United States
Environmental Protection
Agency
Office of Air Quality
Planning and Standards
Research Triangle Park NC 27711
EPA-450/4-79-013
March 1979
Air
Weekend/Weekday
Differences in
Oxidants and Their
Precursors

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                                      EPA-450/4-79-013
 Weekend/Weekday Differences
in  Oxidants  and Their Precursors
                         by

                Yuji Horie, Joseph Cassmassi,
                Larry Lai, and Louis Gurtowski

                Technology Service Corporation
                  2811 Wilshire Boulevard
                Santa Monica, California 90403
                  Contract No. 68-02-2595
              EPA Project Officer: Gerald L. Gipson
                      Prepared for

           U.S. ENVIRONMENTAL PROTECTION AGENCY
               Office of Air, Noise, and Radiation
            Office of Air Quality Planning and Standards
           Research Triangle Park, North Carolina 27711

                      March 1979

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This report is issued by the U.S. Environmental Protection Agency
to report technical data of interest to a limited number of readers.
Copies are available free of charge to Federal employees, current
contractors and grantees, and nonprofit organizations in limited
quantities from the Library Services Office (MD-35), Research Triangle
Park, North Carolina  27711; or, for a fee, from the National Technical
Information Service, 5285 Port Royal Road, Springfield, Virginia  22161
This report was  furnished  to  the  Environmental Protection Agency by
Technology Service  Corporation, 2811 Wilshire Boulevard, Santa Monica,
California  90403,  in  fulfillment of a contract.  The contents of
this report are  reproduced herein as received from Technology Service
Corporation.  The opinions,   findings and conclusions expressed are
those of  the  author and  not necessarily those of the Environmental
Protection Agency.   Mention of company or product names is not to
be considered an endorsement  by the Environmental Protection Agency.
                 Publication No. EPA-450/4-79-013

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                               CONTENTS


FIGURES  	       v

TABLES   	     vii

Section                                                              Page

  1.   INTRODUCTION AND SUMMARY   	       1

      1.1  OBJECTIVES OF THIS STUDY	       2
      1.2  SUMMARY OF CONCLUSIONS  	       3

  2.   SELECTION OF STUDY LOCATIONS AND DATA BASE PREPARATION   .  .       7

      2.1  AIR QUALITY DATA    	       9
      2.2  METEOROLOGICAL DATA   	      12

  3.   DIFFERENCES IN DAILY MAXIMUM OXIDANT LEVELS  	      16

      3.1  SPATIAL VARIATION OF DAILY MAX OX LEVELS	      16
      3.2  WE/WD DIFFERENCES IN DAILY MAX OX LEVELS	      21

  4.   METEOROLOGICAL ADJUSTMENT OF WE/WD OXIDANT LEVELS  	      33

      4.1  WE/WD DIFFERENCES IN METEOROLOGY  	      34
      4.2  WE/WD DIFFERENCE IN OX WITHIN METEOROLOGICAL
             CLASSES   	      38

           4.2.1  AID Decision Tree Analysis   	      39
           4.2.2  Meteorological  Classification  	      48
           4.2.3  WE/WD Differences Within High-OX-Potential
                    Class	      58

      4.3  METEOROLOGICAL ADJUSTMENT OF WE/WD OXIDANT LEVELS   .  .      60
      4.4  CLASSIFICATION  OF  SITES    	     67

  5.   DIFFERENCES IN OXIDANT PRECURSOR LEVELS  	      70

      5.1  DIFFERENCES IN 6-9 A.M. AVERAGE NO AND NO,
             CONCENTRATIONS	*	      70
           5.1.1  WE/WD Differences in NO Concentrations   ....     72
           5.1.2  WE/WD Differences in N02 Concentrations    ...     74

      5.2  WE/WD DIFFERENCES IN N0?/N0v RATIOS   	     73
                                  C,   y\
                                    m

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                               CONTENTS


Section                                                               Page

      5.3  WE/WD DIFFERENCES IN 6-9 A.M. AVERAGE NMHC AND
             THC CONCENTRATIONS	   83

           5.3.1  WE/WD Differences in 6-9 A.M. Average
                    NMHC Levels	   85
           5.3.2  WE/WD Differences in 6-9 A.M. Average
                    THC Levels   	   87

      5.4  WE/WD DIFFERENCES IN CO CONCENTRATIONS  	   90

  6.  RESULTS AND CONCLUSIONS	   96

      6.1  OXIDANT-PRECURSOR RELATIONSHIP IN URBAN CORE AREAS  ...   96
      6.2  OXIDANT-PRECURSOR RELATIONSHIP IN DOWNWIND
             AREAS	103
      6.3  WE/WD OXIDANT CHANGE VS. LONG-TERM OXIDANT CHANGE   ...   108

  7.  REFERENCES    	110

APPENDICES

  A.  STUDENT T-TEST AND WELSH T-TEST   	   112
  B.  WILCOXON  RANK SUM TEST   	114
  C.  CONFIDENCE INTERVAL FOR THE MEDIAN   	115
  D.  THE AID DECISION-TREE PROGRAM	116
  E.  BINARY DECISION TREES DEVELOPED  FOR CLASSIFYING DAILY
        MAXIMUM OX  LEVEL BY METEOROLOGY  	   118

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                                 FIGURES


Number                                                                   Page

  1    Map of the northeastern United  States,  showing  the  five  large
        metropolitan areas  selected for the WE/WD  air quality
        analysis 	    8

  2   Location of 22 monitoring  stations  used for  the WE/WD  oxidant
        air quality analysis  	   11

  3   Location of 22 air monitoring stations, 12 surface  meteoro-
        logical  stations, 3 upper-air meteorological  stations
        used for the analysis	15

  4   Mean weekday levels of  daily maximum OX concentrations 	   17

  5   Mean weekend levels of  daily maximum OX concentrations 	   19

  6   Percentage changes of daily maximum OX  concentrations,
        WE-WD (SS versus MTWTF)	20

  7   Percentage changes of daily maximum OX  concentrations,
        WE-WD (Sun. versus  TWTF)  	   22

  8   Plot of P-values for  testing the WE/WD  difference  in daily
        maximum oxidant levels 	   24

  9   Box-display of WE/WD  daily  maximum  oxidant concentrations
        under normal and Sunday definitionsfor the  Newark,
        New Jersey, site	26

 10   Box-display of WE/WD  daily  maximum  oxidant concentrations 
        under normal and Sunday definitionsfor the  Medford,
        Massachusetts, site.	   28

 11    Box-display of WE/WD  daily  maximum  concentrations--
        under normal and Sunday definitionsfor the  Greenwich,
        Connecticut, site	30

 '.'(.   ';-.->:-- -.nci;-iv- tree for the Newark, New JGrs~y, :"?t~'  ^	1-

 13  Binary decision tree for the Mamaroneck, New  York,  station  ....   44

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                              FIGURES
Number
14
15
16
17
18
19
20
Binary decision tree for the Salem, Massachusetts,
station 	
Preferred wind direction of high-oxidant air pollution
at each of the 22 sites 	
Example of Type I station, which exhibits a consistently
lower weekend OX level in every WE/WD comparison ....
Example of Type II station, which exhibits a consistently
higher weekend OX level in every WE/WD comparison ....
Example of Type III station, which does not exhibit a
consistent relationship among meteorological classes . .
P-values for testing WE/WD differences in 6-9 A.M.
average NO by t-test 	
Graphical presentation of t-test results for WE/WD dif-
Page
45
47
64
65
66
75
       ference in 6-9 A.M. average NO concentrations (Sun.
       vs.  TWTF)   	    76

21   P-values for testing WE/WD difference in 6-9 A.M.  average
       N02 by t-test	    79

22   Graphical presentation of t-test results for WE/WD difference
       in 6-9 A.M. average NO concentrations (Sun. vs.  TWTF)  .  .    80

23   P-values for testing the WE/WD difference in N0?/N0  ratio  by
       the Wilcoxon rank sum test (Sun. vs. TWTF) . 7 ......    82

24   P-values for testing WE/WD differences in CO concentrations
       during the 6-9 A.M., daytime, and 24-hr periods  (SS vs.
       MTWTF)   	    94
                                 VI

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                                   TABLES


Number                                                                   Pa9e

  1    Monitoring Stations Used for Analyzing Oxidant Air
        Quality Data	   10

  2   Meteorologically Grouped Air Monitoring Stations 	   14

  3   Significance of the WE/WD Difference in Daily Max OX
        Level  at Each Station by T-Test and Overall Judgment	   31

  4   Meteorological Station and Corresponding Air Monitoring
        Station Used in WE/WD Meteorology Analysis   	   35

  5   T-Test Results for Testing the WE/WD Differences in
        Each of Seven Meteorological Variables 	   36

  6   List of Meteorological Variables Used for Site-Specific
        Meteorological Classification of Daily Maximum Oxidant
        Concentration by AID 	   41

  7   Qualitative Description of Meteorological Characteristics
        for High, Middle, and Low Classes at Each Oxidant
        Monitoring Site	49

  8   Meteorological Characteristics of High-Oxidant Class
        For Each Site	59

  9   WE/WD Differences in Daily Max OX Levels for All Cases
        and for High Met Class Only (SS vs. MTWTF)   	61

 10   Classification of WE/WD Differences  	   68

 11    Monitoring Stations Used for Analyzing NO and N0? Air
        Quality Data	71

 12   WE/WD Differences in 6-9 A.M. Average NO Concentrations  	   73

 13   WE/WD Differences in 6-9 A.M. Average N02 Concentrations   .  . .  .'  77

 14   WE/WD Differences in 6-9 A.M. Average (N02/NOX) Ratios   	   84
                                    VII

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                                TABLES
Number                                                               Page

  15   WE/WD Differences in 6-9 A.M. Average NMHC
         Concentrations  	    86

  16   Statistical  Significance of WE/WD Differences in 6-9 A.M.
         Average NMHC Concentrations   	    88

  17   WE/WD Differences in 6-9 A.M. Average THC
         Concentrations  	    89

  18   Monitoring Stations Used for Analyzing Carbon Monoxide
         Air Quality Data	    91

  19   WE/WD Differences in CO Concentrations During the 3-Hr
         (6-9 A.M.), Daytime (6 A.M.-9 P.M.), and 24-Hr
         Periods   	    92

  20   WE/WD Means of Daily Maximum OX Concentrations and 6-9
         A.M. Average Precursor and CO Concentrations at Urban
         Core Sites (SS vs. MTWTF)   	    97

  21   Percentage Changes  (WE-WD) in Daily Maximum OX Concen-
         trations and 6-9 A.M. Average Precursor and CO
         Concentrations at Urban Core Sites  (SS vs. MTWTF)   ...    99

  22   WE/WD Means of Daily Max OX and of 6-9 A.M. Average
         Levels of NO, N02, and NMHC:  SUN.  Versus TWTF	   101

  23   Percentage Difference  (WE - WD) in Daily Max OX and
         in 6-9 A.M. Average  Levels of NO, NO?, and NMHC:
         Sun. Versus TWTF	   102

  24   Weekend Reductions  in  6-9 A.M. Urban  Precursor Levels and
         Downwind Daily Max OX Levels (SS vs. MTWTF)   	   105

  25   WE/WD Changes in Daily Maximum Oxidant Levels in Regions
         Upwind of, Downwind  of, and Within  the Large
         Metropolitan Areas (SS Vs. MTWTF)   	   107
                                    viii

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                         1.   INTRODUCTION AND SUMMARY
     The National Ambient Air Quality Standard (NAAQS)  for photochemical
oxidants (ozone) is not attained in many geographical areas,  including Los Angeles,
Houston, and the northeast megalopolis from Maryland to Maine-   These wide-
spread high-oxidant levels indicate the need for more comprehensive control
measures for oxidant precursors than the currently implemented  emission
controls.  To formulate a comprehensive and cost-effective control  policy
requires a better understanding of how sensitive ambient levels of oxidants
are to changes in spatial patterns of precursor emissions, as well  as to
changes in amounts of precursor emissions.  A study of the weekend/weekday
(WE/WD) differences in oxidant and precursor concentrations would be useful
for gaining a better understanding of the precursor emissions/ambient oxidant
sensitivities.
     The WE/WD differences in daily maximum oxidant levels have been examined
by a number of researchers.   In 1974, two studies conducted for different
geographical regions (one for the New York-New Jersey region and the other for
the Los Angeles region) reached, coincidentally, the same conclusion:  That
although primary-pollutant concentrations are measurably lower  on weekends
than on weekdays, daily maximum oxidant levels are not necessarily lower on
weekends [California Air Resources Board (CARB) 1974; Cleveland et al. 1974a
and 1974b],  These earlier findings were later revised when the WE/WD
differences were examined for a wider geographical area and for a larger

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data base.  Recent studies all indicate that although the oxidant level may
be higher on weekends at some very-source-intensive sites, the weekend
oxidant levels are measurably lower in most  areas  [Levitt and  Chock
1975; Cleveland and McRae 1977; Elkus and Wilson 1977; Horie et al.  1977].
     The present study is performed for the northeast megalopolis, extending
from Washington, D.C., to Boston, Massachusetts, and uses large data bases
of air quality and meteorology.  The spatial pattern of the WE/WD differences,
as well as that of weekday levels, is investigated for both oxidant and
precursor concentrations in order to gain some insight into control  strate-
gies that are likely to be the most effective for reducing ambient levels
of oxidants.

1.1  OBJECTIVES OF THIS STUDY
     Despite observed  reductions of ambient precursor levels on
weekends, earlier studies [Cleveland et al. 1974a; CARB 1974;  Elkus  and
Wilson 1977; and Levitt and Chock 1975] have indicated that the WE/WD
differences in ambient oxidant levels are small and inconclusive.  The
weekend levels are higher than the weekday levels at some monitoring sites,
while at other sites the weekend levels are lower or about the  same.   In
view of these findings, the so-called weekend effect raises intriguing
questions about the effectiveness of simultaneous reductions in hydrocarbons
and nitrogen oxides as a policy for controlling photochemical air pollution.
     The objectives of this study are as follows:
     1.  To determine if there are significant differences between WE and
         WD concentrations of precursors and oxidants;

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     2.  To determine the extent to which these differences may be
         functions of meteorological variations;
     3.  To examine differences between WE/WD spatial  and temporal patterns;
     4.  To assess whether further study is indicated  to quantify the effec-
         tiveness of reducing oxidant levels by reducing precursor levels;  and
     5.  To gain greater insight into control strategies that are likely to
         be the most effective for reducing oxidant levels.
To accomplish the above objectives, we compiled a large data base of air
quality and meteorological data, including ambient pollutant concentrations
at 22 sites, surface meteorological observations at 12 sites, and upper-air
meteorological  measurements at 3 sites.  The data analysis is focused on the
five summer months, May through September, during the  years 1973 through 1976.
1.2  SUMMARY OF CONCLUSIONS
     The paragraphs below summarize the findings and conclusions reached in
this study.  For convenient reference, the summary is  organized according
to the order of the sections.

Selection of Study Locations and Data Base Preparation (Section 2)
     Twenty-two air quality monitoring sites in and around each of the five
      large metropolitan areas in the northeast United States were selected
      for study.  The five metropolitan areas are:  Washington, D.C.,
      Baltimore, Philadelphia, New York-Newark, and Boston.
     The 22 study locations represent a wide variety  of site characteristics.
      The study sites were selected from stations upwind of, downwind of,
      and within the metropolitan area.  However, no true rural sites are
      included in the analysis because of inadequate oxidant data at those
      s i tes.
     The 22 air quality monitoring sites are grouped  so that sufficient
      weather data for the analysis can be provided by 12 surface weather
      stations and 3 upper-air weather stations.

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     All the oxidant and precursor-pollutant data, as well as the meteor-
      ological data, were labeled as to the day of week as well as the day
      of year.  These labels were used to compile the data for weekends
      and weekdays, separately.

     Two definitions of weekend/weekday are used:  normal WE/WD defines
      weekends as  Saturdays and Sundays, and weekdays as Mondays through
      Fridays; Sunday WE/WD defines weekends as Sundays only, and weekdays
      as Tuesdays  through Fridays.


Difference in Daily Maximum Oxidant Levels (Section 3)

     The WE/WD differences in daily maximum oxidant levels are examined
      by applying  two different statistical tests to the oxidant data:
      the t-test,  and a visual test of the box-displayed WE/WD oxidant
      data.  The final judgment on the WE/WD oxidant difference at each
      site is made by considering the consistency of the two test
      results.

     Of the 22 stations examined, only three stationsLancaster,
      Pennsylvania; Greenwich, Connecticut; and Danbury, Connecticut-
      exhibit a consistently significant WE/WD difference by the two
      different statistical tests.  These stations all have a significantly
      lower oxidant level on weekends.

t     Only the Newark station exhibits a higher weekend oxidant level with mar-
      ginal significance in the t-test.  However, the box-display of the WE/WD
      oxidant data has revealed that the t-test result is probably biased
      by one extremely high oxidant value in the weekend data set.

t     The percentage decrease in daily maximum oxidant levels on weekends
      is generally higher at sites downwind of the large metropolitan areas
      and is lower at sites within or upwind of the metropolitan areas.

0     The percentage change (WE-WD) tends to be larger when Sundays are
      compared with Tuesday through Friday (Sunday WE/WD) than when Saturdays
      and Sundays are compared with Monday through Friday (normal  WE/WD).


Meteorological Adjustment of WE/WD Oxidant Levels (Section 4)

0     The WE/WD differences in meteorology are examined for seven  meteor-
      ological variables at 12 surface meteorological  stations.   Of the
      seven variables, at least a marginally significant difference between
      the WE/WD levels is found in five variables at two or more meteorological
      stations.

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     The five variables having at least a  marginally  significant  difference
      are visibility,  at six sites;  surface pressure,  at  eight  sites; max
      temperature, at  two sites;  wind  speed,  at  two  sites;  and  mixing height,
      at two sites.

     A pattern recognition  computer program  called   AID was
      applied to find  the oxidant-meteorology relationship  at each
      air monitoring site, using  weekday data only.  The  results
      are expressed  in a binary classification tree, in which the
      relationship between daily  max oxidant  levels  and meteorological
      conditions can be easily  seen.

     The most important meteorological  variable to  explain  the daily max
      oxidant levels is daily max temperature, which appears on 20 out of
      the 22 binary  classification trees developed.  Other  important
      variables are  visibility, on 16  trees;  wind direction, on 14 trees;
      mixing height  and wind through the mixing  layer, on 10 trees; wind
      speed, on 8 trees; and surface pressure, on 5  trees.  Average
      temperature never appeared, probably  because of  the high  col-
      linearity with max temperature.

     Three site-specific meteorological classes,  high,  middle, and  low,
      were developed for each site by  using the  AID  decision tree.
      WE/WD differences in daily max oxidant levels  were computed  for
      five cases:  high, middle,  and low meteorological  classes, all
      days, and all  days with meteorological  normalization.   Then,  each
      site was categorized into one of three station types:   stations
      showing a consistently lower weekend  level  among 4 out of the  5 cases;
      stations showing a consistently  higher weekend level  among 4 out
      of the 5 cases;  and stations showing  np_ consistent WE/WD  difference
      among the 5 cases.

     The stations with a consistently lower weekend level  among 4 out of
      the 5 cases are  found  in the periphery of  or outside  the  large
      metropolitan areas; the stations with a consistently  higher  weekend
      level are found  within the metropolitan areas; and the stations
      without a consistent WE/WD difference among  the  5 cases are  found
      in the transitional zones.


 Differences  in  Oxidant Precursor  Levels  (Section 5)

      WE/WD  differences  in  6-9 A.M. average  concentrations of NO, N02, and
       (N02/NOX)  are examined for  eight  stations reporting an adequate amount
      of NO  and N02 data for the  analysis.   The eight stations are Bethesda,
      Silver  Spring,  Suit!and, and  Baltimore  in Maryland; Camden and Newark
       in New  Jersey;  Welfare Island and  Mamaroneck in New York.

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     Five stations, Silver Spring, Baltimore, Camden, Newark, and
      Mamaroneck, exhibit a significantly lower 6-9 A.M. average NO
      level on weekends.  One station, Bethesda, shows a higher weekend
      NO level with marginal significance.  The remaining two stations,
      Suitland and Welfare Island, do not show a significant WE/WD
      difference in 6-9 A.M. NO concentrations.

t     For 6-9 A.M. average NOg concentrations, all eight stations examined
      exhibit a significantly lower level on weekends than on weekdays.

     The WE/WD difference in 6-9 A.M. nonmethane hydrocarbons (NMHC) and
      total hydrocarbons (THC) is examined for six stations.  Among the
      four stations examined for NMHC, the Baltimore and Suitland stations
      exhibit a significantly lower weekend level under the Sunday WE/WD
      definition.  Under the normal WE/WD definition, all four stations
      showed a significantly lower weekend level.  The two stations,
      Bethesda and Camden, examined for THC under the normal WE/WD
      definition showed a significantly lower weekend level.

     The WE/WD difference in carbon monoxide (CO) was examined for 17
      stations under the normal WE/WD definition.  All but two stations
      exhibit significantly lower 6-9 A.M. concentrations on weekends.

     The results of the analyses indicate that 6-9 A.M. primary-pollutant
      concentrations of NO , NMHC, THC, and CO are indeed lower on weekends
      than on weekdays.


