United States Office of Air Quality EPA-454/R-98-002

Environmental Protection Planning and Standards August 1998

Agency Research Triangle Park, NC 27711

Air

 GUIDELINE ON OZONE

 MONITORING SITE SELECTION
EPA-454/R-98-002

Guideline On Ozone Monitoring Site Selection

United States Environmental Protection Agency

Office of Air Quality Planning and Standards

Emissions, Monitoring, and Analysis Division

Research Triangle Park, NC 27711

August 1998

iii

TABLE OF CONTENTS

Section Page

LIST OF FIGURES .......................................................... v

LIST OF TABLES ......................................................... vii

LIST OF ABBREVIATIONS AND ACRONYMS ................................ viii

EXECUTIVE SUMMARY ................................................. ES-1

1. INTRODUCTION ....................................................... 1-1

2. IMPORTANT CHARACTERISTICS OF OZONE FORMATION ................... 2-1

2.1 OZONE FORMATION CHEMISTRY ................................ 2-1

2.1.1 Basic Principles ........................................... 2-1

2.1.2 Atmospheric Sinks for Ozone ................................ 2-3

2.2 OZONE PRECURSOR EMISSIONS .................................. 2-5

2.2.1 Sources of Ozone Precursor Emissions ......................... 2-5

2.2.2 Spatial Distribution of Emissions .............................. 2-5

2.3 METEOROLOGICAL CONDITIONS CONDUCIVE TO OZONE

FORMATION ................................................... 2-5

3. SPATIAL DISTRIBUTION OF OZONE CONCENTRATIONS .................... 3-1

3.1 EXAMPLE 1-HOUR AND 8-HOUR OZONE PATTERNS ................. 3-1

3.1.1 San Francisco Bay Area .................................... 3-1

3.1.2 Northeastern United States ................................. 3-11

3.2 SUMMARY AND CONCLUSIONS ................................. 3-21

4. MONITORING OBJECTIVES AND NETWORK DESIGN ....................... 4-1

4.1 INTRODUCTION ................................................ 4-1

4.2 PRINCIPLES INVOLVING LOCALIZED VS. REGIONAL MONITORING ... 4-2

4.3 MONITORING OBJECTIVES AND SITE SELECTION .................. 4-3

4.3.1 Regulatory Compliance ..................................... 4-6

4.3.2 Control Strategy Development and Assessment ................... 4-6

4.3.3 Health Effect Studies ...................................... 4-6

4.3.4 Vegetative Impacts Related to a Secondary Ozone Standard ......... 4-7

4.4 OZONE SEASON ................................................ 4-7

5. MACROSCALE CONSIDERATIONS IN MONITOR SITING ..................... 5-1

5.1 DEVELOPMENT OF A NEW NETWORK OR ENHANCEMENT OF AN

EXISTING NETWORK ........................................... 5-1

5.1.1 Maximum Population Exposure Site(s) ......................... 5-1

5.1.2 Upwind and Downwind Site(s) ............................... 5-1

5.2 EXAMINATION AND ADJUSTMENT OF AN EXISTING OZONE MONITORING

NETWORK TO REFLECT 8-HOUR MONITORING

NEEDS ....................................................... 5-11
iv

TABLE OF CONTENTS (Concluded)

Section Page

6. MICROSCALE CONSIDERATIONS FOR MONITOR SITING .................... 6-1

6.1 GUIDANCE FOR MONITOR PLACEMENT ........................... 6-1

6.1.1 Vertical and Horizontal Probe Placement ....................... 6-1

6.1.2 Effects of Obstructions ..................................... 6-1

6.1.3 Separation from Roadways .................................. 6-3

6.1.4 Spacing from trees and other vegetation ........................ 6-3

6.2 USE OF SATURATION MONITORING .............................. 6-3

7. RECOMMENDATIONS .................................................. 7-1

8. REFERENCES ......................................................... 8-1

APPENDIX A: BACKGROUND ON OZONE FORMATION ...................... A-1

APPENDIX B: SUPPLEMENTAL REFERENCE MATERIALS ................... B-1
LIST OF FIGURES

Figure Page

2-1. Schematic diagram of photochemical air pollution from emission to deposition

(NAS, 1991) ......................................................... 2-2

2-2. Typical ozone isopleths used in the EPA’s Empirical Kinetic Modeling Approach 

(EKMA) ............................................................ 2-4

2-3. Spatial distribution of the anthropogenic VOC emissions (tons per year) in the OTR

during 1990 .......................................................... 2-7

2-4. Spatial distribution of the anthropogenic NOx emissions (tons per year) in the OTR

during 1990 .......................................................... 2-8

3-1. Selected cities in the San Francisco Bay Area and downwind regions ............... 3-3

3-2. Selected monitoring sites that operated in the San Francisco Area and downwind

regions between 1992-1995 .............................................. 3-4

3-3a. Exceedances of the 0.12 ppm ozone threshold between 1992-1995 in the

San Francisco Bay Area and downwind regions ............................... 3-6

3-3b. Exceedances of the 0.12 ppm ozone threshold between 1992-1995 in the 

San Francisco Bay Area and downwind regions ............................... 3-7

3-4a. Contour plot of the exceedances of the 0.08 ppm 8-hour threshold between

1992-1995 in the San Francisco Bay Area and downwind regions ................. 3-8

3-4b. Exceedances of the 0.08 ppm 8-hour threshold between 1992-1995 in the 

San Francisco Bay Area and downwind regions ............................... 3-9

3-5. Ozone concentration profiles for Bethel Island and Livermore for July 27, 1995 ..... 3-10

3-6. Ozone concentration profile for Woodland, California (northeast of the 

San Francisco Bay Area) on August 20, 1995 ............................... 3-12

3-7. Major cities in the Northeastern United States ............................... 3-13

3-8. Number of site exceedances of the 1 hour 0.12 ppm ozone threshold (greater than

or equal to 125 ppb) in the Northeast OTR during 1988 ....................... 3-15

3-9. Number of site exceedances of the 1 hour 0.12 ppm ozone threshold (greater than

or equal to 125 ppb) in the Northeast OTR during 1995 ....................... 3-16
LIST OF FIGURES (Concluded)

Figure Page

3-10. Number of site exceedances of the 0.08 ppm 8-hour running average in the

Northeast OTR during 1988 ............................................ 3-17

3-11. Number of site exceedances of the 0.08 ppm 8-hour running average in the

Northeast OTR during 1995 ............................................ 3-18

3-12. Ratio of the concentration of the 0.08 ppm 8-hour running average standard and

the 1 hour 0.12 ppm ozone threshold for the ozone episode of July 20, 1991 ........ 3-19

3-13. Ratio of the concentration of the 0.08 ppm 8-hour running average standard and

the 1 hour 0.12 ppm ozone threshold for the ozone episode of July 15, 1995 ........ 3-20

5-1. Morning wind roses (7-10 a.m.) for Muskegon, Michigan from 1981-1990 for

summer days (June-September) for (a) days that exceeded the 0.12 ppm 1-hour

ozone threshold, and (b) all days (Chu, 1995) ................................ 5-2

5-2. Depiction of the 30, 35, and 40 degree latitude lines for the United States ........... 5-5

5-3. Morning (7-10 a.m.) and afternoon (1-4 p.m.) wind roses for Milwaukee,

Wisconsin (Chu, 1995) ................................................. 5-7

5-4. Morning (7-10 a.m.) and afternoon (1-4 p.m.) wind roses for Atlanta, Georgia

(Chu, 1995) .......................................................... 5-8

5-5. Example network design for a metropolitan area .............................. 5-9

5-6. Morning wind roses (7-10 a.m.) for two stagnation dominated metropolitan areas:

Louisville, Kentucky and Huntington, West Virginia (Chu, 1995) ................ 5-10

5-7. Example network design for a metropolitan area with no predominant wind

direction (Chu, 1995) ................................................. 5-12

6-1. Schematic representation of the airflow around an obstacle such as a building 

(EPA,1978) .......................................................... 6-2

6-2. Monitor locations for the Wasatch Front Ozone Saturation Study summer 1993

(Start, 1994) ......................................................... 6-6
vii

LIST OF TABLES

Table Page

2-1. Summary of Anthropogenic VOC and NOx emissions in the United States during 1994

(EPA, 1994) ......................................................... 2-6

3-1. Monitoring sites in the San Francisco Bay Area and downwind regions ............. 3-5

4-1. Ozone and ozone precursor monitoring site types with corresponding

monitoring objectives ................................................... 4-5

5-1. Criteria for ozone conducive conditions for the eastern United States .............. 5-4

6-1. Separation distance between ozone monitors and roadways ...................... 6-4
viii

LIST OF ABBREVIATIONS AND ACRONYMS

Act - Clean Air Act

CBD - Central Business District

CFR - Code of Federal Regulations

EKMA - Empirical Kinetic Modeling Approach

EPA - United States Environmental Protection Agency

FR - Federal Register

hv - Photochemical energy from ultraviolet radiation or a photon

k - Rate Constant n

M - A molecule which absorbs a reaction’s excess vibrational energy

NAAQS - National Ambient Air Quality Standard(s)

NAMS - National Air Monitoring Stations

NMHC - Non-Methane Hydrocarbon(s)

NO - Nitric Oxide

NOx- Oxides of Nitrogen

NOy- Total Reactive Oxides of Nitrogen

NO2- Nitrogen Dioxide

O - Oxygen Atom

O2- Oxygen Molecule

O3- Ozone

PAMS - Photochemical Assessment Monitoring Stations

PWD - Prevailing Wind Direction

RH - Relative Humidity

SIP - State Implementation Plan

SLAMS - State and Local Air Monitoring Stations

T - Temperature

VOC - Volatile Organic Compound(s)

W - Wind Speed
ES-1

EXECUTIVE SUMMARY

OVERVIEW

This report provides background information for State and local agencies responsible for

designing and implementing an ozone monitoring program. The purpose of this report is to

update a 1978 U.S. Environmental Protection Agency (EPA) report entitled: Site Selection for

the Monitoring of Photochemical Air Pollutants (EPA, 1978). This updated report, Guideline

on Ozone Monitoring Site Selection, is motivated by the need to help air quality agencies and

others incorporate 8-hour monitoring into their ozone monitoring program. Since the 1970s,

ozone monitoring programs have been designed to address the 1-hour ozone National Ambient

Air Quality Standard (NAAQS); however, on July 18, 1997, the U.S. EPA promulgated an 8-

hour ozone NAAQS. This report assists those agencies considering how to design, implement or

revise ozone monitoring networks, particularly in light of the need to collect 8-hour ozone

concentration data. Although this report does not specifically address special purpose monitors

(SPMs), these additional monitors deployed by air pollution control agencies can serve multiple

purposes including :

• fulfilling the need to obtain information on where to locate permanent monitoring stations,

• providing data to support pollutant formation and transport analyses,

• assessing air quality in a particular location, or

• determining where the monitored domain near urban areas should be expanded to ensure

measurement of a maximum 8-hour concentration. 

Where SPM data are used as part of a demonstration of attainment or nonattainment or in

computing an ozone design value, the SPM must meet the requirements for SLAMS. Note that

the EPA is obligated to consider all publicly available, valid (i.e., collected in accordance with 40

CFR 58), and relevant data, including that collected by SPMs, in the NAAQS regulatory process. 

More detail on SPMs may be found in 40 CFR 58.14. 

Existing ozone monitoring networks should be reviewed annually in accordance with the

requirements of 40 CFR 58.20 (d); any changes to the networks to accommodate the 8-hour

ozone standard should be addressed at that time (see also the SLAMS/NAMS/PAMS Network

Review Guidance, EPA-454/R-98-003, for further detail on network review.) The EPA does not

expect major changes to the monitoring networks as a result of these reviews since the analysis in

this document indicates that, generally, the 1-hour and 8-hour concentrations are well correlated

spatially.

The report covers basic issues essential to understanding monitor siting considerations,

including the following:
ES-2

· The physics and chemistry of ozone formation: The report includes a description of

ozone chemistry, the relationship between precursor and ozone concentrations, and

meteorological conditions that are most conducive to ozone formation.

· Spatial distribution of ozone concentrations: Distributions of ozone concentrations

across metropolitan areas and ozone transport regions or broad geographical areas are

examined and related to ozone chemistry and meteorology. Also included is a comparison

of the spatial distribution of 1-hour and 8-hour ozone concentrations.

· Monitoring objectives and network design: Purposes of ozone monitoring are described,

as well as the types and number of monitoring sites that are recommended.

· Macroscale considerations for monitor placement: Techniques are recommended to

identify general locations for the required monitor types for a metropolitan area.

· Microscale considerations for monitor placement: Techniques are recommended for

finding specific locations for ozone monitors.

Appendices include background information on ozone formation, and references to

supplemental materials that can assist those involved with siting efforts.

REVIEW OF KEY FINDINGS AND RECOMMENDATIONS

In this report, improved knowledge and understanding of ozone photochemistry is used to

reach a series of conclusions and recommendations regarding ozone monitoring networks. In

particular:

Ozone Formation (Section 2.)

Section 2. includes a description of the physics and chemistry of ozone formation (a

supplement to this section is included as Appendix A). The following key principles are

illustrated:

· Ozone is a secondary pollutant formed in the atmosphere by reactions between oxides of

nitrogen (NO ) and volatile organic compounds (VOC). x

· The relationship between the precursor concentrations and ozone formation must be

considered when developing control strategies. Ozone formation in some geographic

areas may be limited by the supply of NOx, while in other areas ozone formation may be

limited by the supply of VOC.

· Ozone formation is most conducive during warm, dry, and cloudless days with low wind

speeds; these conditions most often occur during high-pressure systems. Consequently,

ozone monitoring should take place during the warmer periods of the year.
ES-3

Spatial Distribution of Ozone Concentrations (Section 3.)

Using examples from the San Francisco Bay Area and the northeastern United States,

Section 3. includes a discussion of the urban and regional distribution of ozone concentrations. 

The following key principles are discussed:

· Generally, the highest ozone concentrations in a metropolitan area are found downwind of

the urban fringe.

· The locations of the peak concentrations of a 0.12 ppm 1-hour and a 0.08 ppm 8-hour

threshold are similar; however, downwind sites may be more prone to 8-hour exceedances

relative to 1-hour exceedances.

· Subtle differences exist between the 1-hour and 8-hour forms. For example, downwind

sites which receive transported, dispersed ozone plumes and have low NO concentrations

available to titrate ozone, will have flatter diurnal concentration profiles that are more

prone to 0.08 ppm 8-hour exceedances, relative to 0.12 ppm 1-hour exceedances. Also,

the ratio of the 8-hour running average ozone concentration and the 1-hour daily

maximum ozone concentration generally increases from urban centers, suggesting that

rural sites may be more prone to 8-hour exceedances than to 1-hour exceedances.

