United States
Environmental Protection
Agency
Atmospheric Research and Exposure
Assessment Laboratory
Research Triangle Park, NC 27711
Research and Development
February, 1995
EPA/600/R-95/021
&EPA
Interim Findings on the Status of
Visibility Research
Office of Research and Development
Office of Air and Radiation
U.S. Environmental Protection Agency
February 1995
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Interim Findings on the Status of
Visibility Research
Office of Research and Development
Office of Air and Radiation
U.S. Environmental Protection Agency
February 1995
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The National Academy of Sciences has given permission to use
quotations from the copyrighted work, Protecting Visibility in
National Parks and Wilderness Areas, by the National Research
Council (NRC). Copyright 1993 by the National Academy of Sciences.
Courtesy of the National Academy Press, Washington, DC.
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Contents
I. Introduction 1
A. Purpose 1
B. Report Structure 2
II. Synopsis of Prior Reports ... 2
A. National Research Council Reports (NRC), National
Academy of Science Report 3
B. IMPROVE 1993 Report 13
- IMPROVE Network 13
- IMPROVE Findings 14
- IMPROVE Recommended Research 16
C. National Acid Precipitation Assessment Program
(NAPAP) 17
D. Effects of 1990 CAAA on Visibility in Class I
Areas 18
III. Published Results 19
A. Monitoring 20
- Sampling Problems Related to Gas/Particle Phase . 21
- Relative Humidity Effects 22
- Particle Characterization 22
- Direct Measurement of Optical Properties .... 22
- Trends 24
- Remote Sensing and Global Change 26
B. Current Sources of Visibility Impairing Pollution
and Clean Air Corridors 27
- Source Area Atmosphere Characterization 29
- Marine Contribution to Visibility 31
C. Adaptation of Regional Air Quality Models for
Assessment of Visibility 32
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D. Studies of Atmospheric Chemistry and Physics of
Visibility 37
- Optical Efficiencies 37
- Particle Size and Aerosol Formation 37
- Atmospheric Chemistry 39
- Reconstructed Visibility Estimates 40
- Human Perception 41
- Radiative Transfer and Mathematical Models ... 41
- Global Change Research.in Atmospheric Chemistry . 42
IV. Research in Progress 43
A. Grand Canyon Visibility Transport Commission (GCVTC) 43
B. Environmental Protection Agency (EPA) 45
1. MOHAVE 45
2. Clean Air Status and Trends Network (CASTNET) . 46
3. Human Observer Comparison Study (HOCS) .... 47
4. Visibility Cooperative Research (UNC-CH) ... 50
5. Aerosol Equilibrium Model (In-house Research) . 50
6. Regional Particulate Model and Planned
Improvements 51
7. Source Attribution with Trajectories 53
8. Aerosol Modeling 53
9. Other EPA Studies Under Sections 812 and 404
of the 1990 CAAA 53
C. National Park Service (NPS) 54
D. Department of Energy (DOE) 60
E. Electric Power Research Institute (EPRI) 63
F. National Oceanic and Atmospheric Administration (NOAA) 65
V. Concluding Remarks 66
A. EPA/Office of Research and Development (ORD) Plans to
Eliminate Funding of Exclusively-Focused Visibility
Research 66
B. EPA/ORD to Fund Particle Research which Includes
Visibility Implications 66
C. Regional Haze Regulation 66
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D. CENR Subcommittee on Air Quality Research .... 67
E. Global Change Research 67
F. Continuing Research 67
REFERENCES 69
APPENDIX A: ACRONYMS 103
APPENDIX B: 1,05
APPENDIX C: 109
APPENDIX Ds 113
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I. Introduction
I.A. Purpose
This report has been prepared in response to the provisions
of §169B(a) of the Clean Air Act (CAA)a that call for the
Administrator of the U.S. Environmental Protection Agency (EPA)
to produce interim findings of available visibility related
research and information:
(1) The Administrator, in conjunction with the National Park Service and other appropriate Federal
agencies, shall conduct research to identify and evaluate sources and source regions of both visibility
impairment and regions that provide predominantly clean air in class I areas.6 A total of $8,000,000 per
year for 5 years is authorized to be appropriated for the Environmental Protection Agency and the other
Federal agencies to conduct this research. The research shall include-
(A) expansion of current visibility related monitoring in class 1 areas;
(B) assessment of current sources of visibility impairing pollution and clean air
corridors;
(C) adaptation of regional air quality models for the assessment of visibility;
(D) studies of atmospheric chemistry and physics of visibility.
(2) Based on the findings available from the research required in subsection (a)(1) of this section as well
as other available scientific and technical data, studies, and other available information pertaining to
visibility source-receptor relationships, the Administrator shall conduct an assessment and evaluation that
identifies, to the extent possible, sources and source regions of visibility impairment including natural
sources as well as source regions of clear air for class I areas. The Administrator shall produce interim
findings from this study within 3 years after enactment of the Clean Air Act Amendments of 1990.
The purpose of this interim report is to summarize the
results of visibility research published since 1990, both federal
and non-federal, that relate to the research objectives stated in
§169B(a) of the CAA. Evaluation and assessment of the results is
premature at this time. Thus, the report describes or recounts
the research and does not address its merits. Moreover, the
report does not make EPA policy determinations about visibility
protection.
•Section 816 of the 1990 Clean Air Act Amendments (CAAA) added
Section 169B to the Clean Air Act (CAA or Act). See Pub. Law No.
101-549, 104 Stat. 2399, 2695-2697. The Clean Air Act is codified,
as amended, at 42 U.S.C. §§7401-7671q.
"Mandatory class I areas (certain large national parks,
wilderness areas, etc.) where visibility is an important value and
associated integral vistas are identified at 40 CFR Part 81,
Subpart D.
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I.B. Report Structure
Following this introductory Section (I), Section II
summarizes the findings of four major visibility related reports
published since 1990:
• National Academy of Sciences Report 1993, "Protecting
Visibility in National Parks and Wilderness Areas"
• IMPROVE (Interagency Monitoring of Protected Visual
Environments) 1993 Report
• 1992 NAPAP (National Acid Precipitation Assessment
Program) Report to Congress
• EPA 1993 Report to Congress on Effects of 1990 CAAA on
Visibility in Class I Areas
Section III provides a summary of published research papers
relevant to visibility research (full citations are included in a
Reference section at the end of the document). Appendix D
identifies major scientific conferences involving visibility
research not cited elsewhere in this document. Many of these
conferences did not publish formal proceedings, and individual
presenters may or may not have published the research.
Section IV contains a summary of the visibility related
research currently being performed by:
- Grand Canyon Visibility Transport Commission (GCVTC)
- Environmental Protection Agency (EPA)
- National Park Service (NPS)
- Department of Energy (DOE)
- Electric Power Research Institute (EPRI)
- National Oceanic and Atmospheric Administration (NOAA)
Some of this research is conducted in support of programs such as
global climate change, but has implications for visibility
research.
Section V discusses visibility research in the planning
stages.
II. Synopsis of Prior Reports
There have been four significant reports issued since the
passage of the CAAA in 1990. The contents of these reports are
summarized below in chronological order.
In addition to these major reports, conferences addressing
visibility research results were also critical in the transfer
and coordination of visibility research. The primary purpose of
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each of these conferences was to provide a forum for discussion
on active visibility related research. The conference
presentations are not summarized in this document. However,
dates, sponsors, and references are provided in Appendix D.
II.A. National Research Council (NRC) , National Academy of
Sciences Report
The National Research Council report, Protecting Visibility
in National Parks and Wilderness Areas, was issued January 1993,
outlined working principles for assessing the relative importance
of anthropogenic emission sources that contribute to haze in
Class I areas and for considering various alternative source
control measures. Because of the importance of this document,
the Executive Summary is quoted in major part.
Executive Summary
Many visitors to America's national parks and wilderness areas are unable to
enjoy some of the beautiful and dramatic views that would prevail in the
absence of air pollution. Scenic vistas in most U.S. parklands are often
diminished by haze that reduces contrast, washes out colors, and renders
distant landscape features indistinct or invisible.1 The National Park Service
(NPS) has reported that visibility impairment caused by air pollution occurs in
varying degrees at many park monitoring stations virtually all the time. Today,
the average visual range in most of the western United States, including
national parks and wilderness areas, is 100-150 km (about 60-100 miles), or
about one-half to two-thirds of the natural visual range that would exist in the
absence of air pollution.2 In most of the East, including parklands, the average
visual range is less than 30 km (about 20 miles), or about one-fifth of the
natural visual range.3
Visibility degradation in parklands is a consequence of broader regional-
scale visibility impairment. The causes of this impairment are well understood.
Most impairment is caused by fine particles that absorb or scatter light. Some
of these particles (primary particles) are emitted directly to the atmosphere;
others (secondary particles) are formed in the atmosphere from gaseous
precursors. Visibility-reducing particles and their precursors can remain in the
atmosphere for several days and can be carried tens, hundreds, or thousands
of kilometers downwind from their sources to remote locations, such as
national parks and wilderness areas. During transport, the emissions from
many sources mix together to form a uniform, widespread haze known as
regional haze.
1Haze degrades visibility primarily through the scattering or absorption of
light by fine atmospheric particles. Visibility is the degree to which the
atmosphere is transparent to visible light.
Visual range is defined as the greatest distance at which a large black
object can be discerned against the horizon sky.
The natural visual range in the East is less than that in the arid West.
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Most visibility impairment is caused by five particulate substances (and
associated particulate water): sulfates, organic matter, elemental carbon (soot),
nitrates, and soil dust. The major cause of reduced visibility in the East is
sulfate particles, formed principally from sulfur dioxide (S02) emitted by coal
combustion in electric utility boilers. In the West, the other four particle types
play a relatively greater role than in the East. The causes and severity of
visibility impairment vary over time and from one place to another, depending
on meteorological conditions, sunlight, and the size and proximity of emission
sources.
Congress in 1977 established a national goal of correcting and pre-
venting pollution-related visibility impairment affecting large national parks and
wilderness areas, termed "mandatory Class I areas."4 However, the federal
government and the states have been extremely slow in developing an effective
visibility protection program. The present program lacks sufficient resources,
and it targets few of the major types of sources of visibility impairment in Class
I areas. As a result, little progress has been made toward the national visibility
goal established by Congress 15 years ago.
The Clean Air Act includes two emissions control programs specifically
concerned with visibility in national parks and wilderness areas. One of these,
the Prevention of Significant Deterioration (PSD) program, is directed mainly at
new sources; the other, a visibility protection program, largely is aimed at
existing sources.
The PSD program requires that each new or expanded "major emitting
facility" locating in a "clean air area" install the "best available control
technology", and it establishes increments (allowable increases) that limit the
cumulative increases in pollution levels in clean air areas. Many large national
parks and wilderness areas are designated as Class I areas and therefore are
subject to the most stringent increments. The PSD program has protected
visibility to some extent by reducing the growth of emissions of pollutants that
contribute to regional haze. The program's requirement that major new sources
locating in clean air areas install the best available control technology has been
particularly important.
But the limits on growth in air pollutant concentrations established by
the PSD program have been only partially effective. First, the restrictive Class
I increments apply only to large parks created before enactment of the Clean
Air Act Amendments of 1977; many other scenic areas receive no special
protection. Second, it is not even clear that the Class I increments ensure
effective protection against new sources that might cause visibility impairment.
The increments do not distinguish between particles in the 0.1-1.0//m range—
which have the greatest potential to degrade visibility—and larger particles.
Moreover, increments focus on the concentration of pollution at a given time
and place; but visibility impairment depends on the total magnitude of fine
particulate matter between an object and an observer.
4These are national wilderness areas and national memorial parks larger than
5,000 acres, national parks over 6,000 acres, and international parks. Any
such area must have been in existence on August 7, 1977, the date the Clean
Air Act Amendments of 1977 were signed into law, to be considered a manda-
tory Class I area.
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A 1990 report by the U.S. General Accounting Office (GAO) discussed
other flaws in the PSD program. The GAO found that federal land managers
had not fully met their responsibilities to review PSD permit applications, due
to lack of time, staff, and data 8nd also due to the failure of the U.S.
Environmental Protection Agency (EPA) to forward permit applications.
Moreover, many sources of visibility impairment in national parks and
wilderness areas are exempt from PSD requirements, because they are
considered minor sources, or because they existed before the PSD program
took effect in the 1970s.
The other visibility protection program under the Clean Air Act requires
states to establish measures to achieve "reasonable progress" towards the
national visibility goal and to require the installation of the "best available
retrofit technology" on large sources contributing to visibility impairment in
mandatory Class I areas. In 1980, EPA issued rules aimed primarily at
controlling "plume blight" (impairment due to visible plumes from nearby
individual sources). At that time, EPA also expressed its intention to regulate
regional haze at some future date "when improvement in monitoring techniques
provides more data on source-specific levels of visibility impairment, regional-
scale models become more refined, and scientific knowledge about the
relationships between air pollutants and visibility impairment improves." More
than a decade later, despite major advances in monitoring techniques, regional-
scale models, and scientific knowledge of visibility impairment, EPA has yet to
issue rules for regulating regional haze. Instead, EPA's rules require only the
regulation of impairment that is attributable to individual sources through the
use of simple techniques. This has greatly weakened the visibility program's
effectiveness. Fourteen years passed until the first pollution source was
required to control its emissions under this program.
Emission-control measures already adopted or planned will not solve
the nation's visibility problems. The acid rain control program established by
the 1990 Clean Air Act Amendments has been predicted to reduce SOs
emissions in the East by about 36% by 2010. That reduction probably will
improve visibility in much of the East but will eliminate only a fraction of the
anthropogenic visibility impairment. In the West, where most Class I areas are
located, projections done for EPA indicate that the acid rain control program will
halve, but not entirely prevent, expected growth in SO, emissions between now
and 2010.
THE CHARGE TO THE COMMITTEE
This report was prepared by the NRC's Committee on Haze in National
Parks and Wilderness Areas. The committee was convened by the Council'*
Board on Environmental Studies and Toxicology in collaboration with the Board
on Atmospheric Sciences and Climate of the Commission on Geosciences,
Environment, and Resources. The committee's members have expertise in
meteorology, atmospheric chemistry, air-pollution monitoring and modeling,
statistics, control technology, and environmental law and public policy. The
committee's work was sponsored by the U.S. Department of the Interior
(National Park Service, Bureau of Reclamation, and Office of Environmental
Quality), U.S. Department of Energy, U.S. Environmental Protection Agency,
U.S. Department of Agriculture (Forest Service), the Arizona Salt River Project,
and Chevron Corporation.
The committee was charged to develop working principles for assessing the
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relative importance of anthropogenic emission sources that contribute to haze
in Class I areas and for considering various alternative source control measures.
It also was charged to recommend strategies for filling critical scientific and
technical gaps in the information and data bases on (1) methods for determining
individual source contributions, (2) regional and seasonal factors that affect
haze, (3) strategies for improving air-quality models, (4) the interactive role of
photochemical oxidants, and (5) scientific and technological considerations in
choosing emission control measures.
In 1990, the committee published an interim report, Haze in the Grand
Canyon, which evaluated the National Park Service's Winter Haze Intensive
Tracer Experiment (WHITEX) report on the causes of wintertime haze in the
region between the Grand Canyon and Canyonlands National Park. The
WHITEX report by NPS had asserted that the Navajo Generating Station (NGS),
a large coal-fired power plant in Page, Arizona, is a principal contributor to
visibility impairment in Grand Canyon National Park {GCNP). Our committee's
interim report concluded that, at some times during the study period, NGS
contributed significantly to haze in GCNP, but that WHITEX failed to
quantitatively determine the fraction of sulfate particles and resulting haze
attributable to NGS emissions. The committee identified flaws in the models
used to estimate NGS's contribution, in the interpretation of those models, and
in the data base. The committee found that sources other than NGS appeared
to account for a significant fraction of haze observed in GCNP during the study
period. Thus, if NGS emissions were to be controlled, visibility impairment in
GCNP would be reduced but not eliminated.
THE COMMITTEE'S APPROACH TO ITS CHARGE
In this final report, the committee examines patterns of visibility degradation
and haze-forming pollutant concentrations in various parts of the United States
resulting from natural and anthropogenic sources of gases and particles. . . .
It considers the regulatory and institutional frameworks for efforts to improve
and protect visibility, including the Clean Air Act... . This report also reviews
the scientific understanding of haze formation and visibility impairment,
including the meteorological and chemical processes responsible for the
transport and transformation of gases and particles in the atmosphere, as well
as chemical and physical measurement techniques. . . . The approach of first
relating source emissions to aerosol composition, and then relating aerosol
composition to visibility, is fundamental to most of the committee's analyses.
In evaluating methods for source identification and apportionment, this
report considers the technical adequacy (including degree of uncertainty),
flexibility, and difficulty of implementation of the various approaches. ... In
discussing control techniques, the report describes the emissions reduction
potential of various control measures and illustrates the translation of control
measures into a rough prediction of effects on visibility. . . . The report also
considers policy implications of scientific knowledge about visibility and
recommends approaches to remedy scientific and technical gaps that limit
present understanding of source effects on visibility and the ability to evaluate
control measures. . . .
GENERAL CONCLUSIONS AND RECOMMENDATIONS
The complete design of a program for protecting and improving visibility in
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Class I areas must involve many policy issues outside the bounds of science
and the committee's expertise. However, present scientific knowledge about
visibility impairment in Class I areas has several important implications for policy
makers.
Progress toward the national goal of remedying and
preventing man-made visibility impairment in Class I areas
(Clean Air Act, Section 169A(a)) will require regional
programs that operate over large geographic areas and limit
emissions of pollutants that can cause regional haze.
Most visibility impairment in national parks and wilderness areas {Class I
areas) results from the transport by winds of emissions and secondary airborne
particles over great distances (typically hundreds of miles). Consequently,
visibility impairment is usually a regional problem, not a local one. Regional
haze is caused by the combined effects of emissions from many sources
distributed over a large area, rather than of individual plumes caused by a few
sources at specific sites. As a result, a strategy that relies only on influencing
the location of new sources, although perhaps useful in some situations, would
not be effective in general. And of course, such a strategy would not remedy
the visibility impairment caused by existing sources until those sources are
replaced.
A program that focuses solely on determining the contribution
or individual emission sources to visibility impairment is
doomed to failure. Instead, strategies should be adopted that
consider many sources simultaneously on a regional basis, al-
though assessment of the effect of individual sources will re-
main important in some situations.
Because haze is caused by the combined effects of the emissions of
many sources, it would be an extremely time-consuming and expensive
undertaking to try to determine, one source at a time, the percent contribution
of each source to haze. For instance, the efforts to trace the contribution of
the Navajo Generating Station to haze in the Grand Canyon National Park took
several years and cost millions of dollars without leading to quantitatively
definitive answers. Moreover, there are (and will probably continue to be)
considerable uncertainties in ascertaining a precise relationship between
individual sources and the spatial pattern of regional haze.
Assessment of the contribution of individual sources to haze will
remain useful in some situations. For instance, a regional emissions
management approach to haze could be combined with a strategy to assess
whether locating a new source at a particular location would have especially
deleterious effects on visibility. ... the committee has aet out working
principles for attributing visibility impairment to individual sources.
Visibility impairment can be attributed to emission sources on
a regional scale through the use of several kinds of models.
In general, the best spproech for evaluating emission sources
is a nested progression from simpler end more direct models
to more complex and detailed methods. The simpler models
are available today and could be used ss the basis for
designing regional visibility programs; the more complex
models could be used to refine those programs over time.
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After identifying which pollutants are impairing visibility for a given
region, it is useful to apportion visibility impairment among contributing sources
to the extent possible so that the relative effectiveness of alternative control
measures can be evaluated. Source apportionment models of varying degrees
of accuracy and complexity can be used to analyze regional haze problems,
although no single source-apportionment method is necessarily best for all
visibility problems. Simpler methods are most effective in the early stages of
source apportionment, with the more complex methods being applied, if
necessary, to resolve difficult technical issues.
For regional haze problems, the committee recommends use of the following
models, in order of increasing sophistication:
• Speciated rollback models. These are simple, spatially averaged
models that assume changes in pollutant concentrations to be directly
proportional to changes in regional emissions of these pollutants or
their precursors.
• Hybrid combinations of chemical mass balance receptor models a
different source-oriented secondary particulate mass formation model,
and used with empirical data for pollutant scattering and absorption
efficiencies. Receptor models are models that infer source contribu-
tions by characterizing atmospheric aerosol samples, often using
chemical elements or compounds in those samples to identify
emissions from particular source types. Hybrid models are formed by
combining two or more separate modeling techniques.
• Hybrid combinations of mechanistic models for transport and sec-
ondary particulate mass formation with measured particle size-distribu-
tion data to facilitate light scattering calculations. Mechanistic models
are 3-dimensional, computer-based models that simulate the
atmospheric transport, dispersion, chemical conversion, and deposition
of pollutants as faithfully as possible.
