EPA-600/4-77-009
February 1977
Environmental Monitory Series
         DIURNAL VARIATIONS  IN CARBON  MONOXIDE
            CONCENTRATIONS,  TRAFFIC  COUNTS  AND
                                           METEOROLOGY
                                  Environmental Sciences Research Laboratory
                                       Office of Research and Development
                                      U.S. Environmental Protection Agency
                                 Research Triangle Park,. North Carolina 27711

-------
                 RESEARCH REPORTING SERIES

Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped  into five series. These  five broad
categories were established to facilitate further development and application of
environmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The five series are:

     1.    Environmental Health Effects Research
     2.    Environmental Protection Technology
     3.    Ecological Research
     4.    Environmental Monitoring
     5.    Socioeconomic Environmental Studies

This report has  been assigned to the ENVIRONMENTAL MONITORING series.
This series describes research conducted to develop new or improved methods
and  instrumentation for the identification and quantification of environmental
pollutants at the lowest conceivably significant concentrations. It also includes
studies to determine the ambient concentrations of pollutants in the environment
and/or the variance of pollutants as a function of time or meteorological factors.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.

-------
                                           EPA-600/4-77-U09
                                           February 1977
     DIURNAL VARIATIONS IN CARBON MONOXIDE
CONCENTRATIONS,  TRAFFIC COUNTS AND METEOROLOGY
             Gerard A.  DeMarrais
     Meteorology and Assessment Division
  Environmental  Sciences Research Laboratory
     Research Triangle  Park, N.C.  27/11
  ENVIRONMENTAL SCIENCES RESEARCH LABORATORY
      OFFICE OF RESEARCH AND DEVELOPMENT
     U.S. ENVIRONMENTAL PROTECTION AGENCY
     RESEARCH TRIANGLE PARK, N.C.  2/711

-------
                                 DISCLAIMER
     This report has been reviewed by the Office of Research and Developr
 ment, U.S. Environmental Protection Agency, and approved for publication.
 Mention of trade names or commercial products does not constitute endorse-
 ment or recommendation for use.
                            AUTHOR'S.AFFILIATION
     The author is on assignment with the U.S. Environmental  Protection
Agency from the National Oceanic and Atmospheric Administration,
U.S. Department of Commerce.
                                     ii

-------
                                 ABSTRACT
    Although pollutant emission patterns play important roles,  they cannot
adequately explain the diurnal  variations in carbon monoxide concentrations
found in urban areas.  In this  study, hourly data from a large  network of
carbon monoxide monitoring stations, with instrumentation corrected for
moisture interference, are analyzed and compared with traffic flow and
meteorological conditions at several locations in Maryland.   The meteor-
ological phenomena that appear to be important in explaining the diurnal
variations involve the ventilation effects resulting from variable wind
speeds and mixing heights.
                                    iii

-------
                                 CONTENTS

Abstract	 iii
Figures	vi
Tables	vi
    1.  Introduction	   1
    2.  Conclusions and Kecommendations	,  .   .   2
    3.  Data and Method	   3
    4.  Results	   5
    5.  Summary	12
References	13
Appendix
    Site description of carbon monoxide monitoring stations .  	  15

-------
                                  FIGURES


Number                                                                  Page

  1  Locations of CO monitoring stations ...............      17

  2  Diurnal variation of CO concentrations by month, Maryland
       stations, 1973 ........................   18-20

  3  Diurnal variation of occurrence of selected wind speed ranges
       Baltimore, 1951-1960 .....................      21
  4  Diurnal variations of mixing heights, frequency of wind speeds
       greater than 12 mph, traffic flow and CO concentrations.  ...      22
  5  Comparison of diurnal variations in CO concentrations (January).      23
  6  Comparison of diurnal variations in CO concentrations (October).      24
                                   TABLES


Number                                                                  Page

  1  Annual average hourly percentage of total  vehicular traffic  at
       four Maryland highway stations 	      25

  2  Calculated urban monthly average mixing heights and ventilation
       factors for Washington, DC, area, 1973	      26

  3  Calculated urban mixing height and ventilation factors for tne
       Washington, DC, area for January 1973	      27

  4  Calculated urban mixing height and ventilation factors for the
       Washington, DC, area for October 1973	      28
                                     vi

-------
                               INTRODUCTION


    State and local pollution control  agencies  in complying with federal
regulations (1), gather air quality data at a tremendous rate.   The two main
purposes for collecting these data are to determine whether air quality
standards are being exceeded and to provide baseline check points for
assessing changes in air quality.   Little use has been made of  these data,
which have been gathered for many years, as a resource for determining the
effects of variations in meteorology on the observed air quality.

    In this report of a study of data for January through November 197.3,
hourly carbon monoxide (CO) measurements from the air quality network
operated in the State of Maryland are evaluated and compared with specific
meteorological variables.  The major sources of the CO emissions in the
area are motor vehicles (2); traffic counts, therefore, are considered.
Major emphasis is given to the diurnal variation of CO concentrations in
each month, and in two cases, to the differences in CO concentrations between
a relatively unpolluted workweek followed by a  polluted workweek.

-------
                      CONCLUSIONS AND RECOMMENDATIONS


1.  The average diurnal variation of CO concentrations at the nine Mary-
land stations has a bimodal distribution with peaks in the morning and .
late afternoon.

2.  The diurnal variation of CO concentrations at the nine stations is not
well correlated with the local diurnal variation in traffic density.   Further
studies should be undertaken to determine whether this Maryland finding is
typical of other locations.

3.  The diurnal variations of wind speedi mixing height, and traffic  density
explain about 50 percent of the variance in CO concentrations.  More  detailed
studies, with the traffic count and air quality stations in closer proximity
should be undertaken to determine whether a greater amount of the variance
can be explained by the traffic, wind speed and mixing height.

4.  The diurnal variation of CO concentrations consistently shows marked
peaks that persist for 4 hours or less.  New studies should be made to
determine whether averaging CO concentrations for 8 hour periods tends to
minimize a CO problem.

5.  A curbside station and one remote from traffic but in a metropolitan
area showed very little difference in diurnal variations in CO concentrations.
This indicates that CO is not a very localized problem but readily covers
the nearby area.

-------
                              DATA AND  METHOD

CO INSTRUMENTATION AND DATA

    Carbon monoxide concentrations are  determined  by  the  nondispersive
infrared absorption method used  in the  Maryland  air quality  network.  The
instrumentation (3) includes an  infrared  source, sample and  reference cells,
detectors in each cell, a control  unit  and  amplifier,  and a  recorder.
Because water vapor also absorbs infrared radiation,  a standard  drying
technique is used to eliminate potential  interference. The  reference and
sample gas are brought to 60°F and 15 percent  relative humidity  before  being
brought into the cells.

