CR-2-191
MORNING VEHICLE-START EFFECTS ON PHOTOCHEMICAL SMOG
June 1971
J. R. Martinez
R. A. Nordsieck
A. Q.' Eschenroeder
Prepared for
Environmental Protection Agency
Air Pollution Control Office
Under Contract No. EHSD 71-22
GENERAL
RESEARCH W CORPORATION
-------
In addition to approval by the Project Leader
and Department Head, General Research Corporation
reports are subject to independent review by
a staff member not connected with the project.
This report was reviewed by J. R. Brennand, Jr.
The work upon which this publication is based was
pursuant to Contract No. EHSD 71-22 with the National Air
Pollution Control Administration, Environmental Health Service,
Public Health Service, Department of Health, Education and
-------
CR-2-191
MORNING VEHICLE-START EFFECTS ON PHOTOCHEMICAL SMOG
Contract No. EHSD-71-22
June 1971
J. R. Martinez
R. A. Nordsieck
-------
ABSTRACT
The influence of cold-start vehicle emissions on air quality is
investigated using the General Research Corporation photochemical/
diffusion model. Both the time and space distribution of cold starts
are examined. A day from an October 1968 Los Angeles smog episode serves
as a baseline for determining diffusion coefficients, nitrogen balance,
and hydrocarbon reactivities. Vehicular and stationary sources for 1968,
1971, 1974 and 1980 are emission inputs, and pollutant concentrations at
the ground are air quality outputs. Stagnant central basin conditions
govern the time phasing studies.
Emissions introduced during the starting process have the greatest
effect of all on carbon monoxide peaks, the effect being to increase the
peak CO concentration from 9 to 13 percent. The levels of ozone and
nitrogen dioxide that build up later in the day are influenced less be-
cause chemical processes afford dilution time. Thus for ozone the increase
in concentration due to morning emissions ranges from 1 to 7 percent and
for nitrogen dioxide the range is from 0 to 2 percent. Typical west-to-
east morning air movement forms a background for the geographical distri-
bution studies. If vehicle starts are decentralized geographically, only
a slight increase in the pollutant loading is noted because of the low
morning wind speeds. This increase is so small as to be insignificant
compared with the starting emission portion of the total emissions.
Based on the results of the study, three possible weighting schemes are
proposed for combining cold-start and hot-start driving cycle measurements,
-------
CONTENTS
SEC. PAGE
ABSTRACT i
I INTRODUCTION 1
A. Background of the Problem 1
B. Objectives of This Work 2
C. Study Design 3
II SOURCE EMISSION DISTRIBUTION 4
A. Motor Vehicle Emissions 4
B. Stationary Source Emissions 12
C. Input Generation for Photochemical Model 13
III CHOICES OF CHEMICAL MODEL AND BASELINE CASE 16
A. Incorporation of N0_ in Kinetics 16
B. Cases Selected for Atmospheric Modeling 19
C. Establishment of Baseline Case 20
IV TEST RESULTS FOR DETERMINING COLD-START EFFECTS 29
A. Results Under Stagnant Atmospheric Conditions 29
B. Results of Cross-Basin Trajectory Analysis 31
V CONCLUDING REMARKS 34
A. Interpretation of Modeling Results 34
B. Some Alternatives for Weighting Factors to Combine
Cold-Start and Hot-Start Cycle Test Results 35
REFERENCES 41
-------
ILLUSTRATIONS
NO. PAGE
1 Geographical Distribution of Freeway Traffic in the
Los Angeles Basin Area 10
2 Los Angeles Traffic/Time Distributions 11
3 Estimated Growth of Stationary Source NO Emissions 13
4 Cross-Basin Air Trajectory 15
5 Computed Concentrations of Propylene, Nitric Oxide,
and Nitrogen Dioxide Compared with Experimental Values 18
6 Computed Concentrations of Ozone and PAN Compared with
Experimental Values 18
7 Influence of Cold Start and Diffusion Coefficient on
Carbon Monoxide Buildup 22
8 Ground Level Nitric Oxide Concentration on October 23,
1968 in the Central Los Angeles Basin 25
9 Ground Level N02 Concentration on October 23, 1968 in
the Central Los Angeles Basin 26
10 Ground Level Ozone Concentration on October 23, 1968 in
the Central Los Angeles Basin 27
11 Ground Level Reactive Hydrocarbon Concentration on
October 23, 1968 in the Central Los Angeles Basin 28
-------
TABLES
NO. PAGE
1 Federal Cycle Gram-Per-Mile Emissions Based on Measure-
ments 5
2 Equivalent California and Federal Emission Standards 5
3 Hot-Running and Cold-Start Emissions by Model Year 7
4 Vehicle Age and Usage Distribution at Close of
Model Year 8
5 Average Vehicle Emission Factors 9
6 Chemical Kinetic Model for Hydrocarbon/Nitric Oxide
Mechanism 17
7 Emission Effects 29
8 Air Quality Effects 30
9 Carbon Monoxide Effects 30
10 Emission Effects for 1974 Trajectory 31
11 Air Quality Effects for 1974 Trajectory 32
12 Carbon Monoxide Effects for 1974 Trajectory 33
13 Weighting Factors for Air Quality Method for 1971 37
14 Weighting Factors for Vehicle-Activities and Emissions
Methods 38
15 Average 1971 Vehicle Emission Factors Adjusted to
Account for Morning Cold-Start Effects 39
-------
-------
Sec. I A
I. INTRODUCTION
A. BACKGROUND OF THE PROBLEM
Time-phasing of pollutant emissions might exert a powerful influence
on air quality in the case of photochemical smog. During the morning
hours this is possible because on most days a period of atmospheric sta-
bility and stagnation is followed by a rising trend of solar radiation
input.
Many of the analyses of motor vehicle air pollution contributions
allocate emissions in direct proportion to car-miles per hour or traffic
intensities. However, suboptimum fuel mixture conditions during the
vehicle-start phase increase some of the emission rates above those for
normal operating activities. The increases are larger for those starts
occurring when the vehicle engine is not warm. Therefore, the morning
starting contributions are especially significant since most of the
vehicle population is subjected to startup (about 90% of them between
6 and 9 A.M. in Los Angele:
hours preceding the start.
6 and 9 A.M. in Los Angeles ) and most of them have been idle for several
This becomes a key issue in the design of control system certifi-
cation tests because the cold start is a relatively infrequent phenomenon
and running emissions are continuously distributed. Because of the nature
of the morning time-phasing, it may be that a uniform proration of start-
ing emissions over the whole day's activities is not an adequate criterion
for evaluating a control system. Consequently an analysis of the time-
phasing effects is necessary to assess the cause/effect relationship be-
tween start-up contributions and air quality. Then the results of the
analysis must be translated into a weighting scheme for combining the
results of cold-start cycle and hot-start cycle tests for evaluating the
-------
Sec. I B
*
B. OBJECTIVES OF THIS WORK
In this report, the relative importance of morning cold-start emis-
sions is assessed for those effects associated with photochemical smog.
A computer simulation model is used with aerometric data from the Los
Angeles basin to study the buildup of air pollution as it is affected by
starting emissions. The procedure relates meteorological factors, time/
space traffic distributions, and ultraviolet solar radiation with the
photochemical atmospheric mechanisms involved in air pollution. (Aver-
aging over the daily activities of motor vehicles may not give an ade-
quate description of the most severe conditions.)
Using validated input values and chemical parameters, we set out to
determine air quality changes that are ascribable to vehicle start-up.
