EPA - 450 / 2 - 75 - 005
July 1975
CONTROL OF PHOTOCHEMICAL OXIDANTS-
TECHNICAL BASIS
AND IMPLICATIONS OF RECENT FINDINGS
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Air and Waste Management
Office of Air Quality Planning and Standards
Research Triangle Park, North Carolina 27711
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EPA -450/2-75-005
CONTROL OF PHOTOCHEMICAL OXIDANTS -
TECHNICAL BASIS
AND IMPLICATIONS OF RECENT FINDINGS
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Air and Waste Management
Office of Air Quality Planning and Standards
Research Triangle Park, North Carolina 27711
July 15, 1975
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CONTENTS
Page
LIST OF FIGURES iii
LIST OF TABLES iii
SUMMARY iv
INTRODUCTION ' 1
EFFECTS OF OXIDANTS ON THE ENVIRONMENT AND THE
OXIDANT STANDARD 2
FORMATION OF OXIDANTS IN THE ATMOSPHERE 3
OXIDANT CONTROL STRATEGY 8
Federal Programs for Oxidant Control 8
State Programs for Oxidant Control 9
RECENT FINDINGS 12
Recent Trends of Oxidant Concentrations 12
Rural Oxidants 16
Sources of Rural Oxidants 17
Atmospheric Phenomena Which Affect Oxidant
Concentration 18
Assessment of Recent Technical Findings 23
IMPLICATIONS OF RECENT FINDINGS FOR OXIDANT
CONTROL STRATEGIES 27
General Implications 27
Implications for Control of Urban Oxidants 28
Implications for Control of Non-Metropolitan Oxidants 29
EPA PROGRAMS LEADING TO MORE EFFECTIVE CONTROL
STRATEGIES 32
REFERENCES 34
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LIST OF FIGURES
Figure
1 Photochemical Production of Oxidants 4
2 Results of a Typical Smog Chamber Experiment 5
3 Southern California Oxidant Trends 13
4 Mean Diurnal Ozone Concentration at McHenry, Md. 15
5 Hourly Averages of Freon 11 and Ozone, Whiteface, New York 20
6 Ozone Profile Flight, Wilmington, Ohio, August 1, 1974 21
7 Smoothed Variations of Area Average Daily Ozone - Surface-
Pressure - 1973 Time Sequence 22
8 Average Ozone Concentration 06 July 1974 24
9 Average Ozone Concentration 08 July 1974 25
LIST OF TABLES
Page
Simplified Summary of Chemical Reactions for Production of
Oxidants 4
2 Ozone Data June 14 - August 31, 1974 17
3 Nationwide Hydrocarbon Emissions 30
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SUMMARY
As part of the Clean Air Act's mandate to protect public health and enhance the
environment, the Environmental Protection Agency has developed a program to
attain and maintain the National Ambient Air Quality Standard for Photochemical
Oxidants. This program requires the reduction in emissions to the atmosphere of
the precursor chemicals that are responsible for the formation of oxidants. Control
measures have been instituted at both the Federal and State level and include the
Federal Motor Vehicles Control Program, the Federal program for control of
aircraft emissions, development of Federal New Source Performance Standards for
stationary sources, and those measures taken by the States through State
Implementation Plans to attain the oxidant standard. State measures include such
programs as control of stationary sources and Transportation Control Plans to
control in~use motor vehicles and reduce vehicular traffic in urban areas.
Oxidants are not emitted directly into the atmosphere but result primarily from
a series of chemical reactions between oxidant precursor compounds in the
presence of sunlight. The precursors are organic compounds and nitrogen
oxides, primarily emitted from motor vehicles and stationary sources. Several
fundamental facts concerning oxidant formation have been important in developing
a strategy to reduce oxidant concentrations. These are: (1) organic compounds
do not all react at the same rate in the chemical process and while some react
within several minutes, others may take many hours; (2) in addition to the
absolute concentrations of precursors, the ratio of organic compounds to nitrogen
oxides is important in determining the concentration of oxidants formed; and (3)
meteorological conditions, such as sunlight ultra-violet intensity, temperature,
and atmospheric stability affect oxidant production.
These considerations and our knowledge of the nature and distribution of
precursor emissions form the basis of EPA's program for reducing oxidant
concentrations by reducing emissions of organic compounds from all significant
sources through the various Federal and State programs. EPA also requires
reductions in nitrogen oxide emissions to meet the National Ambient Air Quality
Standard for nitrogen dioxide, but does not require their reduction in order to
control oxidant concentrations.
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To insure the technical accuracy of the oxidant control strategy and to refine
that strategy where necessary, EPA has conducted a continuing program of
research studies and data analysis. New findings based on laboratory and field
studies accomplished over the last several years generally support the control
measures currently being taken and further indicate that additional measures may
be necessary in order to meet the oxidant standard nationwide. These recent
studies have documented frequent violations of the oxidant standard in both urban
and rural areas although, in some urban areas, both the maximum concentrations
of oxidants and the frequency of violations have decreased over the past several
years. At rural locations, the number of violations of the standard and the
maximum concentrations are sometimes as high and even higher than in nearby
urban areas. It is thus apparent that oxidants are a rural as well as an urban
problem.
The recent studies show that: (1) man-made emissions are the predominant
source of high levels of oxidants, even in remote rural areas; (2) the contribution
of natural sources of oxidants is usually not more than 0.05 ppm, compared with
the oxidant standard of 0.08 ppm; and (3) transport of oxidants and their
precursor compounds has been demonstrated to about 50 miles downwind of urban
areas and it is likely that transport over longer distances occurs. It also has been
shown that the highest oxidant concentrations in the Midwest in both rural and
urban areas occur during periods of stagnant conditions associated with high
pressure weather systems.
In both urban areas and in many non-urban areas, there appear to be
sufficient emissions of man-made precursors to account for the high oxidant levels
observed. While transport of oxidants and their precursors may occur, most
urban areas probably are responsible for their own oxidant problem. The high
oxidant levels in non-urban areas appear to be the result of both locally produced
precursors and precursors transported from urban and other non-urban sources.
As a result, control strategies for non-urban areas will need to be directed at
measures which reduce emissions from both non-urban sources as well as urban
sources and which meet the specific needs of each of these areas.
The following implications from the new findings are suggested:
Continued application of hydrocarbon control measures in the urban
area. Because of the high precursor emission densities and the great
numbers of people exposed to oxidants, continued emphasis on intensive
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control measures within cities will be necessary to meet the oxidant
standard in major urban areas .
Increased control over wide geographic areas through Federal and State
programs to meet standards in non-urban areas. In addition to the
continued intensive control of hydrocarbon emissions in urban areas, it
may be necessary to extend some measures under present State
Implementation Plans to include non-urban areas as well as cities.
Increased control of stationary sources. Although mobile source controls
(including Transportation Control Plans) will continue to be a major part
of the oxidant control program, there is a need for more stringent control
of precursor emissions from stationary sources.
Increased emphasis on controlling all reactive organics. Both under
conditions of transport and under persistence of stagnant air masses
there can be sufficient time for less reactive hydrocarbons to contribute
to oxidant formation. This indicates the importance of controlling all
organic compounds which can form oxidants.
Control of nitrogen oxides as well as hydrocarbon emissions may become
necessary to meet the oxidant standard nationwide. Nitrogen oxides
emissions may be transported into rural areas and contribute to oxidant
formation by reaction with locally emitted organic compounds. It may
eventually become necessary to consider control of nitrogen oxides,
coordinated with the control of hydrocarbons, as a part of the oxidant
control strategy.