    The CO network in Maryland consisted  of 19 stations in 1973.   A few
stations, however, did not have  the drying  mechanisms  operating  properly
early in the year.  The data analyzed here  are from the file of  the National
Air Data Branch (NADB) for the U.  S. Environmental Protection Agency, which
maintains pollutant data from throughout  the nation.   (No December CO data
were available.).  The NADB hourly average  values  are listed at  the starting
time of each hour.  An individual  station record for  either  a month or  work-
week was not used unless each hourly period had  at least  60  percent of  the
possible measurements; monthly data were  not used  unless  at  least 6 months
of information were available.  The names and  locations of the 12 stations
from which data were used (five  in the  Baltimore area, five  in the
Washington, DC area, and two in  the northwest  part of the state)  are shown
in Figure 1.  A brief description of each station  can be  found in the appen-
dix.
METEOROLOGICAL DATA


    Analysis of the monthly CO data for the individual  sites  showed  signi-
ficant diurnal variations in concentrations as  well  as  month-to-month  and
station-to-station variations.  These variations  are compared with varia-
tions in wind speed for Baltimore later in this report.   Mixing height
variations can also be important in evaluating  air pollution  problems  (4,5),
and the available mixing height information (6) for the Washington-Baltimore
area Include calculated morning and afternoon mixing heights  and ventilation
factors (depth of the mixing layer multiplied by  the average  speed in  the
layer).  These mixing heights are not to be considered  absolute numbers  but
only relative Indicators of vertical  mixing. This condition  1s particularly
true 1n the early morning when, assuming an urban hear  island of 9°F,  the
calculated mixing height is low (for example, 100 meters or less).   Such a
low height indicates the likelihood of a high degree of thermal stability
near the ground 1n urban as well as rural  areas.

-------
TRAFFIC PATTERNS


    Concurrent data on the diurnal variation of traffic flow at the indivi-
dual pollutant monitoring sites are not available.  The Bureau of Traffic
Engineering of Maryland (11) operates 39 continuous-count stations; four
stations were selected as being representative of the areas of concern in
this study.  The diurnal variation in 1973 traffic flow, in percentages,
is shown in Table 1.  The three counting stations on Interstate-695 (the
Beltway around Baltimore) are representative of the ten CO sampling sites
in the Washington and Baltimore areas.  (The counting station west of the
Baltimore-Harrisburg Expressway is only 2 miles west of the CO monitoring
station at Towson and the counting station south of U. S. 1 is 4 miles
northwest of Linthicum).  The fourth station shows the traffic pattern in
a Hagerstown suburb and is representative of the CO sampling sites in
Hagerstown and Cumberland.

    The traffic data in Table 1 show that the lowest hourly traffic flow,
generally 1 percent or less of the 24-hour flow, occurs from 2 to 5 AM.
Then the traffic increases and attains a peak of about 6.6 percent between
7 and 8 AM.  The traffic remains around 5 percent per hour from 9 AM to
3 PM and then increases to a plateau-like peak for about 3 hours.  This
afternoon peak is higher than the morning peak.  During the early evening
and nighttime, the traffic-flow gradually decreases reaching its lowest
point in the early morning hours.  The traffic pattern at the Hagerstown
station differs from the others in that the morning peak is not as high
and the traffic in the evening is a little greater.

-------
                                  RESULTS


DIURNAL VARIATIONS OF CO


    Data for nine of the stations shown in Figure 1  met the criteria discussed
earlier for this study.   Figure 2 shows time from midnight to midnight as  the
abscissa and concentrations in parts per million (ppm,  by volume;  1  ppm =
1.146 milligrams per cubic meter, under standard conditions)  as  the  ordinate
for these stations.  Average hourly concentrations are  plotted as  points at
the starting time of the observation.   Examination of these data reveals a
more complicated relationship among factors causing diurnal pollutant vari-
ations than the observation (12) that peak concentrations coincide with peak
traffic flows; the correlation between the diurnal variation in  CO concen-
trations at Linthicum and the traffic at the 1-695 station south of  U. S.  40
is 0.39 for January and  only 0.05 for July.  The sunrise and sunset  marks
in Figure 2 reveal a tendency for the peak concentrations to correlate with
the times of these phenomena.  Another interesting feature of the  results  is
that Bethesda, a station located about one-quarter mile from heavy traffic,
shows relative concentration peaks similar to those of  the curbside  station
at Cumberland (see station description in Appendix). These detectable peaks
observed away from the sources suggest that CO rapidly  spreads over  the
urban area.

    The patterns of the  diurnal concentration curves for the individual
stations over the study  period vary from little month-to-month consistency
at Essex (Figure 2) to consistent, double-humped curves with morning and
afternoon peaks at Gaithersburg, Cumberland, Hagerstown, and Hyattsville.
At a majority of the stations, however, there is good month-to-month consist-
ency.  Because most stations have individual, unique characteristics, no one
station 1s representative of the area shown in Figure 1.

    Another characteristic that is consistently found at a majority  of
stations is the progressive shift of the morning peak to earlier times in
the first half of the year and to later times from July to November.  A
similar progressive shift at a majority of stations is  not seen  in the
afternoon data because:   (1) peaks are sometimes difficult to detect (Linthicum),
(2) double peaks occur at times (Baltimore), and (3) the month-to-month shift
of the peak is Inconsistent at some places (Linthicum,  Hagerstown, and
Hyattsville).  A reversal of the shift pattern is seen  at three  stations
(Gaithersburg, Bethesda, and Cumberland) 1n that the peaks move  progressively
te later times 1n the early half of the year and to earlier times  from
June through November.  A fairly consistent pattern among the stations 1s
for the higher peak values to occur 1n the colder part  of the year with the
extreme peak generally occuHng in the morning in January.

