United States
Environmental Protection
Agency
Office of Air Quality
Planning and Standards
Research Triangle Park NC 27711
EPA-450/4-80-017
September 1980
Air
Technical Basis for
Developing Control
Strategies for High
Ambient Concentrations
of Nitrogen Dioxide
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TECHNICAL BASIS FOR DEVELOPING CONTROL STRATEGIES FOR
HIGH AMBIENT CONCENTRATIONS OF NITROGEN DIOXIDE
U.S. Environmental Protection Agency
Office of Air Quality Planning and Standards
Monitoring and Data Analysis Division
Research Triangle Park, North Carolina
September 1980
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This document is issued by the Environmental Protection Agency to report
technical data of interest to a limited number of readers. Copies are
available free of charge to Federal employees, current EPA contractors and
grantees, and nonprofit organizations - in limited quantities - from the
Library Services Office (MD-35), U.S. Environmental Protection Agency,
Research Triangle Park, North Carolina 27711; or, for a fee, from the
National Technical Information Service, 5285 Port Royal Road, Springfield,
Virginia 22161.
This document has been reviewed by the Office of Air Quality Planning and
Standards, U.S. Environmental Protection Agency, and approved for publication.
Subject to clarification, the contents reflect current Agency thinking.
Publication No. EPA-450/4-80-017
ii
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TABLE OF CONTENTS
Page
List of Tables v
List of Figures vi
Executive Summary ix
1.0 Introduction 1
2.0 Emissions of NOV 3
X
2.1 Anthropogenic Emissions 3
2.2 Natural Sources of NOY 6
A
2.3 Significance of Manmade Versus Natural Emissions 6
2.4 Implications 7
3.0 Mechanisms for N02 Formation in the Atmosphere 10
3.1 Photochemical Synthesis of Nitrogen Dioxide 11
3.2 Titration 13
3.3 Carryover 18
3.4 Implications 20
4.0 Monitoring, Data Quality and the Observed Extent of High N02
Concentrations . 22
4.1 Network Design and Operation 22
4.2 Data Quality Assurance Techniques 25
4.3 N02 Concentrations Observed in Excess of 0.20 ppm .... 26
4.4 Implications 31
5.0 Transport of N02 34
5.1 Urban Gradients of N02 34
5.2 Levels of N02 Reported in Rural Areas 43
5.3 Chemistry of Transported NO 46
/\
5.4 Implications 51
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Table of Contents (continued) Page:
6.0 Relation-ships Between N02 and its Precursors . 53
6.T ReTatioiTsr Among N02 and Precursors . . 53
6.1.1 Smog Chamber Experiements 54
6.1.2 Empirical Relationships from Ambient Data 58
6.1.3 Relationships Derived from Chemical Kinetics Models 62
6.2 Implications 67
7.0 Impact of Controlling Various Source Categories 71
7.1 Tracer Approaches 73
7.2 Use of Peak-to-Mean Ratios 76
7.3 Impact of Point Sources on Ambient Levels of N02 80
7.4 Implications 95
8.0 Use of Models for N02 . 99
8.1 Multisource Urban Models 100
8.1.1 Linear Rollback 100
8.1.2 OZIPM 108
8.1.3 Photochemical Box Model (PBM) 110
8.1.4 Complex Multisource Models 112
8.2 Models to Assess the Impact from Individual Sources. ... 112
8.3 Implications 122
9.0 References 127
10.0 Acknowledgement 131
IV
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LIST OF TABLES
Number Page
2.1 1977 National NO Emissions by Source Category
(Expressed as N02) 5
2.2 NO Emissions in Selected Urban Counties* 8
/\
4.1 Distribution of N02 Concentration >_ 0.20 ppm for all
California Monitoring Stations, 1975-1977* 30
5.1 High N02 Values Observed in the Vicinity of Los Angeles,
1975-1977 36
5.2 Spatial Distribution of Hourly N02 Concentrations in the
RAPS Network, 1976 41
5.3 N02 Observed at Rural Sites 44
5.4 Concentration of Nitrogen Oxides Observed in Clean
Remote Areas 45
5.5 Chemical Stability of NO Corresponding to Observed
Conversation Rates 48
6.1 Summary of Conclusions from Smog Chamber Experiments . . 57
6.2 Empirically Estimated Dependence of N02 on NO Control . 60
A
6.3 Estimated Impact of Reducing NMOC on Peak N02 Obtained
with Empirical Models 61
6.4 Effect of Separate NO and NMOC Reductions on Peak
N02 Under Various Conclitions* 63
8.1 Sensitivity of Peak N02 to Changes in NMOC with Various
Prevailing NMOC/NO¥ Ratios 104
A
8.2 Available Complex Multisource Photochemical Dispersion
Models 113
8.3 Approaches for Estimating Maximum Impact on Peak N02
from Large Individual Sources 117
8.4 Hypothetical Example Illustrating Use of Multi and
Individual Source Models for Projecting Future Short
Term N02 Concentrations for Use in SIP's 123
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LIST OF FIGURES
Number Page
3.1 Illustration of Photochemical and Titration Formation
of N02 12
3.2 Ozone Limiting Case of Titration 16
3.3 NO Limiting Case of Titration 17
3.4 Diurnal Concentration Profiles Illustrating Carryover. . . 19
4.1 Seasonal Variation in Hourly N02 Levels Exceeding
0.20 ppm at all non-California Stations Reporting to
SAROAD during 1975-77 27
4.2 Frequency of Maximum 1-hour N02 Concentrations >_ 0.20 ppm 28
5.1 Gradients in Maximum hourly N02 Observed in the
Los Angeles Basin 37
5.2 Example of N02 Transport at El Toro Station on
January 25, 1975 39
5.3 Pollutant plots for Riverside, October 1-4, 1975 40
5.4 Gradients in Maximum N02 Concentrations Observed During
the St. Louis RAPS 42
6.1 Nitrogen Dioxide Maximum Concentration versus Initial
Oxides of Nitrogen (means of several experiments)
UNC Study 55
6.2 Dependence of Nitrogen Dioxide Maximum Concentration
on Initial Nitrogen Oxides, Bureau of Mines Study .... 55
6.3 Maximum one-hour Nitrogen Dioxide Concentration
Produced from Irradiation of Multicomponent
Hydrocarbon/NO Mixtures, General Motors Study 56
/\
6.4 Maximum Nitrogen Dioxide Concentration as a Function
of Initial NO for Two Initial Hydrocarbon Levels,
UC Riverside Study 56
6.5 N02, With 8 hours of Emissions and 3%/hr Dilution .... 64
6.6 N02, With 8 hours of Emissions and 10%/hr Dilution .... 65
VI
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List of Figures (continued) Page
6.7 N02, With 10 hours of Emissions and 3%/hr Dilution .... 66
6.8 N02, NO and 03 Versus Time for Point A (Figure 6.7) ... 68
6.9 N02, NO and 03 Versus Time for Point B (Figure 6.7) ... 69
7.1
7.2
7.3
7.4
7.5
7.6
7.7
7.8
7.9a
7.9b
7.10a
7.10b
7.11
Pollutant Plots for Lynwood, November 23-25, 1976. . . .
Example of Potential Point Source Impact at Long Beach
on Sunday, November 16, 1975 ,
Example of Potential Point Source Impact at Whittier
on Monday, August 25, 1975 ,
Dependence of Maximum/mean Ratio on Annual Mean N02
Concentrations ,
Location and Elevation of Clinch River Power Plant
Stations ,
N02/NOV versus NOV at Clinch River on Days with High
NOV .x ,
X
N02 Ratios Observed at Clinch River During Hours with
High Ambient NO ,
Evidence of NO Aloft Over St. Louis
X
Horizontal Plume Traverse 21 km Downwind of Labadie
Stacks on August 14, 1974
Horizontal Plume Traverse 21 km Downwind of Labadie
Stacks on August 14, 1974
Horizontal Plume Traverse 45 km Downwind of Labadie
Stacks on August 14, 1974
Horizontal Plume Traverse 45 km Downwind of Labadie
Stacks on August 14, 1974
Diagrams for a Titration Reaction in a Turbulent
. 75
. 77
. 78
81
, 83
, 85
, 86
, 87
, 89
, 90
, 92
92
Plume 94
8.1 ConceptualView of the Column Model 109
8.2 Schematic Diagram of Modeling Domain for the Photochemical
Box Model Ill
VII
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List of Figures (continued) Page.
8.3 Nitrogen Dixoide (N02) Data from the N02/03 Sampler
Siting Study 115
8.4 Procedure for Applying Rollback in Assessing the Impact
of Control Strategies on N02 125
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EXECUTIVE SUMMARY
This document has two objectives: (1) to provide a concise description of -
the extent and likely causes of high concentrations of nitrogen dioxide (NOpK
and (2) to identify useful procedures for evaluating the effectiveness of strategies
to reduce high concentrations of NOp- Each chapter reviews one facet of the
subject matter. The last section in each chapter summarizes direct implications
posed by preceding information.
According to 1977 estimates, over 95 percent of anthropogenic NO (i.e.,
A
nitric oxide plus nitrogen dioxide) occurs as the result of combustion. Approxi-
mately 90 percent of these emissions occur as nitric oxide (NO). Thus, N02 is
primarily a secondary pollutant arising from oxidation of NO in the atmosphere.
As of 1977, there is roughly a 60-40 split in stationary-mobile source emissions
of NO in the United States. Natural sources emit about 20 percent of the total
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NO nationwide. However, these emissions are diffuse and ambient N0? concentra-
A £.
tions in remote areas are very low. Hence, natural NO emissions need not be
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included in the NO emission inventories used in SIP's for N0?.
A £.
Conversion of NO to NOp in the atmosphere occurs via two principal mechanisms:
(1) oxidation of NO by organic compounds (.Photochemical synthesis), and (2)
oxidation of NO by ozone (.titration). Titration occurs rapidly and is likely to
be the primary means by which large individual sources of NO make their greatest
A
impact on hourly concentrations of N02« A third process is the oxidation of NO
by Op (thermal conversion); however, this process is only important in and near
stacks where NO concentrations are very high. High concentrations of NOp found
at representative monitoring sites 1/2-4 hours travel time from major source
areas of NO and organic pollutants are likely to be largely the result of photo-
s\
chemical synthesis. Observed high concentrations of N02 are most likely to result
from the impact of both synthesis and titration to varying degrees. Thus, peak N02 is
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approximately proportional to N0v emissions, unless the peak results alimost solely
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fvQ,m. tttira-tfiOR in which the conversiiofl. orff NO to. NOp is limited by the amount of
aiv.a]i,Tabl'e-©zone. Data- from, continuous.!^ operated .monitoring, sites suggest this
exception, iis very ira-re. Curves, depicting diurnal patterns of NO, NOo and ozone
observed: at a monitoring site can serve as useful means for assessing whether or
not an observed peak concentration of NOp is entirely attributable to ozone-
limited titration or nearly so.
Proper design of NOp monitoring networks, choice of individual sites and
careful data quality assurance are essential prerequisites for formulating an
appropriate strategy for reducing NOp- At a minimum, the network should incor-
porate (1) a station 1/2-4 hours travel time-(e.g., 15 km) downwind of the most
intense area of NO and organic emissions, and (2) a station within the most
A
intense area of VOC and NO emissions. In order to obtain representative values,
J\
monitors should not be located within 200 meters of large individual sources.. In
screening the data for validity, it should be noted that existing data suggest
that hourly NOp concentrations in excess of 0.40 ppm are unlikely. Values, in
excess of 0.40 ppm should be further scrutinized using statistical tests and, in
some cases, by reviewing concurrent- diurnal data- for NO,- NOp, and ozone.
Long range transport of NOp is not a significant factor leading to
peak hourly concentrations of NO^. Comparison of urban-suburban data
with rural or remote data reveal concentrations in the urban-suburban
areas are very much higher. Consequently,, in most cases, there is
little need to consider intercity transport of NOp in estimating local'
control requirements to meet any prospective short term NAAQS for NOp.
A possible exception to this generalization is the case of several large
cities, aligned by the wind, which are withi'rr several hours travel titne*
of one another. The data base underlying this report is insufficient to
examine such a case. Eleyated concentrations of NOp sometimes observ.ed'
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to persist throughout the night (i.e. the "carryover" phenomenon) most likely
occur as the result of poor dilution in the vicinity of sources of NO .
A
Control strategies for NCL need to be coordinated with strategies for reducing
ozone. First, experimental evidence and modeling exercises indicate that reducing
organic emissions pursuant to ozone State Implementation Plans (SIP's) may also
reduce peak NOp concentrations attributable to photochemical synthesis. Second,
reducing ozone diminishes NOp concentrations, formed as the result of ozone-
limited titration, proportionally. Third, long range transport of NCLj while in
low concentrations, may be sufficient to aggravate violations of the ozone NAAQS
in rural areas. Unfortunately, the present state of knowledge does not enable
reliable quantitative estimates of this latter impact to be made.
Use of SOo and CO data as tracers and use of peak to mean ratios appear to
be useful techniques for qualitatively assessing the impact of NO source categories
A
on observed high hourly concentrations of NOo- These techniques provide justi-
fication for assumptions incorporated in models to quantitatively estimate the
impact of control programs. Tracer and peak-to-mean techniques indicate that
observed concentrations of NOo which are greater than 0.20 ppm are almost always
attributable to ubiquitous area sources, most of which are mobile sources.
Hence, mobile source reductions should ordinarily be more effective on
an area-wide basis than point source reductions in reducing annual
average and hourly peak concentrations of N0~. The infrequent impact of
elevated point sources is attributable to the following factors: (1}
during stable stratification, elevated plumes remain aloft; (2) corres-
pondence of wind direction and distance of maximum plume impact with the
location of a monitor is infrequent; (.3) NOo monitoring networks are
generally sparse. In contrast, area sources have low release heights (i.e.,
xi
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thf surface during all meteorological conditions) and are distributed
throughout the urban region in all directions from the monitors.
The preceding stattroents do not necessarily mean that an elevated point
source's impact on hourly concentration is insignificant. Modeling studies have
shown that areas downwind of major elevated point sources during peak NOp
periods may experience concentrations substantially greater than the general
urban levels associated with area sources, particularly if ambient levels of
ozone are high. A control strategy for a given urban area should consider both
the areawide NCp levels and "hot spots" associated with large elevated point
sources. Control of both mobile and large elevated point sources may be
required to meet a short-term NOp standard. Control of NO emissions
from elevated point sources may also be highly desirable for other
reasons (e.g., reduction in fine particulate, acid rain-forming potential
and impact on downwind ozone formation).
Models to quantify the impact of control strategies can be categorized as
multisource or individual source models. Several multisource models exist.
These range in sophistication from photochemical air quality simulation models
(AQSM's) to proportional rollback. Although they have not been extensively
tested, photochemical AQSM's offer the greatest potential for evaluating pro-
spective NOp control strategies. They appear to be particularly attractive
alternatives in areas which have already compiled a substantial data base for the
design of ozone SIP's.
Key assumptions of the proportional rollback model also appear to hold for
use in evaluating broad strategies to reduce annual and hourly NOp concentrations.
For example, both peak and mean NOp appear to be proportional to ambient NO . In
£ X
addition, use of the rollback model can be supplemented with a crude assessment
of the impact of reducing organic emissions described in Chapter 8. The rollback
xii
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model represents an acceptable approach in an individual urban area only if some
heed is given to source-receptor relationships. To reconcile findings concerning
the lack of point source impact on observed high hourly and mean concentrations
of N02 with the assumptions in rollback, the following approach is appropriate.
First, unless tracer data indicate to the contrary, do not include the emissions
from elevated point sources in the emission inventory used in rollback. This
enables projections of future air quality to be made at existing monitoring sites
which are consistent with present observations. Second, to assess the impact of
major individual sources on peak hourly NO^, superimpose predictions obtained
with a point source dispersion model upon representative projected values of
future N02 obtained with the rollback model. It may then be necessary to repeat
this procedure using different (but consistent) sets of meteorological assumptions
in the rollback and point source models.
There are several available models for estimating the impact of individual
point sources or major roadways on N02. The following approach represents an
acceptable compromise between sufficient detail and data availability. Utilize
a Gaussian point source model to estimate maximum concentration of NO (i.e.,
/\
NO + NOp). Apply the Ozone-Limiting Approach to estimate the peak concentration
of N02. Special cases in which a source emits N02 (rather than NO) directly into
the atmosphere can be considered using Gaussian models and modifying the inputs
of the Ozone-Limiting Approach.
xiii
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1.0 INTRODUCTION
This document has two objectives. The first is to provide a concise
description of the extent and likely causes of high ambient concentrations of
nitrogen dioxide (NO^). The second objective is to identify useful procedures
for evaluating the effectiveness of strategies to reduce unacceptably high con-
centrations of N02, The document is intended to be of use to persons having
responsibilities for:
a) determining the need for a strategy to reduce short term (e.g.,
hourly) and/or annual concentrations of NO^; and
b) designing and evaluating the effectiveness of a strategy for
meeting such an objective. The information presented in this document is based
on reviews of general references on oxides of nitrogen (such as the Oxides of
Nitrogen Criteria Document ), the results of laboratory and field studies conducted
by the EPA and others, ambient air quality data reported to the National Aero-
metric Data Bank (NADB) during 1975-1977 from throughout the nation, and modeling
studies.
The document is organized in the following manner. Chapter 2.0 identifies
sources of emissions of NOV, The relative importance of various major source
X
categories is described. Since only a small fraction of the oxides of nitrogen
(NO ). emissions occur as N0?, it is important that the means by which NO is
A £ /»
converted to NO- be identified. Chapter 3.0 describes the most important pro-
cesses by which N02 is formed in the atmosphere. Having described the processes
by which high leyels of ambient N02 can exist in the two preceding chapters,
Chapter 4.0 describes the extent of the ambient air quality data base which has
been examined, procedures for identifying data which may be spurious and existing
ambient levels of NQ2-
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5^-7 dnscuss factcws whnfeffc may/ eantrftiiite to htfgji shcrr-fc term- ejancerin-
trra.tfonB of N%.. Ehasptear 5,0 exanriines tfre rale- of intermediate and; long range
traraisport: iffi co.nftritb;U(tw§ to' observed high levels of ambient N.CL. Chapter 6:.(T
aidd»?es>ses neJationslrips between, NOp and its two majpr precursors.: nitric oxide;
(NO) and volatile organic compounds (VOC).. Laboratory, modeling and empirfcal
studies are drawn upon to surmise existing relationships, between, ambient NQ^ and
precursors which are subject to control programs. Chapter 7.0 focuses on relation-
ships whfeh appear to exoist between observed high, levels of HQ^ and precursor
emissions froffl several major source categories. Potential of various source
categories for leading to high ambient levels of NO,, is also discussed.
Chapter 8.0 identifies and briefly discusses modeling procedures, wh-ieh. may
prpye use.ful in quantifying the extent of controls needed to. attain air quality
goals. Models which are suitable for urban wide applications, as well as those
which are useful in estimating the impact of individual sources, are enumerated.
The Tast section in; each chapter summarizes the implications of material presented
in that chapter on the formulation of effective: strategies concerning NO-.
Hence, these last sections provide a short hand review of key implications o.f the
present state of scientific knowledge on developing strategies for reducing
ambient levels of NCL. The remaining portions of each chapter provide a more
indepth presentation of information leading to the identified implications for
strategies.
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2.0 EMISSIONS OF NOY
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2.1 Anthropogenic Emissions
Oxides of nitrogen (NO ) emissions ordinarily consist of two components:
A
nitric oxide (NO) and nitrogen dioxide (N02). Nitrogen dioxide is thus both a
primary and secondary pollutant. A primary pollutant is one which is emitted
directly to the atmosphere from a source of pollution. Some sources where most
of the NO is emitted as N0« are nitric acid manufacturing plants, nitrogen
fertilizer plants and munitions manufacturing. The NO emissions may be up to
100 percent of the total NO emissions from such sources. Another source of N0?
X £
emissions is fuel combustion. However, only about 5-10 percent of the NO
A
emissions from combustion sources occurs as NOp. According to the National Air
Quality Monitoring and Emissions Trends Report for 1977, 96 percent of anthro-
pogenic NO emissions within the U.S. arise from the combustion of fuels. Due
A
to the preeminance of combustion sources, the composition of emissions from
combustion sources has important implications on the assessment of ambient NOp.
Combustion sources are comprised of two major categories: stationary fuel
combustion (56 percent of the total NO ) and combustion by mobile sources (40 per-
A\
cent). This leads to the conclusion that only about 10 percent of the NO
A
emissions nationwide occur as NOg. Because of the importance of combustion
processes as a source of NO , nitrogen dioxide can be thought of chiefly as a
A
secondary pollutant. A secondary pollutant is one which is not emitted directly
by a source of pollution. Instead, secondary pollutants arise from chemical
reactions in the atmosphere occurring among other primary and secondary pol-
lutants (i.e., nitric oxide, volatile organic compounds [VOC], ozone, etc.) The
chemical reactions which produce N02 are discussed in Section 3.0.
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Since secondary N02 is chiefly formed through chemical reactions involving
nitric oxide. (NO), it is essential to investigate sources of both N02 and NO
(i.e., sources of NO ). For purposes of impact assessment, manmade sources of
/V
NO can be classified into four types:
A
1. stationary
2. mobile
3. point
4. area
The preceding classifications prove useful for selecting an appropriate mix of
controls (mobile versus stationary) and in modeling applications (area versus
point). Table 2.1 presents national NO emission estimates segmented into several
/\
broad source categories. Note that several of the categories identified in
Table 2.1 include both point and area, as well as mobile and stationary sources.
The distinctions between "area" and "point," and between "mobile" and "stationary,"
are as follow. Individual sources which emit large quantities of NO are considered
A
explicitly in models and are hence regarded as "point" sources. Sources which,
on an individual basis, emit relatively small amounts of NO are aggregated in
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models and designated as "area" sources. For example, a large isolated electrical
generating power plant is considered to be a point source, whereas residential
heating (due to the large number and spread out nature of housing) would be
considered as an area source. The distinction between mobile and stationary is
just one of ability of a source to relocate in a very short period of time.