The first step in the investigation is a determination of the source
emission inventory for stationary and mobile emitters. Peculiar to our
purpose is the segregation of the temporal and geographical distribution
of starting contributions from the running emission distributions. Then,
selecting a stagnant day in the Los Angeles basin we artificially remove
these contributions to assess their effect on air quality. This is done
for 1968, 1971, 1974, and 1980.
After the source characterization, the diffusion coefficient (as a
function of time and height) is found for the central basin using measure-
ments of meteorological conditions and carbon monoxide from late 1968.
Uptake of nitrogen oxides by the ground, by urban surfaces, and by air-
borne particulates is not well known (in fact, it is a subject of active
research). Consequently, the nitrogen balance near the ground is estab-
lished as the next preparatory step in the investigation. Previously
determined reduction factors in the nitric oxide emissions are reconfirmed
to account for the heterogeneous effects. Gas phase removal mechanisms
are included, as shown in our last report, and recently reported results4
for N03 chains are added to update the model. Hydrocarbon reactivity is
-------
Sec. I C
propylene photo-oxidations to those characteristic of the aggregation of
atmospheric hydrocarbons.
C. STUDY DESIGN
In an attempt to capture realism, all our simulation results include
appropriate stationary source emissions as well as mobile source emissions
constructed from vehicle age distributions. In keeping with the use of a
stagnant slab model to approximate the central basin, we selected a base-
line day with little wind, resulting in relatively high pollution. For
the geographical distribution studies, we used average September wind
patterns in the moving air parcel simulation. Four test years were chosen,
some to make best use of the existing data base and others to reflect sub-
stantial changes in emission control systems.
Successive sections describe the modeling of source emission distri-
bution, the choices of chemical model and baseline case, and the simula-
tion results. The final section contains some overall observations and
cautions regarding the results. It concludes with some suggestions for
-------
Sec. II A
*
II. SOURCE EMISSION DISTRIBUTION
In general, contaminant source emissions are distributed in space
and time in a rather complex manner. The model of geographical and
hourly variations used here is based largely on the work of Roberts,
Roth, and Nelson. The SDC driving patterns survey provided a means
for estimating the magnitude and temporal distribution of the morning
start-up phenomenon, but the geographical distribution was chosen some-
what arbitrarily for lack of readily usable data. Annual variations in
average daily source emissions, resulting from urban population growth
and the increasing number of vehicles with various levels of emission
control devices, were drawn from projections by the California Air Re-
sources Board (ARE) and the Los Angeles Air Pollution Control District
7 89
(LAAPCD), and from available test data, standards, and surveys.
A. MOTOR VEHICLE EMISSIONS
1. Average Annual Vehicle Emission Factors
Q
Table 1 shows recent test data from Huls for vehicle emission fac-
tors as measured on the new federal driving cycle, with and without
an initial cold-start. The tabulated differences, multiplied by 7.45
miles (the distance that would be covered during one federal cycle) ,
provide baseline values of average grams per cold start for the five
model years shown.
Lacking measured emission factors for other model years, we chose
to use federal standards where available and "scale" California standards
and pre-standard estimates to the federal cycle to fill in any remaining
gaps between 1960 and 1975. Pre-1960 emission factors were assumed to
be constant at the 1960 levels, and similarly, the 1975 emission factors
were extrapolated as constant for all subsequent years. The scaling
technique used is based on California ARE calculations of "equivalent
standards" for 1972 under the new federal procedure.11 Table 2 is repro-
-------
Sec. II A
Tables 1, 2
TABLE 1
FEDERAL CYCLE GRAM-PER-MILE EMISSIONS
BASED ON MEASUREMENTS (From Ref. 8)
Model Year
1968
(6 tests)
1969
(54 tests)
1970
(57 tests)
1971
(25 tests)
*
1975
(3 tests)
Cold Start
Hot Start
Difference
Cold Start
Hot Start
Difference
Cold Start
Hot Start
Difference
Cold Start
Hot Start
Difference
Cold Start
Hot Start
Difference
NO (as NO )
X jL
6.11
5.35
0.76
5.28
4.40
0.88
6.18
5.39
0.79
4.43
3.71
0.72
0.85
0.99
-0.14
HC
4.80
3.36
1.44
4.79
3.51
1.28
4.11
2.65
1.46
3.63
2.80
0.83
0.61
0.25
0.36
CO
71.34
37.23
34.11
48.14
28.87
19.27
41.91
22.67
19.24
42.08
29.66
12.42
6.68
0.90
5.78
These three tests were carried out on 1971 model cars equipped with
prototype versions of the emission control devices which are to be
used on 1975 production models.
TABLE 2
EQUIVALENT CALIFORNIA AND FEDERAL EMISSION STANDARDS
Pollutant
HC
CO
California
1972 Standards
1.5 gm/mi
23 gm/mi
Equivalent
New Procedure
3 . 2 gm/mi
47 gm/mi
Federal
1972 Standards
3 . 4 gm/mi
-------
Sec. II A
Based on these data, the scale factors 3.2/1.5 and 47/23 were used
(where necessary) to convert California Standards and pre-standard esti-
mates for HC and CO emissions to equivalent federal emission factors.
NO emissions must be scaled slightly differently since the current
X
federal test procedure does not provide for their measurement. To fill
this gap, California has augmented the procedure by requiring that the
federal test cycle be followed by two hot 7-mode (California) cycles for
nitrogen oxides testing. The new equivalent 1972 standard under this
procedure is 3.2 gm/mi NO as compared to 3.0 gin/mi under the 7-mode
11 X
test procedure alone. Hence, the scale factor 3.2/3.0 was used to
convert NO emissions based on California cycle testing to their equi-
X
valent federal values. Once scaled, all emission factors were assumed
to contain the effect of one cold-start as specified in the federal pro-
cedure.
To summarize, pre-1968 emission factors were obtained by the scaling
technique, measured emissions data were used for the years 1968 through
1971 and for 1975, and the more stringent of the scaled California Stan-
dards or the Federal Standards were used for the years 1972, 1973 and
1974.
Given data for the contributions of cold-start emission factor for
the model years shown in Table 1, we extrapolated back from 1968 and for-
ward from 1975 assuming constant levels to obtain estimates of cold-start
contributions to emission factors in other years. Between 1971 and 1975,
the cold-start differentials were assumed to scale down in proportion to
changes in exhaust emission standards in the intervening years. We then
subtracted these cold-start contributions from the scaled federal-cycle
emission factors to yield estimates of "hot-running" emission factors
for the years not included in the test data.
Table 3 summarizes the "hot-running" emission factors and cold-start
emissions used in the study. (As mentioned above, the gram-per-start
cold-start emissions are obtained by multiplying the gram-per-mile cold-
-------
Sec. II A
Table 3
TABLE 3
HOT-RUNNING AND COLD-START EMISSIONS BY MODEL YEAR
Model Year
1960 and
before
1961
19b2
I<»b3
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975 and
after
Hot-Running Emissions (grams /mile)
N0x (as N02)
3.50 (B)
3 50 (B)
3.50 (B)
3 50 (B)
3.50 (B)
3 50 (B)
5.64 (B)
5.64 (B)
5.35 (A)
4.40 (A)
5 39 (A)
3.71 (A)
2.66 (B)
2 49 (C)
2.49 (C)
0.99 (A)
HC
22 06 (B)
2 69 evap.
4 10 blowby
22 06 (B)
2 69 evap
4.10 blowby
22.06 (B)
2 69 evap
22 06 (B)
2 69 evap.