Speciated rollback models are available now; ... the committee uses such a
model to illustrate apportionment of regional haze. The recommended hybrid
combinations could be assembled from available components.
To assess the contribution of an existing single source to visibility
impairment, photographic and other source identification methods could be
used in simple cases. More complex situations require the use of hybrid
combinations of chemical mass-balance or tracer techniques with secondary
particle models that include explicit transport calculations and an adequate
treatment of background pollutants. For complex applications that require the
greatest sophistication, the most advanced reactive plume models available
should be used with measured data on particle properties in such plumes and
should be accompanied by an adequate treatment of background pollutants.
To assess new single sources, the most advanced reactive plume
models available should be used with measured data on particle properties in
the plumes of similar sources and accompanied by an adequate treatment of
background pollutants.
To analyze a single source at the regional scale, a description of the
source in question should be inserted into an appropriately chosen multiple-
source description of the regional haze problem.
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The next step in designing a visibility protection strategy is to deter-
mine whether methods for controlling visibility-impairing emissions exist or can
be developed and to assess the effects of alternative sets of controls. The
committee's analysis of one control scenario indicates that application of
commercially available emission controls would reduce but not eliminate
anthropogenic visibility impairment; the greatest improvement would be in the
East. (This analysis should not be construed as endorsing a technology-based
or any other specific control strategy.)
Visibility policy and control strategies might need to be different in the West
than in the East.
Haze in the East and in the West differ in important ways. Haze in the
East is six times more intense than in the West because of the much higher
levels of pollution in the East. Were all anthropogenic pollution to disappear,
visibility would still be greater (by about 50 percent) in the West. In relatively
clean areas, small increases in pollutant concentrations can markedly degrade
visibility; increases of the same magnitude are less noticeable in more polluted
areas. Hence, visibility in Class I areas in the West is particularly vulnerable to
increased levels of pollution. Moreover, the West contains most of the nation's
large national parks and wilderness areas, which can be fully appreciated only
when visibility is excellent. The East, however, contains a large population to
enjoy the benefits of any improvement in visibility in that region.
In the East, sulfates derived from S02, emissions from coal-fired power
plants account for about one-half of all anthropogenic light extinction.
Reductions in these emissions are expected to occur in the next two decades
as a result of the 1990 Clean Air Act Amendments' acid rain control program.
In the West, no single source category dominates; therefore, an effective
control strategy would have to cover many source types, such as electric
utilities, gasoline- and diesel-fueled vehicles, petroleum and chemical industrial
sources, forest-management burning, and fugitive dust.
Efforts to improve visibBity in Class I areas also would benefit visibility outside
these areas.
Because most visibility impairment is regional in scale, the same haze
that degrades visibility within or looking out from a national park also degrades
visibility outside it. Class I areas cannot be regarded as potential islands of
clean air in a polluted Bea.
Reducing emissions for visibility improvement could help alleviate other air-
quality problems, just as other types or air-quality improvements could help
visibility.
The substances that contribute to regional haze also contribute to a
variety of other undesirable effects on human health and the environment. For
example, S02 is a precursor of sulfuric acid in acid rain, oxides of nitrogen (NO,)
and volatile organic compounds (VOCs) are precursors of lower-atmosphere
ozone, and fine atmospheric particles are a respiratory hazard. Such particles
can influence climate by interacting with incoming solar radiation and by
modifying cloud formation. Policy makers should consider the linkages between
visibility and other air-quality problems when designing and assessing control
strategies.
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Achieving the national visibility goal will require a substantial, long-term
program.
The national visibility goal is unlikely to be achieved in a short time.
Policy makers might develop a comprehensive national visibility improvement
strategy as the basis for further regulatory action, and establish milestones
against which progress toward the visibility goal could be measured.
Current scientific knowledge is adequate and control
technologies are available for taking regulatory action to
improve and protect visibility. Howevei, continued national
progress toward this goal will require a greater commitment
toward atmospheric research, monitoring, and emissions
control research and development.
The slowness of progress to date is due largely to a lack of commit-
ment to an adequate government effort to protect and improve visibility and to
sponsor the research and monitoring needed to better characterize the nature
and origin of haze in various areas. The federal government has accorded the
national visibility goal less priority than other clean-air objectives. Even to the
extent that Congress has acted, EPA, the Deportment of Interior, and the
Department of Agriculture have been slow to carry out their regulatory
responsibilities or to seek resources for research.
RECOMMENDED RESEARCH
The committee addressed the need to alleviate scientific and technical
gaps in the areas of visibility and aerosol monitoring and measurement, source
apportionment, and emissions control technology. The committee considered
wlurt measures might be taken to understand better the sources of hue,
possible means of reducing emissions from those sources, and alternative ways
of preventing future visibility impairment In Class I areas.
The committee emphasizes that the need for additional research does
not imply that further regulatory action, if otherwise warranted, to improve
visibility in Cless I areas would be premature. The authority of regulatory
agencies to act without complete scientific knowledge is claarly Implied in the
Clean Air Act. Moreover, visibility impairment Is probably better understood
end more easily measured than any other air-pollution effect. The remaining
gaps in knowledge of visibility are primarily a symptom of the lack of a strong
national commitment to enforcing the visibility protection provisions of the
Clean Air Act.
Resources for research are limited; therefore, precautions should be
taken to ensure that the visibility protection activities of the federal land
management agencies, EPA, the Department of Energy, and state and local air
agencies are of the highest possible quality. In addition, a greater effort is
needed for formal publication of scientific work in independent, peer-reviewed
literature.
The committee recommends establishing an independent science advi-
sory panel with EPA sponsorship to help guide the research elements of the
national visibility program. This panel could address the need foe wider
participation by the scientific community in addressing visibility problems.
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EPA should build upon and expand its efforts to track the success of
the PSD program. In particular, information is needed about the potential of
new sources to reduce visibility in Class I areas and about the effects on such
areas of the new emissions trading programs of the 1990 Clean Air Act
Amendments. EPA's current visibility-screening model needs to be revised to
consider the contribution of an individual source to regional haze.
Research on relating human judgments of visibility to objective mea-
sures, such as light extinction, should continue. The results should be used to
inform decision makers and the public about the perceptibility of predicted
visibility changes.
Areas in which research is needed include atmospheric transport and
transformations of visibility-impairing pollutants, the development of models
that can better apportion haze among sources, and improved instrumentation
for routine monitoring and for obtaining data that can be used to evaluate
models. Monitoring and research must be closely coordinated. Better models,
however, are not enough. Any model, even the simplest or most refined,
depends on good empirical data on the airborne particles that cause haze and
on their sources. Greater resources are needed to develop these data.
Monitoring Strategies
If national visibility monitoring networks are to achieve their goals, a
long-term commitment to establishing and financially supporting these networks
is essential. Monitoring programs should be able to relate visibility impairment
to its sources on a scale commensurate with regional haze events and the
distribution of major emissions sources. Monitoring networks in the East need
to be expanded to track visibility improvements associated with reductions in
S02 emissions. Wind observations should be evaluated to ensure that
atmospheric transport is represented accurately.
A consensus should be developed on the specific instrumentation to
be used for monitoring light extinction. Standards should be established for the
performance characteristics of the instrumentation. Future measurement
programs should devote increased attention to quality assurance and control.
Strengthening the quality assurance and control program of the Interagency
Monitoring of Protected Visual Environments (IMPROVE) network should be a
high priority.
Greater attention should be given to the implications that planned changes
in airport visibility monitoring hold for research on visibility impairment. Airports
should be equipped with integrating nephelometers sensitive enough to measure
the range of haze levels encountered in the atmosphere.
Measurement Methods
Current measurement methods permit reasonable estimates of the
average contributions of major aerosol constituents to atmospheric visibility
impairment. However, several aerosol measurement methods need to be
developed or improved for the following:
e Accurate measurement of orgsnic and elemental carbon particles,
especially at (ow concentrations
e Routine measurement of the water content of airborne particles
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• Measuring particle size distributions
• Continuous measurement of sulfates, organics, elemental carbon,
nitrates, and elemental composition
• Solar- and battery-powered measurement for use in remote areas.
The committee recommends using high-sensitivity integrating
nephelometry for routine visibility monitoring. This technique, which measures
the scattering of light from an air sample drawn through an enclosed cell, can
provide accurate data at reasonable cost. Nephelometer data can be compared
with measured particle concentrations at the same point to determine the
contributions of different pollutants to visibility impairment. A readily available,
easily serviced, and electronically up-to-date instrument with adequate
sensitivity for good and poor visibility is needed. Nephelometer measurements
of light scattering should be supplemented with independent measurements of
light absorption. Instrumentation for continuous measurements of particle
absorption coefficients should be developed.
Source-Apportionment Modeling
Source-apportionment models require better input data on source emissions,
along with unified procedures for testing individual sources. Emissions data
need to be integrated more accurately into overall emissions inventories. The
inventories requiring the most improvement are those for primary organic and
elemental carbon particles and gaseous VOCs.
Models should be validated using existing data sets from comprehen-
sive field studies. Mechanistic models and hybrid receptor models should be
included in validation studies.
Receptor models require substantial source testing and ambient emis-
sions measurements to improve emissions profiles for sources of haze.
Standard protocols for the release and sampling of tracers should be developed,
along with field studies to verify these protocols. Inexpensive and relatively
short-lived tracers are needed to distinguish the emissions of similar sources.
Research should also continue toward the development of advanced
mechanistic models. Two kinds of mechanistic models are especially needed:
(1) an advanced reactive model for analysis of visibility-impairing plumes from
single sources; and (2) a grid-based, multiple-source regional model for analysis
of regional haze problems. The development of such models will require
significant refinement in the understanding of processes that affect particle size
distributions. Critical processes include atmospheric emissions of particles and
gases that play a role in the production of secondary particles, and gas-to-
particle conversion. Measurement programs that are intended to acquire such
information should be designed in collaboration with modelers to ensure that
the results are suitable for model development and validation. . . .
FUTURE DIRECTIONS FOR PROTECTING AND IMPROVING VISIBILITY
Present scientific knowledge has important implications for the design of
programs to protect and improve visibility. What is needed, overall, is the
recognition that any effective visibility protection program must be aimed at
preventing and reducing regional haze. An effective program must, therefore.
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control a broad array of sources over a large geographic area. Such a program
would mark a considerable break from the present approach of focusing on
visible plumes from nearby sources and of attempting to determine the effects
of individual sources on visibility impairment.
While visibility impairment is as well understood as any other air
pollution effect, gaps in knowledge remain. Filling these gaps will require an
increased national commitment to visibility protection research. We believe that
the time has come for Congress, EPA, and the states to decide whether to
make that commitment.
NOTE: Reprinted with permission of the National Academy of Sciences, from Protecting Visibility
in National Parks and Wilderness by National Academy Press. Copyright 1993.
II.B. IMPROVE 1993 REPORT
The Interagency Monitoring of Protected Visual Environments
(IMPROVE) network provides an important link in the understanding
of visibility. This network, mentioned in the monitoring
discussion in the NRC Executive Summary, is intended to provide a
basis for establishing a consensus on monitoring light extinction
and standards for the quality of the resulting data.
The specific objectives of the IMPROVE program are:
(1) Establish current background visibility in Class I
areas;
(2) Identify chemical species and emission sources
responsible for existing man-made visibility
impairment; and
(3) Document long-term trends.
IMPROVE Network
The IMPROVE network incorporates quality assurance and self-
consistency between measurements. The NRC Committee recommended
increased attention to quality assurance and control and that a
consensus be developed regarding instrumentation to be used for
monitoring light extinction. By measuring visibility routinely
at the IMPROVE network over a period of time, the trends can be
assessed. The following paragraphs present a brief description
of the network and the findings in the IMPROVE 1993 Report which
was issued February 1993 (Sisler, Huffman and Latimer, 1993).
Understanding light extinction is important in understanding
variations in visibility. The light extinction coefficient is
calculated from the measured aerosol species concentrations by
multiplying the concentration of a given species by its light
extinction efficiency, and summing over all species. The light
extinction efficiency for sulfates and nitrates, as well as for
some organics which are hygroscopic, increases with increasing
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relative humidity. The extinction efficiencies used to estimate
the light extinction for soluble species are adjusted for the
seasonal and annual average relative humidity at each site.
The IMPROVE network consists of thirty-six protocol sites,
established to characterize the distribution of visibility and
aerosol concentrations over the United States. Each site has
aerosol monitoring and scene monitoring (automated cameras)
equipment. However, only 20 sites have optical monitoring
equipment (transmissometers) to measure light extinction.
Transmissometers measure the light transmitted from an artificial
source through the atmosphere over a distance (one to fifteen
kilometers) to a detector.
Relative humidity is measured continuously at the
transmissometer sites. Simultaneous measurements are taken of
elemental sulfur and sulfate ions concentrations. The aerosol
monitoring includes a PM-10 sample and three PM-2.5 samples on
Teflon, nylon, and quartz filters. The IMPROVE sampler is
programmed to collect two 24-hour duration samples per week. The
network provides for the estimation of organic mass in two
different ways: hydrogen mass measured on the Teflon filter; and
organic carbon mass measured on the quartz filter.
IMPROVE Findings
The physical and chemical properties of particles and
aerosols in the atmosphere affect the light absorption and light
scattering efficiency of the atmosphere. Among the particle and
aerosol properties that are important are: size; acidity; and
chemical composition.
Elemental carbon is one of the materials that absorbs light.
However, an analysis of the network data suggests a ratio of
light absorption coefficient to elemental carbon mass twice as
large as expected. The difference between these observations and
traditionally accepted values may be explained by two factors:
(a) light absorption is impacted by substances other than
elemental carbon; and/or (b) uncertainties in the absorption and
carbon measurement methods.
Fine aerosols, (diameters less than 2.5 /xm) , are more
efficient at light scattering than coarse aerosols. The network
data in Appendix B indicate that fine aerosol concentrations are
highest in Washington, D.C., in the Appalachian Mountains, and
southern California. The lowest concentrations occur in the
Great Basin in Nevada, the Colorado Plateau in the Four Corners
states, and in Alaska.
The composition of the fine aerosol varies with the
geographic location and season. Of the 19 regions in the IMPROVE
network, organic carbon is the largest single component in nine
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regions. Sulfate is the largest single component of fine aerosol
in six regions, primarily in the East. The contributions of
organic carbon and sulfate are approximately equal in three
regions (Boundary Waters, Sonoran Desert, and West Texas).
Nitrate is the largest component of fine aerosol in Southern
California only. In general, average fine mass aerosols
concentrations are highest in summer, soil concentrations are
highest in spring or summer, and nitrate concentrations are
generally highest in winter or spring.
The acidity of sulfate aerosol measured by the network was
estimated statistically from concentrations of hydrogen, sulfate
and organic carbon. Sites identified as acidic by this procedure
were located in the following national parks: Volcanoes in
Hawaii; Mount Rainier in the Pacific Northwest; Point Reyes,
Redwoods, and Pinnacles in Northern California; Shenandoah in the
East; and Tonto in southeastern Arizona.
A spatial examination of reconstructed light extinction
(Appendix C) demonstrates a variability much like the spatial
variability of fine aerosol concentrations (Appendix B).
Contributions to the light extinction from coarse particles and
fine soil, sulfate, organics, nitrate, and light absorbing carbon
also are summarized in Appendix C. Since relative humidity (and
hence the light scattering efficiency of sulfate, nitrate, and
some organics) is higher in the East than in the West, the
difference between eastern and western light extinction is even
more pronounced than accounted for by the difference in aerosol
concentrations. In the Appalachian Mountains, sulfate accounts
for 2/3 of the total aerosol light extinction on an annual basis,
and 3/4 of the total in summer. Organic carbon is the largest
single contributor to light extinction in four of the 19 regions
(Great Basin, Northern Rockies, Sierra Nevada, and Sierra-
Humboldt) . Higher extinction occurs in summer because of
elevated sulfate and carbonaceous aerosol concentrations.
Direct measurements of light extinction for all periods,
excluding conditions of fog, precipitation, and low clouds, show
the same pattern of high light extinction (low visibility) in the
Eastern U.S. and, to lesser extent, in Southern California. This
agreed with the reconstructed light extinction. Measured light
extinction is generally within 10% of the reconstructed values
calculated from the measured concentrations of the major aerosol
species. However, reconstructed extinction is about 80% of
measured light extinction in the Appalachian Mountains during
summer and in the Pacific Coast, Southern California, Sonoran
Desert, and West Texas regions and 50% of measurement extinction
at Yosemite in Sierra Nevada.
The human eye and mind are not able to detect small changes
in light extinction. However, a measure of human perception, the
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deciview scale5, indicates the effect of aerosol on human
visibility. Based on this index, the best visibility was
indicated to be at Bridger Wilderness, but good visibility was
also indicated at the Great Basin, most of the Colorado Plateau
and parts of the Central Rockies. Other areas were indicated to
have noticeably worse visibility, e.g. the Appalachian region.
The general spatial distribution noted above for the annual
average visibility, as indicated by deciview, generally holds
true for each season as well. The least impairment occurs in
winter, peak impairment occurs in summer.
IMPROVE Recommended Research
Recommendations for future research in the IMPROVE report
include research to better understand organic carbon light
absorption and to develop an adjustment to the current light
absorption calculation. The assumption regarding the type of
hydrocarbon used to calculate the mass fraction of hydrogen and
carbon in organics needs to be studied. Light absorption
calculations are based upon the light absorbing capability of
elemental carbon. Studies should be performed to determine
whether all light-absorbing carbon is elemental, and whether the
pyrolyzed carbon on the quartz filter may absorb light when in
the air. The IMPROVE report suggests that light absorption
correlates equally well with organic carbon and elemental carbon,
and recommends that this be studied.
The IMPROVE report recommends that the relative humidity
correction applied to the sulfate, nitrate, and organic aerosols
be re-evaluated. The studies of sulfate and nitrate relative
humidity factors are based on ammonium sulfate. Ammonium nitrate
has a different deliquescence point than ammonium sulfate and a
specific correction for relative humidity effects on ammonium
nitrate needs to be established. Acidic sulfates, e.g., sulfuric
acid and ammonium bisulfite, also need specific corrections,
because they have higher water contents and higher light
scattering efficiencies than ammonium sulfate. The
hygroscopicity of organics is not well understood and needs to be
investigated.
At many of the IMPROVE sites the calculated light
extinction, as estimated from concentrations of the major aerosol
species, underestimated the measured light extinction. The
general and specific causes need to be determined.
5Deciview scale (Pitchford and Malm, 1994) is such that one
unit change corresponds to about 10% change in extinction
coefficient. This small, but perceptible, change in the index is
zero for pristine atmosphere and increases as visibility is
degraded.
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II.C. National Acid Precipitation Assessment Program (NAPAP)
The "NAPAP 1992 Report to Congress" (Chapter 4, "Ambient Air
Concentrations, Deposition, and Visibility" issued June 1993)
includes key findings related to visibility research. These
findings address only one aspect of the research being summarized
here. The research addressed by the NAPAP Report is applicable
to the assessment of current sources of visibility impairing
pollution and clean air corridors. The key findings are:
• Concentrations and deposition of pollutants are
generally higher in the vicinity of major point sources
and in urban areas than at regionally representative
sites.
• The area of influence of specific sources or particular
source regions on specific receptors varies with the
season and with the prevailing meteorological
conditions.
• Except for areas impacted by local sources, air
concentrations and deposition of pollutants are highest
at high elevation (locations above 1,4 00m) in the
Appalachians.
The fate and transport of acidic-related pollutants in the
atmosphere is important in the understanding of visibility
because these pollutants interfere with the transmission of
light. The acidic-related pollutants form particles which absorb
water vapor during long transport periods, scatter light, and
contribute to regional haze. The main link between acidic
deposition and visibility degradation is through sulfur dioxide
emissions and the production of sulfate aerosols in the
atmosphere. Sulfate aerosol is an important contributor to
visibility reduction.
The information presented in Chapter 4 of the "NAPAP 1992
Report to Congress" is from the IMPROVE network, which, as
described in the previous section, provides basic aerosol
measurements at numerous rural locations, including selected
national parks and wilderness areas in the continental United
States. While the IMPROVE network provides the broadest spatial
resolution of any available data, IMPROVE does not include
measurements in populated areas. These data from IMPROVE are
evaluated for consistency with earlier data, by comparing fine
mass fractions for the important aerosol species. The average
percent sulfate in fine particle mass at IMPROVE rural sites (6
in East, 27 in West) appears to be quite consistent with similar
sulfate data presented in an earlier NAPAP Visibility Report for
rural sites (15 in East, 20 in West). The estimated 50 percent
sulfate contribution to non-Rayleigh extinction in the rural East
also compares well with the earlier NAPAP Integrated Assessment
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Report, which considered only sulfate effects on extinction at
rural sites in the East.
