7
1.1
Exhaust
hydrocarbons
g/mi
8.8
6.0
4.6
4.5
4.4
3.6
2.9
2.7
2.7
0.33
0.23
g/km
5.5
3.7
2.9
2.8
2.7
2.2
1.8
1.7
1.7
0.20
0.14
Nitrogen oxides
g/mi
3.6
3.4
3.4
4.3
5.5
5.1
3.5
3.5
2.3
1.6
0.31
g/km
2.2
2.1
2.1
2.7
3.4
3.2
2.2
2.2
1.4
1.0
0.19
References 4, 8-11. It should be noted that pre-1968 results are not at
low mileage but are arithmetic means from tests of a random sample of
vehicles. There is no reason to present low-mileage emission rates for
these vehicles since they are not subject to exhaust control device
deterioration.
Estimates based on the relationship of low-mileage emissions to standards
for 1971 and earlier controlled vehicles.
Based on EPA low-mileage estimates for the 1975 emission standards.
Based on estimates in Reference 8.
Test data for heavy-duty vehicles are limited and the calculation of such emission factors
requires the use of variables derived for light-duty vehicles (e.g., s and d) .
Test Cycle Emission Rate (c)
Study of heavy-duty vehicle emission rates has been limited to single geographic
areas. The emission rates presented in Tables 11 through 15 are divided into nationwide,
high altitude, and California. The high altitude and California data are simply modifi-
cations of the nationwide data. High altitude values were calculated using the relationship
between high and low altitude light-duty vehicle emission rates. California emission
rates are simply changes in the nationwide values which reflect the earlier promulgation
of emission standards in California.
Deterioration Factors (d)
With the exception of 1975 and later California models, emission standards for
heavy-duty vehicles have resulted in relatively small emission reductions on a mass
basis. For this reason, the available deterioration factors (for light-duty vehicles)
10
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Table 3. LIGHT-DUTY VEHICLE EXHAUST EMISSION FACTORS
FOR HIGH-ALTITUDE CITIES
Model year
Pre-1968
1968
1969
1970
1971
1972&
1973-74&
1975C
1976 and laterd
Exhaust emission factors at low mileage3
Carbon monoxide
g/mi
130
74
48
72
75
42
42
20
1.8
g/km
81
46
30
45
47
26
26
12
1.1
Hydrocarbons
g/mi
10
6.0
5.4
6.1
5.3
4.9
4.9
1.8
0.23
g/km
6.2
3.7
3.4
3.8
3.3
3.0
3.0
1.1
0.14
Nitrogen oxides
g/mi
1.9
2.2
2.6
2.8
3.1
3.1
1.4
1.8
0.31
g/km
1.2
1.4
1.6
1.7
1.9
1.9
0.87
1.1
0.19
References 4, 8-11. It should be noted that pre-1968 results are not at
low mileage but are arithmetic means from tests of a random sample of
vehicles. There is no reason to present low-mileage emission rates for
these vehicles since they are not subject to exhaust control device
deterioration.
Estimates based on the relationship of low-mileage emissions to standards
for 1971 and earlier controlled vehicles.
"Based on EPA low-mileage estimates for the 1975 emission standards.
Based on estimates in Reference 8.
Table 4. CARBON MONOXIDE DETERIORATION FACTORS (d) FOR LIGHT-DUTY
GASOLINE-POWERED VEHICLES IN ALL AREAS EXCEPT CALIFORNIA8.9
Vehicle age,3 years
Model year
1967 and earlier
1968
1969
1970 through 1974b
1975C
1976 and laterC
1
1.00
1.24
1.42
1.18
1.04
1.34
2
1.00
1.35
1.53
1.32
1.30
1.77
3
1.00
1.41
1.59
1.38
1.36
2.14
4
1.00
1.47
1.63
1.40
1.43
2.42
5
1.00
1.53
1.68
1.44
1.44
2.73
6
1.00
1.58
1.71
1.47
1.49
2.99
7
1.00
1.63
1.75
1.50
1.56
3.26
8
1.00
1.67
1.79
1.51
1.63
3.48
9 and
older
1.00
1.72
1.82
1.56
1.69
3.77
a"0"-year-old vehicles have a deterioration factor of 1.00.
bBased on test results for 1970 model year vehicles.