22 06 (B)
2.69 evap
22 06 (B)
2.69 evap.
5 81 (B)
2 69 evap.
5.81 (B)
2 69 evap.
3 36 (A)
2 69 evap
3.51 (A)
2.69 evap.
2.65 (A)
2.69 evap.
2.80 (A)
2.63 (B)
2.63 (B)
2 63 (B)
0 25 (A)
CO
128 9 (B)
128 9 (B)
128.9 (B)
128 9 (B)
128 9 (B)
128 9 (B)
35 39 (B)
35.39 (B)
37 23 (A)
28 87 (A)
22.67 (A)
29.66 (A)
28 70 (C)
28 70 (C)
28 70 (C)
0.90 (A)
Cold-Start Emissions (grams/start)
N0x (as N02)
5 66
5.66
5 66
5 66
5.66
5 66
5 66
5 66
5.66 (A)
6 56 (A)
5.89 (A)
5 36 (A)
4 02 (D)
3 78 (D)
3 78 (D)
-1 04 (E)
HC
10 73
10 73
10 73
10 73
10 73
10.73
10 73
10 73
10 73 (A)
9 54 (A)
10 88 (A)
6.18 (A)
4 25 (D)
4 25 (D)
4 25 (D)
2 68 (A)
CO
254 1
254 1
254 1
254 1
254 1
254 1
254.1
254 1
254.1 (A)
143 6 (A)
143 3 (A)
92.53 (A)
76 74 (D)
76 74 (D)
76 74 (D)
43 Ob (A)
g
(A) Test data (see Table 1).
(B) Obtained by scaling California Standard or estimate of pre-standard emission factor to federal
test procedure and subtracting cold-start contribution as extrapolated from test data (see text)
(C) Obtained by subtracting cold-start contribution from Federal Standard
(D) Larking data in the 1972-1974 time period, it was assumed that the techniques employed to meet
the standards used in obtaining the running emission factors (See (B) or (C) above) would act
proportionally in scaling the cold-start emissions down from their 1971 values toward their
1975 values.
(E) Test data. The negative correction is always used in coniunction with the running emission factor,
thereby reducing each post-1975 car's net N0x contribution to the level predicted for a federal
-------
Sec. II A
Table 4
emission factor" shown in Table 3 for pre-1971 cars was calculated from
data in Ref. 12 by assuming an average gasoline Reid vapor pressure of
8.5 Ib/in , a Los Angeles County auto population of 3.95 x 10 cars,
and an average annual mileage of 10,000 miles per car. The result is in
good agreement with Ref. 9. The evaporative emission controls introduced
on 1971 and later cars were assumed to perform as specified. Blowby
9
emissions on pre-1962 cars were set at 4.1 grams/mile and were likewise
assumed to be eliminated on post-1961 cars by the introduction of posi-
tive crankcase ventilation systems.
The model year emission factors in Table 3 were then combined in
13
accordance with a vehicle age and usage distribution shown in Table 4
to yield weighted average vehicle emission factors for each year to be
analyzed. The resulting average running and cold-start emissions for
1968, 1971, 1974, and 1980 are shown in Table 5.
TABLE 4
VEHICLE AGE AND USAGE DISTRIBUTION AT CLOSE OF MODEL YEAR*
(From Ref. 13)
Fraction Miles Driven in
Age (years) of Population Last Year
1
2
3
4
5
6
7
8
9
10
over 10
0.108
0.105
0.102
0.098
0.093
0.088
0.081
0.072
0.062
0.051
0.140
15,000
13,000
11,000
9,600
8,400
7,000
5,300
5,000
4,400
4,200
3,500
Based on a survey of California vehicles stopped at California Highway
-------
Sec. II A
Table 5
TABLE 5
AVERAGE VEHICLE EMISSION FACTORS
Year
1968
1971
1974
1980
Hot-Running Emissions
(grams /mile)
N0x (as N02)
4.51
4.55
3.55
1.52
HC
16.81
10.00
5.70
0.93
CO
82.49
51.58
37.80
6.55
Cold-Start Emissions
(grams/start)
NO (as NO )
X £,
5.66
5.76
4.81
0.17
HC
10.73
9.69
6.89
3.46
CO
254.10
188.71
128.05
54.69
2. Geographical Distribution
Using an extensive data base of traffic counts in the Los Angeles
area, Roberts, Roth and Nelson, of Systems Applications, Inc. (SAI),
have characterized the spatial distribution of Los Angeles freeway and
surface street traffic intensities in each of 625 grid squares of 2 x 2
miles each. Average daily traffic intensity within each square is repre-
sented by an estimate of the total number of vehicle miles (either free-
way or non-freeway) driven in that square. As an example, Fig. 1 shows
the resulting spatial distribution of freeway traffic used in the study
(where the numbers in the squares are thousands of vehicle miles per day).
A similar but more completely filled grid represents surface street traffic.
The absolute traffic intensities given by SAI were assumed to apply as of
1968.
Two geographical distributions of the total "pulse" of cold-start
emissions were used. For the stagnant slab-model, we assumed that morning
start-ups were distributed uniformly, using the area of the Los Angeles
Basin (1250 sq mi) as an estimate of the size of the densely populated
-------
Sec. II A
Fig. 1
166
169
271
311
383
N,
94
155
242
222
890
352
356
242
101
\
^
119
68
456
137
473
295
\
\
144
156
126
426
141
511
295
v
158
17
494
140
333
51
160
404
348
351
\!
\
}
/
\,
v*
"»>
109
104
178
275
215
426
32
391
^
150
395
194
205
466
326
190
423
290
436
795
531
391
366
305
257
217
625
131
143
A£
^
^V
1
51
335
620
452
449
343
45
.22!
13
193
257
585
623
159
241
236
545
107
ir
37
90
109
257
518
576
283
281
302
131
340
->»
28
297
224
332
129
56
323
**>
306
79
444
527
314
383
278
195
529
X
311
234
205
265
33
30
289
102
349
\
41
254
285
263
104
243
59
213
181
\
74
362
147
156
253
83
144
112
153
\
66
291
70
12
330
183
78
115
\
65
214
59
251
145
25
150
160
80
s18
43
203
42
15
138
68
398
163
97
6
*%.
11
170
21
108
156
257
67
148
95
\
0
173
32
3
207
137
161
131
169
61
159
16
15
16
85
77
91
59
Figure 1. Geographical Distribution of Freeway Traffic in the Los
Angeles Basin Area (Adapted from Roberts, Roth
and Nelson, Ref. 5).
-------
Sec. II A
Fig. 2
was to evaluate the effect of a more suburb-oriented cold-start distri-
bution. Hence, for that case, we modeled both the uniform cold-start
distribution and one in which the density of morning start-ups (essen-
tially equatable to car residence density) was assumed to be 3 times as
high at the outer edge of the populated area as in the basin center
(here taken as the Federal building downtown), varying linearly in be-
tween .
3. Temporal Distribution
The hourly distributions of average daily traffic intensities for
freeway and non-freeway traffic used in the study were those developed
by SAI in conjunction with the geographical distributions described above.
In their report, SAI shows that only small errors are incurred by charac-
terizing all freeway traffic by one time distribution and all non-freeway
traffic by another. The resulting traffic/time distributions used to
time-allocate the freeway and non-freeway mileages in each grid square
are shown in Fig. 2.
