GUIDELINE SERIES
OAQPS NO. 1-2-076
June 1977
OOOR77005
REGULATORY AND TECHNICAL CONTROL
STRATEGIES FOR FINE PARTICLES
US. ENVIRONMENTAL PROTECTION AGENCY
Office of Air Quality Planning and Standards
Research Triangle Park, North Carolina
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Regulatory and Technical Control Strategies for Fine Particles
Joseph Padgett and J. D. Bachmann
U. S. Environmental Protection Agency
Research Triangle Park, North Carolina
Joseph Padgett, B. E. (Mechanical Engineering), M. S. (Mechanical
Engineering) is Director of the Strategies and Air Standards
Division, Office of Air Quality Planning and Standards, U. S.
Environmental Protection Agency.
John Bachmann, B. S. (Chemistry), M. A. (Teaching), M. S. (Environ-
mental Health Engineering) is an Environmental Engineer in the
Strategies and Air Standards Division.
The mailing address for both authors is: Environmental Protection
Agency, Office of Air Quality Planning and Standards, Strategies
and Air Standards Division, MD-12, Research Triangle Park, North
Carolina 27711.
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REGULATORY AND TECHNICAL CONTROL STRATEGIES FOR FINE PARTICLES, J. Padgett and
J.D. Bachmann, U.S. Environmental Protection Agency, Research Triangle Park, N.C,
This paper discusses the implications of available information for the development of tech-
nical and regulatory control strategies for fine particulate matter. Although it has been
widely assumed to be a desirable goal, results of research activities in recent years have
led to questions regarding the effectiveness and desirability of an undifferentiated fine
particulate standard. Atmospheric particulate mass appears to be distributed into two
fractions, which have distinct origins and properties. The fine particle mode is dominat-
ed by the products of gas to particle conversions, mostly sulfates, nitrates, and organics.
Control of these substances means additional S02, NO, and HC control. Some directly
emitted particulates in this fraction have low concentrations, but high toxicity. The
.composition of fine particles varies markedly in different regions of the country. A
general fine particulate standard, unless set at an unrealistically low level, would not
ensure against effects from specific chemical compounds and would not obviate the need for
•separate regulatory programs for the more toxic particles. Control of specific categories
of fine particles, with continued reliance on the current TSP, may be a more effective
regulatory approach. EPA's current research programs are weighted towards specific pollu-
tant categories, but regulations are several years away. In the meantime, existing regu-
latory programs can limit increases in emissions of fine particles and their precursors.
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REGULATORY AND TECHNICAL CONTROL STRATEGIES
FOR FINE PARTICLES
Introduction
The need for control of fine participate matter (roughly defined as
less than 2 ym) has been a topic of continued concern to EPA. The likeli-
hood that these small particles are responsible for most of the adverse
effects associated with total suspended particulate matter (TSP) was /,\
recognized at the time current particulate standards were established.^ '
However, insufficient information existed to relate adverse effects to
specific fine particulate levels. In 1972 the Administrator of EPA
identified the establishment of National Ambient Air Quality Standards for
fine particulate matter as a national priority objective in his guideline
policy statement for the development of 1973-1978 program plans. The.
EPA Office of Research and Development (ORD) committed a substantial
portion of its resources to studying fine particles and their control.
An air quality standard for fine particles still cannot be set with the
limited information available at this time. Furthermore, results of EPA
and other research activities in recent years have led to questions
regarding the effectiveness and desirability of an undifferentiated fine
particulate standard.
This paper discusses the implications of available information for
fine particulate control strategies. Based on current information, it
appears that even with a fine particulate standard it would still be
necessary to regulate certain classes of toxic fine particulate com-
pounds such as lead, sulfates, and nitrates. Quite possibly the current
TSP standard augmented by regulatory programs for specific toxic particu-
lates will be adequate and preclude the need for a new general fine
particulate standard with attendant regulatory disruptions.
Discussion
Current Information
Although available information is still insufficient to initiate
a regulatory program, the results of fine particulate research in recent
years has led to important advances in our understanding of the origin
and impacts of particles in the atmosphere. Figure 1 summarizes many
of the important characteristics of atmospheric particulates typically
observed in a variety of recent studies.which determined particle size
distributions in a number of areas.(2,3r The plot is constructed so
that the area under any section of a curve is proportional to the
concentration in that size range. It is clear that very fine particles
make up most of the total suspended particulate in terms of numbers of
particles and surface area. The mass (inferred from volume) shows a
bimodal distribution.
