EPA
United States
Environmental Protection
Agency
Environmental Research
Laboratory
Corvallis OR 97333
EPA/600/3-87/037a
September 1987
Research and Development
Air Pollution Exposure
Systems and Experimental
Protocols
Volume 1: A Review and
Evaluation of Performance
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EPA 600/3-87/037a
September 1987
AIR POLLUTION EXPOSURE SYSTEMS AND EXPERIMENTAL PROTOCOLS:
.VOLUME 1: A REVIEW AND EVALUATION OF PERFORMANCE
William E. Hogsett
US EPA Environmental Research Laboratory
200 SW 35th Street
Con/all is, OR 97333
David Olszyk
Statewide Air Pollution Research Center
University of California
Riverside, CA 92521
Douglas P. Ormrod
Department of Horticultural Sciences
University of Guelph
Guelph, Ontario NIG 2W1
Canada
George E. Taylor, Jr.
Environmental Sciences
Oak Ridge National Laboratory
P. 0. Box X
Oak Ridge, TN 37830
David T. Tingey
US EPA Environmental Research Laboratory
200 SW 35th Street
Con/all is, OR 97333
ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CORVALLIS, OREGON 97333
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DISCLAIMER
The information in this document has been funded wholly by the United
States Environmental Protection Agency. It has been subjected to the
Agency peer and administrative review, and it has been approved for
publication as an EPA document. Mention of trade names or commercial
products does not constitute endorsement or recommendation for use. ,
ii
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Table of Contents
Acknowledgements v
Preface
Lis-t of Participants \ ^
Volume I. A Review and Evaluation of Performance ............
I. Introduction j^
II. System Descriptions 2-1
A. Dry Deposition Systems !!!!!!!!!!!!! 2-1
1. Gaseous Exposures !!!!!!!!! 2-1
a. Non-Chamber Plume Exposures ........... 2-1
b. Non-Chamber Air Exclusion 2-2
c. Chambers -- Outdoors I!!!!!!." 2-4
d. Chambers -- Indoors I!!!!* 2-5
e. Cuvettes 1 . 1 ! ! 1 ! ! 1 2-6
2. Non-Gaseous (Particulate) Exposures .11! 2-9
3. Pollutant Monitoring !'! 1 ' ' ' ' 2-9
4. Pollutant Dispensing and Control . . . 1 ! 1 ! ! ! ' ' ' " 2-10
5. Environment Monitoring ....... 2-12
6. Data Acquisition 111!*'"" 2-12
B. Wet Deposition Systems ill."!.'!.".'!!!.** 2-13
1. Rainfall Simulation Indoors 11! 2-13
2. Rainfall Simulation Outdoors ....!.' 2-14
3. Mist/Cloud Water Simulation ! ! 2-14
4. Aerosol Simulation ! 11 1 I!!!* 2-15
5. Simulation of Both Wet and Dry Deposition" I!!!"'*" 2-15
6. Pollutant Monitoring ! 1 ! ! * 2-25
7. Pollutant Dispensing and Control I!!!!*" 2-17
8. Environment Monitoring ! ! ! ! ! ! ! ' ' 2-19
9. Data Acquisition . . ' I!!!!!!*** 2-19
III. Criteria for Evaluating Exposure System Performance . . 3-1
A. Pollutant Chemistry in the Atmosphere I!!!'* 3-1
B. Physical and Chemical Features of the Environment 3-6
C. Biological Attributes of the Plant ! 11." 3-7
IV. Evaluations 4 ,
A. Dry Deposition — Gaseous Exposures 1 ! ! ! 4~i
1. Non-Chamber Plume Systems ! ' 4_!
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2. Non-Chamber Air Exclusion Systems 4-2
3. Chambers -- Outdoor . . 4-3
4. Chambers -- Indoors 4-4
5. Cuvettes 4-9
B. Wet Deposition 4-10
1. Indoor Rainfall Simulation 4-13
2. Outdoor Rainfall Simulation — Shelterless Plots 4-13
3. Outdoor Rainfall Simulation -- Sheltered Plots 4-13
4. Outdoor Rainfall Simulation — Automated Exclusion/
Scheduled Simulation 4-14
5. Outdoor Rainfall Simulation -- Automated Exclusion/
Automated Addition 4-14
6. Facilities for Simulating Wet and Dry Deposition ... . . 4-14
7. Cloud Water/Mist Simulation 4-14
8. Aerosol Simulation 4-15
V. Exposure Regimes/Air Quality 5-1
A. Data Bases -- Dry (Gaseous) and Wet Deposition b-l
B. Characteristics of Ambient Air Quality in the U.S. ...... 5-2
1. Air Quality Characterization: Dry Deposition 5-2
2. Air Quality Characterization: Wet Deposition 5-19
3. Co-Occurrence of Pollutants • 5-24
C. Exposure Regimes for Determining the Effects of Deposition on
Plants 5-28
1. Dry Deposition 5~29
2. Wet Deposition 5~32
VI. Recommendations 6-1
A. Workshop Guidelines °-i
B. General °~2
C. Dry Deposition °-4
D. Wet Deposition > 6-9
1. Rainfall Simulation o-11
2. Cloud Water/Radiation For Simulation 6-14
E. Wet and Dry Combinations 6-!6
F. Aerosols J-JJ
G. Exposure Regimes 6~18
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ACKNOWLEDGEMENT
We would like to acknowledge all the individuals, who helped in preparing
this document by submission of descriptions of their exposure systems, and/or
publications, and providing information as requested; in particular, Allen S.
Lefohn for his contributions to the discussions of air quality and exposure
regimes, and William E. Winner for his review and evaluation of cuvette
exposure systems.
The workshop was successful primarily because of the cooperation and the
interaction among the invited participants. We would like to express our
thanks to all these individuals for their participation and subsequent review
of this document, and especially acknowledge the contributions and helpful
guidance of the working group discussion leaders: Dr. Walter W.- Heck, Dr.
Michael H. Unsworth, and Dr. William E. Winner.
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PREFACE
The Forest Response Research Program within the National Acid Precipita-
tion Assessment Program (NAPAP) has requested information and recommendations
for standardization of wet and dry deposition exposure facilities, technology,
and experimental exposure protocols to investigate the role of atmospheric
deposition in forest decline. Standardization of exposure facilities and
protocols will permit close comparability among exposure studies conducted on
forest species at a variety of sites across the United States. The task of
obtaining a collective judgment on the recommendations was best accomplished
through an international workshop in which representatives from a large number
of laboratories active in pollutant deposition exposure studies participated.
The international workshop was hosted by the U.S. Environmental Protection
Agency Corvallis Environmental Research Laboratory on January 27-28, 1986, in
Corvallis, Oregon. Prior to the workshop, the authors prepared a review and
evaluation of all available wet and dry deposition exposure systems and experi-
mental exposure protocols; the draft document was provided to each participant
to serve as a working paper and point of departure for discussion. This review
and evaluation, following critical review by the participants and revision, is
included in this document as Chapters 1-5 of Volume I and the appendices of
Volume II. The consensus view of the participants on the recommended level of
stanardization of exposure studies with forest species is presented in Chapter
6.
The twenty-five invited participants were asked to work in small, inter-
active groups to examine and discuss recommendations for standardization within
the three main categories: (1) dry deposition exposure systems; (2) wet
deposition exposure systems; and (3) deposition exposure regimes. Participants
were asked to discuss the following issues: (1) What is the minimum level of
standardization of exposure studies (wet and dry deposition) to pursue? (2)
Should that level of standardization be applied to specific equipment/facility
or should the level of standardization be applied to performance characteris-
tics? (3) What is the recommended standard of exposure system and protocol for
specific research objectives? A consensus view on recommendations for each _
category was established in the larger discussion group, and these are given in
Chapter 6. The conclusions of this workshop were initially presented to the
NCASI workshop on Controlled Exposure Techniques and the Evaluation of Tree
Responses to Airborne Chemicals, held in Atlanta, Georgia, February 4-5, 1986.
VI
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International Workshop on Standardization of Exposure Systems and Protocol
Corvallis, Oregon
January 27-28, 1986
PARTICIPANTS
Name/Organization
Wayne L. Banwart
Department of Agronomy
University of Illinois
Turner Hall
Urbana, Illinois 61801
Jeff Brandt
Acid Deposition Staff (RD-676)
U.S. Environmental Protection Agency
401 M Street, S.W.
Washington, D.C. 20460
Philip Dougherty
School of Forest Resources
University of Georgia
Athens, Georgia 30602
Walter W. Heck
USDA/NCSU
Raleigh, North Carolina 27650
W. E. Hogsett
EPA/ERL
Corvallis, Oregon 97333
Patricia Irving
Argonnne National Laboratory
Building 203
Argonne, Illinois 60439
Keith F. Jensen
U.S. Department of Agriculture
Forest Service
359 Main Road
Delaware, Ohio
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Garret Kats
Statewide Air Pollution
Research Center
University of California
Riverside, California 92521
Lance W. Kress
Argonne National Laboratory
RER/203
Argonne, Illinois 60439
John Laurence
Boyce Thompson Institute
Tower Road
Ithaca, New York 14853
Allen S. Lefohn
ASL & Associates
111 N. Last Chance Gulch
Helena, Montana 59601
Keith Lewin
Department of Energy and Environment
Brookhaven National Laboratory
2 Center Street
Upton, New York 11973
Richard Mandl
Boyce Thompson Institute
Tower Road
Ithaca, NY 14853
Grady E. Neely
EPA/ERL
Con/all is, Oregon 97333
Douglas P. Ormrod
Department of Horticultural Sciences
University of Guelph
Guelph, Ontario NIG 2W1
Canada
Benjamin Stout
NCASI
260 Madison Avenue
New York, New York 10016
Jack Winjum
Weyerhauser Company
505 N. Pearl Drive
Centralia, Washington 98531
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George E. Taylor, Jr.
Environmental Sciences
Oak Ridge National Laboratory
P.O. Box X
Oak Ridge, Tennessee 37830
David T. Tingey
EPA/ERL
Corvallis, Oregon
97333
Bill Winner
Director, Laboratory for Air Pollution
Impact to Agriculture and Forestry
Department of Plant Pathology and Physiology
Virginia Polytechnic Institute
and State University
Blacksburg, Virginia 24060
George Krause
Landesanstalt fur Immissionsschutz
des Landes Nordrhein-Westfalen
Wallneyer Str. 6
4300 Essen 1
West Germany
Lena Skarby
Swedish Environment Research
Institute
P.O. Box 5207
S-402 24 Goteborg, Sweden
M. H. Unsworth
Institute of Terrestrial
Ecology
Bush Estate, Penicuik,
Midlothian, EH26 OQB, Scotland
Mike Ashmore
Department of Pure and Applied Biology
Imperial College at Silwood Park
Ascot, Berkshire SL5 7PY
Great Britain
Hans-Dieter Payer
GSF, Abt. Toxikologie
Ingolstader Landstrassen 1
8042 Neuherberg
West Germany
IX
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I. Introduction
A first step in the process of determining if, and to what level stan-
dard12ation of exposure systems and protocols shou d be recommended to the
Forest Response Research Program is the integration of current knwledqe of
exposure facilities, experimental protocols for exposure and characterization
of ambient air quality as it relates to exposure prot^o/ This document
provides both descriptions and evaluations of existing dr^ and wet deposition
exposure facilities, monitoring equipment, and microenvironmental sampling
The definitions of dry and wet deposition of Fowler (1980) are used in
T16Mn ^y^°t\^ is ^e direct transfer to and absorpUon of gases
03 NOX) and particles by natural surfaces (e.g., vegetation) Wet
°f an 6lement °r ^bstance from9the ^mosphere to
°P "" ^ SUrface °f' * h^ometeor (e.g', rain,
°" the v^1'0"5 exposure systems has been organized based on
™ environmental control the various systems are capable of rather
than the degree of pollutant control the systems can accomplish (e.g. '
ooen ?nne?h^hr°renTHeTSUre 'hambers » greenhouse exposure chambers,
n£,m 5 * af!!K T116.^65 of systems, are discussed and evaluated as a
group; detailed descriptions of each facility are given in the appendices The
equipment including the various types of chambers for both wet Inl Tory depoll
mpSta/^tff * 110'?^or1n9' Pollutant-dispensing and control, and microenvTon-
mental data acquisition are evaluated using generic criteria Experiment!
protocols for exposure dynamics are discussed and a review of ambient air
quality and its characterization as related to wet and dry deposiiiSn exposure
vegeiHion ^ecl ^^'0"5 ^ 91"Ven f°r -pos^re '605"6
A descriptive evaluation of the exposure system performance is outlined
using generic criteria which reflect conditions necessary to provide reaistic
and reproducib e experimental conditions. The criteria are related totte
pollutant Chemistry in the atmosphere and the plant's edaphic climatic and
atmospheric ;environment during both exposure aSd non-exposure per ods The
'" * dl'SCUSSl°n °f each ^tern's "strengths
The relative importance of the criteria will vary, depending on research
task objective. Thus the evaluation of available research systems and final
recommendations should be tempered with the understanding thaf criteria It w-
gency depends on the biological objective that the facilities or protocols were
originally designed to meet. In addition, the selection of appropriate cri •
teria for evaluating the various systems, the evaluation of the systems and
finally, recommendations regarding standardization are also tempered with
consideration of these exposure systems for use with forest tree speJes since
most were developed for use with crop species and have followed pmoco Is
directed towards annual and perennial crop species. In discussion and consider
9nd Protoco1> the specific requirements for tree
' -commendations'oT
1-1
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1.
2.
Exnerimental tree studies present the obvious problem of size, of both
heightTnS canopy coverage; and, also, increased variability;-slower growth
riles compared to annual crops; length of exposure^enod required? and in the
ra«P of certain conifers distinct growth "flushing" periods. Additional, but
no less imporlant considerations, have been outlined by Cowling (1985, personal
communication):
"Forest trees are perennial plants which accumulate biomass 12 months a
year even in northern latitudes. Roots of forest trees grow whenever the
ground is no? ?rozen. A very important part of^et P^tosynthesis in many
forest trees occurs during the "dormant season," even in deciduous
forests."
"After the first year of growth, most coniferous trees carry from as few
as two to as Was ten or even twelve different year-ages of needles
which differ ^physiological activity and susceptibility to stress
Thus responses to stress in young seedlings with only primaryneedles or
only one-Jear-old and/or two-year old secondary needles may not be the
same as responses of seedlings,Hsaplings, or mature trees with many
different year-ages of needles."
"Loss of foliage due to premature loss of needles may be of no importance
in feteminilg net photosynthesis In forest stands because of the redun-
dancies and normal shading effects within forest canopies.
"Roots of forest trees extend far beyond (often 3 to 4 times) the diameter
of the aboveground canopy."
"Forest trees normally allocate a significant part of current photosynthe-
sis to sySs?s of new feeder roots and to maintaining symbiotic associa-
l ons with mycorrhizal fungi. These so-called "root turnover" or root
maintenance "and/or "mycorrhizal maintenance" functions constitute a
larSe d?ain'(10-40% of gross photosynthesis), on the energy resources of
the tree! Thus, marginal impairment of photosynthesis by airborne
chlmiclll'or other stress factors can result in nutrient deficiencies
indued by roots that are not fed adequately and therefore do not perform
their nutrient-uptake responsibilities adequately.
"Trees grow in diameter long after they stop growing in height."
"Some forest trees have multiple flushes of shoot growth during the same
growing season which are determined by different sets of buds.
"The capacity of some forest trees to grow in height during a 9Jven
growing season is determinedly the nature and vigor of buds set during
the previous growing season."
"Symbiotic associations between forest trees and soil microorganisms are
exceedingly important determinants of growth.
4.
5.
6.
7.
8.
9.
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10.
11.
12.
13.
"Forest soils are essentially never limed or irrigated and rarely culti-
vated or fertilized. Thus, forest trees usually experience some nutrient
and water stress during most growing seasons."
"Forest soils are normally very heterogeneous."
"Most forests (even in the southern pine region) are wild ecosystms which
do not depend on man for their perpetuation."
"The natural range (and therefore the ecotypic and genetic diversity
within) many forest tree species is very much larger than the natural
range, ecotypic variability, and genetic diversity of many agricultural
crops. Wheat and loblolly pine are good examples. Although wheat is
grown all over the world, its natural range was confined to a region of
the Near East which is very much smaller than the natural range of lob-
lolly pine. *
"Forest trees are never free of competition from lesser vegetation."
"The net economic values of forest trees at time of harvest could be
increased by air pollution if the principal effect were to increase the
mortality of suppressed trees in forest stands."
Finally, recommendations are offered by the workshop participants on the
minimum level of standardization for exposure facility types, monitoring
hardware and the exposure protocol. The recommendations reflect the issues
discussed with respect to system evaluations, the evaluation criteria and the
special concerns of the use of these systems and protocols with forest tree
o |J trC* 1 c~ o •
14.
15.
1-3
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II. System Descriptions
All atmospheric pollution-vegetation studies are designed to address
specific research objectives at the biochemical, physiological, whole plant or
population level. The facility used depends upon the objective; systems are
designed to control the pollution environment to test hypotheses and to mini-
mize the facility effect on the environment during the pollution event.
Although the systems discussed in this document were originally designed
primarily to address whole plant and population effects, all have been used in
experiments designed to address various biochemical and physiological hypo-
theses.
A. Dry Deposition Systems
Most facilities for investigating the effects of dry deposition on vegeta-
tion have been designed to study gaseous air pollutants. Very few facilities
have been designed to evaluate the effects of particle deposition, in terms of
either ambient exposures or controlled addition. .