10
6
FREEWAYS
NON-FREEWAYS
L L
2400
0400
0800
1200
TIME OF DAY
1600
2000
2400
Figure 2. Los Angeles Traffic/Time Distributions (Adapted from
Roberts, Roth and Nelson, Ref. 5)
-------
Sec. II B
Data reported by Systems Development Corporation (SDC) on the
distribution of weekday trip start times in Los Angeles indicates that
the temporal distribution of morning car-starts can reasonably be approxi-
mated by a triangular pulse. The model selected starts from zero at
6:00 A.M., rises linearly to a peak at 7:30 A.M. and then falls linearly
back to zero at 9:00 A.M. SDC's data also showed that of an average 4.4
trips per car weekday (in Los Angeles), 20.6% or 0.907 trips per car week-
day were started between 6:00 and 9:00 A.M. Thus about 91% of the regis-
tered automobiles in L.A. County were assumed to contribute one morning
start to the cold-start contaminant pulse.
Annual variations of total pollutant emissions from motor vehicles
were assumed to grow in proportion to the registered vehicle population.
Hence, based on an extrapolation of vehicle registration in L.A. County
by the LAAPCD and using 1968 as the baseline case, the vehicle mileages
in each square of the geographical distributions were increased accord-
ingly for each year analyzed beyond 1968. Of course, the cold-start
contaminant pulse was similarly scaled up.
B. STATIONARY SOURCE EMISSIONS
The stationary source emissions modeled for this study included
oxides of nitrogen and reactive hydrocarbons as characterized by SAI in
Ref. 5. Stationary carbon monoxide emissions were neglected in comparison
with motor vehicle CO emissions. The SAI model uses the same 25 x 25 grid
of 2 x 2 squares described above to provide geographic source distribution.
Within each square, the flux is given in kilograms per hour and is assumed
to be constant between 6:00 A.M. and 6:00 P.M. This data was assumed to
be circa 1968, and subsequent annual growth was modeled on the basis of
ARE projections for the South Coast Air Basin. Reference 6 shows negli-
gible expected changes in total stationary hydrocarbon fluxes over the
next 20 years; Figure 3 shows the growth curve obtained for stationary
source N0x emissions. These factors were used to increase the kilogram-
per-hour NO fluxes in each grid square.
X
-------
Sec. II C
Fig. 3
CO
z
o
to
to
X
o
Ul
o
a:
o
to
o:
o
a:
o
1980
Figure 3. Estimated Growth of Stationary Source NO Emissions
(From Ref. 6)
C. INPUT GENERATION FOR PHOTOCHEMICAL MODEL
1. Central Basin Stagnant Case
The character of the central basin stagnant mixing case is such that
it would seem inappropriate to simply use the source data specified for
the single 4-square-mile grid square containing the central basin measure-
14
ment station. Instead, to simulate more extensive local mixing during the
eight-hour period of interest, source emission fluxes in neighboring grid
squares were combined in an area-weighted average to yield an input flux
history which represents an average over a 5 x 5 mile square centered on
the measurement station.
2. Cross-Basin Trajectory Case
The Lagrangian air parcel formulation used in the current GRC model
3
has been described in a previous report. The air trajectory data
-------
Sec. II C
necessary to determine the schedule of changes in pollutant fluxes
into the air parcel on a realistic cross-basin trajectory was obtained
from Eef. 14. Figure 4 shows the trajectory, which was obtained from
a combination of average September wind speeds and directions measured
at stations near the coast, where the trajectory starts, and downtown at
the Federal building, where the trajectory passes at about noon. The
afternoon portion of the trajectory was obtained by estimating wind speeds
and directions from a similar trajectory in Ref. 14. The resulting tra-
jectory is very similar in path and time history to one constructed in
Ref. 14 by more detailed analysis of average hourly resultant wind stream-
lines for September, and is therefore considered to be a realistic cross-
basin air trajectory. Because the wind speeds near the coast are practi-
cally zero early in the morning, the trajectory shows no appreciable move-
ment until after 8:00 A.M. As the air parcel subsequently moves along the
trajectory, the schedule of pollutant influxes is determined by both the
geographical and temporal source distributions described above.
-------
Sec. II C
Fig. 4
h A J
\ BEVERLY HILLS
HUNTINGTON
PARK
TORRANCE / \
MKHEW \
-A. _
00
00
«0
Ci
fo
4;
Figure 4. Cross-Basin Air Trajectory
-------
Sec. Ill A
III. CHOICES OF CHEMICAL MODEL AND BASELINE CASE
A. INCORPORATION OF N03 IN KINETICS
The basic 12-step kinetic model has been described in detail in
Ref. 3. Table 6 shows the reactions and rate constants included in this
version of the model.
One problem with the 12-step model was that late in the reaction
the ozone and N09 concentrations were high. Although comparison with
15
the measurements of Altshuller, et al showed the computed ozone and
NO, to be within the range of experimental uncertainty, validation of
O -I C
the model against atmospheric data ' showed that the disparity can
become large even after reducing the hydrocarbon rate constants. The
kinetic scheme has now been modified in order to remedy this deficiency
and thus achieves a higher degree of realism. The modifications are
described below.
Two reactions and one species, NO,, have been added to the kinetic
model. These are shown as reactions 13 and 14 in Table 6. The 0, + NO-
reaction, shown in Table 6 as reaction 13, describes the late-time phe-
nomena which lower the concentrations of ozone and NO- . Tests of a
mechanism with the addition of reaction 13 alone displayed anomalously
high concentrations of R02 late in the reaction. This problem was
solved by adding an R02 + NO- chainbreaking reaction (No. 14 in Table 6)
following Hanst's suggestion concerning the role of NO- in the forma-
tion of PAN. The rate constant assigned to reaction 14 is the same as
that of reaction 7 since these two reactions are analogous.
The results of the simulation compared with experimental data for a
propylene-nitrogen oxide system are shown in Figs. 5 and 6. The effect
of the modifications is to lower the concentration of ozone and NO-
late in the process. The production of PAN satisfies the criteria set
4
forth by Hanst. The effect that varying the initial NO /EC ratio has
-------
TABLE 6
CHEMICAL KINETIC MODEL FOR HYDROCARBON/NITRIC OXIDE MECHANISM
(Stoichiometry imbalances may occur because of lumped parameter assumptions.)
Reaction
(1) hv + N02 -*- NO + 0
(2) 0 + 02 + M * 0. + M
(3) 03 + NO -> N02 + 02
(4) 0 + HC -> 2R02
(5) OH + HC -* 2R02
(6) R02 + NO * N02 + 0.5 OH
(7) R02 + N02 -> PAN
(8) OH + NO * HN02
(9) OH + N02 ->- HN03*
(10) 03 + HC -+ R02
**
(11) NO + N02 * 2HN02
(12) hv + HN02 -* NO + OH
Model Values From Validation
0.4 mm
1.32 x 10 ppm mm
40 ppm mm
6100 ppm mm
80 ppm mm
1500 ppm mm
, -1 . -1
fa ppm mm
10 ppm mm
30 ppm mm
0.0125 ppm mm
0.01 ppm mm
0.001 mm~
1 8
Nominal Values for Propylene System
0.4 mm
-5 -2 -1
1.32 x 10 ppm mm
22-44 ppnT'Snin"1
6100 ppm mm
244 ppm mm
122 ppm mm
122 ppm mm
99 ppm mm
300 ppm mm
0.00927 - 0.0125 ppnT^nin"1
H cn
to 0>
& n
i |
(D
H
H
>
Additional reactions included m Model
(13) 03 + N02 -> N03
(14) K02 + N03 -> PAN
.005 ppm mm
, -1 -1
6 ppm mm
.05 ppm mm
122 ppm mm
**
Rate constant lumps third body concentration
Water vapor lumped into rate coefficient
-------
Sec. Ill A
Figs. 5, 6
3 o
EXPERIMENTAL POINTS |
OF ALTSHULLER, et al15 "?