-------
Wind Speed and Mixing Height


    The general pattern of concentration variation (Figure 2) shows a peak
in the morning followed by a rapid decrease; relatively low concentrations
in the forenoon and most of the afternoon; another high value in the after-
noon or evening; and, finally, a slow decrease in concentrations through
the night.  In an attempt to explain this general pattern, the first phenom-
enon sought was one that would allow for diluting the concentrations pro-
duced by the moderate-to-heavy traffic of midday to the same values as those
associated with the light traffic of early morning (see Table I).  Wind speed
is an obvious phenomenon that would contribute to this result.  The climatol-
ogical wind speed data for Baltimore (10)(Figure 3) show the variation of
low speeds (0-3 mi/hr) and high speeds (13-24 mi/hr and 25 mi/hr and greater)
for each midseason month.  The dots in Figure 3 show the 1973 average fre-
quencies of winds in excess of 12 mi/hr (high speed), based on the three-
hourly observations currently available (13;.  The 1973 diurnal variation
of high speed differs from the climatological summary but shows the typical
low frequency at night, the increasing frequency in the forenoon with a peak
around noon, and the rather sharp decrease in frequency in the late after-
noon.  Thus, the wind speeds allow for a limited amount of horizontal trans-
port in the morning; then in the forenoon, they provide greater volumes of
air for dilution and downwind transport.  Finally in the afternoon and evening,
there is progressively less wind for dilution and transport.  The higher
frequency of strong winds in April probably contributes to the resulting
low concentrations shown in April graphs in Figure 2.

    Combined with the wind speed in markedly affecting the concentrations
of pollutants is vertical mixing -- the depth (height) of the surface-based
layer through which the pollutants will be readily dispersed (4).  Mixing
height data for 1973 for urban areas in the vicinity of Washington, DC (6)
are shown in Table 2.  The monthly values are averages of daily values
calculated according to a standard procedure (14) for each workday.  (No
weekend data are available.)  Listed in the table are mixing heights,
average wind speed in the mixing layers, and ventilation factors (layer
thickness times speed) for the morning and early afternoon (that is, the
maximum mixing height, corresponding to the maximum surface temperature).
In the morning, the calculated mixing heights were usually low by factors
of 2.5 to 9, compared with the calculated maximum afternoon heights.  The
high average mixing heights arid wind speeds on April mornings are probably
prime causes of the lack of marked peaks in early morning concentrations
(see Figure 2).

-------
    Unfortunately, these mixing height data give only an indication of how
limited the mixing is in the morning and how extensive it is at its maximum;
the temporal change in vertical mixing is not shown.  This temporal change
is directly related to thermal  stability.  The typical urban pattern (4)
shows that thermal instability is restricted to a relatively shallow surface-
based layer (400 meters or less over most of the nation) until a few hours
after sunrise.  Shortly thereafter, as the surface temperatures continue
to Increase, the height of the top of the layer of marked instability
increases rapidly and along with the surface temperature, levels off at
around 2 to 3 PM.  Thereafter,  the mixing heights increase very little
until about an hour before sunset.  Near sunset, as the ground cools,
vertical mixing is inhibited and the condition persists until shortly after
the following sunrise.  Enhanced vertical mixing begins latest in the day
during the winter months and is a contributing factor in the extreme concen-
tration peaks (Figure 2) observed on winter mornings.  Vertical mixing through
a deep layer persists for only a few daylight hours each day during the
winter months; this is probably a prime cause of high average concentrations
during the mornings of the winter months.


Apparent Dependence of Concentrations on Meteorology and Traffic

    The diurnal variations of CO concentrations have been seen to depend on
a combination of the traffic flow and the dilution patterns.  In January,
peak concentrations generally occur around 7 to 8 AM, the time of peak traffic
flow.  In the forenoon, although the traffic decreases to a moderate flow,
the CO concentrations drop rapidly as the wind speeds increase and vertical
mixing takes place to increasingly greater heights.  By the middle of the
day, the increased speeds and mixing heights are sufficient enough to lower
concentrations produced by the moderate mid-morning traffic (about 50 percent
of that during the morning peak) to levels similar to those that occur in
the pre-dawn hours (when the traffic is about 10 percent of that in the
middle of the day).  At about 3 PM, the traffic volume increases, the wind
speeds decrease, and shortly thereafter, the layer of enhanced vertical
mixing is reduced.  (The change from unstable to stable is marked by a
change from a deep layer with vigorous vertical mixing to a shallow layer
with suppressed mixing).  The concentrations increase until they reach a
maximum at around 5 to 6 PM, the time of- peak afternoon traffic flow.
Thereafter, the concentrations decrease gradually as the ventilation is
markedly reduced and the traffic count decreases.  The changes from January
to June show the peak concentrations in the morning gradually shifting to
earlier times and those of the afternoon and evening shifting to later times.
From June to November the respective shifts are in the opposite directions.
These shifts follow the times of sunrise and sunset, which in turn govern
the beginning and ending of solar-induced thermal instability and the onset
of relatively strong daytime winds.

-------
Comparison of Parameters in One Area

    Simultaneous CO, mixing height, wind speed, and traffic observations for
one site for a month are not available.  There are, however, representative
monthly CO, wind speed, and traffic data for the Linthicum area.  The CO
data are from Linthicum (EPA data bank); the climatological wind data are
from Friendship International Airport (13), which is 2 miles to the south-
west of Linthicum; and the traffic data (11) are from an Interstate-695
location, which is 4 miles to the northwest of Linthicum.  The urban mixing
height data calculated for the Washington-Baltimore area are considered
representative of the Linthicum area.

    Figure 4 shows the diurnal variation of the four parameters for January
and October 1973.  The curves connect points that are averages for an hour
plotted at the ending time of the hour.  Certain assumptions had to be made
with regard to mixing height because only two values are given for each day.
These assumptions were:  (l) the morning mixing height value was represen-
tative of the hours from sunset until an hour after sunrise, (2) the maximum
mixing height occurred at 2 PM and remained stationary until an hour before
sunset, (3) the change in mixing height from an hour after sunrise until
the maximum occurred was linear, and (4) the change from the maximum mixing
height to that which prevailed throught the evening and nighttime was
discontinuous.  These assumptions are in accord with the diurnal variation
of mixing heights described earlier.  (As stated earlier, the mixing height
values should be interpreted qualitatively; they are generally inversely
proportional to atmospheric stability.)