1. Gaseous Exposures
Facilities available that accommodate experimentally controlled exposures
with gaseous pollutants can be divided into two main types based on degree of
manipulation of the atmosphere and plant environment: (1) non-chamber systems
which provide a degree of pollutant exposure control with no appreciable
alteration of the plant environment; and (2) chamber systems which provide much
greater control of the pollutant exposure but modify the environment somewhat
from ambient conditions. A third method, infrequently used, is the ambient
gradient. It incorporates known differences in pollutant doses in the field,
but does not allow for manipulation of ambient to create a range of treatments
(S. V. Krupa, personal communication; Colvill et jfL, 1985; Oshima et al.,
1976). The ambient gradient procedure is not cTTscussed in this reportT"
a. Non-Chamber Systems
Non-chamber field exposure facilities are of two main types: (1) systems
designed to emit a plume of pollutant over a plant canopy and thus dependent on
ambient wind for diffusion and dispersion; and (2) systems designed to exclude
ambient air from plants by blowing filtered air over a canopy during fumigation
episodes, possibly coupled with pollutant addition.
(1) Plume Exposures
All plume systems are designed to treat target plants with pollutants
added above background levels. The pollutant is injected into ambient air and
relies on local winds for dispersion. Very simple emitters, e.g., thin tubes
are used, resulting in no facility effect on the plant environment.
The first plume systems used an array of tubes throughout a plot to
provide exposure regardless of wind direction. One system was designed to
expose native grasses to S02 fumigations similar to those possible from an
electric generating point source (Lee etjfL, 1975). This Zonal Air Pollution
2-1
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System (ZAPS) consisted of a network of long and short aluminum pipes suspended
0.75 m above 0.5-ha plots. Gas release holes were positioned at 3-m intervals
with more than 250 points above the canopy. The design consisted of a series
of plots, each receiving a long-term exposure to a different concentration of
S02. The rate of S02 release was constant, resulting in high variability in
horizontal and temporal concentrations with different wind speeds and direc-
tions.
Another early plume system used an array of 128 vertical tubes distributed
through an exposure plot, with each tube emitting S02 at various heights
(de Cormis et _al_., 1975). This system provided for low variability in vertical
pollutant concentrations for the tree species studied. However, horizontal
variability across the plot and temporal variability were substantial.
Further refinement of plume systems emphasized alteration of the original
designs, either to increase the uniformity of pollutant concentrations across
the exposure plots and/or to use the variability across plots for a gradient of
pollutant exposure concentrations for dose response studies. To alleviate
problems and increase potential uses for plume systems, researchers have
simplified the array of tubes (Thompson et al., 1984; Miller et al_., 1980;
Greenwood et a/L, 1982), devised more efficient shapes for the exposure systems
to consider variable wind directions, i.e., circular (McLeod _et jH., 1985) or
square (Greenwood et jfL, 1982), increased the distance between tubes and
target plants (McLeod et _al_., 1985), or increased system portability '(Northrop,
1983). Recent designsHFfave added'^computer-controlled pollutant dispensing and
monitoring components to regulate exposures based on wind speed and direction
and to improve the uniformity in concentrations across plots. Several plume
systems are described in greater detail in Appendix A.'
(2) Air Exclusion
The second type of non-chamber exposure'system is the air exclusion
facility which permits clean air treatments or removal of ambient air pollut-
ants without the environmental modifications associated with chambers. Most of
these systems use inflatable PVC ducts lying between plant rows to blow
filtered air over the plants during 'ambient fumigation episodes. The duct^air
prevents the incursion of ambient air into the plant canopy and delivers air
filtered by charcoal to the canopy. Air exclusion systems are intended to
impose negligible alteration of the plant environment since the ducts are ;
inflated and lie alongside the plants only during fumigation. When fumigation
is not occurring, the ducts lie flat on the ground, resulting in little or no
effect on light, air temperature, humidity, and other climatic variables.
The first air exclusion system was designed by Jones et _al_. (1977) to
exclude S02 from soybeans during fumigation episodes from a coal-fired elec-
trical generating station. The system has ducts with holes positioned to
release filtered air over a soybean canopy whenever a dedicated S02 monitor
indicates a fumigation event. The system excludes most of the S02 from the
top, but is less successful in excluding S02 from within the canopy.
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Further development of the air exclusion system involved alteration of the
number and size of holes, number of ducts, air flow rates, and other parameters
to provide for pollutant addition as well as exclusion. Shinn et al. (1977)
developed a modified system for adding 03 and H£S to a plant caTiopyT The
system not only excludes ambient polluted air, but also provides for exposure
to a linear gradient of 03 and H2$ concentrations along the length of the
ducts. The gradient is achieved by injecting the air at the blower end of the
system and allowing the gradually decreasing emission rate along the length of
the ducts to lower the adjacent plot concentration; and by increasing the hole
diameter along the length. The exposure plot is surrounded by a 0.6 m high
fiberglass barrier to minimize outside air incursion across the plot. This use
of the air exclusion system for a linear gradient exposure necessitates
extensive air monitoring across the plot to determine plant exposure concentra-
tions for dose-response regression analysis (Bennett et al., 1980; Coyne and
Bingham, 1978).
Further development has emphasized changes in design to provide for a
variety of pollutant,interaction exposures. Laurence et al. (1982) refined the
system to provide for gradients of HF, and S02 plus 03" Wompson and Olszyk
(1985) designed a system adaptable for a variety of uses including ambient air
exclusion, or linear gradients of ambient air or S02. Kuja et al. (1985)
modified their system to exclude ambient ozone and added a rafufcover to
prevent ambient rainfall. Air exclusion systems at particular locations are
described more fully in Appendix B.
b. Chamber Systems
A wide variety of chambers have been used to provide controlled exposures
to defined pollutant doses. The most generalized distinction is (1) outdoor
chamber and (2) indoor chamber. Outdoor chambers are designed to control the
chemical or pollutant atmosphere around the plants with minimal modification
but no direct control of the environment. Indoor chambers control both the
chemical or pollutant atmosphere, and the environment, thus minimizing all
possible nontreatment effects on plant responses.
The general requirements for all types of chambers are similar (Guderian,
1985; Heagle and Philbeck, 1978). Both outdoor and indoor chambers are
designed to: (1) provide environmental and exposure conditions representative
of outdoors; (2) provide an environment as uniform as possible within and
between chambers; (3) provide for uniform concentrations of defined pollutants
both within and between chambers; (4) have transparent coverings to minimize
reductions in irradiance quantity and quality; (5) contain exposed surfaces
made of nonreactive materials; (6) facilitate experimental manipulation of
plants; and (7) be easy to build and maintain. No single design has attained
the ideal for all these considerations; each has particular characteristics
based on the research goals, available technology and materials, and resources
of individual investigators.
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(1) Outdoor Chambers
The outdoor or field chamber was originally designed to exclude ambient
air while blowing filtered air over the plant canopy. The earliest chambers in
use from the early 1950's to early 1970's were closed systems that were
essentially small greenhouses (e.g., Thompson and Taylor, 1966; Van Haut,
1972). Some of the chambers were large enough to be used over young citrus
trees (Thompson and Taylor, 1969); others were smaller, appropriate for plants
ranging from tree seedlings (Miller and Yoshiyama, 1973) to sweet corn (Heagle
et_aj., 1972). All of these early closed chambers were designed to dispense
"aTrThrough the chamber; however, environmental conditions in the chamber did
not represent ambient. Increased air temperatures were especially apparent.
Air flow through the usually cube-shaped chambers was not necessarily uniform,
creating dead air spaces generally in corners.
Current designs of outdoor chambers range from minimal low chambers with
open tops to large fully enclosed domes. The majority incorporate features to
meet requirements (a) through (f) above, but with features important for a
certain type of crop. A versatile 2.4 m high, 3.0 m diameter, PVC-covered
open-top chamber designed by Heag.le et _aj[. (1973) has been adopted for studies
investigating the effects of both ambTent and added air pollutants on a wide
variety of crops as part of the U.S. Environmental Protection Agency National
Crop Loss Assessment Network (NCLAN) (Heck et a.!., 1982; 1984). Either
filtered or ambient air can be blown into the chamber through a perforated duct
(plenum) around the inside of the base of the chamber. Air moves up through
the plant canopy and out the chamber top where it restricts the incursion of
ambient air. A frustrum or baffle around the top of the chamber further
restricts air incursion. The open top allows ambient rainfall, minimizes
ambient light restriction, helps prevent air temperature increases, and permits
entrance of ambient pests.
The essential components of all open-top chambers include: (1) chamber
covering permitting light penetration; (2) a high capacity blower to inject
air; (3) charcoal and/or particulate filters to remove ambient pollutants; and
(4) some structure for dispersing and mixing air entering the chamber, e.g., a
plenum. In general, the materials must be durable and resistant to pollutant-
induced breakdown. The filters must have a high filtering efficiency for the
pollutants in question and, in addition, the filters may have to be chemically
treated to remove gases such as S02 or NOX.
Open-top chambers have been used most extensively with herbaceous crops
during the spring-summer growing season. The environmental modifications that
do occur may limit their usefulness in areas with environmental extremes and
during other seasons, especially winter. However, there have been few year-
round growth studies in chambers and non-chamber field plots to evaluate the
effect of chamber environmental modification. The size and dimensions of the
originally-designed chambers have also limited their usefulness for some crops,
especially larger trees. In addition, the limited incursion of ambient air
that occurs even with the highest air flow rates can affect their usefulness in
more polluted areas or if a very clean treatment is required. Low height
chambers and/or high air flow rates can reduce the chamber effect on the
environment for certain low-growing winter crops such as barley and ryegrass
2-4
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(Fowler, personal communication; Roberts et aj., 1983). Larger chambers can
allow space for growth of small trees or Targe vines (Kats et _al_., 1985; Heck
_et _aT_., 1,985; Brewer, 1983). Other variations of the chamber design are
possible. Outdoor chamber systems for gaseous dry deposition research includ-
ing open-top, semi-open-top, and closed-top designs are described in Appendix
C. Outdoor chambers for studies of gaseous dry deposition and wet deposition
combinations are described in Appendix D.
(2) Chambers -- Indoors
The other general chamber type, indoor chambers, has been widely used in
air pollution research. These chambers may be used in greenhouses in which
natural light is the only or principal light source or in growth rooms in which
all the light is artificial. Greenhouse chambers may be units within a green-
house or entire small greenhouses. The growth room chambers may be units
within a growth room or entire rooms. The chambers differ in size, number of
integrated units, air circulation patterns, air pressure, air mixing, air
exchange rates, and other features. Descriptions of many indoor chambers for
gaseous dry deposition research are given in Appendix E.
Early greenhouse chambers used entire greenhouses or exposure chambers
(Berry, 1970; Hill et _al_., 1959). A later version featured several small
greenhouse-like structures within a large greenhouse (Posthumus, 1978). Other
scientists placed several transparent chambers within a greenhouse (Hill et
_al_., 1959; Lockyer et a!., 1976; Piersol and Hanan, 1975). Some designs of
exposure chambers have~~b~een used to construct multiple units in greenhouses and
in growth rooms (Heck j?t jfL, 1968; Heck j;t jfL, 1978).
Early growth room chambers were converted walk-in storage rooms with
artificial lights added (Menser and Heggestad, 1964). Other researchers
divided walk-in rooms to permit direct comparisons of control and treated
plants (Adams, 1961). Some investigators converted commercial growth chambers
into exposure chambers by adding corrosion-resistant coatings and sealing (Wood
£t^l_., 1973). Another approach has been to place exposure chambers within
commercial growth chambers (Cantwell, 1968; Heck eta±., 1968) or within
walk-in rooms (Heck £t jil_., 1968; 1978; McLaughlin et al., 1976; Payer et al.,
1985). Other investigators have constructed -self-contained growth chamber^like
.units specifically designed for air pollution studies (Hill, 1967; Oliva and
Steubing, 1976). In recent years, some very sophisticated indoor ("air pollu-
tion phytotron") systems have been constructed (Aiga j?t jal_., 1984; Payer et
al., 1985) which provide wide ranges of environment and pollutant concentra-
tions that are electronically monitored and controlled.
Many chambers used in air pollution research were designed to be used in
groups for experiments. Each group typically has a single air conditioning
system and pollutant dispensing and monitoring system. The air exchange is
usually based on a single pass-through of conditioned air to which'pollutant is
added in particular chambers as dictated by the experimental design. There is
no extra mixing 'Within the exposure chamber; the speed of air movement through
the chamber is assumed to be sufficient to maintain adequate levels of carbon
2-5
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dioxide and pollutant in the vicinity of the plants. The chambers designed and
constructed by Heck et a\_. (1968), Jensen and Bender (1977), and Lockyer et jiK
(1976) are of this type.
The continuous-stirred tank reactor (CSTR) exposure chamber was developed
to permit plant growth and gaseous pollutant studies using the principles of
chemical reaction systems (Rogers jjt _al_., 1977). A set of two chambers,
mounted on a cart, was constructed and tested in a walk-in plant growth room in
the North Carolina State University Phytotron. The thorough mixing in the CSTR
chamber ensures that plants are exposed to the concentration of pollutant
measured at the outlet and that leaf boundary layer resistance is small.
Measurement of inlet and outlet pollutant, carbon dioxide, and water vapor
concentrations permit calculations of pollutant flux, net photosynthesis, and
transpiration rates, respectively. This concept for exposure chambers was
extended to the design and construction of a nine-unit greenhouse and four-unit
growth room (Phytotron) facility (Heck et al., 1978). Subsequently, CSTR
chambers have been constructed at severTT locations. Many of the CSTR systems
have been developed to control temperature and humidity as well as provide high
light levels.
There are many other types of controlled environment indoor facilities in
use. Some are quite sophisticated but have not been the subject of methods
publications. Such chamber systems are sometimes described in papers dealing
largely with research results. The chamber systems described in this section
thus represent only examples of the array of chamber systems in use today.
(3) Cuvettes
Use of leaf cuvettes in air pollution research has been limited, even
though they are widely used by plant physiologists in leaf photosynthesis
studies. Photosynthesis research can help clarify plant-environment relations,
is fundamental in the analysis of carbon metabolism, and can help define
physiological mechanisms which alter plant growth rates and form (Mooney,
1972a). A basic introduction to principles of gas exchange studies is pre-
sented by Sestak et jH. (1971). Cuvettes in current use are generally designed
to monitor gas fluxes on a leaf area basis for an individual leaf, or leaf
part, while it is still attached to the plant. Although whole plant and
multiple plant chambers can be used for gas exchange studies, these facilities
are described elsewhere. Cuvettes must have gas handling systems and monitors
for water vapor, C02, and possibly air pollutants. Integration of these
components constitutes a gas exchange system.
Cuvette systems used in air pollution experiments are commonly operated
independently from exposures, i.e., gas exchange rates are determined before or
after fumigations, but fumigations do not occur within the cuvette. Cuvettes
in this case can be used to document the effects of gaseous air pollution
exposures (e.g., Mclaughlin et al., 1979; Norby and Kozlowski, 1981; Olszyk and
Tibbitts, 1981; and Kimmerer aricTKozlowski, 1981).
A few systems exist in which air pollutants can be introduced directly
into the cuvette. These systems constitute air pollution exposure systems and
can be used to calculate air pollution uptake characteristics of single leaves
2-6
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as well as dynamic physiological responses of leave to exposure. The most
common design is the single pass, flow-through system which operates by inject-
ing air pollutants into the air stream before gases flowing through the system
enter the cuvette (Black and Unsworth, 1979; Taylor and Tingey, 1979; Winner
and Mooney, 1980; Hallgren _et a!., 1982; Noble and Jenson, 1983;. Atkinson,
1986),. Thus, monitoring air p'oTlution concentrations in the air streams
entering and leaving the cuvette allows the device to be used as an air
pollution exposure chamber. Studies with; S02 have led to approaches for
calculating total S02 uptake by leaves and for partitioning total uptake
between the fraction adsorbed to leaf, surfaces and the fraction adsorbed into
leaf mesophyll (Taylor and Tingey, 1979; Winner and Mooney, 1980b,c). These
approaches used for analysis of S02 uptake should be appropriate for the
analysis of fluxes of other air pollutants, such as 03, between the leaf and
air.
%
There are four basic approaches to gas exchange studies in cuv,ettes. All
of these cuvette systems are commercially available; however, many scientists
develop their own equipment or modify commercial units.
1. Cuvettes for Sampling Rates of Gas Absorption or Emission from
Leaves. These cuvettes are generally small chambers with a known areaTvolume
and they readily clip on to a portion of a leaf of known area. There is no air
flow through the cuvette but a fan is used to mix air. If the cuvette contains
a humidity sensor and thermocouples to measure leaf and air temperatures, the
rate of increase in relative humidity in the cuvette is equated to the leaf
transpiration rate and this value can be used to calculate stomatal conductance
(Sestak et jj., 1971). Newer systems simultaneously monitor rates of increas-
ing water vapor concentration and decreasing%C02 concentration in the cuvette
which allows calculation of transpiration, stomatal conductance, and photo-
synthesis. These systems are relatively inexpensive, easy to use, and field
portable (Winner and Mooney, 1980a). The cuvettes are simple and lack environ-
mental control. Gas exchange rates are measured quickly (less than 2 minutes)
to prevent leaf heating, the extreme buildup of water vapor, or C02 depletion.