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The distinct minimum at about 2 ym between the two modes provides a
convenient size classification for atmospheric particles. The size range
between 0.1 and 1.0 ym is called the "accumulation" mode and typically
makes up about one third of the total suspended particulate mass. Fine
particulate mass "accumulates" in the 0.1 to 1 ym range by coagulation
of smaller particles or by condensation of gases on existing particles.
It is thought that most of the mass in the accumulation mode is formed
by physical and chemical processes which convert gases into particles,
including trace element fumes from high temperature sources and the
transformation products of sulfur oxides, nitrogen oxides, and organic
compounds.'3' The large particle fraction of the bimodal distribution is
termed the "coarse" mode. Coarse mode particles are generally formed by
mechanical processes such as grinding or rubbing, for.example industrial
processes, soil, street dust, and rubber tire wear/3' Chemical composi-
tion in this range is dominated by compounds of soil and mineral derived
elements such as silicon, iron, and aluminum.(4»5) There appears to be
very little exchange of mass between the fine and coarse particle ranges
in the atmosphere because the far greater number and surface area associ-
ated with the accumulation mode dominate the condensation and coagulation
growth processes.'3' Thus, the fine and coarse particle modes generally
have distinctly different origins and chemical compositions.
Fine particles of varying chemical species share.a number of impor-
tant properties as a result of their physical size.(') Fine particles
have very long life times in the atmosphere, and can therefore be trans-/g\
ported long distances before removal by dry deposition or precipitation/ '
Particles in the 0.1 to 1 ym size range also scatter more light per.unit
mass than larger particles and thus control visibility reduction.!' /
Significant increases in fine particle loading could have climatalogical
consequences.(7) The greater surface area associated with fine particles
provides sites to which more toxic chemical compounds can be attached or
formed. Most importantly, these particles, because of their aerodynamic
size, penetrate deeply into the respiratory system and impose a probable
danger to public health by their own intrinsic toxicity, or by acting as
a transport mechanism for more toxic substances.('' The effective size
is apparently important even for particles generally considered to be
in the respirable range. Toxicological studies have found that sulfuric
acid aerosols of 0.7 ym size produced a four fold greater response than
2.5 ym particles of the same compound.(8)
The chemical properties of small particles have been found to be
at least as important as their physical size in defining the fine particu-
late problem. Considering the differing origins and compositions of fine
and coarse particles, illustrated in Figure 1 and discussed above, chemical
characteristics in some respects determine the ultimate particle size
distribution. In addition, medical authorities believe that the chemical
nature can influence the respiratory penetration of particles, their
solubility and retention in the lungs, and their resultant biological
toxicity/'>9' Chemical composition also determines the extent to which
atmospheric fine particles can cause material damage, crop losses, and .
ecological disruption during removal by precipitation or deposition.v10^
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TABLE I. Results of Ambient Monitoring Studies
Percent Fine
Participate Mass
Secondarily Formeda
(14),(15)
Fine Participate
Mass as
Percent of TSP
1
i^B
*
1
1
Pasadena,
California 75 60
Pomona, California 80 40
New York City, N. Y 60 50
(Welfare Island)
Columbus,
Secaucus,
Ohio 70 NA
N. J.b 80 60
Denver, Colorado 60-75 15-25
1
1
1
•
1
1
1
•
1
1
*
1
1
1
aSul fates,
primary,
Moderate
TABLE II
Regjon
East0
^
Mideast0
South
Midwest6
Mountain
Southwest
West Coast
Measured
5
nitrates, ammonia, and organic compounds (organic material assumed 1/2
1/2 secondary).
pollution conditions.