Monitoring Objectives and Network Design (Section 4.)

This section introduces the basis for establishing a monitoring network. Key points

include:

· Ozone monitoring objectives , which include determining regulatory compliance,

developing control strategies, and collecting data for health studies and air quality studies.

· For both the 1-hour and 8-hour ozone standards, a minimum of two NAMS (National Air

Monitoring Stations) ozone monitors are required by 40 CFR 58, Appendix D, §3.4, for

urbanized areas with populations greater than 200,000. If a nonattainment area is

designated as serious, severe, or extreme for the 1-hour ozone standard, then up to five

monitors are required as a part of the Photochemical Assessment Monitoring Stations

(PAMS) program (40 CFR 58, Appendix D, §4.4).

· Although 8-hour ozone monitoring seasons vary across the United States, in general, 8-

hour ozone monitoring seasons are slightly shorter than those established previously for

the 1-hour standard (based on a 6-year data review). See the “Guideline for Selecting

and Modifying the Ozone Monitoring Season Based on an 8-Hour Standard”, EPA454/R-98-001, for further information on ozone monitoring seasons.
ES-4

Macroscale Considerations in Monitor Siting (Section 5.)

This section focuses on identifying general locations within a metropolitan area to site

ozone monitors. Recommendations include:

· Upwind monitors should be located along the trajectory of the morning prevailing wind

direction (PWD), and the downwind monitor should be located along the trajectory of the

afternoon PWD. Upwind monitoring will help establish background ozone and ozone

precursor concentrations (if PAMS) entering the geographic area, and downwind monitors

will help evaluate the peak ozone concentrations experienced within the geographic area. 

· PWDs should be determined from wind rose plots of days where high concentrations of

ozone are measured, or days conducive to ozone formation. Specific meteorological

criteria based upon temperature, wind speed, and relative humidity can help identify days

conducive to ozone formation. These factors vary depending upon local conditions, and

the section describes how these criteria can be applied to different parts of the country.

· For areas where ozone episodes are dominated by stagnation conditions and for which a

dominant PWD cannot be discerned, ozone monitors should be located along the major

axes of the emission sources within 10 miles of the urban fringe (i.e., no further than 10

miles beyond the outermost portion of the urban fringe).

· For re-evaluating a network pursuant to the 8-hour running average concentration, the

potential for changes in the spatial distribution of exceedances needs to be evaluated.

Generally, 1-hour and 8-hour ozone concentrations are well correlated spatially. However,

since downwind sites are more prone to ozone exceedances under the 8-hour NAAQS, 

the total geographic area monitored may need to be expanded to encompass the expanded,

downwind areas where these 8-hour ozone exceedances may occur.

Microscale Considerations in Monitor Siting (Section 6.)

This section provides guidance on determining specific monitor locations after the general

locations are determined (Appendix B provides references to supplemental materials useful in

developing a monitoring program). Key recommendations from this section include:

· Probe inlets should continue to be placed 3 to 15 meters above ground level. (40 CFR 58,

Appendix E, §2.1)

· Samplers should be separated from any obstruction such as a building by at least twice the

height that the obstruction protrudes above the sampling inlet. (40 CFR 58, Appendix E,

§2.3)

· Monitors should be placed an adequate distance from roadways. The more traffic on a

road, the greater the recommended distance between the roadway and the monitor
ES-5

location. For example, for roads with 10,000 or less vehicles per day, monitors should be

at least 10 meters away from the roadway; roads with 110,000 vehicles per day or more

should have monitors sited at least 250 meters away from the roadway. (40 CFR 58,

Appendix E, §2.5)

· Inlet probes should be placed at least 20 meters from the drip line of trees to minimize

interferences. Large trees or stands of trees should be treated as large obstructions, e.g.,

as buildings, however, the probe must not be located less than 10 meters from the drip line

of such trees treated as obstructions. (40 CFR 58, Appendix E, §2.4)

· “Saturation monitoring” can also be used to site monitors. Saturation monitoring involves

the deployment of numerous portable monitors on a short-term basis to quickly develop a

better understanding of how ozone concentrations vary across a geographic area.

Summary (Section 7.)

The recommendations in Section 7. of this document provide real world examples to

illustrate how geographic areas should conduct their monitoring activities, answer common

questions, and provide practical “rules of thumb” for those engaged in monitoring network

design. As a primer on ozone monitoring issues, this report touches on a number of key issues

and their implications for monitoring network design including: ozone formation and spatial

distribution; monitoring objectives, such as regulatory compliance and scientific research; big

picture geographic issues, such as the need for siting monitors upwind of an urban area; and

micro-scale issues, such as the need to avoid siting monitors too close to trees and buildings.

Readers with an interest in either developing a new ozone monitoring network, assessing the

efficacy of an existing network, or updating an existing network to monitor 8-hour ozone

concentrations should all find this report's contents to be of value. The recommendations are

organized in response to commonly asked questions such as:

· How many monitors are required for a particular region?

· How should monitors be sited to properly account for wind direction?

· How should monitors be sited to avoid problems in the immediate vicinity of the monitor

(e.g., from roads, trees, buildings, etc.)?

· Are existing 1-hour monitoring sites still useful for 8-hour monitoring, or should they be

moved? In particular, does an 8-hour standard mean that the geographic area monitored

needs to be enlarged, and if so, by how much?

· How can monitor sites be evaluated to determine if they are located in appropriate

downwind locations to identify peak ozone concentrations?

· Should 8-hour ozone monitoring be conducted during the same time of the year as 1-hour

ozone monitoring?
ES-6

· How should monitor siting differ depending upon whether an area is an isolated urban

area, is part of an urban corridor which includes numerous metropolitan areas, or is an

urban area with multiple prevailing wind directions?

· What PAMS requirements will be necessary in new ozone nonattainment areas for only

the 8-hour ozone NAAQS?
1. INTRODUCTION

This report provides background information for those responsible for designing and

implementing an ozone monitoring program. Its purpose is to update the 1978 U.S.

Environmental Protection Agency (EPA) report entitled: Site Selection for the Monitoring of

Photochemical Air Pollutants (EPA, 1978). This updated report, Guideline on Ozone

Monitoring Site Selection, is motivated by the need to help air quality agencies and others

incorporate 8-hour monitoring into their ozone monitoring program. Since the 1970s, ozone

monitoring programs have been designed to address the 1-hour ozone National Ambient Air

Quality Standard (NAAQS); however, on July 18, 1998, the U.S. EPA promulgated an 8-hour

ozone NAAQS. This report assists those agencies considering how to design and implement

ozone monitoring in light of the need to collect 8-hour ozone concentration data. In addition,

knowledge of ozone photochemistry has improved in recent years and it is timely to revisit

monitoring network design issues in light of new scientific knowledge.

One of the first steps in recognizing or diagnosing an air pollution problem is the

development of an adequate monitoring network to quantify pollution concentrations.

Specifically, there are five major objectives to air monitoring:

· Determine the highest concentrations expected to occur in the area covered by the

network.

· Determine representative concentrations in areas of high population density.

· Determine the impact of specific sources or source categories on ambient pollution

concentrations.

· Determine general background concentrations.

· Determine the extent of air pollution transport into and out of an area.

Numerous decisions need to be made in designing a monitoring network, including:

· How many monitors are needed?

· What pollutants need to be measured?

· Where should the monitors be placed?

· During which part of the year is monitoring needed?

In the years since the EPA produced its 1978 guidance for designing monitoring networks

for photochemical pollutants (EPA, 1978), there have been significant advances in the

understanding of the chemistry of photochemical air pollution. Also, moving from a 1-hour daily

maximum ozone concentration-based NAAQS to one based on an 8-hour running average

concentration means different geographic areas may be affected. Such differences suggest that

changes to traditional monitoring networks should be considered.

The principal focus of this report is ozone, but a few remarks are made regarding NO

monitoring as it relates to ozone formation. EPA’s 1978 document addressed monitoring

placement criteria for nonmethane hydrocarbons (NMHC), nitrogen dioxide (NO2), nitric oxide

(NO), and ozone (or oxidants, on which the standard was then based). There is no longer a
NMHC standard, and there are presently (as of 1996) no areas in violation of the current NO2

NAAQS. Therefore, these pollutants are not addressed. It is worth noting that since 1991,

hydrocarbons that are considered precursors to ozone are now required to be measured in

selected ozone nonattainment areas at sites called Photochemical Assessment Monitoring Stations

(PAMS). A separate document was prepared by the EPA to address the selection of PAMS sites

(EPA, 1994). 

This report is organized into seven main sections:

1. Introduction (this discussion).

2. Important Characteristics of Ozone Formation: Provides the reader with a general

explanation of ozone formation chemistry, the importance of ozone precursor emissions, and

meteorological conditions conducive to ozone formation.

3. Spatial Distribution of Ozone Concentrations: Describes the spatial nature of ozone

concentrations and how these differ on a 1-hour and 8-hour basis. The section provides

example ozone concentration situations in the western and northeastern United States.

4. Monitoring Objectives and Network Design: Discusses under what conditions to monitor in

neighborhoods versus urban or regional settings, monitoring for regulatory compliance versus

control strategy development, the number of required monitor sites, and the seasons during

which monitoring should occur.

5. Macroscale Considerations in Monitor Siting: Provides guidance for determining the general

location of monitors in a metropolitan area.

6. Microscale Considerations for Monitor Siting: Provides guidance for the specific placement

of monitors, once the general location has been identified.

7. Recommendations: Summarizes the key recommendations included in the report.

The report also includes references and two appendices: Appendix A, which contains

information on ozone formation, and Appendix B, which contains citations for additional

information resources of interest to those involved with monitor siting.
NO h NO O

k

2

1 + n → +

O O M O M

k

+ 2 +  → 3+

2

hn

hn

O NO NO O

k

3 2 2

3 +  → +

2-1

2. IMPORTANT CHARACTERISTICS OF OZONE FORMATION

Ozone is not generally emitted by sources, but instead is formed in the atmosphere by a

series of complex chemical reactions between oxides of nitrogen and organic compounds. The

process of ozone formation is complex. However, a basic understanding of this chemistry is

necessary for predicting the spatial distribution of the ozone concentration within a metropolitan

or regional area, and for developing an ozone monitoring network. This section provides

background information on ozone formation and includes brief discussions on the following

(More detail on the photochemical reactions contributing to ozone formation can be found in

Appendix A.):

· Ozone formation chemistry in the troposphere

· Sources and spatial distribution of ozone precursor emissions

· Meteorological conditions conducive to ozone formation.

Figure 2-1 shows a general schematic diagram of ozone formation in the atmosphere.

This section describes these processes and their interactions. These principles are illustrated

further in Section 3., which describes the spatial and temporal distribution of ozone

concentrations.

2.1 OZONE FORMATION CHEMISTRY

2.1.1 Basic Principles

The basis for ozone formation is the photolysis of nitrogen dioxide (NO2), by the

following reactions (Seinfeld, 1986):

(2-1)

(2-2)

where represents photochemical energy from ultraviolet radiation (or a photon), k represents

a rate constant for the reaction of NO2 and and M represents N2 or O2 or another molecule

that absorbs the reaction’s excess vibrational energy. Once formed, ozone is rapidly dissociated

by reaction with NO, as follows:

(2-3)
2-2

Figure 2-1. Schematic diagram of photochemical air pollution from

emission to deposition (NAS, 1991)
[ ]

[ ]

[ ]

O

k NO

k NO 3

1 2

3

=

2-3

The NO2 molecule is regenerated, and in the absence of other species a steady state is achieved

through reactions (2-1) through (2-3) in which the ozone concentration can be estimated by the

following relationship:

(2-4)

In the natural troposphere, these reactions normally result in a background ozone

concentration of 15 to 45 parts per billion (ppb) (Altshuller and Lefohn, 1996). 

Emissions of volatile organic compounds (VOC) are key contributors to ozone

concentrations above normal background values. Most combustion and biogenic emissions of

oxides of nitrogen are emitted in the form of NO, although some of the NO in the combustion

gases are oxidized to NO . In the atmosphere, VOC facilitate the oxidation of NO to NO , thus 2 2

allowing for continued ozone production while reducing the destruction of ozone by NO. 

VOC sources are both anthropogenic and natural in origin. Anthropogenic sources

include automobiles, chemical production, and other industrial activities. Many species of

vegetation including trees and plants naturally emit VOC. In the natural troposphere, there is a

sufficient amount of natural (or biogenic) VOC to oxidize some of the NO to NO , which results 2

in the background ozone concentration discussed above. When additional VOC is added to the

atmosphere, a greater proportion of the NO is oxidized to NO2, resulting in greater ozone

formation. Additionally, anthropogenic sources of NO, including sources such as automobiles

and electric utility plants, result in greater levels of NO2 in the atmosphere, which is then available

for photolysis to NO and O (an oxygen atom) and, ultimately, NO and ozone.

Using the chemical principles elucidated above, EPA’s Empirical Kinetic Modeling

Approach (EKMA) can be used to depict a simplistic but concise diagram for predicting the

effects of changes in VOC or NOx concentrations. Figure 2-2 shows a typical set of EKMA

ozone isopleths. The ozone isopleths depend on the VOC/NO ratio. For low VOC/NO ratios, x x

the system is VOC limited (i.e., reductions in VOC emissions will reduce the ozone

concentrations). Conversely, for high VOC/NO ratios, the system is NO limited (i.e., reductions x x

in NO emissions will reduce ozone concentrations). The NO -limited region is typical of x x

locations downwind of urban and suburban areas, while the VOC-limited region is typical of

highly polluted urban areas.