It is difficult to make a thorough comparison with the
IMPROVE nitrate data, as nitrate data in earlier reports were
sparse and some measurements were biased. However, the "NAPAP
1992 Report to Congress" suggests that the IMPROVE nitrate
concentrations were somewhat greater in magnitude than those
presented in the earlier NAPAP Visibility Report. "NAPAP 1992
Report to Congress" states that the "discrepancy is not critical,
however, because the nitrate concentrations are still small
compared with sulfate concentrations and because the air quality
changes from acidic deposition controls are primarily for
sulfates, and not nitrates."
The report evaluates trends based upon emission reductions
achieved in the past and projected for the future. The 10-
percent reduction in emissions of sulfur dioxide that occurred
from 1980 to 1990 should have produced about a 6% improvement in
visibility. The analysis of visibility trends for 1980 to 1990
is likely to be ambiguous for this level of change. The trend
should be much more obvious when the CAA acid deposition control
programs have been fully implemented. The NAPAP report estimated
that those programs should result in a 4 0% emissions decrease and
there should be an associated increase of 30% in visual range
between 1980 and 2010 in the rural East. The report suggests
that trends analysis might be enhanced by considering trends in
the concentration of sulfates in the atmosphere, because sulfate
should exhibit nearly twice as much relative change as
visibility.
The main thrusts of the "NAPAP 1992 Report to Congress" are
in the areas of visibility source apportionment, emission control
technology, and fundamental scientific issues of regional haze
management. Important conclusions of this report are: adequate
information exists to justify new visibility protection rules;
and targeting single-point sources may not be the best approach,
since a wide variety of sources can contribute to the regional
haze that distorts visibility.
II.D. Effects of 1990 CAAA on Visibility in Class I Areas
An EPA Report to Congress (EPA, 1993) addressed the changes
that are expected in visibility conditions in Class I areas as a
result of the implementation of the provisions of the 1990 CAAA
(other than the added section 169B visibility protection
provisions). The report-in-hand is closely linked to the 1993
EPA Report to Congress. In the 1993 EPA report, an assessment
was made using key locations and a simple emissions-driven air
quality analysis to ascertain areas likely to see changes in the
distribution of man-made visibility-impairment related
pollutants. The report used two levels of analysis. The first
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or preliminary analysis used a very simple model to determine
geographic regions where visibility improvement might be
possible. The preliminary analysis for the Eastern U.S.
concentrated mostly on changes in sulfur dioxide emissions. For
the Southwestern U.S., the mixture of pollutants is more varied
and less dominated by sulfur particles. Thus the analysis
incorporated changes in emissions of sulfur, nitrogen, organic
and primary particulate matter. The 1993 EPA report indicated
that, based on the preliminary analysis, the following regions
are expected to experience perceptible changes in visibility
conditions: most areas east of the Mississippi River; areas west
of the Mississippi River and south of Minnesota and Wisconsin;
southern California; and the central California coast.
The second level of analysis employed more advanced air
quality models to analyze these two regions. Using these models,
the current annual average visibility conditions, expressed in
standard visual range (kilometers), were estimated along with
seasonal changes. Atmospheric processes which form visibility
impairing particles and the resulting levels of extinction of
those particles (due to humidity) vary by season.
Using the advanced air quality models, Class I areas from
Maine to Georgia are estimated to experience future improvements
in regional visibility conditions. The major improvements
expected for Class I areas are for those areas along the central
and southern portions of the Appalachian Mountains. No areas are
expected to have perceptible decreases in regional visibility.
There is considerable uncertainty in the analysis and single day
changes can not be estimated.
Predictions of visual air quality are most uncertain for
sensitive western areas due to emissions from Mexico.
Uncertainty varies by pollutant type with estimates of changes in
organic particle concentrations being quite uncertain, especially
those formed secondarily in the atmosphere from emissions
(natural and man-made) of gaseous volatile organic compounds.
"Although visibility will improve in many eastern Class I areas,
based on estimates of the natural annual average visibility . . .
there will still be perceptible man-made regional visibility
impairment in all Class I areas nationwide," i.e. the situation
is improving, but there is still a problem.
III. Published Results
The published results in this section are pertinent to the
"assessment and evaluation that identifies, to the extent
possible, sources and source regions of visibility impairment ...
as well as source regions of clear air for class I areas" (CAA
§169B(a)(2)) . Visibility research is a continuing activity. The
central frame of reference for the research results included in
this report is that they were published after the passage of the
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CAAA in November 1990. Most references included in Section III
of the report are to refereed journal articles, or to conference
papers with published preprints or proceedings. Brief summaries
of selected documents have been grouped according to the
following four research areas identified in §169B(a)(1) of the
CAA: monitoring; sources of visibility impairing pollution;
regional air quality models; and atmospheric chemistry/visibility
physics. It is important to recognize that new science published
in the refereed literature is subject to scientific review and
examination. The results and conclusions are sometimes
controversial. The fact that research is published does not
imply that the results are accepted by the entire scientific
community. Rather, the publication makes it possible for others
to question and challenge results. Section III merely reports
the findings claimed in the publications and does not judge the
credibility of the research.
III. A. Monitoring
The topic of monitoring in this report includes
instrumentation, methods, networks, and identification of
visibility trends, changes and patterns. Results from national
and international research are reported.
Monitoring (see Appendix C of NRC, 1993, for complete
background discussion) includes both direct measures of the
visible light transmitted in the atmosphere as well as monitoring
the chemical composition of the atmosphere. If the chemical
composition of the atmosphere is known, models can be used to
estimate visibility. Atmospheric gases that should be monitored
because they are important in light scattering, light absorption
or in particle formation include: sulfur dioxide (S02) , nitric
oxide (NO) , nitrogen dioxide (N02) , ozone (03) , ammonia (NH3) ,
hydrogen peroxide (H202) and non-methane hydrocarbons.
Monitoring of fine atmospheric particles (smaller than 2.5
/zm) is important as they account for much of the reduction in
visibility. Analyses of fine particles can be used to infer
origin as well as quantify chemical composition. Properties of
particles which are important to visibility research include:
size distribution; mass and chemical size distribution; carbon
(organic versus elemental); and water content.
The direct measurement of visible light transmitted through
the atmosphere is an important part of optical monitoring. Other
factors are important to visibility as well, including the amount
and color of light emitted by the viewed object and the
scattering of ambient light. The average extinction coefficient
over a path for which transmittance can be calculated as the sum
of scattering and absorption coefficients for gases and particles
along the path. In some methods, point measurements must be
extrapolated to the sight path. Sight path methods include
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measurements of transmittance over a long path, radiance along a
sight path, and photography. Considerable scientific research
related to monitoring methods was published prior to the passage
of the CAAA of 1990 and is not included here. Many of the
science issues related to monitoring were identified by NRC
(1993) and were covered in the executive summary included in
Section II.A.
Measurement methods for aerosols are varied and the various
instruments have differing capabilities and limitations.
Research has indicated potential problems with some methods for
measuring aerosols and John (1993) addressed instrument
performance and standards for sampling of aerosols.
Sampling Problems Related to Gas/Particle Phase
Sampling and characterization of organic carbon is plagued
with problems including evaporative losses and gas adsorption on
substrate. This is an area of active research, including the
methods for measuring organic carbon and the size of the loss.
The following are research articles that addressed this
problematic area. Some research indicates that particulate
organic compounds are underestimated from filters because semi-
volatiles are lost. The claim is that this effect is much larger
than the overestimate resulting from absorption of organic vapors
on a quartz filter (Eatough et al. 1993, Lewis et al. 1991). A
multi-system, multi-channel, high-volume diffusion denuder
sampler, which measures these as well, was used in Provo, UT and
Los Angeles, CA. Results indicate that lost particles were 0.4-
0.8 /xm in size and included paraffinic and aromatic compounds,
organic acids and esters (Tang et al. 1994).
Sampling losses are not limited to organic compounds. Zhang
and McMurry examined evaporative losses during atmospheric
aerosol sampling of adsorbed or absorbed species (1991) and of
fine particulate nitrates (1992), using the annular denuder and
the cascade impactor.
The transition between gas and particle phase was addressed
by Turpin et al. (1993) who developed a new gas/particle
diffusion separation sampler to provide quality measurements of
semi-volatile organic compounds. Separation is possible because
gases diffuse much faster than particles. The contribution from
the particulate phase is the difference between the total
concentration and the gas-phase concentration. Results from this
sampler agreed well with theory.
An annular diffusion denuder and filter pack system sampled
gaseous HN03, HN02, and S02 and also nitrate, nitrite and sulfate
particles for 12-hour periods in Page, AZ, January 15 - February
4, 1986. Results showed 88% of total nitrate was gaseous HN03,
97% of total nitrite was the gaseous HN02 and 91% of total
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sulfate was the gaseous S02. There was good agreement between
the denuder and filter pack concentrations of the HN03 gas,
better than the agreement for particulate nitrate (Benner et al.
1991) .
Improvements have been made that relate to monitoring
capability. Koutrakis et al. (1993) developed a new system for
atmospheric monitoring that combines a honeycomb denuder with the
filter pack. The system is less costly and allows more denuders
for simultaneous collection of a greater variety of gases. The
system is also more convenient for large-scale monitoring
studies.
Relative Humidity Effects
The NRC (1993) also recognized the need for methods related
to the routine measurement of the water contents of airborne
particles. Thomas and Gebhart (1994) found that measurements and
theory indicate that if the aerosols are mainly in the
accumulation mode, approximately 0.1 to 1.0 /im in size, there is
a fairly linear relationship between photometer response and mass
concentration. However, the relationship did not hold for a case
with high relative humidity.
Relative humidity effects, induced by flow in the
microorifice uniform deposit impactor, on sulfuric acid drop size
were studied by Fang et al. (1991). Stein et al. (1994) used the
DMA-impactor technique to study the relative humidity dependent
bounce and density of atmospheric particles.
Particle Characterization
Other particle characteristics are important to estimating
the visibility: density, vapor pressure and number of
condensation nuclei. Kelly and McMurry (1992) measured the
aerosol particle density by inertial classification with a
differential mobility analyzer. Stolenburg and McMurry (1991)
presented an ultrafine aerosol condensation counter. Zhang et
al. (1993) determined particle vapor pressures using the tandem
differential mobility analyzer.
The size distribution of particles is important to
visibility. For elemental carbon and polycyclic aeromatic
hydrocarbons, Venkataraman et al. (1994) examined size
distributions of vehicular air pollution. Sloane et al. (1991)
found improved precision measuring aerosol particle size by using
an optical counter and a nephelometer.
Direct Measurement of Optical Properties
The NRC (1993) recommended use of high sensitivity
integrating nephelometry which measures light scattering of an
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enclosed air sample. An integrating nephelometer was modified to
sample on an alternating basis, first from fine particles (<2.5
/im) , then from all particles. This nephelometer was collocated
with a transmissometer which monitored total extinction. Results
indicated that the nephelometer underestimated the scattering by
coarse particles by nearly half. When this bias was eliminated,
the results indicated that coarse particles were responsible for
one-fourth to one-third of total particle light scattering (White
et al. 1994). Results apply to spring and summer in the
southwest U.S.
Liquid water in aerosol particles influences light
scattering. Rogers et al. (1991) looked at the estimation of
liquid water using a nephelometer. Optical detectors were
examined by Dick et al. (1994) for size - dependent counting
efficiencies and angular scattering patterns for spherical
particles of known size and composition. Hering and McMurry
(1991) looked at the calibration of optical counters.
Another direct measurement of optical properties is
visibility monitoring with human observations. Airports have
been the source of these observations which were taken to support
aviation. The NRC (1993) recognized that the human observations
of visual range were being automated by the National Weather
Service (NWS). The representativeness and responsiveness of the
visibility sensor, Belfort Model 6220, was addressed by Bradley
and Lewis (1993). The Belfort 6220 is a blend of an electro-
optical sensor that measures point visibility and an algorithm to
convert a time series of measurements into an estimate of surface
horizontal visibility. The estimate, representative of 2-3 miles
around the sensor, is adjusted for the weather for each minute
before calculating a 10-minute harmonic mean. The NRC (1993)
recommended that airports be equipped with integrating
nephelometers. Section IV.F of this report will address some of
the ongoing research on the Belfort Model 6220.
Human perception of visibility, although a direct
measurement, is subjective. Human perception is being addressed
with instrumentation. Henry, Shibata and Chitwood (1994)
describe a new visual colorimeter for quantifying perception of
color and brightness of natural objects seen through the
atmosphere. Laboratory precision of color matches is 4% and
accuracy is about 5%.
Coarse particles do not remain in the atmosphere as long as
fine particles and do not affect visibility as much. However,
coarse particles up to 10 /xm are still important. John et al.
(1991) found particle bombardment can cause reentrainment,
oversampling due to deagglomeration of soil; and John and Wang
(1991) found the sampling effectiveness of oiled collection
surface decreases with loading.
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Surface properties of aerosol properties are also important.
Pandis et al. (1991) presented an epiphaniometer, a new
instrument for continuous monitoring of the Fuchs surface of
aerosol particles. It attaches neutral radioactive lead atoms to
the surface and is used to determine rapid surface changes in the
aerosol.
Trends
Monitoring research includes analyses related to one of the
main purposes in monitoring, identifying trends or spatial
patterns. There have been several papers published which
examined the trend of visibility or of aerosols which are the
major factor influencing visibility.
The trend in visibility in the U.S. has been examined based
on human observations on visible range at airports. In the East,
there were areas of improved visibility in the summer from the
1978 - 1982 to the 1988 - 1992 period, based on the 75th
percentile (Figure 1, from Husar, Elkins and Wilson, 1994). In
the northeast U.S., extinction coefficients calculated from the
visible range show a direct relationship with the sulfur
emissions in both January and July based on data from 1948 to
1983 (Husar and Wilson, 1993).
Visibility concerns extend beyond political borders. In
Canada, airport visibility had not previously been analyzed
because the limits placed on visibility observations were below
the range needed to determine visual impairment from
anthropogenic pollution. Stuart and Hoff (1994) reported a
technique to remove or mitigate the bias. They claimed that this
method may be used to adjust for the limits and to examine
trends.
A decreasing trend (1.6-1.8% per year) of optically active
tropospheric aerosol was observed by Hofmann (1993) using balloon
measurements (1971-1990) over Laramie, WY. Hofmann suggests this
may be related to S02 emission reductions in U.S. during this
time. Pennick et al. (1993) using sites in New Mexico also found
that aerosol loadings decreased slightly from mid 1970's to 1990
although elemental black carbon concentrations were unchanged.
Organic aerosols are very important. Using gas chromatography,
Hildemann et al. (1994) examined seasonal trends in ambient
organic aerosol in Los Angeles and found strong peaks in the fall
and winter.
Temporal patterns are also identified with monitoring data.
Husar and Poirot (1992) found that particles less than 10 jzm had
different weekly patterns in different parts of the U.S. For
example, El Paso, Texas had lower weekend concentrations. In
California, Mondays are highest in San Bemadino while in
Yosemite National Park and Oceanside, south of Los Angeles,
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Figure 1. 75th Percentile of July-September Daily Extinction
Coefficient. (Five-year average is centered on year
shown). Source: Husar, Elkins and Wilson, 1994.
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highest concentrations are on Sunday. These coarse particles do
not affect visibility as much as fine particles so similar
patterns may or may not be observed in visibility.
Eldred and Cahill (1994) examined data from 12 monitoring
sites in remote Class I visibility areas from June 1982 through
August 1992. The sulfate concentration in the West decreased or
showed no change during that period except for concentrations in
winter which increased. The sulfate in the East generally
increased except for winter where it decreased. Summer increases
in sulfate in the Shenandoah Naticjnal Park were more dramatic.
Zinc in the atmosphere, primarily associated with industrial
activities, was high at the eastern sites, but showed no trend.
Remote Sensing and Global Change
The NRC (1993) noted that "satellites can be useful for
characterizing the large scale distribution of haze events," but
use of satellite monitoring for regional haze in Class I areas is
not possible at this time. The quantification of aerosols
contributing to haze over terrestrial areas is difficult.
Aerosol parameters may be measured more accurately over an ocean
surface with no sun glint (Durkee, et al. 1991).
Visibility research and monitoring is related to research in
other programs, such as global change. In research to assess
direct forcing and climate impact of aerosols, Kaufman et al.
(1993) took ground-based measurements of solar transmission and
sky radiance for several aerosol types (smoke, sulfate, dust,
maritime aerosol) around the world. This research is of limited
value, the path radiance of concern for satellite observations,
and for the radiative forcing in global climate, is for the total
column rather than a horizontal path.
Trends and changes in visibility related measurements have
been found beyond North America. Measured global and diffuse
radiation measurements, used to calculate solar radiation
attenuation by aerosols, showed slight increases from 1971 to
1987 in the Slovak Republic (Luk&c, 1994). However, monitoring
results in Chile for a 15-year record of visual range at an
airport had no trend (Frier and Firinguetti, 1994). An
interference light filter spectrophotometer at the center of the
European-Asian continent found small increases from 1985 - 1990,
followed by a sharp decrease attributed to the Pinatubo eruption
(Aref'ev and Semenov, 1994).
Common problems in interpreting monitoring data exist in the
U.S. and abroad. For example, the extinction coefficient varies
with the composition of the air mass, with influence from
humidity, and also from changes in the stability of the boundary
layer. For example, the aerosol loading in Leipzig, Germany,
changes depending on the use of fuel for heating. The humidity
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adjusted extinction coefficient has a strong relationship with
the origin of the air mass (Uhlig, Stettler and von Hoyningen-
Huene, 1994). The classification of air masses is a useful tool
in interpretation of monitoring data and meteorological
parameters explain aerosol extinction to some extent. Nilsson
(1994) modeled the relation between aerosol extinction and
meteorological parameters.
Atmospheric transport can transport aerosols formed over the
oceans to the atmosphere over terrestrial areas. Much of the
research on these ocean aerosols is supported by the global
change program, e.g. over the Pacific Basin where research
aircraft (at altitudes up to 39,000 feet) sampled aerosols.
Results using particle number, size, shape, and assuming the
refractive index of (HN4)2 S04 yielded estimates of extinction
coefficients 2.03 ± 1.20 x 10"4 km"1 for visible wave lengths
(Pueschel et al. 1994). Although this research is not directly
applicable to Class I areas in the U.S., it is relevant to long-
range transport into those areas.
Global change research addresses the radiative properties of
the troposphere. Changes in these properties also affect
visibility. Research performed by the Atmospheric Radiation
Measurement (ARM) program (of the U.S. Department of Energy)
contributed to the understanding of atmospheric visibility. The
ARM Science Team addressed the need for more than ground based
measurements. Clouds, in particular created large errors in
estimating atmospheric radiative properties (Kinne, Bergstrom and
Ackerman, 1994). Clouds are also of key interest in the
formation and characteristics of particles in the atmosphere.
III.B. Current Sources of Visibility Impairing Pollution and
Clean Air Corridors
The NRC (1993) states that visibility in parks and
wilderness areas is largely impaired by haze originating from
many different sources in a region. The degree of haze varies as
do the sources of pollution contributing to that haze. The
conditions under which visibility is "good" need to be
understood. The concept of a Clean Air Corridor refers to the
atmospheric path of air that arrives at an area associated with
good visibility conditions. The Clean Air Act states that the
duties of visibility transport commissions include addressing
the establishment of clean air corridors in which additional
restrictions on increases in emissions may be appropriate to
protect visibility (CAA §169B(d)(2)).
One of the most relevant papers on the topic of the source
of visibility impairing pollution, White et al. (1994) contains a
number of research results of interest. The research examined
back-trajectories for air arriving at the Grand Canyon and
classified, if possible, the source of the air. The four
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quadrants, {NE, NW, SW, SE) were considered as the source zones.
Back-trajectories were calculated for air parcels arriving in the
Grand Canyon during the hours of 1100 and 1700 (LST). The study
found that on forty percent of the days, the two back-
trajectories originated in the same quadrant as they had the
previous 48-hours. They also used a region specific chemical
marker as a tracer to examine one source area. Methylchloroform,
a regional tracer for air from the Los Angeles basin, was
monitored at the mouth of the Grand Canyon along with particulate
sulfate.
In this study, White et al. (1994) found patterns of high
methylchloroform at the mouth of the Canyon during April through
October. The back-trajectories for these days estimated the air
on those days to have been in the southwest quadrant, which
includes Los Angeles, during the previous two days. However,
some days with air from the same quadrant had low concentrations
of methylchloroform. Back-trajectories for fine sulfate
particles on the south rim were not as consistent. High sulfate
particle concentrations were observed on days when the back-
trajectories of the air parcels spent 3/4 of the time exclusively
in only one quadrant. However, no specific quadrant was
determined to be the primary quadrant of concern. Each quadrant
except the northwest was identified as having at least one high
concentration day at the south rim. Low concentrations were
found with back-trajectories from the northwest and southwest
only. High relative humidity and low visual range are associated
with air from the southwest, but not the northwest. Air
transported from the southwest also was coincident with reduced
visual range on several days from October to January.