    During the early morning hours in January, the traffic flow and CO con-
centration curves appear to be very similar in shape, with the peak in con-
centrations following the traffic flow peak by 1 hour.  In the forenoon, the
concentrations decrease by about 70 percent, whereas the traffic flow decreases
by only 30 percent.  Simultaneously with those decreases, the frequency of
high speed (>12 mi/hr) winds increases and mixing heights extend to progress-
ively greater heights; these meteorological factors combine to allow for
substantial dilution of the concentrations.  During the early afternoon, the
increase in the mixing height and decrease in the frequency of high speed
winds combine to lower the concentrations only a slight amount.  In the late
afternoon and early evening, both the traffic flow and CO concentrations show
marked increases with the start of a prolonged plateau-like peak in concen-
trations, which follows the traffic flow peak by 2 hours.  The delay in the
peaking of concentrations occurs because the traffic flow increases to its
maximum at the same time that the mixing height and winds are allowing con-
siderable dilution of the emissions; the peaking in concentrations occurs
after the mixing heights and wind speeds decrease to nighttime low values.
During the last hours of one day and the earliest hours of the next, the CO
concentrations decrease rather slowly although the traffic flow decrease is
rapid.  The CO concentrations with minimum traffic flow are about the same
as they are 1n the middle of the afternoon.
                                     8

-------
    The  results of October are similar to those for January.  The major
differences  are the displacements of the times of peak CO concentrations.
In  the morning, the displacement from January to October is to earlier hours
and in October the peak concentration coincides with the peak traffic flow.
The difference between the January and October meteorology in the morning
occurs because"enhanced vertical mixing, as shown by the initial increase in
mixing heights, starts at an earlier time in October.  In the afternoon, the
shift of peak CO  concentrations from January to October is to later hours,
and the  time delay between peak traffic flow and peak concentrations in
October  is 4 hours.  This afternoon displacement of the concentration peak
coincides with the displacement toward later hours, from January to October,
of  the time  when  vertical mixing subsides (shown by the time when the discon-
tinuity  in mixing height occurs).

    To determine  the statistical significance of the several variables on
the variation of  the CO concentrations, a correlation analysis was applied
to  the January data in Figure 4.  The simple correlation factors between
concentrations and frequency of winds greater than 12 mi/hr, traffic and
mixing height were -0.27, 0.39, and -0.34, respectively.  The multiple
correlation  was 0.72, meaning that 52 percent of the variance in CO con-
centrations  was explained by these three variables.  This correlation is
significant  at the 10 percent level.


DETAILED  DATA ON  POLLUTED  PERIODS

    A previous report (15) sought out the times of day of the greatest CO
concentrations for 8-hour periods in Baltimore and concluded that the night-
time hours were most frequently associated with the highest 8-hour CO con-
centrations. Because that conclusion is inconsistent with the findings of
this investigation, the meteorology that occurred on the five worst days at
the most polluted station (in the previous report) was evaluated.  It was
found that relatively wet weather and high relative humidities prevailed on
all 5 days,  indicating that water vapor may have been a cause of the recorded
high concentrations.  (All moisture problems in the Maryland network have been
corrected; the data for the 12 stations used in this investigation have been
reported to  be free of moisture problems by each local official responsible
for collecting the data).


Meteorological Differences Between Clean and Polluted Workweeks

    The  data from the Maryland network were examined to find a 2-week period
during which a relatively clean workweek (one with relatively low CO con-
centrations) was  followed by a relatively polluted workweek and for which an
abundance of data was available.  Two such periods, one in January and the
other 1n October, were found.  The Dally Weather Maps (10), three-hourly
weather  observations for Baltimore and Washington (9), and the calculated
mixing heights  (6) for the periods were gathered and the differences noted.

-------
    Data from nine stations are used in the January comparison of the clean
(January 8-12) and polluted (January 15-19) workweeks.   The resulting dif-
ferences in hourly concentrations for each combined 5-day period are seen
in Figure 5.  Combining the observations for each hour  did little to minimize
the marked differences in concentrations.  At almost all stations and for a
large majority of hours, the concentrations during the  polluted week are
markedly greater and the greatest differences are observed at the times  of
the peak concentrations.

    A high pressure area centered over the middle of the United States
dominated the weather over Maryland during the clean workweek, January
8-12.  Aloft there were westerly winds, and the surface winds at Washing-
ton National Airport and Baltimore's Friendship Airport were generally
light northwesterly through the week,  the average temperature for the
period was about 10*F colder than normal.  The sky was  clear and the
visibility was 10 miles or more for most of the week.

    A high pressure area over the eastern United States was the major
feature of the weather during the polluted workweek, January 15-19.   On
Friday January 19, a cold front was situated just west  of Maryland.   The
flow aloft showed that air was being brought in from the southwest.   The
surface winds at both airports were northwesterly on Monday but south-
westerly the rest of the week.  The average temperatures started out 1°F
to 2°F warmer than normal and rapidly increased to 13°F to 15°F warmer
than normal by the end of the week.  Approximately a quarter-inch of rain
fell on Friday afternoon.  The visibility was onlv a little less than the
previous week (lowest reported visibility 6 miles) and  most days were
cloud-free.

    The urban mixing height data are shown in Table 3.   The morning  mixing
heights and wind speeds are seen to be markedly less in the polluted week
than in the relatively clean week.  The ventilation factors at the time  of
maximum mixing heights show that there was relatively little relief  in the
afternoon during the polluted week, whereas there was much greater ventila-
tion during the clean week.

    To determine the statistical significance of the variables on the
variation of CO concentrations, a correlation analysis  was applied to the
CO data for Linthicum in Figure 5, the mixing heights in Table 3, the con-
current hourly wind speeds for Baltimore Airport, and the traffic counts for
Station 14 shown in Figure 1.  The simple correlations  between CO concentra-
tions and wind speed, traffic, and mixing height were -0.63, 0.13, and -0.38
respectively.  The multiple correlation was 0.73, indicating that the three
parameters explained 53 percent of the variance in concentrations.  This
correlation is significant at the 5 percent level according to an f-test.

    Data from 11 stations for the October period were available in sufficient
quantity to make comparison of the clean workweek (October 15-19) with the
polluted workweek (October 22-26).  The results are seen in Figure 6.  At a
majority of stations, practically all hours showed markedly higher concen-
trations, and the differences were largest during the evening.  At Riviera

                                    10

-------
Beach, Silver Spring, and Gaithersberg, practically all  the differences
were at times of the peaks, and at these times the concentrations were
markedly greater during the polluted week.

    A cold front passed through Maryland late on Monday, October 15,  1973,
and a high pressure center dominated the weather from Tuesday through Friday.
The flow aloft over Maryland was generally westerly throughout the period.
The temperature started out warmer thanonormal but became colder with the
passage of the front, averaging about 5°F below normal.   There was precipi-
tation at Baltimore and at Washington with the passage of the front on Monday.
Except for one observation during a rain shower, all visibilities were 15
miles or greater, and except for six observations there were no clouds at
Baltimore.  The visibility and cloud observations at Washington were very
similar to those of Baltimore except that the visibility was never less
than 10 miles.

    A high pressure center remained over the eastern United States through
almost all of the polluted workweek.  Late on Friday, October 26, a cold
front passed over Maryland.  The flow aloft over the area was initially
from the west, about 10 knots, but then became light and variable.  No
precipitation occurred during the period, and although the temperatures
started out colder than normal they became warmer than normal, by about
4*F, most of the week.  There were no clouds and the visibilities were
relatively low (but never less than 2 miles) throughout the period.