Experiments generally involve determining gas exchange rates of many leaves
(reference?). These cuvettes can be adapted to collected gases emitted from
leaves, e.g., ethylene (Tingey et _aJL, 1976) and H2S (Winner et
-------
can be used to calculate a number of important physiological parameters includ-
ing stomatal conductance and partial pressures of C02 in mesophyll tissues
(Jarvis et a!., 1971) and gaseous air pollution flux into leaves (Unsworth et
jfL, 19757 Winner and Mooney, 1980b,c).
Monitoring and control of all important environmental factors allow study
of dynamic leaf responses to gaseous pollutants such as S02 under constant
conditions so that results from many experiments can be compared (Winner and
Mooney, 1980d; Winner et jH., 1982). The interaction of gaseous pollutants
with environmental facors can also be studied (Norby and Kozlowski, 1981; 1982;
Jones and Mansfield, 1982). These systems are often used to determine
responses of leaves to wide ranges in light, C02, and vapor pressure deficit.
The resultant response curves are useful for diagnosis of quantum yield and
intrinsic photosynthetic capacity of mesophyll cells. Differential gas
exchange systems are usually developed for laboratory studies and used in
conjunction with computerized data acquisition systems. Recent engineering
advances now make these systems field portable. They are relatively expensive
and require technical expertise (Oechel and Lawrence, 1979; Mahon and Domey,
1979; Lange and Tenhunen, 1984; 1985).
3. Cuvettes for Maintaining Constant Levels of Water Vapor and C02 —
Null Balance, Steady State Gas Exchange Systems^ Null balance systems are
commonly used to survey gas exchange rates for many leaves and in this appli-
cation, temperature and radiation are monitored but control of these environ-
mental factors is unnecessary (Ni Ison et _al_. , 1984). This approach to gas
exchange involves monitoring water vapor and C02 in the air around a leaf
contained within a flow-through cuvette. Dry air or CC>2 are added to the air
stream to bring ambient humidity and C02 to desired levels. Transpiration rate
is calculated from the rate at which dry air is added to the cuvette to reduce
humidity to a predetermined level. Photosynthesis rate is calculated from the
rate at which C02 is added to increase C02 concentrations in the cuvette to
predetermined values. Predetermined humidity is often the level outside the
cuvette and predetermined CC>2 is often 320 ppm. These systems are commercially
available, relatively expensive, field portable, and easy to use (Coyne and
Bingham, 1981).
4. Cuvettes for Carbon Isotope Studies. Cuvettes are used to expose
single leaves to C02 containing unstable carbon isotopes. Radioactivity
emitted during decay of 13-C and 14C can be monitored to document rates of
photosynthesis and photosynthate translocation from source leaves to other
plant tissues. 14C02 experiments require pulse-chase techniques and destruc-
tive sampling (Austin and Longden, 1967) to detect 14C with scintillation
techniques (Smith, 1969).
techniques for generating
cuvette difficult. Most cuvettes used in radiolabeling experiments are simple
and have no environmental control. The 14COp techniques can be used in the
field research (Tieszen et _al_., 1974; Mclaughlin et jfL , 1982). Although this
research approach can illustrate photosynthesis and allocation, it is difficult
to detect the dynamics of plant responses to air pollutants. In addition,
• studies do not describe stomatal conductance, and photosynthesis rates
The nature of pulse-chase experimentation and
14COo make control of CO? concentrations in th
the
measured with i4C02 techniques may not match rates meaured by .infrared gas
analysis techniques (Karlsson and Sveinbjornsson, 1981). The C0£ experiments
2-8
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are nondestructive since the nC can be monitored throughout the plant with
geiger counters. 'The ilC02 experiments can reveal dynamic changes in photo-
synthesis and carbon allocation in environmentally controlled studies, but this
research is expensive and can only be done at a few locations (Magnuson et al.
1982).
2. Non-Gaseous (Particulate) Exposures
There have been few studies on the effects of dry particulate on plants
under controlled exposure conditions. Requirements for quantitative and
uniform applications pose special problems in chamber design and experimental
methodology. Particle size characterization is required along with quantita-
tive and repeatable application methods.
Several non-gaseous dry deposition studies have been conducted outdoors.
For example, Chamberlain and Little (1981) described the nature of the particu-
late pollutant deposition.process by studying radioisotope and lead contamina-
tion of plants set outdoors in the zone of influence of the contaminants.
Other studies have been conducted in more closely controlled conditions
indoors. Plant exposure chambers for dust studies were described by Darley et
aj. (1968) who were studying cement-kiln dust effects on plants. Krause and
Kaiser (1977) dusted their plants with heavy metal particulate for studies of
metal-S02 interactions. The "special dusting chamber" was n'ot described.
Ormrod ^t _al_. (1986) devised a chamber for metal dust application to plants
based on the chamber designed by Marple and Rubow (1983) for studies of dust
effects on electronic instruments. Several chambers for particulate dry
deposition research are described in Appendix H.
3. Pollutant Monitoring
The gaseous air pollutant monitoring program will need to follow approved
Quality Assurance Guidelines. This will assure provision of air pollutant
analyzer data which is of known quality and accuracy and is therefore legally
defensible. The National Crop Loss Assessment Network (NCLAN) Quality Assur-
ance Plan for Air Pollutant Analyzers (dated September 11, 1985, U.S. Environ-
mental Protection, Corvallis, Oregon) identifies procedures which are necessary
to air pollutant analyzer measurement and calibration activities. All
analyzers used in the NCLAN project for measuring pollutant levels in the
ambient atmosphere must use a Reference Method and meet the equivalency
requirements specified by EPA in the Federal Register, 40 CFR 53.
Monitoring of pollutants involves many concerns and precautions including
sampling locations, lengths and characteristics of sampling lines, location of
analyzers, and frequency of sampling (Tibbitts, 1978). In most experiments
involving several treatment levels, time-sharing of monitoring instruments may
be required. The number of locations that can be sampled with a single
instrument is dictated by the response time of the monitor and the degree of
variability expected in the pollutant concentration over exposure time (Heagle
and Philbeck, 1978). Sampling should be frequent enough to ensure use of
monitoring data for the various exposure statistics dictated by the research
objective. Time-sharing systems usually involve continuous flow sampling
lines, a sampling manifold, a scanning valve control unit, and monitoring
2-9
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instruments for each gas under study. Each monitor draws samples directly from
the sampling manifold, allowing simultaneous analysis of different pollutants.
The monitoring instruments must be able to analyze continuously and have a fast
response time with low sample air flow rate.
Analyzers for ozone, sulfur dioxide, and nitrogen oxides are classified by
detection method. Table II-l compares analyzers for the three gases with
respect to various performance parameters and cost (Burmann and Rehme, 1978).
These tables indicate that only some analyzers will be acceptable and useful in
experimental work involving the need for rapid and accurate monitoring of
pollutant gases.
It should be noted that some of these instruments have been modified and
improved since publication of these tables. For example, the response time of
pulsed UV fluorescent S02 analyzers can be reduced to about 90 sec by modifying
standard instruments; the lower detectable limit for this type of analyzer
should be lower than 10 ppb. Other considerations for selection of analyzers
include size, weight, portability, power source and consumption, and require-
ment for accuracy gases.
4. Pollutant Dispensing and Control
The basic requirements for dispensing systems for gaseous dry deposition
have been described by Heagle and Philbeck (1978) (Appendix I). According to
them, an acceptable dispensing system: (1) delivers easily-controllable
concentrations of pollutant to one or more exposure chambers; (2) is free of
leaks; (3) is relatively free of concentration "drift"; and (4) is constructed
with nonreactive materials to prevent corrosion and the production of secondary
substances. A system that dispenses pollutants to several chambers at once is
usually preferable to separate systems for each chamber. Each dispensing
system consists of three major parts: (1) a pollutant source (usually com-
pressed gases or an ozone generator); (2) a series of precision valves or
flowmeters; and (3) an air dilution manifold that carries the gas to the inlet
ducts. Field dispensing systems require greater air-moving capacity than those
for greenhouse or laboratory chambers. Larger initial pollutant concentrations
in cylinders or at the generator are usually required to compensate for the
high volume of air moving through field systems.
Generalized pollutant control systems have also been described by Heagle
and Philbeck (1978). The type of control may depend on the experiment objec-
tive: a single static set concentration may be used; several set concentra-
tions varying with time may be used in one or more chambers; or gradually
changing dynamic concentrations may be used. A simple control may consist of
solenoid valves which are turned on by a microswitch controller mounted on a
recorder. A set of concentrations is added and once flow rates and pollutant
concentrations are adjusted manually to near the desired levels, the method is
satisfactory. A more elaborate system, in which valves open a proportional
amount, uses "servo" valves to maintain the concentration more smoothly over
time. Programmable control systems are available for dynamic dose-response
studies. A monitoring instrument and a programmed exposure regime are compared
electronically and the volume of pollutant injected into the chamber is varied
accordingly.
-------
Table II-l. Comparisons of analyzers with respect to major performance parameters and procurement cost
(from Burmann and Rehme, 1978).
TYPICAL OZONE/OXIDANT ANALYZERS
Analyzer
Parameter
Colorimetric
Gas-Phase
Chemiluminescent
Gas-So nd
Chemiluminescent
UV Photometric Electrochemical
Lower Detectable
Limit
Specificity3
Stability^
Response Time
Working Range
Cost
10-20 ppb
low
low
< 5 rain
up to 10 ppm
3-6 K
< 1-10 ppb
high
high
< 1 min
up to 2 ppm
3-6 K
< 1-10 ppb
high
high
< 1 min
up to 2 ppm
> 6 K
< 1-10 ppb
high
high
< 1 min
up to 10 ppm
3-6 K
10-20 ppb
low
low
< 5 min
up to 10 ppm
1-3 K
TYPICAL S02 ANALYZERS
Analyzer
Parameter
2nd
Conducti- Flame Derivative UV Pulsed UV
metric Voltametric Amperometric Colorimetric Photometric Spectrometric Fluorescent
Lower Detect-
able Limit
Specificity3
Stability**
Response Time
Working Range
Cost
10-20 ppb
moderate
low
< 5 min
up to
10 ppm
1-3 K
10-20 ppb
moderate
low
< 5 min
up to
10 ppm
1-3 K
10-20 ppb
moderate
high
< 5 min
up to 2 ppm
3-6 K
10-20
high
low
ppb < 1-10 ppb
moderate
high
< 5 min < 1 min
up to
3-6 K
4 ppm up to
10 ppm
3-6 K
10-20 ppb
high
high
< 5 min
up to 2 ppm
> 6 K
< 10 ppb
moderate
high
< 5 min
up to 5 ppm
> 6 K
TYPICAL NOX ANALYZERS
Analyzer
Parameter
Voltametric
Amperometric
Colorimetric
Chemi1uminescent
Lower Detectable
Limit
Specificity3
Stabilityb
Response Time
Working Range
Cost
10-20 ppb
low
low
< 5 min
up to 10 ppm
1-3 K
10-20 ppb
low
low
< 5 min
up to 12 ppm
3-6 K
10-20 ppb
high
low
> 5 min
up to 2 ppm
3-6 K
< 1-10 ppb
low
low
< 1 min
up to 10 ppm
< 6 K
3 Specificity: high — < 10% error from species commonly encountered in ambient air; moderate -- scrubber
required to eliminate interferences; low — scrubber may not eliminate interferences under all condi-
tions, and/or data corrections required based on concurrent measurements.
b Stability: high — meets EPA Reference and Equivalent Method specifications; moderate — may be operated
without significant drift for 1-2 days; low ~ requires daily zero/span adjustment.
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The technology available for pollutant dispensing and control is changing
rapidly, and new, more efficient, precise systems are becoming available. For
example, the use of a combination of a microcomputer and electronic flow
controller can provide complex dynamic concentration regimes in both outdoor
and indoor research facilities (Hogsett et jfL, 1985; personal communication).
A varied array of pollutant control systems has been described, usually in
conjunction with descriptions of exposure systems. More details and references
are included with facility descriptions in the appendices.
5. Environment Monitoring
Detailed and complete measurements of the environment are needed because_
environment interacts with pollutant response and because of the differences in
exposure facility designs, surrounding environmental conditions, and cultural
practices in different facilities (Mandl et _al_., 1973; Heagle and Philbeck,
1978; Tibbitts, 1978; Olszyk et jfL, 1980; Unsworth et a/L, 1984a,b). Despite
efforts to standardize the environment, existing differences can create signif-
icant variations in irradiance patterns, airflow patterns, atmospheric composi-
tion, temperature patterns, and other factors which may affect pollutant
response. Inadvertent environmental stresses created by subtle factors such as
water and nutrient supply, excessive vibration or wind action, and unplanned
contaminants also may adversely affect plant response. Thorough environmental
measurements and detailed reporting will help identify interactions affecting
response variables.
Guidelines providing a common basis for the measurement of environmental
parameters promote a uniform, accurate means of reporting data and results from
air pollution experiments. Such guidelines have been prepared by a panel of
plant scientists and published in several journals including HortScience, a
journal published by the American Society for Horticultural Science (Krizek and
McFarlane, 1983) (see Appendix H). Although these guidelines were prepared for
research in greenhouses and growth chambers, they should apply equally well to
intensive field studies.
6. Data Acquisition
Data acquisition is either manual through use of recorders and notes, or
automated through use of a datalogger or interface/computer system. Manual
data acquisition is labor intensive and conservative in terms of retention of
all continuous data traces. However, reduction and averaging of data for
desired periods, e.g., 1-hour ozone concentrations, is subjective for each
worker.
Automated systems with dataloggers facilitate storage of large amounts of
data from a variety of monitors in a form amenable to computer processing and
reduction. Computerized sysems using an instrument interface and microcomputer
allow for on-line storing, processing, and retrieval of pollutant and environ-
mental data. Programs have been developed.at several sites to present a
continuous printout of both summary tabular and graphic data.
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Use of a computerized system further facilitates control of pollutant
dispensing via signal feedback to ozone generators, solenoids, critical
orifices, flow controllers, or other devices. The pollutant dispensing can be
controlled to provide specified exposure profiles ranging from continuous
constant added concentrations to proportional increases based on ambient
concentrations.
Automated data acquisition systems generally are programmed to save only
average concentrations for longer periods of time, typically 1 hour. This
permits storage of continuous data for longer periods than if short averaging
times, i.e., minutes, are used. However, peak concentrations of short duration
are masked by longer averages, even though peak concentrations may be biologic-
ally significant. More sophisticated acquisition systems may be able to retain
both average and peak concentrations.
B. Wet Deposition Systems
Irrespective of the meteorological form of wet deposition or the location
of the facility in a controlled or field environment, most systems have common
structural components providing for water purification and conditioning,
simulant stock solution storage, simulant mixing and flow control, droplet
generation and dispensing, presentation/display of vegetation to the hydro-
meteor, and characterization of the deposition rate and hydrometeor chemistry.
The uniqueness of the facilities for simulating rainfall, mist/cloud water, or
aerosols lies in the selection of methods for droplet generation and dispens-
ing; most of the other components are common to all systems. Consequently, the
facilities will be discussed according to the form of the hydrometeor being
generated. The concerns of the degree of environmental modification and
control resulting from the exposure system employed is the same as for dry
deposition exposure facilities. More detailed descriptions of these faciltiies
are provided in Appendix F. The issue of snowfall deposition is not addressed
in this document.
1. Rainfall Simulation Indoors
Irrespective of whether rainfall simulation is conducted in a controlled
environment or under field conditions, only two methods are commonly used. In
the first, rainfall is injected into the atmosphere via a nozzle that disasso-
ciates the liquid into a range of droplet sizes depending on the configuration
of the nozzle and pressure of the feed solution. Typically, these systems have
one or several nozzles arranged to deliver droplets in a uniform pattern. Many'
controlled environment systems use multiple nozzles of similar or different
configuration to provide either more complete coverage and/or a range of
droplet sizes within a single event. There are alternative methods for droplet
generation (e.g., fixed orifice size) which are discussed in the section on
pollutant dispensing and control. None of the rainfall simulators in con-
trolled environments use enclosures, so that the environment reflects that of
the glasshouse or growth chamber.
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2. Rainfall Simulation Outdoors ,
While the most common method of generating rain in field situations is
identical to that in controlled environments, i.e., droplet generation via
nozzle, the uniqueness of the different types of field exposure systems lies in
their methods of addressing incident ambient rain and the protocol for the
distribution of simulated rain. The use of enclosures (shelters), either
continuously or intermittently, may inadvertently modify the plant's micro-
climate either during the exposure event itself or subsequently (i.e., non-
exposure environment). Techniques that apply simulated rain in excess of that
provided naturally do not suffer from the same problems as those induced by
enclosures, but, alternatively, they must evaluate the influence of additional
water and chemical inputs on the system's microclimate and edaphic properties.
Techniques that apply simulated acid rain in excess of that provided naturally
do suffer from the inherent problem of uniform distribution of applied simu-
lated rains, particularly in windy climates. This may prove to be a major
limitation at high elevations where high winds are common.
Rainfall exclusion is commonly achieved using either permanently-mounted
covers or movable exclusion lids that are activated with an electronic rain
sensor. Permanent exclusion systems create environments that have many
properties comparable to those of a glasshouse (except for the soil component),
and the potential for climate modification must be realized. The method of
automated exclusion uses lids over either small individual plots (e.g., 7.5 m2)
or large surface areas (e.g., 300 m2). Most field systems that exclude ambient
rain (either automatically or permanently) dispense simulated rain on a
prescheduled basis (scheduled addition) rather than simultaneously with the
event and under the same environmental conditions (automated addition). The
most common protocol calls for scheduled additions either early in the morning
(0600-0900 h), at night, or on cloudy days. Field exposure systems are
documented that provide automated methods of exclusion and application in a
realistic and simultaneous fashion with respect to ambient rainfall events.