2.1.2 Atmospheric Sinks for Ozone

Once in the atmosphere, ozone can be removed by wet and/or dry deposition. Ozone in

contact with the surface can be deposited or adsorbed onto vegetation such as trees and other

plants. This scavenging includes ozone uptake into vegetation which may also result in

detrimental effects to the plant or tree. Also, ozone can be scavenged by precipitation, and can be

transformed in the aqueous phase by reaction with other atmospheric constituents such as

hydrogen peroxide.
2-4

Figure 2-2. Typical ozone isopleths used in the EPA’s

Empirical Kinetic Modeling Approach (EKMA). The NO -x

limited region is typical of locations downwind of urban and

suburban areas, whereas the VOC-limited region is typical

of highly polluted urban areas (NAS, 191).
2-5

2.2 OZONE PRECURSOR EMISSIONS

2.2.1 Sources of Ozone Precursor Emissions

Table 2-1 summarizes the total anthropogenic emissions of VOC and NOx in the United

States for 1994. The dominant activity for producing NOx is combustion processes, including

industrial and electrical generation processes, and mobile sources such as automobiles. Mobile

sources also account for a large portion of VOC emissions. Other industries that process organic

materials, such as the chemical industry or others that use solvents, also account for a large

portion of the VOC emissions. Although not included in Table 2-1, the EPA also estimates the

annual biogenic emissions. In 1996, for example, estimated VOC biogenic emissions were 29

million short tons (EPA, 1997), which is higher than the anthropogenic emissions. Biogenic VOC

emissions include the highly reactive compound isoprene. However, it must be noted that a large

proportion of the biogenic VOC emissions are in forested and vegetative areas, where these

emissions may not impact urban ozone formation. In contrast to biogenic VOC, which is large

relative to anthropogenic emissions, biogenic NO emissions (2000 short tons) are much smaller. x

2.2.2 Spatial Distribution of Emissions

Anthropogenic VOC and NO emissions are generally concentrated around urban areas, x

due to the nature of the emission sources (e.g., mobile sources [cars, trucks, buses], area sources

[including non-road mobile sources], and industry being located in the populated areas of the

country). The resulting spatial emission patterns reflect this urban-centered orientation. For

example, Figures 2-3 and 2-4 show the VOC and NOx emissions (both anthropogenic and

biogenic) for 1990 in the northeastern United States. The highest emission densities are near the

urban centers of Boston, New York City, Philadelphia, and Baltimore. The emission densities

drop rapidly expanding outwards from the urban centers. As will be discussed in more detail

later, ozone formation is generally highest downwind of urban centers as a result of the

photochemical reaction of ozone precursor emissions in these areas.

2.3 METEOROLOGICAL CONDITIONS CONDUCIVE TO OZONE FORMATION

The chemistry of ozone formation is complex and dependent on numerous variables.

Similarly, there are a variety of meteorological variables that influence ozone formation. Although

changes in daily emissions can affect daily ozone concentrations, it is the daily variation in

meteorological parameters that best explain the daily variations of ozone concentrations. The

meteorological conditions which are conducive to ozone formation include:

· Days when solar radiation is high: Solar radiation is an important factor because of

ozone formation reactions (see Equation 2-1). Solar radiation is highest during cloudless,

summer days.
Table 2-1. Summary of Anthropogenic VOC and NOx

 emissions in the United States during 1996

(EPA, 1997).

Source Type

NO Emissions VOC Emissions x

Emissions Percentage Emissions Percentage

(thousand short tons) of Total (thousand short tons) of Total

Fuel Combustion, Electric

Utility 6034 25.8 45 0.2

On-Road Vehicles 7171 30.7 5502 28.8

Fuel Combustion,

Industrial 3170 13.6 208 1.1

Non-Road Sources 4610 19.7 2426 12.7

Fuel Combustion, Other 1289 5.5 822 4.3

Miscellaneous 239 1.0 601 3.1

Other Industrial

Processes

403 1.7 439 2.3

Chemical & Allied

Product Manufacturing

159 0.7 436 2.3

Petroleum & Related

Industries

110 0.5 517 2.7

Waste Disposal &

Recycling

100 0.4 433 2.3

Metals Processing 98 0.4 70 0.4

Solvent Utilization 3 0.01 6273 32.9

Storage & Transport 6 .03 1312 6.9

Total 23,392 100% 19,084 100%

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· Days with low wind speed: Low wind speeds lead to poorer dispersion and generally result in

increased concentration of ozone and ozone precursors within a smaller area. However, areas

affected by long-range transport can experience high ozone concentrations during periods of

moderate wind speeds.

· Days when mixing heights are low: Strong temperature inversions can cause low mixing heights to

occur, which generally lead to increased concentrations of ozone and ozone precursors in a shallow

vertical layer.

· Days with low relative humidity: Since days with high relative humidity or precipitation are likely to

be associated with cloudy skies or hazy conditions, which reduce temperatures and solar radiation,

and result in decreased ozone concentrations, days with low relative humidity are more conducive

to ozone formation.

The presence of ozone-conducive meteorological conditions can be indicated by the synoptic-scale

meteorology (i.e., meteorological conditions over a broad regional area). In particular, ozone

concentrations generally accumulate during periods with slow-moving, high-pressure weather systems

(NAS, 1991) for the following reasons:

· High-pressure systems are characterized by widespread sinking of air through most of the

troposphere, which warms the subsiding air. This tends to make the troposphere more stable and

less conducive to mixing, which would disperse ozone and ozone precursor emissions. Temperature

normally decreases with altitude in the troposphere, which allows warm air parcels to rise from the

surface and mix through the upper troposphere. However, the subsidence of air creates a

temperature inversion, which effectively results in a “warm air lid” placed on top of cooler air. The

cooler air near the surface fails to mix with the warmer layer above, and there are higher

concentrations of ozone and ozone precursors near the surface.

· The light winds associated with high-pressure systems allow for ozone and ozone precursors to

accumulate near the emission sources.

· The clear skies associated with high-pressure systems result in increased solar radiation at the

surface, which is favorable for photochemical processes.

Ozone-conducive meteorological conditions normally occur during the summertime in most of the

United States, except in the southern or “sunbelt” areas where ozone is not restricted to the summer

season. Consequently, ozone monitoring is not necessary during the entire year in most areas. Determining

the precise monitoring requirement for the ozone season in each metropolitan area of the United States is

beyond the scope of this report. However, the periods during which ozone monitoring is likely to be

needed for regulatory compliance in each area of the country are outlined in Section 4, and in the

Guideline for Selecting and Modifying the Ozone Monitoring Season Based on an 8-Hour Ozone

Standard, EPA-454/R-98-001.

3. SPATIAL DISTRIBUTION OF OZONE CONCENTRATIONS

When designing a monitoring network for measuring ozone, it is important to understand the spatial

distribution of ozone in the monitoring area. However, the complex process of ozone formation can lead

to complex distributions of ozone within a metropolitan area. Several ozone formation processes influence

this spatial distribution, including:

ssions is normally in the

· Proximity to fresh combustion emissions: NO from fresh combustion emi x

form of NO, which destroys ozone by chemical reaction (sometimes referred to as NO titration).

This results in low ozone concentrations relatively near combustion sources.

· Distance from primary emission sources: The process of ozone formation in the atmosphere takes

several hours. The maximum concentration normally forms several miles downwind of the primary

emission sources because the conversion of precursors requires several hours.

· Direction and wind speeds through the city: The direction and speed of winds through the

maximum emissions area will determine where the ozone and ozone precursors are transported.

Furthermore, 1-hour and 8-hour ozone concentration averaging times have different spatial

implications. For example, high 8-hour ozone concentrations can be observed over a broader geographic

area than high 1-hour ozone concentrations. To illustrate the differences between 1-hour and 8-hour spatial

concentration patterns, the next two sections provide examples using observed ozone data from the San

Francisco Bay Area (in California) and the northeastern United States. Much of the data and plots for

these examples are based on an analysis by Reiss et al., 1995. All of the data portrayed in this report as

representing exceedances of the 1-hour and 8-hour thresholds were based on the following “rules of

thumb”: ozone concentrations equal to or greater than 0.085 ppm over an 8-hour period were considered

an exceedance of the 0.08 ppm 8-hour threshold; all concentrations equal to or greater than 0.125 ppm

over a 1-hour period were considered an exceedance of the 0.12 ppm 1-hour threshold. These conventions

comply with 40 CFR 50, Appendix I, §2.3, which stipulates the following:

The number of significant figures in the level of the standard dictates the rounding

convention for comparing the...ozone concentration with the level of the standard. The

third decimal place of the computed value is rounded, with values equal to or greater than

5 rounding up. 

3.1 EXAMPLE 1-HOUR AND 8-HOUR OZONE PATTERNS

3.1.1 San Francisco Bay Area 

The San Francisco Bay Area has a dense monitoring network encompassing upwind, source, and

receptor areas. Thus, it can be used to provide a good example of the spatial distribution of ozone within

and downwind of a major metropolitan area. The largest urban centers are the cities of San Francisco,

Oakland, and San Jose. Generally, the city of San Francisco, the areas south of the city (the San Jose

area), and the areas east of the San Francisco Bay are urbanized. Most of the ozone precursor emissions

are associated with the urban areas and the industrialized eastern shoreline of the San Francisco Bay and 
3-2

Sacramento River Delta. The areas north of San Francisco (Marin, Sonoma, and Napa counties) are

relatively rural and produce fewer ozone precursor emissions. Figure 3-1 depicts major cities in the San

Francisco Bay Area. The meteorology of the region is dominated by coastal westerly winds and water-toland airflows. Therefore, much of the daytime emissions in the urbanized portions of the San Francisco

Bay Area are transported to the east and south.

Selected monitoring sites that were active between 1992 and 1995 in the San Francisco Bay Area

and downwind areas are shown in Figure 3-2. Table 3-1 lists the sites (city and county) corresponding to

the numbers in Figure 3-2. Figures 3-3a and 3-3b, and 3-4a and 3-4b, show the number of exceedances

for 1991-1995 of a 0.12 ppm 1-hour threshold, and a 0.08 ppm 8-hour threshold. Figures 3-3a and 3-4a

show contours using the number of exceedances to mark each contour interval; Figures 3-3b and 3-4b

show the number of exceedances at specific monitors. The figures show information for the San Francisco

region and nearby downwind areas. For both the 1-hour and 8-hour thresholds, most of the exceedances

are in the eastern and southern portions. There are no exceedances in the city of San Francisco, and there

are very few exceedances in the counties north of the city. Three important factors contribute to the clean

air in San Francisco: (1) the upwind region is the Pacific Ocean, and thus the air blowing into the San

Francisco area is clean to begin with; (2) San Francisco’s automobile and truck traffic contributes NO

emissions that can titrate ozone; and, (3) except for the area immediately south of San Francisco, nearby

areas produce relatively few emissions, so there are fewer emissions that contribute to ozone formation in

the immediate vicinity of the city (counties north of San Francisco are largely rural; the area immediately to

the east of the city is the San Francisco Bay). In contrast to San Francisco, San Jose to the south and east

experiences higher ozone concentrations. Key differences include: (1) winds blowing into San Jose are

already polluted with neighboring areas’ emissions (for example, the southern portion of the San Francisco

Bay Area, near San Jose, provides transported pollutants from upwind portions of the Bay Area including

San Francisco and East Bay cities such as Oakland) and (2) San Jose is bordered by heavily developed

areas with significant emissions activity.

In general, Figures 3-3 (a and b) and 3-4 (a and b) show that the distribution of the 1-hour and 8-

hour exceedances are relatively similar. However, at several of the downwind sites (e.g., Bethel Island and

Livermore) there were considerably more 8-hour exceedances compared to 1-hour exceedances. While

these sites have some local emissions that may contribute to their ozone concentrations, ozone transport

from the San Francisco Bay Area is likely. This simple analysis shows that the area of influence of an urban

area such as San Francisco may be greater for the 8-hour standard compared to the 1-hour standard. 

Therefore, States should consider expanding ozone monitoring networks for the 8-hour standard to larger

geographic areas during the annual network review required by 40 CFR 50.20 (d).

The effects of downwind transport can be graphically illustrated with the ozone data from the San

Francisco Bay Area. Downwind sites can be more prone to exceedances of a 0.08 ppm 8-hour threshold

due to transported pollutants that increase ozone concentrations in the early evening hours. This increased

exceedance count can be explained in part by the diurnal ozone concentration profile of the site. For

example, Figure 3-5 shows a comparison of the diurnal profile for Bethel Island (site #8) and Livermore

(site #1) for July 27, 1995. Livermore exceeded the 1-hour standard 10 times during 1992-1995, and 
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3-5

Table 3-1. Monitoring sites in the San Francisco Bay Area and downwind regions.

The number corresponds to the label used in Figure 3-2.

Number City County

1 Livermore Alameda

2 Oakland Alameda

3 San Leandro Alameda

4 Fremont Alameda

5 Livermore Alameda

6 Concord Contra Costa

7 Richmond Contra Costa

8 Bethel Island Contra Costa

9 Pittsburgh Contra Costa

10 San Rafael Marin

11 Point Reyes Marin

12 Napa Napa

13 San Francisco San Francisco

14 Tracy San Joaquin

15 Redwood City San Mateo

16 Gilroy Santa Clara

17 San Jose (4 Street) Santa Clara th

18 Los Gatos Santa Clara

19 Mountain View Santa Clara

20 San Jose (Carlos Street) Santa Clara

21 San Jose (Piedmont Rd.) Santa Clara

22 San Martin Santa Clara

23 Davenport Santa Cruz

24 Santa Cruz (Airport Blvd.) Santa Cruz

25 Scotts Valley (Vine Hill School Rd.) Santa Cruz

26 Scotts Valley (Scotts Valley Dr.) Santa Cruz

27 Santa Cruz (Bostwick Ln.) Santa Cruz

28 Fairfield Solano

29 Vallejo Solano

30 Vacaville Solano

31 Santa Rosa Sonoma

32 Healdsburg Sonoma

33 Sonoma Sonoma

34 Davis Yolo
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3-7
3-8
3-9
3-10
3-11

exceeded the 0.08 ppm 8-hour threshold 20 times. By comparison, Bethel Island only exceeded the 1-hour

standard one time during 1992-1995, but it exceeded the 0.08 ppm 8-hour threshold nine times. The

diurnal profiles for both sites are very similar in the morning hours until about 10:00 a.m. After 10:00 a.m.,

the concentration at the Livermore site continues to rise rapidly, while the concentration at Bethel Island

levels off. After about 2:00 p.m., the concentration at Livermore declines sharply, while the Bethel Island

concentration actually rises to its peak at 6:00 p.m. For the remainder of the evening, the concentration at

Bethel Island is higher than at Livermore. Livermore exceeded both the 1-hour and 8-hour standards on

this day, while Bethel Island only exceeded the 8-hour standard. 

Both Livermore and Bethel Island receive transported ozone and ozone precursors from the

urbanized areas to the west. However, the difference is likely explained by the difference in urbanization

between Livermore and Bethel Island. Neither are very urban compared to San Francisco and San Jose,

but Livermore is a moderately sized community near a major Interstate, which connects with Oakland;

Bethel Island is a more rural site and not near any major roadways. Livermore likely has some ozone

buildup in the afternoon as the result of local emissions which add to the transported ozone and ozone

precursors, and the rush hour traffic which provides fresh NO to quickly titrate the ozone concentration

after the peak is reached in the late afternoon. Bethel Island likely receives a dispersed plume of ozone late

in the afternoon, adding to ozone formed locally. Unlike Livermore, there is no late afternoon source of

fresh NO to titrate the ozone; therefore, the ozone concentration lingers for a longer number of hours than

at Livermore. The more elongated peak and the lingering of the ozone through the early evening at Bethel

Island make this site more prone to 8-hour exceedances than Livermore. Therefore, with an 8-hour form

of the standard, special attention should be given to rural sites receiving transported ozone without fresh

NO in the late afternoon to titrate the ozone.