Thus, an automobile is a mobile source, whereas a power plant is a stationary
source. Because the distinctions between stationary-mobile and area-point are
not mutually exclusive, it is possible to categorize certain sources as "stationary-
point" etc., as in Table 2.1.
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Table 2.1 1977 National NO Emissions by Source Category
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(Expressed as NOo)
Source Category
Fuel Combustion
Transportation
(mobile-area)
Electric Utilities
(stationary-point)
Industrial
(stationary—point/area)
Residential-Commercial
(stationary-area)
NO., (XI0° metric tons/yr)
9.2
7.1
5.0
0.9
Fraction of
Total NO,, Emissions
X "'"" ' •••--•
0.40
0.31
0.22
0.04
Industrial Processes
Manufacturing
(stationary-area)
Miscellaneous
(area)
0.7
0.1
0.03
0
Total
23.1
1.00
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2.2 Na&uKafl -Sources -df 'HO
Thus far, Ihe -locus has iieen *on manmaxte sources of NO . It is appropriate
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to next examine whether natural sources of NO are likely to contribute signi-
A
ficantly to ambient concentrations of NOp. There is much uncertainty about the
extent of natural emissions of NO . Estimates of xflobal NQV emissions from
A X
fi -3
natural sources are in the order of 20-90x10 metric tons/year. These estimates
are arrived at indirectly by determining what they would have to be in order to
balance the global NO cycle. Natural emissions primarily result from biological
/\
and chemical transformations in the -soil and from lightning. Relatively small
additional amounts (on a global basis.) of NOV arise from conversion of naturally
X
emitted ammonia (NH3) and from large point -sources (i.e., volcanoes). Prorating
natural NO emissions to the United States, on the .basis of land area (i.e., the
/\
U.S. comprises about 6 percent of the world's land area), gives a maximum estimate
of natural N.OX of about 6x10 metric tons/year emitted within the U.S. This
.latter figure is roughly Z5 percent of the annual manmade emissions presented in
Table 2.1. and therefore 20 percent of the total (manmade + natural) NO emitted
/\
nationwide.
2.3 Significance of Manmade Versus Natural Emissions
Although natural emissions constitute as much as 20 percent of the total NO
«
.emissions within the U.S., their impact on ambient N02 concentrations is likely
to be much less. This assertion becomes obvious when one considers emission
density rather than total emissions. It is reasonable to assume that natural
emissions are distributed fairly uniformly about the country. Manmade emissions,
however, are clustered near centers of habitation or in large industrial complexes.
If the estimated 6x10 metric tons/year of natural emissions were distributed
o
over the approximately 3,000,000 mi of the contiguous 48 States, an NO emission
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2
density of two metric tons/year/mi would result. Comparing this emission
density with sample urban emission density figures presented in Table 2.2 indicates
4
dramatic differences.
The disparity between NO emission density attributable to natural sources
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and the emission density in urban areas is reflected by ambient air quality
measurements. Various monitoring studies conducted in remote areas, and in rural
areas of the United States, have typically reported N0« concentrations of less
than 1 ppb to 10 ppb (i.e., .001-.01 ppm). ' Maximum readings as high as 30-
40 ppb have been observed in rural areas several tens of kilometers downwind from
urban areas. However, contributions from anthropogenic sources cannot be ruled
out on these occasions. In contrast to ambient levels of NOp measured in rural
areas, several urban or suburban monitoring sites have reported maximum hourly
p
N0? concentrations in excess of 200 ppb (0.20 ppm). Thus, like NO emission
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density, differences between N02 concentrations in urban and remote areas as
great as two orders of magnitude have been observed. Prevailing concentrations
of N02 in urban, rural and remote areas will be described in greater detail in
Chapters 4 and 5.
2.4 Implications
Implications of the nature and extent of NOV emissions are summarized below.
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1. The major portion of NO emissions occur as NO. Hence, one needs
A
to recognize the role of atmospheric chemistry resulting in the conversion of NO
to N09. Selection of an appropriate NO control strategy will, in part, depend
£* A
on being able to correctly identify factors influencing the conversion of NO to
N0? in the atmosphere.
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TABLE 2.^2 NO Emissions in Selected Urban Counties*
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Urban Area
St. Louis
Chicago
Washington, DC
Minneapolis
Richmond
Houston
County
St. Louis City
St. Louis County
Cook
Washington
Hennepi n
Henrico
Harris
Emissions
tpyt
46,676
80,946
250,644
40,794
54,243
12,980
214,419
County Area
mi2
61
499
954
61
567
232
1,723
Emission Density
tpy/mi 2
765
162
263
669
96
56
124
* NEDS data base
Area emissions as of June 6, 1979
Point emissions as of October 2, 1979
t tpy = tons per year
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2. Although most (90 percent) manmade NO emissions occur as NO,
x\
there are some manufacturing processes which result in significant amounts of N02
being directly emitted as a primary pollutant. Such emissions have the potential
for causing locally severe short-term peaks of N02.
3. On a national basis, 1977 emission estimates indicate approxi-
mately 60 percent of anthopogenic NO emissions result from stationary sources,
/\
with the remaining 40 percent arising from mobile sources. This suggests that
potential for reducing ambient levels of N02 is available through emission reductions
from stationary or mobile sources, or both.
4. Although natural NO emissions are about 25 percent of the manmade
/\
emissions within the U.S., the emission density (i.e., emissions/area) of natural
emissions is very much less than that for manmade emissions in urban areas.
Ambient measurements of NO- in urban and remote areas indicate urban concentra-
tions of N02 are as much as two orders of magnitude greater than concentrations
in remote areas. This information indicates that natural emissions of NO would
contribute an insignificant amount to any conceivable NAAQS for N02. Therefore,
natural emissions of NO can be ignored in the formulation of strategies for
reducing unacceptably high levels of N02.
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3.0 MECHANISMS FOR N02 FORMATION IN THE ATMOSPHERE
Observed concentrations of N0« are chiefly the result of chemical reactions
in the ambient air involving NO and other pollutants. In order to reduce NOp
levels, it is important to know how the NOp is formed in the atmosphere. Since
there are several potential pathways responsible for this formation, proper
identification of the significant path leading to particular violations of NAAQS
for N02 is vital in selecting appropriate controls and in properly designing
SIP's.
In subsequent sections, different mechanisms or formation processes will be
discussed. Each process will be outlined and its implications about the effec-
tiveness of various control strategies described. In the later sections, it will
become apparent that the mechanisms presented either involve ozone directly or
precursors of ozone. This indicates that close coordination is needed between
N02 control programs and ozone control programs.
The three major phenomena with potential for leading to high NO- are:
1. photochemical synthesis;
2. the reaction of nitric oxide with ozone (titration);
3. carryover.
Each process will now be discussed. Typical diurnal patterns of ambient NO^
concentration are presented, illustrating each of the above phenomena. Next,
findings are discussed which concern: (1) observed frequency with which each
phenomenon corresponds with high concentrations of N0?, and (2) the correspond-
ence (or lack thereof) between the occurrence of high NOp concentrations and
physical location of a site. As described in Chapter 4.0, the quantity of avail-
able N0/N02 data is much greater in California than in the rest of the nation.
10
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For this reason a distinction is made in the subsequent discussion between
analyses conducted with California data and with other data.
3.1 Photochemical Synthesis of Nitrogen Dioxide
Photochemical synthesis occurs when chemical reactions are initiated by
radiant energy (sunlight). More specifically, mixing of nitric oxide (NO) and
non-methane organic compounds (NMOC) in the presence of sunlight leads to chemical
reactions which produce NCL. This process usually results in maximum NCL con-
centrations occurring a few hours after sunrise.
Photochemical synthesis of NO^ may occur at any time during the daytime
hours; however, the speed with which the NCL is formed and destroyed is a function
of sunlight intensity and precursor concentrations. Thus, the large concentra-
tions of NO and NMOC in the mid morning (sunrise-1100 LOT) will more readily
produce larger amounts of N0~.
Photochemical synthesis of NO^ occurs throughtout the year. Monitoring data
described in Chapter 4.0 suggest that the severity of wintertime and summertime
NO,, peaks are comparable. One difference between seasonal N02 peaks attributable
to synthesis is that during winter peak NOp concentrations may occur later in the
day. This i;s dye to the -reduced sunlight intensity during the winter months and
the resulting- slower production of organic radicals causing conversion of NO to
N02.
Figgre 3., 1 provides an illustration of summertime photochemical synthesis.
The data shown in Figure 3.1 were observed by averaging pollutant concentrations
at four sites in the St. Louis RAPS on October 1, 1976, Typically, photochemical
synthesis begins shortly after sunrise. The morning traffic peak combines with
11
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°3
Titration
0..
4
S.R.
10 12 14
Time ol Day (C.S.T.)
Figure 3.1 Illustration of Photochemical and Titration Formation of
1C
19
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adverse meteorological conditions to produce the high concentrations of pre-
cursors (NMOC and NO) needed to sustain the production of high concentrations of
N0?. Later, emissions are substantially reduced and/or meteorological conditions
improve (note the diurnal carbon monoxide pattern roughly corresponds to the
decrease in NO concentrations).
/\
A review of diurnal patterns of N02 and ozone using 1975-77 data for California
indicates that photochemical synthesis accounts for 56 percent of the 5400 site
g
hours with N02 concentrations reported in excess of 0.20 ppm. A similar review
of data was undertaken for the rest of the United States. The results indicate
that about 32 percent of the occasions where NO^ exceeded 0.20 ppm can be unambi-
guously attributed to photochemical synthesis.
The physical location of a site which primarly experiences high N02 concen-
trations, due to photochemical synthesis, is one within several (e.g., 1-4 hours)
hours travel time of where heavy NO and VOC emissions occur. These areas are
A
usually located in urbanized or larger industrialized areas where primary emissions
of NOY and VOC are both plentiful.
/\
3.2 Titratjon
Tvtration is defined as the reaction between ozone and nitric oxide.
NO + 03 * N.02 + 02 (3.1)
This reaction can occur at any time of the day, since it is not dependent on the
presence of sunlight. In reviewing diurnal patterns of N0? and associated pol-
l.uta.nts, however, the most marked impact of titration in contributing to high
levels of N02 pccurs in the late afternoon or early evening. Ozone formed in the
late morning or early afternoon, mixes with the fresh afternoon NO emissions to
13
-------
form NOp. This reaction occurs very rapidly "in the atmosphere, unlike the photo-
cheiiiical'ly driven synthesis which proceeds more slowly.
The spee'd 'of tthe "tftrat'ion reaction is clearly il'Tustrated !t>y
measurements collected near a section of the San -Diego Freeway in Los Angelas.
In this study, NO monitors were deployed upwind and at varing "intervals downwind
A
from the roadway. Indications are that the NO in the roadway plume is converted
by the 0, in dilution air.. Th'-is was observed-within 1'0's of feet from the -road-
way, indicating a conversion'time of-a few minutes or less.
A sample diurnal concentration profile illustrating titration 'is presented
in the right-hand portion of Fi-gure 3.1 (page 12'). Note that'the amount'of NOp
produced after 150.0 is not greatly different "than the amount of ozone present at
1400 hours. As Equation 3.1 states, one part NO plus • one part 0.,-yields one part
NOx. Howeyer, when analyzing.ambient data, other processes are taking place
simultaneously. For -example-, direct-em'ts-s'ion's of NOp, some 'photochemical .production
of NOp still occurring, and a smoothing effect of a one-hour ^averaging time may
lead to varied amounts of NOp being observed while CL is "declining. Consequently,
even when titration is the predominant means by which NOp is formed, an observed
increase in NOp may not coincide precisely with an observed decrease in'XL.
The amount of NOp formed through the titration reaction is limited either by
the available NO or by the available 0~, It is important to be able to identify
which of these two factors limits the NOp, because "of the underlying implications
on what would constitute an effective control strategy. Clues as to which factor
is limiting may be obtained by examining diurnal, concentration profiles of N0£,
NO and 0.,. Such an examination leads to one of two possible outcomes: (1) the
14
-------
amount of N02 produced is less than the amount of ozone observed, or (2) the
ozone is depleted and excess HO is present. Either case has the potential to
produce more N0?. The first case is termed "NO limiting." This is where NO
t. A
emissions (primarily NO) are not sufficient to mix with the ozone present, thereby
limiting the amount of N02 produced. The second case is called "ozone limiting,"
and occurs if insufficient ozone is present to convert available NO to N02.
Figures 3.2 and 3.3 are hypothetical diurnal patterns of 03 limiting and NO
limiting cases of titration respectively.
In the case where titration formation of N02 is limited by available ozone,
0, control programs should be effective in reducing peak NOp- In such a case, a
reduction in ambient ozone levels should result in a corresponding reduction in
N02. On the other hand, if the amount of N02 formed is limited by the availability
of NO , programs to reduce ozone will affect ambient N0? resulting from titration
X £
to a lesser extent until the 0., is reduced to a level where its availability limits
the amount of NO converted to N02.
Data bases in California and in the remainder of the nation have been
reviewed to ascertain the frequency with which titration reactions appear to be
the predominant cause of peak observed N02. In California, diurnal NO, N02
and ozone concentration patterns suggest that titration was primarily responsible
for 40 percent of the hourly N02 values in excess of 0.20 ppm. The more limited
data base in the remainder of the nation however indicates that in only about
8 percent of the instances in which N02 is in excess of 0.20 ppm does afternoon
titration appear to be a principal contribution to the high observed N02- An
indication as to what type of site might experience afternoon reaction of 03 and
MO is also available from the California data analysis. In the Los Angeles Air
15
-------
a
a.
c~
o
to .10 —
.05 —
12 1 2 3 4
567
Sunrise
8 9 10
11 12
Noon
8 9 10 -11 12
Time of Day
Fifiuro 3.2 O/one Liniitinq Case of Titration.
-------
I r I~T:
9 10 11 12 1 23
Timf! of Day
Figurn3.3 NO Limitinq Casnnf Titrntinn.
-------
Basin, sites which are usually impacted by transported 0_ frequently experience
peak late afternoon N02 peaks in which titration is an important factor. Presence
of transported.ozone alone is not sufficient however. It is also necessary that
the monitor be located near large sources of NO .
3.3 Carryover
Carryover is a condition in which relatively high NCL concentrations persist
for several hours (e.g., overnight). Carryover can occur at any time of day.> If
it persists throughout the night, it is termed "nighttime" carryover. If it
occurs during the late morning to early afternoon, it is termed "afternoon"
carryover. Carryover, whenever it occurs, is not a formation process. Rather,
it serves as a base upon which other mechanisms add additional NO^. Examples of
afternoon and nighttime carryover are illustrated in Figure 3.4. Note that in
the example shown the carryover in the afternoon is carryover upon which Cu-
limited titration builds. The carryover which occurs in the early morning is
nighttime carryover upon which photochemically produced N0~ subsequently builds.
Presence of carryover NOp as a base upon which NCL peaks formed by synthesis
or titration can build has important implications concerning effectiveness of
reducing NO emissions. It means that it is most likely that an N0~ peak is the
A £
result of both synthesis and titration. Thus, even when diurnal concentration
patterns suggest that a peak is primarily the result of ozone-limiting titration.,
reduction of NO emissions should reduce observed peak N09. The reduction in N09
x c f c
would occur as a result of the reduction of the base (i.e., carryover N0~) upon
which the ozone-limiting titration peak builds.
18
-------
12 1 2 3 4 5 ' 6 7
9 10 11 12
Time of Day
Figure 3.4 Diurnal Concentration Profiles Illustrating Carryover.
-------
There is no single explanation for carryover. In some cases it may result
from N0? emitted or produced upwind and transported to the monitoring site. Use
of the term "transport" in this context implies a movement of an urban plume of
N0? less than a few tens of kilometers. In other cases^ carryover NQp is locally
generated and then trapped in a stagnant or very slow moving air mass. A sea
breeze may also cause a recycling of an air mass, thereby causing the persistance
of high N0~ concentrations. The underlying meteorological basis for carryover is
that poor dispersion will not allow rapid dilution of NOp to occur. This results
in N0? remaining high for several hours.
3.4 Implications
1. Since N0~ is primarily a secondary pollutant, understanding the means
by which it is formed is important in formulating effective control strategies.
Two formation mechanisms have been identified: photochemical synthesis and the
titration reaction with ambient ozone. A third phenomenon, "carryover," results
from the persistence of high N0? over several hours. However, NOp which is
"carried over" originates from either synthesis, titration or both.
2. If synthesis is identified as the major cause of observed violations,
control of NO emissions should be effective in reducing ambient N09. This
X c.
subject will be discussed further in Chapter 6.0. If titration is the prime
cause for high N0?, efforts need to be made to determine whether NO or ozone
£ A
is the limiting factor. If the limiting factor is NO , reduction of NO emissions
X X
should prove effective in reducing ambient NO-. If ambient N02 is limited by the
amount of available ozone, reducing NO emissions should nevertheless be effective,
/\
because the carryover N02 upon which a peak attributable to ozone-limited titration
builds, should be reduced. Only in a rare case where diurnal curves suggest
20
-------
ozone limited titration will little or no carryover would control of NO emissions
A
be relatively ineffective. Even in this unusual case, some reduction of N0~
would result from NO controls by virtue of some of the NO being emitted as N09
A A C-
as a result of instack oxidation reactions.
3. Reviewing the diurnal sequence of NO,,, NO and 03 concentrations is a
useful approach for estimating whether an observed N0? peak concentration is
associated with synthesis or titration. Such information can be effectively
displayed using the graphical procedures illustrated in this chapter. These
graphs may also be useful in detecting the presence of carryover assessing whether
N09 peaks are formed primarily as the result of titration is limited by available
NO or by available ozone.
J\
4. Although the graphical techniques described herein are useful in
identifying N0? formation processes, frequently the information they yield may be
ambiguous. Information on monitor siting, together with the knowledge that the
titration reaction is faster than photochemical synthesis of N02, may help resolve
some ambiquities arising from diurnal concentration profiles. If a monitor is in
close proximity to a major source of NO (e.g., within a few hundred meters of a
A>
major highway or in position to be directly impacted by a point source plume),
titration may be the predominant explanation of high observed N09. If a monitor
is located downwind (e.g., 1/2-4 hours travel time) from major sources of NO and
A
volatile organic compounds (VQC)_, photochemical synthesis may be important in
leading to high concentrations of NO.
21
-------
4.0 MONITORING, DATA QUALITY AND THE OBSERVED EXTENT OF HIGH N02 CONCENTRATIONS
In Chapters 2.0 and 3.0, NO emissions and processes by which these emissions
X
may lead to high concentrations of M02 were discussed. This information serves
as important input in the design of appropriate control strategies. There is an
even more fundamental question, however. How does one know whether controls are
required in the first place? The need for control programs for N0? is largely
determined by reviewing ambient air quality data. Not only that, but the extent
of required emission controls may depend on these data as well. Consequently,
the importance of obtaining high quality data which are representative of what
one is trying to measure cannot be overemphasized. Issues pertaining to data
quality can be subdivided as follows: (1) use of appropriate instrumentation,
siting criteria and operating procedures, and (2) review of recorded data to
check for spurious values.
4.1 Network Design and Operation
Regulations and guidance concerning appropriate instrumentation and operating
12 13
principles is provided elsewhere. ' State and Local Air Monitoring Stations
(SLAMS) exist to meet the data needs of individual State/local agencies. As
such, it is the SLAMS network which must be responsive to the needs for identi-
fying the extent to which a particular city is out of compliance with existing
NAAQS and the extent to which particular sources of NO may deteriorate ambient
X
air quality.
Information and references cited in Chapter 3.0 can be useful in designing a
SLAMS network which is likely to detect high N02 concentrations. Recall from
Section 3.1 that a review of California data suggests that highest N0_ concentra-
tions attributable to synthesis occur within or immediately downwind of areas
22
-------
with greatest NO and VOC emissions. Interpretation of diurnal NCL patterns
A ^*
typifying synthesis suggests that the maximum impact on ambient NCL occurs in the
mid-morning to early afternoon within about 1-4 hours travel time of maximum
emissions. Thus, the location of maximum NCU attributable to synthesis depends
on the prevailing winds. In order to cover both the case of atmospheric stag-
nation and the case of light, but identifiable, resultant winds, it is recommended
that stations be established within the downtown area and within 15 km of the
downtown area in directions of commonly occurring winds.
Because of the need to obtain representative data in making attainment/
nonattainment decisions and in designing control strategies, existing siting
criteria should be adhered to. The salient features of this guidance are:
1) Inlet probe height located 3-15 meters above ground and at least
one meter away from supporting structures.
2) There must be unrestricted air flow in at least three of the four
cardinal wind directions. It is not desirable to locate an instrument within
20 meters of trees nor is it appropriate to position an inlet probe along a
vertical wall.
3) Monitors should not be located within 20 meters of any road nor
within 250 meters of any road with average daily traffic (ADT) in excess of
10,000 vehicles per day. For roads with ADT of 1000-10,000 vehicles/day, the
monitor should be located no closer than 20-250 meters away. The exact limita-
tion in this latter case is determined by linear interpolation based on traffic
flow. For example, if the ADT of a nearby road is 5000 vehicle/day, the moni-
toring probe should not be within 103 meters of the road.
23
-------
If the purpose of a monitoring station is to document the impact of a
particular -source of emissions, siting specifications for the station may be
somewhat different. If a source is a primary source of NOp and its emissions
occur near ground level, station location should be as near to the source as one
could reasonably expect the public to be for a period consistent with that specified
in a NAAQS. The issue of what is a "reasonable" monitoring location for such
purposes is not a technical issue, but a political and philosophical one. As
such, it is outside the scope of this document. If a source's NO emissions
/\
occur primarily as NO, its maximum impact will most likely arise as a result of
titration with ambient ozone. As discussed in Section 3.2, titration of an NO
/\
plume occurs very rapidly upon mixing with dilution air. Hence, siting guidance
should be consistent with that for a primary source of NOp described above.
The problem of monitor siting for estimating the impact of a large, elevated
point source of NO is difficult to resolve. Complications are introduced as a
A
result of several factors:
1) the probability of observing the impact of a source at any particular
monitoring station at a given time is small;
2) there is a tradeoff which occurs between dilution and chemistry
within a plume. If the plume does not undergo substantial dilution, conversion
of NO to NOp via the titration reaction may be suppressed. Conversely, if ample
dilution occurs, significant conversion of NO to NO^ may occur, but it may be
difficult to distinguish between the impact of the plume and that of background
air.