GBased on Reference 8 and EPA estimates.
11
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Table 5. EXHAUST HYDROCARBON DETERIORATION FACTORS (d) FOR LIGHT-DUTY
GASOLINE-POWERED VEHICLES IN ALL AREAS EXCEPT CALIFORNIA®^
Model year
1967 and earlier
1968
1969
1970 through 1974b
1975C
1976 and laterc
Vehicle age,a years
1
1.00
1.12
1.10
1.05
1.00
1.45
2
1.00
1.18
1.16
1.10
1.13
1.95
3
1.00
1.21
1.18
1.13
1.22
2.40
4
1.00
1.23
1.21
1.15
1.29
2.76
5
1.00
1.26
1.23
1.17
1.37
3.14
6
1.00
1.28
1.25
1.20
1.43
3.46
7
1.00
1.30
1.28
1.22
1.50
3.79
8
1.00
1.32
1.29
1.24
1.56
4.07
9 and
older
1.00
1.35
1.31
1.26
1.63
4.42
"0"-year-old vehicles have a deterioration factor of 1.00.
DBased on test results for 1970 model year vehicles.
"Based on Reference 8 and EPA estimates.
Table 6. NITROGEN OXIDE DETERIORATION FACTORS (d) FOR LIGHT-DUTY
GASOLINE-POWERED VEHICLES IN ALL AREAS EXCEPT CALIFORNIA8,9
Model year
1972 and earlier
1973 through 1974b
1975
1976 and laterc
Vehicle age,a years
1
1.00
1.11
1.00
1.34
2
1.00
1.18
1.18
1.77
3
1.00
1.20
1.23
2.14
4
1.00
1.21
1.23
2.42
5
1.00
1.22
1.41
2.73
6
1.00
1.23
1.45
2.99
7
1.00
1.24
1.45
3.26
8
1.00
1.25
1.45
3.48
9 and
older
1.00
1.26
1.45
3.77
a,,
0"-year-old vehicles have a deterioration factor of 1.00.
DBased on test results for 1971 (California) model year vehicles.
'Based on Reference 8 and EPA estimates.
should only be used for 1975 and later California heavy-duty vehicles. A deterioration
factor of 1.00 should be assumed for all non-California heavy-duty vehicles regardless of
model year and for all pre-1975 California heavy-duty vehicles. It is recommended that
1968 CO and HC deterioration factors be applied to 1975 and later California heavy-duty
vehicles.
Weighted Annual Mileage (m)
The determination of this variable is illustrated in Table 16. For this example,
the nationwide model year distribution of heavy-duty vehicles is used.
12
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Table 7., CARBON MONOXIDE DETERIORATION FACTORS (d) FOR LIGHT-DUTY
GASOLINE-POWERED VEHICLES IN CALIFORNIA8.9
Model year
1965 and earlier
1966
1967
1968
1969
1970 through 1974b
1975C
1976 and later0
Vehicle age, a years
1
1.00
1.13
1.11
1.24
1.42
1.18
1.34
1.34
2
1.00
1.21
1.18
1.35
1.53
1.32
1.77
1.77
3
1.00
1.24
1.23
1.41
1.59
1.38
2.14
2.14
4
1.00
1.25
1.29
1.47
1.63
1.40
2.42
2.42
5
1.00
1.28
1.35
1.53
1.68
1.44
2.73
2.73
6
1.00
1.29
1.40
1.58
1.71
1.47
2.99
2.99
7
1.00
1.31
1.46
1.63
1.75
1.50
3.26
3.26
8
1.00
1.32
1.50
1.67
1.79
1.51
3.48
3.48
9 and
older
1.00
1.34
1.56
1.72
1.82
1.56
3.77
3.77
a,,
"0"-year-old vehicles have a deterioration factor of 1.00.
3Based on test results for 1970 model year vehicles.
"Based on Reference 8.
