EPA-AA-TEB-81-9
Summary Report of Several Ambient Carbon Monoxide Studies
by
Mark Wolcott
November 1980
Test and Evaluation Branch
Emission Control Technology Division
Office of Mobile Source Air Pollution Control
Office of Air, Noise, and Radiation
U. S. Environmental Protection Agency
-------
Summary Report of Several Ambient Carbon Monoxide Studies
The Federal Clean Air Act assigns to the United States Environmental
Protection Agency (EPA) the responsibility to promulgate National Ambient
Air Quality Standards (NAAQS) regarding carbon monoxide (CO). On
August 8, 1980 EPA proposed CO standards of 25 parts per million (ppm)
maximum allowable one hour expected concentration level and 9 ppm maximum
allowable expected eight hour average concentration level (Reference i).
In an effort to achieve these standards the Clean Air Act also assigns to
EPA the responsibility to promulgate regulations regarding the amount of
CO new highway vehicles may emit from the tailpipe.
To properly fulfill this second mandate, it is necessary to understand
the conditions associated with high ambient CO concentrations. The EPA
Office of Mobile Source Air Pollution Control (OMSAPC) has conducted
several studies concerning ambient CO concentrations and the relationship
between those concentrations and motor vehicle emissions. The purpose of
this report is to provide a single concise summary of several such
studies. Four of the included studies were conducted by OMSAPC. Anotner
related study, sponsored by EPA Region X, is included in this summary
report for completeness.
The five studies summarized in this report, provide information on recent.
CO emission inventory estimates, the meteorological conditions associated
with high ambient CO concentrations, the distribution of CO concen-
trations within selected cities, the effects on CO concentration levels
of both meteorological conditions and traffic characteristics at typical
hot spot locations and the estimated amount that in-use vehicle emissions
must be reduced to achieve the NAAQS for CO.
This report presents a synopsis of how each study was conducted, the
questions addressed by the studies and the major results from each study.
Synopsis of Studies
The first study is summarized in the Mobile Source Emission Inventory
Report (Reference 2). That study focuses on the development oc emission
inventory estimates for a variety of mobile pollution souices. The
purpose of the study was to evaluate the contributions of various mobile
sources to the total emission burden. The study was performed to
provide one input into the OMSAPC Program Assessment Group's evaluation
and direction of mobile source control programs. Both current and future
year (up to 2005) carbon monoxide county inventories were estimated and
presented in this report. The estimates were based on the 1978 Mobile
Source Emission Factors document (Reference 3) and the 1977 National
Emissions Data Systems (Reference 4). Eighteen different types of mobile
sources were considered. The contribution of unregulated as well as
presently regulated mobile sources were included. Inventories for
non-methane hydrocarbons and oxides of nitrogen were also estimated. The
sensitivity of the emission inventory estimates to low ambient tempera-
tures, different stationary source control and retirement rate assump-
tions, different driving modes and different mobile and stationary source
growth rate assumptions were considered. Finally, the results were
interpreted in terms of their likely air quality effect.
-------
The purpose of the second study (Reference 5) was to determine the
temporal and meteorological characteristics that are associated with
deteriorated air quality. The study used meteorological observations
obtained from the National Weather Service (NWS) for 60 U.S. cities in
calendar year 1975-1977. Corresponding ambient concentration data were
provided by the EPA Storage and Retrieval of Aerometric Data (SAROAD)
system (Reference 6). In general, sites were ranked and chosen according
to the number of times the NAAQS was exceeded in the 1975-1977 period.*
The parameters studied included:
1. Month of the year
2. Hour of the day
3. Observation temperature
4. Rush hour temperature
5. Pre-rush hour temperature
6. Absolute humidity
7. Barometric pressure
8. Wind speed
9. Insolation
10. Atmospheric stability
Ambient CO concentration levels were related to each of these temporal
and meteorological parameters.
EPA Region X sponsored the third set of studies in Seattle, Washington
and Boise, Idaho (References 7 and 8) to find out how high CO concen-
trations were distributed in those two cities. The Seattle study
involved monitoring CO concentrations at 36 outdoor sites, five indoor
sites and two pedestrian walking routes. Data were collected for twenty
days in the fourth quarter of 1977. The fourth quarter corresponds to
the season when high carbon monoxide levels frequently occur. A similar
study methodology was established for Boise. The purpose of these
studies was to determine the magnitude and spatial extent of the carbon
monoxide problem and the representativeness of the central business
districts' permanent monitors. Although this set of studies was not
sponsored by OMSAPC, they have been included in the report because of
their relevance.