In conclusion, White et al. (1994) state that "clean
corridors" for visibility can differ from the corridors with low
sulfur and other aerosol fractions. At the Grand Canyon the best
visibility occurs when air is from the north, however this seldom
happens during the summer tourist season when visibility is most
important. They also noted that the region to the southwest of
the Grand Canyon is a regular supplier of air on days which have
haze.
Joseph et al. (1993) and Eatough et al. (1992) used tracers
to identify sources of sulfur oxides impacting the Grand Canyon.
Using ratios of various tracers and meteorological data they
established source fingerprints for air originating from the
northwest, west and southwest. They found the fingerprints were
distinct enough to distinguish emissions from Los Angeles Basin
from emissions in San Joaquin Valley. The fingerprints for air
from the northwest were similar to that for the San Joaquin for
one ratio, but other ratios differed. The tracers (spherical
aluminosilicate, total fluoride, fine particulate selenium,
arsenic and lead) were judged to be useful in identifying sources
of air impacting the Grand Canyon. Presence of these tracers
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would indicate the source of the emissions affecting visibility.
Tracers also play a key role in other Clean Air Corridor
research. Simonet et al. (1993) examined lignin pyrolysis
products as tracers of plant classes in emissions from biomass
combustion. Heaton et al. (1992) looked at tracer elements found
in Rhode Island precipitation.
Source Area Atmosphere Characterization
Caka et al. (1993) studied the SOx (gaseous S02 plus
particulate sulfate) at the Grand Canyon during the winter
intensive portion of Project MOHAVE (Measurement of Haze and
Visual Effects) (January 14, 1992 - February 12, 1992) for the
purpose of identifying the source and the ratio of the S0X and
its components. They found highest S0X associated with transport
from close sources east of the Grand Canyon. Dilution and
increased conversion to sulfate occurred as the air was
transported farther west. The highest ratio of sulfate to S0X
was associated with wet atmospheric conditions. The average
concentration of sulfate was comparable in air masses from the
east and from the west. Lowest concentration of both S02 and
sulfate was associated with transport from the northwest
quadrant.
In order to effectively understand source regions of both
clean and dirty air, with respect to visibility, it is important
to understand the composition of the atmosphere in, and of air
parcels coming from, the source regions. In this regard, many
researchers have characterized "source area atmosphere."
Research outside the U.S. has addressed methods to help
characterize sources. Different types of air masses have
different aerosol optical characteristics. Identification of
these in Germany allowed for separation of local and temporal
effects. The aerosol optical thickness and phase function have
distinct extinction properties from marine, continental and aged
air masses {von Hoyningen-Huene and Wendisch, 1994).
Classification, if possible for the U.S., would be an asset in
identifying Clean Air Corridor characterization.
The composition of air parcels varies and Malm, Sisler,
Huffman, Eldred, and Cahill (1994) examined spatial and seasonal
patterns in particle concentrations and optical extinction. They
used 36 sites, mostly in the western U.S., for the period March
1988 through February 1991. This monitoring included major
visibility-reducing aerosols (sulfates, nitrates, organics, light
absorbing carbon, wind-blown dust) as well as light scattering
and extinction. The composition of fine aerosol changes across
the U.S. In the East, sulfate is the greatest component. In the
Pacific Northwest organics contribute more than any other type of
aerosol. Nitrate comprises the largest mass fraction of fine
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particles in Southern California. In the Sonoran Desert, in
western Texas and around the Great Lakes the contribution of
organic carbon and sulfate are about the saine. When the aerosol
concentrations are used to estimate the light extinction the
relative humidity must be considered.
The reconstructed light extinction is greatest in the East
and in Southern California (Malm et al. 1994). Since the light
scattering efficiency of nitrate, sulfate and some organics is
greater because of higher relative humidity and different
distribution of size of particles in the East, the differences
between eastern and western light extinction are even more
pronounced than the differences in aerosol concentrations. This
research noted that corrections for relative humidity were based
on the correction for ammonium sulfate. The correction should be
larger for acidic sulfates such as sulfuric acid and ammonium
bisulfate which have higher light scattering efficiencies.
Carbon and the source of carbon in the atmosphere is a key
issue in Class I areas. Carbonaceous aerosol particles include
natural aerosols which contain little elemental carbon, with the
exception of wild fire smoke aerosols. The anthropogenic
particles from combustion have more elemental carbon than
organic, depending on the type of the source (Hildemann et al.
1991) .
Particles from natural sources also received attention in
the literature. Hallock et al. (1992) looked at carbonaceous
aerosol particles from common vegetation in the Grand Canyon.
Mazurek et al. (1991) looked more broadly at the biological input
to visibility-reducing aerosol particles in the remote arid
southwestern U.S.
One method to examine the effects of visibility impairing
pollution is with the air quality models. The changes produced
in visibility as a result of emission change were estimated with
a model. The visibility should improve as S02 emissions are
reduced and expectations are that with the 10 million ton
reduction in S02 emissions called for in the Clean Air Act's
market-based acid deposition control program, there will be a 21%
improvement in visibility, when averaged over space and time
(Malm, Trijonis, Sisler, Pitchford, and Dennis, 1994). The
biggest improvement should be along the Appalachians. This
estimate assumes that other factors which contribute to loss of
visibility remain the same. During the summer months, sulfates
account for from 50 to 78V of the loss of visibility and the
visible range varies from 15-18 km in the Shenandoah and Great
Smokey Mountain National Park. The Acadia National park in Maine
has better visibility with 50-80 km visual range. The changes
due to reduced S02 emissions were estimated with the Regional
Acid Deposition Model (Version 2.1).
3t)
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is implemented) and to determine concentra-
tion trends to measure the effectiveness of the
National Strategy; 2) identifying the other
factors (like wind speed, wind direction and
the mixing depth, source emission profiles,
and the distribution of sources throughout the
urban area) that must be measured in order to
derive an estimate of total area source emis-
sions from the measured ambient outdoor
concentrations; 3) developing data analysis
methods to allow the trend in area source
emissions to be determined despite "noise"
from natural variations (like those caused by
year to year changes in weather) and from the
trends of point sources and mobile sources;
and 4) determining if ambient outdoor data
indicate that all area sources of the controlled
HAPs have been recognized (that is, do the
ambient concentrations reconcile with EPA's
understanding of the emission sources?)
Human Exposures
The key research questions for Human Expo-
sures are:
• What are the human exposures to HAPs?
• What are the routes of exposure?
What is the distribution of human exposures to
the various HAPs? By what route, and how effec-
tively, do the HAPs reach humans?
Data are needed to define how people's activi-
ties and the concentration of the HAPs vary
with time and to characterize how that varia-
tion will affect the distribution of exposures.
Research is also needed to define those cir-
cumstances that will lead to high exposures
and high potential risks, including research to
identify the chemicals and circumstances that
make indirect exposures important.
4.2 Research on Effects Assessment
As with Exposure Assessment, there is a need
for more research into Effects Assessment. Two
areas that need additional research are Internal
Dose and Health Effects.
internal Dose ana Health Effects
Critical issues facing health effects researchers
in trying to define the potential human health
effects of hazardous air pollutant emissions from
area sources are:
• How can the most substantial hazards from
HAPs be identified?
• How can health risks be estimated reli-
ably?
How can the most substantial hazards from HAPs
be identified?
Hazard identification research is needed to
develop, refine, and validate methods for
identifying chemicals and agents that pose
potential human hazards. Faster, more accu-
rate, less expensive, and more reliable tech-
niques are needed to determine cause and
effect relationships between environmental
pollutants and adverse health outcomes than
the methods that are currently available. Bat-
teries of test methods designed to evaluate
potential hazards comprehensively also need to
be validated. A comprehensive program to
collect toxicity data also is needed. Efforts
should include evaluation of realistic scenarios
for concentrations and exposures.
Additionally, field studies that evaluate the
biological effects of exposure to urban air
pollution are needed. These field studies
should combine short-term methods developed
in the laboratory to screen for problem chemi-
cals, mixtures, and/or sources, and longer-
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reacts with gaseous nitric acid, a component of Los Angeles smog.
Kreidenweis et al. (1991) found that the effects of
dimethylsulfide on marine aerosol concentrations varies by
latitude. Kreidenweis et al. (1991) used a smog chamber to
measure aerosols formed during photo-oxidation of dimethylsulfide
and dimethyldisulfide and compared these results to predictions
from a model of aerosol nucleation and growth. There was good
agreement in the presence of N0X/ but maximum total number
concentrations in dimethyldisulfide oxidation in the absence of
N0X were under-predicted.
Airships have been used to measure large aerosol and cloud
droplet distributions over the ocean. Frick and Hoppel (1993)
show an effect of processing marine boundary layer aerosol
through stratus clouds. Parungo et al. (1992) examined the wet
and dry deposition of atmospheric aerosols to the Pacific Ocean.
Other research is in progress, but results are not yet
published, on Clean Air Corridors. This research will be
discussed in Section IV.A., on Research in Progress under the
Grand Canyon Visibility Transport Commission.
III.C. Adaptation of Regional Air Quality Models for Assessment
of Visibility
An EPA project for developing modeling tools for assessing
visibility impairment from single or multi-sources began in 1991.
EPA and the Federal Land Managers6 established an Interagency
Work Group on Air Quality Modeling (IWAQM) (EPA, 1992) to address
the coordination between Federal Agencies on the testing of
modeling methods and to assist in the development of modeling
guidance for Class I Prevention of Significant Deterioration
(PSD) and Air Quality Related Values (AQRVs) impact assessments.
The IWAQM was formed as a result of the 1990 Clean Air Act
Amendments and overlapping Federal jurisdictions. For this
reason, states sought help because there are multi-State/Regional
issues and there were no specific recommendations regarding
modeling pollutant impacts involving long-range transport and
dispersion (50 to 250 kilometers). The technical work group was
formed to pool resources and develop mutually acceptable modeling
techniques.
Under this arrangement, EPA and its IWAQM partners reviewed
"off-the-shelf" modeling techniques that can be employed in the
interim for assessing PSD and AQRV impacts. It recommended (U.S.
EPA, 1993) the MESOPUFF II model; this model mimics continuous
6The term "Federal Land Manager" means, with respect to any
lands in the United States, the secretary of the department with
authority over such lands. See CAA §302 (i).
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release as a series of puffs, which allows simulation of the
meandering transport characteristic of puffs over distances of 50
to several hundred kilometers. Conversion of S02 to sulfate and
N0X to nitrate is parameterized including the equilibrium of the
HN03/NH3/NH4/N03 systems. In comparison to standard plume
dispersion models (routinely employed for impacts involving short
transport distances), the MESOPUFF II model provided a more
realistic characterization of the fate of pollutants of
particular concern for regional visibility impairment. MESOPUFF
II, for instance, was able to better handle extended periods of
near calm wind conditions, during which regional visibility
impairment would be anticipated to be worse. The Federal Land
Managers are developing a demonstration package to provide
assistance in the utilization of this interim recommendation.
Concurrently, IWAQM has been fostering the development and
testing of improved methods. Upon its review, IWAQM was
convinced that improving the time and space resolution of the
meteorological fields would yield a commensurate increase in the
accuracy and confidence of any long range dispersion calculation
scheme. Therefore, it undertook to investigate and develop a
data base for testing this hypothesis. The IWAQM's approach was
to commission the development of such a data base and upon
consideration, selected the Penn State Mesoscale Model (MM4-FDDA)
to generate a year's worth of meteorology data for the
investigation. This system applies four dimension data
assimilation of the NWS upper air and surface data through the
Newtonian nudging technique to generate accurate and high spatial
and temporal meteorological fields for the United States (Ching
and Irwin, 1993).
A data base was generated at 80 km resolution on an hourly
basis for an area including the contiguous United States,
northern Mexico, and Southern Canada, as well as the eastern
Pacific, western Atlantic and the Gulf of Mexico. After a review
of candidate models, the Calpuff model was selected to be run
against the meteorological data base. Using the CAPTEX
(mesoscale tracer experiments), the results clearly indicated
pronounced improvements with these data as compared to the
conventional use of the twice daily sounding method. A user's
guide for the CALMET meteorological model that incorporates the
use of the MM4-FDDA data is in review (Scire, 1994).
The IWAQM (U.S. EPA, 1993) provides the assumptions for a
Level I analysis technique for evaluating effects of long range
transport and regional visibility. The Level I is a relatively
simple analysis expected to provide a conservative estimate of
concentrations due to long range transport. The assumptions are
that all NOx has been converted to nitrate and all S02 to
sulfate.
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Receptor modeling is another important modeling tool that
has been applied to apportionment of sources of primary
particulate emissions on local and regional scales. The chemical
mass balance model's ability to apportion source emissions is
limited to sources with dissimilar source profiles. The first
order principles and implicit assumptions have been documented
(Watson et al. 1991).
There are models for transport only. One such model uses a
hybrid between Eulerian and Lagrangian approaches. The model
calculates long-range pollutant transport and dispersion
(Draxler, 1992). Adaptations of this model are in use for
examining clean air corridor issues.
Mechanistic visibility models calculate, from first
principles, the impact of gases and particles on atmospheric
optical properties. The modeling extends acid deposition models
or regional photochemical smog models, by calculating primary
particulate substances as well as products of gas-to-particle
conversion (Middleton and Burns, 1991).
Air quality modeling has been applied across the U.S.
Middleton and Burns (1991) modeled the air quality in Denver, CO.
Middleton et al. (1991, 1993) used a fine grid version of the
RADM to examine sulfate levels in the eastern U.S. and also to
examine the role of nitrogen oxides in oxidant production.
Nitrogen oxides in air quality models was also addressed by
Russell et al. (1993) who examined the dry deposition flux of
nitrogen containing air pollutants.
Several modeling studies have been published for the Los
Angeles area. Hildemann et al. (1993) modeled urban organic
aerosols. Harley et al. (1993) modeled photochemical smog. They
modeled the concentrations of volatile organic compounds using a
lumped chemical mechanism. Harley et al. (1992) examined
speciation of organic gas emissions and the detecting of excess
unburned gasoline in the Los Angeles atmosphere. One of the big
issues in modeling is the formation of secondary organics.
Pandis et al. (1992) modeled this for Claremont, CA.
Aerosols can be incorporated into existing models, e.g.
Engineering Aerosol Model Version 2 for the treatment of sulfur
species, including size-dependent aerosol transport, dynamics,
and chemistry for sulfuric acid aerosol buffered by ambient
ammonia; it is based on the RADM Engineering Model (EM2)
framework (McHenry et al. 1992).
There remain a number of unresolved issues with respect to
models and their ability to accurately predict important
characteristics. Wexler et al. (1994) examined the important
processes that need to be included in modeling aerosols. New
research has contributed to the direction which should be
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addressed in models. For example, condensational growth
significantly alters particle composition in the atmosphere since
a large fraction of particulate matter has been shown to be of
secondary nature (Turpin and Hutzicker, 1991; Eldering et al.
1991). Condensation and evaporation of volatile compounds occur
due to super- or sub-saturation of chemical compounds in the gas
phase. For some aerosol compounds (e.g., sulfuric acid), the
vapor pressure is so low that evaporation is insignificant,
whereas for other (e.g., water, ammonium nitrate), condensation
or evaporation may occur depending on the meteorological and air
quality conditions. Chemical equilibrium for water vapor seems
to be a reasonable and well-accepted assumption, but both
theoretical arguments and atmospheric measurement show that
departure from equilibrium for the lower-concentration pollutants
is a common occurrence (Wexler and Seinfeld, 1992; Wexler et al.
1994) .
New particle formation by sulfuric acid-water nucleation may
occur in the atmosphere for sufficiently high relative acidities
and relative humidities. This requires a significant rate of
production and a low rate of removal. Production of gas-phase
sulfuric acid is primarily a daytime occurrence because it is
formed by the oxidation of emitted S02 by OH radicals, which are
formed during ozone photolysis in air containing water vapor.
The primary loss mechanisms for sulfuric acid are condensation on
pre-existing particles and deposition to the surface. Deposition
is relatively small under neutral or stable conditions, but may
be significant under unstable conditions or those with
substantial wind shear. Loss due to condensation on pre-existing
particles is significant if the particle loading is high,
otherwise it removes sulfuric acid slowly. Typically, urban and
suburban locations that have substantial sulfur dioxide
emissions, but also lower particle mass loadings, cooler
temperatures, and higher relative humidities, are more suitable
for nucleation (Wexler et al. 1994).
Particle diffusivities are lower than for gases and
typically result in deposition velocities for particles that are
an order of magnitude or so lower than those for gases. For the
largest particles, deposition is enhanced by gravitational
settling, but this is not usually significant unless the
particles are larger than 10 (Wexler et al. 1994).
Clouds contribute to both production and removal of
particulate pollutants. Precipitating clouds provide an
important removal mechanism for aerosols. Nonprecipitating
clouds can also dramatically affect the aerosol size distribution
through aqueous-phase reactions of dissolved trace-gas species in
the portion of the entrained aerosol activated as cloud
condensation nuclei. Cloud processing also affects the sulfate
mass concentrations (Hoppel et al. 1994).
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Some of these aerosol constituents are volatile, i.e., they
are partitioned between the gas and aerosol phases, as with
ammonium nitrate and ammonium chloride. In locations with very
acidic aerosols such that volatile acids will not condense, or
with insignificant ambient nitric and hydrochloric acid, the
equilibrium calculation is not necessary. The Southern
California Board model uses the Aerosol Inorganics Model
equilibrium code (Wexler and Seinfeld, 1991).
Another modeling issue which needs to be addressed is
flexible grid sizes and the nesting. There are certain problems
associated with grid nesting (see, for example, Mathur et al.
1992). Adaptive gridding techniques are possible, but there is
no consensus on which technique is the best, moving nests or
adaptive grid refinements. The algorithms used for readjusting
the grid are computationally less demanding than most other
alternatives, as has been demonstrated in compressible flow
simulations (Benson and McRae, 1992) .
Visibility modeling can also be extended to the modeling of
actual images. Eldering et al. (1993) developed an image based
visibility model assuming theoretical light scattering and
absorption in an atmosphere of parallel planes. These
assumptions are used to determine sky color and the addition of
light to the line of sight. Model calculations are then
transformed for display as synthetic color photographs. These
synthetic color photographs have been tested, both by visual
comparison with standard photographs and by numeric comparison
with radiometric measurements. The model requires: a clear day
base photograph; chemical composition and size distribution of
the aerosol; N02 concentration; relative humidity; temperature;
mixing depth; and sun position. Distance between observer and
objects is also required. The model has been shown to be
successful in demonstrating visual impairment.
Global change research also uses models. Global models use
geochemical mass balance to estimate amount of sulfate. Some
models ignore the microphysical processes and use estimates of
scattering properties from measurements (Charlson et al. 1991).
Other models calculate optical properties from assumed size
distributions (Kiehl and Briegleb, 1993). These models do not
consider the relationship between sulfate and clouds.
Global climate research has a major focus on modeling.
Models which include global sulfur and nitrogen species indicate
that industrial emissions have a large impact on sulfate aerosol
concentrations over large regions in the Northern Hemisphere, not
just within the Northern Hemisphere industrialized regions.
Langner et al. (1992) estimated the global fluxes of sulfur
through the atmosphere. Aerosols are not modeled, but are
estimated from observations.
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III.D. Studies of Atmospheric Chemistry and Physics of
Visibility
There are a number of chemistry and physics issues that are
important with regard to visibility: methods for obtaining
species optical efficiencies; relationship between aerosol size
distributions and chemical conversion mechanisms; and radiative
transfer.
Optical Efficiencies
The methods for obtaining species optical efficiencies
include multiple linear regression (MLR) analysis and
applications of Mie theory to measured size distributions. There
is a strong effect on sulfate scattering efficiency from sulfate
mass median diameter. Zhang et al. (1994) showed that sulfate
and carbonaceous particles were the major contributors to fine
(<2.5 /zm) particle scattering during a three-month measurement
period at Hopi Point, Grand Canyon and that their contributions
were comparable. Scattering by nitrates and soil dust was
typically a factor of five to ten smaller. This result
emphasizes the need to examine carbon and organic compounds in
developing strategies to maintain and improve visibility in class
I areas. Variabilities in ambient sulfate size distributions
caused substantial variations in sulfate scattering efficiencies.
Sulfate scattering efficiencies depended on relative humidity as
well.
Particle Size and Aerosol Formation
The well noted difference in visibility between the eastern
U.S. and the western U.S. is due in part to particle size. The
size distribution of dry particles changes from East to West.