24
-------
The implications introduced by the two preceding observations will be
discussed at greater length in Chapters 7.0 and 8.0. Maximum impact from an
elevated plume results from a complex combination of meteorology, source operating
characteristics and terrain. Deployment of a monitoring network to estimate
maximum impact from an elevated source of NOV is best done on a case-by-case
X
basis. However, generally appropriate monitoring guidance is to orient stations
along the wind axis associated with highest ozone concentrations, paying particular
attention to intermediate distances (e.g., 2-8 km for power plants). At those
intermediate distances, dispersion is sufficient to bring an elevated plume to
the earth's surface and to mix the plume's NO with ambient ozone. At greater
distances, dilution may be expected to diminish concentrations. Even if these
procedures are followed, a network may sometimes have difficulties detecting the
peak impact of a source.. Thus, modeling the impact of an elevated point source
on ambient N0? may sometimes provide a better indication of such a source's
maximum impact than data from available monitoring networks.
4.2 Data Quality Assurance Techniques
Even assuming monitoring instruments are properly deployed and operated,
there is a need to screen the resulting data for spurious values. As many as
three types of screening procedures could occur. First, observed concentrations
of NOp should be compared with existing screened data from other sites. If
values higher than NOo peaks typically reported elsewhere are observed, grounds
for closer scrutiny exist. A second screening test which should be routinely
applied is the use of computerized statistical procedures to identify peculiar
gaps or patterns (e.g., spikes) in the data. These procedures are easy to apply
and have been described at length elsewhere. In the event the first two stages
25
-------
fail to ."esolve whether a questionable data point is or is not valid, procedures
15
described by Richter et. al can be used to provide a more rigorous test. These
latter .procedures require one to plot diurnal patterns of N02, NO, ozone and, if
available, carbon monoxide (CO), and sulfur dioxide (S02) concentrations observed
at a site. Examination of three consecutive days (i.e., bracketing the day
recording the suspicious data) should allow the observer to identify errors which
otherwise would be difficult to detect. Examples of such errors might include
miscoding of N02 as NOX or apparent drifts in the baseline data recorded by an
instrument. The exact procedures are described in detail elsewhere.
4.3 NOp Concentrations Observed in Excess of 0.20 ppm
The first data quality check recommended in Section 4.2 is to note whether a
high recorded data point is consistent with previously observed data. To facilitate
such a comparison, this section summarizes findings of a review of ambient hourly
N02 concentrations of 0.20 ppm or greater which were observed from 1975-1977
inclusive. During these three years, the vast majority of hourly N02 observations
were made within the State of California.
During 1975-77, 161 hourly N0? concentrations of 0.20 ppm or more were
observed from approximately 40 different sites outside of California. All such
values were less than 0.50 ppm, with values in excess of 0.40 ppm being extremely
rare. Figure 4.1 illustrates that, although high M02 concentrations appear to
occur more frequently in summertime, a significant number occur during the other
seasons as well.
At any given location, diurnal patterns of N02 reflect monitor location, as
well as meteorology and source configuration. Figure 4.2 represents a composite
26
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50-
'i'
O-
cx 60
++* H \J ^m
O
CM
O
A|
C
•H
2 30~
4-1
C
01
U
C
O
U
OJ
z 20-
^
3
O
1
CO
ccT 10.
O
O
01
"e
3
•z
37
W
I
N
I
E
R
Nov
Dec
Jan
40
S
P
R
I
N
G
Feb
Mar
Apr
45
S
U
M
M
E
R
May
June
July
_
Fall
A
U
T
U
M
N
Aug
Sept
Oct
SEASONS
Figure 4.1 Seasonal variation in hourly N02 levels exceeding 0.20 ppm at
all non-California stations reporting to SAROAD during 1975-77.
27
-------
ro
oo
p.
o.
o
CM
d 20-
A|
CO
o
o
•H
oncentrat
en
CM
0
:z
M
3
E§ 10
1
,-H
1
1
1
14-1 b
O
cn
s
fc
M
>-i
sj Q-,
CJ
9
•
3
0
2
1
2
2
^n n~
345
3
6
10
7
R
8
17
9
13
10
13
.11
1?
12
3
13
3
14
5
15
5"
16
3
17
n
18
8
19
\
12
20
7
21
11
22
L
6
23
HOUR OF DAY
Figure 4.2 Frequency of tn'aximum 1-hour N02 concentrations >^ Q.20 ppm.
-------
of all monitor stations, outside of California, in which frequency of observing
high N02 is plotted against time of day. It is clear that high NO,, has been
observed at all times. However, the distribution in Figure 4.2 has a distinct
bimodal pattern, with a primary peak between 0700-1200 and a secondary peak
between 1800-2300.
In contrast to the ri02 data base outside of California, the data from that
State are extensive. Further, screening procedures similar to those described in
o
Section 4.2 suggest that the data are of high quality. The California data base
examined for 1975-77 consists of data from 54 stations. Approximately 50 of
these stations were found to have at least one valid observation of hourly NOp
concentrations >_ 0.20 ppm. Approximately half (26) of the stations are located
in the Los Angeles area (Los Angeles, Orange, Riverside and San Bernardino Counties),
with nine stations in the San Francisco Bay and six stations in the San Diego
areas. However, more than 90 percent of the NO values 0.20 ppm or greater occur
in the Los Angeles area. Hence, the California data which are of greatest interest
are heavily weighted by observations in the Los Angeles area.
Table 4.1 presents the distribution of N02 values 21 0.20 ppm observed in
California during 1975-77. The values in Table 4.1 have been adjusted downward
by 13 percent to account for a 10-17 percent reduction which needs to be made to
9
the raw data. The adjustment is necessitated as a result of revisions needed in
the calibration procedure employed during 1975-77 in California. Hence, the data
in Table 4.1 should be viewed as only approximately correct. Nevertheless, they
are adequate to provide a basis for screening questionable data, as described in
Section 4.2. Highlighting some of the information in Table 4.1, one sees that a
full 50 percent of the values reported to be 0.20 ppm or greater are 0.23 ppm or
29
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Table 4.1. Distribution of N02 Concentration >_ 0.20 ppm for all California Monitoring Stations, 1975-1977*
co
o
Concentration
(ppm)t
.20
.21
.22
.23
.24
.25
.26
.27
.28
.29
.30
.31
.32
.33
.34
.35
.36
Number, of
Hourst
631
604
448
628
252
156
169
92
104
81
119
53
40
36
32
25
15
Cumulative
Percentage
of rdgs
> .20 ppm
17.6
34.5
47.0
64.6
71.6
76.0
80.7
83.3
86.2
88.4
91.8
93.2
94.4
95.4
96.3
97.0
97.4
Approximate
Cumulative
Percentage
of all hours1
99.72
99.92
99.96
99.99
Concentration
(ppm)t
.37
.38
.39
.40
.41
.42
.43
.44
.45
.46
.47
.48
.49
.50
.51
.52
.53
.54
Number of
Hourst
31
9
11
10
6
7
2
5
2
2
2
1
2
1
0
2
0
1
Cumulative
Percentage
of rdgs
> .20 ppm
98.2
98.5
98.8
99.08
99.25
99.44
99.50
99.64
99.69
99.75
99.80
99.83
99.89
99.92
99.92
99.97
99.97
100.00
Approximate
Cumulative
Percentage
All Hours
>99.99
* Where simultaneous chemiluminescent and colorimetric measurements were made, only1
colorimetric data have been used in this table.
t Median of all concentrations :> 0.20 = 0.23 ppm
t. Total hours > 0.20 ppm = 3579
J All hours estimated by multiplying (54 stations)(3 years)(365 d/yr)(24 hr/d)(.75 assumed
fraction of reported values ^ 1,060,000.
-------
less. Second, only about 0.3 percent of the estimated total number of hourly N02
concentrations reported are 0.20 ppm or greater. Directing attention to the
upper end of the scale, less than 1 percent of the values reported as 0.20 ppm or
greater were _> 0.40 ppm. These high values constitute less than 0.01 percent of
the estii-nated total number of observations. Out of a sample of approximately
one million site-hours, there are only four hourly values reported as high as
0.50 ppn or greater.
Hence, both the California and non-California data suggest that hourly N02
concentrations of 0.40 ppm or more are likely to be extremely rare. There are
other similarities between the data bases as well. In California, as elsewhere,
NOp concentrations of 0.20 ppm or more occur throughout the year. However, in
California, values in excess of 0.20 ppm appear to occur most frequently during
the winter months. Similarly, high NOp concentrations can occur during any hour
of the day, but are most pronounced during 0600-1300 and 1400-2100 local time.
4.4 Implications
1. Proper design of a N0? monitoring network, use of reference or equivalent
measurement methods, and adherence to careful quality assurance procedures are
essential prerequisites to the design of appropriate control strategies. Published
guidance should be followed on the use of appropriate instrumentation and quality
assurance procedures. Network design needs to recognize the rapidity with
which N0_ is formed, source configuration and prevailing meteorology. Monitoring
stations in the following locations are suggested.
a) Stations should be located one-half to four hours travel time
downwind from the most intense area of VOC and NO emissions (e.g., 15 km) in
A
commonly prevailing daytime wind directions with light winds.
31
-------
b) At least one station should be located within the area of most
intense HOY and VOC emissions to account for stagnation conditions, possible
f\
carryover and high N02 due to titration. However, such station(s) should be
located at least 200 meters from major sources so that the station(s) is not
unduly impacted by nearby individual sources.
c) Efforts to assess the impact on N02 of a single source of NO using
monitoring data will most likely require a special purpose monitoring network.
Such a network is best designed on the basis of case-by-case modeling studies.
However, it is generally most appropriate to orient stations along the wind axis
most closely associated with highest ozone concentrations. Maximum concentrations
may generally be expected to occur at intermediate distances downwind (e.g., 2-
8 km).
2. A second essential prerequisite to designing an appropriate SIP control
strategies is to screen ambient data for erroneous values. Three screening
procedures have been identified:
a) Compare values with high N02 concentrations observed in the city or
in other similar cities elsewhere. If a value is outside the range of existing
observations, it should be subject to further scrutiny.
b) Employ routine computerized tests which have been designed to flag
questionable values.
c) Plot diurnal concentration patterns for N09, NO, 0, and, if possible,
L. O
CO and S0?. Curves should be plotted for 72 hour periods, centered on the day
with questionable N02 readings. The diurnal curves can then be used to identify
inconsistencies among pollutants.
32
-------
Because they are easy to apply, it is recommended that the first two procedures
be applied on a routine basis. The third procedure should be used to help resolve
uncertainties about individual data points which are raised by either of the
first two tests.
3. A review of ambient NO- data from 1975-1977 inclusive suggests the
following:
a) Ambient NCL concentrations as great as 0.40 ppm are extremely rare
indeed. Values in excess of 0.40 ppm should be given close scrutiny prior to
basing a control strategy upon them.
b) Hourly NOo concentrations in excess of 0.20 ppm may occur during
any time of the year. Hence, it is not appropriate to confine monitoring efforts
to certain seasons of the year.
33
-------
5.0 TRANSPORT OF N02
In discussing "transport" of a pollutant, it is important to identify the
geographic seale^of the transport. Transport of high concentrations of NOo,
unlike ozone, appears to be limited to perhaps 100-200 km, even under adverse
meteorological conditions. In support of this contention, Section 5.1 describes
N02 gradients observed in the vicinity of urban areas. Section 5.2 then summarizes
recent observations made in rural areas. Section 5.3 summarizes available informa-
tion on the chemical stability of NO (i.e., NO + N09) in the atmosphere and the
/\ £
nature of the reaction products. This latter information helps provide insight
into possible upper limits to the transport of high NO concentrations and into
/\
relationships with long range transport of ozone. Finally, Section 5.4 identifies
the implications of the foregoing information on the design of appropriate control
strategies to reduce unacceptably high levels of N02 and ozone.
5.1 Urban Gradients of NOo
The two urban areas with the most extensive available NOo monitoring data
within the U.S. are Los Angeles and St. Louis (as a result of the RAPS data
base). These two cities represent distinctly different examples for characterizing
urban N02 gradients. In the former case, the city is surrounded by many miles of
urban development. Further, the area is frequently characterized by unfavorable
meteorology (i.e., light winds and the presence of a subsidence inversion restricting
the extent of vertical mixing). Finally, the terrain in the Los Angeles Basin
frequently serves to channel the flow of pollutants to downwind areas. These
conditions combine to maximize NO^ levels, as well as the area over which high
N02 can occur.
34
-------
In contrast, St. Louis is surrounded by less extensive suburbs. Prevailing
meteorology is more favorable to the rapid dilution of pollutants, and there is
little channelization of windflow. Hence, a review of St. Louis data should
provide insight into the extent of transport from an urban area under more favor-
able (and, perhaps, more typical) conditions.
Table 5.1 presents maximum hourly NCL concentrations and the frequency with
which high (> 0.20 ppm) NO^ concentrations occurred at 23 monitoring sites in the
vicinity of Los Angeles during 1975-77. Figure 5.1 has grouped the data in
Table 5.1 into the following categories: observations made within 10 miles of
downtown Los Angeles; observations made 10-20 miles away; 30-50 miles away; 50-
70 miles away. It is clear that, despite the fact that N02 is a secondary pol-
lutant, the MOp problem is likely to be most severe in the vicinity of source-
intensive areas. For example, in Figure 5.1, the average number of hourly N02
concentrations in excess of 0.20 ppm diminishes from 439 at sites within 10 miles
of downtown Los Angeles to 17 at sites 50-70 miles from Los Angeles. Similar
reductions in maximum hourly N02 concentrations are experienced as well. In
Figure 5.1, for example, the typical maximum hourly N02 concentration of 0.50 ppm
observed within 10 miles of downtown Los Angeles is reduced to 0.27 ppm at sites
50-70 miles away.
It should be remembered that the Los Angeles area is one of sprawling urban
development. Hence, the frequency and magnitude of recorded high hourly concen-
trations of N02 are not entirely attributable to transport from the most source-
intensive area (50-70 miles upwind). Methods similar to those described in
Chapter 3.0 can sometimes be used to distinguish whether observed high hourly
concentrations of N02 are attributable to local emissions or to emissions from
35
-------
TABLE 5.1 High N02 Values Observed in the Vicinity of
Los Angeles, 1975-1977
Distance East
of Downtown
LA, (mi)
-12
-10
-5
-2
0
5
10
14
15
23
23
32
35
38
38
38
52
52
55
55
52
55
67
Site
LA Gault Street
LA Westwood
Lennox
Burbank
LA San Pedro
Lynwood
Pasadena
Temple City
Whittier
La Habra
Azusa
Pomona
Chino
Upland
Upland
Upland
Rubidoux
Rubidoux
Riverside
Riverside
Fontana
San Bernardino
Redlands
No. of
Hourly*
Rdgs > 0.20 ppm
Max. Cone.*
205
628
297
386
633
163
526
264
386
212
166
231
47
13
38
17
8
5
82
31
4
11
9
.35
.62
.40
.48
.62
.39
.46
.47
.62
.47
.38
.41
.38
.34
.30
.27
.28
.26
.35
.30
.24
.25
.29
* Numbers have not been adjusted downward to account for 10-17% over-
estimation resulting from calibration method used during 1975-77.
36
-------
— r>oo
O Maximum Hourly Concentration
A Number of Observations > 0.20 ppm
20 30 10
Miles East or West of Downtown Los Angeles
Fi(|iirf» 5.1 (ir;ulionts in Maximum Hourly MO^ Observed in tho Los Anrjolos Rnsin.
-------
cities which are a few tens of miles upwind. For example, in Figure 5.2, an N02
peak corresponds with increases in CO and 0, during the mid-afternoon, whereas NO
concentrations remain low throughout the day. The site (El Toro) is not one
which is characterized by high local emissions. The N02 peak* in this case is
apparently attributable to transport. In contrast, the N02 peaks illustrated for
October 2-4, 1975 in Riverside, California in Figure 5.3 suggest local formation
of NOp attributable to synthesis (morning) and titration (evening). In the
Los Angeles area data base, instances of local generation of M02 in outlying
areas appear to be much more common than instances of transport.
Because of the role of local NO and VOC emissions in the Los Angeles area,
/\
Figure 5.1 may imply that transport of NOp is more important than it actually is.
For this reason, it is informative to review N02 gradients observed in the St. Louis
area during the RAPS study. Table 5.2 and Figure 5.4 show that, as in Los Angeles,
maximum hourly N02 concentrations diminish with increasing distances from downtown
St. Louis. The difference is that the gradient is somewhat sharper (e.g., within
40 km, maximum, Cgg and C™ concentrations are less than half those observed
downtown). A second difference is that the NOp levels in and near St. Louis are
consistently lower than those in Los Angeles. This second difference is probably
attributable to more favorable meteorology and somewhat less extensive emissions
in the St. Louis area. . The Cgg and C,-Q values shown in Figure 5.4 approach
values which are considerably less than 0.10 ppm. Although maximum N02 concen-
trations 50 km from St. Louis are about 0.08 ppm, the infrequency with which
* The NOp concentrations shown in Figure 5.2 need to be reduced by 10-17 percent,
as a result of the calibration procedure utilized.
38
-------
0.8
0.7
0.6
0.5
0.4
.2
a
I
z
z
UJ
o
o
0 0.3
0.2
0.1
NO.
12
TIME - hours
16
20
24
Figure 5.2 Example of NOa transport at El Tore station on
January 25, 1975. CO has been scaled by 0.01.
.39
-------
:Q.<8
0.7
0.6
2
8: o.5
P 0.4
-------
TABLE 5.2 Spatial Distribution of Hourly N02 Concentrations in the
RAPS Network, 1976
Site
101
102
103
104
105
106
107
110
111
112
108
109
113
114
117
118
119
120
115
116
121
123
124
122
125
Distance
From Center
City (km)
0
1 10
1 10
1 10
1 10
1 10
1 10
£ 10
1 10
1 10
10-20
10-20
10-20
10-20
10-20
10-20
10-20
10-20
20-30
20-30
20-30
30-40
40-50
50-60
50-60
Valid
Hours
6147
6187
5115
5353
4554
5872
6164
7180
6446
6105
6322
6846
5292
7057
6348
6640
6189
6420
7048
6371
5998
6012
6481
6966
6527
Maximum
Cone . (ppm)
.256
.194
.163
.156
.186
.245
.191
.140
.223
.169
.155
.154
.266
.162
.082
.079
.192
.192
.090
.081
.168
.087
.082
.086
.067
C99
(ppm)
.08
.09
.12
.07
.07
.09
.08
.06
.07
.08
.06
.06
.09
.08
.05
.05
.06
.07
.05
.05
.07
.04
.04
.04
.03
CSQ
.026
.031
.020
.024
.026
.029
.029
.015
.022
.025
.014
.010
.022
.012
.008
.009
.015
.016
.009
.009
.008
.005
.003
.002
.001
41
-------
A Maximum Hourly Average
T Runiji; of Observations
{.]—
10
Disunite friini Centor City, km
Fiijiiit! b.4 (jiiiilicnl:, in IVIaxiiiiUnt TOO-, Coiiciiiiliiilions Uhsuivi;d Dtiiniii Hie Si. Lour.,
.1...
50
I
-------
transport appears to be the primary cause of high N02 in outlying areas in
Southern California suggests that it is unlikely that high N02 observed in a city
is significantly impacted by N02 transported from a city more than 50-70 miles
away. This hypothesis is further examined in Section 5.2 where N02 observations
made in rural and remote areas are presented.
5.2 Levels of NOp Reported in Rural Areas
Concentrations of NOo observed in several recent field studies are summarized
in Table 5.3. As with the data collected in outlying areas of St. Louis, ambient
N02 concentrations in rural areas are almost always (Cgg) very low. With rare
exceptions, even the maximum hourly N02 concentrations observed are less than
0.05 ppm.
Table 5.4 shows mean NO/NO^/NO concentrations observed in remote areas.
t A
The distinction between "remote" and "rural" is that a remote location is far
from populated areas. Hence, its pollutant levels are not expected to be influenced
3
by anthropogenic sources. A rural site, on the other hand, is one which is more
likely to be impacted by transport from urban areas. Rural sites may include
small communities which may generate limited amounts of pollutants, but no major
3
sources.
In comparing Table 5.4 with the C™ values in Table 5.3, one sees that there
are, most probably, measurable amounts of residual N02 present at rural sites.
However, these concentrations are usually an order of magnitude of more below
0.10 ppm.
43
-------
TABLE 5.3 ?NQ2 Observed at Rural Sites
Site
Kane, PA
Coshocton, OH
Lewisburg, W. VA
Sampling Period
June 26-September 30, 1973
June 26-September 30, 1973
June 26-September 30, 1973
Approximate
'Max. N02
Cone . (ppm)
.045
.060
.085
.020
.035
.030
.010
.015
.010
Wilmington, OH
McConnelsville, OH
Wooster, OH
McHenry, MD
Dubois, PA
June 14-August 31, 1974
June 14-August 31, 1974
June 14-August 31, 1974
June 14-August 31, 1974
June 14-August 31, 1974
.045
.035
.045
.030
.035
.035
.020
.035
.020
.020
.005
.005
.005
.005
.005
Bradford, PA
Creston, IA
DeRidder, LA
Wolf Point, MT
Simsbury, CT
June 27-September 30, 1975
June 27-September 30, 1975
June 27-October 31, 1975
June 27-September 30, 1975
July 15-August 21, 1975
.020
.010
.025
.025
.023
.015
.010
.010
.010
NA
.005
.005
.005
ND
NA
Montague, MA*
Duncum Falls, OH*
August 1-December 31, 1977
.August 1-December 31, 1977
.073
.043
.016
.018
.007
.008
Giles County, TN*
Lewisburg, W. VA
August 1-December 31, 1977
August 1-December 31, 1977
.055
.028
.024
.009
.011
.004
t Mean daily maximum, approximately equal to Cgg for the sampling period shown
++ Mean rather than C50
ND Not Detectable
NA Not Available
* SURE data provided courtesy of EPRI
44
-------
TABLE 5.4 Concentration of Nitrogen Oxides Observed in Clean Remote Areas
Location
La ramie, Wyoming
Fritz Peak,
Colorado
Fritz Peak,
Colorado
Northern
Michigan
Tropical Areas
Ireland
Concentration (ppb)
:»
X
O.I - 0.4
0.2 - 0.5
0.3 - 0.5
NO
0.01 - 0.05
0.1 - 0.5
0.3 - 0.4
0.3 - 0.7
0.3 - 0.7
SO. 2
N02
<0.1
0.2 - 0.4
0.3 - 0.5
0.3 - 0.6
0.3 - 0.5
0.2 - 1.5
Measurement
Method
Chemi luminescent
Absorption
spectroscopy
Chemi luminescent
Chemi luminescent
Saltzmnn
Chemi luminescent
Date of
Measurement
Summer 1975
Fall 1974;
Summer-
Spring,
1975-1976
Sep 1977
Jun 1977
1965-1966
Jul-Nov
1974
c • — -
Reference
Drummond (1977)
Noxon
(1975, 1978)
Ritter et al.