*CO 10 mg/nj3 (8 hour average)
Ozone 160 ug/m^ (1 hour average)
An exceedence of one of these levels is not necessarily a violation of
the standard. The standard may, for example, recognize only one viola-
tion per day. If the concentration level was higher than the standard
for an entire day, then while there would be only one violation, there
would be 24 exceedences.
-------
EPA also collected data in the vicinity of congested intersections - hot
spots - in another study to find out more about the meteorological,
traffic and CO background conditions in specific areas that have a
history of high CO concentrations (Reference 9). One intersection was
chosen in each of the four cities: San Jose, Phoenix, Seattle and
Chicago. Vehicle and traffic characteristics, as well as meteorological
parameters and ambient concentration levels, were measured. Several
techniques were used to collect these data. Postcard survey and driver
interviews were used to collect vehicle characteristics information. A
traffic analyzer and instrumented pace car were used to collect traffic
speed and volume data. Ten CO sampling devices were placed in the
vicinity of each intersection, some in close proximity to the inter-
section to measure the effects of local traffic and some further away to
measure urban background concentration levels. Each of the vehicle and
meteorological variables were related to the ambient CO level. Further,
an attempt was made to ascertain the degree to which vehicles operating
in the immediate vicinity of the study intersection were responsible for
the observed ambient concentrations.
Another study was performed to estimate the amount in-use vehicle
emissions must be reduced to achieve the NAAQS for CO. This work is
described in a memorandum entitled "Percent Reduction Needed in Mobile
Source Emissions" memorandum (Reference 10). Data from over 50 of the
worst CO problem cities in the U.S. for the three year period 1975-1977
were included in the analysis. Ambient CO concentration data from the
SAROAD system were combined with meteorological data from the National
Weather Service and the 1980 draft mobile source emission factor data
from EPA. These data are the same as those that were used in the second
study of this series. Cities were chosen on the basis of the number of
hourly concentration values in one year that exceeded 10 mg/nH. In
general, the cities with the greatest number of exceedences were
included. The percent reduction required from mobile sources to achieve
the NAAQS was calculated as function of ambient temperature. This
required percent reduction was estimated for each calendar year from 1978
through 1995. In the years that the expected percent reduction needed
became zero, no violations of the CO standard were predicted.
Each of these studies was designed to address a different aspect of the
high CO concentration problem. Together, they attempt to answer the
following questions:
1. How are CO emissions distributed throughout the country?
2. What is the mobile source proportion of total CO emissions?
3. During what seasons and times of day are CO concentrations
highest?
4. What meteorological conditions are associated with high CO
concentrations?
5. What vehicle and traffic conditions are associated with high
ambient CO concentrations?
-------
6. To what extent are high CO concentrations related to activity
within a few hundred meters of the CO monitoring instruments?
7. How does the degree of emission reduction required from mobile
sources to meet the NAAQS change over time?
8. How does the degree of emission reduction required from mobile
sources change with temperature?
The following five sections of this report present the study results as
they pertain to these questions. One section is devoted to each study.
Mobile Source Emission Inventory Report
To answer the first two questions, the Mobile Source Emission Inventory
Report focused on the development of emission inventory estimates for a
variety of mobile pollution sources. In 1977, the latest year for which
data were available at the time the report was written, CO emissions from
all sources totaled 113,715,311 tons. On average, 36 thousand tons were
emitted in each of the nations"s approximately 3200 counties. In most
counties, however, fewer than 12 thousand tons were emitted (Figure 1).
Obviously, the total amount of CO emitted is only one aspect of the CO
inventory. The distribution of emissions within counties is also
important. If emissions are uniformly distributed within counties and if
meteorological conditions are the same for all counties, then emission
densities would be better predictors of ambient concentration differences
among counties than would total tonnage. (The density of CO for a county
is the total amount of CO emitted annually divided by the county area.)
Like the total annual CO emission levels shown in Figure 1, Figure 2
shows that emission density in most, counties is comparatively low. For
the majority of counties CO density is less than 19 tons per square
mile. However, in five percent of counties CO density is greater than
230 tons per square mile. Since the distribution of CO emissions within
counties is not. uniform and meteorological conditions across counties are
not constant, it is also important to relate CO concentrations to popu-
lation density. Most CO emissions are caused by motor.vehicles and tend
to be concentrated in urbanized areas. This assertion is supported by
Figure 3. The high emission density counties are also the high popu-
lation density (urbanized) counties. The relationship is significant for
two reasons. First, it indicates that a large portion of the population
may be exposed to harmful CO levels. Second, since most stationary CO
sources are in rural areas, it supports the hypothesis that the CO
emissions of greatest concern are caused primarily by mobile sources.