Results and Conclusions(Section 6)

     The average oxidant improvement on weekends is 9% when Sundays are
      compared with Tuesday through Friday (Sunday WE/WD), and 5% when
      Saturdays and Sundays are compared with Monday through Friday
      (normal WE/WD).

     A greater oxidant improvement (11% to 12%) is observed in areas
      downwind of the large metropolitan areas than in the source-
      intensive urban core areas.

     The oxidant improvement  (5% to 12%) on weekends is small compared
      with the 40% to 50% reductions in 6-9 A.M. urban NOX and NMHC levels
      on weekends.  However, the weekend reductions in daytime and 24-hr
      precursor levels appear to be considerably smaller than those in
      6-9 A.M. precursor levels.

     The small oxidant improvement on weekends, brought about by simul-
      taneous reductions in NOX and NMHC, may be an indication that the
      simultaneous control of NOX and NMHC is less effective for con-
      trolling urban oxidants than are hydrocarbon controls.

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         2.   SELECTION OF STUDY LOCATIONS  AND  DATA  BASE  PREPARATION
     The areas in this study encompass  the entire northeast  megalopolis  from
Washington,  D.C., to Boston, Massachusetts.  Although  cities and  towns are
scattered throughout the 400-mile-long study region along the Atlantic  Coast,
a great concentration of the population is found  in the  five large metro-
politan areas (Figure 1):  Washington,  D.C., Baltimore,  Philadelphia, New
York-Newark, and Boston.  According  to  the 1970 SMSA population  summary
[U.S. Commerce Dept. 1973], the study region contains  a  population of well
over 26 million people (New York = 11.6, Philadelphia  =4.8, Washington,
D.C. = 2.9,  Boston = 2.8, Baltimore  = 2.1, and Newark  =  1.9  million  people).
     Ambient air monitoring data [U.S.  EPA 1977]  and special studies
[Ludwig and   Skelar 1977; Wight et al. 1977; Wolff et al. 1977; Cleveland
1976] indicate high-oxidant air pollution over the  study region.   To study
the response characteristics of photochemical  oxidants to changes in
precursors-pollutant levels between weekends and weekdays, we compiled a
large data base of air quality and meteorological  data.   The air quality data
include not  only oxidant (OX) concentration data  (measured as ozone),  but
also concentration data of nitric oxide (NO),  nitrogen dioxide (N02),  oxides
of  nitrogen  (NOX), total hydrocarbons (THC),  nonmethane hydrocarbons (NMHC),
and carbon monoxide (CO).  The meteorological  data include both surface
meteorological observations and upper-air meteorological observations.
Because high-oxidant air pollution occurs primarily in the summer season,
the air quality and meteorological data were compiled for the five warmer
months (May  through September) of the years 1973  through 1976.
                                     7

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                        NEWARK-
                        NEW YORK CITY
                                         -  State  Boundary
                                            Metropolitan  Area
                                         0
                                         Miles
4o
Figure 1.        Map of the northeastern United States,
                showing the five large  metropolitan areas
                selected for the WE/WD  air quality  analysis,
                   8

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2.1  AIR QUALITY DATA
     After considering the data completeness of hourly oxidant and pre-
cursor concentration data during the study period,  22 oxidant monitoring
stations were selected from the areas in and around the five large metro-
politan areas (Figure 1),  Table 1  provides the station name, the EPA's
SAROAD I.D. code, the site type, and the number of  valid observation days
for each of the 22 stations used for the oxidant air quality analysis.
These stations were chosen such that the WE/WD differences could be
examined in areas upwind of, within, and downwind of each of the five
 metropolitan areas.   The geographical locations  of  these monitoring  sta-
 tions are shown in Figure 2 by the  abbreviated  station  names; full  station
 names can be found in Table 1.
      Precursor  concentration data are less complete than the oxidant data.
As a  result, fewer monitoring stations are used for the analysis of
precursor-pollutant data.  Adequate NO and N02 data are found at eight
stations:  Bethesda (BE), Silver Spring (SS), Suitland  (SU), Baltimore  (BA),
Camden  (CA), Newark (NE), Welfare Island (WI), and  Mamaroneck (MA)  (see
Figure  2  for their location).  NO  data at these sites are computed by
                                 X
summing NO and  N02 concentration values.  Adequate  NMHC data are found  at
four  stations only:  Silver Spring, Suitland, Baltimore, and Norristown.
Only  two  stations, Bethesda and Camden, are used for the analysis of THC
concentration data.
     CO was also analyzed because in many cases it  can be used as an
indicator of automotive-related pollutant levels.

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               TABLE 1.  MONITORING STATIONS USED FOR ANALYZING
                         OXIDANT AIR QUALITY DATA
Metropolitan
Area
Washington,
D.C.


Bal timore

Philadelphia



Newark -
New York






Boston



Station
Name
Bethesda (BE)
Gaithersburg (GA)
Silver Spring (SS)
Suitland (SU)
Baltimore (BA)
Lancaster (LA)
Camden (CA)
Camden Co. (CC)
Norristown (NO)
Philadelphia (PH)
Asbury Park (AP)
Babylon (BB)
Danbury (DA)
Greenwich (GR)
Mamaroneck (MA)
Middletown (MI)
Newark (NE)
Welfare Is. (WI)
Medford (ME)
Quincy (QU)
Salem (SA)
Worchester (WO)

SAROAD I.D.
210200005 F01
210780004 G01
211480006 F01
211560001 F01
210120018 F01
394660007 F01
310720003 F01
310740001 F01
396540013 F01
397140026 HOI
310060001 F01
330280002 F01
070175123 F01
070330004 F01
334100002 F01
070570003 F01
313480002 F01
334680050 F01
221220003 F01
221880002 F01
221980001 F01
222640012 F01
EPA's
Site Code*
S-R
S-R
S
S
CC-C
S-R
S-R
Ru-A
S-R
CC-C
CC-C
S-I
CC-R
Ru-NU
CC-C
CC-I
CC-C
CC
S-I
S-I
S-R
CC-C
Number of Valid
Observation Dayst
286
140
248
351
253
232
371
224
230
251
197
238
250
340
377
284
278
362
415
458
184
350
*Code key:

 A - Agricultural
 C - Commercial
CC - Center City
 I - Industrial
 R - Residential
Ru - Rural
 S - Suburban
NU - Near-Urban
t
 A "valid observation day" is defined as a day having at least  10  observations
 for the period 6 A.M.4^ P.M., with the maximum hourly concentration not
 exceeding 50 ppb after 9 P.M. and before 6 A.M.
                                  10

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                                N
                                                                              ^Metropolitan Areas
                                                                        0
25    50
75
                                                                                Miles
NOTE:  See Table 1  for station names.
               Figure 2.   Location  of 22 monitoring stations used for the WE/WD
                          oxidant air  quality analysis.

-------
Adequate CO data are found at as many as 17 monitoring stations among the
22 stations listed 1n Table 1.  These 17 stations  supplement the limited
area coverage of the precursor-pollutant data.

2.2  METEOROLOGICAL DATA
     Two types of meteorological data were obtained from the National
Climatic Center:  surface meteorological data and upper-air meteorological
data.  From the surface meteorological data, daily values of the following
six surface meteorological variables were computed:
          T     -  Daily Maximum Temperature
          T     -  Daily Average Temperature
          P     -  Dally Average Surface Pressure
          V1s   -  Daytime (6 A.M.-9P.M.) Average Visibility
          WD    -  Dally Vector Average Wind Direction
          WS    -  Dally Arithmetic  Average Wind Speed
From the upper-air meteorological data, two pertinent meteorological
variable values were extracted:
          MH    -  Afternoon Mixing Height
          V     -  Wind Speed Averaged Through the Mixing Layer
     Most of  these eight meteorological variables have been correlated
with high oxidant air pollution levels.  For example, high oxidant levels
are usually observed on days with high temperature.  The vector average
wind direction and arithmetic average wind speed were selected to indicate
the direction of pollutant air mass transport and the atmospheric dis-
persion conditions, respectively.
                                    12

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     Table 2 lists the three upper-air and  12  surface  stations  at which
the meteorological data used in this  analysis  were  collected.   The
geographical locations of the three upper-air  meteorological  stations,  12
surface meteorological stations, and  22 air monitoring stations are depicted
in Figure 3 with three different symbols.   The name and I.D.  number of
each station are listed in Table 2.   The circles  and dashed  lines  in the
figure indicate how each air monitoring site is grouped to a  nearby surface
meteorological  station and to one of  the three upper-air meteorological
stations.  The groupings of these stations  are given in Table 2.
                                 13

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TABLE 2.  METEOROLOGICALLY GROUPED AIR
          MONITORING STATIONS
Air Monitoring
Station
Medford
Qu i ncy
Salem
Worchester
Middletown
Danbury
Greenwich
Mamaroneck
Welfare Is.
Babylon
Newark
Asbury Park
Camden
Camden Co.
Norristown
Philadelphia
Baltimore
Bethesda
Gaithersburg
Silver Spring
Suitland
Lancaster
SAROAD I.D.
221220003 F01
221880002 F01
221980001 F01
222640012 F01
070570003 F01
070175123 F01
070330004 F01
334100002 F01
334680050 F01
330280002 F01
313480002 F01
310060001 F01
310720003 F01
310740001 F01
396540013 F01
397140026 HOI
210120018 F01
210200005 F01
210780004 G01
211480006 F01
211560001 F01
394660007 F01
Surface
Met. Station
Boston, Mass.
Worchester, Mass.
Windsor Locks,
Conn.
Bridgeport, Conn.
La Guardia, N.Y.C.
J. F. Kennedy,
N.Y.C.
Newark, N.J.
Atlantic City, N.J
Philadelphia, Pa.
Baltimore, Md.
Washington, D.C.
(National )
Harrisburg, Pa.
(Station
No.)
(14739)
(94746)
(14740)
(94702)
(14732)
(94789)
(14734)
.(93730)
(13739)
(93721)
(13743)
(14751)
Upper-Air
Met Station
(Station No.)
J.F. Kennedy
(94789)
Wash. , D.C.
(13743)
Albany, N.Y.
(14735)
                 14

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CJI
                                                                                         Air Monitoring Stations
                                                                                         Surface Meteorological
                                                                                          Stations
                                                                                         Upper-Air and Surface
                                                                                          Meteorological Stations
                     Figure  3,  Location of 22  air monitoring stations, 12 surface  meteorological
                                 stations, and 3 upper-air meteorological stations used for the
                                 analysis. [Several  air monitS'Hng stations are  grouped to a nearby^
                                 surface meteorological station (in circle) which  is,  in turn, grouped
                                 to one of the three upper-air meteorological  stations  (dashed line).]

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               3.  DIFFERENCES  IN DAILY MAXIMUM OXIDANT LEVELS

     This section discusses the spatial variation of daily maximum oxidant
levels and the WE/WD difference at each of the 22 monitoring sites selected
for the analysis.  The oxidant  data used for the analysis are daily maximum
1-hr oxidant concentrations during the five warmer months, May through
September, in the years of 1973 through 1976.  In developing the data base,
both a data quality check for valid observation days and a data screening
were performed.  A "valid observation day" is defined as a day having at
least 10 observations for the period 6 A.M.-9 P.M. (EOT) with the maximum
hourly  concentration  not  exceeding  50  ppb  before 6 A.M. and after 9 P.M.
 (The  50 ppb  threshold  is  used  to  eliminate potentially  erroneous periods
of data since  nighttime oxidant concentrations are usually less than the
threshold  value.   The  station  name,  site code, and sample size of all the
22 stations  are  listed  in Table 1.   The geographical location of each
station can  be found  in Figure  2.

3.1  SPATIAL VARIATION OF DAILY MAX OX LEVELS
     The average weekday level  of daily maximum oxidant concentrations at
each station is plotted in Figure 4.  The average daily maximum oxidant
levels do not vary much from one metropolitan area to another, except for
those of metropolitan Boston, which are somewhat lower than the others.   The
regionwide oxidant level,  as computed by the 22-station average,  is 68 ppb.
                                     16

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Regional  Avg.  68  ppb
                                                                            Metropolitan Areas
                                                                     0     25    50    75
                                                                      '' Miles'      '
      Figure 4,  Mean weekday levels of daily maximum OX concentrations (weekdays as Monday
                 through Friday).

-------
     The average weekend level of daily maximum oxidant concentrations  at
each station is plotted in Figure 5.  The regionwide average of weekend
oxidant levels (63 ppb) is about 7% lower than that of weekday oxidant
levels (68 ppb).  Both figures show that the daily maximum oxidant levels
are higher at stations distant from the large metropolitan areas than at
those near or within the metropolitan areasa phenomenon which indicates
the importance of considering the area downwind of a city when
formulating oxidant-control strategies.
     A comparison of Figures  4 and  5  Indicates that, while the weekend
oxidant levels at stations away  from  the metropolitan areas are measure-
ably lower  than  the  weekday  levels, the same  is not necessarily true for
stations within  the  metropolitan areas.  Figure 6 shows the percentage
change in daily  maximum OX concentrations at  each station between weekends
and weekdays.  The regionwide average improvement of oxidant air quality
on weekends  1s about 7%.   The three stations  in Connecticut exhibit a
greater percentage improvement (= 15%) than the regional average.  The
greatest improvement (18%) of weekend oxidant air quality occurred at
Danbury, Connecticut.  The five  sites in Maryland exhibit a moderate im-
provement of about 9%.
     In contrast to  these  improvements, the weekend oxidant air quality
at several  urban core  sites  is slightly deteriorated.  These urban core
sites are Newark, New  Jersey; Welfare Island, New York; Philadelphia,
Pennsylvania; and Medford, Massachusetts.  However, the weekend deteriora-
tion of oxidant  air  quality at the four sites is small and may be due to
mere sample biases of  the  weekend and the v/eekday oxidant data.
                                   18

-------
Regional Avg.  63  ppb
                                                                            Metropolitan Areas
                                                                     0     25    50    75

                                                                      '" Miles'      '
            Figure  5,  Mean weekend levels of daily maximum OX concentrations (weekends as
                       Saturday and Sunday).

-------
PO
o
               Regional Avg.


               63   68
                       x  100-  -
                                                                                          Metropolitan Areas
                                                                                   0     25    50    75

                                                                                   1      ' Miles'      '
                    Figure 6.   Percentage changes of daily maximum OX concentrations, WE-WD
                               (SS versus MTWTF).

-------
     Figure 7 shows the percentage changes in daily maximum oxidant
levels when the oxidant levels on Sunday are compared with those on
Tuesday through Friday (TWTF).  The percentage decrease on Sundays (Fig-
ure 7) appears to be generally greater than that on Saturdays and Sundays
combined (Figure 6).  Indeed, the regional average percentage reduction
on weekends is increased slightly from 1% to 10% when Sundays are compared
with Tuesdays through Fridays.
     The next subsection examines the significance of the WE/WD differences
depicted in  Figures  6 and 7.
3.2  ME/KD DIFFERENCES IN DAILY MAX QX LEVELS
     This subsection examines the significance of the WE/WD differences
observed in daily maximum oxidant data by applying several different
statistical methods to the original data.  A weekend is normally defined
as Saturday and Sunday (SS)  and a weekday as any day from Monday through
Friday (MTWTF).  However, it has been noted that the oxidant behavior on
Saturdays and Mondays may be somewhat influenced by the carryover effect
from the preceding day.   Therefore, statistical  testing methods are also
applied to the data set of the other WE/WD definition that treats a week-
end as Sunday only (Sun.) and a weekday as any day from Tuesday through
Friday (TWTF).  To distinguish between the two definitions, the SS week-
end and MTWTF weekday will  be referred to as the normal WE/WD, and the
Sunday-only weekend and the  TWTF weekday will be designated the Sunday
WE/WD.
                                    21

-------
ro
ro
          Regional Avg.
 10%


10%)
\
                                                                                        Metropolitan Areas
                                                                                  0      25     50     75

                                                                                  '      ' Miles'     '
         Figure 7.   Percentage changes of daily maximum OX concentrations, WE-WD  (Sun. versus TWTF).

-------
     The statistical  t-test is employed to determine 1f the WE/WD dif-
 ference observed in the mean daily maximum oxidant data is due to mere
chance (see Appendix  A).   The t-test is applied to the two data sets
derived from the normal  and Sunday  WE/WD definitions.   The results  of the
t-test are plotted  in Figure 8 by using the P-value corresponding to
the value T of the  t-test statistics.   A small  P-value, say,  P _< 0.05,
indicates that the  difference in the two means  (WE and WD  oxidant
levels) is real  and significant.  A moderate P-value,  0.05 <  P <_ 0.10,
implies that the difference between the WE/WD levels is likely to be
real but yet may be due to mere chance.  The larger the P-value,  the
more likely that the  observed WE/WD difference  is  caused by mere  chance
and thus is not significant.
     Under the normal  WE/WD definition, four stationsLancaster, Pennsylvania;
Camden County, New  Jersey; Danbury, Connecticut; and Greenwich, Connecticut--
exhibit  a  statistically significant difference between the  WE and WD daily
maximum oxidant levels (Figure 8).   Under the Sunday definition,  three
stations, Danbury,  Greenwich, and Quincy, show a significant  difference.  All
these stations have considerably lower oxidant levels on weekends than on week-
days.  Several other stations show a marginally significant WE/WD difference in
daily maximum oxidant under either the normal or the Sunday definition.  Among
them, only the Newark station exhibits a higher weekend oxidant level.
However, the weekend  levels at the  Newark station  are only significantly
higher than the weekday levels under the normal WE/WD definition.
                                  23

-------
P-Value
1 * U

0.5




0.1


0.05



0.01

0.005

0.001




*










if if










*



V













  
*
A 	
	 ^ 



*






V*'AV^AAJ
*


 
 



*

*















*

*










.  

** '

(Insic
RE


(Marginally
Re<


(Sign
Reg




* . W *A>>*A, W. '^




r * *

nificant
gion)


Significant
ion)


ficant
on)




1 1
&,.**. ^
                                                  \
           --Under the Sunday WE/WD Definition

         *  --Under the normal WE/WD Definition
Figure 8.   Plot of  P-values  for testing the  WE/WD difference
             in daily maximum  oxidant  levels.
                                24

-------
     Note that in most cases,  the P-values  are larger for  the  Sunday
WE/WD definition than for the  normal  definition,  implying  that the  Sunday
WE/WD differences are less significant  than the normal  WE/WD differences.
However, these larger P-values are primarily due  to  the smaller sample  size
associated with the Sunday WE/WD  data set.   As can be seen  from Figures  6
and 7, the percentage difference  between  weekends and weekdays is greater
under the Sunday WE/WD definition for most  of the stations.
     To confirm the t-test results, we  employed a visual test  comparing
box-displayed data, seeking consistency of  the two results.  The box-display
presents, graphically, several  pertinent  statistical  parameters of  the  data:
the sample size, median value,  25  and 75    percentiles,  first- and second-
highest values, and lowest value.   The  box-display of data  is  more  descrip-
tive than the usual presentation  of mean, standard deviation,  and t-statis-
tics.   Also,  the median,  which is less  affected by outlying data points
than the mean,  provides a more robust means of comparing wE/WD differences.
      First, the box-display method is applied to the stations  that
exhibit  a higher oxidant  level on weekends under at  least one  of the
two WE/WD definitions.  These stations are Newark,  Welfare Island,
Philadelphia,  Medford, and Asbury Park (see Figures  6 and 7).    Figure 9
demonstrates the box-display for the WE/WD daily maximum oxidant
concentrations at  Newark, New Jersey.  Although the  Newark station
                                 25

-------
                                              O Highest
                                                (243 ppb)
160
150


130
110
90
70


50


30


10
1 	
-
-
-

-
-
-
-


_




-
-

O

o























A
SUN
(150)


O
o






v 95%
VConfl-
' dence
Interval








 \
/ 95%
- \ Confi-
i dence
' Inter-
val





















A
TWTF
(43)
O

0























A
ss
(186)







75 p-
tlle

HD
^-Mean
median


?; n-
" H
tile



o 2nd Highest










p* HE Mean












A
MTWTF

(88)
Figure 9.  Box-display of WE/WD daily maximum oxidant
           concentrationsunder normal and Sunday defi-
           nitionsfor the Newark, New Jersey, site.
                   26

-------
 exhibits a higher weekend  oxidant  level with marginal  significance
under the normal WE/WD definition,  the box-display of the same data  does
not show any large difference in the medians and the 25   and 75   per-
centiles.  In Figure 9, the confidence interval  of the median is also
 shown (see Appendix C).  The confidence interval  of  the  weekday data
is completely overlapped by that of the weekend  data, indicating that no
significant difference exists between the  weekend and weekday oxidant
levels.
     One reason why the t-test result has  indicated  a  marginally
significant difference between the  WE/WD oxidant levels under the normal
WE/WD definition  can  be found in Figure 9.   As  seen from the box-display
for the norma1  WE/WD data,  one extreme value has significantly affected
the mean of the weekend oxidant levels.  The mean of the  weekend oxidant
levels was re-computed without this high value  (243 ppb).  As a result,
the P-value increases to 0.15, indicating  no significant  difference  between
the WE/WD oxidant levels.
     In addition to the Newark station,  four other stations,  Philadelphia,
Welfare Island, Medford,  and Asbury Park,  also exhibit a  slightly higher
oxidant level on weekends  than on weekdays (see  Figures 6 and 7). Comparison
of the medians  indicates that almost no  difference exists between the WE/WD
oxidant levels.  For example, Figure 10  presents the box-display of  the
WE/WD oxidant data at Medford, Massachusetts.  The WE/WD  median values
are almost identical, and  the WE confidence  interval  overlaps that of the
WD.
                                   27

-------
ro
CO
                      .a
                      a.
                      a.
100





 90





 70





 50





 30





 10


  0
                                         OHighest     O
                                         92nd Hiohest  9
[-H75 p-tile


   <- WE Mean

   Median
                                            25 p-tile
                                         O
                                          Lowest
                                                           WD Mean


                                                         Median
                                        SUN.
                                        (70)
                          TWTF
                          (261)
WE Mean->
95*, Con-
fidence
Interval
                                                                                        I
                                                SS
                                               (149)
                             Box-Display of UE/WD Daily Max 0.. Concentrations at Mndford Site.
                                                                               O Highest

                                                                               9 2nd Hiqhest
                                                                      75 n-tile
WD Mean

95% Confi-
dence> Intent]
                                                                      25 p-tile
                                                                                                        OLowest
                      MTWTF
                      (316)
                     Figure  10.  Box-display  of WE/WD daily maximum oxidant  concentrationsunder
                                   normal  and Sunday definitionsfor the Medford,  Massachusetts, site.