(1978)
Ritter et al.
(1978)
Lodge and Pate
(1966);
Lodge et al.
(1974)
Lodge and Pate
(1966);
Lodge et al.
(1974)
Lodge and Pate
(1966);
Lodge et al.
(1974)
Lodge and Pate
(1966);
Lodge et al.
(1974)
Cox (1977)
Remarks
10-day average
Under forest
canopy
Above forest
canopy
River bank
Seashore
and maritime
Maritime. Maximum
hourly averages
ranged from 0. 3
to 5.0 ppb.
-P.
en
-------
§,.,3 CMffistipy of;
---
preaejitedi m the two previous sections indicates that, only very
of NsEk/NQ- are 1 ikely to. be transported over
£- J\
distances. In this section, observations concerning chemistry of NO as it is
/\
transported are briefly described. This information is pertinent for two reasons:
1) it provides an upper limit estimate of how far high concentrations;
of NOo may be transported under certain conditions;
2) it brings, into better focus the interrelationships, between trans-
ported NO and transported ozone.
In discussing chemical stability, it is more appropriate to consider stability of
NO (NO + N02) rather than N02. The reason for this is that within a weTT mixed
urban Djlume subject to photochemical reactions., NO is rapidly converted to N02
and vice versa.. Stability of NOY, however, allows consideration to be given not
J\
only to N02,, but to NO available to rapidly replenish any lost N02.
Recently, a number of efforts have been made- to estimate the chemical
stability of ambient NOV and to determine its reaction products.16' 17»18» 3
X
Measurements made downwind of Los Angeles have been used to infer NOV decay rates
X
of 2-16 percent/hour, with typical conversion rates being about 10 percent/hour
or slightly less. However, it has been hypothesized that such estimates may
underestimate the conversion rate of NO . This, hypothesis follows from the
A
manner in which the conversion rate estimates are made and from the fact that
there is a continuing infusion of fresh NO from cities downwind of Los Angeles.
The hypothesis that a 10 percent/hour NO conversion rate may be too low has been
A
tested in two sets of experiments. The first set was made downwind of Phoenixr a.
46,
-------
city with relatively little surrounding development. Surprisingly, NO conversion
/\
18
rates of only about 5 percent/hour were observed in these experiments. More
recent experiments conducted in the Boston plume, as it is transported over the
ocean, have indicated NO conversion rates of 14-24 percent/hour, which average
A
18
about 17 percent/hour. Corresponding lifetimes for NO are shown in Table 5.5.
/\
All of the chemical lifetimes in Table 5.5 are based on experimental data
obtained on sunny days in photochemically active urban plumes. It is not possible
to draw definitive conclusions about NO lifetimes at night, outside of urban
X
plumes or under other meteorological conditions, except to say chemical lifetimes
of NO may be somewhat longer under conditions which are not conducive to photo-
A
chemical reactions. However, data presented in Chapters 3.0 and 4.0 imply that
N02 is at its highest within urban plumes and in sunny weather. This allows for
some speculation about maximum distances for transport of high NO. concentrations.
If one is willing to assume that daytime NO conversion rate is at least 5 percent/
/\
hour and most probably greater than 10 percent/hour, maximum travel distance of
significant N02 is probably limited to 100-300 miles.* This estimate represents
a maximum one, because the impact of atmospheric dilution is not taken into
account. It is clear from the minute concentrations of NO reaction products
/\
(mostly PAN and nitric acid) that dilution is the primary means by which ambient
NO is reduced. In addition, the information presented in Sections 5.1 and 5.2
J\
also support the assertion that dilution is the most important factor causing the
distinct differences observed between N02 concentrations in urban and rural
areas.
* This estimate was arrived at by assuming a resultant wind velocity of 10 mph
prevailing throughout the chemical lifetime of NO as reported in Table 5.5.
Nighttime decay rate is assumed to be 0 percent/hour in these calculations.
47
-------
TABLES,. 5 'Chemical StattyiTfty of ?NQ Corresponding to Observed
Gonversafcram
Conversion Rate
(%/hrV
,2
5
10
'14
17
24
Half Life*
(hr.)
35
13
6
5
3.7
2.5
'Lifetime*
(hr.)
-50
1-9
9
7
5.3
3.6
'* .Lif-etatme -Is commonly .as-sumeS 'to be rtfhe 'time required for
-a .pollutant :to '.decay -to V/e or 'to 37% of its original concentration
Time required -for a pollutant to '.decay to 50% of its original
concentration
48
-------
A discussion of transport of NO is not complete without mention of the
X
interrelationships with transport of ozone. There are really two issues which
need airing:
1) What is the impact of ozone, transported from upwind areas, on N0_?
2) What is the impact of transported NO on downwind concentrations of
ozone?
The impact of transported ozone on NOp can be seen by referring once again
to Figure 5.3 (page 40). The dashed lines in the upper set of panels in this
figure indicate that, during the period in question, ozone concentrations on
October 1 and 4 reached Riverside, California from points further west about
4 p.m. This ozone then reacts with local NO emissions resulting in N02 concen-
trations approaching 0.20 ppm in the early evening. This, of course, is an
example of the titration phenomenon discussed in Chapter 3.0. Control programs
resulting in the reduction of transported ozone should therefore reduce peak N02
concentrations resulting from titration of NO with ozone when ozone is the limiting
factor.
The impact of transported NO on downwind ozone is a difficult issue.
/\
Formation of ozone from NO and NMOC is not the result of a chain-ending reaction
A
(i.e., in addition to ozone, NO and additional organic radicals are also formed
/\
as by-products which, in turn, can recombine several more times to form more
ozone). Therefore, concentrations of NO well below 0.10 ppm are sufficient to
/\
sustain synthesis of ozone in excess of the NAAQS for ozone. This observation,
in turn, leads to the following question: should NOX emissions be controlled
even though they pose no threat in terms of unacceptably high concentrations of
N02?
49
-------
At this point, control agencies are confronted with an as-yet-to-be-resolved
dilemma. Model simulations, as well as ambient data, show that the most immediate
effect of NO emissions is to suppress ambient levels oT ozone wi'thrn "the vicinity
of the emissions. Since areas with greatest emissions are usually those in which
the greatest number of people reside, there is a concern that aggressive programs
to curtail NO emissions in such areas may increase population exposure to high
A
concentrations of ozone. Counterbalancing these arguments are those (briefly
alluded to above) which suggest that the availability of NO may aggravate highest
A
ozone concentrations in downwind areas and that reduction of available NO in
• A
rural areas may be needed to curb pervasive ozone problems reported in the north-
eastern U.S. and elsewhere. There is some support for the argument that NO
A
emissions aggravate peak ozone concentrations downwind which can be found in the
19
Los Angeles trend data. Arguments have also been made that NO should also be
A
controlled in order to reduce other oxidants as well (e.g., PAN).
Both the arguments for and against aggressive controls of NO emissions
A
appear plausible. Unfortunately, the state-of-the-art has not sufficiently
advanced to allow quantitative tradeoffs to be made. Indeed, much of the research
cited in this section, as well as other work, has been conducted for the purpose
of developing information needed to make these quantitative tradeoffs. It is not
likely, however, that the problem will be satisfactorily resolved within an
immediate time frame. Therefore, the foregoing information has been presented to
alert control officials to the possibility that more stringent NO controls than
A
those implied by any hypothetical NAAQS for NOo may be needed.
50
-------
5.4 Implications
1. Despite the fact that N02 is a secondary pollutant, it does not appear
that high concentrations are transported over great distances. Highest concen-
trations of N02 occur near areas with high NO emission density. While it is
possible for high NO to persist throughout the night (i.e., the carryover phenomenon),
comparison of urban, suburban and rural data implies that such persistence results
from poor dispersion in the vicinity of source-intensive areas, rather than as a
result of transport from afar.
2. Chemical decay rates of NO (i.e., NO + NO ) serve to provide an upper
A L-
limit estimate for transport of appreciable concentrations of N02. Although
there is some spread in the data, decay rates of about 10 percent per hour appear
to be representative of daytime observations within an urban plume. Assuming a
resultant wind speed of 10 mph, an upper limit for transport of high concentrations
of MOY of about 100-300 miles is estimated.
/\
3. Available urban concentration gradient data, as well as rural data
however, suggest that transport of N02 approaching 0.10 ppm is, in fact, limited
by atmospheric dilution. In the RAPS network, for example, peak hourly N02 is
reduced from .25 ppm to .08 ppm within 50 km of downtown. Rural data indicate that
N02 concentrations seldom exceed .03 ppm. Therefore, if one is primarily concerned
with an hourly N02 concentration of 0.20 ppm or more, impact of long range trans-
port of NO can ordinarily be ignored in the design of control strategies for
4. Use of graphs plotting diurnal concentrations of NO., NO, 03, CO and
S0? may be useful in identifying whether an observed high concentration of NO.
a downwind area is due to transport of N02- Using such a procedure, high N02
51
-------
observed in areas downwind of Los Angeles were frequently found to result primarily
from the titration of locally generated NO with ozone which was transported from
t*pv««d Siau-rees. Heacfr, programs to reduce transported ozone nay also reduce peak
NO- generated in downwind areas under some conditions.
5. Principal reaction products of NO are PAN and nitric acid. These
A
pollutants exist in minute quantities in the atmosphere. Under some conditions,
PAN can regenerate NOp. In addition, comparison of ruraT with remote data
indicates that low concentrations of residual NOo can survive over considerable
distances. Unfortunately, it only requires very small quantities of NOV (well
A
below 0.10 ppm) in the presence of NMOC to sustain ozone concentrations above the
NAAQS for ozone. This suggests that more stringent requirements for controlling
NO emissions may be needed to meet the ozone NAAQS than to avoid unhealthy
A
exposures to MO-.. Unfortunately, quantitative tradeoffs between possible increases
in ozone levels in the vicinity of sources of NO versus possible decreases in
ozone levels observed downwind of sources of NO cannot be made at the present
time. While information is being obtained to develop better tools to quantify
the impact of NO controls on ozone levels in rural areas, it is probably a good
A
idea to minimize any increases in NO emissions as a part of ozone SIP's.
A
52
-------
6.0 RELATIONSHIPS BETWEEN N02 AND ITS PRECURSORS
Previous chapters have established that ambient N0_ is primarily a "secondary"
pollutant. It has also been noted that nitric oxide (NO), non-methane organic
compounds (NMOC),* and ozone (03) play important roles in the formation of N02 in
ambient air. The purpose of this chapter is to discuss relationships between NO
and these other pollutants in greater detail. Three means have been used to
examine relationships between N02 and its precursors: (1) smog chamber experi-
ments, (2) empirical models, and (3) chemical kinetics models. Section 6.1
discusses the results obtained using each of these three approaches. Section 6.2
then summarizes the implications of these results on the design of control strategies
to reduce ambient levels of NOp.
6.1 Relations Among NOp and Precursors
Although it is obvious why peak and average N0? could be sensitive to NO
C- A
levels, it may not be instinctively clear why N02 should be affected by ambient
levels of NMOC. Reducing ambient NMOC reduces the concentration of organic
radicals which serve to oxidize NO to NO . The resulting lower concentrations of
organic pollutants therefore delays the formation of N02. Such a delay allows
greater dilution of NO to occur prior to oxidation. The end result is lower con-
centrations of NO when the NO finally does get oxidized.
* In the discussion in Chapter 6.0, the term NMOC is used rather than VOC
because the smog chamber, empirical and kinetics model evidence all deal with
ambient levels of pollutants rather than emissions. The terms HC and NMHC
(non-methane hydrocarbon) are sometimes used because they are used in the
primary references. In this discussion, NMOC, HC and NMHC may be assumed to be
synonymous.
53
-------
6.1.1 Smog Chamber Experiments
A smog chamber is a transparent container into which known concentrations
of organic pollutants (NMOC) and NO are injected. The resulting mixture is then
/\
irradiated with ultraviolet light (using either sunlight or an artificial source).
The resulting concentrations of N(L and other secondary pollutants can then be
observed. By varying the initial concentration of NO or NMOC and keeping every-
A,
thing else constant, the sensitivity of maximum hourly NO concentrations to
changes in these precursors can be ascertained.
The results of six sets of smog chamber experiements have been recently
20
summarized by Trijonis. Figures 6.1 - 6.4 summarize the dependence of peak NO
on initial NO concentrations in these experiments. As shown in Figures 6.1 -
A
6.4, despite the wide variety of experimental conditions, the relationship between
peak N09 and NO can be approximated by a straight line (i.e., is linear). The
C. . X
slope of the line is generally slightly less than 1:1. Similar relationships
20
were found between initial NO and mean N09.
/\ C—
Smog chamber data are more ambiguous with regard to the dependence of NOp on
initial NMOC concentrations. Most of the data indicate that, within the range of
observations encompassed by the experiment, there is a weak relationship between
peak N02 and initial NMOC concentrations. For example, the experiments cited
indicate that reducing NMOC by 50 percent results in a reduction in peak NOp
ranging from 0-25 percent. The experimental evidence reported is inconclusive
regarding the direction of change in mean N02 corresponding with a reduction in
NMOC. The results of the smog chamber studies are summarized in Table 6.1.
54
-------
a
x
re
.10
.30 .40 .50
Initial Nitrogen Oxides, ppm
Figure 6.1 Nitrogen Dioxide Maximum Concentration vs. Initial Oxides of Nitrogen
(means of several experiments) UNC Study.
a.
u
o
o
(N
O
E
3
'x
a
Initial NOX, ppm
Figure 6.2_ Dependence of Nitrogen Dioxide Maximum Concentration
on Initial Nitrogen Oxides, Bureau of Mines Study.
55
-------
s
a
a
o
TO
4-1
C
a)
u
c
0
u
3
0
I
1
£
'•x
2
u.tu
0.35
0.30
0.25
0.20
0.15
0.10
0.05
o
iiii L^1 '
Baseline Mixture
~ ^~-~a ~
^"^ \
0 S ° Varied
_ y " NO _
/ ^ Levels
x - •*
X° .•'l*~^*'"~ °
° ,f0'~^
~ o '*"' ~
0°
1 1 II 1 1
"0 0.1 0.2 0.3 0.4 0.5 0.6
Initial NOX Concentration, ppm
Figure 6.3 Maximum One-hour Nitrogen Dioxide Concentration Produced from Irradiation
of Multicomponent Hydrocarbon/NO.. Mixtures, General Motors Study.
a
a.
o
I
§
u
CM
O
E
3
X
re
• NMHC~2.1 ppmC
O NMHC ~ 0.7 ppmC
Initial NO Concentration, ppm
Figure 6.4 Maximum Nitrogen Dioxide Concentration as a Function of Initial NOX
for Two Initial Hydrocarbon Levels, UC Riverside Study.
56
\
-------
TABLE 6.1 Summary of Conclusions from Smog Chamber Experiments
tn
MAXIMAL
AVT3AGE NO,
CHAMBER STUDY
University of
finrth Carol ina
Bureau of Mines
i
i
General Motors
UC Riverside !
HEM, Auto
Exhaust i
HEW, Toluene
Dependence
on NOX
Proportional
or si ightly
less than
proportional
Proportions 1
Slightly less
than propor-
tional
Proportional
-------
Dependence
on ilC
50% HC control
reduces maximal
NOg by 10% to
20%
No effect
50% HC control
reduces maximal
N02 by 25%
50% HC control
reduces maximal
N02 by 10% to
15%
-------
-------
Dependence
on NOX
Proportional
or si ightly
less than
proportional
Proportional
Proportional
to slightly
less than pro-
portional
Proportional
Proportional
Proportional
Dc|<-.'.,uf.'ncc
on HC
Uncertain, 50% HC control
may decrease average NQ2
by ?0% or may increase
average NOp
50% 1IC control increases
average N02 ',7 10% to 30%
No effect
No effect
No effect
No consistent ei'.-_-v.t
SOURCE: Reference 20
-------
6.1.2 Empirical Relationships from Ambient Data
The second approach used to determine relationships between NO and
precursors has been to examine ambient data for existing statistical relation-
ships. Empirical models based on these relationships have then been constructed.
This approach has been tried at eight locations -- four in the Los Angeles basin,
20 21
two in Houston and one each in Denver and Chicago. ' For each site, a step-
wise regression approach was used to derive the coefficients in Equations (6.1)
and (6.2). For peak NO,, values occurring between 6:00 a.m. and 4:00 pm:
= A •t"Bl[N02]5 + INTNO ' [B2 + B3 [NMHC] + B4 [RATIO]] (6.1)
where A, B-j , B2, B3, B^ = coefficients of regression
[N02]5 = NOo concentration at 4-5:00 a.m.
[INTNO] = initial NO input (6-9 a.m. NOX - [N02]5)
[NMHC] = 6-9 a.m. non-methane hydrocarbon concentration
[RATIO] = 6-9 a.m. NMHC/NOV ratio
A
For peak N02 values occurring between 4:00 p.m. and 6:00 a.m.,
= A + B1[N02]16 + B2ENITEN°3 + B3 [NITENO][03AFT] (6.2)
where
[NOJ1C = N09 at 3-4 p.m.
L- I O f-
NITENO = initial NO input (4-7 p.m. NOX minus [N02]1&)
OoAFT = ozone at 2-4 p.m.
Analogous equations were also developed for average daytime and nighttime N02.
Data used to derive Equations (6.1) and (6.2) were seasonal Ty disaggregated inta
summer (April -September) and winter (October-March). As described subsequently,
58
-------
such disaggregation allows further insight into relationships between NOp and
reduction in NMOC. Hence, Equations (6.1) and (6.2) were derived and applied
separately to summertime and wintertime data.
The empirical models derived for six of the eight locations* imply that
reductions in NO yield slightly less than proportional reductions in peak N09,
x c.
providing all other factors are kept constant. Reductions in average NO^ are
approximately proportional to reductions in NOV. These results are summarized in
/\
Table 6.2.
As with smog chamber data, more ambiguous results were obtained when
assessing the impact of reducing NMOC on peak and average NO . The results are
presented in Table 6.3. According to the data in Table 6.3, there may be a
slight benefit as measured by reduced peak N0~ accompanying NMOC reduction. This
effect is practically negligible for mean NOo. It is interesting to note that
the effect of reducing NMOC appears to be somewhat more pronounced in winter than
in summer. This is consistent with the hypothesis that since daytime N02 peaks
occur later during winter anyway, additional efforts to delay N02 formation may
be more effective.
An important caveat needs to be stated, before the results shown in Tables 6.2
and 6.3 are taken too far. Equations (6.1) and (6.2) do not explicitly take
account of meteorological differences. Inclusion of meteorological variables in
the relationships may change some of the conclusions, particularly with regard to
* The two Houston sites were excluded because of poor fit of the models with
nighttime data.
59
-------
TABLE 6.2 Empirically Estimated Dependence of NCL on NO Control
£. /\
Maximum N0? Mean N0?
50% NO Control 50% NO Control
Location No Chafige in NMOC No Change in NMOC
Downtown LA -37% -45%
Lennox, CA -40% -47%
Azusa, CA -45% -48%
Pomona, CA -31% -43%
Denver, CO -43% -55%
Chicago, IL -50% -50%
Average 41% 48%
60
-------
Table 6.3. Estimated Impact of Reducing NMOC on Peak N02 Obtained With Empirical Models
Location
Summer Day
% Change in Peak
N02 Accompanying
50% Reduction in
Summer Day
.•.%";.Change/. ia Ave „
NC>2 Accompanying
50% Reduction in
Winter Dav
% Change in Peak
N02 Accompanying
50%vReduction in
Winter Dav
% Change in Ave.
NC-2 Accompanying 50%
Reduction in
Downtown
Los Angeles -19%
Lennox, CA -16%
Azusa, CA 0%
Pomona, CA -17%
Denver, CO - 5%
Chicago, IL 0%
Houston/Mae Dr. 0%
Houston/Aldine 0%
-6%
-4%
+1%
-9%
-7%
0%
0%
0%
-25% -14%
-10% - 5%
-15% - 8%
-20% -19%
- 8% + 8%
-14% - 8%
0% 0%
0% 0%
Average
- 7.1%
-3.1%
-11.5%
- 5,
-------
NMOC. Trijonis, for example, indicates that the apparent impact of NMOC reduction
on N02 may be overestimated by the failure of the models to explicitly consider
21
meteorology. Therefore, the significance of the numbers in Table 6.2 and 6.3
is that they agree qualitatively with smog chamber data, rather than provide
exact numerical values.
6.1.3 Relationships Derived from Chemical Kinetics Models
A chemical kinetics model is a proposed sequence of chemical reactions.
The identity of the reactions and the rates with which they proceed are based on
experimental (e.g., smog chamber), as well as theoretical evidence. These
models enjoy several advantages over the previously described smog chamber and
empirical approaches. These include greater flexibility to test different cases,
use of realistic concentrations, and the ability to vary only the parameter of
interest (i.e., NMOC or NOJ.