Rural or suburban areas that do not exceed 1-hour ozone thresholds can exceed 8-hour ozone

thresholds. One example of this is the ozone concentrations monitored at Woodland, California. Woodland

is a rural site located northeast of the San Francisco Bay Area, not far from the Sacramento metropolitan

area. Figure 3-6 shows ozone concentrations at Woodland on August 20, 1995. This site did not exceed

the 0.12 ppm 1-hour ozone threshold from 1991 through 1995, but exceeded the 0.08 ppm 8-hour

threshold 10 times during this period. The plot shows the concentration was above 80 ppb for 8 hours,

without ever going above 100 ppb. Thus, this site has a steady ozone concentration throughout the

afternoon, without reaching a high peak. This type of site is more likely to exceed a standard based on an

8-hour running average set at 0.08 ppm rather than the 1-hour peak concentration of 0.12 ppm.

3.1.2 Northeastern United States

The northeastern United States includes many major metropolitan areas and has been designated as

an ozone transport region (OTR). In this section, spatial ozone patterns are presented for the entire OTR,

including many metropolitan areas as well as rural areas which are influenced by ozone transport. The

OTR in the northeastern United States extends from Virginia to Maine and contains several large

metropolitan areas, including Washington, D.C., Baltimore, Philadelphia, New York City, and Boston.

Figure 3-7 presents a map of the OTR showing the location of all of the major cities. The close proximity 
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3-13
3-14

of these cities along a southwest to northeast line and a generally southerly or southwesterly flow in the

summertime ozone season results in a complex interaction of ozone and ozone precursors in the region.

Figures 3-8 and 3-9 depict the spatial pattern of exceedances of the 0.12 ppm 1-hour ozone

threshold for 1988 and 1995. These years were chosen for display because in the northeast and

nationwide, 1988 was the year with the highest number of ozone exceedances in the 1980s and 1990s, and

1995 is the most recent year (as of this writing) with available ozone data. Figures 3-10 and 3-11 show

contour plots of the number of exceedances of the 0.08 ppm 8-hour threshold for 1988 and 1995. For both

the 1-hour and 8-hour indices, there are a high number of exceedances in the Baltimore-Washington D.C.

area, Philadelphia, and New York City. Areas such as Maine and western Pennsylvania also recorded a

significant number of exceedances. The exceedances in Maine are likely the result of transport from the

metropolitan areas of Boston and New York. Note that there are significantly more exceedances of the 8-

hour standard than the 1-hour standard in the rural areas.

Husar (1996) conducted a similar analysis for the Ozone Transport Assessment Group (OTAG)

region, which includes 37 eastern states. In Husar (1996) the spatial pattern of exceedances of the 0.12

ppm 1-hour threshold and the 0.08 ppm 8-hour running average were compared from 1991-1995 (June

through August). Over the entire OTAG region, the mean 8-hour average was 86 percent of the 1-hour

daily maximum. Husar (1996) concluded that switching from a 0.12 ppm 1-hour NAAQS to a 0.08 ppm 8-

hour NAAQS would yield an increase in exceedances over the industrial midwestern states from Illinois to

Pennsylvania and a decrease of exceedances around Houston and New York City. However, the overall

exceedance pattern of the 1-hour and 8-hour indices were very similar. Additionally, Husar (1996)

examined the correlation between the 1-hour and 8-hour concentrations among monitoring sites. The

correlation coefficient was extremely good (r =0.96). Husar (1996) shows that there are only subtle 2

differences in the overall exceedance patterns for the 1-hour and 8-hour forms of the standard.

To further examine the subtle differences between the 1-hour and 8-hour forms, contour plots of

the ratio of the 8-hour maximum ozone concentration and the 1-hour maximum ozone concentration were

computed for several high ozone concentration days. The significance of the 8-hour/1-hour ratio is that

higher values tend to indicate that a site is more prone to 8-hour exceedances, and lower values tend to

indicate that a site is more prone to 1-hour exceedances. Figures 3-12 and 3-13 show plots of this ratio

for two regional ozone episodes in the northeastern United States (i.e., ozone exceedances were observed

throughout the OTR on June 15, 1988, and July 20, 1991). Typically, the 8-hour/1-hour ratio is lower in

the center of metropolitan areas. The figures show low ratios in Washington D.C. and New York City
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3-17
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3-21

with the ratio increasing with distance from the city centers. This effect is likely a result of NO titration of

ozone during the morning and afternoon rush hours, which prevents sustained high concentrations of ozone

in the metropolitan areas. Downwind sites away from cities receive transported ozone and ozone

precursors from cities. There is less NO to titrate the ozone in the rural and suburban areas outside of the

cities. For downwind sites that are far from any cities, the transported ozone plume is likely to be spatially

dispersed, resulting in flatter diurnal profiles. Areas such as western Pennsylvania, upstate New York, and

Maine had high values of the ratio, illustrating this effect. These are areas which receive transported ozone

(likely from dispersed plumes) and have low NO concentrations available for titration. This effect points to

the increased importance of rural ozone monitoring outside the urban/suburban core for measuring 8-hour

concentrations as compared to 1-hour values.

3.2 SUMMARY AND CONCLUSIONS

The following general observations can be made from the analysis of the San Francisco and

northeastern United States ozone concentrations:

· The locations of the exceedances of the 0.12 ppm 1-hour and the 0.08 ppm 8-hour average ozone

thresholds were generally similar, with some exceptions.

· The locations of the maximum concentration during exceedances were generally similar for the 1-

hour and 8-hour thresholds within the San Francisco Bay Area, however, far downwind sites were

more prone to 8-hour exceedances relative to 1-hour exceedances. Therefore, the geographic

scope of ozone monitoring for an urban area may need to be expanded for the 8-hour threshold.

· Differences between the 1-hour and 8-hour ozone values among sites can partially be explained by

the ozone diurnal profile at the sites. For example, downwind sites which receive transported,

dispersed ozone plumes and have low NO concentrations available to titrate ozone will have flatter

diurnal concentration profiles and are more prone to 8-hour exceedances, relative to 1-hour

exceedances.

· The ratio of the 0.08 ppm 8-hour running average to the 1-hour maximum concentration increased

with distance from the center of the major cities on episode days. In other words, as you move

away from the center of major cities, the outlying areas become more prone to exceeding the 0.08

ppm 8-hour threshold rather than experiencing peak 1-hour concentrations. Some of the highest

values of this ratio were observed in areas outside the urban/suburban core.
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4. MONITORING OBJECTIVES AND NETWORK DESIGN

4.1 INTRODUCTION

EPA requires different monitoring efforts depending upon an area’s nonattainment status. In

general, the more polluted and populous a region is, the more extensive the required monitoring. For

ozone, there are two monitoring network systems: (1) the State and Local Air Monitoring Stations

(SLAMS), and (2) the National Air Monitoring Stations (NAMS). The objectives of the SLAMS network

are (CFR, 1997):

· Determine the highest concentrations expected to occur in the area covered by the network.

· Determine representative concentrations in areas of high population density.

· Determine the impact on ambient pollution levels of significant sources or source categories.

· Determine general background concentration levels.

The SLAMS stations are used to determine compliance with the ozone NAAQS. The EPA does not

specify the number of required stations, except to prescribe a minimum number of stations . States and

localities are given flexibility in determining the size of their network based on data needs and available

resources.

The NAMS stations are selected from the SLAMS stations. Thus, the NAMS network is a subset

of the SLAMS network. Areas to be monitored must be selected based on urbanized population and

pollutant concentrations. Accordingly, there are two categories of NAMS monitors: category (a) stations

located in areas of expected maximum concentrations, and category (b) stations which combine poor air

quality with a high population density but are not necessarily located in an area of expected maximum

concentrations (sometimes referred to as the maximum exposure monitor). The NAMS monitors are

designed to provide data for national policy analyses and trend analyses, and for providing the public with

information about the air quality in major metropolitan areas. Therefore, it is intended that these stations

remain at the same location. The criterion for designating NAMS sites is any urban area with a population

over 200,000. A minimum of two NAMS ozone monitors are required in each of these urban areas; this

requirement remains unchanged as a result of the promulgation of the 8-hour ozone NAAQS in 1997.

In addition, serious, severe, and extreme ozone nonattainment areas (for the 1-hour ozone

NAAQS) are required to implement enhanced ozone monitoring. This enhanced monitoring is called the

Photochemical Assessment Monitoring Stations, or PAMS program. PAMS requirements are extensive

and include requirements for the monitoring of an array of ozone precursors and meteorological

parameters. At this time, EPA has not extended the PAMS requirements to new ozone nonattainment

areas that violate only the 8-hour NAAQS. Appendix B includes PAMS references for readers seeking

PAMS guidance materials. 

This section describes the issues that determine what type of monitoring network is appropriate for
4-2

a given metropolitan area. Specific topics covered include:

· Monitoring principles involving localized versus regional scale monitoring.

· Monitoring objectives, such as the need to demonstrate compliance with federal air quality

standards, and how they relate to monitor placement.

· Seasonal monitoring requirements.

4.2 PRINCIPLES INVOLVING LOCALIZED VS. REGIONAL MONITORING

As a secondary pollutant, ozone requires appreciable formation time; longer time periods allow

precursor emissions to distribute more uniformly across a region, and thus allow ozone concentrations to

develop more uniformly across subregions and even large-scale regions. This does not imply that ozone

concentrations do not vary across a metropolitan area, it simply means that the gradient in ozone

concentrations is not as great as some other pollutants that derive directly from emission sources. For

example, carbon monoxide can reach significantly high concentrations in localized “hot spots” such as busy

intersections. High ozone concentrations generally occur over larger areas.

In contrast to ozone concentrations, ozone precursor emissions can vary significantly in small areas. 

Monitoring, depending upon whether it is done for ozone or emissions precursors, needs to reflect an

understanding of the spatial scale over which precursor emissions and ozone concentrations occur. Given

the importance of large-scale transport in the ozone formation process, meteorological conditions are

particularly important to the site selection process. For monitoring secondary pollutants, one must identify

areas generally downwind of the primary pollutant sources during periods conducive to formation of the

secondary pollutant. It is important to consider the winds, in combination with the length of time it takes

ozone to form, and the locations of the major sources of the reactants. These factors will be useful for

determining where in an urban region the maximum ozone concentration occurs. Since the mixing of the

ozone precursors will occur over a large volume of air, the monitoring of small-scale variability is usually

not necessary for ozone.

Given these principles, four types of spatial scales can be defined for ozone and ozone precursor

monitoring (40 CFR Part 58, Appendix D; CFR, 1997):

· Middle-Scale: Measurements at this scale represent conditions close to the sources of VOC and

NO emissions such as roads where it would be expected that suppression of ozone concentrations x

would occur. Trees may also have a strong scavenging effect on ozone concentrations and may

tend to suppress ozone concentrations in their immediate vicinity. Measurements at these stations

would represent conditions over relatively small portions of the urban area.

· Neighborhood: Measurements in this category represent conditions throughout some reasonably

homogeneous urban subregion, with dimensions of a few kilometers. These types of stations are

useful for the assessment of health effects because they represent conditions in areas where people
4-3

work and live. These sites, if also monitoring for ozone precursors such as VOC or NO , are useful x

for understanding sources of ozone precursors. 

· Urban: This measurement scale is useful for quantifying concentrations over very large portions of

a metropolitan area with dimensions of 50 or more kilometers. Such measurements can be used for

determining trends, and designing area-wide control strategies. The urban-scale sites can also be

used to measure high concentrations downwind of the area having the highest precursor emissions.

· Regional: This scale of measurement can be used to represent concentrations over large portions

of a metropolitan area and even larger rural areas with dimensions of as much as hundreds of

kilometers. Data from such sites can be useful for assessing the ozone that is transported into or

out of an urban area. Also, data from such stations can be useful for quantifying the amount of

ozone and ozone precursors that cannot be reduced by control strategies within that urban area.

4.3 MONITORING OBJECTIVES AND SITE SELECTION 

The data from an ozone monitoring network serve a myriad of purposes. Air quality program

managers use the aerometric data to determine compliance with the ozone NAAQS. When an area 

violates the NAAQS, the EPA designates the area nonattainment. The Clean Air Act requires that these

nonattainment areas submit plans with sufficient control measures adopted to achieve attainment with the

standards. Some metropolitan areas use ozone monitoring networks to warn residents when unhealthy

ozone concentrations are expected. A detailed understanding of the spatial and temporal distribution of

ozone is needed for researchers that develop models to simulate the atmosphere. These models can be used

by regulators in the development of control strategies or by researchers trying to better understand

atmospheric chemistry, meteorology, and the relationship between precursor emissions and ozone

formation. Yet another purpose of the ozone monitoring network is for human exposure assessment. 

Researchers conducting health studies need to estimate the exposure of residents to ozone and other

pollutants in both high and low concentration areas. Exposure is generally best estimated by sites close to

where people live, work (particularly for outdoor workers), and play (e.g., school playgrounds). Ozone

monitoring networks also provide data useful for evaluating the exposure experienced by urban vegetation

or nearby agricultural/natural areas.

The EPA (CFR, 1997) lists the major objectives of ozone monitoring: 

· Determine the highest concentrations expected to occur in the area covered by the network.

· Determine representative concentrations in areas of high population density.

· Determine the impact on ambient pollution levels by significant sources or source categories.

· Determine background levels. 

This suggests four major types of monitoring sites: (1) maximum downwind concentration sites, (2)

maximum exposure sites, (3) maximum emissions sites, and (4) upwind sites. These site types are

summarized in Table 4-1. Additional monitoring objectives are:
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Table 4-1. Ozone and ozone precursor monitoring site types with corresponding monitoring objectives.

_______________________________________________________________________________________________________________________________________

Type of Site Relevant Pollutants Monitoring Objective Spatial-Scale Notes a

_______________________________________________________________________________________________________________________________________

Maximum Downwind Ozone Regulatory Urban to This site is required as part of the NAMS network and is

Concentration Compliance Regional designed to measure the maximum ozone concentration 

 due to an urban area.

Maximum Exposure Ozone Regulatory Neighborhood to This site is required as part of the NAMS network and is

Compliance Urban designed to measure the highest concentration in a

heavily populated area.

Maximum Emissions NOx, VOC Control Strategy Middle-Scale This site is designed to measure the concentration of NO b

Development and VOC in proximity to a source. This data would be

used in modeling ozone formation.