Of the 113 million tons of CO emitted in 1977, transportation sources
accounted for approximately 85 percent. Figure 4 shows the relative
contribution of mobile source emissions to total county emissions for
each county. The horizontal axis indicates the percent of total
emissions contributed by mobile sources. (The percent of emissions
contributed by stationary sources can be computed as 100 percent minus
-------
this number.) The vertical axis indicates the number of counties at each
mobile source contribution level. As Figure 4 indicates, mobile sources
account for a relatively large portion of CO emissions in most counties.
In 5 percent of counties, mobile sources contribute more than 98 percent
of all the CO emitted. The median contribution is 92 percent. However,
as the portion of the curve at the left hand end of the horizontal axis
indicates, there are a few counties in which mobile sources contribute a
small percentage of county emissions. In most of these counties the
dominant emissions sources are forest wild fires and forest managed
burning. This is particularly true in Alaska, Idaho, Montana and Puerto
Rico. However, there are a few counties in the southwest in which
chemical manufacturing is the dominant CO source. These counties often
contain carbon black manufacturing plants, which, when uncontrolled, emit
large amounts of CO.
In order to narrow the focus to the counties with a CO problem, 143
counties showing violations of the eight hour average NAAQS were
analyzed. (At the time the report was written the primary CO, standard
was 10 mg/m . Also, the National Emission Data System data for Con-
necticut was erroneous, so that state's data were excluded.) Most of the
143 counties include a large urban area. Figure 5 shows the distribution
of total CO emissions for these counties. As would be expected, total
average annual CO emission levels tend to be greater among these counties
than among all the nation's counties. Similarly, emission densities in
these 143 counties also tend to be higher (Figure 6). Further, as can be
seen from Figure 7, the general association of high emission densities
with high population densities also prevails among the counties showing
NAAQS violations. Finally, as Figure 8 shows, most county CO is emitted
by mobile pollution sources. The general shape of Figure 8 follows that
of Figure 4. However, while in Figure 4 several counties are represented
in which mobile sources contributed a negligible portion of total CO,
among 143 counties of Figure 8 which represent areas with ambient CO
violations, mobile sources contributed at least 37 percent of total
county CO. The median CO contribution was 95 percent.
To summarize, three conclusions can be drawn from these figures: 1) the
highest CO emissions levels are found in the nation's urban centers;
2) in these areas mobile source emissions predominate; and 3) large
segments of the population are exposed to high CO emission levels.
Identification of Meteorological Conditions
The second study summarized in this report focused on the temporal and
meteorological characteristics that, are associated with deteriorated CO
air quality. This study addresses the third and fourth questions that
were previously listed. Figure 9 shows the relationship between CO
emission rates and ambient temperature developed for the draft 1980
mobile source emission factor document. The specific curves shown
represent the 1975-1977 calendar year period. Since mobile source CO
emissions tend to increase as temperature decreases, the absolute CO
ambient concentration levels in the nation's urban centers also increase
during cold weather months. In Figure 10 the total number of CO eight
hour moving averages greater than the 10 mg/m^ NAAQS are displayed for
each month of the year. As indicated, most exceedences of the standard
-------
occur during the winter months. Figure 11 shows the number of NAAQS
exceedences as a function of pre-rush hour temperature. Pre-rush hour
temperature was arbitrarily defined as the average temperature during the
three hours preceeding the rush hour associated with the exceedence. For
many vehicles the temperature during this period is the temperature
during the time the vehicle was parked. It is this temperature that is
related to CO emissions. For a given length of parked time, CO emissions
tend to increase as ambient temperature decreases. Therefore, one would
expect that an increased number of exceedences would occur at colder
temperatures. This would skew Figure 11 to the left. Instead, the
distribution in Figure 11 is more or less centered around 50°F. The
confounding factor is that there are comparatively few eight hour periods
anywhere in the country during which the average temperature is below
0°F. Normalizing the data for the total number of hours in each tempera-
ture interval, Figure 12 presents the fraction of time that the ambient
CO concentration level exceeded the NAAQS, given that the ambient temper-
ature was within the range indicated. Thus, within a given temperature
interval, the likelihood of a high ambient CO concentration is greater at
low temperatures than at high temperatures.
The likelihood of high ambient CO concentrations is also greater at low
wind speeds and stable* atmospheric conditions (Figures 13 and 14).
These are conditions under which the CO generated from motor vehicle
operation is not readily dispersed. The low wind speeds and stable
conditions occur most frequently during the late Fall than during the
February - March winter months. That is why more standard violations
occur during the fourth quarter of the year than occur during the first-
quarter. The average temperature difference between the two quarters is
not great. However, there is a difference in the prevalence of unstable
atmospheric conditions.