. Regional Variation Major Chemical Classes Typically Occurring as
Fine Particulate (Annual Arithmetic Average Based on 1966-1968
NASN Data) O8)
SOI 3 BSOb N0oa Pb 3 Total
ItLO/IP3.)- IMS/ID. J_ luS/JBlI. jyc|/m_) (yg/m3)
18.9 7.8 2.1 1.2 30
14.6 7.3 2.9 1.3' 26
10.0 7.8 2.5 1.0 21
5.9 5.3 1.8 1.0' 14
3.4 4.7 1.3 1.3 10
4.4 5.5 2.1 1.0 13
9.4 8.9 3.8 1.9 24
NH* partitioned to SO^, N0~
Benzene soluble organics, about 1/2 to 2/3 of total organics
Providence, R. I. to Washington, D. C>
Ohio, Michigan, Indiana, Illinois
p
Minnesota
, Iowa, Nebraska, Missouri
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Recent epidemiological and toxicologlcal studies appear to support
the relative importance of certain chemical categories of fine particu-
late matter. Preliminary data from the Community Health and Environmental
Surveillance System (CHESS) studies indicates that the respirable fraction
of particulate matter (generally equated to fine particulate matter) has
not shown as pronounced an association with adverse health effects,as have
some specific fine particulates such as nitrates and sulfates.^ ' '
Chemical composition of particles has usually been assumed to be of
lesser importance than size in causing visibility deterioration. However,
several recent studies have indicated that chemical properties of fine
particulate matter are also important in determining visibility reduction.
Studies in the Los Angeles basin found that sulfates accounted for two
to three times the visibility reduction for a given mass than other fine
particulate components.'13^ Hygroscopic and/or deliquescent fine partic-
les can absorb water and increase in size at higher humidities, resulting
in an aerosol/visibility reduction much greater than might be expected on
a mass basis.^ ' At lower humidities, chemical composition can influence
the ability of a particle to reduce visibility by affecting its light
absorptive properties.
Studies which examined both size and chemical composition of atmos-
pheric particulates are too limited in number and time to provide adequate
characterization and source assessment for most urban areas. However,
the results of short-term studies, some of which are summarized in Table I,
provide some insight into the principal sources of fine particulates.
Some 60 to 80 percent of fine particulate mass can be accounted for by
the aforementioned gas to particle transformations which occur in the
accumulation mode.('5) Particles formed in this manner have often been
termed secondary particles; material which leaves a source in particulate
form is termed primary or directly emitted particulate. As indicated in
Figure 1, secondary fine particles have been found to consist almost
entirely of sulfate, nitrate, ammonium, and organic compounds. These
groups were found to comprise 50 percent of the total suspended particulate
levels in the Los Angeles Basin.('6) The principal directly emitted fine
particulate components in most urban areas are lead-related compounds
from automotive,sources, which can account for as much as 10% of the TSP
in some cities.'17' The sum of all other directly emitted metals typically
accounts for only about five percent of the fine particulate mass.('8;
Using National Air Surveillance Network (NASN) hi-vol measurements
of the major chemical components of fine particulates as a surrogate, it was
found that the chemical composition and concentration of fine particulates
varies considerably from one geographic-area to another (Table II).t'8'
It can been seen that the water soluble sulfates are the principal fine
particulate component in the eastern United States, with organics and
nitrates relatively more important in western areas. It is important to
note that benzene soluble organic (BSO) measurements probably underestimate
total organic concentration since benzene is not a good solvent for some
of the more polar oxygenated organic compounds which may be present.04}
In addition, recent data suggests that particulate nitrate results may
be unduly high due to collection of gaseous nitric acid vapor."''
Nevertheless, the general observation that marked geographical variations
exist in the chemical composition of fine particles is still valid. Since
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health and welfare effects associated with these chemical components vary,
it is "likely that different areas of the country may have similar fine
participate concentrations, but with decidedly dissimilar impacts.
Control Strategy Implications
Available information on health and welfare effects does not permit
the establishment of adequate dose/response functions for fine particles.
Given the variability in fine particulate composition, it may not be
possible to develop generally applicable dose/response relationships for
undifferentiated fine particles. However, preliminary research indicates
that some adverse effects may be associated with certain fine particulate
classes even when current TSP standards are met.('2' It is probable that
control programs aimed at the specific classes of fine particles will be
needed. Although developments of such programs will require much addi-
tional research, some assessment of their direction is possible at this
time.