Emissions from natural sources. Oxidant concentrations which can be
attributed to natural sources are usually less than 0.05 parts per million
compared with the oxidant standard of 0.08 parts per million. Because of
emissions from natural sources, more stringent reductions of man-made
emissions may be necessary in some areas.
Further refinement of the oxidant control strategy requires a more quantitative
understanding of the chemical and the meteorological processes leading to high
oxidant concentrations. EPA is engaged in an extensive program of laboratory and
field studies to obtain the needed information. The program includes studies to
quantify the relationship between emissions of precursors and concentrations of
oxidants in the air at both urban and non-urban locations. More extensive data
are being obtained on atmospheric levels of oxidants and precursors as well as
VI
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data on natural and man-made precursor emissions. It is expected that through
this program the current strategy will evolve over the next several years to
include the best measures needed to control oxidants in both urban and non-urban
areas.
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CONTROL OF PHOTOCHEMICAL OXIDANTS -
TECHNICAL BASIS
AND IMPLICATIONS OF RECENT FINDINGS
INTRODUCTION
The strategy used by the Environmental Protection Agency for reducing the
concentrations of photochemical oxidants was largely formulated by 1972. Although
the strategy has undergone some refinements, it still retains the main features
developed at that time. Laboratory and field studies over the last several years
have contributed much new information about oxidants. While the understanding
of these new findings and of the overall oxidant problem is not yet complete, the
new findings generally support the control measures that are being taken.
Furthermore, the accumulating base of data indicates that additional measures may
be needed in order to meet the oxidant standard nationwide.
This paper discusses the current strategy and its technical basis. It reviews
the recent findings and discusses their implications for further increasing the
effectiveness of the strategy. It identifies the topics which must be better
understood before an optimum strategy can be developed and implemented, and it
indicates the study programs underway or planned to provide the needed
understanding.
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EFFECTS OF OXIDANTS ON THE ENVIRONMENT
AND THE OXIDANT STANDARD
Oxidants are strongly oxidizing compounds which are the primary constituents
of photochemical smog. The oxidant found in largest amounts is ozone, a very
reactive form of oxygen. Oxidants also include the group of compounds referred to
collectively as PAN (peroxyacyl nitrates) and other compounds produced in much
smaller quantities. The most accurate technique for determining oxidant
concentrations is one that measures ozone rather than total oxidants. This
measurement method (chemiluminescent) is the one recommended by EPA and was
the method used to develop most of the data in this report. Consequently, although
the term oxidant is used often in the text, the discussion of measurements refer to
ozone values.
Adverse effects on human health resulting from exposures to very low
concentrations of oxidants have been extensively documented for both man and
experimental animals (Ref. 1, 2). Also low levels of oxidants have adverse effects
on many forms of vegetation and microorganisms and on materials such as rubber,
cotton, nylon, and polyesters. National Ambient Air Quality Standards have been
established to protect human health (the primary standard) and to prevent adverse
welfare effects (the secondary standard). For oxidants, both of these standards
have been established at an ambient concentration of 0.08 parts per million (ppm)
which is equivalent to 160 micrograms per cubic meter (Mg/m ) This is an hourly
average not to be exceeded more than once a year.
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FORMATION OF OXIDANTS IN THE ATMOSPHERE
This section presents the technical basis for the approach used by EPA to
control oxidants. Oxidants are not emitted directly into the atmosphere but result
primarily from a series of chemical reactions between oxidant precursors (nitrogen
oxides and organic molecules) in the presence of sunlight. The principal sources
of organic compounds are the hydrocarbon emissions from automobile and truck
exhausts, gasoline vapors, paint solvent evaporation, open burning, dry cleaning
fluids, and industrial operations. There are also natural sources such as seepage
from the ground and emissions from vegetation. Nitrogen oxides are emitted
primarily from combustion sources such as electric power generation units, gas
and oil-fired space heaters, and automobile, diesel and jet engines. Nitric oxide
(NO) is the major form of nitrogen oxide emitted in combustion processes.
Nitrogen dioxide (NC^) is formed from NO and is the compound which decomposes
in sunlight to initiate the formation of ozone.
The factors which determine the concentrations of oxidants formed in the
atmosphere include: (1) the amount and kinds of organic compounds initially
present and the rate at which additional organics are emitted to the atmosphere;
(2) the amount of nitrogen oxides initially present and their emission rates; and
(3) sunlight ultra-violet intensity, temperature, and other meterological factors.
The interaction of these factors and the chemical reactions involved are very
complex and have been the subject of continuing scientific investigation during the
last 20 years, including atmospheric studies, laboratory smog chamber studies,
and computer simulation of the oxidant forming process.
A large number of chemical reactions that may affect oxidant concentrations
are now known (Ref. 3, 4, 5) . A summary of the photochemical process for
producing oxidants is shown in Figure 1 and a simplified summary of the reactions
is presented in Table 1. In the oxidant forming process, organic compounds are
oxidized to form peroxy radicals and aldehyde compounds. The peroxy radicals
rapidly react with nitric oxide (NO) emissions to form nitrogen dioxide (NO2) In
the presence of ultra-violet light the NO2 decomposes back to NO and
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PHOTOCHEMICAL
BY-PRODUCTS
FREE
RADICALS
ORGANIC
COMPOUNDS
Figure 1. Photochemical production of oxidants.
TABLE 1. SIMPLIFIED SUMMARY OF CHEMICAL
REACTIONS FOR PRODUCTION OF OXIDANTS
OXIDATION OF ORGANIC COMPOUNDS TO FORM PEROXY RADICALS
02
CH3CH = CH2+ OH'VCH3CH2-0-0' + H2CO
PROPYLENE PEROXY RADICAL FORMALDEHYDE
ALDEHYDES + 02 + SUNLIGHT-*-ADDITIONAL PEROXY RADICALS
PEROXY RADICALS CONVERT NO TO N02
CH3CH2-0-0-+ NO -»-N02+ CH3 CH2-0-
CH3CH2-0-+ 02-*-H02'+ CH3HCO ACETALDEHYDE
UP TO FOUR CONVERSIONS FOR EACH CARBON ATOM
OZONE FORMING REACTION
N02+02 ^NO + Oi
ULTRA-VIOLET
OZONE SCAVENGING REACTION
PHOTOCHEMICAL BY-PRODUCTS FORMED
ALDEHYDES, PAN, AEROSOLS, NITRIC ACID, H202, OH', C02
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simultaneously forms ozone (O3) . The NO2 can also react with the peroxy radicals
to form peroxyacyl nitrates (PAN) . The importance of the organic compounds in
the process is to convert the NO to NO?- This reduces the concentration of the NO
molecules which react to destroy ozone and also increases the concentration of the
MI>2 needed to produce ozone. In the series of reactions that are possible, as many
as four molecules of ozone can be generated for each carbon atom in the organic
molecules.
Much of the information about the oxidant producing reactions has been
obtained from smog chamber experiments. The results of a typical smog chamber
experiment are presented in Figure 2 (Ref. 6) . These results show a decrease in
the reactive hydrocarbon, propylene, as the aldehyde compounds and the
oxidants, ozone and PAN, are formed. The impact of the various reactions on the
nitrogen oxides leads to low levels of NO and a maximum in the nitrogen dioxide
concentration approximately one hour after the reactions start.
0.54.
trnVVACETYL NITRATE
Pt-- (PAN)
120 180 240
ELAPSED TIME (min)
360
Figure 2. Results of a typical smog chamber experiment. Irradiation of a
propylene-NO-NOo mixture in air. Initial experimental conditions - 0.5
ppm propylene, OA5 ppm NO, and 0.05 ppm NO2 in 760 torr of highly
purified air.
