EPA-AA-TEB-EF-8 2-3
Carbon Monoxide Episodes
by
Mark Wolcott
November, 1981
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
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Carbon Monoxide Episodes
Backg round
Carbon Monoxide is commonly thought of as a local pollutant affecting
relatively small geographic areas* Since most CO emissions result from
the operation of motor vehicles, high CO concentrations are associated
with the congested areas of large urban central business districts. In
the presence of moderate winds and in the absence of a continuing source
of emissions, ambient CO concentrations diminish fairly quickly. High
concentrations measured during evening rush hour traffic, for example,
often diminish to background concentration levels between three and six
a.m.
Against this background, Kreiss and Lansinserfl] postulated that because
of CO's relatively long residence life (about 30 days), certain
meteorological conditions could cause CO to accumulate over a large
area. If such meteorological conditions persisted long enough,
background levels might eventually become great enough to become a
significant proportion of the total CO budget. For the purposes of this
paper, episodes are defined generally as a sequence of days during which
the minimum daily CO concentration increases from one day to the next.
The existence of such episodes would imply that CO is not only a "hot
spot" (localized) problem, but is sometimes an area wide problem as
well. It would also imply that vehicle traffic on one day might
contribute to high ambient CO concentrations on the next day.
Data Base
The data used in this analysis were derived from the Storage and Retrival
of Aerometric Data (SAROAD) system[2], and consists of hourly
concentrations for various U.S. cities during the January 1, 1975 to
December 31, 1977 period. All data were screened for possible errors
prior to analysis. Of the few errors discovered, most were transcription
errors. These were corrected.
Analytical Procedures
For each SAROAD site studied, the hourly data were converted to eight
hour moving averages. Values for one or two missing hours within an
eight hour period were interpolated before the averaging process was
applied. Finally, the minimum and maximum eight hour averages for each
day were calculated and subjected to the episode criteria.
The purpose of the episode criteria was to isolate only the major CO
episodes. Episodes which lasted fewer than five days or during which at
least one daily maximum did not exceed the CO standard were ignored. To
this end, the following criteria were established:
1) The second daily minimum value must be greater than the first daily
minimum value, and any daily minimum after the third day must be
greater than the minimum value three days previous. Otherwise, the
episode terminates.
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2) At least once during the episode period the daily maximum value must
exceed 9 ppm.
3) Only one missing day can exist during the entire episode period, and
that cannot be the first, second or fourth days.
4) The episode lasts at least five days.
Site Selection
The following seven cities were analyzed for CO episodes: Portland, OR;
Seattle, WA; Chicago, IL; Pittsburg, PA; Salt Lake City, UT; Denver, CO;
and New York City, NY. These cities represent a wide range of
geographical areas and provide a good test of the areal CO hypothesis.
Of the seven, only Chicago and New York City failed to report any
episodes. Both Chicago and New York are characterized by moderately high
wind speed which is conducive to high dispersion rates.
, t-
Associated Meteorology
The analysis of 10 major CO episodes from among the cities studied showed
that meteorological conditions accompanying each episode were nearly
identlcal[3]. Immediately before an episode began, an area generally
experienced the passage of a low pressure system and a moderate to strong
cold front. Associated with the low pressure systems were strong winds
and precipitation. These conditions have a cleansing effect and
generally cause ambient CO concentrations to fall to background levels.
After the low pressure system passed, a large strong high pressure system
would gradually move toward the city. As a result, light surface winds
and relatively little atmospheric mixing would prevail over the area for
approximately one week. As the high pressure system moved across the
area, both the maximum and minimum CO levels increased steadily. CO
accumulation was taking place in the high's stable air. The peak CO
values for both the maximum and minimum 8 hour levels occurred when the
high pressure system was directly over the city. This would be expected,
since the strength of a high's inversion is generally greatest at its
center. In fact, fog almost always accompanied the maximum CO level
during an episode. (The inversion prevents the dispersion of both water
vapor and CO and thus causes both to accumulate near the surface.)
The termination of an episode was also generally the same for each site.
After about'seven days, another storm and cold front would pass by and
again cleanse the atmosphere. However, on some occasions, the CO level
fell without a storm passage. In these instances one can often attribute
the decrease in CO to the reduced mobile source weekend activity.
Figures 1-10 show the maximum and minimum daily CO concentrations and the
meteorology associated with each of the ten CO major episodes analyzed.
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Discussion
Other similarties among episodes exist on a broader scale. Episodes
occurred most frequently during the winter months and occurred least
frequently during summer months. This can be attributed to the usually
large increase in CO emissions from motor vehicles at cold temperatures.
Table 1 shows the seasonal distribution of episodes. (In addition to the
ten major episodes displayed in Figures 1-10, 40 less prominent episodes
are included in the tables that follow.)