A
pp
The kinetics model underlying the ozone isopleth model used in EKMA has
been modified to express peak hourly NOp (rather than 0,) as a function of
initial NMOC, NOX and other factors. The shape of the resulting N02 isopleths is
more sensitive to assumptions about dilution rate and emission patterns than is
the shape of ozone isopleths. It is apparent from Table 6.4 and from Figures 6.5-
6.7 that the effectiveness of NOV and VOC controls in reducing peak N09 depends
x C-
on the prevailing ambient NMOC/NO ratio and the levels of N0? experienced as
/\ *•
well. If 0.30 ppm is assumed to be a "typical" peak N0? value, and the NMOC/NO
£ A
23
ratio is assumed to be 9.5:1, results are similar to those observed in smog
chamber experiments and empirically. That is, NOV reductions result in slightly
X
less than proportional reductions in peak N02 and NMOC reductions result in ambi-
guous changes in N02 which are usually slightly beneficial.
62
-------
Table 6.4. Effect of Separate NOX and NMOC Reductions on Peak N02 Under Various Conditions*
U>
Run
1
2
3
4
5
6
7
8
9
10
Dilution Rate
hr-1
3%
3%
20%
3%
10% .
3%
3%
3%
3%
10%
Duration
0800-1500
0800-1500
0800-1500
0800-1500
0800-1500
0800-1500
0800-1500
0800-1500
0800-1500
0800-1500
Subsequent
Emissions
1
No
Yes
Yes
Yes
Yes
Yes
No
Yes
Yes
Yes
Duration
-
0600-1600
0600-1600
0600-1400
0600-1400
0600-1600
-
0800-1800
0800-1400
0800-1400
N02 w. 50%
Reduction in NO
X
-47%
-50%
-47%
-50%
-47%
-50%
-47%
-50%
-47%
-50%
N02 w. 50%
Reduction inWH
-10%
0%
-27%
0%
-13%
0%
-10%
0%
+ 3%
-13%
* Base Case Assumptions: NMOC/NOx Ratio = 9.5:1
N02 =0.30 ppm
-------
0.84
0.72
0.60 —
0.48
CTI
D.
O.
o
z
0.36
0.24 -
0.12
1.0
1.5
Fi(|iiro 6.5
2.0
3.0
3.5
2.5
NMOC, ppmC
with 8 Hours of Emissions and 3%/hour Dilution..
4.0
4.5
5.0
-------
0.84
0.72
0.60 -
0.48 —
01
Q.
a
x
O
z
0.36
0.24 —
0.12 —
0.5 1.0 1.5 2.0
2.5
NMOC. ppmC
Fiijure 6.6 NOo with 8 Hours of Emissions and 10%/liour Dilution
3.5 4.0 4.5 5.0
-------
0.84
0.72
0.60
0.48 -
en
a.
a.
x
O
H
0.24
^~~ °'
/^^ . 0.
0.36 -
0.12 -
1.0 1.5
2.0 2.5
NMOC, ppmC
3.0 3.5 4.0 4.5
5.0
Figure G.7 NC^, with 10 Hours of Emissions and 3%/hour Dilution.
-------
Examining Figure 6.7 somewhat more closely, one finds the curious result
that in some cases decreasing NMOC may increase the peak NO ! How can this be?
The answer can be found by examining in detail how points A and B on Figure 6.7
were derived. This is done by plotting concentrations of NO, N0o» and 03 as a
function of time on Figures 6.8 and 6.9. Upon doing this, one finds that the
lower NMOC level reflected by Figure 6.8 has delayed the N0« peak to late in the
day, when dilution has been reduced. The ozone peak is similarly delayed. NO
emissions occurring late in the afternoon are titrated by the late-forming ozone,
resulting in high N02 in the early evening. Hence, the strange shape of the
isopleths in Figure 6.7 is the result of a different phenomenon (i.e., titration
rather than synthesis), causing the highest level of NO^. These strange looking
isopleths only occur when there are substantial NO emissions late in the afternoon.
A second factor that is apparent from Figures 6.5-6.7 is that at very low
NMOC/NOY ratios (less than about 3-4:1) peak N0? is much more sensitive to NMOC
A C-
reductions than to NO reductions. Thus, under such conditions the generaliza-
A
tions which have been drawn from smog chamber experiments and empirical models
may no longer hold. Because NMOC/NOV ratios appear to be much higher than this
X
however, previous generalization about the efficacy of NO controls versus VOC
/\
controls appear valid using the kinetics model as well.
6.2 Implications
1. Smog chamber data, empirical and chemical kinetics models all suggest
that reductions in NO levels will lead to approximately proportional reductions
A
in both peak and mean concentrations of N02- An exception to this may be the
case in which peak N0? occurs solely as the result of titration reactions with
ozone that are limited by the amount of ozone available.
67
-------
cr>
oo
.205
.175 —
120
240 360
Minutes After Beginning of Simulation
480
600
Figure 6.8 N02, NO and O3 vs. Time for Point A (Figure 6.7).
-------
.228
a
a
o
o
o
.195
.163 -
.130 —
.091 -
.065 —
.033 —
120
240 360
Minutes After Beginning of Simulation
Figure 6.9 N02, NO and Og vs. Time for Point B (Figure 6.7).
480
-------
2. The proportionality constant between peak (and mean) N09 and NO
£- /x
appears to remain approximately the same for a wide variety of NMOC/NO ratios,
J\
emission patterns and dilution conditions. This implies that the linear rollback
model has the potential to serve as a useful means for approximating the impact
of NO controls on ambient N00.
x ^
3. The information presented suggests that programs to reduce NMOC levels
will not have an appreciable impact on mean N02- There may, however, be a. small .
reduction in peak [^ accompanying reduction in NMOC under currently prevailing
NMOC/NO ratios. Data examined imply a reduction in peak N0~ from 0-20 percent
X £-
accompanying a 50 percent reduction in NMOC.
4. Because the proportionality constant between peak N02 and NMOC reduction
appears to be sensitive to a number of factors, the relationship is probably not
a linear one.
5. The preceding conclusions appear valid under currently prevailing
NMOC/NO ratios in urban areas. However, chemical kinetics model simulations
x
suggest that if the prevailing NMOC/NO ratio is reduced to very low values (as a
/\
result of ozone SIP's), the relative effectiveness of NO and VOC controls in
X
reducing N0? is different. For example, for ratios of less than about 2-3:1,
reduction in peak NOp would be more sensitive to further VOC controls than to
further NO controls.
X
70
-------
7.0 IMPACT OF CONTROLLING VARIOUS SOURCE CATEGORIES
Chapter 6.0 has described relationships between peak and mean N02 concen-
trations and NO and NMOC precursors. Chapter 2.0 has identified major categories
/\
of precursor sources (i.e., stationary, mobile, area, point). Chapter 7.0 presents
information which is useful for identifying source categories impacting upon
receptor sites. Thus, the concept of source-receptor relationships is introduced.
One means for surmising source-receptor relationships is through the interpretation
of ambient air quality data. A second approach is through the use of mathematical
models.
The first approach is a qualitative one which is useful in justifying
assumptions which may have to be made in certain models. The second (modeling)
approach is quantitative in that it can be used to estimate reduction in peak or
mean N02 accompanying specified reductions in precursors. Modeling approaches
are discussed in Chapter 8.0.
Prior to reviewing ambient air quality data to better understand source-
receptor relationships, it is important to remember that there are likely to be
relatively few N02 monitoring sites in a given city. Hence, failure to detect an
impact of a source category on observed N02 concentrations cannot necessarily be
used to infer that the category has a negligible impact on N02 concentrations
throughout the city. Nevertheless, review of ambient air quality data can be
extremely useful. Such a review can serve as a means for incorporating source-
receptor, assumptions in various modeling approaches described in Chapter 8.0.
For example, an apparent lack of point source impact at a monitor could justify
elimination of that source category from the emission inventory used in applying
a model without spatial disaggregation (e.g., proportional rollback) to estimate
71
-------
the impact of precursor controls on peak NOp observed at that site. Such a
procedure ma'kes it possible for a relatively simple model to explain why an
observed improvement in air quality has not accompanied emission reductions from
certain source categories. By the same token, control requirements on sources
which d£ impact the monitored air quality data may be less severe.
Review of ambient data collected in the plumes of point sources also serves
an important function. The results can be used to justify modeling approaches
for considering the major impact of these large sources of NO which may occur at
X
locations where there are no monitors.
A third reason for reviewing ambient air quality data in detail is that it
serves to emphasize the importance of proper design of a monitoring network.
For example, further insight is gained into whether a monitoring site is a
representative one. This, in turn, has implications on the use to which the data
are put. It would not be suitable, for example, to devise a citywide emission
control strategy on the basis of monitoring data which clearly reflected the
impact of a single large nitric acid plant.
Two procedures have been used to draw inferences about contributions of
various source categories to observed peak NCL concentrations. These procedures
entail the use of (1) tracer data and (2) peak-to-mean ratios. Results and
inferences of each of these procedures are described in Sections 7.1 and 7.2,
respectively. More detailed observations of point sources and the resulting
implications are described in Section 7.3. Finally, the implications of the
findings reported in this chapter to the design of control strategies
are summarized in Section 7.4.
72
-------
7.1 Tracer Approaches
A tracer approach is one which takes advantage of the correspondence between
emissions of NO and other pollutants emitted by different source categories.
/\
For example, suppose emissions from source category A contain pollutant "a,"
while emissions from category B do not contain "a" but contain pollutant "b"
instead. Further, source category A's emissions do not contain pollutant "b."
In such a case, detection of "a" by a monitor, but not "b," implies that NOp
observed at the site originates from source category A rather than from category B.
In the case of MOV emissions, carbon monoxide (CO) serves as a reliable
A
indicator of NOo originating from mobile sources. In 1977, nearly all (i.e.,
96 percent) of manmade NO was emitted from two source categories: mobile sources
/\
and stationary fuel combustion sources. Ninety-nine (99) percent of the CO
emanating from these two source categories came from mobile sources. Reference 1
also indicates that sulfur dioxide (SO ) should serve as a useful indicator of
NOp emanating from stationary combustion sources. In 1977, 97 percent of S02
from mobile or stationary source combustion sources originated from the latter.
Hence, subject to some caveats which will be identified later, four results are
possible:
1) Close correspondence between NOX and CO but not with S02-
Conclusion: mobile sources are responsible for observed NOp peaks.
2) Close correspondence between NOX and S02 but not with CO.
Conclusion: stationary combustion sources are responsible for observed NO^
peaks.
73
-------
3) Close correspondence between NO and both CO and SO0.
A £
Conclusion: both mobile and stationary combustion sources contribute signifi-
cantly- to observed NO .
4) Lack of correspondence of NO with either CO or S09.
A £
Conclusion: a stationary source not characterized by S02 emissions (e.g.,
nitric acid plant, power plant burning natural gas) may be impacting the moni-
toring site.
Use of CO and S02 tracers in the analysis of ambient NOp data proceeds by
plotting diurnal NO, N02, CO and S02 concentration patterns observed concurrently
at a monitoring station. This procedure has been extensively followed using
1975-1977 SAROAD data to examine coincident NO, N02, CO and SOp peaks. Figure 7.1
exemplifies what was, by far, the most frequent finding — close correspondence
between diurnal CO and NO or NOp concentrations with little apparent correspon-
dence with SOp concentration profiles. This finding is not too surprising when
one considers that mobile source emissions of NO and CO are ubiquitous and occur
x
near the ground. Most of the stationary source emissions of S02 and NO occur as
the result of combustion by electric utilities or industrial sources. Emissions
from such sources are generally confined to restricted areas or "points" defined
by the location of the emitting facility. Frequently, these point source emissions
occur through elevated stacks. Hence, the likelihood of a plume from such a
source impacting at the precise location(s) of a monitoring station(s) for a
better part of an hour or more is slight.
Despite the fact that the impact of point sources of NOV emissions is
A
seldom detected by existing monitoring networks, monitoring data exist which
demonstrate the potential for point sources to contribute to high hourly
74
-------
23
0 8 16
Nov 76
32 40
TIME (HOURS)
56
64
72
25 NOV 76
Figure 7J Pollutant plots for Lynwood, November 23-25, 1976.
Upper panel: 03 ( ). CO (+•
Lower panel: NO ( ). N02 (
). and S02 ( ); CO concentration scaled by 0.01
75
-------
concentrations of NOp. In Figures 7.2 and 7.3, for example, SO* tracer data
demonstrate that superposition of a point source plume on background NOp can
resu.lt in NOp concentrations Above 0.20 .ppm in winter., as well as in summer.
Before leaving the subject of SOp and CO tracers, several caveats regarding
the approach need to be identified. First it should be recalled that the approach
identifies possible causes of high NOp at the monitoring site only. Source
categories not having any apparent impact at existing monitoring sites are not
necessarily unimportant. Second, there may be stationary sources of NO which do
/\
not emit SOp (e.g., nitric acid manufacturing, natural gas pumping stations or
natural gas-fired utilities). These sources could impact on a monitoring site .
without being identified as stationary sources. Third, relatively small amounts
of S09 and CO are emitted by stationary area sources of NO such as residential
L~ f\
oil burners. Hence, S02 is not uniquely identified with large stationary point
sources nor is CO uniquely identified with mobile sources. Finally, relative
amounts of NO versus CO versus SOp emitted from sources within a given source
/\ £-
category vary. For all these reasons, the approach outlined in this section can
only be regarded as qualitative.
7.2 Use of Peak-to-Mean Ratios
Calculating the ratio of peak hourly NOp concentration to mean NOp concen-
tration can sometimes be useful
(a) in determining whether point source impact is likely, and
(b) in assessing whether a short term or annual air quality goal is
more restrictive.
76
-------
0.8
0.7
0.6
E '
a
a
I
z
o
< 0.4
cc
z
LU
o
o
° 0.3
0.2
0.1
.3
, NO
12
TIME - hours
16
20
24
Fiqure 7.2 Example of potential point source impact at
Long Beach on Sunday, 16 November 1975.
CO has been scaled by 0.01.
77
-------
0.8-
0.7
0.6
0.5
^ I
.3
.2'
O
< 0.4
DC
H
Z
UJ
u
0.3
0.2
0.1
12
TIME - hours
16
20
24
Figure 7.3 Example of potential point source impact at
Whittier on Monday, 25 August 1975.
CO has been scaled by 0.01.
78
-------
One would ordinarily expect that a monitoring station which was impacted by
a large, elevated point source of NO would exhibit higher peak-to-mean ratios
/\
than a site which is impacted by ubiquitous area sources of NOV. This assertion
A
is based on the presumption that it is extremely unlikely that a plume from an
elevated point source will impact at a given groundlevel station. Hence, the
mean concentration will be low, but the peak concentration could be high, yielding
a high peak-to-mean ratio. Conversely, there will always be some impact attri-
butable to ubiquitous groundlevel sources, so that one would expect a lower peak-
to-mean ratio when such sources dominate.
Peak-to-mean ratios may also be useful in assessing whether it will be more
difficult to comply with an annual or hourly air quality goal for N02. This
information may, in turn, be useful in the design of control strategies since the
relative effectiveness of different strategies could be different for annual
versus short term problems. To illustrate, suppose the short term air quality
goal were 0.25 ppm NCL not expected to be exceeded more than once per year and
the annual goal were 0.05 ppm. At sites having peak-to-mean ratios in excess of
5:1 (i.e., .25/.05=5), the short term air quality goal is likely to be of greatest
concern.
As suggested in Section 7.1, an extensive review of NO^ data has indicated
that monitoring stations experiencing N02 concentrations in excess of 0.20 ppm
are dominated by mobile (i.e., ubiquitous area) sources of NO .9 Under such
/\
conditions, Martinez and Mitz found peak-to-mean N0~ ratios varying between 3 and
12 with a median value of 6.3 and about 70 percent of the values between 5 and
Q
7.6. These findings are similar to ones found by Trijonis using earlier data
from 120 urban sites throughout the United States.^' Trijonis has also found a
79
-------
distinct difference between urban peak/mean ratios and ratios measured in rural
areas subject to occasional impacts from elevated point sources. This difference
is depicted in Figure 7.4. Figure 7.4 indicates that if the impact of area
sources of NO (e.g., mobile sources) or ambient N0« is insignificant compared to
/\ £
that from elevated point sources, the peak-to-mean ratio is likely to be high.
Unfortunately, the converse of this does not necessarily follow. That is, one
cannot rule out the possibility of a point source significantly impacting at a
monitoring site just because the peak-to-mean ratio is low. Thus, although fewer
data are needed to apply the peak-to-mean ratio versus the tracer approach, the
latter approach is more useful. The principal utility of the peak-to-mean ratio
approach is to identify cases in which point source impacts occur and the impact
of area sources is negligible.
7.3 Impact of Point Sources on Ambient Levels of NOp
Information presented in Sections 7.1 and 7.2 implies that existing N02
monitoring sites tend to reflect the impact of area (mobile) sources. However,
for an analysis of potentially harmful exposures to N02, it is necessary to
consider the impact on ambient N02 of large point sources as well. Air quality
data collected in the vicinity of large point sources give insight into how such
sources should be considered in models to estimate needed controls. In this
section, information collected at a continuously operated groundlevel monitoring
network in the vicinity of the Clinch River (TN) power plant is described. Next,
evidence of point source plumes observed during the St. Louis RAPS study is
described. Third, air quality measurements taken within the plumes of several
large point sources using instrumented aircraft are discussed. Finally, implica-
tions about appropriate methods for modeling such sources are identified.
80
-------
20-
15-
10 _
5 _
3 Rural/Power Plant Stations
Urban Stations
12345678
Annual Mean NO- Concentration, pphm
Figure 7.4. Dependence of maximum/mean ratio on annual mean
N02 concentrations.
81
-------
The Clinch River power plant is a 712 MW coal fired utility located in
complex terrain. NO, NO and S0~ monitoring data were collected for approxi-
X £
mately 1-1/2 years at -six continuously operated monitoring stations shown in
Figure 7.5. For reference in Figure 7.5, the top of the plant's stack is located
approximately 2,175 feet above sea level. Several observations are apparent.
1. The most striking observation is the infrequency with which the
network detected a major impact attributable to the source. Of approximately
120,000 station-hours of observations, hourly NO values in excess of
X
0.20 ppm were observed only 59 times or about 0.05 percent of the time.
The highest hourly reading was 0.83 ppm NO . Only three N05 concentrations
X £
in excess of 0.20 ppm were recorded (0.0025 percent of the time). These
three readings range from 0.35 - 0.74 ppm and are suspicious in that the
corresponding NO data are either missing or agree closely with the
X
recorded N0? value. It should be pointed out, however, that observed
instances of N02 in excess of 0.20 ppm would likely have been more
frequent in an area subject to high concentrations of ozone. Although
the data are not available, it is unlikely that Clinch River experiences
hourly ozone concentrations in excess of 0.10 - 0.15 ppm.
2. Approximately 50 percent of the NO values in excess of 0.20 ppm
}\
occurred between 9 a.m. - 12 noon, giving credence to the hypothesis that the
maximum impact from such sources is likely to be during the period in which
morning fumigation occurs. Twenty-five (25) percent of the time high NO was
X
obseryed between 6 p.m. and 9 a.m. This is surprising, but it could reflect the
presence of katabatic winds. Such a finding is not likely in flat terrain.
82
-------
oo
CO
Stack Heiyht ~ 200 m
Figure 7.5 Location and Elevation of Clinch River Power Plant Stations.
-------
3. During hours in which observed NO was >_ 0.20 ppm, 90 percent of
X
the NO,/NOV ratios varied between .10 and .50, as shown in Figure 7.6. There is
^ A
a slight negative correlation between the N09/N0 ratio and NO on these days
L. X X
with high NO . It is possible that observed N09/N0 ratios during periods with
X c X
high ground level NO attributable to a point source may be somewhat higher in
X
locations subject to high ozone concentrations. As in Figure 7.7
suggests, the median N0,,/N0 ratio observed in this sample is about 0.25. The
w A
three occasions in which hourly NCL concentrations in excess of 0.20 ppm were
observed are all characterized by N09/N0 ratios which lie far outside the range
£ A
observed on every other occasion.
4. As expected, there was an excellent correspondence between hours
with high NO and hours with high S09 observed by the Clinch River network.
A £
In addition to the Clinch River data, air quality data were measured aloft
on 134 days during the RAPS study in St. Louis. A subset of these data have been
24
reduced by Evans. These data are of particular interest, because they were
observed in an urban area. Evans1 data frequently reveal evidence of NO
A
plumes aloft. In a few cases, such as the one illustrated in Figure 7.8, very
high (> 1.0 ppm) NO concentrations were observed for periods of a minute or two.
A
However, in such cases the plume consisted almost entirely of NO. The amount of
NO^ present in such plumes is consistent with the depletion in the ambient ozone
levels found within the plume as a result of the titration reaction. It is also
of interest to note that despite the presence of these plumes aloft, only six
daily maximum hourly N0? readings were observed above 0.20 ppm by the 25 station
RAPS network during 1976. Five of these readings occurred between 8:00 p.m. and
midnight LCT. The sixth reading occurred at 9:00 a.m. Five of the high values
84
-------
.60
<0
.94
IO.08
.50
.40
Of.30
00
en
.20
.10
\
r- O
°o
O
O
O
D
O
D
O
O O OO
g>o o
CO
o
o
o o
o
o o
\
O
00
oco
o
o
o
o
o
o
o
o
t0.90
o
o
o
o
o
o
o
o
NO2 > 0.20 ppm
o
0.20
0.30.
0.40
0.50
0.60
0.70
0.80
0.90
NOX, ppm
1.00
1.10
Ficjure 7.6 NO9/IMOV vs. NOV at Clinch River on Days with High NO,
£- J\ J\ t
-------
30
25
5 20
§ 15
"o
w
a
i 10
(a) All Readings > 0.20 ppm
N =59
Median = .26
.10
.20
.30
NO2/NOX
.40
.50
20
w>
5
'^
(0
I 15
O
"o
w
0)
1
z
10
(b) For NOX Readings > 0.40 ppm
N = 19
Median = .22
.10
.20
.30
.40
.50
Figure 7.7 IMO2 Ratios Observed at Clinch River During Hours with High Ambient NO
86
-------
DaY221,t976 Spiral 31
05:08 to OS: 13
3000
0.050 0.075 0.100 0.125 0.150 ppm
10
12
14
16
18
20 degO
Ozone, NO, NOX, and Temperature vs. Altitude, Day 221
Day 221,1976 Spiral 31
05:08 to 05:13
3000
2000
'£ 1000
I
0
I
0.25 0.50
0.75 1.00 1.25 1.50 ppm
_| I I I
10
12
14
16
18
20 deg 0
Ozone, NO, NOX, and Temperature vs. Altitude, Day 221
Figure 7.8 Evidence of NOy Aloft Over St. Louis.