Collectively, Figures 10-14 indicate that CO violations occur most fre-
quently during the October - January season and that, in general, the
colder the temperature during low wind speed and stable atmospheric
conditons, the greater will be the likelihood of a NAAQS violation.
The frequency of CO violations also changes during the day. This varia-
tion is associated with both traffic and meteorological patterns. As
Figure 15 shows, the likelihood of a NAAQS CO violation increases from a
minimum at 6:00 hours (6 a.m.) to a maximum at 17:00 hours (5 p.m.). The
hour indicated in the figure is the ending hour of the eight hour moving
average. Thus, a 5 p.m. ending hour represents the 10 a.m. - 5 p.m.
eight hour average. Traffic volume generally follows a bi-modal pattern
with peaks during both the morning and evening rush hour period. Since
in most localities the morning rush hour is over by 10 a.m., one hypothe-
sis that may explain the pattern of exceedence frequencies given in
Figure 15 is that the emissions from the morning traffic rush mix with
emissions from vehicles operating during the day. That hypothesis would
explain the increased frequency of a NAAQS exceedence in the 1-2 p.m.
period. The same sort of hypothesis can be set forth for the relatively
*Stability classes range from unstable (A) to stable (G).
-------
8
high likelihood of an exceedence during the late evening hours. High
levels of CO seem to disperse slowly, over a period of three or more
hours. For example, if this hypothesis is correct, then emissions from
vehicles operating after the evening rush hour (8 p.m.) would contribute
to the eight hour average ending at 6 a.m. the next morning. (The 8 p.m.
emissions would contribute to the 11 p.m. ambient one hour CO concen-
tration level and that level would then be averaged with the levels from
the subsequent seven hour period, thus forming the eight hour average
ending at 6 a.m. the next morning.) If CO emissions dispersed imme-
diately, then one would expect the eight hour average CO concentration
level to fall to its minimum value eight hours after the close of the
evening rush hour, about 2 a.m.
A second hypothesis that can be supported by Figure 15 is that the
citywide distribution of high CO concentrations may be more widespread
than is sometimes thought. If this hypothesis is correct, the high CO
emission levels orginating at one congested intersection have time to mix
with high CO emissions originating at other congested intersections. If,
indeed, high ambient CO levels are widely distributed over an urban area,
then all segments of the urban population are exposed to high CO levels
and not just those segments located in a one or two block radius of a few
congested intersections.
Seattle, Washington and Boise, Idaho Carbon Monoxide Studies
Evidence supporting the proposition that high ambient CO levels are
widely distributed over an urban area was collected in Seattle, Washing-
ton and -Boise, Idaho. The study results indicate that the carbon
monoxide problem is widespread and not restricted to the downtown
commercial district. Figure 16 shows the locations of the study
monitoring sites within Boise and the maximum 8-hour values over the
entire sampling period. The bar heights are proportional to those
maximum values. The central business district is detailed in the cutout
section of Figure 16. At several sites within the central business
district the maximum 8-hour value exceeded 9 ppm CO. (The 9 ppm level is
approximately equivalent to the 10 mg/m^ NAAQS in the effect at the
time the study was conducted.) However, as Figure 16 shows, the NAAQS
exceedences were not restricted to the district. In several outlying
areas the maximum 8-hour values also exceed the NAAQS.
To paraphrase the report, the spatial extent of the Boise CO problem
encompasses not only the downtown commercial district but also locations
along traffic corridors outside the core area. When higher concen-
trations were observed in the core area, elevated CO levels also occurred
elsewhere. Altogether about 70 percent of the study sites experienced
one or more days when the eight-hour average exceeded 9.0 ppm.
The study also found that pedestrians were, at times, exposed to eight-
hour average CO concentration levels exceeding the standard. This is
inferred from the fact that for nearly sequential sampling periods
totaling seven or more hours, the average exposure was equal to or above
9.0 ppm on four days. Also, for sampling periods between two to four
hours, pedestrians were on one occasion exposed to 14 ppm.
-------
This study, conducted by EPA Region X, also measured CO concentrations at
indoor sites such as large department stores and office buildings.
Changes in the level of CO concentrations at indoor sites often coincided
with changes in CO concentrations at outdoor sites. At several sites,
late afternoon increases in indoor CO were observed, suggesting an influ-
ence from the afternoon traffic peak. On one or more days the eight-hour
average CO concentration exceeded 9.0 ppm at four of the indoor sites.