Average particle size is larger in the East. Because the East is
humid there is more liquid phase oxidation of S02 which results
in the formation of particles from 0.5 to 0.7 fim in diameter.
Gas phase reactions in the West are predominant because the air
is dryer. This produces sulfates from 0.1 to 0.3 /m in diameter.
Thus there is a strong need to understand the chemistry of
aerosol formation because it has direct effect on the aerosol
optics and visibility. In addition, the size and shape of the
aerosol particles are changed by aqueous-phase chemical
reactions. This can occur many times and increases the
efficiency of the light-scattering (Lelieveld and Heintzenberg,
1992) .
There is still uncertainty in the growth of particles. Meng
and Seinfeld (1994) found that two distinct modes can exist in
the accumulation mode (0.1 - 1.0 fim diameter), the condensation
mode (0.2 /xm)and the droplet mode (0.7 fim) . These modes in the
sulfate size distribution were originally reported by John et al.
(1990) . The growth of condensation mode particles by accretion
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of water vapor or by gas-phase or aerosol-phase sulfate
production cannot explain the droplet mode. The mechanism may be
activation of condensation mode particles to form fog or cloud
drops followed by aqueous-phase chemistry and fog evaporation.
Clouds were found to influence the size distribution of
particles. Hoppel et al. (1994) in a cloud chamber study
examined the effect that non-precipitating water clouds have on
the distribution of aerosol size. Measurements before and after
a cloud cycle showed significant conversion of S02 to H2S04 and a
large change in aerosol size distribution with the cloud
condensation nuclei growing more than smaller particles.
Subsequent clouds cycles had small mass conversion rates. The
final size of cloud condensation nuclei was 2% of the size of the
cloud droplet and the pH was about 5. When H202 was the oxidant,
or sufficient gaseous NH3 was present for neutralization, the
conversion of S02 in a droplet did not have this limit.
Methods have been developed to better determine size
distribution of particles. Wiedensohler et al. (1993, 1994)
compared methods to determine size distributions of low number
concentration ultrafine aerosols.
Research into aerosol formation and growth by Wang et al.
(1992) used measured aerosol size distributions to determine the
rates of gas-to-particle conversion and to study the effects of
additional S02 and/or NH3 on aerosol formation and growth. Kim
et al. (1993) evaluated the sensitivity of thermodynamic
calculations of aerosol composition to the method used to
estimate the activity coefficient. Pandis et al. (1993)
simulated the size distribution of atmospheric secondary organic
aerosols using a Lagrangian trajectory model with gas-phase
chemistry, inorganic and organic aerosol thermodynamics,
condensation/evaporation of aerosol species, dry deposition and
emission of primary gaseous and particulate pollutants.
Pitchford and McMurry (1994) studied size-resolved aerosol
growth, i.e. ratio of moist particle diameter to dry particle
diameter, and chemical composition at the Grand Canyon in winter
1990, using data from the Navajo Generating Station and existing
methods. For relative humidities above 75%, the moist particle
distribution was bimodal indicating an external mixture of
soluble and insoluble constituents. Both constituents grew in
size. The larger, more hygroscopic particles, were composed of
equal volumes of soluble and insoluble materials while the less
hygroscopic were about 85% insoluble.
Koch and Friedlander (1991) presented a theoretical study of
particle growth by coalescence and agglomeration. The dispersed
system will form a condensed phase at high temperature and is
characterized by high density of very small supercritical nuclei.
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Shi and Seinfeld (1991) studied the mass transport
limitation to the rate of reaction of gases in liquid droplets.
They established inequalities for estimating mass transport for
non-first-order chemical kinetics.
Friedlander, Koch and Main (1991) studied scavenging of
coagulating fine aerosol by a coarse particle mode. They derived
an analytical criterion to determine whether diffusion to coarse
mode will suppress growth of fine mode. Friedlander and Wu
(1994) found a linear rate law for the decay of excess surface
area of coalescing solid particle.
Diefenback et al. (1992) simulated multi-droplet
hydrodynamic interactions in liquid water. Kim and Seinfeld
(1992) simulated multicomponent aerosol dynamics. Wolfenbarger
and Seinfeld (1991) examined the inversion of aerosol size
distribution to characterize the solutions.
Atmospheric Chemistry
The chemical composition was considered when Chan et al.
(1992) compared three models of mixed electrolyte solutions.
Mixed ammonium nitrate/ammonium sulfate at relative humidities
from 35 to 70% were measured. Predictions of Zdanovskii-Stokes-
Robinson model were most consistent with data.
Wang et al. (1992) looked at aerosol formation and growth in
atmospheric organic NOx systems. They used outdoor smog chambers
to find aerosol-forming potential of C7 and C8 hydrocarbons with
sunlight when mixed with N0X. Rates of gas-to-particle
conversion were estimated and effects of additional S02 and/or
NH3 found: S02 led to early nucleation burst and rapid growth of
newly formed aerosol; NH3 led to enhanced gas-to-particle
conversion rate; and with both S02 and NH3 there was sustained
particle formation. Wang et al. (1992) simulated the aerosol
dynamics which suggested that over 99% of the mass of condensible
vapor is converted to aerosol by condensation even when a
significant burst of nucleation occurs.
Flagan et al. (1991) studied the distribution of secondary
atmospheric aerosols formed from hydrocarbons. The smog chamber
study used electrical mobility measurements during atmospheric
photo-chemical reactions.
Palen et al. (1992, 1993) used Fourier transform infrared
analysis of aerosol formed in photo-oxidation of isoprene and
beta-pinene. Aldehyde and ketone dominated the aerosols formed
in isoprene photo-oxidation, alcohols and ketones in
photooxidation of beta-pinene.
The scattering efficiency of acidic aerosols when the
relative humidity is high differs from that of neutralized
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ammonium sulfate. Saxena et al. (1993) studied measured
acidities of airborne aerosols and compared them to estimates
based on chemical equilibrium theory. Measured hydrogen ion
concentrations were substantially higher than concentrations
predicted from theory.
The atmospheric chemistry in clouds and fog play an
important role in visibility. Faust et al. (1993) suggested
that the chemistry in air quality models may be missing a
significant reaction which produces hydrogen peroxide in cloud
and fog drops. This is important because hydrogen peroxide is
the limiting reagent in the dominant pathway for the oxidation of
sulfur dioxide to sulfuric acid over eastern North America.
Both clouds and fog are of considerable interest. Mixing
droplets with different pH that are individually in equilibrium
(Henry's law) with atmosphere gives supersaturation with weak
acids and bases (Pandis and Seinfeld, 1991, 1992). Pilinis et
al. (1992) studied aerosol scavenging and processing in fogs.
Kumala et al. (1993) used a one dimensional cloud model to
examine activation and growth of cloud condensation nuclei. The
system has NaN03 particles, condensing water and HN03 vapors.
Simulations suggest enhanced concentrations of atmospheric nitric
acid vapor affect cloud formation by increasing number of cloud
droplets and decreasing mean size (compared to when water is only
vapor). Ulevifiius, Trakumas and GirgSdys (1994) observed this in
winter fog and concluded that the growth rate depends on particle
diameter. They suggest that the aerosol growth is the result of
condensation of low vapor pressure species formed by gas phase
reactions and droplet phase reactions.
Reconstructed Visibility Estimates
Visibility in the Shenandoah National Park is not
reconstructed well, scattering is too low. However, fine mass is
well constructed if water with sulfates is included (Gebhart,
Malm and Day, 1994). Sloane et al (1991) looked at size
segregated fine particles by chemical species and the impact on
visibility.
Sisler and Malm (1994) considered the effect of humidity on
aerosols. They derived from 20 rural sites an empirical
relationship between average relative humidity and average
visibility impairment caused by soluble aerosols. Based on this
relationship, visibility impairment was estimated for an
additional 16 locations where aerosol was monitored, and
reliable, but not concurrent, estimates of relative humidity were
possible. Estimation of scattering by soluble aerosols must take
into account the nonlinear relationship with relative humidity.
Higher sulfate concentrations in the East which coincide with
higher relative humidity explain much of the east-west visibility
dichotomy in the United States. Size distribution of particles
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was not considered.
Human Perception
An important issue in visibility is human perception. A
recent paper proposed a method to measure human perception.
Pitchford and Malm (1994) addressed the human physics of
visibility, specifically human visual perception. A standard
visual index was developed to characterize visibility through
uniform hazes. The index is linear with respect to perceived
visual changes over the entire range of human vision. The index,
called deciview, is near zero for a pristine atmosphere with only
Rayleigh scattering. The index increases by one unit for each
10% change in extinction coefficient. The one unit change is
associated with a small but perceptible scenic change under many
circumstances. This index has potential to be incorporated into
computer simulations of air quality, optical properties and human
perception.
Radiative Transfer and Mathematical Models
Models, other than the Regional Air Quality Models,
addressed in Section III are important in visibility.
Sophisticated aerosol and radiative transfer models are available
on computers. These models incorporate realistic terrain,
multiple scattering, non-uniform illumination, varying spatial
distribution, concentration, optical properties of atmospheric
constituents and relative humidity effects and display these in
synthetic images representing modeled air quality and atmospheric
conditions (Molenar, Malm and Johnson, 1994).
The air quality models discussed in Section III.C indicate
the chemistry and transport particles models (size and
concentration) that have been developed. Many scientists have
Mie scattering models (Wilson and Reist, 1994; Sloane, et al.
1991; Zhang et al. 1994).
Modeling of the physics of aerosols has also included
research results that are based more appropriately in
mathematics, e.g. Zhang et al. (1994) simulated agglomeration of
particles and their breakage using Fibonacci series. Dobbins,
Mulholland and Bryner (1994) used fractals and found a power law
relation between the number of primary particles in an aggregate
and the radius. Kocifej (1994) found that a theoretical solution
of radiation diffusion in a cloudless inhomogeneous molecular
aerosol atmosphere could be used to calculate aerosol
distribution, mean refraction index and the vertical aerosol
concentration gradient. Wu and Friedlander (1993) found enhanced
power law agglomerate growth in the free molecule regime.
The effects of smoke on visibility has been modeled.
Accurate measurements of total scattering from smoke was
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considered by Mulholland and Bryner (1994). They developed a
radiometric model that was accurate within 5% for spherical
particles s 1.1/zm using a transmission cell-reciprocal
nephelometer. They estimated it would be good for up to three
thousand primary spheres in an agglomerate.
Global Change Research in Atmospheric Chemistry
Global change research, driven by radiative forcing,
produced results of interest to visibility, but of somewhat
limited direct application. Visibility is largely concerned with
aerosols in the layer of the atmosphere just above the earth's
surface. Global change is concerned with the distribution of
aerosols throughout the depth of the atmosphere. Because of the
transport of aerosols to and from the boundary layer of the
atmosphere, the results have implications for visibility.
Tropospheric aerosols affect radiative forcing and Charlson
et al. (1992) reported that anthropogenic aerosol, especially
sulfate, in the troposphere is important in the global radiative
balance. The sulfate aerosol particles change the shortwave
reflective properties of clouds as well as directly scattering
the short-wave-length solar radiation.
The chemical composition of aerosols over the Northeastern
Atlantic was classified for November - December, 1989 (O'Dowd and
Smith, 1993). Anthropogenically influenced air masses had 80%,
by number, sulfate particles with soot carbon and sea salt
accounting for the remaining 20%. Sulfate was 65% in Arctic air
masses, in clean maritime air with high wind, sulfate particles
were less than 25%. The latter two air masses were acidic.
Clouds play a key role in global change and empirical
relationships have been estimated between droplet concentration
and aerosol number concentration below cloud base (Raga and
Jonas, 1993). From the measurements of Hudson (1991) it appears
that the growth of cloud condensation nuclei is more rapid for
anthropogenic, than natural, cloud condensation nuclei. Models
indicate that between a day and week is required to transform
dimethylsulfide into cloud condensation nuclei active at .2-.3%
supersaturation, based on assumed oxidation mechanism (Lin and
Chameides, 1993/ Raes, 1993). This mechanism is supported by
observations (Hegg et al. 1991). in severely polluted air,
gaseous nitric acid may enhance cloud condensation nuclei
(Kulmala et al. 1993). The indirect effect of emissions on
radiative forcing is expected to be larger for emissions into
clean air (Twomey, 1991).
If there are no clouds, a method has been developed to
estimate optical properties of the aerosols in the atmosphere.
Wendisch and von Hoyningen-Huene (1994) used ground-based solar
extinction and scattering to infer the optical aerosol properties
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at various geographic locations. The method produced reasonable
estimates of the refractive index which represents the main
physical effects, humidity and non-sphericity.
IV. Research in Progress
This section of the report is arranged by organizational
entity. The discussion presents visibility research information
from several sources but does not set out the research plans of
every single entity conducting noteworthy visibility research.
The information presented was provided by the relevant
organizations. EPA's presentation of the information does not
imply an EPA position about its merits. The information is
intended to provide a frame of reference for the reader who
wishes to follow-up on the research described.
The research plans for the Environmental Protection Agency,
National Park Service, Department of the Interior, and Department
of Energy are presented. The plans of the Grand Canyon
Visibility Transport Commission (GCVTC) are also included. The
GCVTC was established by EPA under §169B of the Clean Air Act and
includes the states of Arizona, California, Colorado, Nevada, New
Mexico, Oregon, Utah, and Wyoming as well as the U.S. Department
of Agriculture and Department of the Interior and the
Environmental Protection Agency. The research plans for the
Electric Power Research Institute (EPRI) are also included. EPRI
provides research support for visibility.
IV.A. Grand Canyon Visibility Transport Commission (GCVTC)
The Clean Air Act (CAA) calls for the GCVTC to assess
scientific and other available information pertaining to adverse
impacts on visibility from potential or projected growth in
emissions from sources in the transport region and to report to
EPA by November 13, 1995 recommendations on what measures, if
any, should be taken under the CAA to remedy adverse visibility
impacts (CAA §169B(d)). The CAA identifies specific measures
that must be addressed (CAA §169B(d)(2)).
The GCVTC has a technical committee composed of four
technical subcommittees, plus a committee on assessment of
alternatives on emission management options, and a committee to
communicate with and to educate the public. The four technical
subcommittees are emissions, aerosol and visibility, meteorology,
and modeling. The tasks and status of these four technical
subcommittees are provided here.
Emissions - A base year emissions inventory has been
developed. Wildfire emissions are to be completed at the end of
March 1995. There was concern that the estimates of emissions
from mobile sources may be too low.
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The inventory of emissions in Mexico will be developed.
Mexican officials are meeting with the subcommittee. The
inventories will be used for large regional models which are
expected to be insensitive to small emissions. The NPS developed
an inventory of these small emissions. Techniques to analyze the
effects of these emissions may be needed.
Aerosol and Visibility - This subcommittee is overseeing
contractor development of a summary of historic aerosol and
visibility data.
This subcommittee, as well as the meteorological
subcommittee is considering an operational definition of clean
air. This definition may be necessary to determine the history
of clean air. Both subcommittees considered a percent of best
visibility days, from 10-30%, with 20% being used for the
majority of the analyses. The definition of clean air does not
predetermine existence or characteristics of Clean Air Corridors.
Meteorology - The Meteorology subcommittee has adapted a
method to characterize clean air corridors. The differences
between models and measured data will need to be reconciled.
This subcommittee is evaluating the meteorological assumptions in
the trajectory analysis for Mexico.
Modeling - Five modeling approaches are being used to
examine the relationship between emission changes and visual air
quality. Modeling will also be used to determine the impact of
emission changes on clear days. A study is in progress to
determine what changes in visibility will occur if emission
sources are placed in areas currently supplying clear air to the
Colorado Plateau.
Clean air corridor research by the meteorology subcommittee
is evaluating several methods to characterize clean air corridor
and explain why clean areas are clean. The years 1982-1992 are
being examined using back trajectory analysis with the
Atmospheric Transport and Diffusion Model (ATAD)7 to examine
meteorological effects. Back trajectory analysis on the cleanest
10, 20, and 30% of the days and as well as the 10, 20, and 30% of
the haziest days will be the basis of the analysis. These days,
determined from transmissometer measurements, will help
understand clear air days. A report on the clean air corridor
characterization is in preparation.
There is another task to examine the sensitivity of clean
air corridors after they are characterized. A major component is
'Heffter, J.L. Air Resources Laboratory Atmospheric Transport
and Diffusion Model (ARL-ATAD). National Oceanic and Atmospheric
Administration, Technical Memo, ERL-ARL-81, 1980.
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meteorology. The air quality modeling directs this, but the
characterization is required first. Only the model ATAD is
available for the 10-year period. Other models have more spatial
and temporal resolution and overlap for shorter time periods.
Several models are being evaluated that will permit
assessment of management options. It is expected that VARED8,
CAPITA9, ASTRAP10, and statistical modeling11 results will be
completed in 1994 . A workshop will be held to evaluate model
performance. The RAMS12 model for some summer days in 1992
should also be available.
Project MOHAVE is contributing significantly to the effort
of the GCVTC. The EPA and Southern California Edison Company are
partners in the project.
IV.B. Environmental Protection Agency (EPA)
The EPA conducts in-house research and also supports
cooperative research with other federal organizations, with
universities and with private concerns.
1. MOHAVE
The EPA is a partner in Project MOHAVE, a tracer study of
emissions from the Mohave Power Plant, a 1580 megawatt coal-fired
steam electric power plant. The study is intended to estimate
impact of Mohave Power Plant on visibility at the Grand Canyon
National Park and other Class I areas. The plant is 120 km
southwest of the Grand Canyon National Park. A field study with
two intensive study periods has been conducted and a final report
is scheduled for November 1994. The air quality monitoring was
performed with full IMPROVE samples at 10 sites and IMPROVE
channel A (fine particles on a teflon filter) plus S02 at 21
sites. During non-intensive periods, sampling was done twice a
week at 10 sites. The winter intensive study period was January
"Utilities Group, Pacific Corp., Arizona Public Service,
Arizona Electric Power, Nevada Power, Public Service Co. of
Colorado, Public Service Co. of New Mexico, Salt River Project,
Sierra Pacific Power, Tucson Electric Power, Los Angeles Water and
Power, Southern California Edison, Tri-State Electric Generation
and Transmission Association.
®R. Husar, Washington University.
"Department of Energy.
"From back-trajectories of models.
12R. A. Pielke, et al., 1992, Meteorol. Atmos. Phvs. 49:69-91.
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14 - February 13, 1992, and the summer intensive study period
was July 12 - early September.
There are many participating sponsors and organizations
involved in this project. The components of the project include
emissions (inventory, source profile); tracer; modeling; aircraft
measurements; meteorological monitoring; air quality monitoring;
optical monitoring; data analysis; and quality assurance.
Emissions data from selected coal burning power plants were
compiled. The emissions were profiled during part of each
intensive period and there was continuous emission monitoring of
S02 and NOx during the intensive studies.
Deterministic modeling of the meteorology was done for each
day with several models at different resolutions. Linearized
chemistry was included.
There was a continuous in-stack release of perfluorocarbon
tracer during each intensive period. Receptor sampling was
accomplished at 31 sites, including a 10-day pre-release
background study. Additional tracers were released from Lake
Powell during the winter, and from Tahachapi Pass and Imperial
Valley during the summer. Aircraft measurements were taken on
six days in the summer. Measurements were made of tracer,
scattering/ ozone, oxides of nitrogen, sulfur dioxide,
temperature, relative humidity, turbulence and solar radiation.
Meteorological data included continuous vertical wind
profiling, radio acoustic soundings to give vertical temperature
and standard surface weather during the entire period. During
the intensive study periods, additional surface and radiosonde
data were taken. Air quality during the intensive periods was
sampled every 12 hours at receptor sites and once a day at
others. One site sampled H202 continuously during the summer.
NH4+ and NH3 were monitored periodically. Medium volume
dichotomous samplers and annular denuders were operated at three
sites. Denuder sampling for gas and particle phase organics was
performed along with size resolved aerosol sampling and other
micro-physics measurements. Optical monitoring was continuous
with nephelometers and transmissometers at several sites. Time
lapse photography was used.
2. Clean Air Status and Trends Network (CASTNET)
The CASTNET, operated by EPA, is an integrated monitoring
network combining different long term monitoring projects and
their data collection activities under one umbrella. The CASTNET
Visibility Network (CVN) is one of the monitoring projects.
Other projects include the Dry Deposition Network, and an Air
Toxics Network. The data which are collected by the different
networks include wet and dry deposition and their constituents,
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ground based 03, fine particle aerosol and its components, light
scattering, light absorption and total extinction, along with
standard meteorological measurements of wind speed, wind
direction, temperature, and relative humidity.
The initial CVN design was completed in 1992 with the intent
to provide spatial coverage for the purpose of national status
and trends in visibility. Accomplishment of its goals requires
the collaboration of other networks, such as the IMPROVE.