    The urban mixing heights and ventilation factor data (6) are shown'in
Table 4.  The morning mixing heights and ventilation factors were much
greater during the clean workweek than during the polluted workweek; there
was generally a four-fold advantage in the ventilation.

    The simultaneous observations of concentrations, mixing heights, wind
speeds, and traffic for the Linthicum area for the October period were
analyzed to determine the statistical significance.  The simple correlation
with concentrations and wind speed, traffic, and mixing height were -0.51,
0.11, and -0.41 respectively.  The multiple correlation was 0.68, indicating
that 46 percent of the variance is explained by the three parameters.  This
correlation is significant at the 5 percent level.
                                    11

-------
                                  SUMMARY
1.  The average diurnal variation of CO concentrations tends to show a
    peak near the beginning of the normal workday, followed by a rapid
    decrease and leveling off that persists until about midafternoon;
    then a rapid increase to high values that are maintained for about
    3 hours in the evening; and then a gradual decrease to low values
    in the early morning.  The low values of midday and early morning
    are comparable.

2.  The diurnal variation of CO concentrations is not well correlated
    with the diurnal variation in traffic density.

3.  The diurnal variation of wind speed or the frequency of winds above
    12 mi/hr, mixing height, and traffic density explain about 50 percent
    of the variance in CO concentrations on a monthly basis and when a
    polluted workweek is compared with a clean workweek.

4.  The diurnal variation of concentration consistently shows marked
    peaks that persist for 4 hours or less.  Averaging CO concentrations
    for 8-hour periods minimizes the acute exposure that frequently
    persists for several hours.

5.  A comparison of the diurnal variation of concentrations between a
    cur.bside station and one in a metropolitan area that was remote from
    traffic indicated there was little difference between the two sites:
    the CO appears to readily move away from the sources and blanket the
    entire area.
                                    12

-------
                                REFERENCES
1.  United Stated Congress,  Clean Air Act of 1970.  P161-604,
    42 USC 1857 et seq.,  Washington, DC, December 1970.

2.  Dadiani. J., Air Quality in Montgomery County. Maryland 1969-1973.
    Department of EnvironmentaT Protection, Montgomery County
    Rockville, MD, 1974.   129 pp.

3.  Bowles, A., A. Solomon,  and N. Wertheimer, Friendship Heights Air
    Quality Study, Bureau of Air Quality Control Technical Memorandum,
    Environmental Health  Administration, State of Maryland, Baltimore,
    MD, 1974.  29 pp.

4.  Holzworth, G. C., Mixing Heights, Wind Speeds, and Potential for
    Urban Air Pollution Throughout the Contiguous United States, AP-101,
    U. S. Environmental Protection Agency, Research Triangle Park, NC,
    1972.  118 pp.

5.  Holzworth, G. C., "Variations of Meteorology, Pollutant Emissions,
    and Air Quality," (presented at 2nd Joint Conference on Sensing of
    Environmental Pollutants, Washington, DC, December 10-12, 1973).

6.  Hand, J. A., (Air Pollution Meteorologist, National Weather Service
    Washington, DC) Inter-office memorandum Mixing Height Data,
    November 30, 1973. 2 pp.

7.  McCormick, R. A., and C. Xintaras, "Variation in Carbon Monoxide
    Concentrations as Related to Sampling Interval, Traffic and
    Meteorological Factors," J.. Appl. Meteor., 1:237-243, 1962.

8.  Tiao, G. C., G. E. P. Box, and W. J. Hamming, "A Statistical Analysis
    of the Los Angeles Ambient Carbon Monoxide Data 1955-1972," J. Air
    Poll. Control Assoc.  25:1129-1136, 1975.

9.  United States Weather Bureau, Local Climatological Data 1973.
    Monthly Summaries for Washington National Airport and Friendship
    Airport, Baltimore, U.  S. Government Printing Office, Washington,
    DC, 1973.

10. United States Department of Commerce, National Oceanic and Atmospheric
    Administration, Daily Weather Maps (for selected weeks), U. S.
    Government Printing Office, Washington, DC,  1973.
                                    13

-------
11.  Bureau of Traffic Engineering,  Traffic Trends, State Highway
     Administration of the Maryland  Dept.  of Transportation, Baltimore
     MD, 1974.  166 pp.

12.  United States Department of Health, Education, and Welfare, Continuous
     Air Monitoring Projects in Philadelphia 1962-1965. Pub!. APTD 69-14,
     National Air Pollution Control  Admin., Cincinnati, Ohio, p. 49, 52,
     1969.

13.  United States Department of Commerce, Weather Bureau, Decennial
     Census of_ United States Climate -  Summary of Hourly Observations
     Baltimore. Maryland 1951-1960.  Climatograpfiy of the United States
     No. 82-18, Weather Bureau, Washington, DC,  1962.

14.  Gross, E., The National Air Pollution Potential Forecast Program,
     ESSA Tech Memo WBTM NMC 477 National  Meteorological Center, Washington,
     DC, 1970.  28 pp.

15.  GCA Corporation, Transportation Controls to Reduce Motor Vehicle
     Emissions in Baltimore, Maryland,  Report APTD-1443, U. S. Environ-
     mental Protection Agency, Research Triangle Park, NC, 1972.  47 pp.
                                     14

-------
                                 APPENDIX

                   SITE DESCRIPTIONS OF CARBON MONOXIDE
                            MONITORING STATIONS


    Each site description, except Cumberland,  is  based on a site visit.   The
Average Daily Traffic (ADT) counts are based on 1973 data that was  provided
by the Traffic Inventory Section of the State Highway Division of the Mary-
land Department of Transportation.  All the sampling intakes were 10 feet
above the ground except where noted otherwise.

1.  Gaithersburg-Sampling intake is 35 feet above the street level  on the
    side of a building, about 75 feet from the nearest curb of Highway 355
    (ADT-27,000 vehicles) and 150 feet from an intersection with a  traffic
    light.

2.  Silver Spring-Sampling device is in a park about 50  feet north  of the
    nearest edge of Highway 1-495 (ADT 89,800 vehicles).

3.  Bethesda-Sampling device is in a meadow about 100 yards from the nearest
    local traffic (a few hundred cars a day),  0.25 mile  from Old Georgetown
    Road (ADT 35,000 vehicles) and 0.5 mile from Rockville Pike (ADT 42,900
    vehicles).