Upwind Ozone, NOx, VOC Control Strategy Regional This site is designed to measure the ozone and ozone b

Development precursor concentrations entering an urban area from an

upwind source region.

Exposure Ozone Data for Health Studies Neighborhood to This site provides additional (i.e., more than the maximum

Urban exposure site required for NAMS) exposure data for

health studies.

Exposure Ozone Data for Vegetation Urban to This site is used to quantify the exposure of vegetation to

Studies Regional ozone to assess the deleterious impact on the vegetation.

Research Ozone, NOx, VOC Research Middle-Scale to This site is established for a specific research purpose

Regional independent of regulations. Often these sites will operate

only temporarily (i.e., during a single summer season).

_______________________________________________________________________________________________________________________________________

 For all MSAs or CMSAs with more than 200,000 people, two ozone NAMS sites are required, the maximum downwind site and the maximum exposure site. If the MSA/CMSA is a a

serious, severe, or extreme nonattainment area; up to five sites (PAMS) are required. (Note that PAMS sites are based on population - see 40 CFR Part 58, Appendix D, Table 2.) 

 EPA requires PAMS VOC monitoring for serious, severe, and extreme ozone nonattainment areas. Up to two of the NAMS monitors may be part of the PAMS program in b

these areas. Appendix B to this document includes references for additional information on PAMS monitoring.

Additional downwind, exposure, and emission sites can be installed as necessary, or as resources permit. Definitions of the spatial scale and type of monitoring site are found in 40 CFR Part 58,

Appendix D, “Network design for SLAMS, NAMS, and PAMS.”
4-6

· Determination of human exposure for health effect studies.

· Data for air quality research studies.

· Determination of vegetation exposure for vegetation effects studies.

These different monitoring objectives require different site selection criteria. The remainder of this

section describes how these different objectives affect site selection.

4.3.1 Regulatory Compliance

For regulatory compliance, the principle objective is to measure the ozone concentration in the high

population density areas and the maximum downwind concentration from the urban region. It is important

to be careful when selecting the high population sites because, particularly in dense urban areas, the

greatest concentration of people may be in an area with heavy automobile traffic, which may result in low

ozone concentration due to NO titration. Therefore, the high population sites must be chosen by

considering both population and the level of nearby fresh NOx emissions. Generally, the area with the

highest population density that might be reasonably exposed to a significant ozone concentration should be

chosen. (Section 5. discusses strategies for determining where the maximum downwind concentration is

located.) For areas needing more than two ozone monitoring sites, additional monitors can be placed in

other areas downwind of the urban region (other than the maximum concentration downwind area) that

may receive transported ozone on a particular day. Note that EPA has not promulgated any special

monitoring requirements for areas designated nonattainment for the 8-hour ozone standard.

4.3.2 Control Strategy Development and Assessment

Another important objective for monitoring is the development of control strategies. This requires

monitoring of both ozone and ozone precursors. For ozone, the sites for regulatory compliance will be

useful for this purpose, as well as additional downwind sites. However, an additional site type is critical

for control strategy development; monitors should be placed upwind of the urban area to determine the

concentration of ozone and ozone precursors entering the urban region. This will help to determine what

portion of an area’s ozone problem is due to local emissions and what portion is due to transported ozone

and ozone precursors. It is also important to measure NOx and VOC concentrations for control strategy

development. In particular, NOx and VOC monitors should be placed near the major NOx and VOC source

regions, which are normally in the areas within the urbanized portion with the highest vehicle traffic. Note

that EPA has prepared separate guidance concerning VOC monitoring. EPA has, for example, detailed

enhanced monitoring requirements for serious, severe, and extreme ozone nonattainment areas, and these

requirements provide for the monitoring of over 50 individual VOC species. Appendix B to this document

includes references for those interested in learning more about VOC monitoring.

4.3.3 Health Effect Studies 

For health effect studies, the emphasis is on neighborhood-scale monitors which can be used to

quantify the exposure of the residents of a particular community. Typically, the more differentiation one

can make in the exposure of residents within an urban area, the more statistical power there is to detect any

health effects that may be associated with the exposure. Therefore, areas with good monitor coverage are

ideal for health effect studies.
4-7

4.3.4 Vegetative Impacts Related to a Secondary Ozone Standard

Secondary standards relate to ecological and welfare effects, as opposed to health effects. Since

ozone adversely affects vegetative growth (e.g., commercially produced crops as well as natural forests),

additional regional-scale monitors for vegetation may be necessary. Information on monitoring network

design to address ecological effects is provided by Reiss et al., 1995. Generally, Class I areas and areas

with substantial acreage devoted to agriculture, are important areas for monitoring the effects for the

secondary standard.

4.4 OZONE SEASON

For the purposes of regulatory monitoring, an ozone season is defined for each state and dominion

in the United States. This ozone season is defined as the period for which exceedances of the standard are

expected. Monitoring for ozone is only required during this season. The EPA recently sponsored an

update to the existing ozone season definitions (EPA, 1998) to better reflect the ozone season for the 0.08

ppm 8-hour threshold. The methodology used to determine the new ozone seasons includes points such as

(EPA, 1998):

1. 8-hour average ozone concentrations were calculated by the U.S. EPA for each day and station in

the United States and its territories using hourly data from the Aerometric Information Retrieval

System (AIRS) database; the EPA provided this data for the six-year period 1990-1995.

2. For determining the ozone season for 8-hour averaging times, exceedances of a conservative

0.080 ppm threshold for each station-month were computed.

3. These data were used to construct state-by-state histograms for the months in which the 8-hour

concentrations were greater than 0.080 ppm in the six-year period 1990-1995 using all available

data for each state.

4. Initial ozone seasons for each state were defined as starting with the first month with any daily 8-

hour concentration greater or equal to the 0.080 ppm threshold, and ending with the month when

the last exceedances of the 0.080 ppm threshold occurred. These seasons vary from state to state

and region to region, but as a whole are generally shorter than seasons based on a 1-hour standard.

With an 8-hour standard, adjustments to these initial ozone monitoring seasons are expected to better

reflect the need for similar ozone monitoring seasons within ozone transport areas; and to consider states

with large geographic areas without available ozone data. Ozone seasons for major metropolitan areas, as

well as a complete description of the methodology used to define these seasons, can be found in the

Guideline for Selecting and Modifying the Ozone Monitoring Season Based on an 8-Hour Ozone

Standard, EPA-454/R-98-001.(EPA 1998).
4-8
5-1

5. MACROSCALE CONSIDERATIONS IN MONITOR SITING

5.1 DEVELOPMENT OF A NEW NETWORK OR ENHANCEMENT OF AN EXISTING

NETWORK

This section provides guidance on the siting of four major types of monitors that are needed in an

ozone monitoring network. As discussed in the last section, these sites are:

• Maximum population exposure sites

• Upwind background sites

• Downwind maximum concentration sites

• Maximum emissions impact (maximum precursor concentration) sites

In addition, this section presents how these monitoring site types are typically deployed to measure

maximum ozone concentrations under two meteorological scenarios: (1) During conditions when

transport occurs from upwind sources or polluted regions (maximum ozone concentration is often

determined by the direction of winds from the emission sources, thus the maximum ozone concentration

often occurs along the path of the predominant wind direction); and (2) during stagnant conditions when

ozone forms primarily from local emission sources. 

5.1.1 Maximum Population Exposure Site(s)

This site is fairly straightforward to locate. Census tract data or other population data may be used

to locate the maximum population area. However, care must be given not to locate this monitor in an area

which is too heavily influenced by local emission sources, given the titration of ozone by nitric oxide, which

is part of fresh industrial emissions or automobile exhaust. Therefore, this monitor should be located in the

area with the highest population that is expected to be exposed to a relatively high ozone concentration. 

This area is likely to be located on the urban fringe in an suburb slightly downwind of the urban area.

5.1.2 Upwind Background and Downwind Maximum Concentration Site(s)

As discussed before, ozone forms in the atmosphere by reactions between VOC and NOx. These

reactions take some time to occur, thus the maximum ozone concentration usually occurs 4 to 6 hours

after maximum emissions, and under conditions of light winds, usually downwind of the urban region

(EPA, 1978; Chu, 1995). Therefore, the key factor for identifying the area where the maximum ozone

concentration is expected to occur is to determine the prevailing wind direction (PWD) from the urban

area. Also, the most significant amount of transported ozone and ozone precursors will come from the

area where the winds enter the city. As meteorological conditions influence the formation of ozone, it is

important to consider PWDs on days when the ozone standard is violated or, if there is no existing

network, on days when conditions are expected to be conducive to ozone formation.

This point is illustrated in Figure 5-1, which shows the morning wind roses for Muskegon,

Michigan from 1981 through 1990 for summer days (June-September) that exceeded the 0.12 ppm 1-hour

ozone threshold, as well as for all summer days. Wind roses should be interpreted as follows:

· The arms of the roses point to where the wind was coming from.
5-2

Figure 5-1. Morning wind roses (7-10 a.m.) for Muskegon, Michigan

from 1981-1990 for summer days (June-September) for (a) days that

exceeded the 0.12 ppm 1-hour ozone threshold, and (b) all days (Chu,

1995). The figure illustrates that prevailing winds on ozone exceedance

days can be distinct from average wind conditions.
5-3

· The length of the arm is proportional to the percentage of time that the wind was coming from that

direction.

· The various pieces of the arm represent speed categories.

A program to construct wind roses is available from the EPA Technology Transfer Network (TTN), which

is operated by the EPA’s Office of Air Quality Planning and Standards (OAQPS) in Research Triangle

Park, North Carolina. Within the TTN, the Support Center for Regulatory Air Models (SCRAM) provides

a program which constructs wind rose plots from user-provided meteorological data. Appendix B to this

document includes references directing the reader on how to access the TTN and other relevant EPA

information.

As shown in Figure 5-1, there is no distinctive morning PWD for Muskegon if all the summer days

are included. However, for days with peak ozone levels above the 0.12 ppm 1-hour threshold, the majority

of winds come from the southwest. Thus, for Muskegon, an upwind monitor should be placed to the

southwest and similarly, the downwind maximum ozone site should be located based on an afternoon

exceedance day wind rose. Therefore, when determining the appropriate downwind location for an ozone

monitor, it is important to concentrate on the meteorological conditions present during an exceedance day. 

Given the generally good correlation between the 0.12 ppm 1-hour and the 0.08 ppm 8-hour thresholds, it

is expected that historical information describing prevailing wind direction during 1-hour exceedance

conditions will generally still be valid during 8-hour exceedances. Using existing data, agencies are

encouraged to conduct similar analyses.

For areas without existing monitoring data or areas which are not certain if their monitors are

located in the maximum downwind direction, Chu (1995) has developed a methodology for determining

from meteorological data which days were most conducive to ozone formation. The methodology is based

on the model developed by Cox and Chu (1993) for relating meteorological variables to the ozone

concentration. Cox and Chu (1993) have found that the maximum ozone concentration is best correlated

with the following meteorological variables: (1) daily maximum 1-hour temperature, (2) morning (7-10

a.m.) winds, (3) afternoon (1-4 p.m.) winds, and (4) midday (10 a.m.-4 p.m.) relative humidity (RH). 

Using these relationships, Chu developed a set of meteorological conditions that are likely to occur during

a day that exceeds the NAAQS. These conditions are summarized in Table 5-1. Figure 5-2 depicts the

latitude delineations used in Table 5-1. The data used to develop these relationships were based on 31

eastern cites, but the relationships were found to be adequate for the entire United States. These criteria

were developed based on the 0.12 ppm 1-hour NAAQS. However, the general correlation between the 1-

hour and the 8-hour ozone thresholds suggests that the 1-hour relationships should be generally applicable

during 8-hour conditions. Nonetheless, it would be useful to re-evaluate and possibly refine the criteria

using the 8-hour threshold.
5-4

Table 5-1. Criteria for ozone conducive conditions for the eastern United States.

1. T $ 26.5 C (79.7 F) for cities north of 40 N

T $ 29 C (84.2 F) for cities between 35 N and 40 N

T $ 32 C (89.6 F) for cities south of 35 N

2. W 5 m/s for cities in transport regions (i.e., Midwest and Northeast) a.m.

W 4 m/s for cities outside transport regions. a.m.

3. W 7.5 m/s for cities in transport regions p.m.

W 6 m/s for cities outside the transport regions p.m.

Wp.m. 5 m/s for Gulf Coast cites and Florida

4. RH 75% for coastal cities north of 40 N

RH 65% for inland cities between 30 N and 40 N

RH 70% for all cities south of 30 N.

Source: Chu, 1995.

Notes: T refers to temperature; W refers to wind speed; RH refers to relative humidity.
5-5

Figure 5-2. Depiction of the 30, 35, and 40 degree latitude lines for the United States
5-6

After determining the ozone conducive days over the analysis period, wind roses should be

calculated for the morning (7-10 a.m.) and afternoon (1-4 p.m.) for these days. The morning wind rose

will be used to determine the location of upwind monitors, and the afternoon wind rose will be used to

determine the location of downwind ozone monitors. Sometimes the morning and afternoon wind roses

can be different. However, over simple terrain areas, the morning and afternoon wind roses and

corresponding PWDs are usually similar except for some urban heat island effect which creates a deflection

immediately downwind of the city. Over complex terrain and over coastal areas, the two wind roses can be

different. This is illustrated in Figures 5-3 and 5-4, which show the morning and afternoon wind roses for

Milwaukee, Wisconsin and Atlanta, Georgia. While the morning and afternoon wind roses are similar in

the flat terrain metropolitan area of Atlanta, the complex interactions between the synoptic winds and the

mesoscale lake breeze of Lake Michigan are evident in the difference between the morning and afternoon

wind roses in Milwaukee. Therefore, it is recommended that both the morning and afternoon wind roses

be examined.

As an example, Figure 5-5 shows a four-monitor PAMS/ozone network design for a metropolitan

area. The sites in this sample monitoring design are as follows: (1) background ozone concentration (i.e.,

upwind site), (2) maximum precursor emissions site, (3) daily maximum ozone concentration (i.e.,

downwind site), and (4) ozone impact on downwind areas. The morning PWD was used as a guide for

siting the background ozone concentration and maximum precursor emission sites, Sites #1 and #2. The

afternoon PWD was used to site the ozone monitors, Sites #3 and #4, downwind of the metropolitan area. 

Chu (1995) recommends that Site #1 be located along the morning PWD upwind from the city limit near

the edge of the photochemical grid model domain, and Site #2 should be located along the morning PWD

near the downwind edge of the central business district (CBD). Site #3 should be located along the

afternoon PWD at the location where the daily maximum ozone concentrations are likely to occur. For

coastal cities, the site should be located as near as possible to the sea/lake breeze convergence zone.