The implementation of CASTNET Visibility began in 1993 with
nine sites being brought on-line, primarily in the East. Before
any site is brought on-line or even selected, it must pass strict
criteria. The criteria for CVN was developed from previous
visibility networks and previous aerosol networks. These
criteria were reviewed to address any differences that may have
impacted criteria adopted for the visibility program and CASTNET
in general. Sites established as of August 1994, are shown in
Figure 2.
Aerosols are collected at all sites with an annular denuder
system (ADS) using a 2.5 /xm cutpoint (PM2.5). The samples are
analyzed for fine particle mass, S04", N03, b^, trace elements,
and organic and elemental carbon. The sampling time is 24 hours
at a frequency of every sixth day. Four out of the nine sites
are planned to be fully complemented visibility sites with
aerosol, optical, and scene monitoring. These sites (NY, OH, LA,
and Arendtsville, PA) have the NGN-2 nephelometer as the optical
instrument selected to measure particle scattering and a camera
with a telephoto lens will document the scene.
Statistical analyses of the network's ability to detect a
trend were undertaken during the past year. The findings
indicated that a 2% annual trend can be detected with 90%
confidence in eight years at a visibility site collecting aerosol
data every third day. When compared to every day sampling, a
reduction of 67% in number of samples, sampling every third day
produced only a 28% increase in the variance of the mean and a
13% increase in the standard error of the mean. The every sixth
day sampling frequency will detect a 2% annual trend with 90%
confidence in nine years.
3. Human Observer Comparison Study (HOCS)
The NRC in its report "Protecting Visibility In National
Parks and Wilderness Areas" recommended the use of nephelometers
in measuring visibility. The NRC was also very concerned about
the transition from human to automated airport visibility
monitoring planned by the NWS, the FAA and the DOD. The change
has unfortunate implications for monitoring and analysis of haze
47
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I
OQ
OS
(/I»
Sfl""3 ^
sr ^ ® q
tO
Q.
Figure 2. Visibility Monitoring Sites - August 1994.
(courtesy of Air Resource Specialists, Inc.,
Ft. Collins, CO).
48
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data. The Automated Surface Observing System (ASOS) will provide
little or no information on the magnitude and extent of haze. In
particular, this system cannot quantify haze levels corresponding
to visual ranges exceeding 10 miles: prevailing range is
typically much greater than this (NRC Report). Recent
indications are that the Belfort instrument which is being used
can provide an indication of visibility out to 35 miles. The
historical record of ambient haze levels is based largely on
airport visibility data from human observers. There is no other
source of such information. If airport visual range measurements
based on human observers cease, no information will be available
for assessing long term temporal trends and spatial patterns. It
is especially important that such trends be documented during the
coming decade, so that the effect of the Clean Air Act's acid
deposition controls on visibility can be determined.
In response to the NRC report EPA has initiated the HOCS.
The purpose for the study is as follows:
* Evaluate and intercompare instrumental optical
techniques used by different visibility measurement
programs including NWS, IMPROVE, and the CASTNET
Visibility Network. (Should include five years of
SCENES13 data) .
* Establish relationships between the human observations
and the NWS replacement instrumentation and EPA's
visibility programs to link the new instrumentation
with the historical human observer database and
encourage archiving by National Weather Service of the
visibility measurement to maximum distance.
* Examine the role of relative humidity on particle
scattering by controlling humidity in one of two
collocated integrating nephelometers.
* Conduct a summer field study at three locations with
human observations: one Class I area, one rural
suburban site, and one urban site.
The HOCS comparisons between nephelometers, human
observations and the new ASOS visibility sensor, as well as
addressing the effect of relative humidity by using nephelometers
capable of controlling humidity, are to be accomplished during a
summer field study at three site locations. Two sites are near
airports with both human observation and the ASOS, and one site
is in a Class I area with human observation data. The Class I
area site will have the different nephelometers and the ASOS
"Publications and presentations by participants have been
documented in a 1993 EPRI report, RP1630 by C.E. McDade.
49
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visibility sensor.
The final reports from the study are expected to be
available in July 1995.
4. Visibility Cooperative Research (UNC-CH)
The EPA will cooperatively, with the University of North
Carolina - Chapel Hill (UNC-CH), use existing data sets and
models related to visibility and optical properties of the
atmosphere to study the relationships between aerosol physical
and chemical properties, meteorology, and those optical
properties. There are some gaps in our knowledge of the factors
influencing visibility. Models of the impact of sources on
regional visibility and plumes could be improved in both accuracy
and efficiency. Among the gaps are: how detailed the input
parameters need to be; how much simplification is possible or
desirable in models; how much time resolution is needed in field
studies of visibility; and the understanding of the complex
solution chemistry of the sulfate-nitrate-ammonium system.
Existing Mie codes will be used to judge the sensitivity of
these complex algorithms to input parameters such as aerosol size
distribution, refractive index, and concentration. Simplifica-
tions of light extinction models will be developed to reduce the
need for the demanding Mie code in production models. Existing
data bases will be analyzed to determine the relative importance
of various species on visibility and to judge the importance of
time resolution in visibility field studies. Chemical models
will be used to investigate the optical impact of S02 oxidation
in droplets and to study the ammonium nitrate replacement
hypothesis. Advanced models of supersaturated solution chemistry
will be introduced in the above calculations to provide more
realistic treatment of liquid particles as relative humidity
decreases below the deliguesence point.
5. Aerosol Equilibrium Model (In-house Research)
The EPA will develop an equilibrium model. Light scattering
properties of atmospheric aerosols are affected by their chemical
composition and therefore visibility models must contain modules
for predicting aerosol compositions for given emission scenarios.
To address this issue, efforts are underway to develop a model
for predicting the gas and particle phase equilibrium
concentrations of airborne inorganic pollutants. The model is
based on a new method for handling the non-ideal nature of
hygroscopic aerosols and will be used to predict aerosol chemical
compositions for a wide range of atmospheric conditions,
including cases of low relative humidities which in the past were
difficult to treat. The model is based on the original mole
fraction model for the (NH4)2S04-H2S04-H20 system developed by
Clegg, Pitzer and Brimblecombe (1992). The first phase of the
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program consists of expanding the model to include the effects of
Na\ N03", and Cl". The new model will be able to predict aerosol
acidity and liquid water contents, aerosol concentrations of
NH4\ Na+, Cl', N03", HS04", and S04", and gas phase concentrations
of NH3, HN03, and HCl. In 1996, model predictions will be
compared with particulate field data and the model refined, if
necessary.
6. Regional Particulate Model and Planned Improvements
The EPA is developing an air quality model to look at
visibility. The Regional Particulate Model (RPM) is specifically
designed to model regional haze. This model is a three
dimensional Eulerian air quality simulation model. The final
stage of model development will have multicomponent aerosol
particles consisting of sulfates, nitrates, elemental and organic
carbon, and soil derived components. Atmospheric chemistry will
consist of gas-phase and aqueous-phase chemistry both in cloud
droplet water and in hydrated aerosols, as well as wet and dry
deposition of gases and particles. The model is an extension of
the Regional Acid Deposition Model (RADM) developed for the
National Acid Precipitation Assessment Program (NAPAP). The
transport, gas-phase and aqueous-phase cloud water chemistry, and
wet deposition are the RADM portion of RPM and have been
extensively exercised and reviewed by an international peer
review panel whose recommendations for improvements have been
incorporated into the current version. The extensions include
explicit representation of particle size distributions (as
lognormals), aerosol dynamics (coagulation and growth), cloud
scavenging and aqueous production of new aerosol material, size-
dependent dry deposition, and wet deposition.
The current status of the model is as follows. A first
generation version has been developed which includes the size
distribution and aerosol dynamics with size-dependent dry
deposition but only for primary and secondary sulfates. This
model includes equilibration with relative humidity and
neutralization by gaseous ammonia. By fall 1994, the model is
planned to also include a representation of cloud-aerosol
interactions commensurate with the representation of clouds in
RADM. This will complete the first generation version of RPM.
By fall 1995, nitrates, organics, and elemental carbon are
planned to be added to the model. This can be accomplished
relatively easily because of a related model which has all of
these species but no representation of the particle sizes. This
model has been used to study aerosol impacts in the metropolitan
Denver area (Middleton, 1993). A module to address the larger,
or coarse model, particles is planned to be added by spring 1996.
RPM is being integrated into the EPA's next generation (Models 3)
system and all future improvements to other parts of the system
are planned to be incorporated into RPM when available.
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Model output was used to make an estimate of Koschmieder
visual range using three approaches, Mie extinction, an
approximate method based on Mie extinction but which uses
particle properties such as size distribution and index of
refraction, and an empirical parameterization based upon total
aerosol mass (Hanna et al. 1993). The results show that the
combination of model output from RPM and a simplified extinction
coefficient produces estimates of visual range consistent with
regional analyses of visibility observations.
Precise numerical calculation of the path radiance resulting
from complex multiple scattering processes requires a large
number of computations, particularly for regional visibility
modeling applications. Using the results of an optical
measurements and analysis program, an approximate technique to
estimate the atmospheric path radiance and directional contrast
transmittance will be used to calculate the vertical distribution
of the atmospheric attenuation and single and multiple scattering
phase functions. Results will be compared with calculations from
Mie theory and other approximate algorithms that are used to
calculate light extinction. The RPM uses bimodal lognormal
distributions to characterize aerosol sizes and will provide
regional aerosol size distributions over the eastern United
States. The optical properties of aerosols using different
algorithms will be used to estimate the regional visibility.
It is recognized that regional scale Eulerian based air
quality models that do not explicitly treat the details of major
point source pollutants within their grid system misrepresent
their contribution and can distort predicted deposition and
concentration patterns. With the advancement of nesting
procedures, models are becoming able to resolve such features.
However, as a matter of practical concern, i.e., until
computational power increases significantly for general users,
and until the details of dispersion and near source chemistry are
better understood, simplified ad hoc approaches for modeling
these sources are currently used. The EPA and North Carolina
State University started a three-year collaboration in reactive
plume modeling. This technical effort is intended to develop and
test a point source model that will treat its chemical and
dispersion evolution from near source to distances at which a
grid model can resolve and handle the remaining details. A final
report is expected in 1996.
A new competitively selected cooperative agreement is
planned to begin before October 1994 to address plumes. The
primary objectives of this 2-year cooperative agreement are to
explore and to evaluate various reactive plume modeling
approaches, and to determine innovative techniques for coupling
reactive plume modeling systems in multi-grid modeling
frameworks. This effort will attempt to advance plume dispersion
and chemical processes of different state-of-science reactive
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plume models systems in order to identify and to recommend a set
of scientifically realistic and reliable plume modeling
techniques for use in air quality grid models.
The EPA in-house investigations continue with the objective
of developing methodologies for improving the parametric
formulation for the aqueous chemistry component of the Calpuff
model. The interest is for situations when puffs are impacted by
low level clouds, and for determining when and where such clouds
are juxtaposed with the puffs.
7. Source Attribution with Trajectories
The EPA is supporting research to investigate source areas
of both clean air and of air arriving on days with severe haze.
This Clean Air Corridor research is funded through an interagency
agreement with NPS. In addition, a competitively selected
cooperative agreement is planned to start October 1994 to further
develop user friendly trajectory models on a PC platform.
8. Aerosol Modeling
New research is planned to begin before the end of
September 1994 in the area of aerosol modeling. The cooperative
agreement was competitively selected and has several objectives.
The first objective is to explore flexible methodologies of
applying current aerosol modeling capability to more accurately
represent relevant chemical and dynamical processes from regional
to urban scales. It will use expanded representation of
inorganic aerosol thermodynamics which recognizes presence of
mixed solid-aqueous particles.
The cooperative agreement will investigate the use of more
efficient mathematical algorithms to represent the thermodynamics
of aerosol chemical systems. Finally, the research will explore
use of parallel computers to increase speed of aerosol models.
9. Other EPA Studies Under Sections 812 and 404 of the 1990 CAAA
Section 812 of the 1990 CAAA calls for EPA to analyze the
impact of the Clean Air Act (CAA) on the public health, economy,
and environment and, in performing such analysis, to consider the
costs, benefits and other effects associated with compliance with
standards issued for specified programs. The report will likely
include valuing increases in visibility attributable to the CAA.
The literature draws distinctions in the methodology for
assessing visibility-related benefits between valuation of
visibility in the eastern U.S. and western U.S. and is based on
evidence that the physical qualities of visibility in the East
are fundamentally different from those in the West. On average,
willingness-to-pay for a given percentage change in visibility is
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higher in eastern cities. Cropper and Oates (1991) in a survey
of environmental economics concluded that visibility benefits
vary regionally and should be classified according to whether the
locations studied are in the eastern or western U.S. Chestnut
and Rowe (1992) state:
"Because of differences in the charac-
teristics of landscape, natural background
visibility conditions, and visual air
pollution impacts, we judge that, for this
application [valuing visibility improvements
in the eastern U.S.], studies conducted only
in the eastern United States should be used
for quantification. Studies conducted in the
western United States might provide some
useful information that would help interpret
some of the eastern studies, such as the
relative importance of health concerns versus
visual aesthetics when respondents give WTP
[willingness-to-pay] estimates for
improvements in air quality."
Research contributing to this report is being compiled by
Industrial Economics, Incorporated. The retrospective report for
costs and benefits 1970-1990 should be available late in 1994. A
prospective report on costs and benefits 1990-2030 should follow
in late 1995.
EPA is currently working to complete the Acid Deposition
Standards Study called for under §404 of the 1990 CAAA. The
purpose of this study is to assess the feasibility and
effectiveness of setting and implementing a standard(s) to
protect sensitive and critically sensitive aquatic and
terrestrial resources from acidic deposition. The study
integrates state-of-the-art ecological effects research,
emissions and source-receptor modeling work, along with
implementation and cost issues.
The study will identify sensitive ecosystems and determine
the environmental impact of acidic deposition levels after
implementation of the CAA, including the sulfur dioxide emissions
trading program, and varied levels of sulfur and nitrogen
deposition. The study's quantitative assessment focuses on
surface water effects. However, a standard and potential
emission reductions may impact visibility, particularly in the
eastern U.S. and this will likely be examined. Finally, the
study will identify and discuss approaches to and issues related
to implementing an acid deposition standard.
IV.C. National Park Service (NPS)
The NPS supports research in visibility monitoring with
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three distinct components: view (scene) monitoring; electro-
optical monitoring; and aerosol monitoring.
Combining the results of these three monitoring components
yields an understanding of how the appearance of a scene is
influenced by the way light is transmitted through the
combination of aerosols present in the ambient atmosphere. There
are 3 0 IMPROVE sites along with 14 additional NPS sites
designated to be operated according to IMPROVE Protocol.
The NPS supports monitoring related research on the IMPROVE.
Many of the results have been published. There is currently
research on S02 collection, nitrate loss on Teflon filters,
Teflon-nylon comparisons of sulfate with very high loadings,
summer soil episodes in the eastern United States and
geographical patterns of trace elements.
The Shenandoah study, conducted during the summer of 1991 at
Shenandoah National Park, was performed to test agreement between
optical instruments (nephelometers and transmissometers), examine
sulfate aerosol acidity, and apportion light extinction to
various aerosol species. The Penn State Study was conducted at
Scotia State Wildlife Preserve during the summer of 1991. This
study was designed to closely examine sulfate aerosol acidity by
examining neutralization of acid sulfate particles during
sampling, comparing various methods of determining sulfate
aerosol acidity, and comparing the UC Davis IMPROVE sampler to
Harvard's HEADS sampler. Results of this study show how some
neutralization of acidic particles can occur during sampling, the
direct measure of ammonium ion is the best method for determining
sulfate particle acidity, and the IMPROVE sampler agreed well
with Harvard's HEADS sampler.
Research in the past year has focused upon development of
improved understandings of factors influencing visibility in
Shenandoah National Park. Comparisons between the Harvard and
IMPROVE methods for determination of the degree of sulfate
neutralization showed that the degree of sulfate neutralization
may be overpredicted in some instances, resulting in
underestimates of the sulfate contribution to scattering.
Impactor-derived sulfate mass distributions, along with
corrected acidity estimates and water uptake estimates, were used
in a Mie scattering model. The computed scattering coefficients
were compared with estimates using statistically-derived mass
scattering coefficients with nephelometer data. Computed b.eat
values were consistently lower than expected from the measure-
ments, which may suggest that water uptake departs significantly
from theoretical values. Possible reasons for such departure
include nonideality of mixtures and kinetic limitations (e.g.,
the effects of an organic coating) to particle response to shifts
in ambient humidity. The possibilities are currently being
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explored from a theoretical standpoint.
The scattering model was applied to the systematic
investigation of the effects of composition, relative humidity,
mass mean diameter, and standard deviation of the aerosol
population upon computed scattering coefficients. For the cases
examined thus far (ammonium sulfate), assumption of geometric
standard deviation near 1.5 can significantly enhance the
predicted scattering, relative to the statistically-derived
values; however, it is not clear that such geometric standard
deviations are realistic. The computational investigation
underscored the need for better-resolution size distribution
measurements during special studies, including measurement of wet
and dry distributions; in addition to assisting in the modeling
of scattering, shifts in the mass distribution that are not due
to humidity changes could then be ascertained and possibly
related to meteorological conditions. This work is continuing,
and will explore more acidic compositions and also internal
mixtures.
A study in the Great Smoky Mountains National Park during
the summer of 1994 was designed to further investigate these
issues. The measurements include: assessment of ambient
aerosol's bulk chemical composition, aerosol size distribution
(by mass and number concentrations), light scattering by ambient
aerosol and by fully neutralized sulfate aerosol, and the
monitoring of meteorological conditions.
The information obtained from this study will be used to
formulate models which are able to predict light extinction from
mass of ambient aerosol. This is a first step to developing a
model which can accurately assess light extinction directly from
emissions data.
The backward Monte Carlo radiative transfer code used in the
visibility modeling effort of the NPS was modified to include
absorption by the atmospheric trace gases 03 and N02 at arbitrary
wavelengths in the visible region of the electromagnetic
spectrum. The program allows for enhanced concentrations of
these gases below the mixing layer or in plume-like structures,
or both. A theory and computer program for the calculation of
the light scattering properties of sphere aggregates, including
cross sections for total scattering, absorption and extinction,
have been developed and made available to the NPS. A theory for
light scattering by spheres possessing arbitrarily placed
spherical inhomogeneities has been derived, along with a new,
very efficient algorithm for calculating the scattering
properties of concentrically stratified spheres. Sphere
aggregates and inclusions are being studied in order to better
understand the optical properties of both externally and
internally mixed soot. Version 1.0 of AGEACT (Aerosol and Gas
Effects on Atmospheric Contrast Transmission), an operational
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model for visual air quality impact studies is expected to be
released by summer of 1995.
Work is expected to proceed on refinements to the AGEACT
model and on the optical properties of internally mixed aerosol
particles. A study, based on exact calculations for densely
packed sphere aggregates, of possible enhancements of absorption
by carbonaceous particles in densely packed filter samples
relative to absorption of free carbon has also been proposed. A
theoretical investigation of Muller matrices, albedos and
asymmetry parameters of haze particles possessing internal
structure, such as sulfate-coated soot aggregates is also
underway. Preliminary work with quasi-Monte Carlo methods has
produced a substantially faster forward Monte Carlo algorithm
that will be applied to the study of upwelling flux from
atmospheric haze layers. This is in connection with questions
regarding climatic effects and remote sensing of particulate
pollution. The radiative transfer and sphere aggregate work is
being combined in a study of the possible effects that various
forms of atmospheric carbon have on visibility and climate.
The NPS is supporting nonlinear regression modeling using
the differential mass balance model for the MOHAVE winter data.
This work attempts to explain simultaneously both S04" and S02
concentrations at Hopi Point using the differential mass balance
model. A by-product of this method will be an apportionment
analysis giving ranges of daily contributions of S04" from
specific sources to Hopi Point. This work is computer intensive
but results obtained so far suggest that this method holds
promise. Eventually, it would be of interest to consider summer
data as well. Also, it would be of interest to consider
simultaneous multiple receptors. Loglinear modeling of data
obtained from psychophysics experiment, to evaluate subjects'
ability to remember haze levels over a 24-hour period indicates
that satisfactory models can be constructed to explain the
experimental data.
The NPS is one of the sponsors of Project MOHAVE. Spatial
and temporal patterns in particle data are being studied. Fine
particle data were collected at approximately over 30 sites
during each of the MOHAVE intensive periods. These two periods
were mid-January to mid-February 1992 and mid-July through early
September 1992. Spatial and temporal patterns in these data are
being examined graphically by time lines and spatial contours and
analytically by summary statistics, inter-site and inter-species
correlations, and empirical orthogonal function analysis.