-------
     Next, the box-display method was applied to the stations that exhibit
a lower oxidant level on weekends in both Figures 6 and 7.   Figure 11  shows
the box-display of the WE/WD oxidant data at Greenwich, Connecticut, where
the t-test result exhibits significantly lower weekend oxidant levels under
the normal and the Sunday WE/WD definitions (see Figure 7).   The box-
display shows that the median of the weekend oxidant levels  is significantly
lower than that of the weekday levels under both the normal  and Sunday WE/WD
definitions.  Furthermore, the confidence intervals of the two medians are
completely separated from each other, indicating that a significant difference
exists between the weekend and weekday oxidant levels.
     Similarly, a clear separation  of the confidence intervals of the two
medians is observed in the box-displayed data for the WE/WD  daily maximum
oxidant levels at the Danbury and Lancaster stations.  At the Camden
County station, such clear separation occurs  only  in  the  SS  vs.  MTWTF
comparison.   The box-displayed data for  Bethesda,  Silver Spring,  Suit!and,
Norristown,  Middletown, and Quincy  all  failed to show a significant difference,
although the t-test results for those stations showed at least a marginally
significant  difference under one of the two WE/WD definitions.  The rest of
the stations do not exhibit a significant difference in either the t-test
results or the box-displayed data.
     The results of  the  significance test  for the WE/WD  difference in daily
maximum oxidant levels are summarized in Table 3.  The t-test results are
categorized  by three levels:  significant,  marginally significant, and
                                   29

-------
o
>.
p Highest
i
i
A 2nd Highest 





" -WE Mean
median

s



o

A A
TWTF SUN.
(230) (93) (189)
(44)
   Figure 11.  Box-display of WE/Wu daily maximum concentrations-
               under normal and Sunday definitionsfor the
               Greenwich, Connecticut, site.
                           30

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                 TABLE 3.   SIGNIFICANCE  OF  THE  WE/WD  DIFFERENCE  IN  DAILY
                           MAX OX  LEVEL  AT  EACH STATION  BY T-TEST AND
                           OVERALL JUDGMENT
Metropolitan
Area
Washington, D.C.
Baltimore
Philadelphia
New York -
Newark
Boston
Station
Name
Bethesda (BE)
Gaithersburg (GA)
Silver Spring (SS)
Suitland (SU)
Baltimore (BA)
Lancaster (LA)
Camden (CA)
Camden Co. (CC)
Norn's town (NO)
Philadelphia (PH)
Asbury Park (AP)
Babylon (BB)
Danbury (DA)
Greenwich (GR)
Middletown (MI)
Mamaroneck (MA)
Newark (NE)
Welfare Is. (WI)
Medford (ME)
Quincy (QU)
Salem (SA)
Worchester (WO)
SS vs. MTWTF
t-Test
0
0
0
e
0
0
0
0
0
0
0
9
e
0
0
0
0
0
Overal }
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Sun. vs. TWTF
t-Test
0
0
0
0
0
e
0
0
0
0
0
0
0
0
0
0
0
0
0
Overall
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Note
   U 9
WE level is significantly higher than WD level
WE level is significantly lower than WD level
WE and WD levels have no significant difference
WE/WD differences are marginally significant
                                    31

-------
insignificant.  The overall judgment on the WE/WD difference is also

made by considering the consistency of test results by the two different

tests employed.  Because the overall judgment involves a subjective judg-

ment (particularly in the case of the box-display results), the overall

test results are categorized by two levels only:  significant and

insignificant.

     The overall test results are as follows:

     0    Under the normal WE/WD definition (SS versus MTWTF), four
          stations exhibit a significant WE/WD difference in daily
          maximum  oxidant levels; the other 18 stations do not
          exhibit a significant WE/WD difference.  All four stations,
          Lancaster, Camden County, Danbury, and Greenwich, have a
          significantly lower oxidant level on weekends.

     t    Under the Sunday WE/WD definition (Sun. versus TWTF), only
          three stations exhibit a significant WE/WD difference in
          daily maximum oxidant levels.  All three stations, Lancaster,
          Danbury, and Greenwich, exhibit a significantly lower
          oxidant level on weekends.

     In the next section, WE/WD differences in oxidant levels are examined

further, taking into account the effect of meteorology.
                                  32

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          4.  METEOROLOGICAL ADJUSTMENT OF WE/WD OXIDANT LEVELS

      In the preceding sections, we have analyzed weekend/weekday (WE/WD)
 differences in oxidant concentrations, using all hourly data during the
 May  1 through September 30 period in four years, 1973 through 1976.  Given
 the  long period of air quality data, the results of such an analysis should
 have  only a minimal meteorological bias occurring between weekdays and
 weekends [Peterson and Flowers 1977; Cleveland et al. 1974a].  Nevertheless,
 there exists a possibility that a meteorological bias occurring between
 weekdays and weekends may have affected the WE/WD oxidant differences
 examined in the preceding sections.
     In Section 4.1, seven meteorological  variables usually correlated
with high oxidant levels are examined for  WE/WD differences in order to
detect a potential bias on either weekends or weekdays.   The impact that
any meteorological differences might have  on oxidant WE/WD differences are
assessed.   In Section 4.2, WE/WD oxfdant concentrations  occurring under
similar meteorological conditions are compared.  Here,  stratifying the oxi-
dant data  by meteorological  class allows examination of  the WE/WD dif-
ferences on only those days  with high oxidant potential.   It is under these
conditions  that reductions in HC and NO levels, are likely to be most im-
                                       A
portant. This  procedure also removes any  masking effect  that might occur
by including days  with meteorological conditions  that are not conducive to
ozone formation.   In Section 4.3,  the meteorological  classes are used to
normalize oxidant  levels  for meteorology,  and these results are compared
to WE/WD differences using the other schemes.   Finally,  each site is cate-
gorized  according  to the oxidant WE/WD differences  observed under the
vario'js  analysis
                                   33

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4.1  WE/WD DIFFERENCES IN METEOROLOGY
     The WE/WD difference in meteorology is examined for each of the 12
surface weather stations whose names and site codes are listed in Table 2.
The WE/WD differences in each of seven meteorological variables are com-
puted for each surface weather station.  Because it is more pertinent to
find the WE/WD difference on sampling days with oxidant data rather than
on all days, each of the 12 meteorological stations is paired to one of
the nearby air monitoring stations.  The paired air monitoring station and
the number of sampling days at that station are listed in Table 4.  The
number of sampling days is gfven separately for weekends and for weekdays.
     Table 5 lists the WE/WD levels and the significance level of the WE/
WD difference in each of seven meteorological variables.  Surprisingly,
WE/WD differences are found in nearly all the meteorological  variables
examined.  Some variables show a significant WE/WD difference at several
sites, while other variables show a consistent WE/WD difference (in
values) among all the sites examined.
     The daytime average visibility is consistently better on weekends
at all 12 sites examined.  At LaGuardia, JFK, Newark, and Philadelphia,
the WE/WD differences in visibility are statistically significant.  Two
other sites, Washington, D.C., and Baltimore, Maryland, exhibit a
marginally significant difference.  This consistently better visibility
on weekends may be due to a decrease in visibility-reducing pollutants
on weekends [Peterson and Flowers 1977].
     The surface pressure 1s consistently lower on weekends than on week-
days at all 12 sites.  Four sitesWindsor Locks, Connecticut; Bridgeport,

                                   34

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  TABLE 4.  METEOROLOGICAL STATION AND CORRESPONDING AIR MONITORING
           STATION USED IN WE/WD METEOROLOGY ANALYSIS
Meteorological Station
Air Monitoring Station
#Valid Cases WE/WD
Boston, Mass.
Worchester, Mass.
Windsor Locks, Conn.
Bridgeport, Conn.
LaGuardia, N.Y.
JFK, New York
Newark, N.J.
Atlantic City, N.J.
Philadelphia, Pa.
Washington, D.C.
Baltimore, Md.
Harrisburg, Pa.
Medford, Mass.
Worchester, Mass.
Middletown, Conn.
Greenwich, Conn.
Welfare Is., N.Y.
Babylon, N.Y.
Newark, N.J.
Asbury Park, N.J.
Camden, N.J.
Suit!and, Md.
Baltimore, Md.
Lancaster, Pa.
 148/368
 129/314
  99/244
 114/283
 126/305
  88/211
  88/214
  92/234
 108/278
 113/301
  80/219
  84/195
 The number of normal weekend days and weekdays with adequate oxidant air
 quality data.
                              35

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                         TABLE  5.   T-TEST RESULTS FOR TESTING  THE  WE/WD  DIFFERENCES  IN EACH
                                      OF SEVEN  METEOROLOGICAL VARIABLES
Surface Weather
Station
Boston, Mass.
Uorchester.Mass.
Windsor Locks,
Com.
Bridgeport,
Conn.
La Guardla.
N.Y.
JFK, N.Y.
Newark, N.J.
Atlantic City,
N.J.
Philadelphia.
Pa.
Washington,
D.C.
Baltimore, Md.
Harrlsburg, Pa.
Max Twp, F
WE/WD Olff.
73.7/74.2 0
73.7/74.0 0
75.2/75.8 0
75.2/75.5 0
75.6/76.4 0
74.2/75.2 0
78.1/78.7 0
77.3/78.4 0
78.6/79.9 0
81.6/82.0 0
78.0/79.7 0
77.7/79.0 0
Avg. Teap, F
WE/WO Dlff.
65.7/66.5 0
65.7/66.2 0
68.0/68.9 0
68.3/68.5 0
68.7/69.5 0
67.4/68.0 0
70.7/71.4 0
68.6/69.2 0
70.4/71.1 0
73.3/73.7 0
69.6/70.6 0
69.0/69.7 0
Avg.V1s1b1l1ty.Ml
WE/WD Dlff.
11.2/10.7 0
11.4/10.9 0
10.5/10.2 0
10.7/10.6 0
9.9/9.0 +
11.1/9.6 +
11.3/9.6 +
7.7/7.4 0
10.5/9.0 +
11.2/10.4 
10.0/9.0 6
9.0/8.3 0
Avg. Wind Speed, kn
WE/WD Dlff.
8.3/8.5 0
8.7/9.1
8.5/8.2 0
8.7/8.4 0
8.0/8.0 0
9.1/9.1 0
7.4/7.2 0
7.6/7.3 0
7.5/7.3 0
6.4/6.6 0
6.2/6.2 0
5.2/4.8 
Nixing Height, m
WE/WD Dlff.
1044.3/998.6 0
1088.5/1059.6 0
1112.3/1067.7 0
1069.8/1069.8 0
1056.4/1018.5 0
1040.1/1099.2 0
1123.2/1038.7 0
1064.0/1029.0 0
1303.6/1363.4 0
1295.6/1389.3 0
1228.2/1382.2 -
1489.5/1381.5 0
Wind Thru M.L.,n/s
WE/WD Dlff.
6.1/6.1 0
6.1/6.2 0
5.0/5.9 0
6.1/6.0 0
6.1/6.0 0
6.1/6.2 0
6.2/6.1 0
6.0/6.0 0
5.0/5.1 0
5.1/5.1 0
5.1/5.0 0
6.5/6.0 
Avg. Pressure, mb
HE/WD Dlff.
1015.6/1016.2 0
1015.4/1016.2 9
1015.6/1016.8 -
1015.9/1017.2 -
1015.8/1016.7 0
1016.1/1017.0 0
1015.3/1016.1 0
1016.3/1017.3 0
1015.7/1016.7 -
1015.9/1017.1 -
1016.3/1017.1 0
1016.5/1017.3 0
00
                         WE level Is significantly lower than WD level
                         WE level Is significantly higher than WD level
                         WE/WD difference Is not significant
                         WE/WD difference Is Marginally significant

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Connecticut; Philadelphia, Pennsylvania; and Washington, D.C.exhibit
a significantly lower pressure level on weekends.   Marginally significant
weekend drops in surface pressure are found at four additional sites:
Worchester, Massachusetts; LaGuardia, New York;  JFK,  New York; and Newark,
New Jersey.  These consistent pressure decreases on weekends  are rather
difficult to explain.  A more detailed study of  the WE/WD meteorological
difference appears to be needed.
     A consistently lower weekend level  is found in both the  daily
maximum temperature and the daily average temperature at all  the 12 sites.
A marginally significant difference in daily max temperature  is found at
the Philadelphia and Baltimore sites.  The WE/WD differences  in daily max
temperature are not significant at the rest of the 12 sites.   The WE/WD
differences in daily average temperature are found to be not  statistically
significant at all  the sites.  However,  because  of the consistent bias
and because of the known high correlation with daily  maximum  oxidant levels
[Breiman and Meisel 1976], the temperature bias  between weekends and
weekdays may have affected the WF/WD differences in daily maximum oxidant
levels, discussed in the preceding section (Section 3).
    The daily average wind speed, afternoon mixing height, and mean wind
through the mixing  layer  also exhibit some difference  between weekends and
weekdays.  However,  the WE/WD differences are not consistent  and  change
randomly among  the  12 sites.  Therefore, we do  not expect any significant
underlying cause for the  WE/WD differences in these meteorological  variables
                                     37

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    Although several WE/WD differences are found in the meteorological
variables examined, we cannot make any quantitative estimate of the impact
of such meteorological biases on the WE/WD oxidant levels.  Instead of
making a quantitative estimate, we discuss, in the following subsections,
a method of removing meteorological biases by comparing the WE/WD oxidant
levels under similar meteorological conditions.

4.2  WE/WD DIFFERENCE IN OX WITHIN METEOROLOGICAL CLASSES
    In this subsection, meteorological classes for daily oxidant potential
are developed for each of the 22 study sites.  Then, WE/WD oxidant levels
are compared within the same meteorological class.  The purpose of
meteorological classification is to determine a common meteorological
condition under which WE/WD oxidant levels can be compared without a
confounding effect of different meteorological conditions.  A difference
in WE/WD oxidant levels in the same meteorological class is then relatable
to differences in WE/WD precursor emissions.
    While the effect of meteorology upon air pollution, in particular,
oxidant air pollution, has long been recognized, very little has been
written concerning actual methods of removing that effect from observed
oxidant levels.  One method is to develop meteorological classes for a
similar daily oxidant potential and to compare WE/WD oxidant levels within
the same meteorological class.  By noting that meteorological classificatior
is a kind of meteorological pattern recognition, we applied a pattern
recognition algorithm called AID (Automatic Interaction Detector, developed
                                    38

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 at Survey Research Center, University of Michigan, some years ago
 [Sonquist et al. 1973]) to develop meteorological classes for daily
 oxidant potential at each study site.
     Because meteorological classification should be independent of
 emissions, we used only weekday data of oxidant and meteorology as
 inputs to AID.  Three meteorological classes, high, middle, and low,
 are  then formed by grouping several original classes produced by the
 AID  program into the high, middle, and low ranges of oxidant concentrations,
 4.2.1  AID Decision Tree Analysis
     After some unsuccessful attempts at using scatterplot approaches, we
 have chosen to  use the AID computer program to develop meteorological
 classes for daily oxidant potential.  The AID program [Sonquist et al.
 1973] simulates the procedures of a good researcher in searching for
 predictors (meteorological variables) that best account for the variations
 of a dependent  variable (daily maximum oxidant concentration).  AID
 searches over all independent variables for the best split that separates
 the  original data set into the two most uniform subsets.  AID keeps
dividing each subset until  no statistically significant split can be made
 (see Appendix D).   The result is a binary decision (classification)  tree
that shows  graphically the  dependence of daily maximum oxidant (OX)  levels
on the meteorological  variables and their ranges.
     The AID program is applied to only the weekday data of oxidant and
meteorology collected for each of the 22 locations in the northeast
United States.  The data for the analysis covers the warmer seasons in
                                    39

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four years  (May through  September,  1973  through  1976).  The reader is
referred  to Section  2  (e.g.,  Figure 2  and Table  2) for a description of
the 22  OX monitoring sites  and  their pairings with surface and upper-air
weather sites.   The  data base for each of the 22 locations includes daily
maximum oxidant concentrations  and  the corresponding values of the eight
meteorological  variables listed in  Table 6.  The meteorological data were
extracted or computed  from  weather  data  obtained from the National Climatic
Center  (NCC)  for 12  surface monitoring sites and 3 upper-air monitoring
sites.  Of  the  eight meteorological  variables used, the first six
variables (Tmav, TA, P,  Vis,  WD, and WS) were obtained from surface
             ma X
                                                          X,
weather tapes (NCC-TDF 14); the last two variables (MH and V) were
obtained  from mixing-height tapes.
     Because AID requires the independent variables to be in discrete
form, the actual  values  of  the  meteorological variables are converted into
a discrete  class by.setting a proper interval of values for each variable.
Table 6 lists the intervals used for the meteorological variables.  The
intervals are determined such that  an  approximately equal number of data
points  fall  into each  interval.
     Because  we  would  like  to obtain meteorological classes that are
independent of  emissions, AID is applied to the weekday data only.  By
excluding all the weekend days  from  the  data base, we can isolate the
relationship  between daily maximum oxidant level and meteorology from
the WE/WD differences  in precursor emissions.  Given meteorological
                                      40

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                           TABLE  6.   LIST OF METEOROLOGICAL  VARIABLES  USED FOR
                                       SITE-SPECIFIC METEOROLOGICAL CLASSIFICATION
                                       OF DAILY MAXIMUM OXIDANT CONCENTRATION BY AID

Variable Name
Daily Maximum Temperature
Daily Average Temperature
Daily Average Sea-Level Pressure
Daytime (6A.H.-9 P.M.) Average Visibility
Vector Average Hind Direction
Daily Average Wind Speed
Afternoon Mixing Height
Mean Wind Speed through Mixing Layer
Symbol
max
TA
F
V1s
WD
MS
MH
7
Unit
F
F
mb
mi
deg
kn
m
m/sec

1
[-.55)
[,50)
[0.1000)
[0.2)
[-45.0)
[0.2)
[0,250)
[0.2)

2
[55,65)
[50,55)
[1000.1004)
[2.4)
[0.45)
[2.4)
[250,500)
[2.3)
Ranges Used to Place the
3
[65.70)
[55,60)
[1004.1008)
[4.6)
[45.90)
[4.6)
[500,700)
[3.4)
4
[70.75)
[60,65)
[1008,1012]
[6,8)
[90.135]
[6,7)
[700.1000]
[4,5)
Variable into a Discrete Class
5
[75.60)
[65.70]
[1012,1016]
[8.10)
[135,180]
[7.8]
[1000.1250)
[5.6)
6
[80,85]
[70,75)
[1016,1020)
[10.12]
[180,210]
[8.10)
[1250,1500)
[6.7)
7
[85.90]
[75.80)
[1020.1024)
[12.16)
[210,240)
[10.12)
[1500.2000)
[7.8]
8
[90.95]
[80.85]
[1024.1028)
[16.20]
[240,270]
[12.14)
[2000,2500]
[8.12)
9
[95.-]
[85.-]
[1028.-]
[20..]
[270.315)
[14.-]
[2500.-]
[12.-]
[  Implies endpoint is included in the range.

(  implies endpoint is excluded.