Finally, although the state monitor in Boise is located in the central
business district, the magnitude of Boise carbon monoxide levels measured
as part of the study was somewhat greater than indicated by the state
monitor. On 19 of the 20 study days the eight-hour standard was exceeded
at one or more sites. Exceedences at the state site occurred on only 9
days. Further, at other locations, eight-hour concentrations above the
standard were up to three times greater than those measured at the
permanent monitor. Since control strategies for an area are based, in
part, upon the levels of CO measured by that area's permanent monitoring
network, it is important that the network accurately reflect the levels
of CO concentration to which the population is exposed. If the measure-
ments made by the permanent monitor are too high, then expensive control
measures may be undertaken unnecessarily. On the other hand, if the
measurements are too low, then the area's population may be exposed to a
significant health risk.
CO Hot Spot Study
The CO hot spot study was designed to characterize the meteorological and
traffic conditions at congested intersections. At each of the four study
sites, ambient CO concentrations were measured in the immediate vicinity
of an intersection. CO monitors were also placed on roof tops, alley
ways and parks away from the intersection in an attempt to measure back-
ground concentration levels. The meteorological parameters were
measured within a few blocks of the study intersection. These parameters
included ambient temperature, barometric pressure, humidity, windspeed
and direction and insolation. (Insolation is a measure of sunlight
intensity.) A traffic counter was placed on the roadway leading into the
intersection while an instrumented pace car monitored traffic speed. As
.vehicles stopped for the red light at the intersection itself, drivers
were asked a series of questions pertaining to their vehicles and the
length of time they had been driving.
Since ten concurrent CO measurements were made for each sampling period,
and since at least a few of the measurements were made away from the
traffic flow, the minimum of those ten measurements was used as an
indicator of the background CO level for the sampling period. The
difference between the maximum and minimum values was used as an
indicator of the CO concentration attributable to traffic at the inter-
section. The difference was then divided by the maximum value to express
the local contribution as a percentage. Thus, a high local contribution
indicates that vehicles at the study intersection are responsible for
most of the measured CO. Conversely, a low local contribution level
indicates that vehicles operating throughout the urban area are respon-
sible. If vehicles removed from the intersection contribute signifi-
cantly to the CO concentrations at the intersection, then CO is a
mesoscale, not a microscale, problem.
-------
10
In San Jose the eight-hour average CO NAAQS was exceeded 10 times during
the study week. On all 10 occasions the sampler recording the highest CO
level was directly downwind of the intersection. Average wind speed was
4 knots; traffic volume was high. The local contribution during these
periods - as we have defined it - ranged from 62 to 98 percent and
averaged 80 percent.
The local contribution during the NAAQS exceedences in Seattle ranged
from 36 to 76 percent and averaged 60 percent. Average wind speed was
2.8 knots. Twice in Seattle high CO concentrations were recorded by all
of the sampling devices in the immediate vicinity of the intersection and
by the "background" devices situated away from the intersection. In
these cases it is probable that the high CO concentrations were not
representative of a typical "hot spot" situation but were widespread and
not restricted to the immediate study area. These particular violations
were likely to be the result of traffic throughout a fairly wide area.
Three other exceedences in Seattle were associated with high local
contributions and followed the pattern seen in San Jose.
Four exceedences of the NAAQS occurred during the test week spent in
Phoenix. High CO concentrations were recorded simultaneously by many
sampling devices there, including those positioned to measure background
concentrations. Since the NAAQS exceedences occurred during eight hour
periods that ended between 1 and 3 a.m., the local contribution averaged
only 35 percent. Thus, the Phoenix location can not be described as a
true hot spot, at least during the study period.
In Chicago there were two eight-hour NAAQS exceedences during the study
week. They occurred after periods of heavy traffic and were associated
with relatively high local contributions - from 79 to 97 percent.
The study findings indicate that the local CO contribution varies
directly with wind speed and insolation and varies inversely with
barometric pressure. It seems that high wind and insolation, and low
barometric pressure are conducive to higher CO dispersion rates. And a
high rate of CO dispersion means that CO from adjacent traffic corridors
tends to be diluted before it reaches the study intersection.
Traffic conditions also influence the local contribution level. The
level varies directly with the proportion of vehicles that are started
after having been parked for several hours. It also varies directly with
the total number of vehicles passing through the intersection and with
the age of those vehicles. Under each of these conditions, vehicle
emissions tend to increase so that the greater the total emission level,
the greater will be the local contribution. This is a typical CO hot
spot situation.
High CO concentrations, at times, were also recorded at all monitoring
locations. This phenomenon was observed most frequently in Phoenix and
Seattle. On several occasions when the highest CO concentration recorded
was greater than 10 mg/m , the local contribution was between 15 and 40
percent. Such a low local contribution indicates that several segments
of the urban area experienced high concentration levels simultaneously.
-------
11
On such occasions, the CO problem is not strictly a hot spot problem but
is a mesoscale problem affecting the entire urban area.