Animations of spatial patterns are being prepared. By
linearly disaggregating the 12-hour average particle
concentrations to 1-hour averages, enough frames of spatial
contours can be developed to animate the spatial patterns in the
data. The resulting video tape aids visualization of the data.
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Rough drafts of the videos for sulfur, organics, elemental
carbon, bromine, lead, and selenium are in various stages of
completion. Eventually, wind vectors will be overlaid on the
contours.
Trajectory models are being compared by NPS. Some simple
intercomparisons between two back trajectory models are being
conducted. The two models are ATAD, a simple 1-layer lagrangian
puff model and a model developed at the Washington University,
which is currently being called the "NGM Model" because it uses
gridded wind fields provided by the NWS's NGM forecast model.
The ATAD has the advantage of being simple and inexpensive enough
to simulate many years of back trajectory estimates. This is
useful for putting the MOHAVE time period in historical context.
The NGM model has several advantages including 3-dimensional
rather than 2-dimensional trajectories, the ability to carry
along several useful variables such as temperature, particle
height, relative humidity, etc., simulation of dispersion.
The first draft of a report examining source attribution of
all species measured at Grand Canyon National Park during 1989-
1991 has been completed. The report includes the results of
approximately a dozen different back trajectory techniques
applied to several particulate species. Data for 1992 and 1993
may also be added. This report may be either a stand-alone
report or be incorporated as part of a larger report to the Grand
Canyon Visibility Transport Commission or included as historical
information in the Project MOHAVE Report.
Examination of "clean air corridors" using several back
trajectory techniques is in progress. Clean air corridors have
been defined as areas which supply "clean" air masses to a
receptor. Clean has been defined by the emissions subcommittee
of the Grand Canyon Visibility Transport Commission as the best
20% of the bext measurements at a given receptor site. Current
plans are to examine data from Grand Canyon, Petrified Forest,
and Canyonlands National Parks. Back trajectory techniques will
be used along with IMPROVE particle data from Big Bend National
Park and Chiricahua National Monument to identify source areas
and estimate emissions in Mexico.
Data from project MOHAVE was used to investigate the
relationship between optical absorption and measured carbon.
Absorption was estimated by two independent methods. One method,
referred to as thermal optical reflectance (TOR), relied on
collecting aerosol samples on quartz substrates and measurement
of evolved carbon as the sample was heated in an oven. "High
temperature" carbon is assumed to be elemental and light
absorbing. A second technique, the laser integrated plate method
(LIPM), relied on collection of aerosols on Teflon filters and
subsequent direct measurement of absorption by optical
techniques. Extinction was measured with long path
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transmissometers, while ambient scattering was measured with
"open air" integrating nephelometers. Coarse (2.5-10.0 fixa) and
fine (less than 2.5 /xm) mass and associated aerosol species were
measured using standard size segregators and analytic techniques.
Comparison of extinction, scattering, and absorption
measurements strongly suggest that optical absorption by TOR
underestimates optical absorption by a factor of approximately
two, while absorption by LIPM is more closely in agreement with
measurements of extinction. It also appears that coarse mass
scattering may have been underestimated by approximately 30%.
Historically, extinction budgets have been calculated using
thermal optical techniques to estimate absorption. New
extinction budgets are calculated for remote areas of the United
States using optical absorption as estimated by LIPM and
comparing them to the more traditional methods using TOR. When
using LIPM to estimate absorption, carbon-derived scattering and
absorption are shown to be responsible for 40-50% of the
extinction at most remote western monitoring sites.
Models to reconstruct absorption using data from IMPROVE
need to be evaluated. Uncertainties in the formulation of
reconstructed fine aerosol mass revolve around the TOR carbon
data. In particular, the identification of light absorbing
carbon as well as the artifact correction need study. Resolution
of the best reconstruction of aerosol mass is, of course, closely
tied to reconstructed aerosol extinction. Specifically, models
that reconstruct absorption as measured by LIPM using IMPROVE
aerosol, optical, and detailed carbon data need to be presented.
A reconstruction requires the use of optical efficiencies.
These are not known, but have been estimated. Determination of
acceptable values is critical and will play an important role in
the quality of the reconstruction.
Given the best reconstruction from IMPROVE, a spatial and
temporal characterization of visibility and aerosol conditions
for the United States will be presented. The characterization
will describe the range of conditions found and the probabilities
of occurrence.
Other issues relate to aerosol acidity and solubility and
the need for more realistic models of aerosol size distributions
and mixtures. Analysis of the effects of size distributions for
ammonium sulfate on scattering have been carried out. Next to be
investigated are scattering by acidic sulfates, ammonium nitrate,
and organics. The more difficult problems of mixtures also need
to be considered. Work in this area will be limited to
literature searches and discussions with other researchers.
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IV.D. Department of Energy (DOE)
The DOE initiated a Visibility Assessment Program in 1990 as
an extension of its general environmental assessment activities.
The research is directed toward maintaining assessments of
environmental and economic benefits of energy supply options.
Initial efforts have used the Argonne National Laboratory
ASTRAP lagrangian model and the DOE Visibility Assessment Scoping
Model (VASM). Updated emissions data bases have been used and
sensitivity analysis performed with respect to assumptions,
uncertainty in aerosols and variability in climate.
During the 1994-1995 period the research focus includes:
organic aerosol high humidity growth; relative contribution to
organic aerosols of fossil versus biogenic; and models. The
models development will provide an easier to use update to VASM,
the optics model for assessments. The ASTRAP will have specific
regional versions and will incorporate fine particulate
transport.
The Atmospheric Radiation Measurement (ARM) Program conducts
some visibility-related research in DOE. Visibility related
research includes the four-dimensional data assimilation and the
relationship between aerosols and clouds.
The ARM Program has two basic objectives: 1) to improve the
treatment of radiative transfer in climate models under all
relevant conditions, and 2) to improve the treatment of clouds in
climate models, including the representation of the cloud life
cycle and the prognosis of cloud radiative properties. The
approach of the program is to establish measurement facilities at
key climate-sensitive locales to acquire measurements of
atmospheric radiative properties (solar irradiance, longwave
fluxes), atmospheric state, and distributions of key
radiatively-sensitive atmospheric constituents, such as water
vapor, aerosols, and radiatively-important trace gases. One
aspect of the measurement program that is related to visibility
impairment studies is solar irradiance measurements obtained with
the Multi-Filter Rotating Shadow and Radiometer (MFRSR). These
measurements can be used to determine direct-to-diffuse
irradiance ratios and aerosol optical depths at several
wavelengths. Estimates of boundary layer height from various
profilers, including lidars and sodars, can in turn be used with
the optical depth measurements to infer a mean boundary layer
aerosol extinction coefficient. Aerosol light scattering
measurements are also obtained at the surface. These quantities
are all related to traditional visibility impairment measures.
The DOE will prepare periodic assessment reports that
evaluate national, long-term visibility alterations that arise
from changes in the emissions of S02, NOx, and VOC resulting from
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different energy supply scenarios of the National Energy Strategy
and CAAA of 1990. They will characterize aerosol light
extinction and visibility over an annual cycle at a variety of
locations in the United States by explicitly treating variable
aerosol chemical composition and relative humidity effects. The
characterization is performed for current conditions using
available data bases and aerosol optics models and is being
applied to potential future emissions conditions using regional
model simulations and other process-related models to estimate
changes in aerosol chemical composition and mixing
characteristics as chemical components are removed from the
system. They will provide an estimate of visibility impairment
distribution curves for non-energy background conditions for
comparison with curves obtained for the various emission change
scenarios. This project depends on data collected by other
projects for improving quantitative relationships among measured
aerosol light extinction (and/or visibility), size-resolved
aerosol chemical species concentrations, and relative humidity.
The project also provides a modest contribution to augment
ongoing aerosol optical characterization field studies.
Three workshops have been convened to frame the technical
and policy-needs aspects of a visibility impairment program plan.
The first workshop defined the technical basis of the assessment.
The workshop participants recommended the use of empirical
relationships between aerosol characteristics and light
extinction in the preliminary assessment and proposed several
options for performing the later assessments. In the second
workshop, the participants reviewed progress in the development
of the assessment plan from the context of policy information
needs of various federal agencies and other organizations. The
third workshop focused on the role of background aerosol in
determining visibility impairment. A preliminary assessment
report was drafted in late 1992 using currently available
relationships between aerosol mass concentrations, relative
humidity and light extinction and existing data bases (e.g., NPS
IMPROVE Network data was used for aerosol concentrations and
visibility, while the NOAA Local Climatology reports and other
IMPROVE data sets were used for relative humidity and
temperature) to establish a baseline for current conditions at
Shenadoah National Park. Recent aerosol thermodynamic
equilibrium and Mie-scattering models and algorithms were adapted
for use in the assessment process and were checked with available
monitoring and high-resolution field data.
An assessment report for 1994 using results generated with
the new models for both current conditions and reduction
scenarios based on the 1990 CAAA for Shenandoah and Grand Canyon
National Parks is in preparation. Additional locations, which
presently are deemed important for assessment studies, are being
selected for analysis with the assessment models. In addition,
the role of organic carbon aerosol, as well as that of elemental
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carbon, to visibility impairment, relative to that produced by
sulfate and nitrate aerosol is being examined with models and
with available field data. Methods for distinguishing organic
carbon originating from natural processes and energy-related
emissions is a key feature of these studies. Finally, regional-
scale atmospheric transport and chemistry models will be used to
establish uncertainty limits of the assessment procedure and
results. The project will also support two Russian aerosol
scientists, under the auspices of the Department of Commerce
Special American Business Internship Training (SABIT) Program, to
provide technical expertise in the areas of aerosol formation
processes and data analysis and interpretation.
The DOE is using technology developed for DOE prototype
instruments to fabricate a multi-filter rotating shadowband
radiometer (MFRSRs). The MFRSR measures total and diffuse
horizontal irradiance, and from the difference and with solar
ephemeris data, computes the direct normal irradiance. The
rotating shadowband is configured for mid-latitudes (20-55
degrees). The NOAA, Colorado State University, and National
Institute of Science and Technology instruments have a central
detector with a broadband ultraviolet-A (UVA) detector. An
additional latitude bracket is required for use in tropical
areas.
Battelle and the State University of New York at Albany will
commercialize the single and multiple wavelength versions of the
rotating shadowband radiometers. A commercial vendor (Yankee
Environmental Systems) was licensed to manufacture these
instruments in June 1993. The same detector is used for the
measurement of both total and diffuse irradiance and thus allows
a precision in calculating the direct normal irradiance
approaching or exceeding sun-tracking radiometers.
Direct-to-diffuse irradiance ratios do not need an absolute
calibration of the detectors. Extraterrestrial solar constants
and total optical depths can also be computed for each wavelength
channel using the Bouguer-Beer-Lambert attenuation law and the
direct normal irradiance as a function of time of day or
atmospheric airmass thickness.
The DOE is investigating how clouds interact with longwave
(infrared) and shortwave (solar) radiation to regulate the
heating of the planet. As mentioned earlier, this is a central
issue in the radiatively forced global climate change issue.
Clouds mask about half the earth's surface at any given
time. Therefore, it is important to know how clouds interact
with outgoing longwave (infrared) and incoming shortwave (solar)
radiation to regulate the heating of the planet. To better
understand how these climatically opposed effects vary in time,
with cloud type and structure, and, to a limited extent, with
geography, there is a nine-station regional network in the
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eastern United States and each station is equipped with standard
instruments for measuring ambient temperature, total
precipitation, and relative humidity. Observations of diffuse
horizontal, total horizontal, and direct normal solar irradiance
are the primary solar measurements and are made with a MFRSR in
six narrow-wavelength bands and one unfiltered broadband that
corresponds to the standard shortwave instruments. For increased
stability and reliability, all three solar radiation components
are determined with the same sensor for each wavelength band.
Several climate-sensitive parameters, derived from the solar
observations, include cloudiness, cloud optical depth, cloud type
(to some extent), and aerosol optical depth and its wavelength
dependence. The goals of this 5-year study, in order of
emphasis, are: 1) to use ground-based measurements in concert
with the satellite results to characterize cloud radiative
forcing for a variety of cloud types and structures over a
limited geographical area; 2) to track changes in cirrus
frequency and optical depth; and 3) to track changes in aerosol
optical depth and its wavelength dependence.
The nine-station Quantitative Links Network (QLN) has been
in operation since late 1991 with a 90% data recovery rate. A
major activity during FY 1993 was the development of data
reduction and analysis methods, including routines to: (1)
perform cosine response corrections on the direct irradiance
data, (2) perform automated Langley regressions to calculate
aerosol optical depths and calibration constants for the six
filtered detectors, and (3) apply general calibration algorithms
to produce engineering data.
IV.E. Electric Power Research Institute (EPRI)
The EPRI is not a government entity, but heavily invests in
research coordinated with both governmental and non-governmental
organizations in a number of cooperative efforts: visual air
quality and climatology in the inter-mountain West; observational
methods; laboratory and theoretical investigations of particles
in their suspended state; human visual perception of changes due
to aerosol; computer models of regional, local and plume effects;
risk analysis models coupling physical and physiological sciences
with socio-economic costs and benefits of emission options.
Much of the research is overlapping with other atmospheric
issues. For example, particles are of interest because of the
health issue when they are inhaled, as well as being of
importance in visibility. The radiative balance affects
visibility as well as climate change.
To determine the impact of emissions on visual air quality,
EPRI research addresses four logical relationships: (1) emissions
to aerosols, (2) aerosols to atmospheric optics, (3) optics to
human perception, and (4) perception to socio-economic values and
policy options. The atmospheric sciences focus is on the first
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three steps, which lead from emissions to human perception. This
involves studies of the relationship between visibility and the
physics and chemistry of the atmosphere, including the seasonal
and geographical variability in visual air quality.
The EPRI's current plans for research are aimed at a
potentially cofunded project to study these topics in the
southeastern United States as a continuation of cofunded projects
carried out in the inter-mountain West since 1982.
The aerosol chemistry and physics part of EPRI's work deals
with issues of importance to atmospheric aerosols: namely the
nature of the particles in their suspended state. The suspended
state of the particles tends to be modified when they are
collected on filters or other surfaces. For example, atmospheric
particles absorb water as relative humidity increases and lose
water when it decreases. Previous work on particulate water
content has focused largely on inorganic components (sulfates,
nitrates, etc.) for which hygroscopic properties are known.
Recent EPRI-supported work shows that water-soluble organics, as
well as the configuration of internally mixed particles, also
play important roles in determining water content and optical
characteristics. Both theoretical and experimental work is
continuing so that these phenomena can also be represented in
numerical simulation models.
Research in the southwestern U.S. has improved the
understanding of the climatology of aerosol concentrations in the
region, and their relation to visibility. In evaluating the
contribution of carbon and sulfate to particle concentrations,
the important role of wind blown dust, less than 2.5 /m in
diameter, was identified.
The aerosol optics part of EPRI's research applies light
scattering theory to measured aerosol size distributions to
determine the contributions of the major fine particle components
to extinction, and to determine the improvement in visibility
that would be expected if the concentration of a given species
were reduced by a specified amount. Previous EPRI-supported work
raised questions about multiple linear regression (MLR) analysis,
which is subject to errors associated with variability in aerosol
size distributions and sampling errors for organic carbon. EPRI
supported a study of aerosol optical properties in the southwest
desert during the summer of 1992 in which theoretical methods
were used to evaluate component optical properties based on
measured size distributions. In the EPRI-supported work aerosol
water content and mixing characteristics are based on size-
resolved measurements. Optical calculations are based on Mie
theory. Statistical assumptions underlying the MLR were not
necessary.
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Models for human visual response continue to be evaluated.
Many previous studies of human perception have taken place in the
western U.S. with diurnal conditions of low and high humidities
and low extinctions. In the eastern U.S. the extinction of
suspended particle concentrations is much higher and high
humidities are of much longer duration, giving rise to shorter
vistas with much more apparent haze than in the West. In the
East, and to some extent in the West, perceived colors and
contrast by human observers remain to be compared with physical
measurements and model calculations. Radiative transfer models
used to create simulated photographs for estimating visibility
impacts for specified aerosol loadings should be evaluated
against actual radiance from key scene elements as perceived by
human observers. EPRI developed hardware provides procedures for
digitizing actual human perception of vistas by relating them to
their photographs.
Laboratory research is planned to develop new measurement
methodologies for parameters of importance for atmospheric
aerosols. A method for direct measurement of time-resolved
aerosol water content and a portable instrument for measuring
aerosol optical absorption coefficients are included.
Diagnostic evaluation of algorithms inside of models is
important because future, comprehensive mathematical models will
be used to determine relationships between natural and manmade
emissions and atmospheric particulate concentrations. Work on
developing and testing such models is already underway in, but
not limited to, support of the GCVTC.
Finally, work is underway to develop models capable of
coupling the climatology of atmospheric aerosols, aerosol
precursor emissions, cost of various emission management options
and the benefits in terms of improved community health and
visibility within a risk analysis framework.
IV.F. National Oceanic and Atmospheric Administration (NOAA)
Visibility related research at NOAA continues to be related
to the implementation of the new Belfort visibility sensor, a
component of the ASOS. The sensor will not necessarily have good
correspondence with human observations which may be limited for
other reasons. The sensor may provide an indication of higher or
lower visibility than the human observer. The human observer
tends to see over the haze layer at high visibilities. Because
of the earth's curvature, the observer is not able to see distant
targets within a thousand feet of the surface and the observer
will report higher visibility than the sensor. The criteria
being used for evaluation of this sensor is not in agreement with
human observations, but it does provide sufficient information
for aviation safety. Thus the location of sensor and other
considerations may lead to measurements that do not agree with
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human observations, but are better for the safety of aircraft.
Research continues to determine the visual range which can be
estimated with the Belfort visibility sensor.
The NOAA also has visibility related research that is part
of the global change program. Marine aerosols are the focus. A
Southern Hemisphere Marine Aerosol Characterization Experiment
will be conducted November-December 1995. The goal is to
document chemical, physical and optical characteristics of the
aerosols and determine the controlling processes of aerosol in
the remote marine atmosphere. Later, a second phase will examine
the marine atmospheric gas/aerosol systems over the North
Atlantic Ocean with a focus on anthropogenic perturbation of
these systems.
V. Concluding Remarks
There has been continued research in visibility and related
scientific areas. As indicated in the preceding discussion, more
results are expected in the next few years. However, Federal
visibility research resources will likely change in the near
future.
V.A. EPA/Office of Research and Development (ORD) Plans to
Eliminate Funding of Exclusively-Focused Visibility
Research
The EPA/ORD plans to fund the research presented in Section
IV. The EPA/ORD currently does not plan to fund additional
research focused exclusively on visibility.
V.B. EPA/ORD to Fund Particle Research which Includes
Visibility Implications
Atmospheric particles are of important human health concern.
Particle research, including size and composition, is a priority
at EPA. Much of the science that is developed on particles in
urban or suburban environments is relevant to visibility in
highly populated areas and can also be utilized for Class I
areas. In particular the regional aspects are baseline to both
human health and visibility.
V.C. Regional Haze Regulation
Mary Nichols, EPA Assistant Administrator, Office of Air and
Radiation, testified April 29, 1994, before the U.S. House of
Representatives' Subcommittee on Environment, Energy, and Natural
Resources of the Committee on Government Operations. In her
written testimony, she stated that EPA plans "to initiate the
technical activities needed to analyze the appropriate scope and
components of a regional haze rulemaking." The most immediate
effort will develop models and monitoring techniques for regional
66
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planning. The EPA will consider options for addressing regional
haze impairment in all Class I areas. The options will be
informed by recommendations in the report of the Grand Canyon
Visibility Transport Commission, due November 1995, and the work
of the Southern Appalachian Mountains Initiative.
EPA's written testimony indicated that full implementation
of the next phase of the acid rain control program will be a
major step in improving visibility in Eastern class I areas and
reducing deposition-related damage. EPA also indicated that
after full implementation of the acid rain program, there will be
the need to have other programs to assure reasonable progress
towards the Congressionally established national visibility goal
to prevent future and remedy existing manmade visibility
impairment in mandatory class I Federal areas. EPA's testimony
indicated that although the NRC (1993) report stated that there
is still some uncertainty regarding the relationship between
human activities and visibility, NRC (1993) reported that the
basic science needed to address regional haze is now available.
EPA indicated that it intended to develop the technical tools to
address regional haze based on the scientific foundation
presented in the NRC (1993) report.
V.D. CENR Subcommittee on Air Quality Research
President Clinton's National Science and Technology Council
has a Committee on Environment and National Resources (CENR)
which contains a subcommittee on Air Quality. This subcommittee
has the responsibility to consider visibility issues and identify
the research necessary to address policy questions. This
subcommittee looks across Federal agencies and considers the full
spectrum of research and needs related to visibility. Changes
and new efforts are planned by individual agencies for FY96 and
beyond. However, the programmatic milestones and schedule
contained in the CENR draft implementation plan extend only to
the fall of 1995.