Table 8. HYDROCARBON DETERIORATION FACTORS (d) FOR LIGHT-DUTY
GASOLINE-POWERED VEHICLES IN CALIFORNIA®^
Model year
1965 and earlier
1966
1967
1968
1969
1970 through 1974&
1975C
1976 and laterc
Vehicle age,3 years
1
1.00
1.14
1.07
1.12
1.10
1.05
1.45
1.45
2
1.00
1.22
1.10
1.18
1.16
1.10
1.95
1.95
3
1.00
1.25
1.12
1.21
1.18
1.13
2.40
2.40
4
1.00
1.27
1.14
1.23
1.21
1.15
2.76
2.76
5
1.00
1.29
1.15
1.26
1.23
1.17
3.14
3.14
6
1.00
1.30
1.17
1.28
1.25
1.20
3.46
3.46
7
1.00
1.32
1.18
1.30
1.28
1.22
3.79
3.79
8
1.00
1.33
1.20
1.32
1.29
1.24
4.07
4.07
9 and
older
1.00
1.35
1.21
1.35
1.31
1.26
4.42
4.42
a"0"-year-old vehicles
Based on test results
cBased on Reference 8.
have a deterioration factor of 1.00.
for 1970 model year vehicles.
13
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Table 9. NITROGEN OXIDES DETERIORATION FACTORS FOR LIGHT-DUTY
GASOLINE-POWERED VEHICLES IN CALIFORNIA8.9
Model year
1970 and earlier
1971 through 1975&
1976 and laterC
Vehicle age,a years
1
1.00
1.11
1.34
2
1.00
1.18
1.77
3
1.00
1.20
2.14
4
1.00
1.21
2.42
5
1.00
1.22
2.73
6
1.00
1.23
2.99
7
1.00
1.24
3.26
8
1.00
1.25
3.48
9 and
older
1.00
1.26
3.77
a"0"-year-old vehicles have a deterioration factor of 1.00.
Based on test results for 1971 model year vehicles.
cBased on Reference 8.
Table 10. SAMPLE CALCULATION OF WEIGHTED ANNUAL TRAVEL
BY LIGHT-DUTY VEHICLES (m)12,13
Age,
years
Ob
1
2
3
4
5
6
7
8
9
10
11
12
13 and older
Nationwide fraction
of vehicles in use
on Dec. 31 (a)
0.038
0.068
0.117
0.111
0.098
0.106
0.105
0.087
0.076
0.059
0.036
0.029
0.016
0.054
Average miles
driven (b)
3,600C
11.900C
16,100
13,200
11,400
11,700
10,000
10,300
8,600
10,900
8,000
6,500
6,500
6,500
a x b
137
809
1,880
1,465
1,120
1,240
1,050
896
654
643
288
188
104
351
ma
0.013
0.075
0.174
0.135
0.103
0.115
0.097
0.083
0.060
0.059
0.027
0.017
0.010
0.032
am =
b,,
a x b
a x b
0" refers to next year's models introduced in the fall.
°This number reflects the fact that some vehicles in this
have been on the road less than 12 months.
age category
14
-------
Table 11. LIGHT-DUTY AND HEAVY-DUTY VEHICLE CRANKCASE AND EVAPORATIVE
HYDROCARBON EMISSIONS FOR ALL AREAS EXCEPT CALIFORNIA14
Model
year
Pre-1963
1963 through 1967
1968 through 1970
1971
1972
1973 and older
LDV hydrocarbons
g/tni
7.1
3.8
3.0
0.5
0.2
0.2
g/km
4.4
2.4
1.9
0.3
0.1
0.1
HDV hydrocarbons
g/mi
8.2
8.2
8.2
3.0
3.0
3.0
g/km
5.1
5.1
5.1
1.9
1.9
1.9
Table 12. LIGHT-DUTY AND HEAVY-DUTY VEHICLE CRANKCASE AND EVAPORATIVE
HYDROCARBON EMISSIONS FOR CALIFORNIA™
Model
year
Pre-1961
1961 through 1963
1964 through 1967
1968 through 1969
1970 through 1971
1972
1973 and older
LDV hydrocarbons
g/mi
7.1
3.8
3.0
3.0
0.5
0.2
0.2
g/km
4.4
2.4
1.9
1.9
0.3
0.1
0.1
HDV hydrocarbons
g/mi
8.2
8.2
8.2
3.0
3.0
3.0
0.2
g/km
5.1
5.1
5.1
1.9
1.9
1.9
0.1
Table 13. HEAVY-DUTY GASOLINE-POWERED VEHICLE EXHAUST EMISSION FACTORS
FOR ALL AREAS EXCEPT HIGH-ALTITUDE AND CALIFORNIA
Model year
Pre- 19703
1970 through 1973b
1974 and laterc
Carbon monoxide
g/mi
140
130
130
g/km
87
81
81
Exhaust
hydrocarbons
L 9/m1
17
16
13
g/km
11
9.9
7.9
Nitrogen oxide
g/mi
9.4
9.2
9.2
g/km
5.8
5.7
5.7
aData extracted from References 5 and 14.