Percent Reduction Needed in Mobile Source Emissions
The purpose of each of the studies discussed so far has been to gather
information about the conditions associated with high ambient CO concen-
trations in an attempt to determine the level of emissions reduction
needed to meet the CO National Ambient Air Quality Standard. Currently,
many areas of the country do not attain the NAAQS. With the principal
exception of the inspection and maintenance strategy, the Federal Motor
Vehicle Control Program has been applied to new vehicles. Thus, as
older, generally dirtier vehicles leave the vehicle pool, the average
level of emissions has declined. As emission rates continue to decline,
so will ambient CO concentration levels. However, the reduction in
average emissions and the resulting improvement in air quality will take
time.
In the final study being summarized the concept of a percent reduction
needed from mobile source emissions was constructed to estimate when and
how many of the nation's counties will meet the NAAQS. The percent
reduction needed is defined in this work as the amount by which mobile
source emissions must be reduced in order to eliminate NAAQS exceed-
ences. As an example, reducing a 40 mg/m^ ambient concentration to 10
mg/nH requires a 75 percent emission reduction. If the maximum CO
concentration measured anywhere in the country is 40 mg/nP, then
reducing emissions throughout the country by 75 percent would eliminate
all exceedences. Reducing emissions by less than 75 percent would
eliminate only a portion of the exceedences.
Figure 17 shows the percent by which the highest eight hour average
ambient CO levels must be reduced in order to meet the CO NAAQS in each
calendar year. Figure 17 assumes that FTP conditions prevail, that no
inspection maintenance program is instituted and that VMT growth is two
percent. Data from over 50 cities with the highest 1975-1977 CO concen-
trations were included in the analysis. The dotted line indicates the
level at which 100 percent of ambient CO standard exceedences will be
eliminated. The solid line indicates the level at which only 90 percent
of exceedences will be eliminated. For example, to eliminate 99 percent
of the total number of eight hour periods that presently exceed the CO
NAAQS, each eight hour average would need to be reduced by 51 percent in
1982 and by 32 percent by 1987.
Several assumptions concerning vehicle miles traveled and stationary
source impact underlie the construction of Figure 17. Vehicle miles
traveled by mobile sources are expected to grow at a two percent annual
compound rate. Stationary source emissions are assumed not to influence
urban concentrations, but to completely disperse before reaching urban
centers. The CO emission factors for the entire highway vehicle fleet
without inspection maintenance are derived from MOBILE2 (Reference 11).
Two points should be made with respect to Figure 17. First, it appears
that 90 percent of NAAQS CO exceedences will be eliminated around 1985.
(The logic here is that if no further reduction is needed to meet the
-------
12
standard, then the standard must have been met.) Elimination of 90 per-
cent of the CO exceedences is a major achievement. However, eliminating
90 percent of the standard exceedences still leaves many eight hour
periods each year during which the public is exposed to unhealthy levels
of carbon monoxide. The remaining 10 percent of exceedences would
generally occur in large urban areas where a significant segment of the
population would be exposed. Second, Figure 17 suggests that, at least
one percent of the exceedences will never be eliminated, since the 99
percent line begins to turn upward in 1995. (Inspection maintenance or
other control programs not currently in place would have the effect of
lowering each of the curves in Figure 17.)
It should be remembered while interpreting Figure, 17 that it represents
only one set of conditions. Other sets of conditions would result in
different curves. Specifically, as we found out in previous studies,
both emissions and ambient concentrations tend to increase at lower
temperatures. Changing the assumptions concerning stationary source
influence, vehicle miles traveled,. vehicle control programs and driving
conditions would also result in different curves. (Temperature was of
prime concern in these studies because CO emission rates are very sensi-
tive to temperature changes.)
The 70 - 75°F temperature interval was used to construct Figure 17.
Figure 18 displays the percent reduction needed from mobile sources as a
function of temperature. Individual vehicle emissions increase at colder
temperatures. At extremely cold temperatures, however, other factors,
such as increased wind speed and decreased atmospheric stability,
disperse the greater per vehicle emissions. As a result, a smaller per-
centage reduction in mobile source CO emissions is required at. tempera-
tures below 0°F than at 0°F. (At the temperature extremes comparatively
few data points determine the position of the curves. Fairbanks, Alaska,
for example, accounts for most of the very low temperatures. Caution
should therefore be used before making any inferences from these sections
of the Figure 18 curves.)
Both the 90 and 95 percent curves of Figure 18 show that the greatest
level of control is needed in the 0 - 5°F temperature interval. A
decreasing level of control is needed over the 0 - 30°F interval. From
30 - 75°F an increasing level of control is needed. Finally, from
75 - 100°F a decreasing level of control is needed. The required levels
of control are summarized in Table 1.