87
-------
occurred on October 1-2, 1976, days with atmospheric stagnation in which several
sites observed above-normal readings. The sixth high reading occurred at 10:00 p.m.,
off a- Friday nigtit in" November 1976. Only one site recorded an abnormally high
reading on this occasion. This tends to support the theory of a point source
impact. However, the time at which the high reading occurred (10 p.m.) makes it
seem more likely that the data reflect the impact of a local ground level source
of are spurious.
The final set of data which sheds insight on the behavior of point source
plumes of NO is obtained from aircraft measurements made within plumes. Studies
A
of this nature are generally conducted by flying traverses at various distances
downwind from the source. By noting cross sectional variations in N02, NO, 0,
and S02, at differing distances (travel times), the validity of various plume
modeling approaches can be checked.
Airborne measurements have been made of several power plant plumes. Perhaps
the most extensively reported set of measurements has been those made within the
or pc
plume of the Labadie power plant as part of the MISTT project. ' The Labadie
source is a 2200 MW coal-fired power plant located about 50 km WSW of downtown
St. Louis. Figures 7.9 and 7.10 show cross sectional concentration patterns 21
and 45 km downwind from the Labadie plant on a particular day. In Figures 7.9a
and 7.10a, S0? measurements are used to trace the cross sectional dimension of
the power plant's plume. Because the "03 + N02 curve" in these figures is constant,
it is apparent that the net formation of N02 within the plume is practically
identical to the net loss of ozone within the plume. This strongly implies that
N0? formation within the plume occurs as the result of the titration reaction
(i.e., reaction 3.1). White et al. ' have shown that a simple model which
88
-------
15
I- /'\S MEASURED 03-I-N02
Q.
Q.
10
cr
t-
2
LU
O
2
O
O
10 km
MEASURED
0
MEASURED
,°0° CALCULATED
! oQ°ooOO°oooybooo
Ambient Conditions
T = 20-25 C
RH = 60-70%
Winds: E @ 3-5 m/sec
This profile is measured
about 1 1/2 hours travel
time downwind from source.
1240
1238 1236
TIME, CDT
1234
76-090
Figure 7.9a Horizontal plume traverse 21km downwind of Labadie stacks on
August 14, 1974. Oxidant (03 + N02) concentrations within
plume were consistent with background levels on either side.
This traverse was flown at 455 m (1500 ft) msl.
89
-------
ol 10
O
o:
h-
-z.
UJ
o
z:
o
o
0
10 km
MEASURED NO x
o°o° CALCULATED NOX
^•X,'' MEASURED NO
. '„ * CALCULATED NO
aoAopxx
1240
1233 1236
TIME , CDT
1234
76-119
Figure 7.9b Horizontal plume traverse 21km downwind of Labadie stacks
on August 14, 1974. Most of the NO emitted by the power
plant had already been oxidized to NC>2. Measured NO profile
agreed well with NO profile scaled from measured S02 concen-
trations. MeasuredTTO profile agreed well with No profile
calculated from measured S02 profile and oxidant-preserving
photochemistry. (The same traverse is shown in Figure 7.9a).
90
-------
10
CL
Q.
2 .
Id
O
O
O
MEASURED 03
MEASURED Oa*
\X' MEASURED S02
O°O° CALCULATED Oi
10km
* / * « / »/
V Wvy v
Ambient Conditions
- T = 20-25C
- RH = 60-70%
- Winds: E @ 3-5 m/sec.
,A ^ - This profile is
N 3-4 hour travel
time downwind from
source.
V
1436
1434
1432 1430
TIME,CDT
1428
1426
76-035
Figure 7.10a
Horizontal plume traverse 45 km downwind of Labadie stacks on 14 August 1974. Ozone
concentrations within the plume at this distance were still depressed below background levels
on either side, although not as strongly as on traverses nearer the stacks. Measured ozone
and oxidant profiles both show large-scale concentration gradient, possible due to urban
plume from Greater St. Louis. The altitude of this traverse was 610 m (2000 ft) msl.
-------
E 10
Q.
Q.
O
cr
h-
z:
ijj
o
o
10 km
o o
MEASURED NOX
CALCULATED NO
MEASURED NO
CALCULATED NO
f\ A
436
1434
1432 1430
TIME,CDT
1428
1426
76-118
Figure t.lOb Horizontal plume traverse 45 km downwind of Labadie stacks on 14 August 1974. Nitric oxide
concentrations were near the detection limit of the monitor, and very little excess within the
plume was observed or predicted. (The same traverse is shown in Figure 7.10a.)
-------
assumes that N02, NO and ozone concentrations within the Labadie plume can be
estimated using an empirical relationship between SO, and NO and the photo-
L. X
OC
stationary state* assumptions. As shown in Figures 7.9 and 7.10, excellent
agreement between the model and observations is obtained. On the basis of these
results, White concludes that, within 40 km of the source, N02 and 03 concentra-
tions within the plume are not affected by reactions with organic pollutants.
Hegg et al. have summarized findings from monitoring studies within the
07
plumes of two coal-fired utilities and two gas-fired plants. These observations
are essentially similar to those found in the Labadie plume. Hegg et al. conclude
that the conversion of NO to N02 within a power plant plume is limited by the
rate with which this NO comes into contact with ambient ozone. In short, conversion
of NO to N02 is limited by the dilution rate. These results can be interpreted
to mean that the fraction of NO which is NO- (i.e., NO./NO ) will be greater
X L. d. X
near the periphery of a plume than at the center!ine. Figure 7.11 illustrates
this concept. Hence, peak N02 concentrations may not occur in the same position
in the plume as peak NO concentrations (i.e., at the centerline). Locations
J\.
within the plume experiencing high NO have, necessarily, been exposed to rela-
/\
tively little dilution air. Exposure to dilution air is necessary for conversion
* The Photostationary State assumes that an equilibrium between the reactions
03 + NO -»• 02 + N02 (i.e., the titration reaction)
k2
and .
hv
NO? + Op -»• 0.5 + NO (reaction of resulting N09
* ki with sunlight) *
occurs. This equilibrium is given by
93
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(b)
- Figure 7.11
Diagrams for a titration reaction in a turbulent
plume: (a) shows a cross-sectional view of the
plume. The reaction zone is an annulus of Area A
and width L . The interior of the plume, where the
concentration of NO is high, has area B. Area C is
the exterior region of ozone. E is a mixing turbu-
lent eddy of velocity Vp, and scale A; (b) shows
concentrations of the gases across section (a).
Source: Reference 27
94
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of NO to N09 to proceed. However, dilution reduces NO concentrations. Thus,
£ A
there are two counteracting phenomena (dilution and chemical conversion) which
should theoretically limit peak N02 concentrations within the plume of a major
point source. A result of these competing phenomena is that N09/N0 ratios are
£ /\
likely to be higher at the plume's periphery than at the centerline. This is
illustrated in Figure 7.11. Maximum N02 concentrations are most likely at
intermediate ranges where sufficient dispersion has occurred to bring the plume
to the surface and to bring the reactants together. Further downwind dilution
will diminish concentrations.
Before ending the discussion of NO point source plumes, one further point
A
needs to be addressed. When concentrations of NO are very high (as they would be
within a few hundred meters of a source), Reaction (7.1) may result in some
initial conversion of NO to N02 without substantial dilution.
2 NO + 02 •> 2 N02 (7.1)
The rate with which Reaction (7.1) proceeds is too slow for it to be significant
2
unless NO concentrations are very high. As a result of Reaction (7.1) and direct
emissions of N09, approximately 5-10 percent of "initial" NO from combustion
£ A
sources will be usually be NOp. N02/N0 ratios can, however, be as high 0.25 as
2
the result of the reaction (7.1). Hence, it is possible to have high N02 concen-
trations at the plume centerline without substantial dilution. However, the extent
to which such an undiluted plume impacts the ground is probably very limited.
7.4 Implications
1. Use of S02 and CO data as tracers is a useful qualitative approach for
assessing whether observed high N02 concentrations are primarily attributable to
95
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stationary or mobile sources. Estimating peak-to-mean ratios of N02 serve as
a useful approach (a) for verifying whether a major point source is impacting a
site sporadically, and (b) for assessing whether a short term or annual air
quality goal for NCL is going to be more difficult to meet at a given site. Such
tools provide qualitative insights (a) on whether to emphasize control strategies
impacting most heavily on ubiquitous area sources or on large point sources, and
(b) on whether to stress control strategies which emphasize reducing short term
peaks or long term averages.
2. Application of tracer data and peak to mean ratios to available NO^
data suggest that observed NOp concentrations in excess of 0.20 ppm are usually
attributable to mobile sources. This finding could, in part, reflect a bias in
the way existing monitors are sited (i.e., frequently near roadways). However,
the data examined indicate that, even when a monitoring network is deployed so as
to maximize the likelihood of detecting the impact of an elevated point source,
observed high hourly concentrations of NO (and N0?) resulting from such a source
3\ £
are very infrequent. The foregoing implies that strategies which stress reduction
of NOV emissions from mobile and area sources will be most effective in reducing
X
observed short term peak N02 concentrations as well as annual average values.
Nevertheless, as a safeguard, when possible a case-by-case review of tracer data
is suggested.
3. The preceding paragraph should not be interpreted to mean that major
point sources of NO necessarily play an insignificant role in the N0~/ozone
A *—
problem. Paragraph 2 referred solely to observed data collected at continuously
operated ground level monitors. Such networks are usually quite sparse. Air-
craft data reveal occasional very high (e.g., > 1.0 ppm) concentrations of NO as
A
96
-------
well as moderate concentrations of NCL within point source plumes. These N02
concentrations, when superimposed upon the N02 resulting from other sources could
lead to hourly concentrations in excess of concentrations as high as 0.20 or
0.25 ppm. Because (a) NO emitted within point source plumes (without substantial
J\
amounts of VOC) may be less photochemically active than NO from well-mixed
/\
sources, and (b) NO from elevated sources may be more isolated from the ground
A
and other scavengers, NO from such sources may be relatively more important in
/\
the transport of low levels of NOV to rural areas. As such, they could be poten-
X
tially more important in the formation of ozone in rural areas, formation of fine
particulates over great distances, and as a contributing factor in the acid
deposition phenomenon.
4. Data collected within point source plumes suggest that N02 within such
plumes primarily results from titration of NO with ambient ozone. Some NO may
A
be emitted as NO as well. Because of the high concentrations of NO which may
L. X
occur within plumes, the ozone titration reaction is likely to. be limited by the
concentration of ambient ozone. This suggests that the most effective way of
reducing peak NOp from a point source may be (a) to limit the amount of NO
emitted as NOp or converted to NO via the oxidation reaction described in
Reaction (7.1). and/or (b) reduce ambient levels of ozone as the result of SIP's
for ozone. However, reduction of NO emissions in any form may be useful in
/\
reducing ozone formation in downwwind areas. Procedures for reducing NO from
/\
large stationary sources are described in the appropriate Control Techniques
28
Document.
97
-------
5. To the extent that St. Louis is a typical city, data examined there
suggest that plumes of NO are frequently present aloft in the early morning
A
hours. Data from Clinch River tend to confirm that the midmorning fumigation
phenomenon is the event most likely to lead to observed major impacts from
elevated point sources. Hence, midmorning may be the most likely time to observe
a major impact from point sources. Even with the fairly dense monitoring network
in the RAPS study and the ideally sited network at Clinch River however, impacts
on ambient NO^ in excess of 0.20 ppm resulting from the fumigation phenomenon
were seldom, if ever, observed.
98
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8.0 USE OF MODELS FOR N02
Previous chapters have discussed sources of NOV, means by which it gets
/\
converted to N02 in the atmosphere, issues relating to measuring representative
values of N02, prevailing levels of N02 observed by existing monitoring networks,
long range transport of N02, and relationships between N02, its precursors and
principal source categories. This information is needed in order to make informed
judgments about collecting and interpreting appropriate data for developing
control strategies to reduce unacceptably high levels of ambient NOo. Questions
that still remain however are:
(1) "how much control is needed to meet an air quality goal for N02?"
and
(2) "what is the impact of an individual source?"
The answers to such questions require quantitative estimates. The role of models
is to provide means for making such estimates.
The purpose of Chapter 8.0 is to describe models available for estimating
the impact of precursor emissions on short term and long term N02 concentrations.
The state-of-the-art for N02 modeling is not so far advanced as for other criteria
pollutants. Although some of the approaches to be described are easy to apply.
such applications cannot be made blindly. Model assumptions or estimates which
appear to contradict existing air quality, meteorological or emissions data need
to be scrutinized and explained if they are to remain credible. The quantitative
predictions provided by models should be regarded as "best estimates" with some
band of uncertainty about them. Unfortunately, in most models for N02, this band
of uncertainty is not precisely known. Such uncertainty is reduced, however,
when use of models is accompanied by high quality input data, and is consistent
99
-------
with qualitative observations described in previous chapters. Under these conditions,
model estimates provide useful quantitative goals for control programs.
The potential for high short term concentration of N0_ is greatest in the
following settings: (1) urban-industrial regions in which NO and VOC are emitted
A
from a large number of mobile and stationary sources; (2) downwind of sources
with large NO emission rates in urban regions or in non-urban regions which
/\
periodically experience high ozone levels (e.g., greater than the MAAQS for 03);
and (3) downwind of certain industrial sources that emit large quantities of N02
directly into the atmosphere. Accordingly, Sections 8.1 and 8.2 present models
available for estimating the impact of controls in multisource urban areas and
from large individual sources of NOV or N09, respectively.
X c
8.1 Multisource Urban Models
Models appropriate for use in multisource urban areas can be categorized as
simple or complex. Simple models include linear rollback, OZIPM and the Photo-
chemical Box Model (PBM). However, the only one of these approaches widely
available as of September 1980 is rollback. Consequently, a more lengthy description
of rollback modeling, as applied in assessing the impact controls on ambient NO^
is provided herein. Complex models include several atmospheric simulation models.
Such models have been primarily used to estimate ambient ozone concentrations,
but can also produce estimates of ambient NOp.
8.1.1 Linear Rollback
Linear rollback is based .on the assumption that changes in emissions of
a pollutant will result in proportional changes in the ambient concentration of
that pollutant. It was concluded in Chapter 5.0 that background concentrations
(be they natural or manmade) transported into the city under review may ordinarily
1QO
-------
be ignored. This simplifies the rollback model still further. The rollback
approach proceeds by first estimating the future design value for NCL which would
occur if no additional controls over and above those already planned were implemented.
- i \ r future -> to -i\
Xf = (xo) [ Q ] (8.1)
^present
where
Xf = future NOp concentrations, any units
Xo = present design value for NOp, consistent units
^future = Pr°Jected N0x emissions, any units
Present = Present NO emissions, consistent units
The amount of additional reduction required in NO emissions to meet either the
A
annual or a prospective short term NOp is given by Equation (8.2)
Xf - X1
R = — x 100 (8.2)
xf
where
R = percent reduction required emissions over and above that already
projected
X1 = the appropriate NAAQS for NO^s consistent units.
Considerable efforts have been made to refine the "Q" terms on the right
hand side of Equation (8.1) in recent years. These emission terms have been
subdivided into different emission categories (e.g., light-duty motor vehicles,
stationary combustion sources, etc.), each of which can reflect different assumptions
29 30
about growth and future control technology. ' As a result of these refinements,
the model has come to be known as the "modified rollback" model. This model has
been widely used in conducting regulatory analyses for prospective N0? air quality
standards and for estimating the impact of various automotive emission standards
101
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on future NO emissions. The modified rollback model computer package and
X
accompanying User's Manual will be available by 1981.
Several underlying assumptions in the linear rollback model need to be
identified.
1) Peak and mean N09 concentrations are directly proportional to NO
£- A
emissions;
2) Peak and mean N02 concentrations are independent of ambient NMOC
(or VOC emissions);
3) Either all source categories are evenly distributed throughout the
city or the atmosphere is perfectly mixed so that the relative contribution a
source makes to ambient concentrations is the same as its relative contribution
to total NO emissions.
X
As discussed in Chapter 6.0, the first assumption is generally valid for
both peak and mean concentrations of N02. The exception occurs when peak N02
concentrations result primarily from titration of NO with ozone and the amount of
N02 formed is limited by the prevailing 03 concentration. Even in such a case,
however, it is probably appropriate to assume that N02 is proportional to NOX
emissions, if the N02 peak is observed to build upon carryover N02. Thus, prior
to applying the rollback model, it is advisable to apply the procedures described
in Sections 3.2 and 3.3 to assure oneself that the observed peak N02 concentration
is not limited by available ozone or that if it is, some carryover N02 is present.
102
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According to the information presented in Chapter 6.0, average N02 concen-
trations are insensitive to changes in NMOC. Hence, the second assumption causes
no problem in considering annual mean N02 values. However, peak NCL concentrations
are slightly sensitive to reductions in NMOC. This sensitivity depends on the
prevailing ambient NMOC/NO ratio, diurnal emissions patterns and atmospheric
/\
dilution. Depending on these factors, reductions in peak N02 ranging from 0-
20 percent have been observed to accompany 50 percent reductions of NMOC in smog
chambers or in chemical kinetics models. It is not possible to consider relation-
ships between peak N0? and NO and NMOC simultaneously using the rollback approach.
£• /\
However, NOp isopleth diagrams similar to those presented in Figures 6.5-6.7 have
been used for several dilution rates and emission patterns to assess sensitivity
of peak N09 to NMOC at various NMOC/NO ratios. Table 8.1 presents the results
£ A
obtained for low dilution rates most likely to accompany high urban levels of NO
A
and NOp. Table 8.1 should not be used for dilution rates in excess of 10%/hour.
The following example illustrates the use of the rollback procedure with the
information in Table 8.1
Example 1
Given:
-Hourly N02 concentration = 0.30 ppm
-Prevailing NMOC/NO ratio on days with high hourly N02 = 9:1
-In the absence of further controls, a projected increase in NO
emissions of 20% over a defined period (e.g., 1987)
-An existing ozone SIP requiring 40% reduction in VOC over the same
defined period (i.e., 1987).
Find:
The required reduction in NO emissions to meet a hypothetical hourly
NAAQS for N02 of 0.25 ppm not expected to be exceeded more than once per
year.
103
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Table 8.1. Sensitivity of Peak N02 to Changes in NMOC with Various
Prevailing NMOC/NO Ratios
J\
% Reduction in Peak N02
Accompanying a 50%
Ratio* Reduction in NMOC
>10:1 0%
8-10:1 10%
6-8:1 15%
4-6:1 20%
* If the prevailing NMOC/NO ratio is not known on days experiencing
N02 in excess of the air Duality control, assume 0% reduction in
peak N02 resulting from reductions in NMOC.
104
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Solution:
(1) Estimate future NOp design value which would occur in the absence of
further controls
Xf = (0.30)0.20) (8.1)
Xf = 0.36 ppm*
(2) Reduce the future design value consistently with the prevailing NMOC/NO
ratio. According to Table 8.1, for a VOC/NO ratio of 9:1, a 50%
reduction in NMOC results in a 10% reduction in peak N02, Since the
anticipated reduction in VOC emissions is 40% rather than 50%, reduce
the future design value as shown:
i+O
Reduction due to VOC Reduction = (—) (10) = 8%
50
New N02 Design Value = (0.36) (1.00 - .08)
= 0.33 ppm
(3) Estimate the required reduction in projected NO emissions (Qfuturp)
needed to attain the NAAQS at the end of the period of interest
(e.g., 1987).
R =[°'330"33'25J x 100 (8.2)
R = 24%
The procedure illustrated in Example 1 is not appropriate in the unusual
case when NOp occurs as the result of ozone-limited titration in the absence of
carryover NOp. In such a case, it would be necessary to estimate reductions in
pp 23 31
ambient ozone using models which have been described elsewhere. '' In the
absence of better information, the resulting reduction in peak NOp would be
proportional to the estimated reduction in peak ozone, after making some allowance
for NO which is emitted directly as NOp. The procedure is illustrated in Example 2.
* Note that the calculations should not be carried out to more than two
significant figures.
105
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Example 2
Given:
(1) An N02 Design Value = 0.30 ppm is observed which is primarily attributable
to ozone-limiting titration occurring in the absence of carryover N02.
(2) A VOC emission control program is estimated to reduce peak ozone by 30%.
Find:
The resulting reduction in peak NO,,.
Solution:
(1) Assume 10% of the N02 results from direct emission of N02
(.10)(.30 ppm) = .03 ppm of N02 would not be affected by reducing
ozone
(2) This leaves 0.27 ppm N02
Estimated Future N02 = (0.27)(1-.30) + .03
= 0.22 ppm
Information presented in Chapter 7.0 casts serious doubt concerning the
validity of the rollback model's assumption that the relative contribution a
source category makes to ambient concentrations is the same as its relative
contribution to total NO emissions. As discussed in Chapter 7.0, monitoring
A
data seldom appear to reflect impacts from major point sources. This could
result in credibility problems if the rollback model is used as the basis for
applying controls on a large source which does not impact the monitoring site.
Failure to observe subsequent improvement in ambient N0? after implementation of
controls could cause criticism of a strategy to reduce emissions from such sources.
Criticism of this nature may stem solely from improper use of the rollback model
rather than from the effectiveness of the strategy per se. A similar problem may
occur if the rollback model is used to assess the impact of a transportation
control measure which is applied at one or a few sites which either do not impact
106
-------
monitoring sites or have a disproportionately high impact on a monitor site
(e.g., as may be the case if a roadway near a monitor were turned into a pedes-
trian mall). Thus, the rollback model is most suitable for estimating the impact
of controls which are applied uniformly to ubiquitous sources. Examples of such
strategies include the Federal Motor Vehicle Control Program (FMVCP) and inspection
and maintenance programs.