Data from Reference 14.
Calculated from information contained in Reference 14. These are low-mileage
emission rates.
15
-------
Table 14. HEAVY-DUTY GASOLINE-POWERED VEHICLE EXHAUST EMISSION FACTORS
FOR HIGH-ALTITUDE AREAS3
Model year
Pre-1 970&
1970 through 1973C
1974 and laterd
Carbon monoxide
g/mi
210
190
190
g/km
130
120
120
Exhaust
hydrocarbons
g/mi
19
18
15
g/km
12
11
9.3
Nitrogen oxide
g/mi
5.0
4.9
4.9
g/km
3.1
3.0
3.0
Based on the light-duty vehicle emissions at high altitude compared with
light-duty vehicle emissions at low altitude.
"'Data extracted from References 5 and 14.
"Data from Reference 14.
Calculated from information contained in Reference 14.
test results.
These are low-mileage
Table 15. HEAVY-DUTY GASOLINE-POWERED VEHICLE EXHAUST EMISSION
FACTORS FOR CALIFORNIA
Model year
Pre-igyoa
1970 through 1971&
1972C
1973 through 1974d
1975C
Carbon monoxide
g/mi
140
130
130
130
81
g/km
87
81
81
81
50
Exhaust
hydrocarbons
g/mi
17
16
13
13
4.1
g/km
11
9.9
8.1
8.1
2.5
Nitrogen oxide
g/mi
9.4
9.2
9.2
9.2
2.8
g/km
5.8
5.7
5.7
5.7
1.7
Data extracted from References 5 and 14.
bData from Reference 14.
c
Based on applicable emission standards.
Calculated from information contained in Reference 14. These are low-mileage
emission rates.
Weighted Speed Adjustment Factor (s)
Again, data based on actual heavy-duty vehicles are not available. Light-duty
vehicle speed correction factors can be used as an approximation for all heavy-duty
vehicles, regardless of model year. The calculation of the weighted speed correction
factor is accomplished using the same technique as that for light-duty vehicles.
16
-------
Table 16. SAMPLE CALCULATION OF WEIGHTED ANNUAL TRAVEL
BY HEAVY-DUTY VEHICLES (m)15,16
Age,
years
0
1
2
3
4
5
6
7
8
9
10
11
12
13 and older
Nationwide fraction
of vehicles in use •
on Dec. 31 (a)
0.011
0.090
0.105
0.085
0.080
0.083
0.075
0.064
0.054
0.045
0.034
0.036
0.031
0.207
Annual miles
driven (b)
3,500
11,700
17,200
15,800
15,800
13,000
13,000
11,000
11,000
9,000
9,000
5,500
5,500
5,500
a x b
38
1,050
1,810
1,340
1,260
1,080
975
704
594
405
306
198
170
1,138
ma
0.003
0.095
0.164
0.121
0.114
0.097
0.088
0.064
0.054
0.036
0.028
0.018
0.015
0.103
a x b
£axb
17
-------
2. PREDICTION OF GASOLINE-POWERED
MOTOR VEHICLE EMISSIONS
Appendix I to 36 FR 22412 describes a method that can be used to approximate
regional air quality levels resulting from the Federal motor vehicle control program.
A simple equation and normalized emission rates for hydrocarbons, carbon monoxide,
and nitrogen oxides are used to calculate expected air quality concentrations for future
years. Calculations are based on emissions data for all sources in the base year and
the future years, the growth factor for stationary source emissions, and the base line
air quality (measured or estimated) in the region. This method cannot be applied to
determine the reduction in photochemical oxidant levels resulting from reductions in
hydrocarbon emissions. Consequently, Appendix J to 36 FR 22412 is to be used to estimate
the percent reduction in hydrocarbon emissions required to achieve the national ambient
air quality standard for photochemical oxidants.