/
Table 1
Average Number of Episodes by Season
Winter
Spring
Summer
Autumn
(Dec, Jan, Feb) (Mar, Apr, May) (Jun, Jul, Aug) (Sept, Oct, Nov) Annual
Seattle 4 (40.3)* 1 (49.2) 1 (66.0) 4 (52.1) 11
Portland 5 (40.5) 1 (51.0) 1 (65.2) 4 (53.8) 10
Denver 5 (31.8) 3 (47.2) 3 (70.2) 3 (51.4) 13
Pittsburg 3 (32.0) 3 (51.7) 1 (73.0) 3 (55.5) 10
Salt Lake City 4 (30.6) 0 (49.0) 0 (72.5) 3 (52.1) 7
Interestingly, no statistical differences could be detected between
winter and summer episodal durations. (See Table 2.) Since the passages
of cleansing weather systems occur much more frequently in the winter
than in the summer, one would expect winter episodes to be shorter.
However, in this instance the five day restriction on the duration of
episodes may have resulted in the lack of seasonal differences.
Table 2
Average Duration of Episodes by Season
(Number of Days)
Seattle
Portland
Denver
Pittsburg
Salt Lake City
Winter
7
7
7
7
Spring
9
7
7
7
0
Summer
6
7
7
7
0
Autumn
7
7
6
8
7
No episodes occurred in Salt Lake City during the spring and summer
months.
Table 3 shows the average increase in background CO levels over the
course of an episode. After the air has been cleansed by the passage of
a low pressure system, daily minimum CO concentrations average 1.1 ppm.
These increase over the life of an episode to 4.5 ppm, roughly three
times the original level.
*Average 30 year temperature in degrees Fahrenheit.
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Table 3
Average Increase in Daily Minimum CO Concentrations (ppm)
Seattle
Portland
Denver
Pittsburgh
Salt Lake City
Average
Starting
Daily Minimum
1.2
.6
1.7
1.0
1.0
1.1
Highest Daily
Minimum
4.5
3.7
5.3
4.3
4.6
4.5
Table 4
Difference
3.3
3.1
3.6
3.3
3.6
3.4
Average Increase in Daily Maximum CO Concentrations (ppm)
Seattle
Portland
Denver
Pittsburgh
Salt Lake City
Average
Starting
Daily Maximum
5.2
5.7
6.5
4.2
5.7
5.5
Highest Daily
Maximum
11.9
12.9
12.3
12.8
12.8
12.5
Difference
6.7
7.2
5.8
8.6
7.1
7.0
In the same fashion, daily maximum concentrations begin an episode at
5.5 ppm and increase to 12.5 ppm. (See Table 4.) If man-made CO
dispersed completely from day to day, then it is likely that the highest
daily maximum concentration measured at these sites would be
substantially below 12.5 ppm. Also, the fact that high CO concentrations
do not appear to disperse completely during episode periods indicates
that during these episodes relatively high CO concentrations permeate the
entire urban area. More than the immediate vicinity of a few congested
intersections is affected. As Table 5 shows, there was at least one
episode in each of the five cities during which the daily minimum reached
7.2 ppm. During one episode in Pittsburgh, the minimum concentration
recorded during one day was 11.1 ppm, well above the CO NAAQS.
Table 5
Highest Daily Minimum and Maximum CO Concentrations
Seattle
Portland
Denver
Pittsburgh
Salt Lake City
Number
of Episodes
34
31
42
31
24
Highest
Daily Minimum
8.1
7.5
9.1
11.1
7.2
Highest
Daily Maximum
16.1
16.7
20.7
21.4
16.7
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While any exceedance of the NAAQS is important, State Implementation
Plans for CO are generally targeted to reduce the second highest eight
hour average CO concentration to a level at or below 9 ppm. The current
standard specifies that the NAAQS is not to be exceeded more than once
per year; one excursion above the standard level is allowed. Since the
second excursion constitutes a violation, states seek to reduce the
second highest eight hour standard.
For each of the cities studied in this report, the twenty highest
measured CO concentrations were obtained for each of the three years,
1975-1977. These values were then classified as to whether or not they
were associated with a CO episode. Table 6 displays the results of 300
possible values, over half were associated with an episode. More than
one-half of the second highest measured CO concentrations were associated
with CO episodes.
Table 6
Incidence of CO Episodes Among Highest Twenty CO Concentrations
1975 1976 1977 Total
Seattle 12* 1 9 22
Portland 0 12 7 19
Denver 5 ** 13 **
Pittsburgh 20 15 8 43
Salt Lake City 12 16 20 48
*Number of highest CO concentrations associated with episodes.
**Missing data.
Conclusions
Clearly, CO episodes contribute in an important way to violations of the
National Ambient Air Quality Standard. During episodes, the average
daily maximum increases from 1.1 ppm to 4.5 ppm; the average daily
maximum increases from 5.5 ppm to 12.5 ppm. In the cities examined for
this report, more than one-half the worst NAAQS violations are associated
with episode periods.
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Figure 1
Seattle
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10
References
1. Kreiss, W. T. and J. M. Lansinger, "Carbon Monoxide Background
Concentrations and Worst Case Analysis", Physical Dynamics, Inc.,
Bellevue, Washington, 98009.
2. Storage and Retrieval of Aerometric Data, U.S. Environmental
Protection Agency, Office of Air Programs, Research Triangle Park,
North Carolina.
3. Daily Weather Maps, U.S. Department of Commerce, National Oceanic and
Atmospheric Administration, Environmental Data Service.
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