The selection of automated versus scheduled additions of rainfall should be
based on the experimental objectives. Scheduled additions may provide greater
control in evaluating the effects of wet deposition on terrestrial vegetation.
Methods of simulating rainfall in excess of ambient rain (shelterless
plots) are many and include both small (e.g., 7 m2) and large (e.g., 625 m2)
plot simulations. The techniques are applicable to both forested and agricul-
tural landscapes. Droplet generation and dispensing are achieved by either a
system of distributed nozzles or a rotating boom that distributes rain droplets
along a radial arm that circulates around the plot providing rain at intermit-
tent intervals. . .
3. , Mist/Cloud Water Simulation
The most common method for delivering wet deposition in mist/cloud water
form is similar to that for rainfall and uses specific "nozzles or rotating ;
discs to create droplets of the size common to cloud water (5-50 urn dia.).
Unlike rainfall systems, most cloud water exposure systems require enclosures
to achieve atmospheric hydrometeor concentrations and/or deposition rates that
realistically mimic ambient. The enclosure size is limited by the ability of
2-14
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the dispensing system to saturate the atmosphere. The use of enclosures may
significantly modify the plants' microclimate in a fashion similar to that
observed for rainfall systems. The issue is less acute if the enclosures are
used only during cloud water events. One common problem in cloud water systems
is the absence of a method for generating the inertia! impaction necessary to
simulate realistic deposition/interception rates by canopies.. Some special
aspects of the cloud water/plant interactions can be addressed using wind
tunnels, notably the biological (e.g., canopy structure) and physical (e.g.,
wind speed) factors governing foliar interception of cloud droplets. It is
important in mist/cloud water systems to characterize deposition rates to
various components of the canopy.
4. Aerosol Simulation Systems
The method of simulating aerosols requires droplet sizes < 1.0 m in
diameter and a well controlled environment with respect to temperature and
vapor pressure. Unlike mist/cloud water exposure systems, high wind speeds are
not required to generate realistic deposition rates. Aerosol generation can be
achieved using specifically designed impingement nozzles. Methods of charac-
terizing the atmospheric chemistry and deposition differ significantly from
those used for simulators of rainfall and cloud water. As with cloud water
systems, it is important to characterize precisely both atmospheric chemistry
and foliar deposition rates if the system is to provide useful information
regarding the response of forest species.
5. Simulation of Both Wet and Dry Deposition
Only a few systems provide automated control of both wet and dry deposi-
tion; examples are available for both controlled environments and field condi-
tions. The most common field systems use: (1) a zonal air pollution exposure
system in combination with rainfall simulation-without excluding natural rain
(i.e., shelterless plot); (2) open-top chambers amended with automated systems
for rainfall exclusion and real-time simulation; and (3) forced air exclusion
of ambient gaseous air pollution (via duct system) combined with exclusion of
natural rain and subsequent real-time rainfall simulation.
6. Pollutant Monitoring
The methodology for monitoring wet deposition pollutants must recognize
the need to estimate both the rate of hydrometeor deposition and interception,
as well as the chemical composition of the hydrometeor. While the methods for
characterizing deposition are.hydrometeor-dependent (i.e., function of hydro-
meteor size and mass), the methods for chemical analysis are generally common
among all forms of wet deposition and involve standard analytical procedures.
The methods of characterizing hydrometeor deposition will be,discussed first.
With respect to rainfall collection either in the field or in controlled
environments, the most common method used in wet deposition research is to
assume a uniform distribution of simulant volume across the surface area
receiving rain so that deposition rates are inferred rather, than measured. In
those systems in which deposition was actually measured, the"coefficient of
variation ranged from 5-20% in .systems with rotating platforms (e.g., Chevone
2-15
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et al., 1984). The degree of variation of hydrometeor deposition in systems
Us'i'ng" fixed platforms will be at least as great. The recommended method of
collecting rainfall is analogous to that used in field settings (i.e., a
wet-only collector) particularly if the collection will subsequently be
analyzed chemically. The most suitable container is plastic (polyethylene),
positioned to collect incident rain simultaneously with exposure of vegetation
(Shriner, 1972).
The monitoring methods used in studies of mist/cloud water deposition are
not well documented. Many systems distribute mist via a manual dispenser
"uniformly" over all vegetation and calculate by inference a deposition rate in
mm (per leaf area). Empirical approaches in simulation studies are encouraged
to include: (1) collection of both incident and throughfall components; and
(2) gravimetric analysis of vegetation before and after cloud water events with
subsequent deposition rates calculated for foliar surface area. Under field
conditions, the collection method depends on the advective component of the
precipitation event. In areas where inertial impaction is the major factor
governing hydrologic input (e.g., high elevation forests immersed in clouds), a
passive water collector can be used (Lovett _et _aj_., 1982), in which cloud water
is impacted on lengths of monofilament nylon line strung sloping in a plastic_
frame. The impacted droplets coalesce, run down the line, and are collected in
a bottle at the base for subsequent chemical analysis. Given the collection
rate per surface area of line, wind speed, and capture efficiency of the line,
the cloud's liquid water content can be estimated. Lovett (1984) estimated
that the uncertainty in the collection efficiencies of this technique is + 25%.
Active methods of collecting cloud water/mist are more commonly used in
radiation fogs and mists in which deposition to tree species is not governed by
inertial impaction. Using forced air (either positive or negative pressure), a
defined rate (1/min) of mist-laden air is impacted on teflon line or a tube;
the droplets coalesce, run down the line, and are collected in a bottle for
subsequent analysis (e.g., Global Geochemistry Corporation, Fog Samples).
These systems are also subject to error, most likely related to the less than
complete collection of large droplets.
Suspended aerosols/particles are commonly collected by a combination of
three techniques: filtration, impaction, and impingement, with most field
applications using filtration and impaction. With due consideration to
collector design, the analysis can focus on the particle size distribution of
the aerosol/particles. The problems associated with collecting aerosols are
not trivial and in general are a greater source of error than that associated
with the chemical analysis (Lindberg and Mclaughlin, 1986). The same conclu-
sion should be extended to both rainfall and cloud water/mist.
Irrespective of the form of wet deposition, the following assumptions
should be empirically evaluated for any laboratory or field-based exposure
system:
1. inferred deposition rates (calculated based on simulant volume
dispensed and platform surface area) are accurate;
2. distribution of deposited hydrometeor is uniform in space and time;
2-16
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3. collector efficiency (particularly important for mist/cloud water and
aerosols).
In
practice, few systems satisfy these requirements.
An equally important parameter to characterize in pollutant monitoring is
the site of hydrometeor deposition within the canopy. The observation that
nearly all of the deposition in the form of mist/cloud water is in the upper
50% of the forest canopy in the high elevation forests of North America
(Lovett, 1984) indicates that deposition is not uniform throughout the canopy.
Current field techniques for characterizing patterns of within-canopy deposi-
tion rely on direct measurement of throughfall and stemflow during cloud water
immersion coupled with fog droplet interception models that estimate sites of-
deposition in the canopy depending on leaf area index, wind speed, and droplet
size (Lovett, 1984). In a controlled environment setting, cloud water deposi-
tion can be estimated by throughfall analysis (e.g., Reiners and Olson, 1984)
or gravimetric techniques. For aerosol deposition, gravimetric techniques are
insensitive, and one must resort to a tracer tagging of the aerosol and subse-
quent elution from the foliage (e.g., Thome .et ^1_., 1982).
The chemical analysis of incident hydrometeors is a function of the
experimental design. The relevant classes of components in precipitation for
consideration include pH* conductivity, multiple inorganic cations (H+, Ca+2,
+-3 ra+-J anH Mn+<- \ = *A on-;/-,«r- fcr\ _ -2 kin - ri- ' unn -
Na, K , NH , A
and
) and anions
NOo"
>'y > ""-, i» , iiiifl , r\ i , i c , «MU mi / anvj an luiii VO<->4 , i»Vx > ^ ' >
and P04~°), dissolved trace metals (Pb, Zn, Cu, Ni, Cd, and Cr), reactive
HCOo
a
chemical species (H^Og), organic acids (Shriner, 1979), and solids (Unsworth,
personal communication). Several individual components may require on-site/
rapid analysis prior to preservation (e.g., H^, NH4+, pH). The degree of
variation in the chemistry of simulated rain is not well characterized in space
or time for any type of exposure system (batch or in-line mixing),. In general,
most systems rely on select indices of simulant chemistry (e.g., pH, conductiv-
ity) to characterize composition. In fact, only a few facilities routinely
analyze the rain simulant; most assume the composition is accurately reflected
in the recipe.
7. Pollutant Dispensing and Control
The critical features with respect to physically and chemically simulating
wet deposition in the form of rainfall, mist/cloud water, and aerosols are the
following:
a. chemical composition of the hydrometeor;
b. spatial distribution of the hydrometeor;
c.' event intensity or rate (cm/h);
d. event duration (h);
e. frequency of events (e.g., events/wk, if events occur at regular
intervals);
2-17
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f. temporal dynamics of events relative to intermittent periods of
dry deposition if events are distributed in either a stochastic
or time-series fashion;
g. distribution of hydrometeor sizes (mm dia);
h. terminal velocity of hydrometeor (m/s); and
i. changes in chemical composition within and among events.
The components involved in hydrometeor dispensing and control in wet
deposition systems commonly focus on the hydrometeor itself rather than the
chemical species in solution. The conditioning of makeup water is typically
achieved using deionization and/or distillation of process or tap water.
According to a recipe of desired chemical composition, stock solutions are
mixed in concentrated form and subsequently stored. Dilution is achieved
manually or through an automated mixing and control system that proportions the
volume of deionized water relative to that of the concentrated simulant. This
mixing and dilution is achieved either by the batch or in-line mode, the latter
requiring a higher degree of quality assurance. The actual dispensing is
achieved through nozzle injection or droplet release from an orifice (e.g., a
distributed array of needle tips). To minimize non-uniform distribution of the
hydrometeor, many systems place the plants on a rotating platform.
Both dispensing techniques (nozzles and needle tips) operate under
positive pressure and are flexible enough to accommodate most of the important
features of rainfall simulation. Nozzles disassociate the simulant solution
into droplets, the size of which depends on the nozzle specification and feed
solution pressure. Most systems inject droplets upward, allowing the hydro-
meteors to fall by gravity. This technique facilitates a realistic terminal
velocity of the droplet and usually promotes a more uniform distribution. The
method of release of simulant under pressure through a series of openings along
a boom or hub has been successfully used in both field (e.g., Abrahamsen et
al., 1977) and laboratory (e.g., Chevone et _al_., 1984) studies. The selection
"of needle tips with defined small openings can effectively control droplet
diameter, although the tips must be geometrically distributed to achieve
uniform dispensing along the length of the boom due to in-line pressure drops.
In either case, control of dispensing rate is provided through the
regulation of flow rate and pressure. Programmable variable-speed motors can
be used in conjunction with flow controllers to regulate the intensity of rain
and intermittent frequency of events. Under field and laboratory conditions,
most systems dispense simulants in a pre-scheduled protocol providing a fixed/
constant duration of events and intervening dry periods. Control or computer-
assisted systems for field application have been documented to simulate
rainfall deposition to shielded plots on a real-time basis by providing on-line
feedback to match deposition rates between ambient and simulated plots (e.g.,
Johnston et al., 1986). This matching is provided by a tipping bucket rain
gauge that sTgnals a control whenever the rain increment exceeds a set point.
2-18
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8. Environment Monitoring
The degree of environmental monitoring in wet deposition studies is
usually limited to the evaluation of light, temperature, and photoperiod in
controlled environments and light, temperature, and gaseous pollutant level
(i.e., ozone) in field systems. The majority of monitoring data is reported as
a single season mean value without a corresponding dispersion statistic or
analysis of temporal dynamics. There are notable exceptions to this practice
depending on the individual researcher's interest. Efforts to characterize
those environmental factors governing the residence time of droplets on the
leaf surface (i.e., temperature, vapor pressure, light) should be a high
priority.
9. Data Acquisition
Few systems include data acquisition features, and the only documented
examples are those from field exposure systems using either open-top chambers
(e.g., Johnston et aj_., 1986) or track exclusion systems (Kuja et. aJL, 1985).
These field systems have acquisition capabilities comparable to those described
in the section on field exposure systems for dry deposition research. The only
unique features relate to the wet deposition environment and include records of
the ambient rainfall rates and times, corresponding deposition data in simu-
lated plots, duration and frequency of rainfall events, simulant dispensing
rate, and the status of the various reservoirs for storage of stock solutions.
It is suggested that acquisition capabilities be extended to characterize soil
water potential, leaf surface wetness, and some technique for monitoring
simulated rainfall distribution and chemistry in treated plots. Most systems
use a personal computer (e.g., Apple He, IBM AT) with either "off the shelf"
or personalized software packages.
2-19
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-------
III. Criteria for Evaluating Exposure System Performance
Irrespective of the deposition mode (i.e., wet or dry), the facility
location (i.e., outdoors or indoors), and/or the study objective, generic
criteria can be specified to evaluate the performance of any exposure system.
Criteria were identified which convey in a quantitative sense the "realism" in
exposure systems with regard to chemical and physical features of the atmos-
phere (including the pollutant), physiological and growth processes of the
plant, and physical and chemical properties of the soil. The criteria are
listed in Table III-l and represent aspects that should be considered from the
widest point of view. The intent is to assure that the exposure systems: (1)
select and characterize the pollutant exposure regime in the atmosphere with
respect to those occurring under ambient conditions (in both a chemical and a
temporal fashion) or in a desired fashion; (2) select and characterize the
environment (during exposure and non-exposure periods) with respect to the
ecological and physiological requirements of the species; and (3) provide an
environment that generates pollutant deposition rates and patterns that can be
compared to those occurring under ambient conditions in agricultural or
forested landscapes. The relative importance of the various criteria will
depend on the objective of the research task and the capabilities of the
exposure system. Conseauently, the criteria should not be applied indiscrim-
inantly to all systems. Moreover, the selection of relevant criteria for even
a single exposure system will differ as a function of experiment-specific
objectives.
The criteria for evaluating a system's performance fall into three general
categories: (1) pollutant chemistry in the atmosphere; (2) the edaphic,
climatic, and atmospheric environment both during exposure and non-exposure
periods; and (3) the biological attributes of the plant.
A. Pollutant Chemistry in the Atmosphere
The principal concerns with dry .deposition facilities relate to the
chemical's concentration on both a spatial and temporal scale (Table III-l).
Within a given exposure system, it is important to characterize the distribu-
tion of the pollutant within the canopy. While some systems foster instantane-
ous mixing throughout the canopy (e.g., CSTR), others promote pollutant
gradients, in either a downward (down-draft chambers) or an upward (open-top
chambers) direction. Depending on the degree of turbulence and canopy closure
(either within or among chambers), pollutant distribution on a horizontal plane
may vary significantly. With respect to temporal exposure patterns, it is
important to characterize the gas concentration dynamics as a function of the
exposure period relative to the plant's daily physiological functions (e.g.,
stomatal conductance) and seasonal phenology. These features include time of
exposure relative to the time of photoperiod initiation and the growth stage of
the plant. Equally important are characterizations of the duration of exposure
and the stochastic nature of exposure events and intervening respite periods.
This criterion may require statistical characterizations that convey the
episodic/temporal dynamics of exposure patterns rather than simply a mean
concentration.
3-1
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Table III-l.
Potentially relevant criteria to use in evaluating .exposure
system performance. Criteria will vary depending on deposition
mode and experiment design.
A. Chemical Properties of the Pollutant in the Atmosphere
1. Dry Deposition
a. Spatial Features
(1) Concentration Distribution (Uniform versus Non-Uniform)
(a) Vertical Plane (e.g., Profiles)
(b) Horizontal Plane (e.g., Patches)
b. Temporal Features
(1) Exposure Profiles-
(a) Square Wave
(b) Time Series
(2) Daily Exposure Regimes
(a) Diurnal
(b) Noctural
(3) Seasonal Exposure Regimes
(4) Annual Exposure Regimes
(a) Growing Season
(b) Winter Season
(5) Episodes
(a) Episode Duration
(b) Respite Duration
(c) Stochasticity/Periodicity of Events
2. Wet Deposition
a. Solution Chemistry (Pre-Droplet Formation)
(1) Cations
(2) An ions
(3) Organics
(4) Reactive Chemical Species
(5) Acidity/pH
(6) Conductivity
b. Incident Hydrometeor Chemistry [same chemical species as (a) above]
(1) Spatial Features
(2) Temporal Features
(a) Exposure Profiles
(b) Daily Exposure Regimes
(c) Seasonal Exposure Regimes
(d) Annual Exposure Regimes
(e) Episodes
c. Incident Hydrometeor Physics
(1) Spatial Features
(a) Horizontal-Plane Distribution
(b) Vertical-Plane Distribution
(2) Temporal Features
(a) Rate
(b) Duration
(continued)
3-2
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Table III-1 (continued)
(c) Frequency
(d) Time of Day/Season
(e) Inter-Event Features
d. Incident Hydrometeor Physical Features
(1) Droplet Size Statistics
(2) Terminal Velocity
(3) Chemical-Specific Deposition Rates
(a) Spatial Distribution
(b) Temporal Distribution
(1) Event
(2) Seasonal
(3) Annual
3. Combination of Pollutants (Dry/Dry, Wet/Wet, Wet/Dry)
a. Characteristics of Co-Occurrence
b» Characteristics of Independence
c. Relationship Between Wet Deposition Event and Antecedent Dry
Deposition Period
B. Physical and Chemical Features of the Environment
1. Atmosphere
a. Radiation
(1) Spectral Quality, Including the Ultraviolet Component (PAR)
(2) Quantity
b. Temperature/Moisture
(1) Air Temperature
(2) Leaf Temperature
(3) Leaf-to-Air Differentials (vapor pressure deficit)
(4) Dew and Frost Formation
(5) Thermoperiod
c. Turbulence and Mixing
(1) Canopy Aerodynamic Resistance to Turbulent Transfer
(2) Leaf Boundary Layer Resistance to H?0
(3) Wind Speed
d. Turnover Time of Air Reservoir
e. Ambient Hydrometeor Deposition
f. Trace Gases
(1) Carbon Dioxide
(2) Ethylene
(3) Water Vapor
2. Soil
a. Chemistry
(1) Cation Exchange Capacity
(2) Partial Pressure of Oxygen
(3) Soil Solution Chemistry
(4) Soil Nutrient Analysis
3-3
(continued)
-------
Table III-l (continued)
b. Physics
(1) Temperature
(2) Water Potential
(3) Porosity/Solution Infiltration.