Several observations are related to this oxidant forming process. They are:
1. It has been shown in smog chambers that organic compounds do not all
react at the same rate in the photochemical process (Ref. 7). However,
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given long irradiation times and sufficient amounts of nitrogen oxides,
almost all organic compounds react to form oxidants. Very reactive
compounds lead to high oxidant levels within a few hours of their
emissions. Less reactive compounds form oxidants over longer times and
may react as they move with an air mass and thus contribute to observed
oxidant concentrations later in the day or far downwind of their source.
There are thousands of organic compounds in typical use. Gasoline may
contain more than two hundred compounds whose composition will vary
by brand and by season of the year. Since the organics emitted to the air
include many of these compounds, there is an extensive variety of
reactions occurring in the atmosphere.
In the immediate vicinity of nitric oxide sources,such as near power
plants and highways, relatively little ozone is found. Ozone reacts
rapidly with nitric oxide and is destroyed, but NO2 is formed. In the
presence of sunlight, the NO2 can later react to form ozone again. These
interactions between ozone and the nitrogen oxides help explain several
observations. Ozone concentrations are usually low at night in urban
areas because the NO emissions act to destroy ozone and the absence of
sunlight prevents additional ozone formation. City ozone concentrations
are often lower than in adjacent suburban or rural areas because the NO
emissions in the city are high and thus reduce the urban ozone
concentrations.
In addition to the absolute concentration of precursors and the
meteorological factors, such as sunlight and temperature, which directly
affect the concentration of oxidants formed, the ratio of organic
compounds to nitrogen oxides is important in determining the
concentrations of oxidants formed. Smog chamber studies have shown
that both very high and low ratios of organic compounds to nitrogen
oxide suppress the amount of oxidants formed (Ref. 8) . For intermediate
ratios, the rate of formation and concentrations of oxidants formed
depends on the type of organic compound and the ratio of organic to
nitrogen oxide. For most urban areas, the ratio of organic compounds to
nitrogen oxides in the atmosphere is such that reducing the ratio will
help to reduce the oxidant concentration.
The highest concentrations of oxidants are most frequently observed in
the afternoon hours. These highest levels of oxidants are formed by
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reactions of the precursor organic compounds and nitrogen oxides that
were emitted earlier in the day or had accumulated from previous days.
Since both organics and nitrogen oxides are removed from the
atmosphere during the photochemical reactions, the highest
concentrations of these pollutants are thus usually observed during the
early morning hours at the start of the photochemical process.
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OXIDANT CONTROL STRATEGY
On the basis of the knowledge and understanding of oxidant formation pre-
sented above, EPA has approached the control of photochemical oxidants by re-
quiring the reduction of atmospheric emissions of hydrocarbons and other organic
compounds. Although a National Ambient Air Quality Standard has been
established for hydrocarbons, this standard serves only as a guide for achieving
the oxidant standard and is not based on health criteria. Since methane is the most
naturally abundant and the most unreactive hydrocarbon species, only non-
methane hydrocarbon compounds are specified in the air quality standard. This
standard is 0.24 ppm (160 micrograms/cubic meter) for the 3-hour concentration
measured from 6 to 9 a.m. A National Ambient Air Quality Standard also has been
established for nitrogen dioxide (NO2) . This standard is based on the health
effects of nitrogen dioxide. Reductions in nitrogen oxide emissions are required to
meet this standard but are not required for the control of oxidant concentrations.
The reductions in emissions of hydrocarbons and other organic compounds are
to be achieved through Federal and State programs which have been formalized in
regulations (Ref. 9) promulgated under the Clean Air Act. The Federal programs
provide for the reduction in emissions nationwide through the Federal Motor
Vehicle Control Program, the Federal program for control of aircraft emissions,
and the development of New Source Performance Stadards. The State programs
provide for additional control measures through State Implementation Plans in
those areas of the country where the Federal programs will not be sufficient to
meet the air quality standard for oxidants. These programs are discussed in the
following sections.
FEDERAL PROGRAMS FOR OXIDANT CONTROL
Because roughly half of the hydrocarbons emitted to the ambient air in the
United States are attributable to motor vehicles, a large portion of hydrocarbon
reduction is to be achieved through the Federal Motor Vehicle Control Program.
This program has required progressively stricter hydrocarbon controls on all
light duty motor vehicles since the 1968 model year. In 1970, Section 202 of the
Clean Air Act required that hydrocarbon emissions from light duty vehicles and
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engines be reduced by at least 90% from emissions permissible in 1970 model year
vehicles. This will eventually require emissions to be reduced to 0.41 grams per
mile. Interim standards of 1.5 grams per mile (g/m) are now required on 1975
through 1977 model year vehicles. Regulations also exist to limit hydrocarbon
emissions from other types of vehicles such as motorcycles, trucks, buses, and
aircraft.
Section 111 of the Clean Air Act also authorizes EPA to promulgate "Standards of
Performance for New Stationary Sources." These are standards for new sources
which reflect the best demonstrated system of emission control, taking the cost of
emission reduction into account. New sources are sources which are constructed
or modified after the standard has been proposed. So far only one new source
performance standard has been promulgated for hydrocarbon reductions. This is
the standard for Storage Vessels for Petroleum Liquids (March 8, 1974, Federal
Register). There are many other hydrocarbon emitting industrial operations
where standards of performance may be appropriate. Some possible sources are
industrial degreasing, fabric dry cleaning, service station gasoline transfers,
automobile finishing and other industrial surface coating operations. EPA is
currently studying each of these areas to gather background data. Standards of
Performance for these sources are scheduled for promulgation between- 1976 and
1978.
STATE PROGRAMS FOR OXIDANT CONTROL
Because the Federal programs will not be sufficient to meet the oxidant
standard in all locations, each State has been required to develop plans for
additional reduction in emissions of organic compounds in those areas which will
not meet the oxidant standard.
To assist States in preparing Implementation Plans, EPA published guidelines
in the August 14, 1971 Federal Register entitled, "Requirements for Preparation,
Adoption and Submittal of Implementation Plans." Twenty-seven States have
promulgated regulations to reduce hydrocarbon emissions in some portion of the
State. In the other States, hydrocarbon regulations are left to local jurisdictions.
The State Implementation Plans include regulations for existing stationary sources
as well as the implementation, where necessary, of Transportation Control Plans.
For the development of Implementation Plans, it is necessary to determine how
much the emissions of hydrocarbons and other organics must be reduced. It was
realized that the relatively simple methods used for directly emitted pollutants
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were not appropriate for the complex oxidant formation processes. Therefore, an
attempt was made to quantify the amount of hydrocarbon reduction required to meet
the oxidant standard. In 1970, the data to relate non-methane hydrocarbon to
oxidant concentrations were available only from Denver, Los Angeles,
Philadelphia, and Washington, B.C. The 6 to 9 a.m. average non-methane
hydrocarbon concentrations were plotted against the peak hourly oxidant
concentrations observed later in the day at the same measurement sites (Ref. 10).
The points scattered because of the variations in the conditions which affect
oxidant formation. A curved line was drawn to enclose these data points within an
upper boundary. This upper limit curve depicted the highest oxidant levels
observed for a given hydrocarbon concentration. From this curve the minimum
hydrocarbon reductions required to meet the oxidant standard could be calculated.
EPA published the result of this calculation as Appendix J in the August 14, 1971
Federal Register. Given a measured maximum oxidant level in an urban area,
Appendix J is to be used to determine the amount of hydrocarbon control required
in that area. In some cities the measured oxidant levels were above (greater than
0.28 ppm) the applicable range of Appendix J. These cities were permitted to
reduce hydrocarbon emissions proportional to the amount that oxidant levels
exceeded the standard.