A PROPYLENE
O NITROGEN DIOXIDE
D NITRIC OXIDE
12-STEP MECHANISM (REF. 3)
14-STEP MECHANISM (SEC. III.A, THIS REPORT)
1.0
0 5 -
60 80
TIME
100 120 140 160 rmn
Figure 5. Computed Concentrations of Propylene, Nitric Oxide, and
Nitrogen Dioxide Compared with Experimental Values
3 0 PPm
2.5
2 0
z:
o
1 IB
LlJ
O
8 ,»
0 5
0
1
A OZONE | EXPERIMENTAL POINTS OF 2
O PAN J ALTSHULLER, et al15
12-STEP MECHANISM (REF 3)
~ 14-STEP MECHANISM (SEC III. A, THIS REPORT)
-
^=m«3
" , , ^^'^MH
3 20 40 60 80 100 120 140 160 mm
TIME
Figure 6. Computed Concentrations of Ozone and PAN Compared
with Experimental Values
-------
Sec. Ill B
on the new model is the same as that reported in Ref. 3: ozone concentra-
tion decreases with increasing NOx/HC ratio. The only difference is
that, with the new model, the ozone levels are reduced by the additional
removal mechanism.
B. CASES SELECTED FOR ATMOSPHERIC MODELING
1. Baseline Case
In order to provide a data base against which to compare the results
of the atmospheric model, we selected a day during a smog episode, October
23, 1968. For this date, we used data recorded by Scott Research Labor-
19
atories at Huntington Park. Because Huntington Park is centrally
located in the Los Angeles Basin, its aerometric data is likely to re-
flect the effects of vehicular sources. This is an important consider-
ation in this study. The day of October 23 was chosen for several reasons:
1. Readily available aerometric and meteorological data
2. The day is typical of high-oxidant, heavy-smog days in
Los Angeles.
3. The prevalence of low wind speeds up to about 9:30 A.M.
and the presence of a low inversion base (approximately
200 meters) indicate stable atmospheric conditions which
lead to a worst-case approach in our computed results.
20
4. Previous modeling experience with this date
2. Choice of Years
Four years were chosen for the cold-start vs hot-start comparison:
1968, 1971, 1974, 1980. The first year, 1968, has extensive atmospheric
data used for validation as mentioned above, but the accuracy of the
vehicular emission data base can only be classified as fair due to the
large quantity of pre-1966 cars still on the road in 1968. By contrast,
1971 has the best and most reliable auto emissions data base of all the
-------
Sec. Ill C
years and it was chosen for this reason. The year 1974 was selected
because it is the last model year before automotive emissions must satisfy
the stringent 1975 standards. Thus for 1974, effects of the cold-start
pulse are unlikely to be overshadowed by emissions from stationary sources.
Finally, by 1980 over 70% of all car-miles in Los Angeles should be tra-
veled by cars with 1975 or post-1975 emission controls. Hence 1980 was
chosen because it is the year when the full impact of the 1975 auto emis-
sion standards is supposed to be felt.
3. Choice of Air Trajectory to Study Decentralization of Starts
The possibility has been suggested that a nonuniform spatial distri-
bution of vehicle starts may reduce the effects of the cold-start contri-
bution. To test this hypothesis, an air trajectory was selected that
would traverse the Basin from West to East during the morning and early
afternoon. The chosen trajectory passes over Downtown Los Angeles and
ends in the eastern part of the San Fernando Valley. Figure 4 shows a
diagram of the trajectory. The trajectory is typical for a September
day which has low winds and stable atmospheric conditions. Using this
trajectory we intend to compare the pollution levels for a single year
under two different assumptions for the distribution of cold starts: a
uniform and a nonuniform spatial distribution. The nonuniform distri-
bution has been described in Sec. II A 2. The 1974 emissions data were
used to generate source inputs for the trajectory.
C. ESTABLISHMENT OF BASELINE CASE
The baseline case was used successively to determine three critical
parameters of the model:
1. The size of the diffusion coefficient
2. The scaling factor for the NO flux
3. The scale factor of the hydrocarbon rate constants
In addition, the base case was also used to ascertain the sensitivity of
the CO buildup to the cold-start pulse and to generate the results re-
ported in the next section.
-------
Sec. Ill C
1. Determination of Diffusion Coefficient
The functional dependence of the diffusion coefficient on height and
21
wind speed has been described elsewhere. Here we are concerned simply
with the proper scaling of the entire diffusivity profile. This was
accomplished by comparing the observed and computed buildup of carbon
monoxide. Figure 7 shows the calculated and measured data. The upper
solid curve was deemed to approach most closely the early-morning buildup
prior to the onset of advection due to increasing horizontal wind speed.
From 0600 to 0930 the wind speed is very low and the model essentially
reproduces the CO buildup. The wind speed increase after 0930 and the
model is no longer effective in estimating the CO concentration at a
point.
Figure 7 also shows the sensitivity of the computed results both to
a change in the value of diffusivity and to the removal of the cold-start
contribution. Increasing the diffusivity by 50% causes the computed
values to drop by several ppm. Removing the cold-start pulse causes an
even more dramatic drop in the calculated early-morning buildup as well
as a qualitative shape change in the curve.
2. Necessity for Scaling NO Flux
2 20 21 22
It has been observed in previous studies ' ' ' that morning NO
A
buildups do not fully account for the emitted NO obtained from source
inventories. In other words, the nitrogen balance between inventories
and measured NO concentrations is poor; sometimes it may be off by as
X 21?
much as a factor of four. This has been shown to be true particularly
22
on high-oxidant days such as October 23, 1968. This phenomenon is not
yet fully resolved, but it is speculated that gas-solid reactions may
account for a large part of the discrepancy. This is known to be the
24
case in smog chamber experiments, and recently Gay and Bufalini have
published experimental evidence supporting the existence of wall reac-
tions which account for a major fraction of the nitrogen deficit.
-------
Sec. Ill C
Fig. 7
100 Ppm
g
o
(J
10
0600
10
00
WITH COLD START
D = NOMINAL
D = 1.5 x NOMINAL
\
LINES CONNECT SRL
WITHOUT COLD START
D = NOMINAL
\DATA POINTS
\
19
-VENTILATION PHASE-
(D = ATMOSPHERIC DIFFUSION
COEFFICIENT)
0800
1000
1200 PST
TIME
Figure 7. Influence of Cold Start and Diffusion Coefficient on Carbon
Monoxide Buildup (See Text for Test Conditions)
-------
Sec. Ill C
Since the nitrogen imbalance involves rapid physical processes which
are still not well defined, we resort to the expedient of artificially
reducing the NO flux obtained from the source inventories in order to
account for the N0x losses during at least the early-morning buildup.
In our particular case, which includes the cold-start contribution, scal-
ing the NO flux by 1/4 yields satisfactory results. This is consistent
with our previous work over the past few years.
3. Hydrocarbon Reactivity
22
It has been shown in earlier reports that the reactivity of the
hydrocarbon mixture in the atmosphere is generally lower than the reac-
tivity of propylene by at least a factor of 2. It is thus necessary to
adjust the rate constants of the reactions involving hydrocarbons since
the validation of the kinetic model was done for a propylene system.
