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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                                                      #76-30.07
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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                                                               76-30.07
            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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                                 76-30.07


               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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                                 76-30.07


     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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I
I
I
               Cit\
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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                                76-30.07


     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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                                76-30.07


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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                                76-30.07


     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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                                76-30.07


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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                                76-30.07


     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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References

 1.  Ai r Qua! i ty Cr i ten a_ for Parti culate  Matter,  U.  S.  Department of  Health,
     Education,  and WeTfare,~Publi c Health Service,  Publication  No. AP-49,
     Washington, D. C., January,  1969.

 2.  K.  T. Whitby, R.  E.  Charlson,  W.  E. Wilson,  R.  S.  Stevens,  "The Size of
     Suspended  Particle Matter in the  Air", Science  183:   1098-99,  (March
     15, 1974).

 3.  K.  T. Whitby, Modeling of Atmospheric Aerosol  Size  Distrjjbiution_,  Progress
     Report, EPA ResearcTGrajTOo.  R80097T7 AprTTT, T9757

 4.  T.  C. Dzubay, M,.  Garneau, 0.  Durham,  R. Patterson,  T.  Ellestad, J.  Durham,
     X-Ray Fluorescence Analysis  of Denver Aerosol,  Unpublished  Report,  U.  S.
     Environmental Protection Agency,  Research Triangle  Park,  N.C.,  (undated).

 5.  R.  E. Lee and D.  J.  VonLehmdin, "Trace Metal  Pollution in the  Environment",
     Journal of Air Pollution Control  Association 23:  853, (1973).

 6.  R.  J. Charlson, A. H.  Vandorpohl, P.  S. Covert, A.  P.  Waggoner, and
     N.  C. Alquist, "H«SO./(NH,)?SOd Background Aerosol:   Optical  Detection
     in St. Louis Region. Atmos.  Environ.  8(12):   1257-1268, (December 1974).

 7.  Inadvertent Climate Modification, Report on Man's Impact  on Climate (SMIC),
     MIT Press, Cambridge, Mass., 1971.

 8.  M.  0. Amdur, J. Boylis, V. Ugro,  M.  Dubriel, and D.  W. Underbill, "Respira-
     tory Response of Guinea Pigs to Sulfuric Acid and Sulfate Salts", Presented
     at Sulfur Pollution and Research  Approaches Symposium, Durham,  N. C.,
     (May 27-28, 1975).

 9.  M. 0. Amdur, T, R. Lewis, M. P. Fitzhand, and K. I.  Campbell,  Toxicology
     of Atmospheri c SuKyr_Dip_xj de! Decay Products, U. S.  Environmental Protection
     Agency, ResearcFTTrTan^le ParkT N.C., Publication Number  AP-111,  July,
     1972.

10.  I.  Nisbet, "Ecological Effects",  In:   Air Quality and Stationary  Emission
     Control, Commission on Natural Resources, National  Academy of Sciences,
     Washington, D.C., Prepared for Committee on Public Works, United  States
     Senate, Washington, D. C., March 1, 1975.

11.  J. French, U.S. Environmental Protection Agency, Research Triangle Park,
     N.C., Private Communication, April 1, 1976.

12.  Health Consequences of Sulfur Oxides:  A Report From CHESS 1970-1971,
     U.S. Environmental Protection Agency, Research Triangle Park,  NC,
     Publication Number EPA-650/1-74-004, May, 1974.

13.  An Assessment of  the Aerosol-_Visibility Problem in the South Coast Air
     Basin. State  of California Air Resources Board, Staff Report 75-20-3,
     October 28, 1975.

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14.   D.  Miller,  W.  E.  Schwartz,  J.  L.  Gemma,  and A.  Levy, Haze Formation:
     Its Nature  and Origin,  U.  S.  Environmental Protection Agency, EPA-650/
     3-75-010, March,  1975.

15.   W.  E.  Wilson,  U.  S.  Environmental  Protection Agency, Research Triangle
     Park,  N.  C.,  Private Communication,  May,  1974.