Appendix B of the August 14, 1971 Federal Register also gives emission
reductions which are attainable through the application of reasonably available
emission control technology. These emission limitations emphasize reduction of
total organic compound emissions, rather than substitution of less reactive organic
compounds, because of the evidence that few organic compounds are
photo chemically unreactive. For organic solvent usage, however, such as in
surface coatings and dry cleaning, the guidelines present a list of low reactivity
compounds that may be considered for exemption for control. At present only
fifteen States have rules controlling emissions from organic solvent usage. The
rules differ in detail from State to State, but most of the States have adopted rules
for organic solvents patterned after the Los Angeles Rule 66 (Ref. 11) rather than
the Appendix B guidelines. This rule defines a group of photochemically highly
reactive compounds and regulates the emissions of these defined compounds.
Transportation Control Plans (Ref. 12) provide for reductions in hydrocarbon
emissions beyond the reductions achieved by the Federal Motor Vehicle Control
Program and stationary source regulations set forth in approved State
Implementation Plans. Pollutant reductions are to be achieved by such measures
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as decreases in vehicle miles traveled, inspection and maintenance programs,
retrofit emission controls for certain vehicles, gasoline supply limitation, and
gasoline transfer vapor control. There are currently twenty-one Air Quality
Control Regions which have transportation control plans for oxidant control. The
compliance dates for six of these plans is May 31, 1975, while the other fifteen have
attainment dates between 1975 and May 31, 1977.
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RECENT FINDINGS
In addition to the control measures being taken under the regulations
discussed above, EPA and others have been conducting field investigations and
laboratory studies to better understand the oxidant problem. The recent studies
have provided several new insights and strengthened the understanding of the
oxidant formation process. Oxidant monitoring at various locations has provided
data for determining concentration trends. Studies in the eastern United States
have shown that the oxidant standard is often exceeded over large parts of that
region and that oxidant concentrations in rural areas can be equal to or exceed
those in urban areas. The studies have related the observed oxidant
concentrations to atmospheric phenomena which affect oxidant formation and
transport and to the man-made and natural sources of oxidants and their
precursors. These studies and phenomena are summarized and discussed in the
following sections.
RECENT TRENDS OF OXIDANT CONCENTRATIONS
Hydrocarbon emissions in the United States reached a maximum in 1968 and
have decreased by about 7 percent between 1968 and 1972 as a result of emission
control measures. At the same time nitrogen oxides emissions have increased by
about 25 percent from 1968 to 1972 and had doubled from I960 to 1970 (Ref. 13) .
Thus, in the typical urban environment, both the hydrocarbon emissions and the
hydrocarbon to nitrogen oxide ratio has decreased. The result has been that in
Los Angeles, in urban areas of New Jersey, arid in other center cities, both
oxidant levels and the number of violations of the oxidant standard has decreased
(Ref. 14-16). For example, in Los Angeles and in the New Jersey areas, the
number of days in which the oxidant standard has been exceeded decreased more
than 50 percent between 1965 and 1972. In smaller urban areas of southern
California, such as Riverside, Palm Springs, and Indio, maximum oxidant
concentrations have also decreased from 1971 through 1974 although the number of
violations has remained high (Ref. 17) . These trends in maximum oxidant
concentrations are presented in Figure 3. While these downward trends in oxidant
levels are generally typical, there of course may be locations where levels have
not been decreasing. The large increase in downtown Los Angeles for 1973 is
because of unusual meterological conditions.
12
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1966
1968
1972
1974
Figure 3. Southern California oxidant trends.
13
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Figure 4 shows how variable the year to year behavior of oxidant levels can
be. This figure presents the average ozone readings by hour of the day for a
rural site near McHenry, Maryland (Ref. 18) . A possible reason for the variability
was the fact that frontal movements recorded in 1973 were almost all classified as
weak. Therefore, they may have lacked the vigorous vertical mixing and flushing
action normally experienced during frontal passages. This may help to explain the
generally higher ozone levels experienced in 1973 at McHenry relative to 1972 and
1974.
A related observation is that ambient levels of oxidant in urban areas are
sometimes higher on the weekends than during weekdays, even though traffic and
industrial emissions are presumably lower, particularly during the morning hours
on Sunday. These results have been observed in a number of cities, including Los
Angeles, Indianapolis, and Linden, New Jersey (Ref. 19).
The general observations are that daily average oxidant concentrations are
higher on weekends and that, in some cities, levels above 0.08 ppm occur more
frequently on weekend days than on weekdays. However, the highest oxidant
concentrations do not appear to occur as frequently on weekends as on weekdays.
The detailed investigation of temporal and spatia] distribution of emissions, the
meteorology, and chemical composition of the air in the cities has not yet been
accomplished in sufficient detail to fully explain these observations. As explained
below, many factors may be operating simultaneously to cause the higher weekend
oxidant concentrations. Because these factors may be specific to weekend
behavior, the observations of weekend oxidants should not be construed as a test
of oxidant strategy measures. In urban areas where reductions in hydrocarbon
emissions have been achieved, long-term reduction in oxidant concentration has
occurred both during the week and on the weekends.
Several important observations can be made about the weekend oxidant
concentrations. First, the air entering or remaining in the city on weekends may
contain oxidants, precursors, and partially reacted intermediates which have
persisted from earlier days. Secondly, detailed emission inventories are not
available to completely specify the differences in emission patterns between
weekdays and weekends. Saturday driving patterns are no doubt different from
weekday patterns, but automobile emissions in the total metropolitan areas may not
be significantly lower on Saturdays. In industrial areas on Saturday and Sunday,
and in all urban areas on Sunday, it is likely that emissions of both hydrocarbons
and nitric oxide (NO) are reduced considerably. Thirdly, there are data from Los
14
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Angeles, urban New Jersey, and other cities to indicate that the ratio of NO to NC>2
is decreased on weekends. The hydrocarbon to nitrogen oxide ratio also increases
on weekends. These last observations indicate that less NO may be available on
weekends to scavenge ozone molecules, and that N©2 concentrations, which can
form oxidant, may have endured from prior days. Moreover, the weekend
increase in the hydrocarbon to nitrogen oxide ratio cannot be explained by
changes in traffic patterns alone. Finally, aerosols have been shown to be lower
on weekends and solar radiation higher with the possible impact that scavenging
reactions are reduced and photochemical reactions increased. The combination of
these interacting factors results in the observations of higher average weekend
oxidant concentrations.
RURAL OXIDANTS
Prior to 1970, measurements of oxidants in rural areas were made infrequently
and did not indicate the presence of particularly high concentrations. But in the
course of a study conducted by EPA in 1970, investigators found oxidant
concentrations in a rural area of western Maryland and eastern West Virginia
which frequently exceeded the national ambient air quality standard during the
summer months. Follow-up studies of increasing magnitude were conducted in
1972, 1973, and 1974 (Ref. 18, 20, 21). The most extensive measurements were
carried out in the summer of 1974 at widely separated sites in the eastern Midwest.
It was found that the oxidant standard was exceeded on a significant number of
days at both urban and rural sites. The rural sites exceeded the standard more
often than urban sites and higher maximum concentrations were measured at the
rural locations. Table 2 (Ref. 18) presents data on maximum ozone observed and
-frequency of ozone above the standard of 0.08 ppm (160 micrograms/cubic meter) .
Note that at DuBois, a small city in rural Pennsylvania, the oxidant standard was
exceeded 341 hours during the period June 14 to August 31, 1974. During the
same period at Pittsburgh, approximately 100 miles southwest of DuBois, the
standard was exceeded only 106 hours. The maximum hourly concentration
measured at DuBois was 0.20 ppm; at Pittsburgh it was 0.14 ppm. Similar high
values of ozone have also been measured in rural areas of New York, New Jersey,
Wisconsin, and Florida (Ref. 22). It thus appears that in many areas of the
eastern United States high concentrations of oxidants are found in both rural and
urban areas.