4.  Linthicum-Sampling device is in a 20-car,  school yard, parking  field
    about 80 feet from the nearest local traffic (probably less than 100
    cars a day) and 200 yards from Highway 1-695 (ADT 60,000 vehicles).
    Friendship Airport is about 2.0 miles to the southwest of this  station.

5.  Baltimore (Calvert and 22nd Sts)-Samp!ing device is  in a 100-car parking
    field about 90 feet from Calvert St. (No ADT) and 50 feet from  22nd  St.
    (No ADT).  Calvert St. is a major, one-way throughfare that carries
    heavy outbound traffic in the afternoon.

6.  Essex-Sampling device is in a 10-car parking field about 10 feet from
    the nearest local traffic (a few hundred cars a day) and 100 yards north
    of Eastern Blvd.  (ADT 36,000 vehicles).
7.  Cumberland-Sampling device is in a grassy field 20 feet from the curb
    of Industrial Blvd. (ADT 21,500 vehicles').

8.  Hagerstown-Sampling device is at rear portion of a 50-car parking
    field about 175 feet from U. S. Highway UN.  (ADT 10,000 vehicles)
    and 175 feet from Maryland Highway 64 (ADT 10,000).
                                     15

-------
9.  Hyattsville-Sampling intake is 10 feet above the ground but below the
    level of the nearby road.  Sampling device is in a meadow 75 feet from
    the nearest edge of Highway 410 (ADT 30,000 vehicles).

10. Towson-Sampling intake is 12 feet above ground on the top, rear end of
     a service building on a college campus, next to a large wooded area.
    The closest road, Highway 1-695 (ADT 76,500 vehicles) is about 150 yards
    to the north of the sampling station.

11. Suitland-Sampling device is adjacent to a 500-car parking field 150 feet
    southwest of Suitland Road (ADT 18,900 vehicles).  Station is 0.3
    mile from Silver Hill Road (ADT 34,000 vehicles) and 0.5 mile from
    Suitland Parkway (ADT 18,500 vehicles).

12. Riveria Beach-Sampling devices is in a 20-car, school yard, parking field
    about 10 feet from the nearest local traffic ( a few hundred cars a day)
    and 0.25 miles from a major throughfare, Fort Smallwood Road (ADT 14,734
    vehicles).
                                    16

-------
                                                                                   PENNSYLVANIA
                                                                                     MARYLAND
                                7 CUMBERLAND
                                8 HAGERSTOWN
                                9 HYATTSVILLE
1 GAITHERSBURG
2 SILVER SPRING
3 BETHESDA
4 LINTHICUM
5 BALTIMORE 22"* 6 CALVERTSTS.
6 ESSEX
                               12 RIVIERA BEACH
13 1-695 WEST OF BALTI-
  MORE HARRISBURG
  EXPRESSWAY
14 1-695 SOUTH OF U.S.
  40
15 1-695 SOUTH OF U.S.I
16 U.S.40WESTOF
  HAGERSTOWN
                                                                   /   \s            I
       Figure 1.  Locations of CO monitoring stations (numbers corresponding to names and numbers shown in Table 1).

-------
                               3.0


                               1.5

                                0


                               4.5

                               2.5

                               0.5


                               3.1

                               1.6

                               0.1


                               3.0

                               1.5

                                0


                               3.5

                               2.0

                               0.5


                               6.0

                               3.0

                                0
                               4.0

                               2.5
                               1.0
                                 12
                                       SUNRISE
                                                   SUNSET
                                         6   9  NOON  3

                                         SILVER SPRING
                                                            9  12
6   9  NOON 3   6

  BETHESDA ' 3')
                                                                                                      12
Figure 2.  Diurnal variation of CO concentrations by month, Maryland stations, 1973.
                                                18

-------
     3.0
   5 1.5
       0
     3.0
   jjj 1.5
       0
     3.0
   I'-5
           SUNRISE
                           SUNSET
 E
  3.0
5"
   0
  3.0
| 1.5
UU
o
O
o
               SUNRISE
                             SUNSET
      3.0
        12
                     9  NOON   3   6
                   LINTHICUM  (4)
                                          12
                                             4.0
                                             2.0
                                              0
                                                3.0
                                                1.5
                                             3.0
                                             1.5
                                              0
                                             3.0
                                             1.5
                                              0
                                             4.0
                                             2.0
                                              0
                                             3.0
                                             1.5
                                                       SUNRISE
                                                                      SUNSET
                                                         SUNRISE
                                                                        SUNSET
                                               12    3    6    9  NOON  3   6
                                                        CALVERT & 221"1 STS. (£l
                                                           BALTIMORE
3.0
1.5
 0

3.0
1.5
 0
3.0
1.5
 0
3.0
1.5
 0
4.0
2.5
1.0
4.0
2.5
                                                                                            1.0
                                                                                          12
                                                                                                  SUNRISE
                                                                                                                       SUNSET
                                                                                                     SUNRISE
                                                                                                                   SUNSET
               9  NOON
                 ESSEX (
                        3   6   9   12
  Figure 2.  (continued) Diurnal  variation of CO concentrations by month, Maryland stations,  1973.
                                                          19

-------
      3.2




   I  "


      0.2



      3.0




   S  '-5
   u.


       0



      3.9



   |  2.4



      0.9
   SUNRISE
                  SUNSET
2
Q.
CL
J.U
1.5
n

-
	 .


-~_/^

	 -
' ^
                                                3.0



                                                1.5



                                                 0



                                                3.0



                                                1.5
       0

        12
                                                       SUNRISE
                                                                         SUNSET
                                             SUNRISE
                                                            SUNSET
3.0



1.5



 0



3.0



1.5



 0



3.0



1.5



 0




3.0



1.5



 0



3.0



1.5



 0
                                                                                      SUNRISE
                                                                                                     SUNSET
                                                                                                                 i
                                                                                                 SUNRISE
                                                                                                                    SUNSET
369  NOON  3   6    9   12       12   3   6   9  NOON  3   6    9   12       12   3    6    9  NOON,


      CUMBERLAND  (7)                            HAGERSTOWN (?)                             HYATTSVULE
                                                                                                                 3    6    9   12
           Figure 2.  (continued)  Diurnal  variation of CO concentrations  by month, Maryland  stations, 1973.