Current limited information summarized in the previous section sug-
gests essentially three approaches to controlling fine particulates, either
as a single group or as chemical subgroups: (1) Control of emissions of
gaseous precursors to fine particulates which account for the majority of
fine particulate mass; (2) Control of the mechanisms which promote trans-
formation of gases to particles; (3) Control of direct emissions of fine
particles.
Since important pollutants in the fine particulate category are
emitted directly (e.g., lead) and formed in the atmosphere (e.g., sulfates),
it is likely that effective control strategies will require integration
of all three approaches. In view of the high proportions of gas derived
aerosols, conventional particulate control plans stressing direct emissions
control alone do not appear likely to be sufficient to provide adequate
protection against fine particulate effects. Since sulfates, nitrates9
and organics appear to be the major fine particulate components, the
control of fine particulates may require more stringent control of sulfur
dioxide (S02), nitric oxide (NO), and hydrocarbons (HC) emissions. It is
of note that only certain chemical fractions of total HC emissions are
thought to be of major significance in forming organic aerosols.^20)
The above substances are converted to particles by a number of incom-
pletely understood mechanisms which involve atmospheric variables such as
levels of other pollutants, humidity, sunlight intensity, and temperature.
Control of certain pollutants may slow these reactions and reduce the
amount of gases which form parti cul ates/ For example., aerosol formation
appears to be accelerated by photochemical activity.''°' Hence, reductions
in photochemical oxidant levels would indirectly influence fine particulate
levels by reducing the amount of gases which are transformed into aerosols.
Since reactive HC emission reduction- is the principal method of oxidant
control, organic aerosols would be both indirectly and directly affected
by oxidant reduction programs.H3) Although some benefit can be expected
from controlling pollutants which influence aerosol formation mechanisms,
many other important variables, such as humidity, cannot be controlled.
Thus, control of mechanisms which form particles in the atmosphere is
likely to be a supplement to control of direct precursor gases, rather
than a complete strategy. Additional understanding of aerosol formation
mechanisms is needed.
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Although it cannot be considered as the likely principal control
approach, control of directly emitted fine particulates is of continuing
importance. As previously discussed, direct emissions appear to account
for 20 to 40 percent of the fine particulate mass. These emissions could
dominate small particle loading in the vicinity of strong sources. Many
directly emitted compounds are both toxic and tend to volatilize in high
temperature processes, recondensing as fine particles before or during
emission.(21) Lead, arsenic, selenium, and benzo(a)pyrene are examples
of such toxic materials, and are typically found in the fine particle
mode. Although these substances are not generally found in high concen-
trations, levels around certain stationary sources may be of concern. In
.addition, toxicologically innocuous directly emitted particles such as
elemental carbon or iron oxide may provide reaction sites for and/or
catalyze aerosol formation processes.(22) Improved characterization of
both direct particulate emissions as well as the ambient aerosols is
needed throughout the nation.
Besides accounting for complex gas/particle transformations and
direct emission characteristics, control strategies may have to consider
the impact of long range transport of significant quantities of fine
particulates and their precursors. A growing body of data suggests that
this phenomenon influences ambient concentrations of oxidantst") ancj
sulfates.(2^) In the case of sulfates, it has been noted that geographical
and temporal correlations exist between S02 emissions increases in the
northeast quadrant of the U. S. and observed or suspected increases in
regionally high urban and non-urban sulfate levels, acidic precipitation,
and visibility deterioration. (") Future fine particulate control pro-
grams may have to be based on broader considerations than the effects of
local emissions on local air quality. Methods must be developed to permit
evaluation of region wide source/receptor relationships as mediated by
fine particulate transformation/transport/removal processes.
Fine Particulate Control Strategies
Original plans for a fine particulate air quality standard were based
on the belief that this approach could protect against adverse effects
related to particle size. This approach was also recognized.as a possible
improvement over the ability of existing TSP standards to safeguard public
health and welfare on a national basis. Ambient concentrations of TSP in
some regions can be greatly influenced by larger particles, such as wind-
blown road dust, which may not be of major interest in protecting public
health and visibility.(2°) Hence, TSP may be a less adequate indicator of
health risk from particulates than are fine particles, although few
epidemiological studies have attempted to relate ambient levels of general
fine particles to health effects.