16
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TABLE 2. OZONE DATA JUNE 14 - AUGUST 31, 1974
CITY
MAXIMUM
CONCENTRATION
(ppm)
DAYS
EXCEEDING
STANDARD
(%)
NUMBER
OF
VIOLATIONS
(TOTAL)
RURAL
WILMINGTON, OH.
MCCONNELSVILLE, OH.
WOOSTER, OH.
MCHENRY, MD.
DUBOIS, PA.
0.18
0.16
0.17
0.17
0.20
58
56
55
43
54
259 HOURS
239 HOURS
262 HOURS
262 HOURS
341 HOURS
URBAN
CINCINNATI, OH.
DAYTON, OH.
COLUMBUS, OH.
CANTON, OH.
CLEVELAND, OH.
PITTSBURGH, PA.
0.18
0.13
0.15
0.14
0,14
0.15
44
35
27
44
26
37
54 HOURS
114 HOURS
113 HOURS
148 HOURS
51 HOURS
106 HOURS
SOURCES OF RURAL OXIDANTS
There are several possible sources of rural oxidants that must be considered
in efforts to account for the observed concentrations. These sources include the
large quantities of man-made precursors that are emitted in urban areas and which
may move into rural areas while reacting to form oxidants, as well as oxidants
formed from man-made precursors which originate in the non-urban areas. There
are also two possible natural sources of rural oxidants. These are (1) downward
transport from the ozone-rich layers in the stratosphere into the lower troposphere
near the surface, and (2) photochemical generation from hydrocarbons emitted by
vegetation. The available evidence strongly indicates that frequent and persistent
concentrations of ozone near the surface above the oxidant air quality standard of
0.08 ppm are not caused solely by natural sources and that the background that
can be attributed to natural sources is usually less than 0.05 ppm.
The amount of ozone transported from the stratosphere may be estimated from
the numerous vertical ozone soundings of the atmosphere made in past years (Ref.
23) . These generally show ozone concentrations at a maximum in the lower
17
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stratosphere decreasing to very low levels at or near the tropopause and with a
slight decrease downward toward the surface. Ozone levels near the surface that
appear to be largely of stratospheric origin may at times range ifrom 0.03 to 0.05
ppm. Temporary higher readings, however, are possible with unusually deep and
vigorous vertical mixing induced by strong cold fronts, jet streams,
thunderstorms, or some combination of these. These are sporadic, usually short-
lived events lasting on the order of minutes or, less often, a few hours. Lightning
from thunderstorms may also cause brief rises in ozone, but lightning by itself is
considered an insignificant contributor to the ozone levels observed in rural areas
(Ref. 24).
The natural organic compounds emitted by vegetation may react to form
additional ozone or they may sometimes decrease it by scavenging reactions. The
ozone added by reactions of natural hydrocarbons may increase the ambient ozone
by 0.02 to 0.05 ppm (Ref. 25). However, the atmospheric conditions that are
conducive to ozone production by the photochemical process are not usually the
same conditions associated with transport from the stratosphere. Therefore, the
high values of non-urban ozone that are frequently above the standard cannot be
attributed only to natural sources, but rather, they appear to be primarily of man-
made origin. The evidence for this conclusion is presented in the following
section.
ATMOSPHERIC PHENOMENA WHICH AFFECT OXIDANT CONCENTRATION
Recent measurements downwind of urban centers (Houston, Phoenix, several
Ohio cities, and Philadelphia) demonstrate that an identifiable urban plume of
oxidants and oxidant precursors travels as far as 30-50 miles from the urban
center (Ref. 18, 26, 27). In Los Angeles, where the magnitude of oxidant
generation is greater, the distance can be extended to 75 miles or more downwind
(Ref. 28). Beyond these distances, the individual urban contribution of pollutants
becomes so mixed with other urban and non-urban contributions that the
individual urban effect is difficult to distinguish. Therefore, while the transport
of oxidants can partially explain the high oxidant levels at short ranges downwind
of urban centers, it has been difficult to show definitely that transport is the
principle cause of high readings at more remote sites. However, as described
below, there is new evidence that oxidants and their precursors can be
transported over longer distances.
To date, the most positive method for demonstrating that the high ozone
concentrations observed at more remote locations are at least partially due to
18
-------
emissions from man-made sources is by simultaneous measurements of ozone and
compounds that are only emitted by human activity. Freon 11 is such a compound
since it has no natural sources, is very stable in the lower atmosphere and has a
measurable but slowly increasing global background concentration of
approximately 90 parts per trillion. An increase above this background
concentration is strong evidence for transport of air from areas of human activity.
In studies during 1973 and 1974 at Whiteface Mountain, New York, at Elkton,
Missouri, and in the Pacific Ocean between Seattle and San Diego, no levels of
ozone above 0.08 ppm have been found in which Freon 11 levels were not also
above the background. Figure 5 presents ozone and Freon 11 measurements at
Whiteface Mountain from July 24-27, 1974 (Ref. 25). On July 24 ozone moved into
the study area at 3 to 4 a.m. accompanied by an elevated Freon 11 level. July 25
was heavily overcast and there was little ozone formation. July 26 was sunny,
Freon 11 was at normal background concentrations, and there was ozone
production to 0.05 ppm during the day. On July 27 there were elevated Freon 11
concentrations, so that the ozone concentrations may have been due to local
production and to ozone or precursors transported to the area. Thus, the higher
levels of ozone measured on July 24 and July 27 are associated with high Freon 11
levels, which implies that these pockets of air had been transported from some
area where man-made emission sources are present.
Certain meteorological phenomena may also help explain the high rural ozone
concentrations. Figure 6 presents vertical ozone profiles taken at three times
during one day during the 1974 summer studies (Ref. 18). These profiles are
similar to other vertical measurements reported by other observers over
Indianapolis and Canton, Ohio and are consistent with ozone soundings of previous
years. The concentrations of ozone decrease above the subsidence inversion near
8000 feet as shown by the temperature profiles. The early morning profile
indicates low ozone concentrations (0.02 ppm) beneath a radiation inversion that
occurred between the surface and 2000 feet. Trapped between these two
inversions is a layer of ozone above 0.09 ppm which had formed on previous days
and had persisted through the night. In this layer, ozone has been separated from
ground based scavenger pollutants and may persist for long periods of time and
may be transported over long distances. At the surface below the radiation
inversion, ground emissions of natural destructive agents and surface features
provide a sink for the ozone. During the day, as the low level radiation inversion
layer dissipates, the lower atmosphere becomes well mixed and oxidant, formed by
19
-------
120 -
80 -
40 -
120 -
80
40
rsi
o
a
~ 120
o
UJ
cc
80
40
120
80
40
OZONE
.'*-*«.. J
-."-V
7/26/74
7/25/74
'4-
7/24/74
I I I
0 2 4 6 8 10 12 14 16 18 20 22 24
HOURS
Figure 5. Hourly averages of Freon 11 and Ozone,
Whiteface, New York.
20
-------
12
11
10
9
8
n~
o
* 1
£
co 6
LU"
I s
i-
* 4
3
2
1
1754 1414
0704
1320
1656
I
10
I
15
r
20
i
25
I
30
I
0.05
TEMPERATURE (°C)
I I
0.075 0.10
OZONE CONCENTRATION, ppm
0.125
Figure 6. Ozone profile flight, Wilmington, Ohio, August 1, 1974.
photochemistry from precursors emitted at the surface or from hydrocarbons left
over from the previous day, mixes with the layer of ozone that had persisted
through the night. As the day progresses , more oxidant is formed as shown by the
high oxidant concentrations measured in the afternoon. This typical pattern
explains the usual diurnal oxidant concentrations measured at ground based rural
monitors. This pattern usually shows low concentrations at night with build-up to
high concentrations in the afternoon. Exceptions occur when high oxidant
concentrations have been measured at night under atmospheric conditions that
dissipate or prevent formation of the surface-based inversion.