The impact of widespread control programs (e.g., inspection and maintenance)
on observed peak and mean concentrations of NCL may be underestimated using the
rollback model unless emissions from large, elevated point sources are somehow
discounted. Use of available S02 data (see Chapter 7.0) as a tracer for N02
originating from large, elevated stationary combustion sources can be used as
justification for discounting such emissions. If (1) NO emissions from large
A
point sources with elevated stacks is accompanied by SO^ emissions, and (2)
ambient data indicate no correspondence between peak N02 and observed ambient SOp
patterns, NO emissions from large elevated point sources can be eliminated from
A
the "Q" terms on the right hand side of Equation (8.1). Because of the unlike-
lihood of observing an impact from an elevated source at any given point, in
applying the rollback model, NO emissions can be eliminated from the "Q" terms
A
on the right hand side of Equation (8.1) in estimating controls needed to meet
the annual N02 standard regardless of whether there are any S02 data.
It should be emphasized that the suggestions in the previous paragraph are
intended to insure that the rollback model provides estimates which result in
more credible prescriptions for ubiquitously applied control measures. The
suggestions do not imply that elevated point sources of NO are unimportant. If
~ A
elevated point source emissions are eliminated from the rollback model as suggested,
107
-------
their potential impact on peak NCL values needs to be considered in some other
way. However, it is first necessary to; describe available models for estimating.
the impact from point sources. Therefore, the discussion of appropriate procedures
for supplementing the rollback model to consider point sources will be deferred
until Section 8.2.
Because the impact of elevated point sources are not ordinarly expected to
impact appreciably on annual average NO^ concentrations at any giveir site, it is
sufficient to use Equations (8.1) and (8.2) to estimated needed NOV emission
J\
controls excluding emissions from elevated point sources. It is not ordinarily
necessary to supplement this analysis with applications of a point source model .
in this case.
8.1.2 OZIPM
The OZIPM (Modified Ozone Isopleth Plotting Package) is similar
22 23 31
conceptually to the OZIPP/EKMA model described extensively elsewhere. ' ' A
well-mixed column of air is assumed to migrate within an urban area so that it
encompasses the monitoring site observing the peak N02 concentration at the time
of the observation. This is pictured conceptually in Figure 8.1. The OZIPM
model provides the user with a graph of peak hourly N0? (rather than 03) as a
function of initial NMOC and NO concentrations. Figure 6.5 (page 62) is an
A
example of such a diagram. The positions of the NO isopleths are also functions
of the NO and VOC emissions encountered by the column along its trajectory,
/\
diurnal variation in the mixing height, sunlight intensity and ozone transported
from upwind. As with the EKMA procedure, one uses 6-9 a.m. NMOC/NO ratios and
J\
the peak observed N02 concentration to identify a starting point on the isopleth
diagram. Control requirements needed to attain a specified air quality goal are
calculated from the starting point.
108
-------
INITIAL
CONDITIONS
WIND
1
/ YYY
Mi
Mi
Mi
7/7"
DILUTION RATE, x%/hr.
ETC.
KEY
ti = TIME PERIOD i
Mi = MIXING DEPTH AFTER TIME i
*:
Qi = PRECURSORS INJECTED INTO
COLUMN DURING TIME i
~LJ- = SUNLIGHT INTENSITY DURING
TIMEi
Figure 8.1 Conceptual view of the column model.
109
-------
The OZIPM model has several potential advantages over rollback. First,
the isopleth diagram can be applied to cases in which NOp results from either
synthesis or titration. Second, the impact of VOC emission controls on peak N02
can be more readily determined. The problem with uniform distribution of point
sources still remains however. Other problems are that the OZIPM model is only
appropriate for short term NCL concentrations and its applicability to winter NO^
episodes is uncertain. Finally, and most importantly, the model needs to be
further evaluated. Thus, it is not expected to be widely available for use sin*
control strategy evaluations until late 1981.
8.1.3 Photochemical Box Model (PBM)
The PBM model has been recently developed and is currently undergoing
further evaluation. In this model, a large stationary cell is superimposed over
32
the urban area as shown in Figure 8.2. The change in estimated poTTutant
concentrations with time is dependent upon the flux of" emissions within the box,
the inflow and outflow of pollutants, the dilution which occurs as the mixed
layer rises, entrainment of species from aloft and formation and destruction of
species through chemical reaction. The model assumes that all emissions are
instantaneously mixed throughout the box and ignores horizontal dispersion.
The model requires overall urban emissions of CO, VOC (divided into
five classes), and NO for each hour of the modeling period (i.e., 0500 to
J\
1700). No spatial resolution of emissions is required. The output gives a
single city-wide prediction for each pollutant for each hour in the modeling
period. The box model has been exercised using several days during the summer of
1976. Air quality data from twelve monitoring sites were used in comparing
predicted and observed concentrations. The comparison showed generally good
110
-------
RISIMC MIXED
HEIGHT
Figure 8.2 Schematic diagram of modeling domain for
the Photochemical Box Model.
Ill
-------
agreement between predicted and observed concentrations of CO, NO, N02 and 03
averaged among numerous monitoring sites within a city.32
The PBM has many of the same advantages and problems as the OZIPM
model. An additional advantage is that it is not dependent on the proper location
of NO/NOX monitors. One problem with the Box Model which has yet to be solved is
that it predicts the spatially averaged N02 concentration throughout the box. A
way must be found to relate this spatial average to peak N02 concentrations for -
the model to be directly usable to assess the impact of controls to reduce high
short term concentrations of N02> It is not certain whether or when this latter
difficulty can be overcome.
8.1.4 Complex Multi source Models
This section identifies models which are distinctive from those described
in Sections 8.1.1 - 8.1.3 in that they reflect greater attempts to incorporate
physical/chemical phenomena believed to occur in urban areas. However, any
resulting increase in credibility occurs at the expense of increased data and
computer requirements for running the model. Unless a suitable data base has
already been compiled for ozone modeling, it is not likely that such models can
be exercised to assess the effectiveness of control strategies to reduce N02
without a lengthy and expensive effort to assemble an adequate data base.
Table 8.2 identifies the complex multisource photochemical models discussed in
greater detail in Reference 33.
8.2 Models to Assess the Impact from Individual Sources
The models identified in Section 8.1 have been primarily used to estimate
the impact of numerous rather than individual sources. Simple models like roll-
back which have no spatial resolution are not well suited for estimating the
112
-------
Table 8.2, Available Complex Multisource Photochemical Dispersion Models
Model
ELSTAR
IMPACT
LIRAQ-2
MADCAP
TRACE
URBAN AIRSHED
Type
Trajectory
Grid
Grid
Grid
Trajectory
Grid
Additional
References
34
35
36, 37
38, 39
40, 41
42
113
-------
Impact of an individual source. Monitoring data collected in the vicinity of
large sources of NO suggest that, at any given time, the maximum impact of an
/v
individual source may be quite localized. For example, Figure 7.9a (page 87)
shows that the plume width from the Labadie power plant is only about 4 km across
pc
as far as 21 km downwind from the plant. As shown in Figure 8.3, Cole et al.
found that the identifiable impact on NOp of a major freeway occurred within a
43
few hundred meters of the roadway. Even the most complex multisource grid
models typically have no greater horizontal spatial resolution than 2-4 km. This
limitation is necessary in order to assure computational feasibility. Conse-
quently, models which focus on individual sources may be useful for new source
review or, if it is desirable, to assess the impact of controlling a major source
in the vicinity of that source.
Cole and Summerhays have described several different approaches which have
44
been used to estimate NOp concentrations in point source plumes. In addition,
Cole et al. have applied a relatively simple approach to estimate NOp concentra-
43
tions downwind from a major freeway with some success. Only the more simple
approaches will be described herein. Information concerning a refined reactive
plume model is available in Reference 45.
The principal distinguishing feature between simple and complex models for
individual sources of NO is that simple approaches do not account for the inter-
/\
action of the NO plume with organic pollutants. However, this limitation is not
/\
a serious one insofar as predicting peak N02 is concerned. A number of field
investigations have indicated that peak N02 within an identifiable plume of an
individual source occurs far too rapidly for reactions with organic pollutants to
pr nc•py *o
cause a significant part of the problem. ' ' ' Further, several of the
114
-------
N02 (ppm)
1.14
L12
D.N
D.06
0.04
O ).02
LOW OZONE LEVEL .
MEDIUM OZONE LEVEL
HIGH OZONE LEVEL
D
O
A
•
4
9
BACKGROUND NO,
AVERAGES
ARBITRARILY PLOTTEl
At 500 METERS
'
r ).oo
1
0 40
1
80
1
120
1
160
1
200
1
240
1
280
1 1
320 360
1
400
I
440
1
1
1 1 1
480 52
DISTANCE DOWNWIND FROM FREEWAY, METERS
Figure 8.3 Nitrogen dioxide (N02) data from the N02/03 sampler siting study.
-------
simple approaches appear to agree quite well with observed data. Hence, they
should be adequate for use in evaluating N02 control strategies.
Four approaches are identified in Table 8.3. These approaches are described
26 44 45
in detail in several references. ' ' All of the approaches require the user
to first estimate maximum hourly NO concentrations using widely available models
/\
for inert pollutants. * '* In the case of major roadways, a "line source"
47 48
model such as HIWAY ' would be used for these purposes. Appropriate point
source models are available on the EPA's UNAMAP system. ' Of the models
described in Table 8.3, the Ozone-Limiting and the Photostationary State Approaches
are preferable to the others, because they consider the limitations on peak N02
posed by the oxidizing potential of dilution air. The Ozone-Limiting Approach is
more practical than the Photostationary State Approach because of the general
lack of rate constant data available to persons likely to be formulating control
strategies. Preliminary testing of the Ozone-Limiting Approach and several other
N02 models for individual sources has been performed using aircraft measurements
of power plant plumes in the St. Louis region. The Ozone-Limiting Method was
found to be reasonably accurate and performed slightly better than a complex
reactive plume model, as well as other simple models. Consequently, a recom-
mended procedure for applying the Ozone-Limiting Approach is presented in greater
detail.
The ozone-limiting approach involves two initial comparisons: (1) comparison
of the estimated maximum ground level NO concentration [NO ] and the maximum
X X nlflX
ambient 0, concentration prevailing at the time of day when [NOV]__V is estimated,
O X il'clX
and (2) comparison estimated [NOV] during midafternoon with prevailing ozone
x max
levels at that time. If 0, concentration is greater than [NO L-,v, all of the
•3 x max
116
-------
TABLE 8.3 Approaches for Estimating Maximum Impact on Peak N02 from Large Individual Sources
1) Partial Conversion
Exponential Decay
Hane
3) Ozone Limiting Approach
4) Photostationary State Approach
Description
1) Multiply predicted maximum NOX concentration
by a constant factor to predict peak N02--
Multiply predicted NOX concentrations by an
exponential decay factor
N02 = Hx
where [NO ] ax - peak ground level N0x con-
centrations estimated with a model for inert
pollutants
k, « the ratio constant in reaction with
which N02 photodisassociates in the preset
of sunlight: k
NO, £- NO + 0
ice
k, = rate constant depicting the rate with
which 03 confines with NO to form N02:
NO
03 - N02
02
Remarks
It is difficult to determine what the factor should
be. Ambient measurements of NO?/rJOx ratios within
pluses vary widely. (See fig. 7.6) Generally, the
greater the dilution, the hicher the ratio. As a
rule, routinslv collectec .T.on "Storing date a"E not
useful for these nurposes, because ihey do rot refie
the impact of point sources. Monitoring cata col-
lected in the vicinity 'vZOOm of a major roed/:ay may
be of use for roadway applications.
Conversion of fJO to N02 appears to be li^:-ri by the
oxidizing potential of the dilution air. Also, the
half life of the NO is determined by dilution rather
than chemistry. Any value chosen for the half life
would have to be arbitrary.
Remarks
) As discussed in Chapter 7.0, the summation of
(NO + N02 + 03) appears to be constant across
point source plurces (see Figures 7.9 and 7.10).
This implies that NO to NP.2 conversion in plumes
with high NO is limited by prevailing ambient
ozone. Fewer arbitrary assumptions are required
using this method than methods (1) or (2). Method
assumes that complete mixing of the NO plumes oc-
currs at the point where maximum ground level NO
occurs. /It also ignores the likelihood of further
photodisassociation of NO; upon- formation. Measure
rrents or estimates of prevailing ar.bient Oi levels
at the time (NO ) occurs are needed.
4) White26 and Cole et al.1*3 have found this ap-
proach to work well in predicting maximum N0? down-
wind from a major point source and a major line
source. The approach assumes thorough mixing of
the plumes with anbient air at the point where
(N0x)ma occurs. Measurements of the k,.rate con-
stant are not typically available. The approach also
requires measurement of prevailing 03 concentrations
at the time (NO )max occurs.
117
-------
NOV is assumed to be NQ9. If [NO ]_,„ is greater than the prevailing 0, con-
X <- X fflciX *j
centration, N02 formation is "ozone-limited." In this latter case, the maximum
N(>2 concentration is assumed to be equal to the Og concentration plus a correction
factor which accounts for NO emitted as N09 and near-stack thermal conversion.
X £-
For combustion sources, the correction factor is assumed to be 10 percent of the
estimated [NO ] . The Ozone-Limiting Procedure is outlined in four steps
X HldX
below.
1. A standard dispersion model for inert pollutants is used to
calculate [N0x]max>
2. [N0v]mav is separated into two components:
X niaX
a) portion emitted as N02 or thermally converted, equal to
t> ' [N0v]m,v, where "b" is assumed to be: .10 for combustion sources;
X maX.
b) the remaining NOV fs assumed to be NO subject to conversion by
X
0.,. This NO is 0-b)[NOvl v or .90[NOv]mav for combustion sources.
j x max x max
3. a) If [0,] > (1-b) [N0jm .. then assume that all of the NO is
J X UlaX
converted to N02 (i.e., [NO^^ = [N02]max).
b) If (1-b)[NOx]max > [03], then
This procedure should be used to address two situations: (i) when NO is
/\
maximum, and (ii) when 0, is maximum. Note that the above procedure can also be
readily applied to assess the impact from other industrial point sources (e.g.,
nitric acid plants, natural gas pumping compressor stations) which emit a larger
118
-------
fraction of NO as N09. One would simply increase the "b" value, described in
A £
step 2a), to whatever fractional value is appropriate for these sources.
The procedure is illustrated with two examples.
Example 3a
Given: A point source dispersion model for inert pollutants estimates
[NO ] = 0.30 ppm to occur in midmorning as the result of
fumigation of a power plant's plume. Existing ozone monitors
suggest that the highest 03 concentration for this time of day
is 0.10 ppm.
Find: Maximum impact of the point source on hourly ambient NO^ levels.
Solution:
(1) Since a power plant is a combustion source, assume b = 0.10
(2) Since (0.90)[NOw]maw = 0.27ppm > 0.10 ppm = [0,],
x max 3
is ozone-limited
C°33 + b ' [NOAax
= 0.10 + (.10)(.30)
=0.13 ppm
Example 3b
Given: For the power plant in Example 3a, [NO ] for midafternoon is
estimated to be 0.15 ppm. Peak ozone at tins time is measured to
be 0.20 ppm.
Find: Maximum impact of the point source on hourly ambient NO^ levels.
Solution:
HT~ b = 0.10
(2) (1-b) [N0x]max = (.90)(.15) = 0.14 ppm < 0.20 = [03]
Therefore, [NO^] max is NO limiting
- 0.15 ppm
119
-------
One additional facet of model fug, procedures for N02 needs to be discussed.
This is the integration of multisource and individual source techniques described
in Sections 8.1 and1 &..2y respectively. In the complex multisource models enumerated
in Section 8.1.4, this integration has already occurred. That is, these models
already include both area and point sources in their emission inventories.
Because complex models can more accurately simulate source-receptor relationships,
the impact of elevated sources on ground!evel concentrations have a greater
chance of being correctly estimated. The only occasion when it may be desirable
to separate a point source from an inventory used in a complex multisource model
would be if one were interested in assessing the microscale impact of a parti-
cular individual source. In such a case, the procedure would be to:
(1) eliminate that particular source from the inventory;
(2) use the multisource model to estimate NO, NOp and ozone concen-
trations attributable to all other sources;
(3) apply an individual source model (e.g., the Ozone-Limiting Approach)
to simulate the impact of the source of interest, using the multisource model's
estimates as the means for characterizing the oxidizing potential of the dilution
air.
In order for the rollback model to be an acceptable approach for estimating
the impact of control strategies on peak short term N02 concentrations, it is
necessary to account for source-receptor relationships in some rudimentary way.
As discussed in Section 8.1.1, it will usually be appropriate to exclude elevated
point sources from the inventory in applying the rollback model to short term
situations. If this is done, however, use of the rollback model in individual
120
-------
areas needs to be supplemented by separate consideration of large individual
sources not impacting a monitoring site. The recommended procedure is as follows.
(1) Review available SCL and N02 data on occasions with high NCL using
techniques described in Chapter 7.0.
a. If there is a clear correspondence between N02 and S02 peaks,
include emissions from elevated point sources in the emission terms on the right-
hand-side of Equation (8.1). Estimate required controls using Equation (8.2).
b. Otherwise, project future N02 concentrations using Equation (8.1)
excluding elevated point source emissions from the emission terms. Proceed to
step (2).
(2) Apply the Ozone-Limiting Approach to estimate peak impact of
selected large individual sources on ground level NCU.
(3) Repeat (2) for several scenarios as needed (e.g., fumigation case,
afternoon impact during periods with maximum 03> etc.).
(4) Repeat (l)(b) using N02 design values which are appropriate for
the times of day assumed for each scenario in (3) and occur nearest to the location
where the peak industrial source impact occurs. If there are no monitoring data
near this location, use a spatially averaged value for the appropriate time of
day.
(5) For each scenario assumed in steps (3) and (4), superimpose the
estimated peak point source impact on the projected future level of N02 obtained
with the rollback model.
121
-------
The foregoing procedure is illustrated numerically in Table 8.4.
8.3 Implications
T. Two major categories of models have been discussed: multisource
models and individual source models. In order to provide comprehensive quanti-
tative estimates of controls needed to attain a short term air quality goal for
NOp, both kinds of models need to be utilized. In designing control strategies
to meet an annual NOp air quality goal, it is generally sufficient to utilize
multisource models alone. If rollback is the multisource model utilized, emissions
from large individual point sources may be ignored in applying the model to
estimate the impact of controls in a achieving annual average, air quality goals.
2. Although several multisource models exist, the one that is most widely
available and easiest to apply is the linear rollback model. Rollback is suitable
for use in both annual and short term applications. Peak and mean NCL concen-
trations appear to be proportional to NO under most circumstances. Consideration
A
of the impact of reducing NMOC can be made by supplementing the rollback model as
illustrated in Example 8.2 (Section 8.1.1).
3. Several relatively simple approaches for estimating the maximum impact
from large individual sources of NO (e.g., a public utility or a major highway)
/\
have been described in Section 8.2. Of these, the Ozone-Limiting Approach appears
to represent the best available compromise between accuracy and data requirements.
This approach is also readily adaptable for modeling sources which emit large
proportions of NOX directly as N02-
122
-------
TABLE 8.4 Hypothetical Examples Illustrating Use of Multi and Individual Source Models
for Projecting Future Short Term N02 Concentrations for Use in SIP's
Scenario
1) Observed nighttime-early morning peaks. Im-
pact from elevated point source assumed to be
negligible due to nocturnal inversion.
Time of Day
1900-0800
Estimated Peak Individual
Source Contribution
Projected Multi -
Source Contribution
0.30 ppm
Projected Peak N02
0.30 ppm
ro
CO
2) Observed mid-morning peaks. Multisource syn-
thesis of N02 superimposed by fumigation im-
pact from an elevated point source.
0900-1100
0.13 ppm
0.20 ppm
0.33 ppm
3) Observed mid-afternoon peaks. Potential
point source impact is relatively high due
to high observed 0^'levels at this time.
Projected multisource impact is low due to
low N02 concentrations observed by existing
monitors during this time.
1200-1800
0.15 ppm
0.06 ppm
0.21 ppm
-------
4. Where available, SOp data should be used to verify the impact of large
stationary point sources on high observed hourly concentrations of NO- using the
procedure outlined in Chapter 7.0. In cases where a review of available evidence
indicates a lac.k of point source impact, NO emissions from major elevated-point
/\
sources should be eliminated from the emission inventory when using rollback.
Peak impacts on NO- estimated from large individual sources with the Ozone-
Limiting Approach should be superimposed on projected NO- concentrations thus
obtained with the rollback model. This procedure is illustrated more completely '
by the flow chart in Figure 8.4.
5. The procedure described in paragraph 4 can be repeated for several
scenarios using consistent estimates of impacts from multiple sources and large
individual sources. An example of several scenarios is illustrated in Table 8.4.
Such repetition should better insure protection of the public against excessively
high short term exposure to NO-.
6. To be implemented, the procedure outlined, in paragraphs 4 and 5 requires
several pieces of data and judgments to be made which may not at first be obvious.
a. A certain amount of expertise regarding local meteorological conditions
is needed in order to arrive at a realistic set of scenarios.
b. Measurements of peak hourly NO- concentrations and ozone at times
of day which are consistent with selected scenarios need to be made at monitoring
sites which are likely to represent NO- levels encountered in widespread areas of
the city.
c. It is desirable, but not essential, to have estimates of the
prevailing NMOC/NO ratio in the city being modeled.
,j\
124
-------
(5)
Use line source model
& Ozone limiting approach
to estimate the impact
of roadway. Add pro-
jected rollback cone.
using N02 design value
from upwind site to
estimate future N02.
Exclude major point
sources from rollback
inventory.
(4)
Are we estimatin
impact of indiv
roadway?
No
(6f
Do
high N02 and
coincide
?
(3) \
Use rollback model \
without emissions !
From elevated point /
sources /'
(7)
Use rollback model
including emissions
from elevated point
sources
(8)
Project future N02
using rollback with-
out elevated point
source emissions in
base or future
oeriods_
1
(9)
Likely impact on
future observed N02
(io) !