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classes that are independent of emissions, we will be able to compare
the WE/WD oxidant levels under similar meteorological conditions.
     Examples  of AID decision  trees are  given  in  Figures  12,  13, and 14.
Figure  12  shows a binary decision tree for the Newark, New Jersey, station,
which Is the most source-intensive station among  the 22 stations analyzed.
The decision tree Indicates that  the highest oxidant air  pollution occurs
on days with daily maximum temperature (Tmaw)  185F, daytime average
                                         max
visibility  (Vis)  < 8 mi, and mean wind speed through mixing layer
2.0 V<6.0 m/sec.   In  other  words, the highest  oxidant  pollution days
are characterized by high  temperature, low visibility, and low wind speeds
throughout the mixing layer.  Such  high-pollution days with mean daily  OX
of Y = 98.7 ppb occurred 21 days  (N=21)  out of the 199 days sampled during
the summer-month periods (May through September) of four years, 1973 through
1976.  On the other  hand,  the lowest oxidant air pollution occurred on  cool
days (T_av < 75F).   On those days,  the mean level of daily maximum OX  con-
       ma x
centrations over the 54 sample days was as low as Y = 24.4 ppb.
     A similar interpretation can be derived easily for other classes from
the binary decision trees of Newark and the other two stations:
Mamaroneck, New York, and  Salem,  Massachusetts.  A complete set of binary
decision trees for all the 22 air monitoring sites 1s given 1n Appendix E.
By scanning these decision trees, one may find that  the dally maximum
temperature, T   , 1s, by  far, the most  Important of the  eight meteoro-
logical variables considered for  explaining the dally maximum  oxidant level.
Other meteorological variables that  have appeared frequently on the
                                 42

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                            0)
                           f4 - 199
                          Y = 49.1
                               Tmax >- 85
                 W
               N -
               Y = 36.
                           <85   8 < Vis     Vis <  8
Class VIII

      1000
-------
             Tax < 75
               (4)
             N = 97
             Y = 30.7
75Tn,a*<85
     -45 < WD < 135135  1 WD<-45
    02)
   N --56
  Y = 25.6
          10 _< US   2  <. HS < 10
                                                 8 < Vis     V1s < 8
Figure  13,  Binary decision tree for the Mamaroneck, New York, station.

-------
                                    T   <85
                                     max
85
-------
 binary  decision trees are:   vector average wind direction (WD), daily
 average wind speed (WS), daytime average visibility (Vis),  and mean wind
 speed through mixing layer (V).  Meteorological variables that have
 appeared only seldom are:  daily average temperature (TA),  daily  average
 sea-level  pressure (P), and afternoon mixing height (MH).
     The output of the AID computer program contains much more information
 than that expressed by the decision tree.  One other useful kind  of
 information  derived from the  AID output is an approximate pollution rose
 for  daily  maximum  oxidant levels.   The sample mean  of daily maximum OX
 concentration and  the number  of cases  are given for each  45-degree sector
 of daily resultant wind direction.   From this information,  we have con-
 structed a map of  an  approximate pollution rose for each  site (Figure 15),
 indicating the wind direction associated with high-oxidant  air pollution.
 From Figure  15 we  can see that the  preferred  wind direction for high-
 oxidant  air  pollution is  from the  southwest quadrant.   It should be noted
 that even  those sites located to the south or the west of the major
 metropolitan  areas have a higher oxidant air  pollution  when the wind
 blows from the southwest  quadrant  instead of  from the  metropolitan area.
Only the Asbury Park, New Jersey, station  has a preferred wind direction
from New York  City  as well as  from the  southwest quadrant.  It is
 interesting to  note that  the  Welfare Island,  New York,  station does
not  have any  preferred  wind direction for  high-oxidant  air  pollution.
                                   46

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                                                                                Metropolitan Area



                                                                          0     25    50    75
                                                                           I _.____._ i ...   - - L     f

                                                                                  Miles
Figure 15.  Preferred wind direction  for high-oxidant air pollution at each of the 22 sites.

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4.2.2  Meteorological Classification
     The site-specific meteorological classes Identified by a binary
decision tree provide us with easily interpretable Information about the
dependence of oxldant concentration on meteorological factors.  However,
the detailed meteorological classes for each monitoring site are rather
inconvenient for 1nterstat1on comparison when we want to quantify
oxidant-concentratlon dependency on meteorology from one site to
another.  To make such interstation comparisons easily, individual
meteorological classes exemplified by nodes of a decision tree were
combined to form three standard meteorological classes for each station:
a high, middle, and low oxidant-forming-potential class.
     Table 7 provides a  summary description of site  location and meteor-
ological characteristics for each of the three standardized classes.
In addition, Table 7 lists, for each site, the WE/WD oxidant levels in
each meteorological class and the terminal nodes of  the decision tree that
form each of the three standardized classes.  For example, the table
provides us with the following Information on the Bethesda station:
     t   The Bethesda station 1s located to the northwest of
         Washington, D.C., and 1s close to a major traffic artery.
     t   The high meteorological class 1s defined by dally max temp
         > 80 F, with wind speed < 6 kt or with wind direction from
         FForth-northeast to southwest (clockwise).
        The high class  has mean oxldant levels of 105  ppb on
         weekends and 114 ppb on weekdays.
     t   The low meteorological class 1s defined by  dally max temp
         < 80F and has mean oxldant levels of 40 ppb on weekends and
         50 ppb on weekdays.
                                  48

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                          TABLE 7.  QUALITATIVE DESCRIPTION OF METEOROLOGICAL CHARACTERISTICS FOR
                                    HIGH, MIDDLE, AND LOW CLASSES AT EACH OXIDANT MONITORING SITE
U3
SITE/SAROAD No.
Bethesda, Md. (BE)
210200005 F01
Gaithersburg, Md.(GA
210780004 601
Silver Spring, Md.
(SS)
211480006 F01
Site Description
The Bethesda monitoring site is located to the
northwest of Washington, D.C.in a trailer and 1n
an open field. Located at the National Institute
of Health close to Wisconsin Ave., a major traffic
artery leading into Metro-Wash., D.C. The area is
suburban and subject to rush-hour emissions.
The monitoring site is located at the environment
lab on Fredrick St. The building is recessed from
the street with the monitor placed 35 ft above
ground. Major emissions come from traffic.
The Silver Spring monitoring station 1s located
due north of downtown Washington, O.C. The site Is
located 50 ft north of interstate 495, in a
trailer 12 ft above ground. Emissions are heavily
traffic oriented. The immediate area surrounding
the site is dominated by a golf course and
residential neighborhood with little heavy industry.
Class/
Terminal
Node #
High
7,13
Middle
6,8,12
Low
10,11
High
5
Middle
4,7
Low
6
High
6,8,9
Hiddle
4,11
Low
12.13
Avg.
Cone.
JpPb)
WE=105 '
WD=114
WE=69
WD=76
WE=IO
WD=50
WE=81
WD=81
WE=53
W0=62
WE=60
WD=51
WE=71
W0=86
WE=52
WD=54
WE=33
WD=36
Qualitative Description
of Meteorology
Max temps >80F, avg visibility
<8 miles and avg surface wind
speed <6 knots, or max temp >80F,
avg visibility >8 miles and avg
wind direction from north-northeast
to southwest (clockwise)
Max temp >80F and avg visibility
<8 and avg surface wind speed
>6 knots, or max temp >80"F
and avg visibility >8 miles and
wind from the southwest to the
northeast (clockwise)
Max temp <80F
Max temp >85eF and avg
visibility <8 miles
Max temp >85F and avg visibility
>8 miles, or max temp <85F
Fut >80F
Hax temp <80F
Max temp >85F and avg surface
wind speed" <8 knots.
Max temp >85F ancTavg surface
wind speed" >8 knots, or max temp
<85F and avg visibility >.16 miles
Max temp <85F and avg visibility
<16 miles

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                           TABLE 7.  CONT'D
SITE/SAROAO No.
Sultland, Hd.(SU)
211560001 F01
Baltimore. Nd. (BA)
210120018 F01
Site Description
Site 1s located at the Sultland Federal Center.
Major arteries 1n the vicinity are Sultland Parkway
and Silver H111 Road. The station will probably
reflect rush-hour traffic emissions and some
Industrial emissions. The monitor 1s elevated 10
ft above ground.
The Baltimore air monitoring site 1s located on
22nd St. and Calvert St. The station 1s located
1n a parking lot and 1s subjected to mainly traffic
emissions. There is some industry; however, the site
is mainly a center-city type.
Class/
Terminal
Node 1
High
10,11,
14,15
Middle
8,17,19
Low
6,12,18
High
4.8,9
Middle
10,11
Low
6
Avg.
Cone.
(DDb)
WE=107
W0=107
WE=73
WD=74
WE=47
HD=49
WE=75
MD=88
WE=59
WD=54
WE=35
WD=32
Qualitative Description
of Meteorology
Max temp >85F and mean velocity
through tTie mixed layer <4.0 m/s
or max temp >85F and the mean
velocity > 470 m/s and avg. visibil-
ity <12 miles
Max temp >85F and mean velocity
through tFe mixed layer >4.0 m/s
and avg visibility >12 mTles, or
max temp <85F and mixing height
>750 m and avg surface wind speed
<8 mph and either max temp >80F
or max temp <80F but >55F and
avg pressure >.1020 mb
Max temp <85F and mixing height
<750 m, or max temp <85F and
mixing height >750 m and avg
surface wind speed >8 knots, or
max temp <85F with mixing height
>750 m, surface wind speed <8 mph
max temp <80F and avg pressure
<1020 mb.
Max temp >_ 85F
Max temp <85F but >75F
lax temp <75F
01
o

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TABLE 7.  CONT'D
SITE/SAROAD No.
Lancaster, Pa. (LA)
394660007 F01
Camden, N.J. (CA)
310720003 F01
Site Description
Located In a residential area to the northeast of
downtown Lancaster. Emissions are mainly traffic
oriented and are generally light. Main arteries are
Broad St. and Lehigh Street. The monitor is located
on Abraham Lincoln Jr. High School on Gofftown Rd.
The site 1s located in a trailer in a residential
area. Davis & Copewood Sts. are not subject to
heavy traffic emissions. Industrial emissions are
located to the northeast and west at about a
distance of two miles. There is a significant
number of parks and cemeteries in the general
vicinity making the site representative.
Class/
Terminal
Node S
High
5,7
Middle
6,13
Low
8,10,12
RT^JT"
5,12,13
Middle
9,17,19
Low
10,14,
16,18
Avg.
Cone,
Jppbl
WE=104
WD=114
WE=90
WD=94
WE=57
WD=61
WE=95
WD=103
WE=72
WD=70
WE=49
WD=41
Qualitative Description
of Meteorology
Max temp ^ 80F with an avg. visibil-
ity < 6 miles or with an average
visibility ^ 6 miles and a mean
velocity through the mixed layer
^ 6.0 m/s.
lax temp > 80F and an avg. visibil-
ity >^ 6 mTles and a mean velocity
through the mixed layer < 6.0 m/s,
or max temp > 70 F but < 80F and a
mixing height >_ 1000 m and an avg.
visibility < 10 miles.
1ax temp < 70F, or max temp > 70F
but<80F and a mixing height < 1000 m
or max temp > 70F but < 80F with a
mixing height > 1000 m and an avg.
visibility >. 10 miles.
Max temp > 90F or max temp >_ 80F
)ut < 90F and an avg. surface wind
speed < 6 knots.
Max temp > 80F but < 90F and avg.
wind speeds > 6 knots and either
winds from tFe southwest to northwest
or winds from northwest to southwest
(clockwise) with mixing heights
j_ 1250 m, or max temp < 80F but
>_ 70F with winds from east to north-
west (clockwise) and mixing height
,1 1000 m.
lax temp < 70F, or max temp _> 70F
but < 80F and avg. wind from north-
west to east, or max temp 21 70F
but < 80F,winds from east to north-
west and mixing height < 1000 m, or
nax temp > 80F but < 90F with avg.
wind speed" > 6 knots, winds from the
southwest to northwest and a mixing
height < 1250 m.

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                                  TABLE  7,   CONT'D
               SITE/SAROAD No.
CMden Co., N.J.(CC)

310740001 F01
en
ro
              Norrlstown, Pa. (NO

              396540013 F01
         Site Description
                                   The site 1s located on the grounds of the N.J.  State
                                   Hospital at Ancorla.  The surrounding area 1s malnlj
                                   agricultural.  The oxldant monitor Is 45 ft  above
                                   ground on the main building of the hospital.  The
                                   site 1s subject to local traffic emissions from
                                   Rt. 30 and Is southeast of Camden.
The site Is located 10 miles to the northwest  of
Philadelphia.  Placed on Belvolr Rd.,the site  1s
subjected to moderate traffic emissions.  Minor
Industrial emissions come from the southwest and
Norrlstown proper while the station Is within  1  mile
of the Penn. Turnpike.  The monitor Is placed  12 ft.
above ground at the St. Armory 1n a residential  area
Class/
Terminal
Node I
                                                     High
                                                      5
                                                    Middle
                                                      7
                                                     Low
                                                     2,6
High
7,12,13
                                                                                        Middle
                                                                                        4,8,15
                                                                                        Low
                                                                                        10,14
  Avg.   I Qualitative Description
           of Meteorology
         HE=112
         HD=102
         WD=82
         WD=57
WE=108
HD=120
         WE=84
         WD=89
                                                             NE-65
                                                             WD=55
         Max temp >_ 75F and avg. surface
         wind from the southwest to northwest
                   (clockwise).
                   Max temp > 1
          lax temp >_75F and avg.  winds from
         northwest to southwest (clockwise)
         and an afternoon mixing height
         > 1250 m.
         Max temp < 75F or max temp > 75F
         and avg. winds from northwest" to
         southwest (clockwise)  and afternoon
         mixing height < 1250 m.	
                                                                                           Max temp ^ 90F and avg. wind speed
                                                                                           < 8 knots or a max temp >_ 80 F but
                                                                                           < 90F and a velocity through the
                                                                                           mixed layer >_ 3.0 m/s.
         Max temp >_ 80F and avg.  wind speed
           8 knots, or max temp >  80F, avg.
         wind speed < 8 knots and" a max temp.
         <90Fw1th a mean velocity through the
         mixed layer < 3.0 m/s, or a max temp.
         < 80F but > 75F and winds from
         east to northwest.	
                  Max temp < BOTwlth winds from
                  northwest to east (clockwise), or max
                  temp < 75 and winds from east to
                  northwest (clockwise).	

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                         TABLE 7.  CONT'D
SITE/SAROAD No.
Philadelphia, Pa.
(PH)
397140026 HOI
Asbury Park, N.J.
(AP)
310060001 F01
Babylon, N.Y. (BB)
330280002 F01
Site Description
The Broad and Spruce St. monitoring site is located
at the corner of a parking site in downtown Phila-
delphia. Emissions are heavy and traffic oriented.
Broad St. is a main artery running north-south. The
site is subject to both industrial and vehicular
emissions and is considered characteristic of a down-
town metropolitan area.
Located on Mattison Ave. between Bond and Main Sts 
the commercial center of Asbury Park. The site is
elevated (second floor) and is subject to mainly
traffic emissions and minor local industry. The
location of the monitor is within one mile of the
Atlantic Ocean.
The site is located in the center of an industrial
park. The general area is residential (parks and
cemetery) with major traffic emissions from Rt. 110
which is heavily traveled. In addition Interstate
495 is within 1 mile to the north with continuous
heavy traffic. The site is downwind of NYC with the
Atlantic Ocean 5 miles to the south.
Class/
Terminal
Node #
High
3,8,9
Middle
10,11,15
Low
12,14
High
5
Hi d'dle"
4,7
Low
6
High
4,5
Middle
9
Low
6,8
Avg.
Cone.
(PPbj
WE=93
WD=81
WE=61
WD=51
WE=38
WD=31
WE=82
W0=89
WE=51
WD=56
WE=52
WD=46
WE=107
WD=101
WE=60
WD=72
WE=50
WD=42
Qualitative Description
of Meteorology
T max >_ 90F, or T max > 80F but
< 90F and with a mixing height
>. 1500 m.
T max > 80F but < 90F and mixing
height < 1500 m, or T max > 75F but
< 80F and a mixing height > 1250 m.
T max < 75F, or T max > 75F but
< 80F with a mixing height < 1500 m.
Mixing height > 1250 m with T max
>_ 80F.
Mixing height > 1250 m with T max
< 80 F, or mixing height < 1250 m
and winds from the southwest to
northwest (clockwise).
Mixing height < 1250 m and winds from
northwest to southwest (clockwise).
Max temp. >_ 80F.
Max temp < 80F, wind from the south-
east to northwest (clockwise) with
max temp >_ 70F.
Max temp < 80F and wind direction
from southeast to northwest (clock-
wise) but 'max temp < 70F, or max
temp < 80F and wind direction from
northwest to southeast (clockwise).
en
CO

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                                   TABLE  7.   CONT'D
                SITE/SAROAO No.
              Danbury, Conn. (DA)

              070175123 F01
in
              Greenwich, Conn.(GR)|

              070330004 F01
              Mamaroneck, N.Y.
                (MA)

              334100002 F01
          Site Description
 Located on a college campus 1n a residential  portion
 of Danbury.  Local emissions are light and come fron
 vehicular traffic and local boilers.   The site 1s
 downwind (40 miles) of central NYC and 1s considered
 very representative of the general area.
rhe site 1s located on Bruce Golf Course near  the NY-I
Conn, state line.   The site 1s considered represents-]
tlve of the general area with local  emissions  being
light, coming from King St., Rt.  684 and Hestchester
County Airport.  NYC 1s approximately 25 ml  to the
southwest.
.ocated at the exit to the New England Thruway35 ft
above ground and 50 ft from the curb on 5th St. and
lad 1son.  The site Is subjected to heavy traffic
emissions and not necessarily representative of the
general area, which 1s commercial  and situated 1n  a
residential  community.
Class/
Terminal
Node I
High
12,13
                                                                                        TTOfle
                                                                                        4,6,11
                                                                                        Low
High
6.7
                                                                                        8,9
                                                                                        Low
                                                                                        10,11
High
3,11
                                                                                        Middle
                                                                                        6,8,10
                                                                                        Tow
                                                                                        12.13
  Avg.
 Cone.
 (ppb)
WE=88
W0=127
         WD=94
                                                              HE=54
                                                              WD=51
         MD=92
         WE=63
         HD=64
WE=79
HD=87
         WD=53
                                                              WD31
 Qualitative Description
  of Meteorology
Avg. visibility < 8 miles and avg.
wind direction from the southeast to
southwest (clockwise).	
         Svg. visibility _ 8 miles with winds from
         southeast to northwest and max temps
           75F.
                  Avg. visibility ^ 8 miles with winds
                  from the northwest to southeast, or
                  avg. visibility ^ 8 miles and winds
                  from the southeast to northwest with
                  max temps < 75F.
         Max temp > 75F and avg. visibility
         s n ml lac
         < 8 miles.         _
         Hax temp >_ 75F and avg. visibility
         > 8 miles.
         Max temp < 75F,
T max >_ 85F, or T max >. 75F but
< 85F and avg. wind speed < 10 knot'
with visibility < 8 miles and max
temp < 80F.	
T max >_ 75F but < 85F "an3~avg.
wind speed >^ 10 knots, or T max
>_ 75F but < 85F and avg. wind
speed < 10 knots and avg. visibil-
ity > 8 miles or T max >, 75F but
< 80*F with avg. wind speed < lOmph
     tvg. visibility < 8 miles.
     enip. < 75F.               ~~"

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                                     TABLE  7.   CONT'D
                  SITE/SAROAD No.
                Middletown, Conn.
                 (MI)

                070570003 F01
tn
en
                 Newark, N.J. (NE)

                 313480002 F01
         Site Description
The site 1s located at the city hall, approximately
20 ft. above ground.  The general  area emissions  are
mixed,Including both Industry and  traffic sources.
The site is approximately 70 miles to the northeast
of NYC, south of Hartford and north of New Haven.
Located In downtown Newark on Washington St.  and
Branford St.  The area has Industry but Is  mainly
commercial.  The station Is subjected to moderate-
to-heavy traffic from urban congestion.  The  monitor
is placed 12 ft above the ground.
Class/
Terminal
Node #
High
6,7
Middle
 4
                                                                                          Low
                                                                                           2
High
12,13,15
                                                                                         Middle
                                                                                         6,10,14
                                                                                         low
                                                                                         4,8
  Avg.
 Cone.
  fnnhl
WE=100
WD=124
WE=77
WD=78
                                                             WE=46
                                                             WD=47
WE=87
WD=77
                                                             WE=63
                                                             W0=49
                                                             WE=33
                                                             WD=26
 Qualitative Description
  of Meteorology
Avg. wind direction from southeast to
northwest with avg. visibility
< 8 miles.
Avg. wind direction from southeast to
northwest and an avg. visibility
> 8 miles.
                  Avg. wind direction from northwest to
                  southeast (clockwise).	
Max temp ^ 85F and avg.  visibility
< 8 miles, or max temp >_ 80F but
< 85F and avg. pressure  >_ 1012 mb,
and mean velocity through the mixed
layer < 6.0 m/s.	
                  Max temp ^ 85F and visibilitity
                  ^ 8 miles, or max temp >^ 75F but
                  < 85F and pressure > 1012 mb and
                  either max temp < 80TF or if max temp
                  > 80F then having a mean velocity
                  Through the mixed layer >_ 6 m/s.
                  Max temp < 75F, or max temp >_ 75F
                  but < 85F and an avg. pressure
                  < 1012 mb.	