For the 0.12 ppm 1-hour NAAQS, the maximum 1-hour ozone concentration is likely to occur in

the early afternoon about 4 to 6 hours after the precursors are emitted. Therefore, the morning and

afternoon wind roses can be used to find an approximate location of the maximum ozone concentration. 

For the 0.08 ppm 8-hour threshold, the maximum concentration generally occurs roughly in the same

downwind area (or slightly further downwind) as the 1-hour daily maximum. Site #4 should be located

along the afternoon PWD near the downwind edge of the photochemical grid model domain.

5.1.2.1 Stagnation Conditions

Chu (1995) notes that the approach described above is not applicable for ozone episodes occurring

during extreme stagnation conditions (average winds < 1.5 m/sec) because, in some

cases, no PWD can be resolved. However, for most stagnation dominated areas, distinct PWDs are still

identifiable. As an example, morning wind roses are shown in Figure 5-6 for two stagnation dominated

metropolitan areas: Louisville, Kentucky and Huntington, West Virginia. While the winds in these cities

are much lighter than the winds in the other cities that were shown, the light but steady winds in these

cities appear to play an important role in the transport, mixing, and accumulation of pollutants.
5-7

Figure 5-3. Morning (7-10 a.m.) and afternoon (1-4 p.m .) wind

roses for Milwaukee, Wisconsin (Chu, 1995). The figure illustrates

that in an area with complex terrain, wind patterns may shift during

the day; therefore, upwind and downwind sites will not necessarily

be located in a straight line through the region.
5-8

Figure 5-4. Morning (7-10 a.m.) and afternoon (1-4 p.m.) wind roses for Atlanta, Georgia (Chu,

1995). In contrast to the Milwaukee illustration, the Atlanta example illustrates that in an area

with a relatively flat terrain, prevailing wind conditions may remain steady throughout the day. In

this example, upwind and downwind monitors would be sited along a straight path through the

region.
5-9

Figure 5-5. Example network design for a metropolitan area. Sites

include: (1) background ozone concentration sites (i.e., upwind sites), (2)

maximum precursor emission sites, (3) daily maximum ozone concentration

(i.e., downwind sites), and (4) ozone impact on downwind sites (Chu,

1995).
5-10
The terminology in Figure 5-7, #3/#1, indicates that a site can be used both as a maximum downwind ozone 1

site as well as an upwind site depending upon the time of day/wind direction.

5-11

For stagnation dominated areas where a well defined PWD may not be discernible, Chu (1995)

recommends that the major axes of the precursor emission sources be substituted for PWDs. Figure 5-7

shows an example PAMS/ozone monitoring network using this strategy. As shown, the axes of the

emission sources extend across the industrial park and CBD. Chu (1995) recommends that monitors be

located within 10 miles of the urban fringe (i.e., no further than 10 miles beyond the outermost portion of

the urban fringe). The reasons for this choice are as follows: (1) completely calm conditions seldom last

more than an hour during the day because most calm days have light variable winds, and (2) ozone

concentrations are likely to be highest when the winds are along the axis of emissions because the

precursor concentrations are likely to be highest.1

5.2 EXAMINATION AND ADJUSTMENT OF AN EXISTING OZONE MONITORING

NETWORK TO REFLECT 8-HOUR MONITORING NEEDS

The need to monitor ozone during 8-hour running average periods raises the issue of whether

monitoring networks designed around the 1-hour standard are adequate to measure compliance with

the 8-hour standard. The analysis presented in Section 3 suggests that, generally, the 1-hour and 8-

hour concentrations are well correlated spatially, and that major changes in the monitoring network

are not needed. However, there are subtle factors that need to be considered to monitor 8-hour ozone

concentrations, including:

· Downwind monitoring sites which receive dispersed, transported ozone plumes are more prone to

8-hour exceedances than 1-hour exceedances. This is particularly true for areas without afternoon

rush hours which provide fresh NO emissions to titrate the transported ozone. 

· The increased stringency of a threshold such as 0.08 ppm 8-hour (in comparison to 0.12 ppm 1-

hour) may encourage monitoring further downwind of metropolitan areas than is currently done,

especially in situations where these areas contain a public welfare area of concern (e.g., agriculture,

Class I Areas, etc.).

Several methods are available to analyze where to site 8-hour ozone monitors; these were described in

Section 3, and include the following:

· Calculate, tabulate, and graphically display the number of exceedances for 1-hour and 8-hour

thresholds.

· Compare locations of the 1-hour and 8-hour exceedances.

· Prepare and compare diurnal ozone profiles.

· Evaluate the potential for farther downwind sites to exceed the 8-hour standards.

· Evaluate the potential for NO titration to influence ozone concentrations.

· Examine the correlation between the 1-hour and 8-hour standards.
5-12

Figure 5-7. Example network design for a metropolitan area with no predominant

wind direction (Chu, 1995)
5-13

For all SLAMS monitoring networks, EPA requires that each agency conduct an annual review to

determine how well it is achieving its required air monitoring objectives, how well it is meeting data users

needs, and how it should be modified (e.g., through termination of existing stations, relocation of stations,

or establishment of new stations) to continue to meet its monitoring objectives and data needs. The main

purpose of the review is to improve the network to ensure that it provides adequate, representative, and

useful air quality data (EPA, 1998). Adjustment to ozone monitoring site locations to respond to the 8-

hour standard should be considered in this annual review. Annual network review criteria may be found in

the SLAMS/NAMS/PAMS Network Review Guidance, EPA-454/R-98-003.

6-1

6. MICROSCALE CONSIDERATIONS FOR MONITOR SITING

6.1 GUIDANCE FOR MONITOR PLACEMENT

The previous section provided guidance on determining the general location in a metropolitan area

for siting photochemical pollutant monitors. Once the general location is selected, care must be given to

selecting a specific location. The site should be selected so that it is in an area of small concentration

gradients (i.e., the concentrations measured by the monitor are not affected by small changes in the

location of the station). This criterion will be met if the site is located such that no single source

contributes disproportionately to the concentrations that are measured, but rather that the measured

concentrations reflect the contributions of numerous sources in the area. This section provides criteria for

selecting station locations that are representative of the larger area around the station by avoiding large

influences of nearby sources and sinks. These recommendations reflect those found in Title 40, Code of

Federal Regulations (CFR), Part 58, Appendix E (Probe Siting Criteria for Ambient Air Quality

Monitoring).

6.1.1 Vertical and Horizontal Probe Placement

Human exposure is a principal concern associated with high ozone concentrations. The monitor’s

ozone inlet probe should be placed at a height and location that best approximates where people are usually

located. However, due to complicating factors such as obstructions and adsorbing surfaces, probes

sometimes need to be more elevated. Considering these issues, probe inlets should be located 3 to 15

meters above ground level. The probe must also be located more than 1 meter vertically and horizontally

away from any supporting structure. See also 40 CFR 58, Appendix E, “Probe and Monitoring Path Siting

Criteria for Ambient Air Quality Monitoring” for further information on probe placement.

6.1.2 Effects of Obstructions

The effects of obstructions such as buildings, trees, and nearby surfaces are important

considerations when siting monitors for ozone and nitric oxide because of the effect obstructions have on

airflow and pollutant mixing, and the destruction of these species upon contact with some surfaces. 

Therefore, it is important that sampling be done at a location where the air has had as little contact as

possible with nearby surfaces. Figure 6-1 shows a schematic representation of airflow around a sharpedged building based on the work of Halitsky (1961) and Briggs (1973). This figure shows that air in the

“cavity zone” (the region beyond the building where the airflow is influenced by the obstruction of the

building) will make considerable contact with the building. Conversely, air outside the cavity zone will

have passed over the building with minimal contact. According to Briggs (1973), the cavity zone extends

to roughly 1.5 building heights downwind of the building. Using this as a guide, the minimal allowable

distance separating samplers and obstructions should be at least twice the height of the obstruction

protrudes above the inlet. Figure 6-1 also illustrates why it is recommended that inlets along the side of a

building be avoided. There is airflow up the side of the building which has considerable contact with the

building and allows a substantial possibility for deposition of the pollutant on the surface.
6-2

Figure 6-1. Schematic representation of the airflow around an obstacle such as a

building [EPA, 1978, based on the work of Halitsky (1961) and Briggs (1973)].
6-3

The inlet for sampling must extend above the roof of the building to avoid the complicated airflow

within the cavity zone. If the building which houses the instrument is small (i.e., less than 2 meters high),

then an extension of the inlet above the roof by a distance of about 1.5 meters should be sufficient (1 meter

minimum). For taller buildings it may not be possible to avoid the cavity zone on top of the building

without using an inlet line that is so long that it will introduce pollutant losses of its own. If sampling from

the top of a tall building is unavoidable, then it is recommended that the inlet be placed toward the upwind

side of the building. Airflow must be unrestricted in a horizontal arc of at least 270 degrees around the

inlet probe, and the predominant wind direction for the season of greatest pollutant concentration potential

must be included in the 270 degree arc. If the probe is located on the side of a building, a 180-degree

clearance is required.

6.1.3 Separation from Roadways

For oxides of nitrogen, most fresh combustion emissions are in the form of NO, which rapidly

reacts with ozone to form NO2 resulting in a decrease in ozone concentrations. EPA (1978) included a

kinetic model to predict the change in the ozone concentration for incremental additions of NOx. Using

this model, it was possible to estimate the proper distance from a roadway for placement of an ozone

monitor so that the ozone concentration is not depressed to a level where the concentration measurement

is not representative of the surrounding area. The EPA has updated this analysis, and the results are shown

in Table 6-1. This table provides the minimum separation distance between roadways and ozone monitors

based on the vehicle traffic of the road.

6.1.4 Spacing from trees and other vegetation

Ozone is known to deposit on trees and other vegetation, thereby reducing the ambient ozone

concentration in that area. Additionally, large trees or stands of trees can obstruct normal wind flow

patterns. To minimize the effects of individual trees on measured ozone concentrations, inlet probes

should be placed at least 20 meters from the “drip line” of trees (the area around the tree where water

dripping from the tree may fall). If the tree or stand of trees could be considered an obstruction, the probe

must be located at least 10 meters from the drip line and at least twice the height of the obstruction

protrudes above the inlet, whichever is furthest.

6.2 USE OF SATURATION MONITORING

Following the use of the sector analysis described in Section 5. for locating a suitable area for a

monitor, one approach for finding a specific location is to develop a short-term sampling study. In this type

of study, numerous portable monitors are employed during a period of high ozone concentration at various

candidate sites within the sectors identified by the meteorological analysis as having the potential for

maximum ozone concentrations. Analysis of data over the period of about a month could be used to

determine the best monitor location.
6-4

Table 6-1. Separation distance between ozone monitors and

roadways (i.e., the minimum distance between the edge of the

nearest traffic lane and the monitor).

Roadway Average Daily Distance Between Roadways

Traffic Vehicles Per Day and Stations (meters)

Minimum Separation

a

10,000 >10

15,000 20

20,000 30

40,000 50

70,000 100

$110,000 >250

 Distances should be interpolated based on traffic flow. a

Source: 40 CFR Part 58, Appendix E, “Probe siting criteria for ambient air quality monitoring.”
6-5

This technique was successfully applied during the Wasatch Front Ozone Saturation Study (Start et

al., 1994). The State of Utah was required to install two new monitoring sites in the Wasatch Front and

conducted this study to determine the optimal location of these monitoring sites. The study was also useful

for assessing whether the existing ozone monitoring sites were properly located. The study employed

Ogawa passive samplers (Koutrakis et al., 1993), which are small, inexpensive diffusion samplers (i.e., no

pump is required) that are useful for obtaining integrated average ozone concentrations over a sampling

period, usually on the order of 12 to 24 hours. The study made measurements at 62 passive sampler

locations, and also placed passive samplers at each of the current ozone monitoring sites. The locations of

the sites are shown in Figure 6-2. Sampling occurred over a 30-day period. However, sampling only

occurred on 7 of the 30 days which were expected to have elevated ozone concentrations. Each sampler

was exposed for 24 hours to give a one-day integrated sample. The passive samplers performed well, and

the study provided an inexpensive means of better understanding the spatial distribution of ozone in the

Wasatch Front.
6-6

Figure 6-2. Monitor locations for the Wasatch Front Ozone Saturation

Study summer 1993 (Start, 1994)
7-1

7. RECOMMENDATIONS

As a primer on ozone monitoring issues, this report has touched on a number of key issues and their

implications for monitoring network design including: ozone formation and spatial distribution; monitoring

objectives, such as regulatory compliance and scientific research; big picture geographic issues, such as the

need for siting monitors upwind of an urban area; and micro-scale issues, such as the need to avoid siting

monitors too close to trees and buildings. Readers with an interest in either developing a new ozone

monitoring network, assessing the efficacy of an existing network, or updating an existing network to

monitor 8-hour ozone concentrations should all find this report's contents to be of value. In summarizing

the report the focus has been to organize information in a way that answers commonly asked questions. As

has been mentioned throughout the text, the reader is referred to the Appendices for more detailed

information concerning the science behind ozone formation, as well as the many additional resources

available to assist in designing and implementing monitoring networks. Each of the question and answer

discussions in this section concludes with a reference to the earlier report chapters that provides additional

relevant information.

How many monitors are required for a particular region?

The EPA has established minimum network regulatory requirements on how many NAMS and

PAMS monitors are required. The EPA guidelines serve as minimum requirements beyond which additional

monitoring through the SLAMS network provides better documentation of air quality in a community. 

Adequately defining the ambient ozone problem in any given area, especially the larger urban areas,

generally necessitates the establishment of additional SLAMS monitors, in addition to the minimum

requirements.

EPA requirements vary depending upon the population and nonattainment status of a particular

area. For all urbanized areas with more than 200,000 people, two ozone NAMS sites are required, the

maximum downwind site and the maximum exposure site. In addition, EPA requires PAMS VOC, ozone,

nitrogen oxides, and meteorological parameter monitoring for serious, severe, and extreme ozone

nonattainment areas (for the 1-hour standard). Areas with less than 500,000 people are required to have at

least two PAMS sites, areas with 500,000 to 1,000,000 people are required to have at least three sites,

areas with 1,000,000 to 2,000,000 people need four sites, and areas with more than 2,000,000 people need

at least five PAMS sites (EPA, 1994). Appendix B to this document includes references for more PAMS

related information. These PAMS requirements have not been extended to include areas which are

nonattainment for the 8-hour standard only.