C. Biological Features of the Environment
1. Vegetation
a. Structure
(1) Leaf Area/Leaf Area Index
(2) Canopy Architecture
(3) Leaf Developmental Stage/Phenology
(4) Canopy Height Relative to Chamber
(5) Canopy Girth Relative to Chamber
(6) Belowground Root Architecture
(7) Mycorrhizae
b. Function
(1) Net Photosynthesis
(2) Transpiration
(3) Leaf Water Potential
(4) Leaf Conductance to H20
(5) Growth Rates
(6) Biomass Partitioning Above- and Belowground
(7) Chlorophyll Content
2. Deposition of Pollutants
a. Wet Deposition
(1) Foliar Interception of Hydrometeor
(a) Horizontal-Plane Variation
(b) Vertical-Plane Variation
(2) Foliar Retention of Hydrometeor
(3) Chemical Processing of Intercepted Hydrometeor
(a) Throughfall Chemistry
(b) Stemflow Chemistry
(4) Deposition to Soil Surface
(5) Chemical Processing of Hydrometeor Chemicals in Soil/Litter
b. Dry Deposition Rates and Amounts
(1) Deposition to Individual Leaves
(a) Leaf Surface
(1) Dry Surface
(2) Wet Surface
(b) Leaf Interior
(2) Deposition to Canopy
(3) Deposition to Chamber Walls/Surfaces
(a) Wet
(b) Dry
(4) Deposition to Soil/Litter Surface
— : ~~ (continued)
3-4
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Table IH-1 (continued)
D. Hardware
1. Dry Deposition
a. Chamber Air Handling
(1) Air Exchange Rate -- Turnover Time
(2) Air Movement -- Recirculation, Single-Pass
(3) Equilibration Time
(4) Direction of Air Flow
(5) Air Pressure
(6.) Air Leakage
(7) Air Filtration
b. Chamber Characteristics
(1) Pollutant Sorption
(2) Size
(3) Volume-to-Leaf Area Ratio
c. Pollutant Concentration
d. Environment
(1) Monitoring
(2) Control
e. Data Acquisition
2. Wet Deposition
a. Hydrometeor Generation
(1) Makeup Water Conditioning
(2) Generation of Droplets
(3) Distribution/Dispensing
(4) Monitoring/Control
b. Environment
(1) Monitoring
(2) Control
c. Data Acouisition
3-5
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For wet deposition systems, criteria should address the physical form and
chemical composition of the incident hydrometeor (Table III-l). It is impor-
tant to recognize that analyses must be conducted on the incident hydrometeor
— rain, mist/cloud water, and aerosol -- in addition to the stock solution.
Equally important is a rigorous analysis of deposition rate ,and, at least in
some situations, an evaluation of the deposition site. This aspect is critic-
ally important in systems designed to simulate cloud .water and aerosols. As
with dry deposition, the event1s temporal dynamics must be characterized with
respect to duration, intensity, and frequency. Of lesser importance is an
analysis of physical features characterizing droplet size and velocity.
Depending on the experimental design, it may be necessary to evaluate the
chemistry of the hydrometeor after deposition either on the leaf surface
(droplet evaporation rate) or in throughfall/stem'flow.
those systems providing pollutant mixtures either in the form of gases
5 plus wet deposition, not only must the individual pollutant regimes be
In
or gases . . . . .
characterized, but their temporal relationships evaluated with respect to the
ambient environment. This requires attention focused on exposure protocols
that convey the characteristics of pollutant co-occurrence, characteristics of
pollutant independence, the relationship between wet deposition events and
antecedent periods of dry deposition, and the phenology of the. plant.
B. Physical and Chemical Features of the Environment
These criteria address the climatic, edaphic, and atmospheric environment
during both exposure and non-exposure periods (Table III-l). It is critically
important to recognize the potential for inadvertent, modification of the
plant's microenvironment by the exposure system'sju.cn that the plant's response
to the air pollutant is not typical nor is its interaction with other biotic
(e.g., pathogens) and abiotic (e.g., drought, winter temperature) stresses.
Attention must focus on both the aboveground and'belowground environment of the
plant. ., .
Most exposure systems influence both physical and chemical properties of
the atmospheric environment; the degree of modification varies not only among
facilities of dissimilar design but also within a given facility as a function
of time of year, time of day or night, and canopy architecture and biomass.
The most relevant physical properties include: (1) the radiation balance
(spectral quality and quantity of incident light) and photoperiod; (2) heat and
moisture relationships (e.g., thermal gradients, leaf/air temperature, dew and
frost formation, vapor pressure deficit); (3) turbulence and mixing of the air
reservoir which affects the canopy's aerodynamic resistance to turbulent
transfer, latent energy exchange, individual leaf boundary layer resistance to
diffusion of gases, and residence/turnover time of the air reservoir; (4) wind
speed; and (5) normal wet deposition processes as rain or cloud water. It must
be recognized that these changes in a plant's microclimate are due to imposi-
tion of an exposure system and may be perpetuated even after the system is
removed.
The potential changes in the chemical properties of the atmosphere are
equally important. Exposure systems may inadvertently influence the concentra-
tion and distribution of physiologically-important gases such as carbon
3-6
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dioxide, water vapor, and trace contaminants. Depending on the quality of the
source materials used to generate pollutants or provide enclosures, as well as
the residence time of the air reservoir in the enclosure, plants may inadvert-
ently experience elevated levels of trace gases (e.g., ethylene) that affect
physiological and growth processes (Morison and Gifford, 1984). Regarding wet
deposition facilities specifically, the deposition of rain in excess of ambient
may significantly affect the systems' performance. The physical and chemical
modifications of the environment with excess wet deposition warrant evaluation
with particular attention focusing on leaf surface wetness and edaphic proper-
ties, including soil water potential, partial pressure of oxygen, and soil
solution chemistry. The use of lysimeters for soil water sampling at various
depths in the profile is strongly encouraged.
C. Biological Attributes of the Plant
Biological (plant) factors are of great importance in the selection
process for a suitable exposure system (Table III-l). The biological objec-
tives (e.g., strain testing, physiological studies, dose-response determina-
tions) may dictate the precision required in the research facility. The nature
of the plants themselves will determine such aspects as size of facility and
may affect the levels of both chemical and physical factors of the experimental
environment.
This is particularly relevant to woody plant species whose growth charac-
teristics (i.e., growth rate, periodicity in the initiation/elongation of new
foliage, the substantial photosynthate allocation to structural and storage
tissues in the aboveground and belowground tissues) render insensitive many of
the more common indicators used in agricultural settings (e.g,. degree of
canopy closure, leaf/plant stage of development, anthesis, podfill). If the
biological status of the plant differs markedly from that in more natural
landscapes, the data are of less merit in efforts to predict how woody plants
will respond to air pollution stress under field conditions. It is entirely
possible that, while the exposure system may meet the established criteria for
physical and chemical performance, it may be unacceptable based on the biolog-
ical criteria.
The selection of biological criteria focuses on two aspects of plant/
environment interactions: the rate of growth and associated physiological
activity in both the root and shoot, and the foliar uptake of trace gases or
interception of hydrometeors on vegetation surfaces (Table III-l). Examples
exist in the published literature in which: (1) the physiological/morpholog-
ical factors governing deposition fostered abnormally high or low pollutant
fluxes; or (2) the rate of carbon gain in control plants only slightly exceeded
the allocation of photosynthate to maintenance costs (i.e., no net growth). In
either case, the presence or absence of statistically significant effects
becomes biologically irrelevant.
The functional criteria with which to evaluate a system's and a plant's
performance with respect to biological features include photosynthesis,
respiration, transpiration, leaf water potential, components of leaf conduct-
ance to water vapor, and chlorophyll content. Foliar gas-exchange rates should
be normalized to projected leaf area for broadleaf species and needle dry
3-7
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weight or area for conifers (i.e., rates for performance evaluation purposes
should not be reported on a per plant basis). The evaluation should be
species- and environment-specific, using as a guideline values that are within
+ 10% of the corresponding rate in open air/companion plots. In controlled
Tndoor exposure systems, rates of net photosynthesis and transpiration should
be >_ 70% of those reported in more natural settings (Table 111-2). These
criteria assume that any influence of the exposure environment on foliar
gas-exchange processes causes a proportional change in the allocation of
photosynthate to structure, storage, and maintenance sinks. If that assumption
is not realized, the applicability of the data is questionable.
Leaf conductance to water vapor is a measure of foliar porosity to the
diffusion of gases. Boundary layer conductance should be >_ 200 pjnol/cnr/sl
(i.e., ca. 0.2 s/cm resistance), while stomatal conductance will vary as a
function of species, environmental conditions, and phenology. Reasonable
values under optimum growing conditions in the light range from 4-20 pjnol/cm2/s
for broadleaf species and 1-4 nmol/cm^/s for conifers (Table III-2). Substan-
tially higher conductances to water vapor are characteristic of herbaceous
species grown under optimal conditions (Table II1-2).
Leaf water potential is a direct measure of the plant's water relations
and is particularly important in exposure systems that either maximize canopy/
leaf ventilation rates (e.g., open-top chambers, forced air exclusion systems)
or simulate wet deposition. Estimates of leaf/twig/needle water potential
should characterize both predawn and midday potentials at intervals throughout
the season with more frequent sampling during periods of low soil water
availability. Chlorophyll content may relate to photosynthetic capacity and
the progression of foliar senescence and chlorosis/necrosis, and thus this
criterion may serve as an indicator of plant vigor.
In woody plants, the selection of features to characterize growth rate
should focus not only on leaf/needle initiation and elongation/expansion, but
the allocation of photosynthate to structure and storage tissues. Reasonable
rates of dry matter accumulation normalized for leaf area per day are 0.1-0.2
ng/cm^/d for woody species and an order of magnitude higher for herbaceous
species (Table III-2). Attempts should be made to assure that the chamber
environment does not significantly alter the allocation patterns. This should
include estimates of biomass in shoot and root tissues, with the latter further
partitioned into large and fine root fractions.
Structural features are particularly important in woody plants, whose
growth rates and architecture are far more variable between and among species
than those of the more genetically uniform, domesticated crop species.
Important aboveground criteria to consider include leaf area/leaf area index
(LAI), canopy architecture and height (relative to the chamber's dimensions),
and canopy girth (relative to the chamber's dimensions). Because the LAI
affects pollutant deposition/interception rates as well as the chemistry of
through-fall and stemflow, this criterion is particularly important in wet
deposition studies under field conditions. Canopy architecture in general and
height specifically may significantly limit the utility of some chamber
systems, because many systems for dry deposition research in the field cannot
adequately restrict the incursion of turbulent eddies into the chamber's air
3-8
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Table III-2. Reasonable rates for select biological criteria as reported under
near optimum growing conditions (Larcher, 1980; Korner et al.,
1979).
Criteria
Agricultural Crops
C Plants
Woody Plants
Broadleaf Conifer
Net Photosynthesis
(nmol/cm2/h)
(timol/g/h)
4.4-8.8
2.2-4.4 0.9-3.3
68-410
Transpiration
(mmol/cm2/h)
1.9-2.2
0.4-0.9
0.5-0.6
Leaf Conductance to H20
(M.mol/cm2/s)
Boundary Layer
Stomate
80-400
20-130
80-400
4-20
1-4
Growth Rate
(ng/cm2/d)
1.0-2.5
0.1-0.2
3-9
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reservoir. Moreover, most wet deposition systems are designed to deliver
droplets in a distributional uniformity that decreases with proximity to the
nozzle.
Belowground criteria include root architecture, mycorrhizal infection and
colonization. Root architecture is particularly important in stress inter-
action studies addressing the role of drought or soil nutrient status because
species and even half-sib families differ substantially in their ability to
explore and extract resources {water and nutrients) from different horizons of
the soil profile. While the significance of mycorrhizae in resource acquisi-
tion in the soil (nutrients and water) and trace metal toxicity is recognized
in woody species, and qualitative and quantitative analysis of mycorrhizal
abundance should be a deliberate criterion of performance in exposure systems.,
it should not be forgotten that the existence of desirable mycorrhizal flora in
the rhizosphere dictates that a significant fraction of the total carbon gain
(as much as 20%) will be diverted away from other s'inks to the fine roots.
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IV. Evaluations
A. Dry Deposition -- Gaseous Exposures
Gaseous dry deposition systems in the field can be categorized into groups
and subgroups based on degree of environmental modification and configuration
of the system.
1. Non-Chamber Plume Systems
Plume systems can be grouped within three categories: grid configuration
(de Cormis et a!., 1975: Lee et jTL, 1975; Runeckles et al., 1981); encircling
systems where th~e entire area is surrounded by emitters TSreenwood et al.,
1982; McLeod et al_., 1985; Roberts, M., personal communication); andTlTnear
configurationTMiller et al., 1980; Moser et al., 1980; Northrop, 1983;
Thompson et a\_., 1984)T~~ ~ ~
Two generic, advantages of the plume systems are: (1) a lack of environ-
mental modifications caused by hardware; and (2) pollutants are dispersed
through the plant canopy the same as in ambient air. The systems result in
variable temporal distribution of pollutant concentrations. General disadvan-
tages are: (1) that no pollutant treatment is possible at concentrations lower
than ambient, thus the effects of ambient pollutants cannot be specifically
identified as they can be with air filtration methods; (2) dispersion of
pollutants is completely dependent on ambient wind patterns and their temporal
nature, resulting in episodic exposures not necessarily representative of
ambient exposure conditions; and (3) exposure plots must be widely separated to
prevent cross-contamination, resulting in practical difficulties in area
requirements for large dose-response experiments.
The grid configured-type plume system has the advantages of: (1) rela-
tively simple system of emitter tubes without a requirement for large blowers
with concomitant high electrical power requirements; and (2) a uniform hori-
zontal distribution of pollutants irregardless of wind direction. Vertical
concentrations, however, are dependent on an appropriate distribution of
emitters at different heights. Grid systems were originally developed without
computer control of pollutant emissions, which required a wide dispersion of
emitters over the plant canopy. The addition of computer control capabilities
has allowed for much simpler arrays of emitters.
Encircling systems have the advantage of providing a uniform distribution
of pollutant over the exposure area by releasing pollutants from any prevailing
wind direction. The current circular and square systems are under computer
control, allowing for rapid feedback control of concentrations, and for monitor-
ing of pollutant concentrations at many points in the exposure area for precise
determination of pollutant dose to the plant canopy. This type of exposure
system provides a totally ambient pattern of pollutant transport over the plant
canopy without any environmental modification. However, the system disadvan-
tagejs the requirement for a large surface area for pollutant dispersion.
Vertical pollutant profiles have not been measured for most systems. A
computer and instrument interface system is required for all exposures.
4-1
-------
Linear configuration plume systems provide a range of pollutant concentra-
tions over a relatively small space for regression analysis of pollutant
exposure and plant response. However, an intensive array of pollutant sampling
points is required to characterize the exposure adequately, and a sophisticated
computer system is required for feedback control of the pollutant release.
Linear plume systems are more dependent on wind direction for pollutant
emission than encircling systems, resulting in more unpredictable pollutant
concentrations and durations of exposure than the other plume configrations.
This characteristic greatly reduces the flexibility of experimental designs and
feasibility of experiment replication. The gradient of exposure concentrations
also may be much greater than that appearing with ambient air, i.e., concentra-
tions are much higher than ambient near the emitters.
2. Non-Chamber Air Exclusion Systems
Air exclusion systems can be grouped within two categories: duct systems
(Jones et^l_., 1977; Kuja et a!., 1985; Laurence et _al_., 1982; Shinn et aJL,
1977; Thompson and 01szyk,T9SSl; and the "gun" system (Spierings, 19F7).
Air exclusion systems permit determination of the effects of ambient air
on vegetation with the capability of adding pollutants to the plant canopy.
Duct systems are midway between plume systems and chambers in terms of sim-
plicity of design, construction, and operation. The ducts themselves are
simple and inexpensive to construct and flexible in terms of length, hole
direction, and row width. However, the blowers generally are of higher
capacity than for chambers, resulting in increased construction and electrical
costs. For relatively short plants, duct systems can provide effective
exclusion or addition when inflated, with little modification of the plant
environment. There are physical limitations to plant row width between the
ducts, and a plant height limitation. Duct systems at present are suitable
only for row crops. Air speed over the plant canopy is high enough to avoid
problems with low leaf boundary layer conductances. The air flow pattern is
below and across the plant canopy, and not necessarily representative of
ambient conditions. Also, there is a large vertical gradient i,n pollutant
concentrations. Concentrations' decline rapidly above the ducts. There
normally is not a large horizontal gradient in pollutant concentrations across
the row, or down the row, unless the hole diameters are specifically designed
to reduce or increase air flow along the length of the ducts.