Preliminary analysis of data from the 1974 summer studies indicates that ozone
concentration variation also is associated with large scale weather features related
to high pressure weather systems (Ref. 18) . Figure 7 presents a smoothed graph
21
-------
aanss3Ud aovduns BDVUBAV vaav
3
I
0)
0)
o
co
O)
c
o
N
O
(13
0)
O5
CD
I
CO
CD
CD
C
O
CO
"S g
(uidd) 3NOZO 3DVU3AW V3dV
22
-------
of surface pressure and surface ozone concentrations for 1973 averaged over
several eastern study sites. This shows that the highest oxidant readings
occurred during periods of high pressure; however as explained below, high
pressure in itself does not lead to high ozone.
On a spatial basis, the higher ozone concentrations generally seem to occur
near the central portion of high pressure cells and decrease outward. The spatial
distribution of ozone in a high pressure system is shown on weather maps for July
6 and July 8, 1974 in Figure 8 and Figure 9 (Ref. 18). These are two of several
cases studied. The five numbers in large boxes are the 8-hour afternoon averages
of ozone concentrations in parts per million observed at the rural monitoring sites.
The six numbers in smaller boxes are the ozone concentrations at urban locations.
The contour lines are lines of constant pressure. The center of the high pressure
system is over Cleveland on July 6 (Figure 8) and is indicated by the line
numbered 22 (1022 millibars) . On July 8 (Figure 9) the high pressure cell is
centered over western Pennsylvania. The wind direction and velocity is shown by
the arrows. The shading indicates areas of high man-rnade hydrocarbon emissions
where emission densities are greater than 10 tons per square mile per year. In the
urban areas emission densities are between 100 and 1000 tons per square mile per
year. Emission densities are high in most of this region and are sufficient to
account for the measured ozone concentrations. The evidence suggests that near
the center of a high pressure area, perhaps 100-200 miles in diameter, envi-
ronmental conditions are most conducive to high oxidant build-up. Apparently,
near the center of the high pressure area, there are light disorganized winds,
sufficient ultra-violet radiation and high temperatures as well as sufficiently high
emissions of precursor compounds. Thus, in a slow moving high, a virtual
outdoor smog chamber develops in which locally generated oxidants are added to
oxidants and precursors which may have been transported into the area as the
high pressure cell was developing.
ASSESSMENT OF RECENT TECHNICAL FINDINGS
The recent field investigations and laboratory studies of oxidant formation and
control have provided several new insights. It is now apparent that oxidants are a
rural as well as an urban problem. Oxidants can be formed over long time periods
during stagnant conditions in high pressure systems or during transport of
oxidants and precursors. This implies that the long-term behavior of oxidants and
precursors is an important contributor to oxidant concentrations. It also implies
23
-------
1. AVERAGE DAYLIGHT OZONE CONCENTRATION
(1200 TO 2000 EOT) IN PPM
A. SMALL BOX - URBAN
B. LARGE BOX -RURAL
2. SURFACE PRESSURE (1700 EOT) IN MILLIBARS
3. 24-HOUR RESULTANT SURFACE WINDS
4. HYDROCARBON EMISSION DENSITY (> 10 tons/
year-sq. mile) (SCREENED AREA)
Figure 8. Average ozone concentration 06 July 1974.
24
-------
1. AVERAGE DAYLIGHT OZONE CONCENTRATION
(1200 TO 2000 EOT) IN PPM
A. SMALL BOX- URBAN
B. LARGE BOX - RURAL
2. SURFACE PRESSURE (1700 EOT) IN MILLIBARS
3. 24-HOUR RESULTANT SURFACE WINDS
4. HYDROCARBON EMISSION DENSITY ( > 10 tons/
year-sq. mile) (SCREENED AREA)
Figure 9. Average ozone concentration 08 July 1974.
25
-------
that less reactive organic compounds as well as the more reactive compounds can
contribute to observed oxidant levels. There are also natural sources of oxidants
which at times may contribute to oxidant concentrations reaching levels near the
oxidant standard. The studies indicate that man-made emissions of hydrocarbons
are, however, the predominant source of the highest levels of oxidant.
26
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IMPLICATIONS OF RECENT FINDINGS
FOR OXIDANT CONTROL STRATEGIES
Recent trends indicate that the current strategy is leading to reduction in
oxidant levels in urban areas. However, it appears that measures in addition to
those planned under the current strategy will need to be developed in order to
achieve the oxidant standard nationwide. The implications suggested are
discussed in detail in the following sections.
GENERAL IMPLICATIONS
It is clear that high oxidant levels are a problem in many non-urban as well as
urban areas. The large cities and major urban areas are, of course, concentrated
major sources of organic compounds and nitrogen oxides. However, non-urban
areas also generate substantial man-made precursor emissions. These areas are
not necessarily rural, but may include many medium and smaller sized cities that
may account for significant fractions of population. In the state of Ohio, where the
rural oxidant studies discussed in the preceding section were largely conducted,
the ten largest cities (population greater than 100,000) account for approximately
60 percent of the total man-generated hydrocarbon emissions. This leaves 40
percent of these emissions coming from the non-metropolitan areas of the state. It
would follow then that the major steps needed to control oxidants in both urban and
non-urban areas are those measures that effectively reduce the emissions in each
of these areas and which take into account the individual characteristics of each
area. These findings in no way indicate that there should be less control of the
urban fraction of hydrocarbons .
It will, of course, be necessary to consider the interaction of urban and non-
urban areas since transport of oxidants and their precursors occurs. As
discussed in the previous section, there is evidence that transport can take place.
The highest oxidant levels in both urban and non-urban locations of the eastern
U.S. , however, appear to occur within areas of a high pressure system. Since
winds are usually light within these systems, transport beyond about fifty miles
may not be important under these conditions. This would support a view that
within high pressure systems the urban centers and the non-metropolitan areas
are both principal contributors to their own oxidant problems.
27
-------
Because controlling urban and non-metropolitan oxidants may present
different problems they are discussed under separate headings in the following
sections.
IMPLICATIONS FOR CONTROL OF URBAN OXIDANTS
Most urban areas are characterized by a large population at risk and high
precursor emission densities. The oxidant problem in these metropolitan areas
can be largely attributed to those mobile and stationary source emissions generated
within the community itself. Based on recent studies in the Midwest, this is
particularly true during meteorological conditions characterized by stagnant high
pressure systems, the same conditions during which the highest oxidant levels
have been observed. For these reasons, the continued application of intensive
control measures in our urban areas is essential if we are to attain the oxidant
standard and prevent adverse effects to human health.
Of particular importance in achieving this objective are those measures
designed to reduce mobile source emissions. Estimates for several major
metropolitan areas show that as much as 70 percent of total urban hydrocarbon
emissions are attributable to light and heavy duty vehicles. To effectively handle
this problem, continued emphasis will be required on Transportation Control Plans
to augment the Federal Motor Vehicle Emission Control program. These plans are
a key element in achieving the oxidant standard in urban areas and offer the only
viable approach to achieving mobile source emissions reductions beyond those
resulting from the Federal Motor Vehicle Emission programs .
Although the present strategy recommends controlling all reactive
hydrocarbons or organic compounds, it does provide for substitution of less
reactive for moderate and highly reactive materials where reduction of all organics
is not feasible or economic. Within large cities this approach can contribute to
reduced oxidant levels, and it may be necessary to continue to permit these
substitutions where measures for total hydrocarbon reduction are not practical.