The revised emission factors presented earlier do not invalidate Appendix I
to 36 FR 22412 when it is used to determine normalized emission rates of hydrocarbons,
carbon monoxide, and nitrogen dioxide on a national basis. State Implementation Plans
submitted in accordance with 40 CFR part 51 on or before January 30, 1972, generally
used this method for estimating whether transportation control measures were required
and for estimating the percent reduction in any pollutant concentration required to meet
national ambient air quality standards. However, those metropolitan areas in air quality
regions requiring such control measures may have air quality and traffic data and technical
capability available that warrant a more accurate prediction of air quality levels and
evaluation of transportation control measures to effect the desired reduction in national
ambient air quality levels. *•'
The basic method for predicting total motor vehicle emissions is to multiply emission
factors, modified to represent on-the-road emission rates, by the vehicle miles of travel
(VMT). The former National Air Pollution Control Administration used a similar simplified
approach for estimating nationwide vehicle emissions.1 Two types of vehicle operating
conditions were assumed, urban and rural. All urban travel was assumed to be at an
average speed of 25 miles per hour beginning from a "cold start"; i.e., the vehicle
was assumed not to have been driven prior to beginning travel at the urban driving
speed. All rural travel was assumed to be at an average speed of 45 miles per hour,
beginning from a "hot start." In this case, the vehicle was assumed to have been operated
before being driven at the rural speed. The emission factors were then adjusted for
these average speeds. A further seasonal adjustment was made. No correction was
19
-------
made for altitude. It should be noted that the emission factors presented in Chapter
1 have been tabulated according to areas where the tests were made, e.g., California,
low-altitude (excluding California) , and high-altitude (above 3,500 feet) . The national
miles of travel for passenger cars, trucks, and buses were taken from Highway
Statistics. *-° The future projections of national vehicle miles of travel were estimated from
the "medium" projections presented in Resources in America's Future. •*•" It should
be noted that forecasts made prior to the 1970 census assumed a higher growth rate than
is now occurring. ^O The total VMT were divided into passenger car and truck miles and
further into rural and urban driving, according to the assumed weighting. The national
emissions for each pollutant were then obtained by multiplying these vehicle miles
traveled by the appropriate emission factor.
Subsequently, a method that modified this approach on a regional basis was develop-
ed from appropriate emission factors and the motor vehicle population and driving pattern
for the particular regional
Total vehicle travel can be determined from regional transportation studies, local
traffic surveys, U .S . Department of Transportation data, and Federal Highway Administra-
tion publications. 13,22, 23 The statistics available from Highway Statistics-*-" include,
by state, miles of public roads and streets, average daily traffic loads, number and
type of vehicles registered, and estimated motor vehicle (passenger and truck) travel
by highway system. Also included are motor fuel consumption and speed trends by
roadway type and vehicle type. Highway Statistics-^" assumed that since the total
emissions from all gasoline-powered motor vehicles in a region is a function of the vehicle
emission factors, the vehicle miles traveled in the region, and the percent travel that
is urban or rural, a proportional relationship could be made to obtain regional emissions
from the average national emissions. This assumption is valid if the regional vehicle
mix of ages, types, makes, and deterioration rates, as well as the percentages of road
types, average speeds, and miles of travel, are the same as the national average.
The following equations were presented to obtain regional emission estimates: 24
TE = UE + RE
VE = UF-VMT-a'k
RE = RF-VMT'1-a-k
where:
TE = Total emission of a pollutant, tons/year
UE = Urban emission of a pollutant, tons/year
RE = Rural emission of a pollutant, tons/year
UF = Urban emission factor, g/mile
RF = Rural emission factor, g/mile
VMT = Vehicle miles of travel
20
-------
a = fraction of travel that is urban
k = 1.1023 x 10~6 ton/g (conversion factor)
The emission factors (for both cars and trucks) needed for the calculations were
presented for each pollutant (hydrocarbons, carbon monoxide, and oxides of nitrogen)
by year and by urban and rural driving. The VMT and the fraction of urban travel
were to be obtained from local traffic studies or assumed. (Vehicle miles of travel
and projections for future years for most cities are available as a result of the Federal
Aid Highway Act of 1962, which required cities with populations over 50,000 to initiate
transportation studies in order to qualify for federal aid for road construction.)