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                                  TABLE  7.   CONT'D
               SITE/SAROAD No.
              Welfare Is., N.Y.
               (HI)

              334680050 F01
en
              Nedford, Mass.  (ME)

              221220003  F01
         Site  Description
Hie site Is  located at ground  level on  the north
snd of the Island.   The Island  lies between mldtown
Manhattan and Queens with  the East River Drive along
:he Manhattan shore.  Traffic on  the Island was
limited for the study period;however,  the  site Is
lowmdnd of heavy-Industrial, power-plant  and traf-
fic emissions.  The site Is well  ventilated and
probably gives a good representation of  East River
Dxldant.
Located to the northeast of downtown Boston.The
station Is situated on a grass-covered median strip
at the Intersection of Rt.  16  and  Rt. 128 and Is
subjected to moderate-to-heavy traffic volumes.  The
area 1s basically commercial and Industrial.  The
station 1s well ventilated  and 1s  considered to be a
representative neighborhood station.
Class/
Terminal
Node I
 High
 11,14,15
                                                                                        Middle
                                                                                        8,10,17
                                                                                            ,16
HTgh
7,10,11
                                                                                        Middle
                                                                                        14,15.
                                                                                        16,17
                                                                                       low
                                                                                        8,12
  Avg.

 IDDb}
WE=93
WD=94
         HE=64
         W0=63
                                                              ME=39
                                                              WD=36
WE=62
WD-69
         WE-53
         WD-47
                                                              WD=31
 Qualitative Description
  of Meteorology
Max temp ^ 85F, or max temp >. 75F
but <85Fand winds from the north to
southwest with a mixing height ^
>.750  mand avg. visibility < 10 miles.
                k > i * ~if~ o co c
         Max tempyT5F but < 85"F with winds
         from the northwest to southwest with
         either a mixing height < 750 m or
         with a mixing height ^ 750 m and an
         avg. visibility >_ 10 miles, or a max
         temp < 75F with wind speed < 8 knots
         and max temps %_ 70F.
                  Max temp >. 75F and < 85F with winds
                  from the southwest to northwest, or
                  max temp < 75F and wind speed
                  < 8 knots and a max temp < 70F, or
                  a max temp < 75F and an avg. wind
                  speed >_ 8 knots.
Max temp > 85, or max temp >_ 75"F
and < 85Fw1th mean wind 1n the
mixed layer >_ 5.0 m/s.        	
         Max temps >. 75" but < 85 and mean
         wind speeds 1n the mixed layer
         < 5.0 m/s, or max temp >. 70F but
         <-75F and surface winds from
         the east to northwest (clockwise)
                  Max temp < 70F, or max temp < 7
                  and wind from northwest to east
                  (clockwise).	

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                         TABLE  7,  CONT'D
SITE/SAROAD No.
Quincy, Mass. (QU)
221880002 F01
Salem, Mass. (SA)
221980001 F01
Worchester, Mass.
222640012 F01
Site Description
.ocated to the southeast of Boston on Rt. 3A and
Washington St. The site is a curbside location sub-
jected to periodically heavy traffic emissions. The
site is well ventilated with numerous industrial
sources in the area (i.e., General Dynamic Shipyard,
Proctor & Gamble and Edison Power). Site experiences
noderate-to-heavy summer daytime traffic volume.
-ocated approximately 10 miles northeast of Boston.
The area surrounding the site is residential, located
on Highland Ave., and it is subject to moderate
traffic. Industrial sources consist of local boiler:
and a power-generating station approximately 2 miles
away.
Located in a commercial district of Worchester, the
station is primarily subjected to traffic emissions.
The monitor is situated at the corner of new Salem &
Washington Sts. in a monitoring trailer.
Class/
Terminal
Node #
High
8,9,14,15
Middle
11,13
Low
10,12
High
3,7
Middle
6,11
Low
8,10
High
4.5
Middle
8,9
Low
6
Avg.
Cone.
_(DPb)
WE=87
WD=89
WE=52
WD=54
WE=43
WD=42
WE=69
WD=86
WE=42
WD=46
WE=43
WD=36
WE=74
WD=79
WE=52
WD=58
WE=46
WD=40
Qualitative Description
of Meteorology
Max temps >^ 80F.
Max temps < 80F but >_ 75F and avg.
visibility < 10 miles, or max temps
< 75F and the avg. surface winds
from the east to northwest (clockwise)
Max temp < 80F but >_ 75F and avg.
visibility >^ 10 miles, or max temp
< 75F and avg. surface winds from
the northwest to east (clockwise).
Max temp >_ 85 F, or max temp > 75F
and avg. surface winds from northwest
to southwest (clockwise).
Max temp >_ 75*F but < 85F and avg.
surface, wTnd from southwest to north-
west (clockwise), or max temp > 65F
but < 75F and mean velocity through
mixed layer >^ 7.0 m/s.
Max temp < 65 "F or max temp >^ 65 "F
but < 75F and mean velocity
through mixed layer < 7.0 m/s.
Max temps ^ 80 F.
Max temps ^ 70F but < 80F.
Max temps < 70 F.
en

-------
     For the other stations, similar information can be obtained from
Table 7.
     Because we are particularly Interested in knowing the meteorological
conditions existing on h1gh-ox1dant days, the meteorological characteristics
of the high met class depicted from AID decision trees are summarized in
Table 8.  For example, the high-oxidant days at Newark, New Jersey, are
characterized by a high  temperature >_ 85F with low visibility < 8 mi
or by moderate temperature of 80F to 85F with low wind speed aloft
< 6 m/sec and high pressure .>! 012 mb.  Among the 22 sites examined, as
many as 20 sites show high temperature as a key meteorological factor
for high-oxidant air pollution.  Three other important meteorological
factors are low visibility, moderate-to-low wind speed, and south-to-
 southwesterly wind.   Surprisingly, mixing  heights  greater than 750 m are
 associated with  high  oxidant levels  at  three sites.  The high mixing
 height  probably  does  not indicate a  necessary  condition for high-oxidant
 air  pollution but  simply means  low mixing  heights  are  not always associated
with high oxidant  levels.
4.2.3  WE/HP Differences Within High-OX-Potential  Class
     This section discusses the WE/WD differences  in daily maximum
oxidant concentrations on high-oxidant-potential days.  Here, the high-
oxidant-potential days are determined solely by the meteorological  condi-
tions existing on those  days.  Because AID decision trees are developed
from weekday data only,  we can expect that the meteorological classes
derived from the AID decision trees provide a means of sorting each
individual  day according  to the meteorological oxidant formation potential
on that day.
                                   58

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              TABLE  8.   METEOROLOGICAL  CHARACTERISTICS  OF
                         HIGH-OXIDANT CLASS FOR  EACH SITE
Metropolitan ' Station Name
Area '

Washington,
D.C.


Baltimore


Philadelphia


Bethesda (BE)
Gaithersburg (GP)
Silver Spr (SS)
Suitland (SU)
Baltimore (BA)
Lancaster (LA)
Camden (CA)
Camden Co.(CC)
Norristown (NO)
Philadelphla(PH)
Asbury Park(AP)
 Babylon (BB)
Oanbury (DA)

New York-
Newark



Boston

Greenwich (GR)
Middletown(MI)
Mamaroneck(MA)
Newark (NE)
Welfare Is.(WT)
Medford (ME)
Quincy (QU)
Salem CSA)
T_,y i Vis
max
F mi
 >80 <8
i "80 >8
" 785 <8
785
(>85
>85
e >80
/>90
1180,90)
>75
/>90
1180,90)
/>90
.1 TJ30.90)
>80
X30

>75

>85
P5.85)
>85
180,85)
>85
175,85)
/>85
\TJ75.85)
1
<6
i6





<8
<8
<8
<8
<8
<10

>80
<~85 '
1^5.851
Worchester (WO) ' ~>80 J
MS

kts
<6

<8



<6
<8
<8






<10





7

m/sec



<4
>4

I6


13






<6

>5


WO

deg.
[-45,210)






[240,315)



[135,240)

[135,315)


[-45,240)

[-45,210)

MH

F P

m mb








^1500
>1250






>750


















>1012




MOTE:
      [  ~  ImpHes endpolnt is Included 1n the range.

      )    implies endpolnt is excluded from the range.
                                   59

-------
       In  Table  9,  the  WE/WD differences  in  daily maximum oxidant levels
  for  all  days and  for  high-oxidant-potential  days only are shown for each
  of the 22  stations.   At 14 of the 22 stations, the WE/WD difference is
  greater  on high-oxidant-potential days.  However, the percentage dif-
  ference  is significantly different only at a few sites.   The average
  percentage difference is about the same C-7%) between the two cases,
  while the  absolute WE/WD difference is  slightly greater on high-oxidant-
  potential  days.
  4.3   METEOROLOGICAL ADJUSTMENT OF WE/WD OXIDANT LEVELS
       Fluctuations in  meteorology cause  most  of the day-to-day variation
of oxidant  concentration levels [Breiman  and  Meisel 1976].  Consequently,
it is desirable to account for the meteorological fluctuations by adjusting
the air quality data.   One meteorological adjustment method is to compare
the WE/WD air quality  levels under similar  meteorological conditions, say,
the high  met class developed in the preceding subsection.  Another meteor-
ological  adjustment method would be to compute a frequency weighted
average of  the  means of high, middle, and low classes for weekends and
that  for weekdays.  Zeldin and Meisel  [1978]  show that meteorological
normalization of air quality parameters  (such as the arithmetic mean and
geometric mean) can be  performed if one is  able to determine meteorological
classes that explain a  significant fraction of air quality variance.  For
example, a meteorologically normalized arithmetic mean can be computed as
follows:
                                   60

-------
TABLE 9.  WE/WD DIFFERENCES IN DAILY MAX OX LEVELS
          FOR ALL CASES AND FOR HIGH MET CLASS ONLY
          (SS vs. MTWTF)
Metropolitan
Area
Washington
D.C.


Baltimore

Philadelphia



Newark -
New York






Boston




Station
Name
Bethesda
Gaithersburg
Silver Spring
Suit! and
Baltimore
Lancaster
Camden
Camden Co.
Norri stown
Philadelphia
Asbury Park
Babylon
Dan bury
Greenwich
Mamaroneck
Middle town
Newark
Welfare Is.
Medford
Quincy
Salem
Worchester
22 Station
Avg.*
WE Level
(ppb)
70
61
52
73
54
82
68
64
82
58
55
66
71
79
49
73
56
62
50
60
49
56
63
All Cases
WD Level
(ppb)
77
64
59
79
59
91
70
75
91
57
57
70
87
92
54
82
49
61
49
62
55
58
68
% Change
(WE-WD)
-9%
-4%
-13%
-8%
-9%
-8%
-2%
-14%
-9%
+2%
-4%
-7%
-18%
-15%
-8%
-11%
+13%
+1%
+2%
-4%
-12%
-3%
-7%
High
WE Level
(ppb)
105
81
71
107
75
104
95
112
108
93
82
107
88
102
79
100
87
93
62
87
69
74
90
Met. Cl
WD Level
(ppb)
114
81
86
107
88
114
103
102
120
81
89
101
127
125
87
124
77
94
69
89
86
79
97
ass
% Change
(WE-WD)
-8%
0%
-17%
0%
-15%
-9%
-8%
+10%
-10%
+15%
-8%
+6%
-31%
-18%
-9%
-19%
+13%
-1%
-10%
-2%
-20%
-6%
-7%
 The station-average percentage, difference is computed from
 the station averages of WE/WD concentration levels.
                               61

-------
                           k,Non =    CM P1
     where C^ Norffl = the meteorologically normalized mean for a
                     pollutant  (e.g., oxldant) 1n the kth period
                     (e.g. weekends),
                 M = the number of meteorological classes,
               C^ = the sample mean of meteorological class "1"
                     (e.g.,  high class) during the k   period,
                P.J = the average frequency of occurrence of meteor-
                     ological class "1" during ajl_ periods (e.g.,
                     all days).
    The following paragraphs describe a meteorological adjustment of the
actually observed WE/WD difference according to Equation (2).  In making
such a meteorological adjustment, we utilized the three standardized
meteorological classes developed in the preceding subsection.  Figure 16
is a bar graph showing the WE/WD mean concentrations in the high-, middle-,
and low-oxidant-potential classes at Lancaster, Pennsylvania.  The figure
also shows the number of cases  in each class on weekends and weekdays.
    The meteorologically adjusted weekend mean for all cases represented
by the figure is given by
                                  3
                      CWE,Norm =     CWE,1 P1
                                   62

-------
where 1 = 1, 2, 3 Indicates, respectively, high,  middle,  and low classes,
and CWE,i 1s the mean weekend oxidant level  of the 1th class.   P1 is the
average frequency of occurrence of each of the three  classes during  all
days, i.e., weekends and weekdays combined.   Similarly, the  meteorologi-
cally adjusted weekday mean is given by
                                  3
                      CWD,Norm *     CWD,i  Pi                       (2'b)
where CWQ ^ is the mean weekday OX level  of the  ith  class.
                                                                 X
     As seen from Figure 16,  the Lancaster station exhibits  a  consistently
lower oxidant level on weekends among  the class  means  of  each  of the  three
meteorological classes, the actual means, and  the meteorologically ad-
justed means.  Because of the consistently lower weekend  levels  among
all the five different WE/WD  comparisons, one  can say  that the weekend
OX level is lower at this station.  The above  consistently lower type
of station is termed a "Type  I station."
     Figure 17 is a bar graph for the  Newark,  New Jersey, station.  This
station exhibits a consistently higher weekend OX level among  the five
different WE/WD comparisons.   Therefore,  we can  expect that  the  weekend
OX level is higher at this station. The  above type  of station is termed  a
"Type II station."
      Figure 18 snows  the bar graph comparisons  of the WE/WD oxidant
 levels  at Baltimore,  Maryland.   Unlike the above two  examples,  the WE/WD
 relationship 1s reversed from one comparison  to another.  While
 the weekend  OX  level  is  lower 1n  the  high  class, those in the middle and
                                   63'

-------
.a
ex
Q.
0)
O
i
+J
2
4J
Q)
O
O
O
X
O
    120 1
    110 "
    100 -
    90-
70 
     60 
4*   50
&
 o   40
 ie
*   30

     20  -

     10
            62
                WD
                 25
                  WE
                 High
                 Class
                                  46
                                    19
WD    WE
  Middle
  Class
                                                        Station:  Lancaster,  Pa.
                                                53
                                                       27
WD    WE
  Low
  Class
                                                                            71
WD    WE
 Actual
 WD    WE
Met. Adjusted
                  Figure 16.  Example of Type I station, which exhibits a consistently  lower
                              weekend OX level 1n every WE/WO comparison.

-------
CT)
    .0
     Q_
     Q.
     VI
     c
     o
     ra
     cu
     o
     o
    o
    X
    o
     X
     rcs
     tO
    Q
     0)
iio-l



100-



 90-



 80



 70



 60'



 50-



 40



 30-



 20-



 10-
                                                        Station:   Newark,  N.J.
                          14
                    60
                                            34
68
                                            =  79
                                   =  199
                                                               31
                                                        71
                    WD    WE

                     High
                     Class
WD    WE

 Middle
 Class
                                                WO

                                                  Low
                                                  Class
WE
WD    WE
 Actual
WD    WE
Met. Adjusted
                 Figure 17.  Example of Type  II station, which  exhibits  a consistently higher
                             weekend OX level in every WE/WD  comparison.

-------
    120-,
-.
Q.
(/I

O
s
+J
Q)
 O
O

X
o
iio-


100"


 90-


 80


 70


 60


 50


 40


 30


 20


 10
                60
18
                 28
                                  74
                WO    WE

                 High
                 Class
           WD    WE

            Middle
            Class
                                                     Station:  Baltimore, Md,
                                               180
                                                          27
                                                    46
                                                      73
                                                WD    WE

                                                 Low
                                                 Class
WD    WE

 Actual
WD     WE

Met. Adjusted
             Figure 18.   Example of Type III station, which does not exhibit a consistent relationship
                         amonq meteoroloa-ical  -"	

-------
low classes are higher.   While the  actual  WE/WD means  show a  lower week-
end level, the meteorologically adjusted WE/WD means indicate a  slightly
higher weekend level.   The Baltimore  station  does  not  show a  consistent
WE/WD relationship among the five comparisons.  Therefore, we cannot  say
whether the weekend OX level is higher  or  lower than the weekday level
for this station.   The above type of  station  is termed a "Type III
station."
      In the next section, these classifications are used  to  categorize
the WE/WD differences in oxidant concentrations at each  site.
4.4   CLASSIFICATION OF SITES
      Table 10 summarizes the results  of grouping  the 22  oxidant  monitoring
stations  into the above-defined three station types.   The  table  lists six
Type  I stations that exhibit a consistently lower OX level  on weekends,
and an additional seven stations having a  consistently lower weekend  level
with  one  exception.  These 13 stations  are all  located either in the  peri-
phery or  outside the large metropolitan areas.  Table  lOb  presents  the
four  Type II stations that exhibit a  consistently higher OX level on
weekends:  Philadelphia, Newark, Welfare Island,  and Medford. These  four
stations  are all located in the most  urban, source-intensive areas.
      Table  lOc  lists  the  five Type III stations that  do not exhibit'a con-
sistent  relationship  among  the  five different WE/TO comparisons.  The
reason why, at  these  stations,  the WE/WD  oxidant  relationship is altered
from  one class  to  another is  not known.   However,  one possible  explanation
could be that  these stations  classed into Type III  have only a  weak  WE/WD
                                     67

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            TABLE  10.   CLASSIFICATION OF  WE/WD DIFFERENCES
a.  Type  I stations,  which exhibit a consistently loner OX level on weekends
Station
Bethesda, Md.
Silver Spring, Md.
Greenwich, Conn.
Mlddletown, Conn.
Lancaster, Pa.
Norrfstown, Pa.
With One Exception
Camden Co., N.J.
Danbury, Conn.
Hamaroneck, N.Y.
Qulncy, Mass.
Salem, Mass.
Suit! and, Md.
Uorchester, Mass.
Sample
Size XWE
286 31
248  27
340 29
284 29
232 31
239 31

224 28
250 30
377 29
458 30
185 29
351 29
350 32
High Class
% Change
(WE-WD)
- 8
-17
-18
-19
- 9
-10

+10
-31
- 9
- 2
-23.
0
- 6
Middle Class
% Change
(WE-WD)
- 9
- 4
-12
- 1
- 4
- 6

-17
-16
-19
- 6
- 9
- 1
-10
Low Class
% Change
(WE-WD)
-20
- 8
- 2
- 2
- 7
-18

-12
+ 6
+16
+ 1
+19
- 4
+15
Actual
% Change
(WE-WD)
- 9
-13
-15
-11
- 8
- 9

-14
-18
- 8
- 4
-12
- 8
- 3
Met.
Adjusted
% Change
(WE-WD)
- 8
-12
-11
-10
- 6
- 3

- 8
-17
- 6
- 3
- 6
- 1
- 3
 b.  Type  II stations, which exhibit a consistently higher OX level on weekends.
Newark, N.J.
Philadelphia, Pa.
With One Exception
Medford, Mass.
Welfare Is., N.Y.
278 28
293 30

415 30
362 29
+13
+15

- 7
- 1
+29
+20

+13
+ 2
+27
+23

+16
+ 8
+13
+ 2

+ 2
+ 1
+24
+17

+ 2
+ 2
 c.  Type  III stations, which do not exhibit a consistent relationship
     among meteorological classes.
Asbury Park, N.J.
Babylon, N.Y.
Baltimore, Md.
Camden, N.J.
Galthersburg, Md.
197 28
238 29
253 29
371 29
140 32
- 8
+ 6
-15
-8
0
-~9 ""'
-17
+ 9
+ 3
-15
+13
+19
+ 9
+20
+18
i -4" '
- 7
- 9
- 2
- 4
! + 1
+ 1
+ 1
+ 2
- 3
                                   68

-------
difference in daily max OX levels so that the WE/WD oxidant relationship
can be altered by sample variation from one meteorological  class  to another.
Those stations classed into Type I and Type II have a  strong enough WE/WD
difference in daily max OX levels so that the WE/WD oxidant relationship
cannot be altered easily by sample variation from one  meteorological
class to another.
                                   69

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                5.  DIFFERENCES IN OXIDANT PRECURSOR LEVELS





      In Sections  3 and 4,  WE/WD differences  in  daily maximum oxidant levels were



examined.  This section discusses the WE/WD differences in oxidant precursor



levels.  The 6-9 A.M.  (EOT) average concentrations of nitric oxide (NO),

  rr.

nitrogen dioxide (N02), nonmethane hydrocarbons (NMHC), and total hydro-



carbons  (THC) are  examined.  The two major oxidant precursors are NO  and
                                                                    J\


NMHC.  NOX is defined  as the sum of NO and N02 concentrations.   Most NOX



is emitted to the  atmosphere in the form of NO and is subsequently oxidized



to NOg.  Nonmethane hydrocarbons are defined as the difference between total



hydrocarbons (THC) and methane  (CH4).



     In  addition to the four pollutants, the 6-9 A.M. averages of



(N0/N0  ) ratios and carbon monoxide (CO) concentrations are also
      /\


examined.  Because NO  works as  a scavenger of OX while NO* works as a



promoter of OX, an ambient (N0/N0 ) ratio provides an Indication of
                                A


net oxidant formation  potential of the atmosphere [Jeffries et al. 1976].