In general, the environmental conditions between ducts are the same as in
outside areas. However, during the winter, air and soil temperatures are
generally greater between ducts than outside. Light intensity can be higher or
lower between the ducts than outside, depending on the sun angle.
The "gun" air exclusion system was developed specifically to expose
particular branches of trees to air pollutants. The "gun" has not been tested
for pollutant uniformity and no information is available regarding its ability
to exclude ambient air, or its effects on environmental conditions around the
branch or gas exchange by the branch. The "gun" system is not adaptable for
multiple tree exposures, but can give a carefully defined pollutant treatment
to a specific target plant.
4-2
-------
3. Chambers — Outdoor
Field chambers can be grouped within three categories: open-top (Heagle
et_al_., 1973; Mandl et _al_., 1973; Brewer, 1979, 1983; Ashmore et _aj., 1980;
Buckenham et aj., 198T; Brewer, 1983; Fowler D., personal communication; Heck
et jjl_., 19M; Roberts et _al_., 1983; Seelinger et al., 1985; Thompson and
UTszyk, 1981); semi-open-top (Runeckles _et a_l_., 1978; Hogsett et £L, 1985;
Krause, G., personal communication, Olszyk, D., personal communication); and
closed-top (Van Haut, 1972; Keller, 1976; Guderian, 1977; Roberts, 1981;
Musselman et a±., 1985b; Lucas P., personal communication).
The generic advantage of field chambers is the ability to exclude ambient
pollutants, and to enclose an experimental area for an increased range of
treatments at near-ambient environmental conditions. The advantage of having a
filtered enclosure (i.e., chamber) for a treatment range is also the basis for
the generic disadvantage, i.e., the accompanying environmental modification
during plant exposures.
Open-top chambers provide for the least amount of environmental modifica-
tion of any outdoor chamber, coupled with ambient air exclusion of up to 80%.
However, even a small degree of chamber-induced environmental modification
(e.g., increased temperature and decreased solar radiation) may have a signif-
icant effect on plant growth, and this modification may be greatest during the
cooler months. In addition to temperture change, all open-top chambers exclude
at least some ambient rain and some designs can have "rain-shadows" resulting
in variation in rainfall patterns within the chamber. The air flow in most
chambers is across and/or from under the plant canopy which produces a pattern
of air movement atypical of field conditions. Only those chambers with
modifications designed to force air down and over the canopy are fully repre-
sentative of ambient conditions. Open-top chambers have relatively low
variability in pollutant concentrations horizontally across chambers. Vertical
variability also is low if the chambers have some type of frustrum or baffle at
the top to limit ambient air incursion. Smaller open-top chambers with high
air exchange rates have the least deviation from outside air temperature and
relative humidity, coupled with a high air speed over the plant canopy.
Exclusion of ambient pollutants is dependent on the filtration system in
place on the air-intake for the open chambers. Activated, charcoal, commonly
used with most field chamber blowers, does not effectively remove all ambient
air pollutants. Activated-charcoal removes much of the 03, but little of the
S02, NO, or N02 from the air. Chemically treated materials have a much greater
filtering efficiency for a variety of air pollutants (D. Fowlers, 1985,
personal communication). Purafil®, potassium permanganate impregnated alumina
particles, removes much of the ambient N02, S02, and also NO via a chemical
reaction of NO to N02 (D. Mandl, 1986, personal communication). Fresh Purafil®
also will remove ethylene. However, Purafil® does not have a high efficiency
for removing 03. Ideally, a series of filters should be used to remove differ-
ent ambient air pollutants prior to injection of air into the chambers.
However, some chemically treated filtering materials, e.g., brominated char-
coal, are fairly corrosive, thus requiring more resistant and expensive
materials for filter containers.
4-3
-------
Most current open-top field chamber designs have been widely used for
growing season studies with many crop species, but have diameters and heights
which currently limit their use for larger plants. Larger chambers that appear
to provide the necessary volume for a small tree's canopy, have more environ-
mental modification and fewer air exchanges compared to smaller chambers.
Larger chambers may be adequate only for a single or a few trees per chamber,
necessitating numerous chambers for replication within pollutant treatments.
Semi-open-top chambers permit control of the plant canopy atmosphere in
areas where environmental conditions preclude open-top chambers. In particu-
lar, the semi-open-top designs may be best for exclusion of ambient precipita-
tion.
Closed-top chambers allow for total control of the atmosphere in the plant
canopy. Unlike open-top chambers, ambient pollutant exclusion is unaffected by
outside wind speed, and is generally greater than open-tops, but still not 100%
efficient. Vertical and horizontal variability in pollutant concentrations is
lower than with any other system for field studies. Closed top chambers
generally have lower irradiances and greater variability in relative humidity
compared to other chamber types.
A comparative performance evaluation of all groups of gaseous dry deposi-
tion systems in the field is presented in Tables IV-1 and IV-2.
4. Chambers -- Indoors
Most of the indoor exposure chambers used in gaseous dry deposition
research can be categorized into four types: chambers in greenhouses (Berry,
1970; Heck et al., 1978; Hill et _aJL, 1959; Lockyer et al., 1976; Piersol and
Hanan, 1975~Posthumus, 1978); self-contained chambers TAdams, 1961; Aiga et
al. 1984; Hill, 1967; Jensen and Bender, 1977; Menser and Heggestad, 1964;
TJTiva and Steubing, 1976; Payer et al_., 1985; Wood et_al_., 1973); chambers
within chambers (Cantwell, 1968; Heck et al., 1968; McLaughlin et art., 1976;
Payer et al., 1985); and CSTR's (Heck et aT., 1978; Rogers et aT7,T:977). They
have mah"y~characteristies in common, but there are some variations. A perform-
ance evaluation of indoor exposure chambers is presented in Table IV-3. A
partial quantitative assessment based on reported values or characteristics is
presented in Table IV-4.
There are many versions of the chamber-within-greenhouse approach to
indoor gaseous dry deposition research, varying from using entire greenhouses
as treatment chambers to using sets of small chambers or CSTR's within green-
houses. This approach has the advantage of providing large chambers, if
needed, and several chambers, if needed, for concurrent differential-dose
experiments. Partial control of temperature and humidity is provided by the
greenhouse structure and light can be augmented by artificial sources placed
over the chambers. The air movement required to minimize environmental differ-
ences between chambers and greenhouses may be sufficient to minimize leaf
boundary layer resistance and to minimize gradients of environment and pollut-
ants within chambers. Natural cycles and levels of light spectrum and irradi-
ance are provided. This may help meet the objectives of some experiments. The
disadvantages of greenhouse chambers include the marked seasonal and diurnal
4-4
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Table IV-3. Performance evaluation for indoor exposure chambers for gaseous
dry deposition research.
Criteria
A. Chemical Properties
1. Concentration
a. Vertical Distribution
b. Horizontal Distribution
c. Chamber Losses
2. Temporal Patterns/Dynamics
a. Exposure Regimes
b. Capabilities
3. Multiple Pollutant Exposures
4. Non-Pollutant Chemicals
a. Carbon Dioxide
b. Water Vapor (Humidity)
c. Trace Gases
B. Physical Properties
1. Radiation
a. Spectrum
b. Qualitative Features
2. Heat Energy
a. Air Temperatures
b. Leaf Temperatures
c. Gradients
d. Dew and Frost
3. Air Movement
a. Turbulence (Mixing)
b. Air Exchange Rate
c. Chamber Equilibration
d. Direction
e. Wind Speed
f. Overall System
g. Pressure
4. Soil Environment
5. Unit Size
Chambers
in Green-
houses
Y/ya
Y/N
Y/N
Y/N
Y/N
Y/N
Y/N
Y/Y
Y/N
Y/N
Y/N
. Y/Y
Y/N
Y/Y
Y/N
Y/N
Y/Y
Y/N
Y/Y
Y/N
Y/Y
Y/Y
Y/N
Y/Y
Self-
Contained
Chambers
Y/Y
Y/Y
Y/N
Y/N
Y/N
Y/N
Y/N
Y/Y
Y/N
Y/N
Y/Y
Y/Y
Y/N
Y/Y
Y/N
Y/N
Y/Y
Y/N
Y/N
Y/Y
Y/N
Y/Y
Y/N
Y/Y
Chambers
within
Growth
Rooms
Y/Y
Y/Y
Y/N
Y/N
Y/N
Y/N
Y/N
Y/N
Y/N
Y/N
Y/N
Y/N
Y/N
Y/Y
Y/N
Y/N
Y/Y
Y/Y
Y/N
Y/Y
Y/Y
Y/Y
Y/N
Y/Y
CSTR's
Y/Y
Y/Y
Y/Y
Y/N
Y/N
Y/N
Y/N
Y/N
Y/N
Y/N
Y/Y
Y/Y
Y/N
Y/N
Y/N
Y/Y
Y/Y
Y/Y
Y/Y
Y/Y
Y/Y
Y/Y
Y/N
Y/Y
a Important/Characterized, Y = Yes, N = No for this type of system.
4-7
-------
Table IV-4. Performance values for indoor exposure chambers for gaseous dry deposition
research.
Criteria
Number of Different Chambers Described
A. Chemical Properties
1. Concentration
a. Vertical Distribution
b. Horizontal Distribution
4. Non-Pollutant Chemicals
a. Water Vapor (Humidity)
B. Physical Properties
1. Radiation
a. Quantitative Features
2. Heat Energy
a. Air Temperatures
b. Gradients
3. Air Movement
a. Turbulence (Mixing)
b. Air Exchange Rate
c. Direction
(1) Down
(2) Horizontal
(3) Up
d. Overall System
(1) Recirculation
Chambers in
Greenhouses
6
14%a
—
+ 5%
—
+0.38-3. 3 °C
+ 1.1°C
lb
30-120
4b
1
1
1
(2) Partial Recirculation 0
(3) Single Pass-Through
e. Pressure
(1) Negative
(2) Neutral
(3) Positive
5. Unit Size (Dimensions)
a. Smallest
b. Largest
5
I
1
4
61x61x92 cm
11x2.8x2.5 m
Self-
Contained
Chambers
7
+ 3-5%
+ 3-5%
1-10%
+ 10-15%
+0.3-1.0°C
+0.6-2.2°C
0
53-138
3
3
1
3
1
3
3
2
2
122x107x122 cm
3x2.6x2.3 m
Chambers
within
Growth Rooms CSTR's
5
+ 5%
+ 5%
—
—
—
—
0
15-120
2
2
0
1
0
4
2
0
2
152x107x122
10x10x2.5 m
2
~ 0%
~ 0%
+ 3.6%
+0.5-2.8°C
—
2
Variable
—
—
—
0
2
0
2
0
0
cm 81x137 cm
152x180 cm
a Variation or range of variation reported for one or more different chambers.
b Numbers of different chambers with this characteristic.
4-8
-------
light patterns, and the shading by superstructures. There are likely to be
gradients of temperature and humidity; the air movement required to minimize
these may be excessive for plants. Temperature and humidity will vary accord-
ing to the greenhouse conditions. A lack of repeatability due to changing
seasonal environments may interfere with replication of experiments. Experi-
mental designs must consider replication in both time and space.
Self-contained chambers have the advantage of providing the controlled and
programmed temperature, light, and humidity characteristics of plant growth
chambers. The environmental conditions are, independent of outdoor environment
conditions. This ensures repeatability of experiments on a year-round basis.
The air movement required for environment control may provide the mixing to '
minimize gradients of pollutants within the chamber and crop canopy as well as
to minimize leaf boundary layer resistance. Fluxes of gases to the plants may
be calculated in well-sealed chambers for which inlet and outlet air volumes
and composition are known. The disadvantages of self-contained chambers
include the limitation of one treatment per chamber which necessitates several
separate chambers for experiments. Corrosion of components by pollutant gases
will be a problem with self-contained chambers. Where recirculation of air is
used to permit flux fate studies or to minimize air conditioning requirements,
there may be some buildup of these contaminant gases and water vapor and
depletion of carbon dioxide during experiments. Most pollutant gases may be
removed in water condensation on heat exchangers which set the dew point.
Chambers within growth rooms or within larger chambers also have the
advantage of controlled programmed environments. With several chambers housed
in a larger facility, additional treatment levels can be administered at the
same time during an experiment. Chambers within large units may also permit
greater flexibility in optimizing ventilation rates to meet plant needs.
Replication both within and among runs is facilitated by this kind of equip-
ment. Disadvantages include the limited size of chambers that can be accommo-
dated. The artificial conditions created in all controlled environment
chambers may be unsatisfactory for studies which require close simulation of
outdoor conditions.
Continuous stirred tank reactor chambers (CSTR's) have distinct advantages
for gas exchange studies because fluxes can be readily calculated. Chambers
can be placed in greenhouses or growth rooms. The rapid air mixing minimizes
gradients within chambers as well as leaf boundary layer resistance. In terms
of disadvantage, the lack of gradients of temperature, humidity, and gaseous
pollution may not represent natural ecosystem conditions. The rapid air
movement may cause wind injury to sensitive plants. The limited size of these
chambers, as constructed to date, will be a disadvantage in studies with large
plants. Lighting systems are often a problem and appropriate minimum light
levels and temperature- and humidity-controlled air will need to be supplied to
each chamber.
,-•'
5. Cuvettes
Four basic types of cuvettes are utilized in gas exchange systems:
4-9
-------
Type 1: Cuvettes for Sampling Rates of Gas Absorption or Emission from
Leaves;
Type 2: Cuvettes for Continually Monitoring the Amount of Gas Absorption
or Emission from Leaves -- Differential Gas Exchange System;
Type 3: Cuvettes for Maintaining Constant Levels of Water Vapor and C02
— Null Balance, Steady State Gas Exchange Systems;
Type 4: Cuvettes for Carbon Isotope Studies.
The basic descriptions of these systems are presented in Chapter II. In Table
IV-5 we evaluate the features usually associated with each of these systems and
classify some well-known models of cuvettes/gas exchange systems. This table
can be used to evaluate the potential for several gas exchange systems to be
used for a wide range of purposes including maintenance of environmental
conditions which can affect foliar air pollution absorption characteristics.
In addition, the gas exchange systems are evaluated for long and short-term
experimental capability as well as the capability for controlled air pollution
exposures. The evaluation of single leaf cuvette systems as fumigation devices
has been limited because cuvettes are designed to have high degrees of gas
mixing, to have uniform distribution around the leaf, and to have the smallest
leaf boundary layer possible. Also, the small volume of these cuvettes
(usually less than 1 liter) makes direct measurements of gradients in gas
concentrations across a leaf surface a difficult technical task. Larger
laboratory fumigation chambers which may contain one or several whole plants
have complex patterns of atmospheric mixing which reflects air flow rates,
chamber shape and size, physical surfaces in the chamber such as chamber walls,
pots, soils, as well as the distribution of plants, plant architecture, and
environmental conditions in the chamber (Unsworth and Mansfield, 1980). Thus,
whole plant chambers must be thoroughly investigated in order to understand air
pollution fluxes between the atmosphere, plants, and other surfaces; failure to
do so may result in misleading expressions of air pollution exposures and other
gas exchange characteristics.
B. Wet Deposition
The performance evaluation of exposure systems simulating rainfall, cloud
water, and aerosols is presented in Table IV-6. Eight generic systems of
rainfall simulation are evaluated, including indoor systems and four types of
systems for outdoor exposures (no exclusion of ambient rainfall, permanent
exclusion, automated exclusion/scheduled addition, and automated exclusion/
automated addition of rain). The general performance of each type of system is
characterized according to the criteria outlined in Section III. Because some
of the criteria are not important for selected systems, the evaluation process
provides two-fold remarks to indicate relevance and degree of characterization
reported (relevant/characteristics).
4-10
-------
Table IV-5. Performance evaluation for cuvette/gas exchange systems.
Cuvette Type9
234
Evaluation of Operational Characteristics
Closed air flow system
Flow-through air flow system
Temperature control
Humidity control
Clip-on cuvette
. Whole-leaf cuvette
Short-term measurements only
Long-term measurement capability
Diagnostic gas exchange capability
Air pollution fumigation capability
Classification of Some Models
Diffusive resistance porometer (numerous models)
Analytical Developmental Corp. Delta T System
LiCor steady state porometer (1600)
LiCor photosynthesis system (6000 and 6200)
Sieman cuvette systems
Data Design Group system (PACsys 9900)
Analytical Development Corporation system (LCA-2)
Armstrong Enterprises system (custom design)
Walz system (custom design)
Yb
N
N
N
Y
Y
Y
N
N
N
X
X
N
Y
Y
Y
N
Y
N
Y
Y
Y
Y
Y
Y
Y
Y
Y
N
Y
Y
Y
Y
N
N
N
Y
Y
Y
N
Y
N
X
X
X
a See text.
b Important, Y = Yes, N = No for this type of cuvette.
4-11
-------
Table IV-6. Performance evaluation for wet deposition systems.