This simplified method could be updated by using the latest emission factors
presented in Chapter 1 of this report and compiled as shown in Tables B-l, B-2, and B-
3 for the vehicle age mix of the region or the age mix of the nation. A sample calculation
for a metropolitan area is shown in Appendix A. Adjustment of the emission factor
for the speeds of the roadway type, with the miles of the roadway type, vehicle type
(light-duty or heavy-duty), and the respective vehicle miles traveled would provide
an even more accurate calculation of emissions.
The accuracy of the gasoline-powered motor vehicle emissions prediction is not
only dependent on the emission factors, it is also very sensitive to traffic data (vehicle
type and age mix, miles of roadway type, average speed) that are best developed by
local traffic surveys. Where air quality levels are developed by a proportional or roll-
back model, data must be obtained on at least a county-wide basis." The use of a
dispersion model requires that the data be developed on a grid basis.25 The grid size
is dependent on the sophistication of the calculation; # grid cells down to 1 mile or 1
kilometer are general. The Chicago area transportation study2° used traffic zones
in the study area that varied from 0.25 square mile in the central business district
to 36 square miles in the outlying areas. A study of Washington, B.C.27 divided the
metropolitan area into 48 irregular subareas that were smaller in the central business
district than in the suburbs (Figure 4). This unique approach to the evaluation of trans-
portation alternatives provided a method for estimating emissions from transportation
data without trip distribution and traffic assignment models. The method makes use of
vehicle trip forecasts along with highway network information to estimate future travel,
the speeds at which this travel will occur, and the emission levels produced. A simplified
flow chart of the method is shown in Figure 5. It should be noted that this method does
not directly provide air quality level forecasts, but it can be particularly useful in a
gross evaluation of transportation system alternatives.
A typical approach to the calculation of air quality levels from a general transporta-
tion model is shown in the flow chart in Figure 6. A further refinement, shown in the
flow chart in Figure 7, was done by the Argonne National Laboratory in a study for the
City of Chicago.26 This study also developed the sensitivity of relating emissions to local
21
-------
27
Figure 4. Study areas in metropolitan Washington, D.C. for auto emissions model.
vehicle registration data (age mix) and speed adjustment. The emission estimates for
27
Washington, D.C. , showed a substantial reduction, up to 25 percent, in carbon monoxide
estimates when the local vehicle age mix was used in place of the national average. The
larger number of late model automobiles with lower emissions that are driven proportionally
more than older ones caused the reduction.
EPA's Guide for Compiling a Comprehensive Emission Inventory28 provides detailed
procedures for preparing stationary and mobile source emissions inventories. The section
on gasoline-powered motor vehicles is particularly useful. Although this approach requires
automatic data processing equipment, it will produce a uniform format that allows ready
comparison with other regions and that is compatible with stationary source data now
being accumulated. This approach is recommended because of its relationship to EPA's
National Environmental Emissions Data System. If the Guide^ is used, the gasoline-
powered vehicle emission factors in Chapter 1 of this report should be used until Compila-
tion of Air Pollutant Emission Factors^ is revised to include them.
22
-------
DATA REQUIREMENTS
SOCIO-ECONOMIC FORECASTS'
HOUSEHOLD INCOME
DISTRIBUTION OF POPULATION
AND EMPLOYMENT
IPHASE 1
ITRIP GENERATION MODEL
IPHASE 2 I PHASE 3
iTRAVE LID ESCRIPTI ON MOD EL (POLLUTANT EMISSIONS MODEL
TRANSIT SYSTEM
ALTERNATIVE
HIGHWAY SYSTEM
ALTERNATIVE
ENVIRONMENTAL PROTECTION
AGENCY EMISSIONS FACTORS
/ VEHICLE
ORIGINS
HIGHWAY NEEDS MODEL
HICLE \
VEHICLE
MILES OF
TRAVEL AND
SPEED BY
FACILITY TYPE
EMISSION MODEL
EMISSIONS
OF
CARBON
MONOXIDE
EMISSIONS
OF
OXIDES OF
NITROGEN
Figure 5. Flow chart of auto emissions model.
ro
CO
-------
Figure 6. Flow chart of the general transportation-air pollution model.