V.E. Global Change Research
Global change research has addressed aerosols, but not
because of visibility concerns. Aerosols are an important
component in radiative forcing. The IPCC (Intergovernmental
Panel on Climate Change) is preparing a report on "Climate System
Radiative Forcing". Chapter 3 of that report deals with aerosols
and Chapter 4 with radiative forcing. This report is not
available for attribution at this time. Much of the atmospheric
chemistry research related to aerosol effects that is important
for the climate system is also relevant to visibility research.
V.F. Continuing Research
A review of research in progress and planned as presented in
67
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Section IV of this report indicates that research will be
continued in visibility related areas. The research is spread
across a wide spectrum of entities. Communication and
cooperation are essential to assure quality research and data.
68
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APPENDIX A: ACRONYMS
ADS Annular Denuder System
AGEACT Aerosol and Gas Effects on Atmospheric Contrast Transmission
ANL Argonne National Laboratory
AQRV Air Quality Related Values
ARM Atmospheric Radiation Measurement
ASASP Active Scattering Aerosol Spectrometer Probe
ASOS Automated Surface Observing System
ASTRAP Advanced Statistical Trajectory of Regional Air Pollutants
ATAD Atmospheric Transport and Diffusion Model
CAA Clean Air Act
CAAA Clean Air Act Amendments of 1990
CALMET CALifornia METeorological Model
CAPITA Center for Air Pollution Impact and Trend Analysis
CAPTEX Cross-Appalachian Tracer Experiment
CASTNET Clean Air Status and Trends Network
CENR Committee on Environment and National Resources
CSU Colorado State University
CVN CASTNET Visibility Network
DOD Department of Defense
DOE Department of Energy
EPA Environmental Protection Agency
EPRI Electric Power Research Institute
EQUILIB EQUILIBrium code (Atmos. Environment. 21(11):2453-24S6 (1987)
FAA Federal Aviation Administration
GCVTC Grand Canyon Visibility Transport Commission
HEADS Harvard-EPA Annular Denuder Systems
HOCS Human Observer Comparison Study
HY-SPLIT Hybrid Single-Particle Lagrangian Integrated Trajectories
IPCC Intergovernmental Panel on Climate Change
IMPROVE Interagency Monitoring of Protected Visual Environment
IWAQM Interagency Work Group on Air Quality Modeling
LIPM Laser Integrated Plate Method
MESOPUFF MESOscale Lagrangian PUFF dispersion model
MFRSR Multi-Filter/detector Rotating Shadowband Radiometer
MLR Multiple Linear Regression
MM4-FDDA Mesoscale Meteorological Model - Version 4 -
Four Dimensional Data Assimilation
MOHAVE Measurement of Haze and Visual Effects
MUSCAT Multiple SCATtering
NAPAP National Acid Precipitation Assessment Program
NGM Nested Grid Model
NIST National Institute of Science and Technology
NOAA National Oceanic and Atmospheric Administration
NPS National Park Service
NRC National Research Council
NWS National Weather Service
ORD Office of Research and Development
PSD Prevention of Significant Deterioration
QLN Quantitative Links Network
RADM Regional Acid Deposition Model
RAMS Regional Atmospheric Modeling System
RH Relative Humidity
RPM Regional Particulate Model
RSR Rotating Shadowband Radiometer (also "Rotating
SABIT Special American Business Internship Training (SABIT)
SCE Southern California Edison
SCENES Subregional Cooperative EPA, NPS, and Electric Utility Study
TOR Thermal Optical Reflectance
UC-Davis University of California at Davis
UNC-CH University of North Carolina - Chapel Hill
103
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UVA Ultraviolet-A
VARED Visibility Assessment of Regional Emission Distributions
VASM Visibility Assessment Scoping Model
VOC Volatile Organic Compound
WTP Willingness to Pay
104
-------
Appendix B
Measured fine and coarse aerosol concentrations (in fig/m3) for the 19 regions
in the IMPROVE network, averaged over the three-year period, March 1988
through February 1991. (Sisler, Huffman and Latimer, 1993).
Season
Fine
Mass
Sulfate
Nitrate
Organics
Elemental
carbon
Soil
Coarse
Mass
ALASKA
Winter
1.6
0.7
0.1
0.6
0.1
0.2
4.0
Spring
2.4
0.9
0.1
0.7
0.1
0.6
3.9
Summer
2.7
0.5
0.0
1.5
0.1
0.4
5.4
Autumn
1.2
0.4
0.1
0.6
0.1
0.1
3.2
ANNUAL
1.9
0.6
0.1
0.9
0.1
0.3
4.2
APPALACHIJ
IN
Winter
6.5
3.0
0.8
2.0
0.4
0.3
3.1
Spring
10.6
6.0
0.8
2.7
0.5
0.6
4.5
Summer
16.6
6.0
0.3
4.4
0.5
0.8
11.2
Autumn
9.7
5.6
0.5
2.7
0.5
0.4
5.5
ANNUAL
10. 9
6.3
0.6
3 . 0
0.5
0.5
6.2
BOUNDARY 1
fATERS
Winter
5.2
2.0
1.4
1 1.4
0.2
0.2
3.2
Spring
5.4
2.6
0.4
1.8
0.2
0.4
5.1
Summer
6.2
2.2
0.1
3.1
0.3
0.5
8.2
Autumn
4.3
1.6
0.4
1.8
0.2
0.3
5.8
ANNUAL
5.3
2.0
0.6
2.1
0.2
0.3
5.7
CASCADES
Winter
3.8
0.6
0.1
2.6
0.5
0.1
2.9
Spring
5.2
1.4
0.2
2.7
0.5
0.3
3.1
Summer
6.7
2.4
0.4
3.0
0.5
0.3
4.6
Autumn
5.3
1.3
0.2
3.1
0.5
0.2
3.9
ANNUAL
5.1
1.3
0.2
2.8
0.5
0.2
3.5
COLORADO I
•LATEAU
Winter
2.9
0.9
0.5
1.1
0.2
0.3
3.2
Spring
3.4
0.9
0.2
1.0
0.1
1.1
5.3
Summer
4.1
1.3
0.2
1.6
0.2
0.9
6.4
Autumn
3.2
1.2
0.1
1.2
0.2
0.5
3.7
ANNUAL
3.4
1.1
0.2
1.2
0.2
0.7
4.7
105
-------
Season
Fine
Mass
Sulfate
Nitrate
Organics
Elemental
Carbon
Soil
Coarse
Mass
CENTRAL ROCKIES
Winter
2.0
0.5
0.2
0.9
0.1
0.3
3.0
Spring
3.4
0.9
0.3
1.1
0.1
1.1
4.3
Summer
4.8
1.0
0.1
2.4
0.2
0.9
7.5
Autumn
2.9
0.8
0.1
1.3
0.1
0.5
4.0
ANNUAL
3.3
0.8
0.1
1.3
0.1
0.5
4.0
CENTRAL COAST
Winter
5.6
0.9
1.9
2.3
0.4
0.2
7.7
Spring
4.2
1.4
0.8
1.5
0.2
0.3
9.3
Summer
4 . 5
1.9
0.8
1.4
0.1
0.2
10.7
Autumn
5.7
1.4
1.0
2.7
0.4
0.3
7.8
ANNUAL
5.0
1.4
1.1
1.9
0.3
0.2
8.9
FLORIDA
Winter
5.5
2.4
0.7
1.9
0.4
0.2
8.5
Spring
7.7
3.8
0.9
2.1
0.3
0.7
8.0
Summer
9.1
2.5
0.5
3.0
0.3
2.7
13.6
Autumn
6.9
3.1
0.5
2.3
0.4
0.5
8.6
ANNUAL
7.1
2.9
0.7
2.3
0.4
0.9
9.6
GREAT BASIN
Winter
1.1
0.3
0.1
0.5
0.0
0.1
1.0
Spring
2.4
0.5
0.1
0.9
0.0
0.9
3.7
Summer
4.5
0.7
0.1
1.7
0.1
1.9
8.2
Autumn
3.1
0.6
0.1
1.4
0.1
1.0
5.1
ANNUAL
2.8
0.5
0.1
1.1
0.1
1.0
5.0
HAWAII
Winter
4.0
2.8
0.1
0.9
0.1
0.1
3.0
Spring
3.6
2.5
0.1
0.8
0.1
0.2
7.4
Summer
1.6
0.9
0.1
0.5
0.0
0.1
10.3
Autumn
3.4
2.5
0.1
0.8
0.1
0.1
9.3
ANNUAL
3.2
2.2
0.1
0.7
0.1
0.1
8.2
106
-------
Season
Fine
Mass
Sulfate
Nitrate
Organics
Elemental
Carbon
Soil
Coarse
Mass
NORTHEAST
Winter
6.6
3.3
0.8
1.8
0.5
0.2
3.1
Spring
6.1
3.6
0.4
1.5
0.3
0.3
4.1
Summer
8.6
4.5
0.3
3.0
0.4
0.3
6.7
Autumn
5.6
3.0
0.4
1.6
0.4
0.2
4.1
ANNUAL
6.7
3.6
0.5
2.0
0.4
0.2
4.5
NORTHERN C
SREAT PLAINS
Winter
3.4
1.2
0.6
1.1
0.1
0.5
3.9
Spring
5.0
1.9
0.6
1.3
0.1
1.0
6.0
Summer
5.6
1.8
0.2
2.2
0.2
1.2
9.7
Autumn
4.0
1.2
0.2
1.5
0.1
1.0
5.8
ANNUAL
4.5
1.5
0.4
1.5
0.1
0.9
6.3
NORTHERN ROCKIES
Winter
5.3
1.0
0.6
3.0
0.5
0.3
2.5
Spring
4.6
1.1
0.2
2.4
0.3
0.6
4.2
Summer
5.4
0.9
0.2
3.0
0.3
1.0
9.2
Autumn
6.7
0.9
0.3
4.3
0.6
0.6
5.7
ANNUAL
5.5
1.0
0.3
3.1
0.4
0.6
5.5
SOUTHERN CALIFORNIA
Winter
4.6
0.5
2.2
1.2
0.2
0.4
4.2
Spring
13.6
1.7
6.9
3.2
0.6
1.2
9.8
Summer
13.8
2.4
4.6
4.2
0.8
1.8
15.2
Autumn
8.1
1.1
3.1
2.0
0.4
1.5
13.2
ANNUAL
9.8
1.4
4.2
2.5
0.5
1.2
10.4
SONORA
Winter
3.2
1.2
0.3
1.1
0.2
0.4
3.3
Spring
4.4
1.2
0.3
1.3
0.1
1.5
7.5
Summer
5.6
2.1
0.2
1.8
0.2
1.2
7.6
Autumn
4.5
1.7
0.2
1.7
0.2
0.8
5.8
ANNUAL
4.4
1.5
0.3
1.5
0.2
0.9
6.0
107
-------
Season
Fine
Mass
Sulfate
Nitrate
Organics
Elemental
Carbon
Soil
Coarse
Mass
SXBRRA
Winter
2.5
0.4
0.7
1.1
0.1
0.2
2.1
Spring
4.3
1.0
0.6
1.7
0.2
0.8
4.8
Summer
7.2
1.7
0.6
3.6
0.5
0.9
7.0
Autumn
4.4
0.9
0.6
2.1
0.3
0.5
5.3
ANNUAL
4.5
1.0
0.6
2.1
0.3
0.6
4.7
SIERRA/HU1
IBOCiDT
Winter
1.7
0.2
0.1
1.0
0.1
0.3
2 . 9
Spring
3.0
0.6
0.2
1.4
0.1
0.6
2.9
Summer
4.0
0.7
0.2
2.2
0.3
0.6
5.6
Autumn
2.8
0.4
0.1
1.7
0.2
0.4
2.7
ANNUAL
2.9
0.5
0.2
1.6
0.2
0.5
3.7
WASHINGTON, DC
Winter
16.3
5.4
3.4
4.9
2.0
0.6
30.1
Spring
16.8
7.3
2.6
4.2
1.7
1.0
10.2
Summer
16.7
8.6
1.2
4.4
1.6
0.9
13.5
Autumn
15.3
6.6
1.6
4.4
2.0
0.8
8.4
ANNUAL
16.2
6.9
2.2
4.5
1.8
0.8
16.4
WEST TEXAS
Winter
3.6
1.5
0.2
1.1
0.1
0.6
5.1
Spring
6.4
2.2
0.3
1.7
0.2
2.1
10.4
Summer
6.6
2.5
0.3
1.7
0.1
1.9
7.4
Autumn
4.8
2.3
0.2
1.4
0.2
0.8
7.0
ANNUAL
5.4
2.1
0.3
1.5
0.1
1.4
7.5
108
-------
Appendix C
Seasonal and annual averages, averaged over the three-year period from March
1988 through February 1991, of percentage contributions to the reconstructed
aerosol light extinction coefficient (light extinction budget) for the 19
regions in the IMPROVE network for sulfate, nitrate, organic carbon, light
absorbing carbon, and coarse particles/fine soil. (Sisler, Huffman and
Latimer, 1993).
Season
Sulfate
Nitrate
j Organics
Elemental
carbon
Soil and
Coarse
ALASKA
Winter
49.7
7.3
20.4
3.9
18.7
Spring
53.3
4.1
22 . 0
3.2
17.3
Summer
30.0
1.8
44 .0
4.6
19.6
Autumn
41.5
5.6
28.8
5.4
18.7
ANNUAL
43.3
4.4
29.8
4.1
18.4
APPALACHIAN
Winter
53.8
15.1
19.6
7.6
3.9
Spring
66.1
9.2
15.6
5.3
3.8
Summer
75.6
2.3
15.3
2.6
4.2
Autumn
68.6
5.8
16. 7
5.1
3.8
ANNUAL
68.3
6.7
16.3
4.5
4.2
BOUNDARY WATERS
Winter
46.2
33.0
14.1
3.2
3.4
Spring
60.9
8.6
20.2
3.6
6.6
Summer
50.4
2.9
33.9
4.2
8.6
Autumn
51.4
13.6
23.3
4
7.6
ANNUAL
51.1
14.5
24.2
3.7
6.5
CASCADES
Winter
27.1
6.6
50.4
11.3
4.6
Spring
39.5
6.9
38.6
10.1
4.9
Summer
47.2
8.0
30.1.
9.4
5.3
Autumn
38.4
5.7
39.6
10.9
5.4
ANNUAL
39.0
6.8
39.4
10.0
4.8
COLORADO PLATBAU
Winter
37.7
14.8
25.5
9.5
12.4
Spring
31.5
7.9
25.1
6.0
29.5
Summer
32.3
4.4
29.9
8.9
24.4
Autumn
39.1
5.0
28.9
9.6
17.3
ANNUAL
35.3
7.0
27.6
8.6
20.5
109
-------
Season
Sulfate
Nitrate
Organics
Elemental
Carbon
Soil and
Coarse
CENTRAL ROCB
:ies
Winter
33.8
13 .1
31.0
6.0
16.1
Spring
38.2
10 .6
26.6
4.2
20.4
Summer
28.5
3.8
37.4
8.9
21.3
Autumn
35.3
5.6
33.8
7.6
17.7
ANNUAL
32.7
7.3
33.6
7.1
19.3
CENTRAL COA£
5T
Winter
21.5
35 .6
26 . 9
6.8
9.3
Spring
37.4
20.9
21.7
4.9
15.1
Summer
44.2
17.2
18 .2
4.0
16.4
Autumn
30.0
19.7
30.3
9.3
10.7
ANNUAL
33.0
24.0
24 .5
6.2
12.2
FLORIDA
Winter
53.0
15.5
17.7
5.4
7.5
Spring
59.0
13.6
16.6
3.7
7.1
Summer
44.9
9.7
26.5
4.2
14.6
Autumn
59.4
10.1
18 .2
5.3
7.0
ANNUAL
54.6
12 .2
19.8
4.6
8.6
GRBAT BASIN
Winter
38.8
18 .5
32.3
1.8
8.7
Spring
31.3
8.4
31.4
2.4
26.6
Summer
16.9
2.8
34.8
5.7
39.7
Autumn
24.4
6.4
35.9
5.7
27.7
ANNUAL
25.3
6.5
34.1
4.1
29.9
HAWAII
Winter
81.5
1.8
11.2
1.8
3.6
Spring
74.4
2.5
11.6
1.4
10.1
Summer
52.8
5.0
13.7
1.8
26.8
Autumn
74.5
1.6
9.9
1.5
12.5
ANNUAL
72.8
2.4
11.6
1.6
11.7
110
-------
Season
Sulfate
Nitrate
Organics
Elemental
Carbon
Soil and
Coarse
NORTHEAST
Winter
58. 8
13.3
16.7
7.8
3.4
Spring
65. 0
7.9
14.9
6.6
5.6
Summer
62.7
4.8
21.5
5.4
5.5
Autumn
63.3
8.4
17.2
6.4
4.7
ANNUAL
62.4
8.4
17 .9
6.5
4.8
NORTHERN GREAT PLAINS
Winter
43.5
21. 0
20.8
4.5
10.2
Spring
49.6
15.2
18.8
3.4
13.1
Summer
39.4
3.5
29.4
5.6
22.1
Autumn
39.9
6.9
28.4
6.1
18.7
ANNUAL
44.0
11.0
24 .5
4.8
15.8
NORTHERN ROCKIES
winter
28.8
16 .3
41.5
9.9
3.5
Spring
36.3
8.1
39.4
8.1
8.1
Summer
25.4
4.7
42.7
9.2
18.0
Autumn
21.9
7.4
52.3
11.2
7.2
ANNUAL
28.0
9.0
44.3
9.8
8.9
SOUTHERN CALIFORNIA
Winter
12.0
50.6
17.8
8.8
10.8
Spring
13.3
55.7
15.7
6.8
8.4
Summer
16.8
32.5
22.3
11.8
16.5
Autumn
12.7
36.7
17.3
10.1
23 .2
ANNUAL
14.4
44.4
18.2
9.0
13.9
SONORA
Winter
44.6
9.7
24.4
8.8
12.5
Spring
28.0
7.3
25
7.0
32.8
Summer
40.8
4.0
25.7
7.0
22.5
Autumn
34.4
3.8
27.8
10.8
19.2
ANNUAL
38.8
5.9
25.7
8.4
21.1
Ill
-------
Season
Sulfate
Nitrate
Organics
Elemental
Carbon
Soil and
Coarse
SIERRA
Winter
16.9
30.9
34.1
7.5
10.6
Spring
31.7
18 .8
29.3
6.5
13 .8
Summer
22 .1
7.6
38 .1
15.6
16 .6
Autumn
21.0
13 .4
35.6
13 . 0
16.9
ANNUAL
24.5
15.3
34.8
10.8
14.6
SIERRA/HUMBOLDT
Winter
22 .1
11.1
42.3
9.0
15.5
Spring
28 .6
12 .2
39.7
7.3
12.2
Summer
22.7
5.7
42.0
11.8
17.8
Autumn
22.1
4.9
46.9
13.1
13.0
ANNUAL
24 .4
7.9
42 .8
10.1
14 .9
WASHINGTON, DC
Winter
34.9
22.0
16.9
13.6
12 .6
Spring
50.0
17.7
15.2
12.0
5.0
Summer
62.0
8.9
15.2
8.9
4.9
Autumn
52. 7
12.8
17.4
13.3
3.9
ANNUAL
49.0
16.0
16.2
11.9
6.9
WSST TSXAS
Winter
44.2
6.8
22.7
7.0
19.3
Spring
36.6
5.1
21.6
5.8
30.9
Summer
49.0
6.0
21.1
4.1
19.8
Autumn
51.3
3.8
20.5
6.1
18.3
ANNUAL
45.5
5.4
21.4
5.6
22.2
112
-------
Appendix D
| Visibility Related Conferences
Title
Date
Location
Sponsors
Reference
Plumes and Visibility:
Measurements and Model Components
Nov. 1980
Grand Canyon National
Park, AR, USA
EPA/NPS
Atmospheric
Envi ronment, Vo1.
15, 1981
Visibility Protection: Research
and Policy Aspects
Sept. 1986
Grand Teton National
Park, WY, USA
AWMA
Transactions, AWMA
Visibility and Fine Particles
Oct. 1989
Estes Park, CO, USA
AWMA/EPA
Transactions, AWMA
Visibility and Fine Particles
Sept. 1992
Vienna, Austria
Institute of
Experimental
Physics
Atmospheric
Environment,
Vol. 28, 1994
Aerosols and Atmospheric Optics
Radiation Balance and Visual Air
Quality
Sept. 1994
Snow Bird, VT, USA
AWMA/AGU
Transactions will
be published in
specialty issues
of journals
113
------- |