The initial 70 - 75°F interval was chosen as one aspect of study in this
analysis to be consistent with the Federal Test Procedure. Motor
vehicles are tested in that program at a temperature within the 68 - 86°F
range. However, since the likelihood of a standard exceedence increases
as temperature decreases and since the percent reduction required from
motor vehicles is greater at. colder temperatures, Figure 19 was con-
structed with the zero to five degree temperature range. In this colder
range, the elimination of 90 percent of the NAAQS exceedences will noi: be
achieved under the study assumptions. Around 1995 growth in the number
of vehicles increases total emissions more than the reduction in
emissions from each individual vehicle decreases total emissions. Beyond
1995 sufficiently more vehicles enter the vehicle pool to more than
compensate for the reduced emissions from each of them.
-------
13
Conclusion
Each of these studies has addressed a different aspect of the high
ambient CO concentration problem. The National Emission Inventory report
showed that the highest CO concentration levels are found in the nation's
urban centers, that in these areas mobile source emissions predominate
and that large segments of the population are exposed to high CO emission
levels. The meteorological study showed that the likelihood of a CO
National Ambient Air Quality Standard exceedence tends to increase at low
temperatures, low wind speeds and stable atmospheric conditions. These
conditions are most prevalent during the late fall months. Within any
season, the likelihood of an exceedence also changes during the day,
reaching a maximum during the evening rush hours. Evidence from the EPA
Region X Seattle, Washington and Boise, Idaho studies indicates that the
citywide distribution of high CO concentrations may be more uniform than
has sometimes been thought. The study results indicate that the carbon
monoxide problem is widespread and not restricted to downtown commercial
districts. Further, both pedestrians and those individuals indoors are
at times exposed to eight hour average CO concentrations exceeding the
standard. The "hot spot" study characterized the meteorological and
traffic conditions at congested intersections. Under conditions
conducive to high dispersion rates, the local contribution to CO concen-
tration levels tends to be high. Under conditions conducive to low
dispersion rates, the local contribution tends to be low. At low
dispersion rates, the CO problem may be more of an areawide than a hot
spot problem. The percent reduction needed study focused on when and how
many of the nation's counties will meet the NAAQS. That study indicated
that 90 percent of warm weather standard exceedences would be eliminated
by 1985. However, during cold weather, when the likelihood of an
exceedence is greater, an additional 20 percent emissions reduction from
mobile sources beyond that which is already accounted for by current
control measures will be required to eliminate even 90 percent of current
exceedences.
-------
Distribution of CO Emissions For All Counties
FIGURE t
FIGURE 2
goo
720
610
SS60
320
160
80
1000
900
800
u! 700
\ 600
o
° 500
u.
o
c 100
u
r 300
z
200
100
<5 <20 <80 <320
-------
Distribution of CO Emissions For Worst Counties
FIGURE S •
FIGURE 6
ns.o
uo.o
35. 0
-30.0
x
o2S.O
o20.0
|.5.0
a
Z10.0
s.o
0.0
•
^n
^™*
^^
IO <160 <610 <2S60
TOTflU CO EMISSIONS 11000 TONS)
27
2M
221
1"
Sis
u.
o
UJ
" 9
3
6
3
0
'•
__
n
-
-
—
—
~
n
In
0
__
1 1
_r-innl 1
•••
—
IS
tu
i
o
°IO
Ik
0
flC
UJ
A
X
x S
0
'
._/
I ir
h W
. AAA AAA AA/W
<100 <100 <1600 <6>IOO <25600 <80000
<200 <800 <3200 <13800
-------
FIGURE 9 MOTOR VEHICLE EMISSION TEMPERRTURE EFFECTS
155
ISO
125
£120
§110
95
S BO
I 75
•" 70
o
u 65
60
55
\
LDV CO
EMISSION
FflCTOR
MOBILE
SOURCE FLEET
CO EMISSION
FRCTOR
10 20 SO
MO 50 BO 70
TEMPERRTURE IF)
80 90 100 MO
en oo
5600
I "4800
! uooo
i
i
, 3200
' 2UOO
I
I 1600
800
FIGURE 10 EXCEEOEHCES OF TME 8 HOUR CO NRROS
&
JRN. MRR. MRT JULT SEPT. NOV.
FEB. RPR. JUNE RUG. OCT. DEC.
MONTH
FIGURE 11 EXCEEOENCE3 OF THE 8 HOUR CO HRR03
2800.