REGIONAL
TRANSPORTATION
SYSTEM
TRAFFIC
SIMULATION
ORIGIN - DESTINATION
TRIP DATA
(GRIDDED)
SPEED DATA
BY ROADWAY
TYPE (GRIDDED)
VEHICLE-MILE
DATA
• BY ROADWAY
TYPE (GRIDDED)
EMISSION
FACTORS
BY AGE
VEHICLE
POPULATION
CHARACTERISTICS
(AGES, TYPES, ETC.)
DETERIORATION
RATES
GEOGRAPHICAL
CHARACTERISTICS
AFFECTING EMISSIONS
(CBD, EXPRESSWAYS, ETC.
Figure 7. Flow chart of the Argonne transportation-air pollution model.26
24
-------
Where a detailed transportation emissions inventory, air quality data, and the
necessary technical expertise are all available, rather sophisticated evaluations of
transportation control and highway system alternatives are possible. Such an approach
was developed by the Argonne National Laboratory and is summarized in a flow chart
(Figure 8). 29 This approach requires the detailed gasoline-powered motor vehicle
emission factors developed in Chapter 1 of this report.
TRANSPORTATION
DEMAND MODEL
i
CWAI HATP
FEDERAL
STANDARDS
1
FORMULATE
CANDIDATE
CONTROL
STRATEGIES
VEHICLE
ACTIVITY
MODELS
l| CBD
}1 EXPRESSWAY
1 REGIONAL
,__- 1--_.
•
1
1
,
-'
MODEL
(PROJECTED
ACTIVITIES)
ACQUIRE TRANSPORTATION
DATA (VEHICLE TYPE
AND AGE DISTRIBUTION,
MASS TRANSIT, ETC.)
EVALUATION
PROCEDURE
(RANKING
ALTERNATIVES)
AIR QUALITY
MODELS
Figure 8. Models for evaluating alternative transportation-related pollution control strategies.29
The use of a computerized diffusion model to calculate air quality levels requires
that the transportation data be obtained or assumed on a grid network as small as 1
mile or 1 kilometer square. A simplified description of the steps used in the Six Cities
Transportation Study^4 is quoted below:
Step 1 - Assignment of VMT and speed to elements of grid network. This required
superimposing a rectangular grid network consisting of 1-mile squares
or 1-kilometer squares over a base map of the metropolitan area and sum-
mation of VMT from each of the individual roadways which may fall within
one of the small grids to obtain the total VMT for each element or small grid
in the grid network. The speed for each grid of the grid network is obtain-
ed by averaging the speeds from each element or roadway within the grid
for each time period required, using the VMT along each element of road-
way as a weighting factor. In this manner the many vehicular sources
moving within an individual grid can be represented by a single stationary
source, which produces the same amount of emissions, equal to the size of
25
-------
the individual grid. This equivalent source is called an area source
because the emissions from the grid or area source are now considered as
evenly distributed or evenly produced over the entire area of the individual
grid.
Step 2 - Use of vehicle emission factors (adjusted for vehicle speed and type) to
calculate emissions on a per-grid basis.
Step 3 - Conversion of emission rates to pollutant concentration on a per-grid basis
by the method of Gifford and Hanna. •*•'
Step 4 - Application of transportation control strategies to the data base to obtain
predicted concentration patterns for each control strategy for the year 1977.
The resulting concentrations are presented graphically as isopleths over a map
of the metropolitan area. One of the isopleths for a particular strategy is shown in
Figure 9. A study of this complex-makes full use of the revised emission factors,
particularly when subregional measures are applied for air pollution control.
In summary, it is emphasized that the gasoline-powered motor vehicle emissions
prediction is no better than the poorest of the many factors that comprise the calculation.
Further, although highway system alternatives and transportation control alternatives
can be evaluated in terms of air quality levels, there is much economy in time and money
in initially evaluating total emissions. The most promising transportation control measures
being considered can then be selected for detailed study by diffusion modeling.
26
-------
Isopleth No. D.C.30
City Washington.D.C. Year 1977 Strategy 3
Pollutant CO Case 4 Time Period 8 Hour Total
Units mg/m3x103 Federal Standard 10 mg/m3
Figure 9. Sample isopleth from a computerized diffusion model.30
27
-------
APPENDIX A.