                                                                 20

-------
70
60
SO
40
30
20
10
     T~T
          v>

          EC
"  0


  V
      369  NOON  3    6    9   12

              JANUARY
                                                     70
                                                     60
                                                     50
                                                     40
                                                     30
                                                     20
                                           10
                                                                          /••;
                                                         /   v       \
                                                               u   I
                                                               <2   /

                                  o
                                  u
                                  o
369 NOON 3   6   9   12

          APRIL
                                                           369  NOON 3   6   9    12


                                                                    OCTOBER
       03 MILES PER HOUR
                         13-24 MILES PER HOUR
                                                                     25 MPH AND OVER
   Figure 3.  Diurnal variation of occurrence of selected wind speed ranges Baltimore,  1951   1960.

                     (•"FREQUENCY OF SPEEDS GREATER THAN 12 IN 1973

                           BASED ON 3 HOURLY OBSERVATIONS)
                                            21

-------
12
9      NOON

    JANUARY
                                                                           MH

                                                                          MIXING
                                                                          HEIGHT
                                                                            (m)
                                                       %OF
                                                       WINDS
                                                      >12 mph
  TR

HOURLY
  %OF
TRAFFIC
                         CO

                       CONC.
                        (ppm)
-  900  —  60  —  12   — 3.0 -



-  800  —  50  —  10   — 2.5 -



-700—40—8   — 2.0 -



-600—30  —  6   — 1.5 -



-500—20—4   — 1.0 —



-400—10—2   — 0.5 -



-300—0   —  0   — 0.0 —
                                                                         - 1300 —  60  —  12  — 3.0 -i
                                                                         — 1100 — 50  —  10  — 2.5 -
                                                                         -  900 — 40  —  8  — 2.0 -
                                                                         -700—30  —  6   - 1.5 -
                                                                         -500—20  —  4  — 1.0 -
                                                                         -300—10  —  2  - 0.5 -
                                                                         L-  100 —  0  —  0  — 0.0 —'
                                  NOON

                                OCTOBER
      Figure 4.  Diurnal variations of mixing heights, frequency of wind speeds greater than 12 mph,
      traffic flow and CO concentrations (hourly data plotted at hour ending time - eastern standard)
      1973, Linthilum Area.
                                                   22

-------
IU.3
9.0
7.5
6.0
4.5
3.0
1.5

1 1 1 1 1 1 1
-
\ S~\ DIRTY
\ / \ WORKWEEK
_ \-X JANUARY 22-26 1973 _
_ —
\ X \ CLEAN
- v^ ^- WORKWEEK -
JANUARY 15-19 1973
1 1 1 1 1 1 1
                                                                       i    i   i    I    I   i    r
                                                                                  ©BALTIMORE
                                                                                CALVERT AND 22nd STS. _
 10.5

  9.0

  7.5

  6.0

  4.5

  3.0

 i 1.5
I    I   I   T   I    I   I

       (f3) TOWSON
                                       i    i   i    i   i    i
                                    n    i    r  T   i   r
                                    (7)CUMBERLAND
o 9.0
u
  7.5

  6.0

  4.5

  3.0

  1.5
  16.0

  14.0

  12.0

  10.0

  8.0

  6.0

  4.0

  2.0
I    T   I    I   I    I   I

     ©HYATTSVILLE
    n    i   i    i    i   r
       
-------
  10.5


  9.0


  7.5


  6.0


  4.5


  3.0


  1.5
1—I—I—I—I

  MARYLAND CO DATA

            DIRTY
          WORKWEEK
        -OCTOBER 15-19
             1973

            CLEAN
      ^   WORKWEEK
     /    OCTOBER 8-12
    '         1973
        I	I   I    I    I
                          V-M  I
                                                                         0BALTIMORE

                                                                       CALVERT&22"''STS
  9.0

  7.5


  6.0


  4.5



 I3'"
 a.
z 1.5
4
CC
     I   I    I


     (TtfJTOWSON
             I    I
 T   I   T   II

^CUMBERLAND
           I   I    I    I    I   I

               (T)BETHESDA
                                    I   I    I    I   I

                                      (Tl)SUITLAND
                                                                I   I    I    I   I    I    I

                                                                        (12) RIVIERA BEACH
  16.0


  14.0


  12.0


  10.0


  8.0


  6.0


  4.0


  2.0
  i    i    r  \    T

7)SILVER SPRING
    12   3   6   9  NOON  3   6   9  12
                                                9  NOON  3

                                                 TIME, hour
                                                                       I    T   I    I    \

                                                                     (T)HAGERSTOWN
                                                                      12   3
                                                                     9  NOON  3   6   9  12
Figure 6.  Comparison of diurnal variations in CO concentration.  A clean workweek vs
a dirty workweek, October 1973, Maryland stations.

-------
Table 1.  ANNUAL AVERAGE HOURLY PERCENTAGE OF TOTAL VEHICULAR TRAFFIC
          AT FOUR MARYLAND HIGHWAY STATIONS,  1973a.


                                  Location
1-695 1-695
West of Baltimore South of
Hour Harrisburg Expressway U. S. 40
of day (13) (14)
12-1 AM
1-2
3-4
4-5
5-6
6-7
7-8
8-9
9-10
10-11
11-12
12-1PM
1-2
2-3
3-4
4-5
5-6
6-7
7-8
8-9
9-10
10-11
11-12
1.6
1.1
0.5
0.5
1.1
3.6
7.0
6.8
5.2
4.9
5.1
5.3
5.3
5.7
6.8
8.2
7.8
5.7
4.6
3.8
3.5
2.8
2.3
1.7
1.1
0.6
0.6
1.0
3.6
6.7
6.3
5.0
4.8
5.1
5.3
5.3
5.6
6.8
8.4
7.8
5.9
4.9
4.0
3.6
2.8
2.3
1-695
South of
U. S. 1
(15)
1.9
1.3
0.6
0.6
1.4
4.4
7.5
6.0
4.4
4.3
4.5
4.8
5.0
5.5
6.6
8.3
7.9
5.7
4.8
4.0
3.8
3.1
2.6
U. S. 40
West of
Hagerstown
(16)
2.0
1.5
0.5
0.5
1.2
4.0
5.3
4.7
4.6
4.7
5.1
5.5
5.5
5.9
7.0
8.1
7.0
6.2
5.4
4.5
4.3
3.1
2.6
 The Maryland Department of Transport describes the first three stations as
roads with consistent yearly traffic (little difference among the 12 months)
and the Hagerstown road as one with a consistent yearly traffic load and a
moderate seasonal traffic peak.