Rainfall Simulation
Outdoor Systems
Shelter- Permanent Automated Automated Combin- Cloud
less Shelter Exclusion Exclusion ation Water Aerosol
Indoor Scheduled Scheduled Scheduled Automated Wet and Simu- Simu-
Systems Addition Addition Addition Addition Dry lation lation
A. Hydrometeor/Pollutant:
Chemistry and Physics
1. Simulant Chemistry
2. Incident Hydrometeor
Chemistry
a. Spatial Features
b. Temporal Features
3. Incident Hydrometeor
Deposition
a. Spatial Features
b. Temporal Features
4. Chemical-Specific
Deposition Rates
a. Spatial Features
b. Temporal Features
5. Incident Hydrometeor
Physics
6. Chemical Processing of
Deposited Pollutants
a. Foliar Retention
b. Throughfall/
Stemflow
c. Soil
7. Pollutant Mixtures
(Het/Het and Wet/Dry)
B. The Environment: Physical,
Chemical, and Biological
Features
1. Atmosphere
a. Radiation
b. Temperature/
Moisture
c. Turbulence/
Mixing
d. Trace Gases
(Pollutant and
Nonpollutant)
2. Vegetation
a. Structure
b. Physiology and
Growth
3. Soil
D. Hardware
1. Hydrometeor Dispens.
a. Droplet Distrib.
b. Deposition Mont.
and Control
2. Environment
a. Monitoring
b. Control
3. Data Acquisition
YAa
Y/N
Y/Y
Y/N
Y/Y
Y/N
Y/N
NA
N/N
Y/N
Y/Y
NA
N/N
N/N
N/N
Y/N
N/N
N/N
Y/Y
Y/Y
YA
Y/Y
Y/Y
N/N
N/N
Y/Y
Y/N
N/Y
Y/N
Y/Y
N/N
Y/N
Y/N
N/N
Y/N
Y/Y
NA
N/N
Y/N
N/N
Y/N
N/N
Y/N
YA
Y/Y
Y/Y
Y/Y
Y/Y
N/N
N/N
Y/Y
Y/N
Y/N
Y/N
Y/Y
N/N
Y/N
Y/N
N/N
Y/N
Y/Y
NA
Y/Y
Y/N
Y/Y
Y/Y
N/N
Y/N
N/N
Y/Y
Y/Y
Y/Y
Y/Y
Y/N
N/N
Y/Y
Y/N
Y/N
Y/N
Y/Y
N/N
Y/N
Y/N
Y/N
Y/N
Y/Y
NA
Y/N
Y/Y
Y/Y
Y/Y
N/N
Y/Y
Y/Y
Y/Y
Y/Y
Y/Y
Y/Y
Y/N
N/N
Y/Y
Y/Y
Y/Y
Y/Y
YA
N/N
Y/Y
Y/Y
Y/N
Y/N
Y/Y
Y/Y
Y/N
Y/N
Y/Y
Y/Y
N/N
Y/Y
Y/Y
Y/Y
Y/Y
Y/Y
Y/Y
Y/N
Y/Y
Y/Y
Y/Y
Y/Y
Y/Y
YA
N/N
Y/Y
Y/Y
YA
Y/N
Y/Y
Y/Y
Y/N
Y/N
Y/Y
YA
N/N
YA
Y/Y
Y/Y
YA
Y/Y
Y/Y
Y/N
Y/Y
Y/Y
Y/N
Y/N
Y/N
Y/Y
Y/N
Y/N
Y/N
Y/Y
Y/Y
YA
Y/Y
N/N
N/Y
N/N
N/N
YA
N/N
N/N
Y/Y
Y/Y
Y/Y
Y/Y
Y/N
N/N
Y/Y
Y/N
Y/Y
Y/Y
Y/Y
Y/Y
N/N
N/N
Y/Y
N/N
N/N
NA
Y/Y
Y/Y
Y/Y
Y/N
Y/Y
Y/Y
N/N
Y/Y
YA
Y/Y
Y/Y
YA
N/N
a Important/Characterized, Y = Yes, N = No for this type of system.
4-12
-------
1. Indoor Rainfall Simulation -- Controlled-Environment or
Greenhouse
The indoor exposure systems for rainfall are described in Appendix F,
Section A. The principal advantage of rainfall simulation in a controlled
environment is that by rigidly controlling exogenous physical, chemical, and
biological factors, one can identify the role of individual factors (or combin-
ations of factors) in governing the influence of wet deposition on the growth
and physiology of wood plants. Consequently, these systems have superior
capabilities for resolving mechanisms and processes governing plant response.
The disadvantage of rigidly controlled exposures is the artificial nature of
the exposure environment. In many agricultural and forested landscapes, the
interaction of edaphic, climatic, and atmospheric factors during and after wet
deposition events may play a substantial role in injury expression. Conse-
quently, data and patterns observed in controlled exposures cannot be indiscrim-
inately applied to predict effects of wet deposition in forests or agricultural
settings.
2. Outdoor Rainfall Simulation -- Shelterless Plots
Shelterless plot systems experience simulated rain in excess of ambient
levels and are described in Appendix F, Section B.I. The most significant
advantage is their ability to accommodate variable plot sizes in terms of both
surface area and canopy height. The absence of exclusion lids and the avail-
ability of relatively inexpensive irrigation systems makes this design worthy
of consideration for many large scale projects. The use of these exposure
systems in more pristine ecosystems has particular merit, especially in efforts
to evaluate the effect of a particular chemical addition. The disadvantages
include: (1) a requirement for a significant water conditioning and simulation
mixing system (for large plot simulation); (2) difficulty in achieving uniform
distribution, particularly in areas experiencing moderate winds; and (3) the
interpretational problem of evaluating the significance of excess water and
chemical input to plant growth. This interpretation should not be regarded as
simply an additive factor, for it may exhibit characteristics of joint action
(i.e., synergism, antagonism).
3. Outdoor Rainfall Simulation -- Permanent Sheltered Plots
Sheltered plot exposure systems are described in Appendix F, Section B.2.
Unlike controlled-environments, sheltered plots under field conditions offer
many features of the environment at near ambient levels. Consequently, these
systems can be used to test general hypotheses on the influence of wet deposi-
tion on woody vegetation. The resulting data can help establish priorities for
subsequent research by identifying hypotheses worthy of more careful evaluation
under more realistic field conditions. The major disadvantage is that some
critical environmental factors cannot be rigidly controlled, including tempera-
ture, light, and moisture conditions. In all systems, rainfall simulation is
by scheduled addition.
4-13
-------
4. Outdoor Rainfall Simulation -- Automated Exclusion/Scheduled
Addition of Rainfall
The automated exclusion systems are described in Appendix F, Section B.3.
The major advantage of these systems is their ability to maintain a near-normal
microclimate (edaphic plus atmospheric) during non-exposure periods, since the
exclusion lids are positioned away from the canopy. Consequently, the systems
provide more realistic data with which to evaluate the influence of wet
deposition on plant growth and development. The major disadvantage is the
prescheduled protocol for rainfall simulation so that simulated rain events may
or may not co-occur with ambient. Because the canopy's microclimate experi-
ences wet deposition on a more frequent basis than ambient plots (e.g.,
saturated vapor pressure, reduced temperature, light attenuation), these
systems may present interpretational problems in evaluating the dose-response
functions. A second, less important disadvantage is the stability of the
exclusion track system in high winds.
5. Outdoor Rainfall Simulation — Automated Exclusion/Automated
Addition of Rainfall
The chief advantage of these systems described in Appendix F, Section B.4,
is that the canopy's microclimate experiences wet deposition events that mimic
ambient conditions including both the time of delivery and the deposition rate.
The simultaneous exclusion and addition is a significant advantage. The
disadvantages are: (1) the system may inadvertently modify the microclimatic
conditions through other hardware devices (e.g., open-top chamber); (2) the
fabrication and maintenance costs are significant; and (3) some experimental
designs cannot be accommodated with automated addition given the natural
variation in rainfall.
6. Facilities for Simulating Wet and Dry Deposition
These systems combine various individual methods of wet and dry simulation
to achieve the objective. The systems available for this type of mixture
exposure studies are described in Appendix D. They have the advantage of being
able to partition sources of phytotoxicity to either wet or dry deposition,
singly and in combination. The chief disadvantage is the additional hardware
required to maintain the system throughout the growing season, and the inadver-
tent modification of the exposure environment which comes with the exclusion of
ambient pollutant.
7. Cloud Water/Mist Simulation
Most cloud water system designs are primitive (Appendix G) and unsuitable
for field application, particularly in areas with advective cloud water.
Existing laboratory facilities are capable of evaluating potential mechanisms
of phytotoxicity. The resulting data, however, should not be indiscriminately
applied to predict effects under ambient conditions.
4-14
-------
8. Aerosol Simulation
The significance of aerosols in terms of vegetation response remains a low
priority. The most important issues are likely to be deposition and fate of
particular chemicals in the aerosol. The disadvantage of these systems is the
need for a rigidly controlled growth environment, commonly provided through a
dedicated chamber. The available systems are described in Appendix G.
4-15
-------
-------
V. Exposure Protocol/Air Quality
A discussion of exposure protocols for deposition studies involves first
the acceptance that exposure studies must relate, at least at some level, to
the characteristics of air quality in the region, site, or elevation of
interest for the particular forest species under study. The characteristics of
the ambient air Quality can be used to develop the appropriate exposure
regime(s)/protocol(s), whether for exposure-response studies in the field or
for controlled-exposure studies.
Reliable, up-to-date monitoring programs and accessible data bases for
both wet and dry deposition are crucial to understanding.the nature of occur-
rences of the pollutant(s) and for developing exposure protocols. Toward this
end, the availability and completeness of available monitoring data are
reviewed with details of the databases included in Appendix J.
Characterization of air quality to establish the dynamic elements of
biological exposures, including temporal and spatial distributions of the
pollutant(s) concentrations, has only recently been the subject of attention
for biological studies, and thus there is not an extensive literature base. In
contrast, annual trends and compositional information for both wet and dry
deposition have been extensively reviewed (e.g., USEPA, 1984;' USEPA, 1982;
Altshuller, 1984). Highlights of these general trends of air quality are
included in Appendix K. In this chapter only the general temporal and spatial
distribution characteristics observed for the various pollutants concerned with
forest health are discussed.
The types of exposure regimes used historically in both dry and wet
deposition studies are discussed, including the air quality characterization
information needed in their development, the degree to which the exposure
protocols relate to current air quality, and the limitations in interpretation
of the experimental results imposed by their use.
A. Data Bases -- Dry (Gaseous) and Wet Deposition
Over the years, a number of programs have measured ambient air quality and
wet deposition in various areas of the United States. The sampling has ranged
from short-term (a few months) to multi-year projects. The sampling programs
have had disparate goals, consequently, the monitoring data do not have the
same level of resolution, are not readily available, and have not all been
entered into data bases. Another consequence of the disparate objectives is
that monitoring has been done extensively in some areas of the country with
little or no data available for other locations.
Gaseous Air Pollutants: Dry deposition is the direct transfer of com-
pounds (gases or particles) from the atmosphere to vegetation or soil surfaces.
Gaseous air pollutants have been more extensively monitored than the dry
particle deposition. The data are usually reported as hourly mean concentra-
tions. Most of the available monitoring data for gaseous air pollutants are
from sites in urban or near-urban areas or in the vicinity of major pollution
emitters. There are several readily available sources of air quality monitor-
ing databases such as; EPA-SAROAD, EPRI, TVA, and the National Park Service
5-1
-------
that can be used to obtain information concerning potential vegetation expo-
sures. The data bases are discussed in detail in Appendix J. Individual
investigators may be able to locate other site-specific data from various
agencies or research projects that will assist in their studies.
B. Characteristics of Ambient Air Quality in the U.S.
The hypotheses that invoke various dry and wet deposition components,
individually or in combinations, to explain the observed decline in forest
species requires first demonstrating the occurrence and general trend of these
various pollutants in the regions, sites, and/or elevations of interest.
Testing these hypotheses further requires an understanding of the dynamics of
the various components and characteristics of ambient air quality, including
the temporal and spatial distribution of concentrations over specific time
periods, for developing experimental exposure regimes. Chronic exposure
studies, specifically, need to incorporate the elements of temporal dynamics of
the pollutant(s) in question into the experimental protocol.
Although reasonably good monitoring databases are available for character-
ization of current air quality over most of the regions of the United States,
the long-term trends in dry and, especially wet deposition, in the U.S. are
difficult to assess because of the lack of a really significant historical
monitoring database for any of the various pollutants. Given this limitation,
general trends of the various component pollutants in both wet and dry deposi-
tion have been reported and are briefly revised in Appendix K. These trends in
pollutant concentrations are important as a first level understanding of the
dynamics of air quality; and the incorporation of this information into experi-
mental protocols for exposure studies of forest species requires some means of
characterization of the air quality data that is relevant to the postulated
effect on tree vigor.
Air Quality Characterization: Dry Deposition
Characterization of air quality with long-term mean concentrations such as
annual, quarterly, monthly, and even 7-hr means is the most prevalent approach,
but has questionable biological relevance. An average of hourly concentrations
over a set time period minimizes the contributions of the peak concentration
values, which is inconsistent with the literature describing the importance of
peaks in the plant response to a pollutant such as ozone (e.g., Heck et al.,
1966; Amiro et jil_., 1984). Other studies have shown the importance of the
spatial and temporal distributions of the hourly concentrations of ozone in
plant growth (e.g., Musselman et al., 1983; Hogsett et aj_., 1985). The rela-
tionship of air quality to the biological reponse of plants requires more
detailed analyses of ambient air to understand the spatial and temporal
distributions of concentrations of the pollutant(s) in question. Characteriza-
tion to define these temporal patterns and concentration-frequency occurrences
provides not only the starting point for relating deposition exposure studies
and the actual air quality of interest across the various regions of the U.S.,
but also the information needed to design experiments that reflect the temporal
and spatial nature of the region, site or elevation of interest (Lefohn et al.,
1986a,b). A particular site for one particular year can be characterizecTf
another approach is to characterize a site over a number of years or a number
5-2
-------
of sites over a region by calculating average characterization information with
standard deviations giving variation over years or regions. The hourly monitor-
ing data is characterized with the following criteria: (1) percentile distri-
bution of hourly concentrations; (2) number of hourly occurrences equal to or
greater than a specified minimum concentration; (3) duration of the episodes or
pre-defined peak concentrations; (4) respite time between these episodes; (5)
average diurnal pattern (over the growing season or annually) of hourly
concentrations for daily temporal distribution of concentrations and identify-
ing, the time of day when the daily maximum occurs. Such characterization would
describe, in large part, the chemical and/or temporal pollutant under study.
Ozone
Ozone percentile distributions from a number of sites across the country
are shown in Table V-l. Most of the sites, with the exception of the
California sites, are similar in distributions at the 95th percentile. In
addition, the year-to-year variation at a given site is small, varying only
10-15% at the 95th percentile and even less at the 50th percentile. All sites
display an infrequent occurrence of the higher concentration values; 70% of the
time, hourly concentrations of 0.050 ppm or less are reported. The relative
infrequency of the higher concentrations is also shown in histograms of the
number of hourly occurrences at specified minimum values (Figure V-l). Rural
sites in the upper Midwest and the Northeast are similar (Figure V-la, Ib), as
well as sites in the Southeast which are not shown. Hourly concentrations of
0-0.03 ppm occur most frequently and values greater than 0.08 ppm occur
infrequently over a growing season. A remote site in the Shenandoah National
Park at 3500 ft. has a somewhat different distribution (Figure V-lc); the
majority of hourly occurrences are around 0.05-8.06 ppm, rather than the
log-normal shape of the rural site distributions under influence of anthro-
pogenic sources. These differences are not necessarily a function of remote
versus rural site designation, or a function of elevation, but simply an
observation of two different types of concentration distributions. These
distributions are also shown in diurnal plots in Figures V-2 and V-3. The
spatial pattern of occurrence of these higher concentrations or episodes is
important in biological response and requires a knowledge of the duration of
the episodes, the probability of the episode occurrence, and the length of time
between the episodes or respite times. Analysis of rural and remote sites in
the Midwest, Great Lakes states, Northeast,, and Southeast (Lefohn et aj.,
1986a; 1986b) indicates that all regions display average growing season"
patterns of hourly concentrations where occurrences of higher concentrations
are infrequent; the episodes or peaks lasting relatively short periods and time
between these episodes being relatively long. Duration of episodes and the
time between episodes or respite times, for a number of sites in the Northeast
and Southeast are given in Tables V-2 and V-3, respectively. Using a different
approach in analysis of air quality, Taylor and Norby (1985) calculated the
probability of episodes and periods of respite lasting defined time periods
(days). The analysis was based on a 4-year monitoring data set from the
Shenandoah collected by Skelly and co-workers. In their analysis, an episode
was defined as any day(s) in which a 1-hr ozone concentration exceeded 0.08
ppm. The Shenandoah National Park monitoring site experienced an average of
five episodes over the growing season for the 4-year monitoring period.
Single-day episodes were infrequent (P = 16%), but there was an 80% probability
5-3
-------
Table V-l. Ozone percentile distribution in some rural sites across the U.S. in the
SAROAD data base {concentration in ppm). From Lefohn and Benedict (1985);
Lefohn (personal communication).
Percentile
Site State
Fontana (RC)a
Fontana (RC)
Fresno Co. (RE)
Oxnard (RA)
Riverside (RA)
San Bernardino Co.
(RNU)
San Bernardino Co.