-65 -US -25 -5 15 35 55 75 95
-55 -35 -15 S 25 145 65 85 105
PRE-RUSH HOUR TEMPERRTURE (F)
-------
Likelihood of a 8 hour CO NAAQS Exceedeiice
ricunt 12
-65 -US -25 -5 IS 35 55 75 95
-SS -35 -IS 5 25 US 65 85 IDS
PRE-RUSH HOUR TEMPERATURE (F>
.12
.11
.10
.09
.08
.07
.06
.05
.04
.03
.02
.01
.0
FIGURE 13
0 4 8 12 16 20 24 28
2 6 10 14 18 22 26 30
WIND SPEED (XT)
.18
.14
g.12
e
x.
~.10
u
X
^ .08
ul
Cl
* .06
o
UJ
u
x -011
UJ
.02
.0
FIGURE 14
FIGURE IS
8 C D E F
ATMOSPHERIC STABILITY
.UOU
.055
.050
S .045
O
" .040
u
5 .035
u .030
£.025
w
S.020
X
".OIS
.010
.005
MI
|
^
^
^
0
1
i.
i
i
2
]
bull
\\i\\ii
4 6 8 10
S 7 9 II
MO
1
1
2
Jfl
I
\
1
S
ii
II
M
i|
1 \
'$
ii
«
IS
IP
03 ^
j|
H
! 1
i I
Ii
( f
li
6 1
17
^
Mi
f
s 1 1
I
ill
III
t 20
19
J
I
1
\
M
»
1
|
1
I
12
23
-------
Figure 16
Scale in PPM CO
T
Denotes 20-Day Sites (11/25/77)
1 Denotes First 10-Day Sites (11/25/77-12/8/77)
na
I Denotes Last 10-Day Sites (12/9/77 - 12/22/77)
U Black Denotes Values Greater than 9 PPM CO
-------
19
Reduction Needed In Mobile Source CO Emissions
flGURE 17
80
HJ70
060
u
o
wSO
2»o
>-
u
§30
5"
u
u
SlO
o o
1978 1980 1982 1981 1986 1988 1990 1992 1994 1996
VMT CROMTMi 2X 70-7SF NO 1/M
FIGURE 18
80
70
60
§50
t-
u
u
•
i-SO
u
u
Sto
-so
-20 10 40 70
PRE-HUSH HOUR TEHPERRTURE
100
130
(Fl
FIGURE 19
w70
a
uSO
§«0
20
lO
O
u o
1978 1980 1982 198U 1986 1988 1990 1992 1994 1996
VHT GROHTHi 2X 0-SF NO 1/M
-------
20
Table 1
Level of Control Required to Meet CO NAAQS
Temperature
Range (F)
0-5
25-30
70-75
95-100
Exceedences
90%
51(a)
36
42
23
of CO NAAQS
95%
56
41
50
24
Eliminated
99%
64
51
64
24
(a) Percent reduction in ambient CO concentrations required to eliminate
90% of NAAQS exceedences in the 0-5°F temperature range. This is a
reduction from the emissions levels observed in the 1975-1977 vehicle
fleet.
Source: Reference 10
-------
21
References
1. Federal Register, Part IV, "Environmental Protection Agency, Carbon
Monoxide; Proposed Revisions to the National Ambient Air Quality
Standards and Announcement of Public Meetings," Monday, August 18,
1980.
2. M. Wolcott, Mobile Source Emission Inventory Report, U.S. Environ-
mental Protection Agency, Discussion Draft, May, 1980.
3. Mobile Source Emission Factors: Final Document, EPA 400/9-78-006,
U.S. Environmental Protection Agency, Washington, D.C., March, 1978.
4. 1977 National Emissions Report, EPA-450/4-80-005, U.S. Envii-onmental
Protection Agency, Research Triangle Park, North Carolina, March,
1980.
5 • Identification of Meteorological Condition's^ During Oxidant, CO and
Nitroge Dioxide Ambient Violation Conditions, U.S. Environmental
Protection Agency, Discussion Draft, April, 1979.
6. Storajye^ and Retrieval of Aerometric Data, U.S. Environmental Protec-
tion Agency, Research Triangle Park, North Carolina.
7. Carbon Monoxide Study, Seattle, Washington, EPA 910/9-78-054, U.S.
Environmental Protection Agency, Region X, Seattle, Washington,
December, 1978.
8. Carbon Monoxide Study, Boise, Idaho, EPA 910/9-78-055, U.S. Environ-
mental Protection Agency, Region X, Seattle, Washington, December,
1978.
9. Analysis of Pollutant and Meteorological Data Collected in the
Vicinity of Carbon Monoxide "HoJ^ Spots", Stanford Research Institute,
International, Discussion Draft, May, 1979.
10. M. Wolcott, "Percent Reduction Needed in Mobile Sources", U.S
Environmental Protection Agency, personal communication to Karl
Hellman, October, 1980.
11. MOBILE2 is a highway mobile source emissions computer model based on
data released in November, 1980.
------- |