Scientifically, the number of ozone monitors needed is a function of the region’s size and

topographic and meteorological complexity. At a minimum, two monitors can be sufficient for a relatively

small metropolitan area. EPA recommends that a region using only two monitors site these as (1) a

downwind site to measure peak ozone concentrations in a populated area and (2) a site to measure

maximum population exposure. A relatively low-cost enhancement would be a monitor at a third location

designed to measure upwind, or background, ozone concentrations. Examples of when additional monitors

would be necessary include: 

· Additional monitors will be needed if the region’s high ozone concentration days are triggered by any

one of several different meteorological scenarios. When prevailing winds differ depending upon the

high-ozone meteorological scenario, additional monitors are needed to evaluate each scenario’s upwind
7-2

and downwind locations.

· Multiple monitors will be needed if an area is quite large geographically, with multiple communities in

downwind locations. Multiple downwind monitors will be needed to adequately measure peak ozone in

the affected areas.

Additional monitors would also be necessary to measure concentrations in an urban core; to address

special studies focused on particular areas; to monitor areas with complex terrain; to monitor the effects of

specific sources; and to quality assure the existing monitoring network from time to time by collocating

monitors near existing monitors, or by conducting saturation monitoring to ensure that the highest upwind

and downwind concentrations are being measured. 

[Refer to Section 4 of the report for more detailed information.]

How should monitors be sited to properly account for wind direction?

Some straightforward meteorological evaluations can help determine how to site monitors to

properly account for prevailing winds. Wind roses need to be constructed for the metropolitan area to

identify what direction the wind originates from at given times of day, as well as the wind strength during

these time periods. Note that the wind strength is important to help determine how quickly pollutants will

travel downwind and be dispersed. Some general “rules of thumb” can assist in this evaluation:

· Identify days when high ozone concentrations are known to occur. If these days are not yet known,

identify days when high ozone concentrations are likely to occur. Methodologies are available to help

with this analysis, which utilize wind, temperature, and relative humidity data (Cox, 1995).

· Construct wind roses for morning (7:00-10:00 a.m.) and afternoon (1:00-4:00 p.m.) winds for high

ozone days. Morning wind direction can help determine where to site upwind monitors; morning and

afternoon wind directions can help determine where to site downwind monitors.

Note that it is important to distinguish between average summer days, and days when high ozone

concentrations are likely to occur. Prevailing average winds may be different than winds on high ozone

days. Also note it is possible that more than one meteorological regime is contributing to high ozone

formation.

[Refer to Section 5 of the report for more detailed information.]

How should monitors be sited to avoid problems in the immediate vicinity of the monitor (e.g., from

roads, trees, buildings)?

Large obstructions, such as tall buildings, and small obstructions, such as trees, can each contribute

to inaccurate pollutant measurements if monitors are sited too close to the obstruction. “Rules of thumb”

to consider include:

· Generally, monitoring probe inlets should be located 3 to 15 meters above ground, and at least 1 meter

vertically and horizontally away from the supporting structure.

· The downwind sides of large obstructions such as buildings or stands of trees include wind pockets that

are not representative of the surrounding air mass. Multiplying the height of the obstruction by 10
7-3

yields a distance equal to the desired distance between the base of the obstruction and the monitor. If

space of that magnitude is not available, the minimal acceptable distance between such obstructions and

monitors is 2 times the height of the obstruction.

· Fresh NO emissions titrate ozone. Thus, ozone is quickly reduced in the vicinity of roadways with any

significant amount of traffic. Monitors should be placed anywhere from at least 10 to at least 250

meters from a roadway, depending upon the extent of traffic. Existing monitors can be quality assured

by analyzing their monitoring data. Frequent measured ozone values in the range of 0 to 30 ppb of

ozone may be an indicator that the site is too close to fresh NO emissions. Natural ozone background

concentrations are on the order of 40 ppb, prior to pollutant contributions that increase observed

concentrations. Measured concentrations in the 0 to 30 ppb range are an indicator of ozone titration

and should trigger a review of the monitor site and its proximity to emission sources.

[Refer to Section 6 of the report for more detailed information.]

Are existing 1-hour monitors still useful for 8-hour monitoring, or should they be moved? In

particular, does an 8-hour standard mean that the geographic area monitored needs to be enlarged,

and if so, by how much?

The locations of existing 1-hour monitors are likely to remain appropriate for monitoring upwind

contributions and downwind peak concentrations. There will possibly be a need however, for additional

monitors to be located further downwind than those established for 1-hour monitoring. These will help

determine the maximum downwind distance where 8-hour exceedances occur.

There is a strong correlation between 0.12 ppm 1-hour and 0.08 ppm 8-hour ozone concentrations,

in terms of the general locations of exceedances. There is also a strong correlation between the locations

of peak 1-hour and 8-hour ozone concentrations. A key difference between 1-hour and 8-hour ozone

concentrations, however, is that downwind of a metropolitan area, 8-hour concentrations tend to exceed

the 0.08 threshold more often than 1-hour concentrations exceed the 0.12 ppm threshold. This means that

when monitoring 8-hour ozone concentrations, it is probable that the area to be monitored needs to be

increased in geographic size in comparison to the area monitored for 1-hour ozone concentrations. 

A “rule of thumb” may be as follows: 1-hour peak ozone concentrations are observed

approximately 4 to 6 hours downwind of the location of the maximum emissions region (usually the urban

core). When monitoring for 8-hour ozone concentrations, however, ozone exceedances may be observed

over a broader geographic area than the typical 1-hour exceedance locations. An 8-hour monitoring

network would likely benefit by adding monitors to the existing 1-hour network. One suggestion might be

to place the additional monitors approximately 1 to 2 hours further downwind from where monitors are

normally sited to measure peak 1-hour concentrations. Consideration of the location for the monitor for

the potentially affected population must be balanced with the need to capture maximum downwind

concentrations outside populated areas.

[Refer to Sections 3 and 5 of the report for more detailed information.]

How can monitor sites be quality assured to determine if they are located in appropriate downwind

locations to identify peak ozone concentrations?

The only practical way to assure whether downwind monitors are observing peak ozone

concentrations is to locate additional monitors further upwind and downwind to see whether measured
7-4

ozone concentrations are higher or lower than the existing monitor. Another possibility is to conduct

photochemical air quality computer modeling to predict the location of peak ozone concentrations, and to

compare the modeling results to the existing or planned network (EPA has issued separate guidance with

detailed computer modeling guidelines). An important consideration is how long ago the existing

monitoring network was established. If the monitors were sited many years ago, and the area has since

undergone significant growth, peak ozone concentrations may be migrating further downwind and/or in the

direction of the growth. Additionally, special studies, such as those described in Section 6.2 of this

document, can be used as an adjunct to network/site evaluation.

[Refer to Section 3 of the report for more detailed information.]

Should 8-hour ozone monitoring be conducted during the same time of the year as 1-hour ozone

monitoring?

The EPA has established ozone seasons for the 1-hour standard which are published in the Code of

Federal Regulations (40 CFR 58, Appendix D). However, given differences in the spatial distribution of

1-hour and 8-hour exceedances, and because the 8-hour standard is stricter than the 1-hour standard,

adjustments to these seasons may be necessary. The EPA is recommending (EPA, 1998) initial 8-hour

ozone seasons for each state to be defined as the period of time beginning with the first month with any

daily 8-hour concentration greater to or equal to a 0.080 ppm threshold, and ending with the last month

with such exceedances during a 6-year period. Adjustments to these initial ozone seasons are expected to

better reflect the need for similar ozone monitoring seasons within ozone transport areas; and to consider

states with large geographic areas without available ozone data.

[Refer to Section 4 of the report for more detailed information.]

How should monitor siting differ depending upon whether an area is an isolated urban area, is part of

an urban corridor which includes numerous metropolitan areas, or is an urban area with multiple

prevailing wind directions?

Answering this question helps summarize a number of the issues presented in this report. The

information presented here draws upon the real world examples provided by the San Francisco Bay Area’s

monitoring network. The San Francisco Bay Area offers a good example of a reasonably complex area

with monitors in a variety of locations that serve multiple needs. Among the San Francisco Bay Area’s

numerous monitoring locations are several major metropolitan areas: San Francisco itself, San Jose to the

south and east of San Francisco, Oakland to the east, and a number of less populous areas to the north,

east, and south (Figure 3-1 illustrates the geographic relationship among Bay Area cities). In addition,

Sacramento is approximately 90 miles to the northeast of San Francisco, and the proximity between the

two regions helps to illustrate the value of monitoring sites located between neighbor regions. This section

concludes with five different monitoring scenario discussions that utilize the San Francisco Bay Area as a

real world example.

· San Jose: an urban area influenced by pollutant transport. San Jose experiences pollutant transport

from its upwind neighbors in the Bay Area, and contributes pollutants to downwind sites. Prevailing

winds influencing San Jose’s ozone concentrations are generally from the northwest. Therefore,

useful monitor locations include San Jose itself, upwind locations to the northwest, and downwind

locations to the southeast. Bay Area cities within this geographic sphere include Oakland, San

Leandro, Hayward and Fremont to the northwest, and Gilroy to the southeast, all of which have
7-5

monitors. San Jose is one of the areas where peak ozone concentrations are observed in the San

Francisco Bay Area. The Gilroy monitor to the southeast of San Jose both monitors the effects of

downwind transport from San Jose on the Gilroy area and serves to confirm that ozone

concentrations monitored in San Jose are higher and, therefore, representative of peak

concentrations. The monitors stretching to the northwest of San Jose provide a range of upwind

ozone concentration measurements that enable San Jose to track the transport of ozone and ozone

precursors into its metropolitan area.

· San Francisco: an urban area influenced by clean upwind air conditions. Metropolitan areas that are

generally located downwind from and near other metropolitan areas should expect to receive and be

influenced by pollutant transport unless one of two conditions occur: (1) extremely fast-rising mixing

heights occur which enable ozone concentrations to mix and become dispersed aloft or (2) strong

afternoon winds with clean air sweep through the region and remove locally generated and

transported pollutants. San Francisco is a coastal city with the Pacific ocean to its west. Afternoon

winds tend to be strong due to the sea breeze, and are relatively clean coming from the Pacific ocean. 

In contrast to its neighbor city, San Jose, San Francisco is relatively free from the influence of

pollutant transport, and thus has little need for upwind monitoring.

· Bethel Island: a downwind area that exceeds the 8-hour but not the 1-hour ozone thresholds. The

San Francisco Bay Area’s peak 1-hour and 8-hour ozone concentrations occur in the same general

areas: to the east and to the south of the Bay Area. However, there are areas in the far downwind

portions of the Bay Area that exceed a 0.08 ppm 8-hour ozone threshold without exceeding the 0.12

ppm 1-hour threshold. Bethel Island, east of the Bay Area, is an example site exceeding the 8-hour,

but not the 1-hour threshold. Bethel Island receives transported ozone and ozone precursors late in

the day that contribute to higher late afternoon ozone concentrations. These peak concentrations are

on the order of 120 ppb ozone, but are not quite high enough to exceed the 1-hour 125 ppb threshold. 

However, when added to the locally generated ozone formed earlier in the day, this transported

pollution creates an 8-hour concentration that exceeds the 8-hour threshold. 

· The greater San Francisco Bay Area: an area with several predominant wind directions contributing

to high ozone concentrations. The greater San Francisco Bay Area experiences high ozone

concentrations to the northeast, the east, and the southeast of downtown San Francisco. These

coincide with the varying meteorological regimes that contribute to ozone formation in the Bay Area,

as well as the region’s topographic complexity, which includes two mountain ranges that run

northwest to southeast (to the south and east of the Bay Area); a river channel that heads to the

northeast; a southeastern basin surrounded by San Francisco Bay to the north; and the two mountain

ranges that sweep toward one another at the Bay Area’s southern rim. To accommodate these

meteorological and topographic conditions, the Bay Area has established ozone monitors in numerous

locations. Several are located in a line extending through the region from the southwest to the

northeast, to the east, and to the southeast. There are also numerous background monitors located in

rural areas to the north. By arranging monitors in this manner, the Bay Area is able to measure ozone

concentrations along the predominant wind directions that occur during conditions favorable to ozone

formation. The large number of monitors enables the Bay Area to measure peak concentrations with

relative confidence, since the area also measures downwind concentrations which are less than peak

concentrations. 
7-6

· San Francisco to Sacramento: metropolitan areas with neighboring monitoring networks. When two

metropolitan areas are located in close proximity to one another, their monitoring networks provide

an opportunity to measure pollutant concentrations relevant to each other’s area. The San Francisco

and Sacramento, California areas provide an example of this situation. Under some conditions,

pollutants are transported to the northeast of San Francisco as far as the Sacramento metropolitan

area, some 90 miles away. The San Francisco to Sacramento corridor is an area of continued growth,

and several monitoring sites are located along the northeast path that connects the two cities. 

Monitors located in between these metropolitan areas can serve a dual purpose: for example,

monitors to the southwest of Sacramento may have been sited to provide background or upwind

readings for the Sacramento area. Depending upon the meteorological conditions present, these same

monitors may also serve to provide downwind ozone concentrations for the San Francisco Bay Area. 

Note that in areas where pollutant transport is a significant ongoing event, such as in the northeastern

U.S. ozone transport region (OTR), monitors between major urban areas may record more uniform

pollutant concentrations in comparison to the San Francisco example cited here. In those cases, the

appropriate location of downwind monitors will be a function of (1) downwind ozone concentrations

and (2) proximity to neighbor metropolitan areas. 

[Refer to Section 3 of the report for more information and maps of the areas discussed.]

What PAMS requirements will be necessary in new areas designated nonattainment only for the 8-

hour standard ?

PAMS requirements are extensive and include requirements for the monitoring of an array of ozone

precursors and meteorological parameters. At this time, EPA has not extended the PAMS requirements to

new ozone nonattainment areas which are nonattainment only for the 8-hour NAAQS (a new 8-hour ozone

nonattainment area in an area EPA designates nonattainment for the 8-hour NAAQS that was not

previously nonattainment for the 1-hour NAAQS). Appendix B to this document includes PAMS

references for readers seeking PAMS guidance materials. 
8-1

8. REFERENCES

Altshuller A.P. and Lefohn A.S. (1996) “Background ozone in the planetary boundary layer

over the United States”. J. Air & Waste Manag. Assoc. 46, 134-141.

Briggs G.A. (1973) Diffusion Estimation for Small Emissions. Draft No. 79 in 1973

annual report of the Atmospheric Turbulence and Diffusion Laboratory, Oak Ridge,

TN., Report No. ATDL-106, 83-195.

Carter W.P.L. (1994) “Development of Ozone Reactivity Scales for Volatile Organic

Compounds”. J. Air & Waste Manag. Assoc. 44, 881-899.

Chu S.-H. (1995) “Meteorological Considerations in Siting Photochemical Pollutant

Monitors”. Atmos. Environ. 29, 2905-2913.