However, recent findings suggest that the less reactive hydrocarbons within an
urban area can persist from one day to the next under stagnant conditions and thus
have time to react and contribute to oxidant levels. Also, under conditions when
transport of oxidants and precursors occurs, there can be sufficient time for the
less reactive organics to contribute to oxidant levels in neighboring areas.
Therefore, while continued use of reactivity oriented measures can be of value to a
city, the recent findings place increased emphasis on the need to control all
organic compounds that can participate in oxidant formation.
28
-------
Another implication concerns the ultimate need to control nitrogen oxides
emissions as part of the strategy for achieving the oxidant standard. The oxidant
strategy relies on reduction of hydrocarbon emissions to meet this objective. Our
understanding of the underlying chemistry continues to indicate that emphasizing
hydrocarbon control is the most effective way to reduce the high levels of oxidants
seen in major urban areas. However, this strategy can leave unreacted nitrogen
oxides in the atmosphere which may be transported into suburban and rural areas
to interact with further infusions of organic material and give oxidant
concentrations which exceed the standard in these areas. The observation of
elevated weekend oxidant levels in some cities may also be related to varying
relative rates of organic and nitrogen oxides emissions.
The strategy leading to control of oxidants in urban, suburban, and rural
areas will continue to be based on the reduction of organic compounds.
Eventually, however, as oxidant levels are reduced toward the standard, it is
likely that coordinated reductions of both organics and nitrogen oxides will be
required. In a coordinated approach, cities could take advantage of the beneficial
effect of a low organic to nitrogen oxides ratio while sufficient control of nitrogen
oxides would be necessary to control oxidant formation downwind or during
extended stagnation conditions.
IMPLICATIONS FOR CONTROL OF NON-METROPOLITAN OXIDANTS
Non-metropolitan areas, in contrast to the major urban centers, have their
emission sources spread over wide geographic areas. Control measures that need
to be taken must be effective over these broad areas. The Federal control
programs for mobile and stationary sources apply nationwide and therefore have
this character. The States can also extend their hydrocarbon control regulations
statewide. These regulations now tend to be confined to major urban areas within
the States. Through the State Implementation Plans, further reductions in
emissions can be achieved from both stationary and mobile sources in non-
metropolitan areas that have an oxidant problem. Thus, in the future, it may be
determined that certain States need to adopt statewide stationary source controls
and possibly certain transportation measures such as vehicle inspection/-
maintenance. This latter measure employed throughout a State would help assure
maximum effectiveness of the Federal Motor Vehicle Control Program.
To characterize the need for stationary source control, a preliminary estimate
can be made of the effectiveness of Federal programs in reducing oxidants in the
29
-------
non-urban areas throughout the U. S. under present nationwide control
timetables. Table 3 shows estimates of national emissions of hydrocarbons from
various mobile sources and from stationary sources projected through 1985. These
estimates include factors for expected growth in each category and reductions
based on Federal programs now being implemented or required in the near future.
In the case of light duty vehicles the estimates are based on EPA's recent
recommendations on extension of interim and statutory standards. For the mobile
sources, the retirement of older vehicles and the projected deterioration of control
devices is also included.
TABLE 3. NATIONWIDE HYDROCARBON EMISSIONS (MILLIONS OF TONS PER YEAR)
YEAR
1972
1975
1985
LIGHT-DUTY
VEHICLES
10.3
9.1
3.6
LIGHT-DUTY
TRUCKS
2.0
1.7
1.3
HEAVY-DUTY
VEHICLES
2.4
2.2
2.1
OTHER
MOBILE
SOURCES*
1.9
2.2
3.4
TOTAL
MOBILE
SOURCES
16.6
15.2
10.4
STATIONARY
SOURCES
12.5
13.7
14.7
TOTAL HC
EMISSIONS
29.1
28.9
25.1
'AIRCRAFT, VESSELS, OFF-HIGHWAY, RAIL
Since rural oxidant levels in excess of 0.16 ppm have been measured as was
reported in Table 2 and natural background concentrations of 0.04 ppm and
possibly higher may occur, it is possible that at least a two-thirds reduction in
man-made oxidants will be needed to meet the oxidants standard in some non-
urban regions. Assuming that the relative reductions in nationwide emissions
shown in Table 3 approximate the effect of the Federal programs over broad
geographic areas, it is not likely that they will be sufficient to achieve the needed
oxidant reductions. It appears from the above estimates that more needs to be done
than is currently planned to solve the non-urban problem. Although it may be
possible to improve the controls for heavy duty vehicle emissions and other mobile
sources, the most promising area for achieving the broad area control needed for
non-urban areas would appear to be stationary sources. In 1972 these sources
accounted for less than half of the total emissions nationwide whereas by 1985 they
will account for almost two-thirds of the nationwide hydrocarbon emissions. This
distribution may not be true in some rural areas where motor vehicles could
remain the dominant source of emissions.
The recent findings have additional implications for control of the non-urban
oxidants problem. Both the more reactive and the less reactive organics, under
the conditions of light winds associated with persistent high pressure systems,
30
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may have sufficient time to generate oxidants. Thus, reduction of total emissions
and not just reduction of the more reactive compounds would need to be
emphasized in non-urban areas. Control of organic rather than nitrogen oxides
emissions appears to be the most effective route to reduce oxidant levels in rural
areas. However, to attain the standard, coordination of both organic and nitrogen
oxides emissions control may be necessary.
Finally, consideration of natural contributions of oxidants and precursors is
likely to be an important consideration in non-urban areas. While recent findings
suggest that natural sources do not in themselves lead to oxidant levels above the
standard, they may in combination with man-made emissions lead to levels in
excess of the standard. Therefore, stricter controls on the man-made emissions
may be needed to compensate for the contributions from natural sources. It may
also be found that the natural source contribution is sufficiently high in some areas
that small additions of man-made emissions may cause the standards to be
exceeded. In these cases it will still be possible to significantly reduce oxidants
but it may not be possible to attain the standards at all times and places.
The implications of the recent findings for the control of oxidants in both
urban and non-urban areas may be summarized as: most organic compounds need
to be controlled; eventually the coordinated control of both organics and nitrogen
oxides may be necessary; present regulations need to be expanded into non-
metropolitan areas; continued emphasis on mobile sources must be supplemented
by reductions in stationary sources; and natural sources should be considered in
determining required emissions reduction measures.
31
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EPA PROGRAMS
LEADING TO MORE EFFECTIVE CONTROL STRATEGIES
Substantial progress has been made in the last several years in understanding
the occurrence and formation of photochemical oxidants in the atmosphere. It is
apparent from all the available information that the reduction in emissions of
organic compounds will lead to reduction of oxidant concentrations. The recent
studies indicate that additional measures will be needed in order to meet the
standard everywhere. There is still not sufficient information, however, to define
exactly what control measures will be needed to meet the oxidant standard
everywhere. EPA is sponsoring an extensive program of laboratory and field
studies, including development of improved mathematical modeling techniques , to
obtain the needed information. The studies will:
Determine and quantify the effect of intermediate and long-range
transport on oxidant levels in both urban and rural areas.
Quantify the role of local emissions of hydrocarbons and nitrogen oxides
on the generation of rural and urban oxiclants.
Determine the long-term behavior of oxidant levels in air masses in
which oxidant precursors are being intermittently or continually
injected.
Quantify the relative roles of hydrocarbons and nitrogen oxides in
oxidant formation particularly at oxidant levels near the standard.
Determine the contribution of natural sources and sinks of hydrocarbons,
nitrogen oxides and oxidants on oxidant ]evels in the lower atmosphere in
both rural and urban areas.