CO is an inert gas that is mainly emitted by automobiles.  Because



ambient  CO levels  may  very well be an indicator of automotive-related



precursor levels,  WE/WD differences in those data were also examined,





5.1  DIFFERENCES  IN 6-9 A.M. AVERAGE NO AND NOp CONCENTRATIONS



     Ambient NO and N02 data are not as complete as the oxidant data



analyzed in  Sections  3 and 4.   After  checking  the  data completeness



at each  air monitoring site, we found adequate NO and NOp data at the



eight stations whose names, site codes, and sample sizes are listed in



Table 11.  The sample  size is given for both the normal (SS vs. MTWTF)



and Sunday (Sun vs. TWTF) WE/WD definitions.  The eight stations are





                                    70

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                TABLE 11.  MONITORING STATIONS  USED  FOR ANALYZING
                          NO AND N02 AIR QUALITY  DATA
Metropolitan
Area
Washington,
D.C.
Bal timore
Philadelphia
New York City
Station
Name
Bethesda
Silver Spring
Suit! and
Baltimore
Camden
Mamaroneck
Newark
Welfare Is.
SAROAD I.D.
210200005 F01
211480006 F01
211560001 F01
210120018 F01
310720003 F01
334100002 F01
313480002 F01
334680050 F01
EPA's
Site Code*
S-R
S
S
CC-C
S-R
CC-C
CC-C
CC
Number of Valid
Observation Dayst
SS" vs. MTWTF
115
121
in
194
202
224
194
213
Sun. vs. TWTF
85
87
82
137
149
166
140
151
  Code key:

 C - Commercial
CC - Center City
 R - Residential
 S - Suburban
  A "valid observation day" is a day which  has  at least  2  out  of 3  possible
  observations during the 6-9 A.M.  (EOT)  period.
                                     71

-------
all located within or near one of the large metropolitan areas (see
Figure 2).
5.1.1  WE/UP Differences in NO Concentrations
     The WE/WD differences in 6-9 A.M. average NO concentration levels
are computed for both the normal and Sunday WE/WD definitions.  The results
are summarized in Table 12.  All the stations except for Bethesda exhibit
a  large  drop  in  6-9  A.M. average NO  levels on weekends.  The weekend re-
ductions in 6-9  A.M.  average NO levels range from 40% at Mamaroneck, New
York,  to 83%  at  Camden, New Jersey,  under the normal WE/WD definition.
Under  the Sunday WE/WD definition, the drop is generally greater than that
under  the normal definition.  Also,  the weekend reductions in the eight-
station  6-9 A.M. NO  levels are 60% and 67% under the normal and Sunday
WE/WD  definitions, respectively.  The Sunday WE/WD definition (Sunday
versus Tuesday through Friday), therefore, yields a slightly larger change
between  the WE/WD 6-9 A.M. average NO levels than the normal WE/WD
definition.
     The NO behavior at the Bethesda station is different from that at
the other stations.   In contrast to  the large weekend drops at the other
stations,  the Bethesda station exhibits a large increase in 6-9 A.M. average
NO concentrations on  weekends.  The  increases are 134% and 252%, respec-
tively,  under the normal and Sunday WE/WD definitions.  A close examination
of the Bethesda NO data revealed that the weekend NO data contained a few
extremely  high NO concentrations.   The standard deviation of the weekend
NO data  is much larger than that of the weekday NO data (5.3 pphm versus
1.4 pphm under the normal  WE/WD definition and 7.6 pphm versus 1.4 pphm
                                 72

-------
                          TABLE 12. WE/WD DIFFERENCES IN 6-9 A.M.
                                    AVERAGE NO CONCENTRATIONS
CO
Station
Name
Bethesda
Silver Spring
Suitland
Baltimore
Camden
Mamaroneck
Newark
Welfare Is.
8-Station
Avg.
SS vs. MTWTF
Mean
WE level
(pphm)
1.5
5.3
0.5
1.0
0.9
1.9
2.2
1.4
1.8
Mean
WD level
(pphm)
0.7
12.8
1.3
4.6
5.4
3.2
6.3
2.4
4.6
% Change
(WE-WD)
+ 134%
- 58%
- 60%
- 79%
- 83%
- 40%
- 65%
- 43%
- 60%*
Sun. vs. TWTF
Mean
WE level
(pphm)
2.3
3.6
0,1
0.9
0.9
1.4
1.8
1,3
1.5
Mean
WD level
(pphm)
0.7
13.0
1.4
4.4
5.4
3.4
6.6
2.4
4.7
% Change
(WE-WD)
+ 252%
- 72%
- 93%
- 80%
- 84%
- 60%
- 72%
- 45%
- 67%*
                The station-average percentage difference is  computed  from the
                station averages of WE/WD concentration levels.

-------
under the Sunday WE/WD definition).  Because of this large difference
between the WE/WD standard deviations, the results for the Bethesda
station should be viewed with cautton.
     The WE/WD differences in 6-9 A.M. average NO levels are examined
by using the t-test.  Figure T9 shows the significance level of the WE/WD
difference at each  station under the normal and Sunday WE/WD definitions.
Among the eight stations examined, fiveSilver Spring, Baltimore, Camden,
Newark, and Mamaroneckexhibit significant WE/WD differences in 6-9 A.M.
average NO levels under both the normal and Sunday WE/WD definitions.  The
remaining three stations, Bethesda, Suitland, and Welfare Island, show a
marginally significant difference  under one of the two WE/WD definitions.
     The t-test results are also presented in Figure 20, in which the
standardized  values of Sunday/WD 6-9 A.M. average NO levels are plotted
 on the x-y axes.   The standardized value  is a dimensionless number
 given  by  a  ratio  of the mean  to  the square root of the pooled variance
 (see Appendix A for definition).   Therefore, the standardized values of
WE and  WD  can be  compared directly, and their difference is exactly the
t-value.   Figure  20 clearly shows  that the WE NO level is lower than the
WD level at all the stations except Bethesda.
5.1.2  WE/WD Differences in NOp Concentrations
     The WE/WD differences in 6-9 A.M. average N02 levels are examined
for the same eight  stations as were used for the analysis of NO concen-
trations.   The mean WE level, the mean WD level, and the percentage change
between weekends and weekdays are listed in Table 13.   On the average for
the eight stations, the weekend N02 level  is 42% lower than the weekday
                                  74

-------
 P-Value

 1.0

 0.5
0.10  .	    ^
                                                           (Marginally Significant Region)
0.05
0.002


0.001
                                                           (Insignificant Region)
                                                       *
0.02

  01Q                                _                     (Significant  Region)


0.005
       .*

*
                   .. p-value under Sunday WE/WD definition
                    p-value under normal HE/HD definition
                                             75

-------
(Tl
 I
vo
0)
Ot
ID



I
0)
o


a
0)
N
^
o


to
               1234567

        Standardized Value* of Averaae.WE  6r9 A.M. NO
              Standardized value  =  mean//pooled variance
  Figure 20.  Graphical presentation  of t-test results for

              WE/WD difference  in  6-9 A.M.  average NO concentrations
              (Sun. vs. TWTF).
                              76

-------
          TABLE 13, WE/WD DIFFERENCES IN 6-9 A.M.
                    AVERAGE N02 CONCENTRATIONS

Station
Name
Bethesda
Silver Spring
Suitland
Baltimore
Camden
Mamaroneck
Newark
Welfare Is.
8-Station Avg.
SS vs. MTWTF
Mean
WE level
(pphm)
1.3
3.4
3.2
3.4
1.8
1.5
2.9
1.8
2.4
Mean
WD level
(pphm)
3,1
5.6
5.7
6.6
3.3
2.0
4.0
3.2
4.2
% Change
(WE-WB)
- 59%
- 39%
- 44%
- 48%
- 47%
- 22%
- 27%
- 42%
- 42%*
Sun. vs. TWTF
Mean
WE level
(pphm)
1,1
3.0
3.0
3.0
1.7
1.2
3,0
1.7
2.2
Mean
WD level
(pphm)
3,0
5.6
5.7
6.7
3.3
2.0
4.2
3.3
4.2
% Change
(WE-WD)
- 64%
- 47%
- 47%
- 55%
- 47%
- 43%
- 28%
- 48%
- 48%*
The station-average percentage difference is computed from the
station averages of WE/WD concentration levels.

-------
 level, under the normal WE/WD definition; while under the Sunday VIE/WD defini
 tion, the mean weekend N02 level  is 48% lower than the weekday level.  The
 percentage drop of 6-9 A.M. average N0 levels on weekends is slightly
 smaller than that of  6-9 A.M. average NO  levels under both the normal and
 Sunday WE/WD definitions.
       Unlike the NO case,  the  N02  concentration data  for  the  Bethesda
 station do not behave strangely.   However;  the weekend percentage drop of
  6-9 A.M.  average N02 levels is largest at the Bethesda  station  among
 the eight stations examined.   Under  the normal definition, the Mamaroneck
 station has the  smallest  percentage  drop, 22%, on weekends,  while under
 the Sunday definition, the  Newark station exhibits the smallest drop, 28%.
       As seen from Figure  21,  the  t-test results of the WE/WD N02 data
 indicate  that, at all  eight station  sites,  the WE/WD differences in
 6-9 A.M.  average N02  levels are significant under both the normal and
 Sunday WE/WD definitions.  The standardized values of WE/WD  6-9 A.M.
 average N02 levels are plotted in Figure  22,  showing that all the
 stations  have significantly lower N02  levels  on weekends than on
 weekdays.

5.2   WE/WD DIFFERENCES IN N0q/N0..  RATIOS
^^^^fc^^^"^"^   ^\
      The preceding subsection  has  revealed that both  NO and N02 concentra-
tions are significantly lower  1n the 6-9 A.M.  period  on the weekends than
1n the same period on weekdays.  The remaining question 1s whether the
(N02/N0 ) ratio 1s also significantly different between weekends and

                                    78

-------
   P-Value

1.0


0.5


                                                         (Insignificant Region)
0.10
     r	
                                          (Marginally Significant Region)
O.OZ


0.01


0.005




0.002


0.001
0.05	    	



                                                               D
                p-value under Sunday HE/HP definition
              r-j_- p-value under normal HE/HP 
-------
                                       (BA)*
                                (CA)*
          1234567

     Standardized Value* of Average WE 6-9 A.M. N02
       "Standardized Value = mean// pooled variance



Fiaure 22.  Graphical presentation of t-test results for WE/WD
Figure      ;JV?er.enceH1n 6.9 A.M. average NO  concentrations

             (Sun. vs. TWTF).
                           80

-------
weekdays.  The important oxidant precursor,  NO ,  consists  mainly  of NO
                                              A


and N02.  While NO scavenges ambient OX  by reductive  reactions, N02 pro-



motes OX formation by oxidation reactions.   Therefore,  the (N02/NOX)



ratio indicates how ambient NO  works on ambient  OX levels.   Because  N07
                              A                                        t-


is formed by oxidation of NO, the (N0?/N0 )  ratio also  may indicate the
                                     ^   A


age of the ambient  '0  gas mixture.   The greater  the  (N0?/N0  ) ratio, the
                     *                                     A


older the pollutants measured in the air mass  are likely to be.   In actual



calculation of the ratio, we use CN02 +  NO)  as an approximate level of  NOX.



      In testing the WE/WD differences in 6-9  A.M. average (N02/NOX)



ratios, we encountered a difficulty:  The t-test  cannot be used to  validate



such differences because the distribution of (N02/NOX)  is  a bounded one,



always lying between 0 and 1.  Fortunately,  the Wilcoxon rank sum test



(see Appendix B) is known to be efficient over a  wide range of distribu-



tions, including a bounded one.  We  therefore  applied the  Wilcoxon  rank



sum test to test observed WE/WD differences  in 6-9 A.M. average  (N02/NOX)



ratios at each of the eight stations.



      The P-values of the Wilcoxon rank  sum test  for  the eight stations



are plotted in Figure 23.  Three stations,  Camden, Newark, and Mamaroneck,



exhibit a significantly higher (N02/NOX) ratio on weekends, indicating



that the 6-9 A.M. weekend NOX gas mixture is more aged  than the  weekday



NO  gas mixture at these sites.  As  in the case of NO concentrations, the
  A


Bethesda station exhibits strange behavior in its 6-9 A.M. average



(N09/N0 ) ratios.  Only this station shows that the weekend level of
      A


6-9 A.M. average (N02/NOX) ratios is significantly lower than the weekday



level.



                                   81

-------
00
ro
                        P-Value
                     1.0




                     0.5
0.10




0.05





0.02




0.01



0.005






0.002




0.001




                                                                                 0
                                                                                         O
                                                                                     (Nonsignificant Region)
                                                                            (Marginally Significant Region)

                                                                       *
                                             HE level  1s higher than WO


                                             WE level  Is lower than WD
                                                                                    (Significant Region)
                          Figure 23.   P-values  for testing  the WE/WD difference 1n N02/N0
                                        ratio by  the Wllcoxon rank  sum test (Sun. versus TWTF)

-------
     The WE/WD differences in 6-9 A.M.  average  (N09/NOV)  ratios  at  each
                                                  u    A


of the eight sta^  ^s are si^.narized in Tab! * 14.  Because  the  (N02/NOX)



ratios are n t dis. 'ibuted normally, the mean values  alone  cannot indicate



the WE/WD d1 /erei. e  in the (N09/NOY) ratios.   Therefore, Table  14  lists
                               ^   /\


both the mean and median values of 6-9 A.M.  average  (N02/NOX) on weekends



and weekdays.



     Note th?t while  the mean value at the Silver  Spring  station is lower



on weekends than on weekdays, the opposite is true for the  median value.



The statistical test  result shown in Figure 23  is  consistent with the



WE/WD difference found in the median values.  On the  average, N02 accounts



for about 55% (59% in median) of NO  on weekends and  59%  (62% in median)
                                   /\


on weekdays.





5.3  WE/WD DIFFERENCES IN 6-9 A.M. AVERAGE NMHC AND  THC  CONCENTRATIONS



     Since methane is practically inert in photochemical  reactions, non-



methane hydrocarbons  (NMHC) are used as the organic  precursor  of photochemical



oxidants.  Since adequate NMHC data are found  only at four  locations, total



hydrocarbon (THC) data at two other locations  are  also included in  the analysis



to supplement the insufficient area coverage of NMHC data.   THC data may be



an indicator of NMHC  when viewed on a relative  basis.  Because  of the known



unreliability of NMHC measurements at lower concentration levels and because



of the poor area coverage of hydrocarbon data,  the results  of this  section's



analysis should be viewed with caution.
                                    83

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         TABLE  14.  WE/WD DIFFERENCES IN 6-9 A.M.  AVERAGE
                   (N02/NOX) RATIOS
Station
Name
Bethesda
Silver Spring
Suitland
Baltimore
Camden
Mamaroneck
Newark
Welfare Is.
8 -Station
Avg.
Sun vs. TWTF
Mean
VIE level
0.242
0.482
0.752
0.808
0.611
0.448
0.618
0.451
0.552
Mean
WD level
0.641
0.496
0.800
0.691
0.584
0.416
0.533
0.594
0.594
% Change
WE-UD
- 62%
- 3%
- 6%
+ 17%
+ 5%
+ 8%
+ 16%
- 24%
- 7%*
Median
WE level
0.163
0.500
0.911
0.889
0.728
0.433
0.732
0.342
0.587
Median
WD level
0.666
0.430
0.933
0.709
0.633
0.417
0.550
0.617
0.619
% Change
WE-WD
- 76%
+ 16%
- 2%
+ 25%
+ 15%
+ 4%
+ 33%
- 45%
- 5%*
The station-average percentage difference is computed from the
station averages of WE/WD concentration levels.
                                84

-------
5.3.1  WE/WD Differences in 6-9 A.M.  Average NMHC  Levels
     Adequate NMHC data are found only at four monitoring  sites:
Suit!and, Silver Spring, Baltimore,  and Norristown.   The first  two
sites are in the vicinity of Washington, D.C.   The last site is located
northwest of Philadelphia (see Figure 2).   The weekend mean  and weekday
mean of 6-9 A.M. average NMHC concentrations,  as well as the percentage
difference of (WE-WD), are computed  for the four sites under both the
normal and  Sunday WE/WD definitions.  The results  are summarized in Table 15.
     Under the  normal  WE/WD definition  (SS versus  MTWTF),  all four stations
exhibit a considerable drop in 6-9 A.M. average NMHC levels  on  weekends,
ranging from 24% at Norristown to 65% at Suitland.  Under  the Sunday WE/WD
definition  (Sun. versus TWTF), three stations, Suitland, Silver Spring,
and Baltimore, exhibit a measurable  decrease in 6-9 A.M.   NMHC  levels
on weekends, while the Norristown station shows a  slight  increase on
weekends.  The four-station average  of weekend reductions  in 6-9 A.M.
average NMHC concentrations is 48% under the normal WE/WD  definition and
32% under the Sunday WE/WD definition.
     Some of the NMHC data exhibit a large difference in  variance
between the weekend and weekday data sets.  The ordinary t-test is based
on the assumption of an equal variance for the two populations.  The
assumption of equal variance is questionable for  the NMHC  data being
examined.  Therefore, the Welsh t-test, which was  designed to determine
the significance of differences between two populations  with unequal
variances [Snedecor and Cochran 1976] is used to  correct the bias of an
                                     85

-------
                             TABLE 15.  WE/WD DIFFERENCES IN 6-9 A.M.  AVERAGE NMHC  CONCENTRATIONS
Station Name
Suitland
Silver Spring
Baltimore
NorHstown
4-Stat1on
A.vg.
SS vs. MTWTF
# Valid
Obs. Dayst
191
207
199
213

Mean
WE level
(pphm)
30
49
56
87
55
Mean
VID level
(pphm)
87
69
153
114
106
% Change
(WE-WD)
- 65%
- 29%
- 64%
- 24%
- 48%*
Sun. vs. TWTF
# Valid
Obs. Dayst
141
148
146
159

Mean
WE level
(pphm)
31
56
60
121
67
Mean
WD level
(pphm)
74
61
154
109
99
% Change
(WE-WD)
- 58%
- 8%
- 61 X
+ 11%
. 32%*
00
           A "valid observation day" is defined as a day which has at least 2 out of 3 possible observations
           during the 6-9 A.M. period.
           The station-average percentage difference is computed from the station averages  of WE/WD
           concentration levels.

-------
ordinary t-test caused by the large unequal  variances,  whenever (maximum
variance/minimum variance) >_ C.   Here C is the critical  value  which depends
on the sample sizes and can be found from the above reference.
     The results of statistical  testing of the WE/WD differences in
6-9 A.M. average NMHC concentrations are summarized in  Table -J6.  At the
Suitland and Baltimore stations, the WE/WD differences  in 6-9  A.M.  average
NMHC concentrations are significant; at the other two stations, Silver
Spring and Norristown, the WE/WD differences are not statistically
significant.  The above conclusions are consistent under both  normal
and Sunday WE/WD definitions.
5.3.2  WE/WD Differences 1n 6-9  A.M. Average THC Levels
     WE/WD differences 1n 6-9 A.M.  average THC concentrations  are
examined for only two sites, Bethesda, Maryland, and Camden, New Jersey.
The results are summarized 1n Table 17.  The weekend mean of 6-9 A.M.
average THC concentrations is lower than the weekday mean by 13% at
Bethesda and 27% at Camden.  The average percentage decrease (21%) in
THC over the two stations is smaller than that (48%) in NMHC,  which
implies that ambient THC contains a considerable amount of methane,
whose concentration levels change little between weekends and  weekdays.
     The t-test indicates that the WE/WD difference in 6-9 A.M. average
THC levels is significant at both stations.
                                    87

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          TABLE 16.   STATISTICAL  SIGNIFICANCE OF  WE/WD  DIFFERENCES  IN
                     6-9 A.M.  AVERAGE NMHC  CONCENTRATIONS
Station Name
SuWand
Silver Spring
Baltimore
Nor r 1s town
WE/WD Definition
New WE/WD
Old WE/WD
New WE/WD
Old WE/WD
New WE/WD
Old WE/WD
New WE/WD
Old WE/WD
Max, Var/M1n. Var *
5.132
11,565
1.660
1.072
6.014
6.960
1.192
1.156
Test
Welsh t-test
Welsh t-test
t-test
t-test
Welsh t-test
Welsh t-test
t-test
t-test
P-Value
0.005 (Significant)
0.000
0.832 (Non-Significant)
0.275
0.000 (Significant)
0.000
0.728 (Non-Significant)
0.283
*When Max. Var/M1n Var.  > 1.8, the Welsh t-test 1s used  to  correct  the  bias  of ordinary  t-test
 caused by large unequal""variances.