SAMPLE CALCULATION
OF
GASOLINE-POWERED MOTOR VEHICLE EMISSION FACTORS
29
-------
Table A-l.
Pollutant
CALCULATION SHEET FOR GASOLINE MOTOR VEHICLE
EXHAUST EMISSION FACTORS
HYDROCARBONS
Calendar year
Vehicle weight class
[II HDV C2 LDV
1970 Metropolitan area SAMPLE
Model
year
1971
1970
1969
1968
1967
1966
1965
1964
1963
1962
1961
1960
1959
1958
Ci
2.9
3.6
4.4
4.5
8.8
8.8
8.8
8.8
8.8
8.8
8.8
8.8
8.8
8.8
di
1.00
1.05
1.16
1.21
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
mia
0.013
0.075
0.174
0.135
0.103
0.115
0.097
0.083
0.060
0.059
0.027
0.017
0.010
0.032
Si
0.79
0.79
0.79
0.79
0.79
0.79
0.79
0.79
0.79
0.79
0.79
0.79
0.79
0.79
cidimiSi
0.03
0.22
0.70
0.58
0.72
0.80
0.67
0.58
0.42
0.41
0.19
•0.12
0.07
0.22
5.85 g/mi
See Table 14 for sample calculation.
30
-------
Table A-2. CALCULATION SHEET FOR WEIGHTED SPEED
ADJUSTMENT FACTOR FOR EXHAUST EMISSIONS
Pollutant HYDROCARBONS
Metropolitan area SAMPLE
Model year(s) ALL
Average speed (j),a
miles/hour
20
30
40
50
60
-------
APPENDIX B.
GASOLINE-POWERED MOTOR VEHICLE
EMISSION CALCULATION SHEETS
33
-------
Table B-l. CALCULATION SHEET FOR GASOLINE MOTOR VEHICLE
EXHAUST EMISSION FACTORS
Pollutant
Vehicle weight class
CH LDV
Calendar year.
I IHDV
Metropolitan area
Model year
si
g/mi
See Tables 14 and 20 for sample calculations of m-j.
34
-------
Table B-2. CALCULATION SHEET FOR WEIGHTED SPEED
ADJUSTMENT FACTOR FOR EXHAUST EMISSIONS
Pollutant
Metropolitan area
Model year(s)
Average speed (j)
fJvd
35
-------
Table B-3. CALCULATION SHEET FOR GASOLINE MOTOR VEHICLE
CRANKCASE AND EVAPORATIVE EMISSION FACTORS
Calendar year
Vehicle weight class:
HDV
Pollutant
LDV
HYDROCARBONS
Model year
hi
himi
36
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Pollution Control Administration, Durham, N.C., April 1970, 21 p.
2. McGraw, M.J. and R.L. Duprey. Compilation of Air Pollutant Emission Factors
(Preliminary Report) . U.S. Environmental Protection Agency, Office of Air Programs.
Research Triangle Park, N.C. Publication No. AP-42. April 1971.
3. Compilation of Air Pollutant Emission Factors (Revised). U. S. Environmental Pro-
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No. AP-42. February 1972. p. 3-2.
4. Study of Emissions from Light-Duty Vehicles in Six Cities. Preliminary Report. Auto-
motive Environmental Systems, Inc. Los Angeles, California. Prepared under Con-
tract No. 68-04-0042. July 1972. p. 4-161.
5. Surveillance Study of Control Equipped Heavy-Duty Gasoline-Powered Vehicles - South-
western USA. Southwest Research Institute. San Antonio, Texas . Prepared under
Contract No. EHS 70-113. Oct. 1972. p. 49.
6. Definition of Atmospheric Areas. Federal Register. 3_3(10): 548, January 16,1968.
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Cities at Two Different Altitudes. U.S . DHEW, Public Health Service. Cincinnati,
Ohio. Presented at APCA Annual Meeting. Paper No. 65-22.
8. Semiannual Report by the Committee on Motor Vehicle Emissions of the National
Academy of Sciences to the Environmental Protection Agency. National Academy of
Sciences. Washington, D.C. Prepared under Contract No. 68-01-0402. p. 78.
Jan. 1972.
9. Hocker, A.J. Exhaust Emissions from Privately Owned 1966-1970 California Automo-
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