Using the rates shown in Table 6 results, as expected, in the production
of amounts of ozone which are far larger than the atmospheric measurements
indicate. Our experience has shown that adjustment of the hydrocarbon
rate constants by about 1/3 to 1/2 the propylene values yields ozone con-
centrations consistent with aerometric data.
For the case of October 23, 1968, we have determined that a 1/3
rate adjustment is necessary to obtain the correct ozone concentration.
Further details on testing other values of the rate constants can be
found in Ref. 20.
4. Summary of Steps Required to Establish the Baseline Case
In establishing the baseline case, the following steps were taken:
1. Determination of the diffusion coefficient using the CO
buildup in the morning
2. Decreasing the NO flux by 1/4 to achieve the nitrogen balance
3. The rate constants of the hydrocarbon reactions were multiplied
by 1/3 to simulate the lower reactivity of the atmospheric
hydrocarbon mixture (as indicated by gas chromatographic
data ' , thus producing the correct amounts of ozone.
-------
Sec. Ill C
Figures 7 to 11 show comparisons of the computed results with the atmos-
19
pheric measurements collected by Scott Research Laboratories. The
carbon-monoxide modeling in Fig. 7 was discussed in Sec. Ill C 1, and
the other figures show the reactive pollutants. The NO (shown in
Fig. 8) follows the data rather accurately, whereas the N02 (shown in
Fig. 9) lags behind the measured data. The time lag is the result of
lowering the hydrocarbon rates coupled with the omission in this simula-
tion of the ventilation phase. It seems paradoxical that the NO should
show such good correspondence with the data but that for NO the agree-
ment is poorer. One possible explanation of this effect is that for NO
fast chemical reactions coupled with vertical diffusion tend to minimize
the susceptibility of NO to advection. By contrast, it is well known
that the residence time of N02 over an urban area is much longer, and
NO- is thus highly sensitive to advective forces.
Time lags can also be seen in Figs. 10 and 11 for ozone and reactive
hydrocarbon, respectively. The ozone buildup is reproduced quite well
by the model, but the model fails to agree with the measured ozone decay.
Again, fast chemical reactions can account for the ozone buildup. The
decay of ozone via chemical reactions is relatively slow, thus ozone
decay at the point modeled is the result of ventilation. For reactive
hydrocarbon, the buildup during the stagnation phase (0600-0930) is well
reproduced, but the decay is chemically very slow and advection is the
dominant effect.
-------
Sec. Ill C
Figure 8
100 PPhm
10
o
o
0600
oo
0800
1000
1200 PST
TIME
Figure 8. Ground Level Nitric Oxide Concentration on October 23, 1968
in the Central Los Angeles Basin
-------
Sec. Ill C
Figure 9
100 pphm,-
o
3
10
CS.
00
to
SRL
19
\
I
0600
0800
1000
1200 PST
TIME
Figure 9. Ground Level N02 Concentration on October 23, 1968
in the Central Los Angeles Basin
-------
Sec. Ill C
Figure 10
100 pphm
10
o
o
0.1
0600
0800
00
00
1000
1200 PST
TIME
Figure 10. Ground Level Ozone Concentration on October 23, 1968
in the Central Los Angeles Basin
-------
Sec. Ill C
Figure 11
100 pphm ^
o
o
0600
0800
1000
1200 PST
TIME
Figure 11. Ground Level Reactive Hydrocarbon Concentration on
October 23, 1968 in the Central Los Angeles Basin
-------
Sec. IV A
Table 7
IV. TEST RESULTS FOR DETERMINING COLD-START EFFECTS
A. RESULTS UNDER STAGNANT ATMOSPHERIC CONDITIONS
The contribution of cold-start emissions to air pollution was deter-
mined for a location in the central Los Angeles basin on a day character-
ized by very light winds. Concentration histories of various pollutants
were computed, both with and without the morning vehicle start-up emissions
in the 0600-0900 hour time interval.
The effect of cold-start emissions of NO , CO , and reactive hydro-
carbon on atmospheric loading during the early morning hours is demonstrated
in Table 7 in the form of emissions ratios for the 25-square-mile test re-
gion; i.e. , ratios of emissions with cold-start to emissions without cold
start.
TABLE 7
EMISSION EFFECTS
(Ratios of emissions with cold-start to
emissions without cold-start, each for 0600-0900 time interval
in central L.A. basin)
Year
Species
NO
CO
Reactive hydrocarbon
1968
1.153
1.479
1.077
1971
1.157
1.568
1.103
1974
1.156
1.526
1.105
1980
1.009
2.297
1.107
To show the effect of cold-start emissions on ambient air quality,
we have computed concentration ratios for two product pollutants, ozone
and nitrogen dioxide, and for carbon monoxide. Shown in Table 8, the ratit
are ground concentration with cold-start divided by ground concentration
-------
Sec. IV A
Tables 8,9
without cold-start, taken at 1300 hours (the end of the simulation run)
for 0_ and NO, , and at peak concentration (whenever it occurs) for
J £
CO .
TABLE 8
AIR QUALITY EFFECTS
(Ratios of concentration with cold-start to
concentration without cold-start
in central L.A. basin)
Time
1300 hours
1300 hours
Peak
Species
°3
N02
CO
1968
1.024
1.011
1.093
Yea
1971
1.028
1.015
1.132
r
1974
1.014
1.014
1.127
1980
1.071
1.002
(no peak)
Table 9 combines carbon monoxide data from Tables 7 and 8 with the
addition of the CO emissions ratio for the 0600-1300 time period. Two
features of these data are notable; first, cold-start contributions to
TABLE 9
CARBON MONOXIDE EFFECTS
(Figures shown are ratios for quantities with cold-start
to quantities without cold-start)
Year
0600-0900 Emissions
0600-1300 Emissions
Peak Concentration
1968
1.479
1.216
1.093
1971
1.568
1.257
1.132
1974
1.526
1.238
1.127
1980
2.297
1.586
(no peak)
-------
Sec. IV B
Table 10
total CO emissions are considerable, even when taken over the complete
7-hour span of the simulation; and second, in spite of a sizeable increase
in CO emissions in the relatively short vehicle start-up period, only
modest increases in peak CO concentration are seen, giving us a measure
of the mitigating effects of atmospheric dispersion and dilution on non-
reacting species.
B. RESULTS OF CROSS-BASIN TRAJECTORY ANALYSIS
To determine the effect of a suburb-oriented cold-start distribution,
the moving air parcel model was employed to compute various pollutant con-
centrations along a cross-basin trajectory typical of September wind pat-
terns (see Sec. II C 2). Using 1974 emissions data, two cold-start geo-
graphical distributions were simulated, one uniform and one weighted 3-to-l
between the suburbs and downtown (see Sec. II A 2).
The cold-start contributions to pollutant loading in the air parcel
are shown in Table 10 as the ratios of emissions with cold-start to emis-
sions without cold-start for NO , CO , and reactive hydrocarbons. Note
that the decentralized start distribution loads the air parcel with more
pollutants than the uniform one because of relatively high morning expo-
sure of the air parcel to areas away from the basin center.