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               TABLE 17.  HE/WD DIFFERENCES IN  6-9 A.M.
                          AVERAGE THC CONCENTRATIONS
Station
Name
Bethesda
Camden
2-Station
A VOL
 Valid
Obs. Dayst
175
160

SS vs. *TW7F
Mean
WE level
(pphm)
174
185
180
Mean
WD level
{pphm)
201
255
228
'"r. Change
(HE-WD)/WDxlOO
- 13S
- 27%
- 212
KE/WD Difference
P-Value
0.04 (Significant)
0.0005 (Significant)

A "valid observation day" is defined  as a day which  has at  least  2 out  of 3
possible observations during the 6-9  A.M. period.
                                   89

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5.4  WE/HP DIFFERENCES IN CO CONCENTRATIONS
     Carbon monoxide (CO) is not an oxidant precursor but 1s an inert
gas emitted primarily by automobiles.  Since ambient CO data are rela-
tively complete, the CO data are used here as an indicator pollutant of
the two precursor emissions, NO  and NMHC, from mobile sources.  Adequate
                               /\
CO data are found at as many as 17 monitoring sites, names and site
codes for which are listed in Table  18.
     The WE/WD differences in CO concentrations are examined for three
overlapping time periods:  3-hr (6-9 A.M.), daytime (6 A.M.-9 P.M.),
and 24-hr.  The intent 1s to estimate the temporal variation of WE/WD
differences in automotive emissions.  The average CO concentrations
over the three different time periods are computed for weekends,
defined as Saturday and Sunday (SS), and for weekdays, defined as
Monday through Friday (MTWTF).
     Table 19 summarizes the WE/WD differences in CO concentrations
computed for the 6-9 A.M., daytime (6 A.M.-9 P.M.), and 24-hr periods.
Except for the Camden County station, the stations exhibit a considerable
decrease in CO concentrations on weekends during all three time periods.
The Camden County station shows the  smallest WE/WD differences in CO
concentrations among the 17 stations and exhibits a small increase (1%)
in the 24-hr average CO levels.  The Camden County site is located on
the grounds of the New Jersey State  Hospital, which could explain why
WE CO levels are not much lower than WD levels.
                                     90

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              TABLE 18.   MONITORING  STATIONS USED FOR ANALYZING
                         CARBON MONOXIDE AIR QUALITY DATA
Metropolitan
Area
Washington,
D.C.
Baltimore
Philadelphia
New York
City
Boston
Station
Name
Bethesda
Silver Spring
Suitland
Baltimore
Lancaster
Camden
Camden Co.
Norri stown
Philadelphia
Asbury Park
Babylon
Mamaroneck
Newark
Welfare Is.
Medford
Quincy
Worchester
SAROAD I.D.
210200005 F01
211480006 F01
211560001 F01
210120018 F01
394660007 F01
310720003 F01
310740001 F01
396540013 F01
397140026 F01
310060001 F01
330280002 F01
334100002 F01
313480002 F01
334680050 F01
221220003 F01
221880002 F01
222640012 F01
EPA's
Site Code*
S-R
S
S
CC-C
S-R
S-R
Ru-A
S-R
CC-C
CC-C
S-I
CC-C
CC-C
CC
S-I
S-I
CC-C
Number of Val id
Observation Dayst
266
283
277
299
236
224
219
202
213
210
145
272
198
247
433
377
441
 Code key:

 A - Agricultural
 C - Commercial
CC - Center City
 I - Industrial
 R - Residential
Ru - Rural
 S - Suburban
 A "valid observation day" is a day which has at least 10 observations for
 the 6 A.M.-9 P.M. period.  For the 6-9 A.M. average concentration, fewer
 observation days may result because of stricter criteria:  2 out of 3
 possible observations.
                                    91

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          TABLE  19.  WE/WD DIFFERENCES IN CO CONCENTRATIONS DURING
                     THE 3-HR  (6-9 A.M.). DAYTIME (6 A.M.-9 P.M.),
                     AND 24-HR PERIODS



Metropolitan
Area
Washington,
D.C.

Baltimore

Philadelphia



New York
City



Boston







Station
Name
Bethesda
Silver Spring
Suit! and
Baltimore
Lancaster
Camden
Camden Co.
Norristown
Philadelphia
Asbury Park
Babylon
Mamaroneck
Newark
Welfare Is.
Medford
Quincy
Worchester
!7-Stat1on
Avg.

6-9 A.M. Average
Mean
WE level
(ppm)
0.4
0.9
1.1
0.6
0.6
1.3
1.6
0.9
1.5
2.5
1.2
2.6
2.3
1.8
1.7
1.9
1.9
1.46

Mean
WD level
(ppm)
0.8
1.3
2.2
1.9
1.1
1.9
1.7
1.9
2.9
2.9
1.8
3.2
3.6
2.0
3.3
2.6
3.0
2.24


% Change
(WE-WD)
- 50%
- 31%
- 50%
- 68%
- 46%
- 32%
- 6%
- 53%
- 48%
- 14%
- 33%
- 19%
- 36%
- 10%
- 49%
- 27%
- 37%
- 35%*

Daytime
Average

% Change
(WE-WD)
- 24%
- 33%
- 35%
- 42%
- 38%
- 25%
- 1%
- 43%
- 30%
- 12%
- 22%
- 15%
- 22%
- 14%
- 27%
- 14%
- 29%
- 25% *


24-Hr Avg.

% Change
(WE-WD)
- 19%
- 19%
- 11%
- 24%
- 23%
- 15%
+ 1%
- 29%
- 15%
- 5%
- 20%
- 8%
- 10%
- 6%
- 19%
- 10%
- 20%
- 15% *

The station-average percentage difference 1s computed from the
station averages of WE/WD concentration levels.
                                     92

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     The percentage reduction of CO concentrations on weekends  is  the
greatest during the 6-9 A.M.  period and  the  smallest during  the 24-hr
period.  Of the station averages,  the 6-9  A.M.  average  CO concentration
on weekends is 35% lower than that on weekdays;  the daytime  average  CO
concentration is 25% lower than that on  weekdays; and the 24-hr average
CO level is 15% lower.
     All stations except Welfare Island  and  Silver Spring exhibit  the
largest weekend drop in the 6-9 A.M. average CO concentration,  followed
by the daytime average and then by the 24-hr average.   At the Welfare  Island
and Silver Spring stations, the weekend  drop in daytime average concentra-
tions is slightly greater than the drop  in the  6-9 A.M.  average which,
in turn, is greater than that in the 24-hr average reductions.  Because
of the smaller weekend drops in CO concentration during the  non-6-9  A.M.
time periods at all the stations other than  the above two, the  weekend
decreases in 6-9 A.M. precursor-pollutant  (NMHC, NO, and NO?) concentrations
should not be interpreted directly as the  same  decreases in  precursor
emissions.  The actual  decreases in precursor emissions on weekends  are
likely to be smaller than those found in the 6-9 A.M. average precursor
concentrations.
     The t-test results of the WE/WD differences in  CO  concentrations
during the three time periods are plotted  in Figure  24, using  P-values.
As seen from Figure 24, the WE/WD differences in both  the  6-9  A.M. average
CO concentrations and daytime average CO concentrations are significant at
15 out of the 17 sites examined.  The two  stations that do not exhibit a
                                    93

-------
  P-Value
1.0
0.5
0.01
0.05
0.01
0.005
 0.001

(Noi

(Ma,
(Si



*
j
A 
 a
+ +


significant

ginally Sigr
inificant Re<



4 *
*

*
legion)

if leant Regi
Ion)



t 





on)


A

t *





A

A

a
. - ^
 -
a 


A

^


B
+
6-9 A.M. Av
Daytime Avg
24-hr Avg
1 1 1 1 1 1 1 I 1 1 1 1 1 1
gocuiae *
-------
significant WE/WD difference in CO concentrations  are Welfare Island,
New York, and Camden County, New Jersey.   All  15 stations  with signifi-
cant WE/WD differences have, of course,  a lower CO concentration on
weekends.
     In the 24-hr average CO concentrations,  11 out of 17  stations
exhibit a significant WE/WD difference,  with  two additional  stations
having a marginally significant difference.
                                    95

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                       6.  RESULTS AND CONCLUSIONS
     The preceding sections discuss the WE/WD differences in observed daily
maximum oxidant levels (Section 3), in meteorologically adjusted oxidant
levels (Section 4) and in 6-9 A.M. average concentrations of oxidant
precursors and carbon monoxide (Section 5).  This section reviews the
analysis results of the  preceding sections and discusses the implications
of those results for oxidant sensitivity to WE/WD changes in precursor
emissions.   In Section 6.1, the oxidant-precursor relationship in urban core
areas  is examined, and that in source/receptor areas is examined in Section 6.2.
In Section 6.3, the oxidant response to WE/WD emission changes is compared
with that of long-term emission changes.
6.1  OXIDANT-PRECURSOR RELATIONSHIP IN URBAN CORE AREAS
     NO, N02> CO, and OX data are available for eight stations:  Bethesda,
Silver Spring, Suitland, Baltimore, Camden, Newark, Welfare Island, and
Mamaroneck.   Furthermore, three stations, Silver Spring, Suitland, and
Baltimore, have NMHC data.  These eight stations are used to examine the
relationship  among the WE/WD differences found in the five pollutants.
     Table 20 summarizes means of daily maximum OX levels and 6-9 A.M.
average concentrations of NO, N02, NMHC, and CO under the normal WE/WD
definition.   All eight stations listed in Table 20 are located within or
near the large metropolitan areas (see Figure 2).  Therefore, the oxidant-
precursor relationships at the eight stations may be representative of
those in urban core areas.  In Table 20, large WE/WD differences are found
                                    96

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TABLE 20 .  WE/WD MEANS OF DAILY MAXIMUM OX CONCENTRATIONS
           AND 6-9 A.M. AVERAGE PRECURSOR AND CO
           CONCENTRATIONS AT URBAN CORE SITES (SS vs. MTWTF)
Station
Name
Bethesda
Silver Spr.
Suit! and
Baltimore
Camden
Mamaroneck
Newark
Welfare Is.
8-Statlon
Avg.
Daily Max
OX (ppb)
WE WD
70
52
73
54
68
49
56
62
61
77
59
79
59
70
54
49
61
64
6-9 A.M. Avg.
NO (pphm)
WE WD
1.5
5.3
0.5
1.0
0.9
1.9
2.2
1.4
i.a
0.7
12.8
1.3
4.6
5.4
3.2
6.3
2.4
4.6
6-9 A.M. Avg.
NO? (pphm)
WE WD
1.3
3.4
3.2
3.4
1.8
1.5
2.9
1.8
2.4
3.1
5.6
5.7
6.6
3.3
2.0
4.0
3.2
4.2
6-9 A.M. Avg.
NMHC (pphm)
WE WD

49
30
56




45

69
87
153




103
6-9 A.M. Avg.
CO (ppm)
WE WD
0.4
0.9
1.1
0.6
1.3
2.6
2,3
1.8
1.4
0.8
1.3
2.2
1.9
1.9
3.2
3.6
2.0
2.1

-------
in 6-9 A.M. average concentrations of OX precursors and CO, but much smaller
WE/WD differences are found in daily maximum OX levels.  Another tendency
is apparent:  The higher the 6-9 A.M. NO level, the lower the daily maximum
OX level.  This tendency is noted among stations but is not always noted
between weekends and weekdays.
     The percentage changes in each pollutant concentration between
weekends and weekdays are computed and summarized in Table 21.   On the
average, the weekend level of 6-9 A.M. average NO concentrations is 60%
lower than the weekday level, while the N02 weekend level is 42% lower.
Since NO  concentration is approximately equal to the sum of NO and N09
        A                                                             
concentrations, the weekend level of 6-9 A.M. NOV concentrations should
                                                J\
be lower than the weekday level by some similar percentage, say, 50%.
     NMHC data are available for three stations only:  Silver Spring,
Sultland, and Baltimore.  The three-station average NMHC level in
6-9 A.M. concentrations is 56% lower on weekends than on weekdays.
Because fewer stations were used, an eight-station average decrease in
6-9 A.M. NMHC concentration on weekends may be different from the three-
station average value.  In any case, the weekend precursor levels are
lower than the weekday levels by about 50% 1n 6-9 A.M. average NO  con-
                                                                 A
centrations and 50% to 60% lower in 6-9 A.M. average NMHC concentrations.
Under the above decreases in 6-9 A.M. precursor concentrations, the
eight-station average drop in daily maximum OX concentrations on weekends
1s only 5%.
                                    98

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          TABLE 21.  PERCENTAGE CHANGES (WE-WD) IN DAILY
                     MAXIMUM OX CONCENTRATIONS AND 6-9 A.M.
                     AVERAGE PRECURSOR AND CO CONCENTRATIONS
                     AT URBAN CORE SITES (SS vs. MTWTF)
Station Name
Bethesda
Silver Spring
Suit! and
Baltimore
Camden
Mamaroneck
Newark
Welfare Is.
8-Stat1on
Avg.*
Daily Max
OX
- 9%
- 13%
- 8%
- 9%
- 2%
- 8%
+ 13%
+ 1%
- 5%
6-9 A.M. Average
NO
+134%
- 58%
- 60%
- 79%
- 83%
- 40%
- 65%
- 43%
- 60%
N02
- 59%
- 39%
- 44%
- 48%
- 47%
- 22%
- 27%
- 42%
- 42%
NMHC

- 29%
- 65%
- 64%




- 56%
CO
- 50%
- 31%
- 50%
- 68%
- 32%
- 19%
- 36%
- 10%
- 35%
*The station-average percentage difference is computed from the station
 averages of WE/WD concentration levels listed in Table 20.
                                   9.9

-------
     Table 22 summarizes means of daily maximum oxidant concentrations
and of 6-9 A.M. average precursor concentrations for the Sunday WE/WD
definition.  As noted in Figures 6 and 7, the WE/WD differences in
daily max OX levels are slightly greater for the Sunday definition than
for the normal definition.  However, a similar magnification of the
WE/WD differences is not seen in the precursor-pollutant levels.
     Table 23 briefly lists the percentage differences for oxidants
and precursors using the Sunday WE/WD definition.  Except for NO at the
Bethesda station, the 6-9 A.M. precursor levels are consistently lower
on weekends among the eight stations.  The daily maximum oxidant levels
are also consistently lower on weekends except for the Newark station,
which exhibits an increase (12%) in daily max OX levels.  The station
average reductions on weekends are 9% for daily max OX levels and 67%,
48%, and 49%, respectively, for 6-9 A.M. average NO, N02, and NMHC levels.
Since only three stations have NMHC values, the average percentage reduc-
tion for NMHC, 49%, is a less reliable estimate than those for NO and N02-
     A comparison of Tables 21 and 23 indicates that, in 6-9 A.M. average
NO and N0 levels, the percentage decreases for Sundays alone are 12% to
14% greater than those for Saturdays and Sundays combined.  In daily maximum
OX levels, the percentage decrease for Sundays only is nearly twice as large
as that for Saturdays and Sundays combined.  For NMHC, the percentage
decrease on Sundays is slightly smaller than that on Saturdays and Sundays
combined.  However, because of the small number of stations (three) used,
such a difference may be due to sample fluctuation.
                                   100

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TABLE 22 .   WE/WD MEANS OF DAILY MAX OX AND OF 6-9  A.M.
           AVERAGE LEVELS OF NO, NO,,  AND NMHC:
           SUN.  VERSUS TWTF        *
Station
Name
Bethesda
Silver Spr.
Suitland
Baltimore
Camden
Mamaroneck
Newark
Welfare Is.
8-Station Avg.
Daily Max
OX(ppb)
WE WD
64
51
74
48
68
46
56
57
58
76
59
79
57
69
54
49
63
63
6-9 A.M. Average Concentration
NO (pphm)
WE WD
2.33
3.63
0.10
0.87
0.85
1.37
1.84
1.32
1.54
0.66
13.00
1.43
4.38
5.39
3.39
6.58
2.39
4.65
N02 (pphm)
WE WD
1.09
3.00
3.05
2.97
1.75
1.16
3.02
1.69
2.22
3.00
5.61
5.73
6.66
3.31
2.05
4.18
3.27
4.23
NMHC (pphm}
WE WD

56
31
60




49

61
74
154




96

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TABLE 23.   PERCENTAGE DIFFERENCE (WE - UD) IN DAILY MAX OX
           AND IN 6-9 A.M. AVERAGE LEVELS OF NO, N0?) AND
           NMHC:  SUN. VERSUS TWTF
-
Station Name
Bethesda
Silver Spring
Suitland
Baltimore
Camden
Mamaroneck
Newark
Welfare Is.
*
8-Station Avg.
OX

- 17%
- 13%
- 6%
- 16%
- 2%
- 14%
+ 12%
- 9%

- 9%
NO

+252%
- 72%
- 93%
- 80%
- 84%
- 60%
- 72%
- 45%

- 67%
NO,

- 64%
- 47%
- 47%
- 55%
- 47%
- 43%
- 28%
- 48%

- 48%
NMHC


- 8%
-59%
-61%





-49%
 The station-average percentage difference is  computed  from the station
 averages of WE/WD concentration levels.
                        102

-------
     Regardless of the WE/WD definition used,  WE  oxidant  reductions  at
the urban sites are not nearly as large as  those  associated  with  early-
morning precursors.  To some degree,  this may  be  expected.   In  the
source-intensive urban areas, emissions are generally  high throughout
a weekday due to traffic.   Because NO is a  very effective scavenger  of
oxidants, it is very likely that oxidant levels in  the urban area may
actually be suppressed during the week. (Recall  from  Section 3 that
weekday maximum daily oxidant levels  within urban areas were generally
lower than those outside.)   The early-morning  reductions  in  NO  and the
daytime reductions in CO on weekends  indicate  that  NO  levels in the  urban
areas may very well be lower throughout the weekend, leaving less NO
available for oxidant scavenging.  Consequently,  in the urban core,  any
reductions in precursor emissions and subsequent  oxidant  formation upwind
of the urban area may be offset by the lowered scavenging effect  on
weekends.

6.2  OXIDANT-PRECURSOR RELATIONSHIP IN DOWNWIND AREAS
     The preceding subsection discusses the relationship  between  weekend
oxidant reductions and weekend precursor reductions at the  eight  urban
core sites.  However, because a higher oxidant level  is usually found in
the downwind area rather than in the urban  source area, it  would  also be
useful to contrast weekend reductions in urban precursor levels with oxidant
reductions in downwind areas.
                                   103

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     By referring to Figure 2, we have formed source-receptor pairs in
Table 24 to compare the weekend precursor reduction at the source site
with the weekend oxidant reduction at the downwind receptor site.  Because
ambient precursor data are available at fewer sites than oxidant data, each
source site is paired with more than one downwind receptor site.  By
considering the location of each monitoring station and the availability
of precursor data, we chose to use the Baltimore station as the urban-
source site for the Washington, D.C.-Baltimore metropolitan area and the
Camden station for the Philadelphia metropolitan area.  The Newark-New York
and Boston metropolitan areas are represented by the Newark and Medford
stations, respectively.  The downwind receptor stations listed in Table 22
were selected by considering the geographical location and the WE/WD
oxidant behavior (a consistently lower weekend level is used as the
criterion).
     Table 24 indicates that while the 6-9 A.M. weekend urban precursor
levels are 64% to 83% lower in the four metropolitan areas (five if the
Washington, D.C., and Baltimore areas are counted separately), the weekend
downwind oxidant levels are only 7% to 14% lower.  On the average over the
four regions, the NO and NMHC levels are 75% and 64% lower, respectively, on
weekends.  Those reductions in 6-9 A.M. precursor levels bring about
11%-12% reductions in daily maximum oxidant levels.  In Table 24, percentage
reductions in 6-9 A.M. and daytime (6 A.M.-9 P.M.) CO levels are also
presented to facilitate some estimate of precursor reductions for those
                                   104

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              TABLE 24.  WEEKEND REDUCTIONS IN 6-9 A.M. URBAN PRECURSOR
                         LEVELS AND DOWNWIND DAILY MAX OX LEVELS
                         (SS vs. MTWTF)
Metropolitan
Area
Washington D.C.-
Baltimore
Philadelphia
Newark-
New York
Boston
4-Region
Avg.a
% Changes in Urban Precursor Levels
Station 6-9 AM 6-9 AM 6-9 AM
Name NO NMHC CO
Baltimore
Camden
Newark
Medford

-79%
-83%
-65%
NA
-75%
-64%
NA
NA
NA
-64%
-68%
-32%
-36%
-49%
-45%
Daytiiud
CO
-42%
-25%
-22%
-27%
-27%
% Changes in Downwind
Station
Name
Bethesda .
Gaithersburg
Silver Spring
Suitlandb
Lancaster
Norristown
Camden Co.
Greenwich
Middletown
Dan bury
Mamaroneck
Salem .
Quincy

OX
-9%
-4%
-13%
-8%
-10% J
-9% )
-14% J
-15% )
-11% 1
-18%
-8%
OX Levels
Regional
Avg.a
-9%
(-14%)c
-14%
-12% \ -7%
-4% ) (-12%)c
-11% c
NA
a.

b.
c.
Data are not available.
The regional average percentage difference is computed from the station
averages of WE/WD concentration levels.
These stations are located somewhat off the typical  downwind area.
Regional average value when nontypical downwind stations are excluded.