In recent years, however, it has become more apparent that a general
fine particulate standard may not go far enough toward improving current
standards. As noted earlier, the fine particulate fraction is made up
of a variety of compounds with varying toxicity. Two of the major com-
ponents, sulfates and nitrates, have shown an association with health
effects. Some fine particulates, such as arsenic and polycyclic organic
mater, may be toxic or carcinogenic even in relatively small amounts.
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Given that the chemical composition varies from region to region, attain-
ment of a given level of fine particles would not guarantee that all fine
participate components of interest would be reduced to safe levels, unless
a national fine particulate standard was set at an unrealistically low
level or supplemented by standards for specific toxic components. It
would appear that proceeding directly to control of fine particulate chemi-
cal components while maintaining current TSP standards may well represent
the most effective means to adequately safeguard public health and welfare.
Although no decisions can be made concerning the ultimate form of
future control programs, work 1s proceeding towards regulation of
specific fine particulate compounds. EPA has estimated that it will take
several years to develop information needed to regulate sulfates, a
major fine particulate component, in many areas. Less is known about
nitrate and organic aerosols.
Since it will take years to initiate regulations for the major fine
particulate components, it is important to provide some assessment of the
potential impacts of current air pollution control programs in the light
of current knowledge on fine particles. Based on available information,
current regulatory programs for S02, N02, HC, TSP, and photochemical
oxidants will have an impact on fine particulate levels. These regulatory
programs include ambient air quality standards, state implementation plan
emission limits, new source performance standards for selected stationary
source categories, and emissions standards for mobile sources. Table III
presents Nationwide Emissions Projections which include the effect of
these programs and anticipated growth.
Table III - Projected Natiowide Emissions of Fine Particulate
Related Pollutants under Current Regulatory Plans
(106 tons/yr)
Particu1.at.es $02 NOX Hydrocarbons
1972 23.2 32.6 22.2 33.7
1975 18.1 33.2 24.5 30.7
1980 18.8 34.0 26.2 31.0
1985 19.7 34.8 26.4 33.8
1990 20.9 38.8 28.6 38.5
a (27 28 29
Projections were obtained by normalizing published estimates^ ' *
to a common nationwide emissions inventory'^) for base year 1972.
Adjustments on some source categories were made to reflect recent
information.
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Although a linear relationship between reductions in emissions and ambient
fine particulate concentrations is unlikely, these projections do provide
some insight into future trends in ambient levels of fine particulates.
With the possible exception of nitrogen oxides, it is apparent that major
emission increases in fine particulate related pollutants are not antici-
pated in the next 10 years. The pattern of increasing emissions of these
pollutants which held through the 1960's appears to have been slowed or
stopped. Although trends data are limited, it is likely that associated .
apparent increases in non-urban sulfate levels,(") acid precipitation,(31)
and regional visibility degradation,HO) which also occurred during the
1960's, have been limited as well. Current regulatory programs should
permit adequate time for research efforts to document effects levels and
source/receptor relationships, and to develop improved control technologies
for fine particles in the next few years without marked increases in ambient
concentrations of fine particulates.
Conclusions
A National Ambient Air Quality Standard for fine particulate matter as
a group can no longer be regarded as the certain choice for a long range
strategy to control fine particulates. Recent evidence suggests an approach
which places greater emphasis on the control of selected categories of fine
particulate matter, especially sulfates and nitrates, with continued reli-
ance on the current TSP standard for overall control of particulates. This
strategy provides continuity in existing particulate control programs, and
has the potential for more effectively achieving necessary health and wel-
fare benefits by placing priority on the control of the more harmful
components of fine particulate matter. A general fine particulate
standard would not necessarily ensure against effects from specific
chemical compounds and would not obviate the need for separate regulatory
programs for such toxic particulates as lead and other trace elements, acid
aerosols such as sulfates and nitrates, and carcinogens such as certain
polycyclic organics.
However, control of fine particulates may become increasingly impor-
tant for achieving the TSP standard in many metropolitan areas. Further-
more, much of the general fine particulate research is useful in developing
criteria for regulation of specific toxic particulates. Therefore, work
necessary to develop a general fine particulate standard should continue,
but it should not be given as high a priority as research directed
specifically to developing control programs for known toxic particulates.
In the meantime, implementation of existing air pollution regulations can
limit increases in emissions of fine particles and their precursors.
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