Use ozone limiting
approach to estimate
impact of large point
source of N02 for a
meteorological scenario'
similar to that in (8) :
(11)
Add impacts estimated
in (8) and (10)
meteorological
scenarios of interes
Jseen considere
(12)
Estimated impact on
peak future N02 for
specified meteorological
scenario
(14)
Consider next scenario
using appropriate design
valuesin rollback and
met. assumptions in the
ozone limiting approach
Figure 8.4. Procedure for Applying Rollback 1n Assessing the Impact of
Control Strategies on N02
125
-------
d. A judgment needs to be made about future prevailing levels of
ozone. Since it is anticipated that areas violating any prospective short term
air quality goal for NO^ are also likely to violate the hourly MAAQs for ozone,
estimates of future peak afternoon ozone values should be available. Estimates
of future nighttime or morning ozone concentrations should be consistent with
those made about future transported ozone in ozone SIP's.
7. The modeling approaches described in this chapter imply that-it may be
possible to estimate a prospective violation of a short term NO^ NAAQS even
though one has not been observed and may not be likely to be observed by existing
monitoring networks. For example, if an hourly NCL NAAQS of 0.25 ppm were estab-
lished, hypothetical Scenario 2 in Table 8.4 would be a case in point.
126
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9.0 REFERENCES
1. National Air Quality, Monitoring, and Emissions Trends Report 1977,
EPA-450/2-78-052, pages 5-11, Office of Air Quality Planning and Standards,
U.S. Environmental Protection Agency, Research Triangle Park, North Carolina,
December 1978.
2. Air Quality Criteria for Oxides of Nitrogen (draft), Chapter 6, revised
June 1980.
3. J. R. Martinez, Survey of the Role of MO in Nonurban Ozone Formation,
EPA-450/4-79-035, September 1979. x
4. National Emissions Data System (NEDS), National Air Data Branch, U.S.
Environmental Protection Agency, Research Triangle Park, North Carolina.
5. Research Triangle Institute (1975), Investigation of Rural Oxidant Levels
as Related to Urban Hydrocarbon Control Strategies, EPA-450/3-75-036,
March 1975.
6. C. E. Decker, et al., Formation and Transport of Oxidants Along the
Gulf Coast and in Northern U.S.. EPA-450/3-76-033, August 1976.
7. W. C. Eaton, C. E. Decker, J. B. Tommerdahl, F. E. Dimmock, Study of
Nature of Ozone, Oxides of Nitrogen and Nonmethane Hydrocarbons in Tulsa,
Oklahoma, Research Triangle Institute, RTI/1466/000, F. April 1979.
8. SAROAD, 1975-1977, SAROAD Data Summaries, maintained by National Air
Data Branch, U.S. Environmental Protection Agency, Research Triangle Park,
North Carolina.
9. J. R. Martinez, K. C. Nitz, Analysis of High NO? in California. 1975-
1977. EPA-450/4-79-034a, b, c, August 1979.
10. SAROAD 1975-1977, SAROAD Hourly Concentrations, maintained by National
Air Data Branch, U.S. Environmental Protection Agency, Research Triangle
Park, North Carolina.
11. D. H. Sennett, personal communication concerning a Brief Analysis of
LACS Data Base, LACS Study, summer 1978, Los Angeles, California.
12. Personal communication of A. J. Hoffman to J. D. Sableski, subject "N02
Siting Guidance," July 14, 1978.
13. U.S. Environmental Protection Agency, "Ambient Air Quality Monitoring,
Data Reporting and Surveillance Provisions," Federal Register
(40CRF 51,52,53,58), 44 No. 92 (May 10, 1979), pp. 27558-27604.
14. T. C. Curran, Screening Procedure for Ambient Air Quality Data,
EPA-450/2-78-037, U.S. Environmental Protection Agency, Research Triangle
Park, North Carolina, July 1978.
127
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15. H. G. Richter, E. L. Meyer, and D. H. Sennett, "A Graphical Procedure
for Screening and Analyzing High N02 Concentrations," Paper 79-12.4,
presented at 72nd APCA Meeting, Cincinnati, Ohio, June 1979.
16. C. W. Spicer, The Fate of Nitrogen Oxides in the Atmosphere,
EPA-600/3-76-030, March 1976.
17. C. W. Spicer, D. W. Joseph, P. R. Sticksel, G. M. Sverdrup., and G. "F. Wood,
Reactions and Transport of Nitrogen Oxides and Ozone in the Atmosphere.
draft, EPA Contract 68-02-2439, U.S. Environmental Protection Agency,
Research Triangle Park, North Carolina, June 1979.
18. C. W. Spicer, J. R. Koetz, G. W. Keigley, G. M. Sverdrup, and .G. F. Wood,
A Study of Nitrogen Oxide Reactions Within Urban Plumes Transported Over
the Ocean, draft, EPA Contract No. 68-02-2957, U.S. Environmental Protection
Agency, Research Triangle Park, North Carolina, November 1979.
19. J. C. Trijonis, T. K. Peng, G. J. McRae, and L. Lees, Emissions and Air
Quality Trends in the South Coast Air Basin, EQL Memo No.. 16, January 1976,
California Institute of Technology, Pasadena, California.
20. J. C. Trijonis, "Dependence of Ambient M02 on Precursor Control," pap.er
presented at EPA Technical Symposium on Implication of a .Low NO Vehicle
Emission Standard, May 2-4, 1979, Reston, Virginia.
21. J. C. Trijonis, Empirical Relationships Between Atmospheric Nitrogen
Dioxide and Its Precursors, EPA-600/3-78-018, February 1.978.
22. Uses, Limitations, .and Technical Basis of Procedures for Quantifying
Relationships Between Photochemical Oxidants and Precursors,
EPA-450/2-77-021a, U.S. Environmental Protection Agency, Research Triangle
Park, North Carolina, November 1977.
23. Procedures for Quantifying Relationships Between Photochemical Oxidants
and Precursors: Supporting Documentation. EPA-450/.2-77-021b. U.S.
Environmental Protection Agency, Research Triangle Park, North Carolina,
February 1978.
24. R. B. Evans, The Contribution of Ozone Aloft to Surface Ozone Maxima,
PhD Dissertation, University of North Carolina, Chapel Hill, North Carolina,
1979.
25. W. H. White, J. A. Anderson, W. R. Knuth, D. L. Blumenthal, J. C. Hsiung,
and R. B. Husar, Midwest Interstate Sulfur Transformation and Transport
Project: Aerial Measurements of Urban and Power Plant Plumes. Summer 1974,
EPA-600/3-76-110, November 1976.
26. W. H. White, NO - 0$ Photochemistry in Power Plant Plumes: Comparison
of Theory with Observation, Environmental Science and Technology II (10)
October 1977, page 995.
128
-------
27. D. Hegg, P. V. Hobbs, L. F. Radke, and H. Harrison, "Reactions of Ozone
and Nitrogen Oxides in Power Plant Plumes," Atmospheric Environment 11, (6),
page 521.
28. Acurex Corporation, Control Techniques for Nitrogen Oxides Emissions from
Stationary Sources - 2nd Edition, EPA-450/1-78-001, January 1978.
29. N. deMevers and J. R. Morris, "Rollback Modeling: Basic and Modified,"
JAPCA 25, page 943, September 1975.
30. J. H. Wilson, Jr., "Methodologies for Projecting the Relative Air Quality
Impacts of Emission Control Strategies," Paper 78-4.1, presented at 71st
Annual Air Pollution Control Association Meeting, Houston, Texas, June 1978.
31. G. Z. Whitten and H. Hogo, User's Manual for Kinetics Model and Ozone
Isopleth Plotting Package. EPA-600/8-78-014a, July 1978.
32. K. L. Schere and K. L. Demerjian, "A Photochemical Box Model for Urban
Air Quality Simulation," Proceedings 4th Joint Conference on Sensing of
Environmental Pollutants, American Chemical Society, 1978.
33. R. Y. Wada, M. J. Wong, and E. Y. Leong, Applicability of Selected Models
for Addressing Ozone Control Strategy Issues. EPA-450/4-79-026, December 1979.
34. F. Lurmann, D. Godden, A. C. Lloyd, and R. A. Nordsieck, A Lagrangian
Photochemical Air Quality Simulation Model. EPA-600/8-79-015a, b, June 1979.
35. A. Fabrick, R. C. Sklarew and J. G. Wilson, "Point Source Model Evaluation
and Development Study, Appendix C, User Guide to Impact," prepared for
California Air Resources Board and the California Energy Resources
Conservation and Development Commission by Science Application, Incorporated,
Westlake Village, California, SAI Contract No. A5-058087, March 1977.
36. M. C. MacCracken, et al., "The Livermore Regional Air Quality Model: I.
Concept and Development," Journal of Applied Meteorology 1_7, March 1978,
pages 254-272.
37. W. H. Duewer, et al., "The Livermore Regional Air Quality Model: II.
Verification and Sample Application in the San Francisco Bay Area,"
J. Applied Meteorology 17, March 1978, pages 273-311.
38. R. C. Sklarew, M. A. Joncich, and K. T. Tran, "An Operational Air Quality
Modeling System for Photochemical Smog in the San Diego Air Basin," pre-
sented at the Annual American Meteorological Society Meeting, Reno, Nevada,
January 1979.
39. R. C. Sklarew, "Verification of the MADCAP Model of Photochemical Air
Pollution in the San Diego Air Basin," presented at the Annual American
Meteorological Society Meeting, Reno, Nevada, January 1979.
129 .
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'4*6. P. J. Orivas, '"TRACE (Trajectory Atmospheric Chemistry and Emissions) User's
Guide,*' Pacific Efiivrowmantal Services, Incorporated, Santa Monica, California,
PES Report TPF016, November 1977.
41. P. J. Drivas and L. G. Wayne, "Validation of an Improved Photochemical
Air tjaallty Sfmilatitwi "Model," Pacific Environmental Services, Incorporated,
Santa -Monica, California, PES Report TP-014, March 1977.
42. S. D. Reynolds, L. E. Reid and T. W. Tesche, "An Introduction to the
SAI Airshed Model and Its Usage," Systems Applications, Incorporated,
San Rafael, California, SAI Report EF 78-53R, December 1978.
43. H. S. Cole, N. Mayer, and H. Fowler, "The Impact of an NO Diesel Waiver
on N02 Concentrations Downwind of a Major Line Source," U.S. Environmental
Protection Agency, Office of Air Quality Planning and Standards, internal
report, May 1979.
44. H. S. Cole and J. E. Summerhays, "A Review of Techniques Available for
Estimating Short Term N02 Concentrations," JAPCA 29 (8), August 1979,
pages 812-817.
45. D. Stewart and M. A. Yocke, User's Guide to the Reactive Plume Model -
RPM-II, SAI Report EI-79-93R, EPA Contract No. 68-02-2775 (1980) in
press.
46. C. Hosier, "Meteorological Program of the Environmental Protection Agency,
Bull American Meteorological Society 56 (12), December 1975, pages 1261-
1270.
47. D. B. Turner, "Atmospheric Dispersion Modeling: A Critical Review," JAPCA
29_ (5), May 1979, pages 502-519.
48. J. R. Zimmerman and R. S. Thompson, User's Guide for HIWAY, A Highway
Air Pollution Model. EPA-650/4-74-008, February 1975.
49. Guidelines for Air Quality Maintenance Planning and Analysis, Volume 9
(revised), Evaluating Indirect Sources, EPA-450/4-78-001, September 1978.
50. M. A. Yocke, et. al, Evalution of Short Term N02 Plume Models for
Point Sources. SAI Report 103-EF-80-91, EPA Contract No. 68-02-2775,
(1980), in press.
130
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10. ACKNOWLEDGEMENT
The principal authors of this document are Edwin L. Meyer, Jr.,
Donald H. Sennett, Henry S. Cole and Harold G. Richter. Valuable contributions
have been made by David Iverach. The authors are also indebted to
Carole J. Mask for her very able typing and clerical support.
131
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(5)
Use line source model
& Ozone limiting approach
to estimate the Impact
of roadway. Add pro-
jected rollback cone.
using N02 design value
from upwind site to
estimate future H02.
Exclude major point
sources from rollback
inventory.
(4)
e estlmatlri
impact of Indlv
roadway?
6)
Do
high N02 and SOS
coincide
Use rollback niodel
without emissions
from elevated point
sources j
(7)
Use rollback model
Including emissions
from elevated point
sources
(8)
Project future N02
using rollback with-
out elevated point
source emissions In
base or future
periods
>
s
(9)
Likely Impact on
future observed NO?
(10)
Use ozone limiting
approach to estimate )
impact of large point '
source of N02 for a
meteorological scenario1
similar to that in (8) I
(ii)
Add Impacts estimated
In (8) and (10)
r^
(12)
Estimated Impact on
peak future N02 for
specified meteorological /
scenario ^J
A
Have all meteorological
scenarios of 1 nteres
een considered^
Consider next scenario
using appropriate design
values in rollback and
met. assumptions in the
ozone limiting approach
_\___
I
Figure 8.4 Modeling procedures for N02 SIP'S
-------
TABLE 8.3 Approaches for Estimating Maxima iapact on Peak Ho2 from Large Individual Sources
Kan,
1) Partial Conversion
Exponential Decay
Name
3) Ozone Uniting Approach
4) Photostationary State Approach
. 1) Multiply predicted naxlmum NO concent'ratio
i by a constant factor to predict peak NO,--
'2)
Multiply predicted NO, concentrations by an
exponential decay factor
N02 •
• where t - travel time from source, tine center
T • half 11ft conversion time of HO to N02.
consistent units
b • fraction of NO. emitted as NO,, as smart to
be 0.10 for combustion sources, could be
higher for Industrial process sources.
(NO,) • laxlnum estimated ground level
concentrations, of nOK using an Inert pol*
lutant model.
.3) Method assumes that maximal N02 concentration
can be Halted by dilution of an NO, plume
and/or the ozone concentrations In the di-
lution air. If oione Is the limiting factor.
•here [0,]_ • art lent ozone concentration* It •
the tine peak NO. occurs.
b • frarttnn of NO emitted as NO,
[NO J • Maximui ground level X0f estimated
wltR an inert point source Model .
4) Approach confines (NO )HI estimated with an
:1nert pollutant model nun the photo*'»f ionary
state relationship. As a result, [NO} L, Is
estimated as follows:
•here [NO^]^, • peak grcund level NOj con.
ctntrations estimated «1th a •odel for inert
pollutants
k) ' the ratio constant In reaction with
•Men N02 photodisassoclates 1n the presence
of sunlight: n
S_
NO,
NO * 0
k, * rate constant depicting" the rate wit*'
•filch 0, coifclnes «1tti NO to fora NO,:
«0 » 0, - NOj
02
Remarks-
1) It 1s difficult to determine what the factor should
be. Artfent measurements of NOi/NO, ratios within
plumes vary widely. (See fig. 7.6) Generally, the
greater the dilution, the higher the ratio, '-l a
mle. routinely collected mon'taring date irt not
useful for these purposes, because they do not refle
tke impact of point sources. Monitoring data col-
lected in the vicinity vJOOm of a najpr roac<.-ay may
•« of use for roadway applications.
Conversion of NO to N02 appears to be lirr.fte- by the
Oxidizing potential of the dilution air. ~,-,i. the
half life of the no It dettrcined by dilution rather
than checistry. Any value chosen for the half life
Mould have to be arbitrary.
Remarks
3) As discussed in Chapter 7.0. the sumption of
(NO » NO; » Oj) appears to be constant across
point source plunes (see Figures 7.9 and 7.10).
This Implies that NO to NO? conversion In plums
with high NO is limited by prevailing ant lent
ozone. Fewer arbitrary assumptions are required
«1ng this method than methods (1) or U). Method
assumes that complete mixing of the NO piumes oc-
._c«rrs at the point where maximum ground level NO
•ccurs. /It also Ignores the likelihood of further
photodisassoclatlon of NO- upon- formation. Measure-
ments or estimates of prevailing ancient Oi levels
at the tine CO,),,, occurs are needed.
Unite" and Cole et al." have found this ap-
proach to work well 1n predicting maximynt NO* down-
wind from a aajor point source and a major line
source. The approach assumes thorough mixing of
the plumes with anbient air at the point where
(NO ) occurs. Measurements of the k. rate con-
stant Ire not typically available. The approach also
requires measurement of orevallino 03 concentrations
at the time (*>„)„,„ occurs.
-------
T-'"' r P. 3 Ajprr.-icre'i for Estimating Maximum Impact on Peek DC? froa> Large Individual Sources
; 1) Partial Conversion
Description
1) Multiply predicted maximum NOX concentration
by a constant factor to predict peak N02.'
Remarks
1) It is difficult to determine whejt the factor should
be. Ambient measurements of N0i/N0x ratio* within
plumes vary widely. (See Fig. 7.6) Generally, the
greater the dilution, the higher the ratio. As a
rule, routinely collected monitoring data are not
useful for these purposes, because they do rvst reflect
the Impact of point sources. Monitoring data col-
lected in the vicinity -v-ZOOm of a major roadway may
be of use for roadway applications.
Exponential Decay
2) Multiply predicted NOX concentrations by an
exponential decay factor
N02 = (rb)(NOx) (l-e'"jT/T) + (b)(N°x)
• where t = travel time from source, time center
T = half life conversion time of NO to N02,
consistent units
b = fraction of N0y emitted as NO,, assumed to
be 0.10 for combustion sources, c6uld be
higher for industrial process sources.
(NO ) = Maximum estimated ground level
x m3 x _ . _
concentrations of NO usmc an Inert pol-
lutant model.
2) Conversion of NO to N02 appears to be llrri-.e. by the
oxidizing potential of the dilution air. Aiso, the
half life of the NO 1s determined by dilution rather
than chemistry. Any value chosen for the half life
would have to be arbitrary.
-------
TABL- "J..1 Approaches for Estimating Maximum Impact on Peak .'IP. From Larno Individual Sources
Nane
3) Ozone Limiting Approach
3)
Description
Method assumes that maximum N02 concentration
can be limited by dilution of an NOx plume
and/or the ozone concentrations in the di-
lution air. If ozone is the limiting factor.
[NO,] = TO,] = [HO ]
L ?J max L 3Jt xj
max
where [0-,] = ambient ozone concentrations at
the time peak NO-* occurs.
b = frarHon of NO emitted as N02
[NO ] = Maximum ground level NO estimated
with an inert point source model.
Remarks
3) As discussed in Chapter 7.0, the summation of
(NO + N02 + 03) appears to be constant across
point source plumes (see Figures 7.9 and 7.10).
This Implies that NO to N02 conversion in plunes
with high NO is limited by prevailing ambient
ozone. Fewer arbitrary assumptions are required
using this method than methods (1) or (2). Method
assumes that complete mixing of the NO plums oc-
currs at the point where maximum ground level NO.
occurs. /It also ignores the likelihood of further
photodisassodation of N02 upon formation. Measure-
ments or estimates of prevailing ambient 0, levels
at the time (NO ) occurs are needed.
4) Photostationary State Approach
4) Approach combines (NO )max estimated with an
-.inert pollutant model witn the photo^fl Honor
state relationship. As a result,[NO?] is
estimated as follows: max
[NO,]
max
where [NO ] = peak ground level NO con-
x max x
centrations estimated with a model for inert
pollutants
k] = the ratio constant in reaction-with
which N02 photodisassociates in the presence
of sunlight: n
N02 i^-- NO + 0
k, = rate constant depicting" the rate with
which 0-, combines with NO to form N02:
NO + 03 -* N02 + 02
K 3
4) White" an(j cole et al.^ have found this ap-
proach to work well in predicting maximum NO? down-
wind from a major point source and a major line
source. The approach assumes thorough mixing of
the plumes with ambient air at the point where
(N0x)max occurs. Measurements of the kj rate con-
stant are not typically available. The approach also
requires measurement of prevailing 03 concentrations
at the time (W)) v occurs.
X HiaX
-------
TABLE 8.4 Hypothetical Example Illustrating Use of Multi- and Individual Source Models for Projecting Future Short-Term
concentrations tor use in
Scenario
1) Observed nighttime-early morning peaks. Im-
pact from elevated point source assumed to be
negligible due to nocturnal Inversion.
Time of Day
1900-0800
Estimated Peak Individual
Source Contribution
Projected Multi -
Source Contribution
0.30 ppm
Projected Peak N0g
0.30 ppm
2) Observed mid-morning peaks. Mu1t1source syn-
thesis of N02 superimposed by fumigation Im-
pact from an elevated point source.
0900-1100
0.13 ppm
0.20 ppm
0.33 ppm
3) Observed mid-afternoon peaks. Potential
point source Impact is relatively high due
to high observed 0^'levels at this time.
Projected multlsource Impact Is low due to
low N02 concentrations observed by existing
monitors during this time.
1200-1800
0.15 ppm
0.06 ppm
0.21 ppm
-------
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
i. REPORT NO.
EPA-450/4-80-017
2.
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
5. REPORT DATE
Technical Basis for Developing Control Strategies
for High Ambient Concentrations of Nitrogen Dioxide
September 1980
6. PERFORMING ORGANIZATION CODE
7'AUTEHciwin L. Meyer, Donald H. Sennet, Henry S. Cole and
Harold G. Richter
8. PERFORMING ORGANIZATION REPORT NO
9. PERFORMING ORGANIZATION NAME AND ADDRESS
U. S. Environmental Protection Agency
Office of Air Quality Planning and Standards
Monitoring and Data Analysis Division (MD-14)
Research triangle Park, North Carolina 27711
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
12. SPONSORING AGENCY NAME AND ADDRESS
13. TYPE OF REPORT AND PERIOD COVERED
Same
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRACT
Report presents information which may prove useful in designing control strategies
to reduce high short-term and/or annual concentrations of nitrogen dioxide (N02).
Specific implications of findings of the design of control strategies are identified.
The following topics are discussed: (1) nature and significance of manmade and natural
sources of NOX; (2) mechanisms by which N02 is formed in the atmosphere; (3) monitor-
ing network design, data quality checks, and the extent of high N02 concentrations
observed in the U.S. from 1975-77; (4) the extent to which N02 is transported from
urban to suburban areas and over longer distances; (5) derived relationships between
N02, NO and organic pollutants; (6) procedures for examining monitoring data to
estimate impacts of various source categories observed on high levels of N02; and
(7) modeling procedures for N02 which are available.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS C. COSATI Field/Group
Nitrogen Dioxide
Air Quality Management
Photochemical Pollutants
Air Pollution Control Strategies
18. DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
145
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (Rev. 4-77)
PREVIOUS EDITION IS OBSOLETE
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