 The numbers in parentheses correspond to the site identification numbers
in Figure 1.
                                     25

-------
Table 2.  CALCULATED URBAN MONTHLY AVERAGE MIXING HEIGHTS  AND  VENTILATION
          FACTORS FOR WASHINGTON, DC, AREA, 1973.
Morning conditions



Month
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Morning
mixing
height,
m
332
346
369
556
359
395
312
431
482
360
389
Speed in
mixing
layer,
m/sec
6.0
5.7
5.2
.7.4
4.6
2.8
3.3
3.3
4.2
, 3.9
5.6

a
Ventilation3
mz/sec
1,992
1,972
1,919
4,114
1,651
1,106
1,030
1,422
2,024
1,404
2,178
Afternoon conditions
Maximum
mixing
height,
m
882
1,045
969
1,618
1,883
1,483
1,704
1,788
1,517 ,
1,568
1,470
Speed in
mixing
layer,
m/sec
7.8
7.8
7.5
9.0
9.0
4.5
4.8
4.4
5.8
6.5
9.2

a
Ventilation
m2/sec
6 ,880
8,151
7,268
14,562
16,947
6,674
8.T79
7,867
8,799
10,192
13,524
 Mixing height times wind speed.
                                     26

-------
Table 3.  CALCULATED URBAN  MIXING  HEIGHT  AND  VENTIALTION  FACTORS  FOR
          THE WASHINGTON, DC,  AREA FOR JANUARY  1973.
                  Morning conditions
                                   Afternoon  conditions

Day
of
Month
Morning
mixing
height,
m
Speed
mixing
layer,
m/sec
in
a
Ventilation*
m2/sec
Maximum
mixing
height,
m
Speed i
mixing
layer,
m/sec
n
a
Ventilation
.nr/sec
 8
 9
10
11
12
15
16
17
18
19
420
480
120
100
480
 90
310
 50
 40
 80
6.7
5.7
4.9
3.6
7.7  ,
(0.5)'
4.1
2.1
0.5
5.1
2,814
2,736
  588
  360
3,696 ,
  (45)1
1,271
  105
   20
  408
  650
1,350
1,320
1,290
1,470
  355
  230
  210
  130
  280
5.7
8.6
6.4
9.6
8.8
8.6
7.9
5.3
4.6
6.5
 3,705
11,610
 8,448
12,384
12,936
 3,050
 1,817
 1,113
   598
 1,820
aMixing height times wind speed.

 Value in parentheses indicates estimated speed.   Reported speed was  zero.
                                     27

-------
Table 4.  CALCULATED URBAN  MIXING HEIGHT AND VENTILATION  FACTORS  FOR
          THE WASHINGTON, DC, AREA FOR OCTOBER 1973.
Morning conditions

Day
of
Month
15
16
17
18
19
22
23
24
25
26
Morning
mi xi ng
height,
m
170
1,040
1,020
270
540
180
140
100
170
170
Speed
mixing
layer,
m/sec
4.3
11. 6
S.O
3.4
4-° u
(0.5)b
1.0
1.0
3.1
3:3
in

Ventilation3
mvsec
731
12,064
-8,160
918
2,160
(90)b
140
TOO
527
561
Afternoon conditions
.Maximum
mi xi ng
height,
m
2,300
1,380
.2,300
1,970
2,000
1,200
1,220
1,150
1,550
1,800
Speed in
mixing
1 ayer ,
m/sec
11 .5
11.8
10.4
10.9
5.0
4.0
2.0
4.4
4.3
7.1


Ventilation9
m2/sec
26,450
16,284
23,920
21 ,473
10,000
4,800
2,440
5,060
6,665
12,780
 Mixing height times wind speeds.

"Value in parentheses indicates estimated speed.   Reported speed was  zero.
                                     .28

-------
                                   TECHNICAL REPORT DATA
                           (Please read Instructions on the reverse before completing)
1. REPORT NO.
 EPA-600/4-77-009
                             2.
                                                           3. RECIPIENT'S ACCESSIOI*NO.
4. TITLE AND SUBTITLE
  DIURNAL  VARIATIONS IN CARBON MONOXIDE  CONCENTRATIONS,
  TRAFFIC  COUNTS  AND METEOROLOGY
                             5. REPORT DATE
                              February Iy77
                             6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
                                                           8. PERFORMING ORGANIZATION REPORT NO.
  Gerard  A.  DeMarrais*
9. PERFORMING ORGANIZATION NAME AND ADDRESS
  ENVIRONMENTAL SCIENCES RESEARCH  LABORATORY
  OFFICE  OF  RESEARCH AND DEVELOPMENT
  U.S.  ENVIRONMENTAL PROTECTION  AGENCY
  RESEARCH TRIANGLE PARK, NC   27711
                             10. PROGRAM ELEMENT NO.

                               1AA603
                             11. CONTRACT/GRANT NO.
12. SPONSORING AGENCY NAME AND ADDRESS
  ENVIRONMENTAL SCIENCES RESEARCH  LABORATORY
  OFFICE  OF RESEARCH AND DEVELOPMENT
  U.S.  ENVIRONMENTAL PROTECTION  AGENCY
  RESEARCH TRIANGLE PARK, NC   27711
                                                           13. TYPE OF REPORT AND PERIOD COVERED
                               In-house 9/75-2/76
                             14. SPONSORING AGENCY CODE

                               EPA - ORD
15. SUPPLEMENTARY NOTES
  *0n assignment from the National  Oceanic and Atmospheric  Administration,
   U.S.  Department of Commerce.
16. ABSTRACT
        Although pollutant  emission patterns play  important roles, they cannot adequate-
   ly explain the diurnal variations in carbon monoxide concentrations found  in urban
   areas.   In this study, hourly data from a large network of carbon monoxide
   monitoring stations, with  instrumentation corrected for moisture interference,  are
   analyzed and compared with traffic flow and meteorological conditions  at several
   locations in Maryland.   The meteorological phenomena that appear to be important
   in explaining the diurnal  variations involve  the ventilation effects resulting  from
   variable wind speeds and mixing heights.
17.
                                KEY WORDS AND DOCUMENT ANALYSIS
                  DESCRIPTORS
                                              b.lDENTIFIERS/OPEN ENDED TERMS
                                           c. COS AT I Field/Group
   * Air pollution
   *Carbon monoxide
   * Meteorological  data
   * Traffic surveys
   * Diurnal variations
   * Wind velocity
* Mixing
* Height
~T3~B~
 07 B
 04 B
 13 B
 13 M
18. DISTRIBUTION STATEMENT
   RELEASE TO  PUBLIC
                                              19. SECURITY CLASS (This Report!
                                                 UNCLASSIFIED
                                           21. NO. OF PAGES
                                                34
                                              20. SECURITY CLASS (Thispage)
                                                 UNCLASSIFIED
                                                                         22. PRICE
EPA Form 2220-1 (9-73)
                                           29

-------