-------
stations at which data are not available for one or both of the two
precursor pollutants.  The weekend reductions in CO levels are somewhat
smaller than those in the two precursors.  However, their percentage
reductions are still much greater than those in downwind oxidant levels.
     The ll%-to-12% reduction in downwind oxidant levels is small,  compared
with the 64%-to-75% reduction in 6-9 A.M. urban precursor levels.   However,
the percentage reduction in downwind oxidant levels is more than twice
as large as that in urban oxidant levels (see Table 21).  These greater
responses of downwind oxidant levels to changes in precursor levels can
be seen from Table 25, where percentage changes in weekend oxidant  levels
are computed for stations in each of three types of regions:  upwind areas,
metropolitan areas, and downwind areas.  Although the grouping of stations
into each of the three types of region is somewhat arbitrary, the following
general pattern emerges from such a spatial grouping of stations:
        The oxidant levels in urban source-intensive areas are
         relatively low, and the weekend reductions are also small.
        The oxidant levels in areas downwind of a large metropolitan
         area tend to be higher than those within the metropolitan  area.
         The weekend reductions in these downwind areas are also much
         greater than those within the metropolitan area.
     t   The oxidant levels and weekend reductions in the upwind area
         are slightly higher than those in the metropolitan area
         itself, but are substantially lower than those in the
         downwind area.
     The above results should be viewed with some caution.  The analysis
of precursor data indicated substantial reductions in 6-9 A.M. concentra-
tions in urban areas on weekends.  These 6-9 A.M. concentrations are thought
                                   106

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   TABLE 25.  WE/WD CHANGES IN DAILY MAXIMUM OXIDANT LEVELS
              IN REGIONS UPWIND OF, DOWNWIND OF, AND WITHIN
              THE LARGE METROPOLITAN AREAS  (SS VS. MTWTF)
Region
Upwind
Areas
Metro-
politan
Areas
Downwi nd
Areas
Station Name
Bethasda (BE)*
Gaithersburg(GA)
Asbury Park (AP)
Worchester(WO)
Norristown(NO)*
Suitland (SU)*
Baltimore (BA)
Philadelphia(PH)
Camden (CA)
Newark (NE)
Welfare Is.(WI)
Mamaroneck(MA)*
Medford (ME)
Quincy (QU)*
SilverSpring(SS)
Lancaster (LA)
Camden Co.(CC)
Babylon (BB)*
Greenwich (GR)
Dan bury (DA)
Middletown(MI)
Salem (SA)
Regional Average
% Difference % Difference WE Level WD Level
(WE - WD) (ppb) (ppb)
-9
-4
-4
-3
-9
-8
-9 }
+2
-2
+13
+1
-8
+2
-4 '
-13
10
-14
-7
-15
-18
-11
-12 )
-7% 65 69
-2% 59 60
-12% 67 76
This station could be classified differently.
                             107

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to be indicative of the oxidant-forming potential of a city.   However,
due to the limited spatial resolution of precursor data, the  changes in
concentrations outside the urban area are unknown.  Precursor emissions
occurring outside the urban area can affect the oxidant levels formed on
any given day.  A second precautionary note is that the 6-9 A.M.  reduc-
tions in weekend precursor levels may be misleadingly high as a result
of changes in temporal emission patterns.  As Table 19 suggests,  reductions
in weekend CO levels are considerably less than implied by the 6-9 A.M.
levels alone.

6.3  ME/WD OXIDANT CHANGE VS. LONG-TERM OXIDANT CHANGE
     By comparing the WE/WD oxidant change with the long-term oxidant
change occurring in the Los Angeles region, this subsection gives a brief
overview of the effectiveness of simultaneous control of NOX  and NMHC for
controlling oxidant air pollution.  The hydrocarbon controls  employed in
the Los Angeles area  in the late 1960s brought about a remarkable oxidant
improvement in the source-intensive areas [Horie and Trijonis 1977]. The
152-to-18% reduction in NMHC and the 35%-to-36% increase in NO  brought
                                                              A
over a 30% improvement in daily maximum oxidant levels during the 1965-1974
period.  Over that 10-year period, the oxidant improvements in the source-
intensive western portion of the basin were greater than those in the
downwind, eastern portion of the basin [Trijonis et al. 1976].
     On weekends, the eight-station average 6-9 A.M. NO and N02 levels  are,
respectively, 60S and 42% lower than the weekday level, while the three-station
average 6-9 A.M. NMHC level is 56% lower than the weekday level (Table  21).
                                   108

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These reductions in morning precursor levels brought about a  (Saturdays



and Sundays) to 9% (Sundays) improvement in daily maximum oxidant levels in



the source-intensive metropolitan areas.  A slightly greater improvement



(about 12%) occurred in areas downwind of the metropolitan areas.



     Compared with the Los Angeles example, the weekend oxidant improvement



that is brought about by a simultaneous reduction in NO  and NMHC is much
                                                       /\


less dramatic.  In Los Angeles, there has been a reduction in NMHC and an



increase in NO , thereby resulting in a lower NMHC/NO  ratio.  These three
              A                         -~'            X


factors appear to have contributed synergistically to the 30% reduction in



daily maximum ox'dant levels.  On the other hand, about the same percentage



reductions in both NMHC and NO  occurred on weekends, thereby leaving the
                              J\


NMHC/NO  ratio virtually unchanged.  Therefore, the reduction in NMHC
       ^


alone contributed to lowering oxidant levels on weekends.



     The above examples, however, should not be used to compare the



effectiveness of hydrocarbon controls and that of simultaneous control



of NO  and NMHC.  Because the Los Angeles area has higher oxidant levels
     J\,


than are found in the northeast United States, a relative contribution of



transported oxidant to the ambient oxidant level would be smaller in the



Los Angeles area than in the northeast.  As a result, the effect of the



same oxidant control on ambient oxidant level will show up more in the



Los Anqeles area  than in the northeast United States.
                                   109

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                             7.  REFERENCES
Breiman, L. and W. Meisel  (1976),  "The Change in Ozone Levels Caused by
Precursor Pollutants:  An  Empirical Analysis," Proceedings of the EPA
Conference on  Environmental Modeling and Simulation, April 19-22, 1976,
Cincinnati, Ohio.

California Air Resources Board  (1974), "Weekday vs Weekend Oxidant Concen-
trations," California Air  Quality  Data, Vol. VI, pp. 5-6.

Cleveland, W.  S., B. E. Kleiner and J. L. Warner (1974a), "Using Robust
Statistical Methods  in Analyzing Air Pollution Data with Applications to
New York-New Jersey  Photochemistry," Presented at the 67th Annual Meeting
of the Air Pollution Control Association, June 9-13, 1974, Denver, Colorado.

Cleveland, W.  S., T. E. Graedel, B. Kleiner, and J. L. Warner (1974b), "Sunday
and Workday Variations in  Photochemical Air Pollutants in New Jersey and
New York," Science,  Vol. 186, pp.  1037-1038.

Cleveland, W.  S. and J. E. McRae (1977), "Weekday-Weekend Ozone Concentrations
in the Northeast United States," Presented at the Conference on Air Quality,
Meteorology and Atmospheric Ozone, July 31-August 6, 1977, Boulder, Colorado.

Eldon, J. A.,  J. C.  Trijonis and K. Yuan (1978), "Statistical Oxidant/Precursor
Relationships  for the Los  Angeles  Region, Final Report: Creation of Empirical
Oxidant/Precursor Models," Prepared for the California ARB by Technology
Service  Corporation, Contract #A5-020-87.

Elkus, B. and  K. R.  Wilson (1977), "Photochemical Air Pollution:  Weekend-
Weekday  Differences," Atmos. Environ., Vol. 11, pp. 509-515.

Horie, Y., A.  S. Chaplin,  and E. D. Helfenbein  (1977), "Population Exposure to
Oxidants and Nitrogen Dioxide in Los Angeles, Volume II:  Weekday/Weekend and
Population Mobility  Effects," U.S. EPA Publication No. EPA-450/3-77-004b.

Horie, Y. and  J. Trijonis  (1977),  "Population Exposure to Oxidants and
Nitrogen Dioxide in  Los Angeles, Vol IV:  Analysis and Interpretation of
Trends," U.S.  EPA Publication No.  EPA-450/3-77-004d, U.S. EPA/Office of
Air Quality Planning and Standards, Research Triangle Park, North Carolina.

Jeffries, H. E., J.  E. Sickles, II, and L.  A. Ripperton (1976),  "Ozone
Transport Phenomena:  Observed  and Simulated," Presented at the  APCA meeting
in Portland, Oregon, June  1976.

Levitt, S. B.  and D. P- Chock (1975), "Weekday-Weekend Pollutant and Meteoro-
logical Studies of the Los Angeles Basin," Presented at the APCA Annual
Meeting, June  15-20 1975.
                                     no

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Ludwig, F. L. and E. Skelar (1977), "Ozone in the Northeastern United States,"
U.S. EPA Publication No. EPA 901/9-76-007, U.S. EPA/Office of Air Quality
Planning and Standards, Research Triangle Park, North Carolina.

Peterson, J. T. and E. C. Flowers (1977), "Interactions Between Air Pollution
and Solar Radiation," Solar Energy. Vol.  19,  pp.  23-32.

Snedecor, G. W. and W. G. Cochran (1976), Statistical  Method,  Sixth Edition.
Ames, Iowa:  The Iowa State Univ. Press, pp.  115-118.

Sonquist, J. A., E. L. Baker and J. N.  Morgan (1973),  "Searching for
Structure," University of Michigan, The Institute for  Social  Research.

South Coast AQMD (1976), "Preliminary Emissions Inventory and  Air Quality
Forecast 1974-1995," South Coast Air Quality  Management District, El Monte,
California.

Trijonis, J. C. et al. (1976),  "Emissions and Air Quality Trends in the South
Coast Air Basin," EQL Memo No.  16,  Calif. Inst. of Tech.,  Pasadena, California.

Trijonis, J. C., and K. Yuan (1977), "Visibility  in the Southwest,"
Final  Report prepared by Technology Service Corporation for the U.S. EPA,
Grant No.  802815.

Trijonis, J. C. and K. Yuan (1978), "Visibility in the Northeast,"
Final  Report prepared by Technology Service Corporation for the U.S. EPA-
Grant No.  802815.

U.S. Dept. of Commerce (1973),  1970 Census of Population:   Graphic Summary
of the 1970 Population Census,  PC(Sl)-55, Bureau  of the Census, Social  and
Economic Statistics Administration, Washington, D.C.

U.S. EPA (1977), "National Air Quality and Emissions  Trends Report, 1976,
U.S. EPA Publication No. EPA-450/1-77-002, U.S. EPA/Office of Air Quality
Planning and Standards, Research Triangle Park, North  Carolina.

Wight, G.  D., G. T. Wolff, P.  J. Lioy,  R. E.  Meyers and R. T.  Cederwall (1977),
"Formation and Transport of Ozone in the Northeast Quadrant of the U.S.,"
Proceedings of the ASTM Conference on Air Quality, Meteorology and Atmospheric
Ozone, August 3-6, 1977, Boulder, Colorado.

Wolff, G. T., P. J. Lioy, R. E. Meyers, R. T. Cederwall, G. D. Wight, R. E.
Pasceri and R.  S. Taylor (1977), "Anatomy of Two Ozone Transport Episodes  in
the Washington, D.C. to Boston, Mass., Corridor," Environ. Sci. Techno!.,
Vol. 11, pp. 506-510.

Zeldin, M. D.,  and W. S. Meisel (1978),  "Use of Meteorological Data in Air
Quality Trend Analysis," U.S.  EPA Publication No. EPA-450/3-78-024, U.S.
EPA/Office of Air Quality Planning and Standards, Research Triangle Park,
North Carolina.


                                     Ill

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                              APPENDIX A
                    STUDENT T-TEST AND WELSH T-TEST

     Let x-j  and  Xg be  sample means  of n. weekend values and ng weekday
                                2      2
values, respectively,  and  let s^  and  s be  sample  variances of the weekend
values and the weekday values,  respectively.  Then, t corresponding to
the differences  tn sample  means is  computed by
                                      xl:"  X2
                                    2       J:          *
                                    s /n, + s /n
                                 V      I       L.
        2
where s  is the pooled variance, which, in turn, is given by
                      2
                     S  =
                                       Cn2-l)
     The two-tailed probability for the t-value computed and the
(n-j + n - 2} degrees of freedom, which can be found in most statistical
textbooks, is compared with the chosen confidence level (1-p).
If p  0.05, the difference between the two means x^ and x^ is significant,
i.e., not merely due to chance.  If 0.05 < p <_ 0.10, the difference
between the two means is marginally significant.  If p > 0.10, the
difference is insignificant, i.e., likely to be caused by mere chance.
     The preceding test is based on the assumption that the two
population variances are the same.  When the two population variances
                                   112

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differ quite a lot, a Welsh t-test,  instead of an ordinary t-test, has
to be used.  The Welsh t-statistic  is computed by
                     t1 =
                             xl " X2
                         V sl/nl  + S2/P2

The significance level of t1 is, approximately,
                          (w1 + w2)
where w-, = s^/n-,, w2 = s2/n2, and t1 and t2 are the significance levels
of t found in th_ t-table for (n,-1) and (n2-l) degrees of freedom,
respectively.
                                   113

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                               APPENDIX B
                         WILCOXON RANK SUM TEST
     The significance probabilities (the P-value) of W can be obtained by
the use of a normal approximation.  The mean of W  can be approximated
by M = l/2n(n-Hn+l), and the variance of W  can be approximated by
VAR = [mn(nH-n+l)]/12.  Thus the P-value of Z = (Ws-M)/VvAR can be found
in the normal table.
                                    114

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                              APPEND I'X C
                  CONFIDENCE INTERVAL FOR THE MEDIAN
                                                      /\
     Let {Y..}, i=l , ... n be a sample of size n.  Let Y be the median.
Then a robust confidence interval for the median can be computed by
the following procedure, suggested by Alan M. Gross ("Confidence
Interval Robustness with Long-Tailed Symmetric Distributions," JASA,
Vol. 71, June 1976, pp. 409-416) and simplified by John Tukey and
Mosteller "Data Analysis and Regression".
      Procedure:
        /s
1.  Let Y be  the median.
2.  I = Interquantile  range
      = upper hinge-lower hinge.
3.  Compute the standardized value
                                 >\
                                 v
4.  Compute the asymptotic variance for the median:
                                    A
             Var Y =	o	sr
                                         d-u2)4
 5.  The  confidence  interval,  then,  is
    where ty is the t percentage with df
                         v = 0.7(n-l)
                                   115

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                              APPENDIX D
                     THE AID DECISION-TREE PROGRAM
     The purpose of the AID program is to illustrate the relationship
between a dependent variable A and independent variables x, y, z	
by dividing the available data into classes  (defined by the independent
variables) in such a way as to best explain  the variance in A.  The
data base for AID consists of a set of "n" measurements: A-.,...,A
and x.|,...,xn; y-j...,yn; z-|,...,zn; etc.  Each of the independent variables
is represented by discrete numbers: e.g., x might assume values of
1, 2, 3, or 4; y might assume values of 1, 2, 3, 4, 5, 6, or 7;
z might assume values of 1, 2, 3, 4, or 5; etc.  To achieve discreteness,
the raw data for the independent variables are usually divided in
ranges.  For example, if the variable z represents wind speed (WS),
one might define z = l if 0 .1 WS < 3 mph; z  = 2 if 3 1 WS < 7 mph;
z = 3 if 7 1 WS < 15 mph.; z = 4 ff 15 1 WS < 25 mph; and z = 5 if
25 mph 1 WS.
     The first step of AID determines the independent variable that
can be split into two groups (say, z = 1 and 4 versus z - 2, 3, and 5)
which maximize  the residual sum of squares  (RSS) explained in the
dependent variable.  If the two groups (G1 and G2) have group means
AT and 7C", then the RSS explained by splitting the data is
                                  116

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                              ARSS = RSSQ -  RSS1  -
where
                                       n      _ _
                               Rssn = Z (A- - Ar  =  RSS  for entire data set;
                                  u   -i=i  J
                               RSS1  =    (A,  - fij")   =  RSS  for GI ;  and
                               RSS9 =L  (A.  -  ATT  =  RSS  for G9.
                                  L   G2   J     L               c
The program considers all possible partitions of the data  among all  the
independent variables and selects the specific  independent variable  and
specific partition that maximizes ARSS.
     The partitioning process is repeated on  each  of the groups G^ and
Gp to yield subgroups.  These and all further subgroups  are divided
until either 1) no subgroup can be split to achieve  a  ARSS above a
preset lower bound, 2) further division  will  produce a subgroup with
number of elements less than a preset bound,  or 3) the number of
terminal subgroups reaches a prespecified limit.
                                   117

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                              APPENDIX E

                   BINARY DECISION TREES DEVELOPED FOR
            CLASSIFYING DAILY MAXIMUM OX LEVEL BY METEOROLOGY
     This appendix shows the 22 AID trees analyzed.  Briefly listed are
meteorological variables and their corresponding symbols used 1n the tree
structure.
      1.  T                - Maximum Temperature  (F)
           max
      2.  T  n             - Average Temperature  (F)
           avg
      3.  P                - Surface Pressure (mb)
      4.  Vis              - Visibility (miles)
      5.  WD               - Wind Direction ()
      6.  WS               - Wind Speed (knots)
      7.  MH               - Mixing'Height (meters)
      8-  V"                - Mean wind speed through  the mixing  layer Cm/sec)
     The other symbols Included 1n each decision tree are:
H:  Indicating that the terminal node belongs to the high class.
M:  Indicating that the terminal node belongs to the middle class.
L:  Indicating that the terminal node belongs to the low class.
P.V.E.:  Percentage of variance explained by the decision tree.
                                   118

-------
                       MH<1250
           /      \
     -45fi:WD<240    240=sWD<-45
  W
N =  74
Y =  45.6
1250
-------
                                         /     \
-45
-------
          W85
       max<80
             80
-------
                     210
-------
-45
-------
T   <75
 max
75
-------
01
                                               8 < Vis
.Vis < 8
                          -45  < WD < 135    135 - WD < "45               -45 < WD < 135     135 < WD < -45
               10 < WS    4 < WS < 10
                                                   /    \
 M
                       240 < WD < -45
                                  135 < WD < 240
                                                                                     M
                                                                                                  0
-------
                         T   <85
                          max
                  (2)
               N    50
               Y  *  56.3
                8    8lTmax
    (6)
 N = 25
JT = 51.4
8
-------
                                               Vis<8
-45
-------
                W60
        N = 70
        Y = 69.5
    /     1
W70
MH<1000     10CQS1H
    6
-------
                                                       H
           Tmax <  75
                           75 i Tn,ax < 85
-45
-------
           MH<1000   KWO
-------
-45
-------
                      (1)
                     N - 199
                    Y = 49.1
             Tmax<85
           W
          N  =  136

          Y  =  36.8>
                    63

                    75.7
T    <75
 max
751  T    185  8 
-------
-45
-------
         250
-------
CO
01
                                           W75
-45
-------
                          T   <85        85
-------
                               (1)
                             N = 179
                             Y = 59.6
                          /     \
                       T   <85     85
-------
                                       (1)
                                     N = 248

                                     Y = 78.9
                                 /         \
                               T ' <85
                                max
85
-------
                       T    < 75   T    > 75
                       'max         max -
         8 < WS    4 < WS  < 8
 (12)
N = 56
Y = 27.2
           W85
        T   <70   70
-------
Tmax<7     70
-------
                                   TECHNICAL REPORT DATA
                            (Please read Instructions on the reverse before completing)
1. REPORT NO.
  EPA-450/4-79-013
                                                           3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
 Weekend/Weekday  Differences in Oxidants and
 Their Precursors
             5. REPORT DATE
                March 1979
             6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
 Yuji Horie, Joseph  Cassmassi, Larry Lai and
 Louis Gurtowski
                                                           8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
 Technology Service  Corporation
 2811 Wilshire Boulevard
 Santa Monica, California   90403
             10. PROGRAM ELEMENT NO.

                       2AA635
             11. CONTRACT/GRANT NO.

                   68-02-2595
12. SPONSORING AGENCY NAME AND ADDRESS
 Air Management Technology Branch
 Monitoring and Data  Analysis nivision
 Office of Air Quality Planning and Standards
 U.S. Environmental  Protection Agency
 Research Triangle  Park,  North Carolina  27711
              13. TYPE OF REPORT AND PERIOD COVERED
                       Final
             14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRACT
      Differences  between weekend and weekday oxidant and oxidant  precursor concentra-
 tions  in  the  Northeastern United States  are examined.  Statistical  tests are employed
 to assess whether any differences in daily  maximum oxidant concentrations are signi-
 ficant.   Weekend/weekday differences in  meteorology are also examined for any potential
 impact on weekend/weekday differences  in oxidants.  Oxidant concentrations are adjusted
 in an  attempt to  account for variations  in  meteorology, and weekend/weekday differences
 in the adjusted values are examined.   Finally, statistical tests  are used to determine
 significant differences in early morning oxidant precursor concentrations.  The analyse
 revealed  that oxidant concentrations downwind of major urban areas  tend to be lower on
 weekends.  However, the reductions are not  as large as those observed in precursor
 concentrations within the major urban  areas.  Reductions in weekend oxidant concentra-
 tions  at  locations within the major urban areas are not as great  as those at downwind
 locations.
17.
                                KEY WORDS AND DOCUMENT ANALYSIS
                  DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS  C.  COS AT I Field/Group
 Air Pollution
 Ozone
 Oxidants
 Meteorology
18. DISTRIBUTION STATEMENT
                                              19. SECURITY CLASS (This Report)'
                                                Unclassified
                           21. NO. OF PAGES
                              149
                                              20. SECURITY CLASS (Thispage)
                                                Unclassified
                           22. PRICE
EPA Form 2220-1 (Rev. 4-77)   PREVIOUS EDITION is OBSOLETE

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