TABLE 10
EMISSION EFFECTS FOR 1974 TRAJECTORY
(Ratios of emissions with cold-start to emissions without
cold-start, each for 0600-0900 time interval)
Species
NO
CO
Reactive Hydrocarbon
Spatially Uniform Start
Distribution
1.168
1.546
1.134
Decentralized Start
Distribution
1.183
1.594
1.146
-------
Sec. IV B
Table 11
The effect of vehicle start distribution on air quality in the air
parcel may be seen in Table 11 by comparing the concentration ratios com-
puted for the uniform and decentralized start distributions. These con-
centration ratios (ground concentration with cold-start to ground concen-
tration without cold-start) were computed at peak value for CO , and at
the end of the air-trajectory (1400 hours) for the chemical product pol-
lutants 0- and NO- .
TABLE 11
AIR QUALITY EFFECTS FOR 1974 TRAJECTORY
(Ratios of concentration with cold-start to
concentration without cold-start)
Time
1400 hours
1400 hours
Peak
Species
°3
N02
CO
Spatially Uniform
Start Distribution
1.039
1.024
1.125
Decentralized
Start Distribution
1.042
1.026
1.136
Finally, as in the previous section, we compare the contributions
of cold-start CO emissions to total CO emissions, and to the resultant
peak concentrations of CO in the air parcel (see Table 12). (The 0600-
1400 time period is the complete duration of the simulated trajectory.)
As before, the noteworthy features of these data are the considerable
size of the cold-start emission contributions, and the weakness of the
coupling between emissions and air quality effects for a non-reacting
species.
-------
Sec. IV B
Table 12
TABLE 12
CARBON MONOXIDE EFFECTS FOR 1974 TRAJECTORY
(Figures shown are ratios for quantities with cold-
start to quantities without cold-start)
Spatially Uniform
Start Distribution
Decentralized
Start Distribution
0600-0900 Emissions
0600-1400 Emissions
Peak Concentration
1.546
1.250
1.125
1.594
1.273
-------
Sec. V A
V. CONCLUDING REMARKS
A. INTERPRETATION OF MODELING RESULTS
Considering the wide variety of emission sources and control measures
we find that the pollution input of cold starts can be very large or very
small as shown in Table 7. Its nitric oxide contribution (between 6 and
9 A.M.) stays near 15% of all emissions until the end of the present
decade, when it drops below one percent. Dominant effects of stationary
sources compared with vehicles cause this sharp decrease. Reactive hydro-
carbon starting emissions comprise about 10% of the 6 to 9 A.M. inputs.
Because of choked engine operation, carbon monoxide starting contributions
range from 48% to 130% of running CO emissions during the morning traffic
peak hours.
It is of central interest here to examine the incremental change in
air pollution due to the vehicle starting process. As illustrated in
Table 8, 1300-hour levels of oxidant species, 0- and N0» , are only
raised by a few percent. Apparently, the reaction times to produce these
compounds are long enough to allow considerable atmospheric dilution.
Carbon monoxide is treated as nonreactive in the simulations and it is
more strongly affected than the oxidant species. Because carbon monoxide
is emitted directly and oxidant is not, it may be more meaningful to use
the emission ratios (Table 7) instead of the air quality ratios (Table 8)
for setting standards. This procedure would more nearly reflect the
increased exposure of receptors in the area where the cars are started.
Expressed as percentages, cold-start emissions (Table 7) exhibit a
long-term upward trend. This tendency is attributable to lower running
emissions achieved by more sophisticated control systems, accompanied by
thermal inertia in catalytic devices, which aggravates the cold start
problem in advanced systems. The oxides of nitrogen actually assume a
lesser role as time goes on because they pose less of a cold-start con-
trol problem than do the other contaminants. NO compounds are also
-------
Sec. V B
special in that stationary inputs are projected to grow significantly
over a period when vehicles are experiencing more stringent controls.
Tables 7 through 9 demonstrate that the time phasing of vehicle
activities and reactions influences the morning startup effects. The
relatively high deposition rates in the 6 to 9 A.M. time interval have
these effects superimposed on them. Spreading the averaging period over
the 6 A.M. to noon time interval reduces the relative fractional contri-
bution. Although it was not modeled, delaying the startup time and the
traffic peak would also drive downward the air pollution levels, because
higher wind speed and surface heating would enhance the dispersion.
Spatial distribution of cold-starts was also investigated in the
study. An air trajectory, going from the western Los Angeles basin over
the downtown area and up into Burbank, was chosen. Essentially no dif-
ference was noted between one case with uniformly distributed cold-starts
and another case with three times the cold-start density at the coast as
that in downtown (Tables 10-12) - This lack of sensitivity to spatial
distribution occurred for all pollutants for the 0600-1400 time interval
covered by the trajectory.
B. SOME ALTERNATIVES FOR WEIGHTING FACTORS TO COMBINE COLD-START
AND HOT-START CYCLE TEST RESULTS
It is clear from the results previously discussed that cold-start
emissions cannot be neglected in setting automotive emission standards.
The question is, how should the emissions be weighted to reflect the
relative influences of cold and hot starts? We now examine some alter-
native weighting methods.
Based on our analyses, three possible methods might be considered:
1. Air Quality Method
2. Vehicle Activities Method
-------
Sec. V B
Each of these is related in some way to the morning emissions in contrast
with all-day averaging. The objective of each method is to obtain a
weighting factor w for the formula
E = we + (l-w)h
w
in which E = weighted average of cold and hot-start cycle emissions
w
c = cold-start cycle emissions
h = hot-start cycle emissions
w = cold-start weighting factor
The air quality method uses our simulation results summarized in
Tables 7 and 8. It is the ratio of percentage deterioration in air quality
to percentage increased emissions. If, for example, a five-percent con-
taminant concentration increase is caused by a ten-percent emission in-
crease (due to cold-start) , then w = 0.5 by the air quality method.
This method accounts for the leverage factor of emissions upon air quality.
A problem arises in determining the percentage deterioration in air qual-
ity due to secondary pollutants such as NC>2 and 0, since no simple
cause/effect relationship exists between these pollutants and their pre-
cursors, NO and HC (reactive). We chose to average between the percen-
tage increases in pollution level due to N0_ and 0_ , and similarly,
to average between HC (reactive) and NO emission percentages to get
the emission term in the denominator.
From Table 7, we find that the average NO and HC (reactive) per-
centage emission increase for 1971 is 13%. Similarly, from Table 8 we
obtain an average increase of 2.15% for (NO, + 0,). This yields a value
^ -J
of w = 2.15/13 = 0.17. The relatively low value of w reflects the
dilution which occurs during the reaction phase. Applying the procedure
to CO in a straightforward manner, one obtains w = 0.23 . The larger
weighting factor for CO arises because it is not considered to be reac-
tive. The air quality method results are summarized in Table 13.
-------
Sec. V B
Table 13
TABLE 13
WEIGHTING FACTORS FOR AIR QUALITY METHOD FOR 1971
Pollutant w
Reactive 0.17
CO 0.23
The vehicle activities method is based on vehicle-miles per cold
start during the morning hours. Let
_ (mileage per vehicle during time period)
a ~ (cold-starts per vehicle during time period)
Then this method gives w from the following formulas
w = ^^ ; for a > 7.45
3.
w = 1 ; for a <_ 7.45
because the driving cycle covers 7.45 miles of operation. Driving pat-
terns for Los Angeles obtained from Ref . 1 (page 1-45) show that for
the 0600-0900 time period, the average trip length is 10.6 miles. Hence,
a = 10.6 and w = 0.70 . Likewise, for the interval 0600-1200 we find
a = 12.5 , and hence, w = 0.60 . (Note that this method does not dif-
ferentiate between reactive and nonreactive emissions.) These results
are summarized in Table 14.