16.   G.  M.  Hidy, Characterization  of Aerosols  in California  (ACHEX), Final
     Report, Volume I  Summary,  Air Resources  Board,  State of California,
     Contract No.  358, April,  1975.

17.   E.  J.  Lillis  and  D.  R.  Dunbar,  "Impact of Automotive Exhaust Particulate
     Emissions on  Air  Quality",  Unpublished Report,  U.  S. Environmental  Protec-
     tion Agency,  Research Triangle Park, N.C..(November 13, 1975).

18.   A.  P.  Altshuller, Principal  Species  in Atmospheric Fine Particulate
     Matter, in  Minutes of Meeting of  U.S.  Environmental Protection Agency
     Air Pollution Chemistry and Physics  Committee,  Alexandria,  Virginia,
     April  17-18,  1970, P. 15.

19.   C.  W.  Spicer,  The Fate  of Nitrogen Oxides in  the  Atmosphere, Coordinating
     Research Council, Inc., Contract  (CAPA-9-71),  U.S. Environmental  Protection
     Agency, Contract  No. 68-02-0799,  Columbus, Ohio,  September  13, 1975.

20.   D.  Schuutzle, D.  Cronn, A.  L. Crittonden, and  R.  J. Charleson, "Molecular
     Composition of Secondary Aerosol  and Its Possible Origin",  Environmental
     Science and Technology  9(9):   838-845,  (September, 1975).

21.   P.  F.  S. Natusch  and J. R.  Wallace,  "Urban Aerosol Toxicity:  The
     Influence of Particle Size",  Science 186: 695-699,  (November 22, 1974).

22.   T.  Novokov, "Sulfates in Pollution Particulates", Presented at 67th
     Air Pollution Control Association Annual Meeting, Denver, Colorado,
     (June 9-13, 1974).

23.   Control of Photochemical  Oxidants—Technical  Basis and  Implications of
     Recent Findings.  EPA-450/2-75-005, U.S.  Environmental  Protection  Agency,
     Research triangle Park, N.C., July, 1975.

24.   N.S. Stasiuk, P.  E. Coffey, and R. F. McDermott,  "Relationships  Between
     Suspended Sulfates and Ozone at a Non-Urban  Site", Presented at  68th
     Annual Air Pollution Control  Association Meeting, Boston, Massachusetts,
     (June  15-20,  1975).

25.  Position Paper on Regulation of Atmospheric  Sulfates,  U:  S. Environmental
     Protection Agency,  EPA-450/2-75-007, Research Triangle Park, N.  C.,
     September, 1975.

26.  W.  L.  Faith,  "An  Evaluation of Fugitive  Dust Regulations",  Presented at
     the 67th Annual Meeting of the Air  Pollution Control  Association,
     Denver,  Colorado,  (June 9-13, 1974).

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27.  L. J. Habeggar, R.  R.  Cirillo, and N.  F.  Sather, Priorities and
     Procedures for the Development of Standards of Performance for New
     Stationary Sources of Atmospherlc Emisslons, Draft Contractor Report,
     U.S. Environmental  Protection Agency,  Contract Number EPA-IAG-D4-0463,
     Argonne, Illinois,  April 1, 1975.

28.  Implication of Alternative Policies for the Use of Permanent Controls
     and Supplementa1 Contro1s, U.S. Environmental Protection Agency,
     Washington, D. C.,  July 7, 1975.

29-  Air Quality and Stationary Source Emission Control, Commission on
     Natural Resources,  National Academy of Sciences, Washington, D.C.,
     Prepared for the Committee on Public Works, United States Senate,
     Washington, D. C., March 1, 1975.

30.  OAQPS Data File of Nationwide Emissions, Preliminary Estimates, Un-
     published Report, U.S. Environmental Protection Agency, Research'
     Triangle Park, N. C., May, 1975.

31.  C. V. Cogbill, G. E. Likens, "Acid Precipitation in the Northeastern
     United States", Water Resources Res. 10(6). (December, 1974).

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