Develop more reliable methods for relating emissions of precursors and
the concentrations of oxidants observed in the atmosphere.
Expand the aerometric data base for oxidants and precursors through-
out the United States .
Develop detailed emission inventory data by place, time of day, and year
for both anthropogenic and natural sources of oxidant precursors.
32
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Develop control technology for stationary sources at reasonable cost.
Much of the needed information from these studies will become available over
the next few years. In some other areas several more years of effort may be
required. However, as soon as the information in any area is sufficient to support
an addition or change in strategy, the improvement can be made. Thus, it is
expected by this process that the current strategy will evolve into an optimum
strategy over the next several years.
33
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REFERENCES
1. "Air Quality Criteria for Photochemical Oxidants," U .S .Department of Health
Education and Welfare, Publication No. AP-63, March, 1970.
2. "Health Effects of Air Pollutants," Volume 2 of Air Quality and Automobile
Emissions Control , National Academy of Sciences and National Academy of
Engineering, Prepared for the U.S. Senate Committee on Public Works, Serial
No. 93-24, September, 1974.
3. Altshuller, A.P. andBufalini, J.J., "Photochemical Aspects of Air Pollution:
A Review," Environ. Sci. Technol. 5, 39, (1971).
4. Demerjian, K.L., Kerr, J.A. and Calvert, J.G., "The Mechanism of
Photochemical Smog Formation," pps. 1-262, in Advances in Environmental
Science and Technology, Vol. 4, John Wiley and Sons, 1974.
5. Hecht, T.A., Seinfeld, J.H., and Dodge, M.C., "Further Development of
Generalized Kinetic Mechanism for Photochemical Smog," Environ. Sci.
Technol. 8, 327-339, (1974).
6. University of California, Riverside, California Air Environment 4, No. 3,
1974.
7. Glasson, W.A. and Tuesday, C.S., "Hydrocarbon Reactivities in the
Atmospheric Photooxidation of Nitric Oxide," Environ. Sci. Technol. 4, 916-
924, (1970).
8. Dimitriades, B., "Effects of Hydrocarbon and Nitrogen Oxides on
Photochemical Smog Formation," Environ. Sci. Technol. 6, 253-260, (1972).
9. Code of Federal Regulations 40, Parts 50-87, July 1, 1974.
10. Schuck, E.A., et. al. , "Relationship of Hydrocarbons to Oxidants in Ambient
Atmospheres," Jr. APCA 20, 297-302, May, 1970.
11. County of Los Angeles, Air Pollution Control District, Rules and Regulations,
Rule 66, 66.1, 66.2.
12. Horowitz, J. and Kuhrtz, S., "Transportation Controls to Reduce Automobile
Use and Improve Air Quality in Cities," U.S. Environmental Protection
Agency, EPA-400/11-74-002, November 1974.
13. "The National Air Monitoring Program Air Quality and Emissions Trends,
Annual Report," Volume 1., U.S. Environmental Protection Agency, EPA-
450/1-73-001-a, p. 1-13, August 1973.
14. "Monitoring and Air Quality Trends Report, 1972," U.S. Environmental
Protection Agency, EPA-450/1-73-004, pp. 4-14-28, December, 1973.
34
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15. "Monitoring and Air Quality Trends Report, 1973, "U.S. Environmental
Protection Agency, EPA-450/1-74-007, pp. 77-79, October, 1974.
16. Altshuller, A.P., "Evaluation of Oxidant Results at CAMP Sites in the United
States," Jr. APCA 25, pp. 19~24, January, 1975.
17. State of California Air Resources Board, "Ten Year Summary of California Air
Quality Data, 1963-1972," January, 1974 and California Air Quality Data
Quarterly Reports, 1973-1974.
18. Decker, C.D., et. al. , "Investigation of Rural Oxidant Levels as Related to
Urban Hydrocarbon Control Strategies," Research Triangle Institute,
Prepared for U.S. Environmental Protection Agency, EPA-450/3-75-036,
March, 1975.
19. Cleveland, W.S. , et.al. , "Sunday and Workday Variation in Photochemical Air
Pollutants in N.J. and N.Y.," Science 186. 1037-1038 (1974) and Science 186,
257(1974).
20. Johnston, D.R. , "Investigation of High Ozone Concentrations in the Vicinity of
Garrett County, Maryland and Preston County, West Virginia," Research
Triangle Institute, Prepared for U.S. Environmental Protection Agency, EPA-
R4-73-019, January, 1973.
21. Johnston, D.R., et. al. , "Investigation of Ozone and Ozone Precursor
Concentrations at Non-urban Locations in the Eastern U.S.," Research
Triangle Institute, Prepared for U.S. Environmental Protection Agency, EPA-
450/3-74-034, May, 1974.
22. "Monitoring and Air Quality Trends Report, 1973," U.S. Environmental
Protection Agency, EPA-450/1-74-007, pp. 117-125, October, 1974.
23. Dutsch, H.U., "Photochemistry of Atmospheric Ozone," in Advances in
Geophysics, Vol. 15, pps. 219-322, Academic Press, 1971.
24. Shlanta, A., and Moore, C.D., "Ozone and Point Discharge Measurements
under Thunderclouds," Jr. Geo. Res. 77, pps 4500-4510, August 20, 1972.
25. Rasmussen, R.A., and Robinson, E., "The Role of Trace Atmospheric
Constituents in a Surface Ozone Model," Washington State University,
Pullman, Washington, 1975.
26. Rasmussen, R.A., et. al., "Measurement of Light Hydrocarbons in the Field
and Studies of Transport of Oxidant Behind an Urban Area," Final report,
Contract No. 68-02-1232, Washington State. University (In preparation).
27. Cleveland, W.S., and Kleinger, B., "The Transport of Photochemical Air
Pollution from the Camden-Philadelphia Urban Complex," Bell Laboratories,
presented at AAAS Annual Meeting, New York, 1975.
28. Blumenthal, D.L., et. al. , "Determination of the Feasibility of the Long-Range
Transport of Ozone or Ozone Precursors," Meteorology Research, Inc.,
Prepared for U.S. Environmental Protection Agency, EPA-450/3-74-061,
November, 1974.
35
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1 REPORT NO.
EPA-450/2-75-005
3 RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
Control of Photochemical Oxidants - Technical Basis
and Implications of Recent Findings
5. REPORT DATE
July 15, 1975 (date of issue)
6. PERFORMING ORGANIZATION CODE
7 AUTHOR(S)
8. PERFORMING ORGANIZATION REPORT NO
9 PERFORMING ORGANIZATION NAME AND ADDRESS
U.S. Environmental Protection Agency
Office of Air and Waste Management
Office of Air Quality Planning and Standards
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
policy paper
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRACT
The report assesses laboratory and field studies conducted over the past
several years and discusses how the findings affect current and projected
programs to control oxidants. Both maximum concentrations and the frequency
of violations of the air quality standards for oxidants have decreased in some
urban areas as a result of recently initiated controls. However, long thought
to be primarily an urban problem, oxidant levels well in excess of the standard
have been observed in broad areas in the eastern third of the U.S. Although
naturally occuring sources such as vegetation and the stratosphere do contribute
to these high levels, man's activity is their predominant cause. Instances
are noted in which oxidants and their precursor compounds have been carried up to
50 miles, and probably farther. More extensive control of oxidant-producing
compounds will be required in rural areas while emphasis continues to be
placed on control in the cities.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COS AT I Field/Group
photochemical oxidants
8. DISTRIBUTION STATEMENT
release unlimited
19. SECURITY CLASS (This Report)
Unclassified
21 NO OF PAGES
42
20 SECURITY CLASS (This page)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
37
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