Code of Federal Regulations (1997) Title 40, Part 58, Appendix D. “Network Design for

SLAMS, NAMS, and PAMS”., published by United States Government.

Cox W.M. and Chu S.H. (1993) “Meteorologically Adjusted Ozone Trends in Urban Areas:

a Probabilistic Approach”. Atmos. Environ. 27B, 425-434.

Halitsky J. (1961) “Estimation of Stack Height Requirements to Limit Contamination of

Building Air Intakes”. J. Amer. Ind. Hyg. Assoc. 26, 106-115.

Husar R.B. (1996) Pattern of 8-hour Daily Maximum Ozone and Comparison with the 1-

hour Standard. Report prepared by the Center for Air Pollution Impact and Trend

Analysis for the OTAG Air Analysis Workgroup, posted on the OTAG Air Analysis

Web Site, URL='http://capita.wustl.edu/otag/'.

Koutrakis P., Wolfson J.M., Bunyaviroch A., Froehlich S.E., Hirano K., and Mulik J.D.

(1993) “Measurement of Ambient Ozone Using a Nitrite-coated Filter”. Anal.

Chem. 65, 209-214.

National Research Council (1991) Rethinking the Ozone Problem in Urban and Regional

Air Pollution. National Academy of Sciences/National Research Council, National

Academy Press, Washington, DC.

Reiss R., Chinkin L.R., and Main H.H. (1995a) “Ozone NAAQS Review--ambient Air

Monitoring Support Target Strategy. Work assignment 1-95. Final report prepared

for U.S. Environmental Protection Agency, Research Triangle Park, NC by Sonoma

Technology, Inc., Santa Rosa, CA, EPA Contract No. 68D30020, STI-94510-1535-

FR, September.
8-2

Reiss R., Chinkin L.R., Roberts P.T., Main H.H., and Eisinger D.S. (1995b) Investigation

of Monitoring Networks for an Alternative Ozone NAAQS. Work assignment 7-

95. Final report prepared for U.S. Environmental Protection Agency, Research

Triangle Park, NC by Sonoma Technology, Inc., Santa Rosa, CA, EPA Contract No.

68D30020, STI-94571-1553-FR, December.

Seinfeld J.H. (1986) Atmospheric Chemistry and Physics of Air Pollution. Wiley

Interscience, New York, NY, pp. 154-156.

Start G.E. (1994) Wasatch Front Ozone Saturation Study Summer 1993. Report

prepared for the State of Utah Department of Environmental Quality by U.S.

Department of Commerce, National Oceanic and Atmospheric Administration, Idaho

Falls, ID.

 U.S. Environmental Protection Agency,(1978) Site Selection for the Monitoring of

Photochemical Air Pollutants. Office of Air Quality Planning and Standards,

Research Triangle Park, NC, EPA-450/3-78-013, April.

U.S. Environmental Protection Agency (1989) Quality Assurance Handbook for Air

Pollution Measurement Systems. Volume IV - meteorological measurements. 

Report prepared by the Office of Research and Development, Atmospheric Research

and Exposure Assessment Laboratory, U.S. Environmental Protection Agency,

Research Triangle Park, NC, EPA/600/4-90-003.

U.S. Environmental Protection Agency (1996) Clearinghouse for inventories and emission

factors. On U.S. Environmental Protection Agency electronic bulletin board.

U.S. Environmental Protection Agency (1998) Guideline for Selecting and Modifying the

Ozone Monitoring Season Based on an 8-Hour Standard, Office of Air Quality

Planning and Standards, Research Triangle Park, North Carolina, EPA-454/R-98-

001, June.

U.S. Environmental Protection Agency (1994) Photochemical Assessment Monitoring

Stations Implementation Manual, Office of Air Quality Planning and Standards,

Research Triangle Park, North Carolina, EPA-454/B-93-051, March.

U.S. Environmental Protection Agency (1998) SLAMS/NAMS/PAMS Network Review

Guidance, Office of Air Quality Planning and Standards, Research Triangle Park,

North Carolina, EPA-454/R-98-003, March.

U.S. Environmental Protection Agency (1998) National Air Quality and Emissions Trends

Report, 1996, Office of Air Quality Planning and Standards, Research Triangle Park,

North Carolina, EPA-454/R-97-013, January.
APPENDIX A

BACKGROUND ON OZONE FORMATION

NO hv NO O

k

2

1

+ → +

O O M O M

k

+ 2 + → +

2

3

O NO NO O

k

3

3

+ → 2 + 2

[ ]

[ ]

[ ]

O

k NO

k NO

3

1 2

3

=

RH + OH⋅ → R⋅ + H2O

A-3

APPENDIX A: BACKGROUND ON OZONE FORMATION

OVERVIEW

Ozone is formed as a product of photochemical reactions involving several components,

the primary contributors being volatile organic compounds (VOC) and oxides of nitrogen (NO ). x

This appendix includes a brief discussion of the atmospheric chemistry governing ozone

formation. The material in this discussion is based on Seinfeld (1986).

As discussed in the main body of the report, the photolysis of NO in sunlight produces 2

NO and O, and the free oxygen atom rapidly combines with O and forms ozone (O ): 2 3

(A-1)

(A-2)

where hv represents photochemical energy from ultraviolet radiation (or a photon), k1 and k2

represent rate constants for the reaction of NO and hv, and M represents N , O or another 2 2 2

molecule that absorbs the reaction’s excess vibrational energy. Once formed, ozone is rapidly

dissociated by reaction with NO, as follows:

(A-3)

The NO2 molecule is regenerated, and in the absence of other species a steady state is achieved

through reactions (A-1) through (A-3) in which the ozone concentration can be estimated by the

following relationship:

(A-4)

where k and k are the rate constants for reactions (A-1) and (A-3). 1 3

Ozone in the clean troposphere is governed almost solely by the relationship described by

reaction (A-4). In the more polluted troposphere, oxidation of VOC by free radicals have

significant effects on ozone formation by converting NO to NO2. The key to this oxidation

process is the OH radical (a radical is a highly reactive molecule) which is formed by ozone

photolysis and other reactions. Its reaction with many VOC leads to the formation of peroxyalkyl

radicals, as follows:

(A-5)
OH ⋅

R⋅ RH

R⋅ +O2 → RO2 ⋅

RO2 ⋅

RCHO RCO ⋅

RC(O)O2 ⋅

RO2 ⋅ + NO → NO2 + RO⋅ → RONO2

RC(O)O2 ⋅ + NO → NO2 + RC(O)O ⋅

RC(O)O2 ⋅

RO ⋅ +O2 → HO2 ⋅ + R′CHO

R′CHO R R′

HO2

HO2 ⋅ + NO → NO2 + OH ⋅

OH ⋅

A-4

Where RH is a general symbol for a hydrocarbon (which in this context is synonymous with a

VOC); is a hydroxy radical (the dot following the OH symbolizes a free electron, which

defines a radical); and, is the molecule without the H (it is thus a free radical

hydrocarbon). In the presence of oxygen, the next step in this process is:

(A-6)

where is a free radical oxidized hydrocarbon (a peroxy radical).

Similarly, OH radicals react with aldehydes ( ) to form acyl ( ) and

acylperoxy [ ] radicals.

The peroxy radicals react rapidly with NO to form NO and other free radicals, as follows: 2

(A-7)

(A-8)

The [ ] radicals normally decompose to alkyl radicals which leads to the generation

of another peroxyalkyl radical. This peroxyalkyl radical can oxidize additional NO to NO .2

Typically, smaller alkoxy radicals react with O to form HO radicals and a carbonyl 2 2

compound, as follows:

(A-9)

where is an aldehyde or carbonyl, is a general term for a carbon chain, and is a

different carbon chain. Like other peroxy radicals, the hydroperoxy radical ( ) can oxidize

NO to NO as follows: 2

(A-10)

Note that the OH radical is regenerated in this reaction, which completes the cycle. Therefore,

this radical is available to react with other VOC species and continue the chain of reactions.
APPENDIX B

SUPPLEMENTAL REFERENCE MATERIALS

APPENDIX B: SUPPLEMENTAL REFERENCE MATERIALS

On-line Resources

The U.S. EPA operates a world wide web site with a variety of useful information. The Internet

address is ‘www.epa.gov’. The Office of Air Quality Planning and Standards (OAQPS) has a

sub-directory on the site at ‘www.epa.gov/oar/oaqps’.

EPA Quality Assurance Guidelines

1. Quality Assurance Handbook for Air Pollution Measurement Systems, Volume I: A

Field Guide to Environmental Quality Assurance U.S. EPA, EPA-600/R-94-038a,

April 1994 (or later revision).

2. Quality Assurance Handbook for Air Pollution Measurement Systems, Volume II: 

Ambient Air Specific Methods U.S. EPA, EPA-600/R-94-038b, April 1994 (or later

revision due out Spring 98).

3. Quality Assurance Handbook for Air Pollution Measurement Systems, Volume IV: 

Meteorological Measurements U.S. EPA, EPA/600/4-90/003, August 1989

These 3 quality assurance documents are available through the EPA's Center for Environmental

Research Information, 26 W. Martin Luther King Drive, Cincinnati, Ohio 45268, telephone (513)

569-7562, fax (513) 569-7566.

Photochemical Air Monitoring System (PAMS)

Photochemical Assessment Monitoring Stations Implementation Manual U.S. EPA, EPA454/B-93-051, March 1994

This reference document is available through the National Technical Information Service (NTIS),

5285 Port Royal Road, Springfield, VA 22161, telephone (703) 487-4650, fax (703) 321-8547. 

The PAMS NTIS document number is PB 941 873 82.

Meteorological Data

National Climatic Data Center (NCDC), Asheville, North Carolina, (704) 271-4476. NCDC

stores a wide variety of meteorological data from the United States, which can be used for

analysis.
B-4

EPA Technology Transfer Network (TTN) contains a program to construct wind rose plots. The

program can be found within the directory for the Support Center for Regulatory Air Models

(SCRAM). The TTN electronic bulletin board can be reached by modem at (919) 541-5742 or

via the Internet at http://www.epa.gov/ttn/.

The National Weather Service (NWS) operates regional climate centers, which can provide

meteorological data. The regional centers include:

1.) High Plains Climate Center

L.W. Chase Hall

University of Nebraska

Lincoln, NE 68583-0728

(402) 472-6706

2.) Midwestern Climate Center

Illinois State Water Survey

2205 Griffith Drive

Champaign, IL 61820-7495

(217) 244-8226

3.) Northeast Regional Climate Center

1123 Bradfield Hall

Cornell University

Ithaca, NY 14853

(607) 255-1751

4.) Southeast Regional Climate Center

1201 Main Street

Suite 1100

Columbia, SC 29201

(803) 765-0849

5.) Southern Regional Climate Center

245 Howe-Russell Complex

Louisiana State University

Baton Rouge, LA 70803

(504) 388-5021

6.) Western Regional Climate Center

Desert Research Institute

P.O. Box 60220

Reno, NV 89506-0220

(702) 677-3106
B-5

Code of Federal Regulations

The following citations include specific monitoring requirements as outlined in the Code of

Federal Regulations.

40 CFR Part 58, Appendix D - Network Design for State and Local Air Monitoring Stations

(SLAMS), National Air Monitoring Stations (NAMS), and Photochemical Assessment

Monitoring Stations (PAMS).

40 CFR Part 58, Appendix E - Probe Siting Criteria for Ambient Air Quality.
TECHNICAL REPORT DATA

(Please read Instructions on reverse before completing)

1. REPORT NO. 2. 3. RECIPIENT'S ACCESSION NO.

 EPA-454/R-98-002

4. TITLE AND SUBTITLE 5. REPORT DATE

 Guideline on Ozone Monitoring Site Selection

 August 1998

6. PERFORMING ORGANIZATION CODE

7. AUTHOR(S) 8. PERFORMING ORGANIZATION REPORT NO.

 

 

9. PERFORMING ORGANIZATION NAME AND ADDRESS 10. PROGRAM ELEMENT NO.

 U.S. Environmental Protection Agency

 Office of Air and Radiation

 Office of Air Quality Planning and Standards

 Research Triangle Park, NC 27711

11. CONTRACT/GRANT NO.

 

12. SPONSORING AGENCY NAME AND ADDRESS 13. TYPE OF REPORT AND PERIOD COVERED

 Director

 Office of Air Quality Planning and Standards

 Office of Air and Radiation

 U.S. Environmental Protection Agency

 Research Triangle Park, NC 27711

14. SPONSORING AGENCY CODE

EPA/200/04

15. SUPPLEMENTARY NOTES

16. ABSTRACT

This report provides background information for State and local agencies responsible for designing and implementing an ozone monitoring program. The

purpose of this report is to update a 1978 U.S. Environmental Protection Agency (EPA) report entitled: Site Selection for the Monitoring of Photochemical Air

Pollutants (EPA, 1978). This updated report is motivated by the need to help air quality agencies and others incorporate 8-hour monitoring into their ozone monitoring

program. Since the 1970s, ozone monitoring programs have been designed to address the 1-hour ozone National Ambient Air Quality Standard (NAAQS); however, on

July 18, 1997, the U.S. EPA promulgated an 8-hour ozone NAAQS. This report assists those agencies considering how to design, implement or revise ozone monitoring

networks, particularly in light of the need to collect 8-hour ozone concentration data. Existing ozone monitoring networks should be reviewed annually in accordance

with the requirements of 40 CFR 58.20 (d); any changes to the networks to accommodate the 8-hour ozone standard should be addressed at that time (see also the

SLAMS/NAMS/PAMS Network Review Guidance, EPA-454/R-98-003, for further detail on network review.) The EPA does not expect major changes to the

monitoring networks as a result of these reviews since the analysis in this document indicates that, generally, the 1-hour and 8-hour concentrations are well correlated

spatially. The report covers basic issues essential to understanding monitor siting considerations, including the physics and chemistry of ozone formation, spatial

distribution of ozone concentrations, monitoring objectives and network design macroscale considerations for monitor placement, and microscale considerations for

monitor placement. Appendices include background information on ozone formation, and references to supplemental materials that can assist those involved with siting

efforts. 

17. KEY WORDS AND DOCUMENT ANALYSIS

a. DESCRIPTORS b. IDENTIFIERS/OPEN ENDED TERMS c. COSATI Field/Group

 Ozone, Monitoring, Site Selection, Air Quality Air Pollution control

Standards, Ambient Air, Air Pollution

 

18. DISTRIBUTION STATEMENT 21. NO. OF PAGES

 Release Unlimited

19. SECURITY CLASS (Report)

 Unclassified 85

20. SECURITY CLASS (Page)

 Unclassified

22. PRICE

EPA Form 2220-1 (Rev. 4-77) PREVIOUS EDITION IS OBSOLETE