The third method, the emissions method, simply reduces the cold-
start weighting factor obtained from the vehicle activities method to
account for stationary source background emissions. The reduction is
accomplished by applying a scale factor that is the ratio of vehicular
-------
Sec. V B
Table 14
Method
TABLE 14
WEIGHTING FACTORS FOR VEHICLE-ACTIVITIES
AND EMISSIONS METHODS
Time Period
Type of Emissions
(HC + NO)
CO
Vehicle Activities
Emissions
0600-0900
0600-1200
0600-0900
0600-1200
0.70
0.60
0.52
0.43
0.70
0.60
0.70
0.60
Central L.A. Basin location. Emissions used are for 1971. Stationary
CO emissions are considered to be negligible.
emissions to total emissions for the same time period used in the acti-
vities method. It should be noted that in this method the scale factor
used to multiply w will in general be a function of geographical loca-
tion, since the ratio of vehicular to stationary emissions is not con-
stant throughout an urban area. However, average total emissions for an
urban area could be used. Another possibility is to follow a worst-case
approach and choose the geographical region with the highest vehicular/
stationary emissions ratio. For illustrative purposes, we have computed
scale factors for reactive emissions (HC + NO) and for CO for the cen-
tral L.A. Basin location used throughout the study. These calculations
require hourly average emissions data for the various sources. We used
those for 1971. Table 14 summarizes the results obtained for the vehicle-
activities and emissions methods.
Finally, to illustrate how the incorporation of cold-start emissions
on a weighted basis would affect the emissions rating of an average 1971
-------
Sec. V B
Table 15
car, we have computed some sample weighted emission factors. The range
of w's shown in Tables 13 and 14 is 0.17 - 0.70 for the reactive pol-
lutants (HC and NO) and 0.23 - 0.70 for CO . Table 15 contains the run-
ning emission factor for each pollutant and the range of corrected emis-
sion factors associated with use of the lower and upper values of w for
that pollutant.
TABLE 15
AVERAGE 1971 VEHICLE EMISSION FACTORS ADJUSTED
TO ACCOUNT FOR MORNING COLD-START EFFECTS
NO , gm/mi
X
HC, gm/mi
CO, gm/mi
Running Emission Factor
(w = 0)
3.71
2.80
29.66
w = w .
mm
3.83
2.94
32.52
w = w
max
4.21
3.38
38.36
The choice of weighting method rests on the air quality objective
of the control program. The vehicle activities method, which is the most
stringent of these three for morning emissions, would be used for protec-
tion against excessive roadside CO exposure. On the other hand, the
air quality method might be applied to the reactive pollutants to reflect
the yield factors and dilution factors affecting photochemical pollutants.
If the morning hydrocarbon air quality standard is imposed on photochem-
ically active pollution, however, the emissions method might be most
satisfactory. Clearly, the selection of approach is influenced by the
direction taken in making policy in air pollution abatement. The choice
of method, therefore, is not easily amenable to analysis.
-------
-------
REFERENCES
1. D. H. Kearin and R. L. Lamoureaux, A Survey of Average Driving
Patterns in the Los Angeles Urban Area. System Development Corpor-
ation TM-(L)-4119/000/Ol, February 28, 1969.
2. A. Eschenroeder and J. R. Martinez, Mathematical Modeling of Photo-
chemical Smog. General Research Corporation IMR-1210, December 1969.
3. A. Eschenroeder and J. R. Martinez, Further Development of the Photo-
chemical Smog Model for the Los Angeles Basin. General Research Corpor-
ation CR-1-191, March 1971.
4. P. L. Hanst, "Mechanism of Peroxyacetyl Nitrate Formation," Journal
of the Air Pollution Control Association. Vol. 21, No. 5, May 1971,
pp. 269-271.
5. P.J.W. Roberts, P.M. Roth, and C.L. Nelson, "Contaminant Emissions
in the Los Angeles BasinTheir Sources, Rates, and Distribution,"
Appendix A of Development of a Simulation Model for Estimating Ground
Level Concentrations of Photochemical Pollutants. Systems Applica-
tions, Inc., Report 71SAI-6, March 1971.
6. Air Pollution Control in California; 1970 Annual Report. Air Resources
Board, January 1971.
7. Profile of Air Pollution Control in Los Angeles County. Air Pollution
Control District, County of Los Angeles, January 1969.
8. T. Huls, Environmental Protection Agency, Air Pollution Control
Office, Private Communication, May 18, 1971.
9. M. J. McGraw and R. L. Duprey, Air Pollutant Emission Factors (Pre-
liminary Document), U.S. Environmental Protection Agency, April 1971,
p. 24.
10. "DREW Urban Dynamometer Driving Schedule," The Federal Register.
Vol. 35, No. 219, Part II, November 10, 1970, Appendix A, p. 17311.
11. "Test Procedures, Emission Standards Revised," California Air Re-
sources Board Bulletin, Vol. 2, No. 9, Nov.-Dec. 1970, p. 4.
12. Gasoline Modification; Its Potential as an Air Pollution Control
Measure in Los Angeles County. A Joint Project by California Air
Resources Board, Los Angeles County APCD, and Western Oil and Gas
Association, November 1969, p. 28.
-------
13. A. J. Hocker, California Air Resources Board, Private Communication,
May 19, 1971.
14. M. Neiburger and J. G. Edinger, Meteorology of the Los Angeles Basin
with Particular Respect to the "Smog" Problem. Air Pollution Founda-
tion Report No. 1, April 1954.
15. A. P. Altshuller, S. L. Kopczynki, W. A. Lonneman, T. L. Becker, and
R. Slater, "Chemical Aspects of the Photooxidation of the Propylene-
Nitrogen Oxide System," Environmental Science and Technology. Vol. 1,
No. 11, Nov. 1967, pp. 899-914.
16. Atmospheric Reaction Studies in the Los Angeles Basin. Vols. I - IV,
Scott Research Laboratories, February 1970.
17. P. A. Leighton, Photochemistry of Air Pollution. N.Y.: Academic Press,
1961.
18. K. Westberg and N. Cohen, The Chemical Kinetics of Photochemical Smog
as Analyzed by Computer. AIAA Third Fluid and Plasma Dynamics Confer-
ence Paper No. 70-753, Los Angeles, June 29-July 1, 1970.
19. Final Report on Phase I. Atmospheric Reaction Studies in the Los
Angeles Basin. Vol. I and II, Scott Research Laboratories, June 30,
1969.
20. A. Q. Eschenroeder and J. R. Martinez, A Modeling Study to Characterize
Photochemical Atmospheric Reactions in the Los Angeles Basin Area.
General Research Corporation CR-1-152, Nov. 1969.
21. A. Q. Eschenroeder and J. R. Martinez, Concepts and Applications of
Photochemical Smog Models. General Research Corporation TM-1516,
June 1971.
22. A. Q. Eschenroeder and J. R. Martinez, Analysis of Los Angeles Atmos-
pheric Reaction Data from 1968 and 1969. General Research Corporation
CR-1-170, July 1970.
23. R. Gordon, H. Mayrsohn, R. Ingels, "C^C,. Hydrocarbons in the Los
Angeles Atmosphere," Environmental Science and Technology. Vol. 2,
pp. 1117-1120, Dec. 1968.
24. B. W. Gay and J. J. Bufalini, "Nitric Acid and the Nitrogen Balance
of Irradiated Hydrocarbons in the Presence of Oxides of Nitrogen,"
Environmental Science and Technology. Vol. 5, No. 5, May 1971
pp. 422-425.
------- |