(RNU)
Ventura Co. (RA)
Columbia Co. (WI)
Juneau (WI)
Sherburne Co. (MM)
Will Co. (IL)
Allen Co. (IN)
Porter Co. (IN)
Kent Co. (MI)
Preble Co. (OH)
Hamilton Co. (RI)
Hamilton Co. (RI)
Madison Co. (RNU)
Madison Co. (RNU)
Jefferson Co. (RNU)
Jefferson Co. (RNU)
Clay Co. (RNU)
Omaha (RNU)
Mobile (R)
CA
CA
CA
CA
CA
CA
CA
CA
WI
WI
MN
IL
IN
IN
MI
OH
OH
OH
IL
IL
KY
KY
MO
NE
AL
Hillsborough Co. (R)FL
St. Petersburg (R)
Pensacola (R)
Tarpon Springs (R)
Jefferson Co. (RNU)
North Little Rock
(RNU)
Iberville Parish
(RNU)
Arlington (RA)
Arlington (RA)
Denton Co. (RA)
Camden Co. (RA)
PL
PL
PL
AL
AR
IA
TX
TX
TX
NJ
Year
1980
1981
1979
1979
1981
1979
1981
1979
1979
1979
1980
1981
1981
1982
1981
1981
1978
1980
1978
1980
1978
1980
1980
1980
1978
1978
1979
1981
1981
1981
1979
1978
1979
1980
1981
1979
Min.
Obs.
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.002
0.002
0.000
0.000
0.005
0.003
0.000
0.000
0.003
0.003
0.005
0.003
0.003
0.003
0.003
0.003
0.000
0.000
0.000
0.000
0.000
0.003
0.003
0.003
0.003
0.003
0.003
0.003
10
0.010
0.003
0.030
0.003
0.003
0.010
0.010
0.003
0.018
0.006
0.010
0.006
0.005
0.003
0.013
0.005
0.003
0.003
0.005
0.003
0.003
0.003
0.010
0.010
0.000
0.008
0.005
0.001
0.005
0.003
0.012
0.003
0.010
0.003
0.003
0.003
30
0.020
0.010
0.050
0.010
0.010
0.030
0.020
0.020
0.034
0.024
0.022
0.018
0.013
0.018
0.024
0.017
0.003
0.003
0.014
0.014
0.016
0.016
0.020
0.020
0.007
0.015
0.015
0.015
0.015
0.007
0.024
0.007
0.030
0.020
0.010
0.015
50
0.030
0.020
0.060
0.020
0.020
0.040
0.040
0.030
0.044
0.035
0.030
0.029
0.023
0.030
0.030
0.030
0.010
0.012
0.027
0.025
0.027
0.030
0.030
0.030
0.022
0.025
0.025
0.027
0.030
0.030
0.034
0.017
0.040
0.030
0.030
0.025
70
0.050
0.040
0.070
0.040
0.040
0.070
0.060
0.040
0.054
0.045
0.038
0.039
0.035
0.040
0.040
0.040
0.025
0.025
0.041
0.035
0.042-
0.047
0.041
0.042
0.040
0.035
0.040
0.040
0.040
0.049
0.046
0.031
0.050
0.040
0.040
0.035
90
0.110
0.010
0.090
0.060
0.100
0.140
0.120
0.080
0.072
0.060
0.050
0.055
0.054
0.060
0.057
0.060
0.055
0.055
0.069
0.062
0.064
0.077
0.060
0.070
0.060
0.052
0.060
0.056
0.060
0.074
0.067
0.061
0.070
0.060
0.060
0.060
95
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
160
160
090
070
130
170
150
100
085
070
058
063
066
068
066
072
077
072
083
077
076
093
070
085
070
062
070
064
070
086
081
078
,080
0.080
0.
0.
1
,080
,073
Max.
Obs.
0.420
0.350
0.150
0.230
0.280
0.400
0.350
0.220
0.172
0.125
0,115
0.118
0.108
0.142
0.108
0.127
0.180
0.172
0.187
0.170
0.182
0.186
0.160
0.170
0.120
0.138
0.120
0.112
0.115
0.166
0.144
0.205
0.190
0.160
0.150
0.147
5-4
-------
Table V-l (continued)
Site
Camden Co. (RA)
Anne Arundel Co.
Greenwich (RNU)
Greenwich (RNU)
Agawam (RNU)
Worcester (SR)
Glen Falls (SR)
Kutztown (R)
Keene (CO
State
NJ
(R)MD
CT
CT
MA
MA
NY
PA
NH
Green Mountain (RNU)VT
Chequamegon N.F.
Croatan N.F. (RE)
Big Meadows Shen.
WI
NC
Year
1980
1981
1979
1980
1979
1979
1981
1980
1982
1982
1983
1982
1980
1981
1981
1980
1982
1983
Min.
Obs.
0.003
0.003
0.003
0.003
0.003
0.000
0.000
0.000
0.001
0.000
0.000
0.000
0.000
0.000
0.005
0.000
0.000
0.005
Percent! le
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
10
.005
.003
.005
.005
.003
.019
.021
.004
.005
.003
.003
.005
.005
.005
.020
.010
.005
.030
30
0.017
0.007
0.013
0.019
0.012
0.028
0.032
0.015
0.017
0.017
0.019
0.018
0.020
0.015
0.030
0.020
0.020
0.045
50
0.026
0.020
0.024
0.029
0.025
0.036
0.040
0.025
0.028
0.030
0.032
0.029
0.035
0.030
0.035
0.030
0.025
0.055
70
0.037
0.034
0.034
0.040
0.040
0.046
0.050
0.035
0.038
0.046
0.045
0.041
0.050
0.040
0.040
0.045
0.040
0.065
90
0.066
0.061
0.054
0.062
0.065
0.064
0.067
0.054
0.056
0.071
0.071
0.059
0.065
0.055
0.055
0.065
0.055
0.075
95
0.080
0.076
0.070
0.082
0.082
0.080
0.078
0.065
0.067
0.085
0.086
0.069
0.075
0.060
0.060
0.075
0.060
0.080
Max.
Obs.
0.141
0.170
0.204
0.216
0.175
0.185
0.139
0.106
0.116
0.150
0.134
0.135
0.115
0.105
0.080
0.150
0.095
0.010
(VA)
Dickey Ridge Shen.
N.P. (VA)
Sawhill Run Shen.
N.P. (VA)
Mark Twain N.F. (MO)
Mark Twain N.F. (MO)
1983 0.000 0.030 0.045 0.055 0.065 0.075 0.085 0.110
1983 0.000 0.020 0.030 0.040 0.055 0.070 0.080 0.120
1981 0.000 0.015 0.025 0.030 0.040 0.050 0.060 0.115
1983 0.000 0.025 0.035 0.045 0.055 0.070 0.080 0.110
RE = Remote; RNU = Rural Near Urban; RA = Rural Agricultural; RC = Rural Commercial;
RI = Rural Industrial; R = Rural (unspecified).
5-5
-------
B
(XOJ 0.04 OOt 0.01 0.10 0.13
Ozone p.p.m.
0.00 0.02 0.04 0.09 0.04 0.10 013
Ozone p.p.m.
to
at
u
o
u
O
-------
E
CL
Q.
o
o s
m
CL
Q.
B
tois
Ho u r
10 12
Hour
Figure V-2.
Average diurnal concentration pattern from rural and remote sites
A. Croatoan National Forest, North Carolina (May-September 1983")•'
B. Juneau, Wisconsin (April-October 1979); C. Kutztown,
Pennsylvania (May-September 1983); D. Sawmill Run, Shenandoah
National Park (May-September 1983). From Lefohn (personal
communication).
5-7
-------
a
o.
Q. .
O
ol
§
_l I 1
e
Q.
8
B
6 3 4 i i io 13 14 is it 20 22 24 , 0 3 4 i 810 \3 14 It it 20 32 4 |
Ho u r Ho ur
_1 I I 1 1 1 I—
1 4 I I 10 12 TJ Is IS 20~
Hou r
c
o
0 °
F. AZ (0300SOUCAOS)
-' • Custer IIF. HT (2703I0101A08)
Octioco KF. OR (380420111*03)
- <• i-
10. II. 14. !(. II. 20. 22-
Ho u P
Figure V-3 Average diurnal concentration patterns from remote monitoring
sites. A. Dickey Ridge, Shenandoah National Park, Virginia (May-
September 1983); B. Big Meadows, Shenandoah National Park (May-
September 1983); C. Cheauamauga National Forest, Wisconsin (April-
October 1981); D. Three National Forest sites (June-August 1981).
5-8
-------
Table V-2. Average duration (hrs) of events that meet or exceed the indicated
concentration, and the number of events (in parentheses) (Lefohn,
personal communication).
Average Length of Events That Meet or Exceed
the Ozone Concentration
ppm
Worcester
(1979)
Glen Falls, NY
(1980)
Kutztown, PA
(1983)
Keene, NH
(1982)
Green Mountain N.F., VT
(1981)
Croaton N.F., NC
(1983)
Dickey Ridge,
Shenandoah National Park
(1983)
Big Meadows,
Shenandoah National Park
(1983)
Sawmill Ridge,
Shenandoah National Park
(1983)
0.05
7.9
(113)
4.8
(87)
67
(131
6.4
(107)
6.8
(61)
6.1
(124)
19.9
(95)
19.6
(95)
8.8
(162)
0.07
5.9
(47.)
3.4
(36)
5.8
(63)
5.1
(30)
4.4
(10)
3.3
(18)
8.2
(114)
6.2
(111)
5.3
(84)
0.09
3.3
(29)
2.9
(11)
3.7
(35)
3.2
(13)
3.5
(2)
0
(0)
4.3
(31)
3.1
(20)
3.4
(26)
0.12
2.0
(5)
0
(0)
1.6
(8)
2.0
(1)
0
(0)
6
(0)
0
(0)
0
(0)
2.0
(1)
5-9
-------
Table V-3. Average respite time (hr) between episodes which equal or exceed
indicated ozone concentration (Lefohn, personal communication).
Worcester, MA
(1979)
Glen Falls, NY
(1980)
Kutztown, PA
(1983)
Keene, NH
(1982)
Green Mountain N.F., VT
(1981)
Croatan N.F., NC
(1983)
Dickey Ridge,
0.05
22.9
33.2
19.6
27.3
47.4
46.2
9.4
ppm
0.07 0.09 0.12
63.9 110.8 789.5
89.6 251.0
44.1 68.2 290.1
106.7 254.8
219.7
194.4
22.2 92.3
Shenandoah National Park
(1983)
Big Meadows, 17.8
Shenandoah National Park
(1983)
Sawmill Run, 11.3
Shenandoah National Park
(1983)
25.1
27.6
130.6
• 83.4
5-10
-------
that the episode would last 2 or more days. A respite time of less than 2
weeks had a probability of 50%, and the median respite time between episodes
was 10 days. The seasonality of peak occurrences was also suggested by this
analytical approach. An episode was most likely to occur during May, June, and
July (about equally), 40% less likely to occur during August and even less
likely during the fall months. In general, the episodes occur infrequently and
with less duration than the respite times (< 0.08 ppm) between the episodes.
An additional temporal and spatial feature important in analysis of air
quality and the response of vegetation is the diurnal concentration pattern.
How these hourly concentration values are distributed over a day is- shown in
average diurnal plots (Figures V-8 and V-9). The majority of sites analyzed
display a diurnal rise and fall in concentration with the maximum occurring in
the afternoon hours, obviously closely tied to diurnal climatic patterns and
influenced by nearby anthropogenic sources (Figure V-8). Another diurnal
pattern observed at some remote sites is one in which there is very little, if
any, rise and fall in concentration; and most hours of the day experience
concentrations above 0.03 ppm (Figure V-3). With these sites no clearly
discernible maximum concentration is observed. The percentile distributions
for these sites show 80-90% of the hourly values eaual to or above 0.030 ppm,
but still relatively infrequent occurrence of values above 0.080 ppm over the
growing season (Table V-l). This lack of diurnal variation in concentration
has also been associated with increased elevation in the U.S. and Germany
(Taylor and Norby., 1985; Blank, 1985) and attributed to the reduction in
atmospheric scavenging processess at higher elevation (e.g., nitric oxides).
The explanation for this el'evational distribution pattern is not clear and
caution is urged in generalizing this observed phenomenon until further monitor-
ing data is available from a variety of sites and elevational gradients. The
diurnal patterns shown in Figures V-2 and V-3 are from monitoring sites located
at elevations of 13 m (Croatan N.F., N.C.), 440 m (Chequamegon, N.F., MI),
1200 m (Custer N.F.); 1500 m (Sawmill Run S.N.P.), 2100 m (Dickey Ridge,
S.N.P.), 2500 (Apache N.F.), and 3500 m (Big Meadows, S.N.P.). With these
examples, the lack of diurnal variation does not hold true for all increased
elevation sites.
Sulfur Dioxide
In the case of S02, the mean values describing regional air quality are
most often derived from point-source monitoring sites, and it is difficult to
estimate how relevant these data are for quantifying air quality in regional
terms. Point-source S02 concentration and frequency of occurrence depend upon
the emission strength, meteorological conditions (e.g., wind speed), and
distance between the source and monitoring site, thus bringing into question
the use of these data to characterize of air quality in assessing S02 impact on
forests located 25-50 miles, from a source. There remains a need for site-
specific monitoring to characterize S02 impact. ' .
The percentile distribution from a number of S02 monitoring sites indi-
cates 70% or more of the hourly values are below 0.01 ppm (Table V-4). This
distribution points out the ineffectiveness of an annual mean for characteriz-
ing S02- An example of the distribution of hourly concentration values for SO?
is given in Figure V-4 and, unlike the lognormal-like distribution for ozone at
5-11
-------
Table V-4. Percentile distribution for sulfur dioxide (ppm), January-December. From Lefohn et
al. (1986b).
Percentile
Site State
North Little Rock (RNU)a
North Little Rock (RNU)
Pensacola (RNU)
Pensacola (RNU)
Tarpon Springs (RC)
Tarpon Springs (RC)
Trigg Co.
Trigg Co.
Jay (R)
Jay (R)
Rumford (005 JOS) (RI)
Pittsfield (RI)
Lake Co.
Lake Co.
Iron Co. (RI)
Iron Co. (RI)
Berlin (RI)
Pembroke (RNU)
Camden Co.
Essex Co. (RE)
Rensselaer (SI)
Rensselaer (SI)
Columbus Co. (RI)
Columbus Co. (RI)
Monroe Co. (RE)
Monroe Co. (RE)
Burlington (CC)
Dickey Ridge, Shen. (NP)
Salem
Lewisburg
Chippewa Co. (RA)
Porter Co. (R)
AR
AR
FL
FL
FL
FL
KY
KY
ME
ME
ME
MA
MN
MN
MO
MO
NH
NH
NJ
NY
NY
NY
NC
NC
TN
TN
VT
VA
VA
WV
WI
IN
Year
1981
1982
1981
1982
1981
1982
1982
1983
1982
1983
1983
1982
1978
1979
1982
1983
1983
1983
1982
1983
1980
1982
1982
1983
1979
1980
1982
1983
1982
1979
1980
1982
Min.
Obs.
0.000
0.000
0.000
0.000
0.000
0.000
0.001
0.000
0.001
0.002
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.003
0.003
0.000
0.000
0.001
0.000
0.000
0.000
0.001
0.001
10
0.000
0.000
0.000
0.000
0.000
0.000
0.001
0.001
0.002
0.002
0.001
0.000
0.002
0.000
0.000
0.000
0.000
0.000
0.002
0.000
0.001
0.001
0.003
0.003
0.000
0.000
0.001
0.000
0.002
0.000
0.001
0.001
30
0.000
0.000
0.001
0.002
0.000
0.000
0.001
0.001
0.002
0.002
0.002
0.002
0.002
0.000
0.000
0.000
0.000
0.000
0.003
0.002
0.004
0.004
0.003
0.003
0.000
0.000
0.002
0.000
0.003
0.000
0.001
0.001
50
0.000
0.000
0.003
0.005
0.000
0.000
0.002
0.002
0.003
0.003
0.008
0.005
0.002
0.000
0.000
0.003
0.002
0.000
0.004
0.003
0.009
0.007
0.003
0.003
0.000
0.000
0.005
0.006
0-.006
0.000
0.001
0.003
70
0.000
0.000
0.006
0.008
0.000
0.000
0.004
0.004
0.004
0.004
0.013
0.010
0.002
0.000
0.006
0.007
0.005
0.004
0.009
0.004
0.015
0.012
0.003
0.003
0.000
0.010
0.007
0.005
0.005
0.003
0.001
0.007
90
0.005
0.000
0.014
0.018
0.010
0.005
0.011
0.011
0.008
0.006
0.036
0.018
0.002
0.000
0.020
0.019
0.016
0.013
0.016
0.006
0.029
0.0215
0.076
0.009
0.010
0.010
0.015
0.010
0.010
0.011
0.001
0.014
95
0.007
0.003
0.028
0.027
0.015
6.010
0.017
0.016
0.011
0.008
0.059
0.025
0.002
0.000
0.030
0.034
0.023
0.031
0.020
0.007
0.040
0.036
0.015
0.014
0.010
0.020
0.020
0.015
0.015
0.017
0.002
0.022
99
0.015
0.008
0.107
0.095
0.040
0.035
0.032
0.031
0.022
0.020
0.126
0.055
0.002
0.000
0.117
0.120
0.051
0.092
0.036
0.014
0.067
0.059
0.039
0.029
0.030
0.030
0.032
0.025
0.025
0.037
0.004
0.046
Max,
Obs.
0.1C
0.05
0.4]
0.2i
0.1'
0.1^
O.K
O.K
O.li
0.0!
0.3]
0.15
o.o:
0.0(
1.0(
0.51
0.1!
0.2!
O.Oi
0.0<
0.2;
0.1'
0.1'
0.0!
0.01
O.Oi
O.li
O.Oi
O.Oi
0.1
o.o;
0.1
All sites are 0-60 miles from a National Forest. RE = remote; RNU = rural near urban; RA = ruj
agricultural; RC = rural commercial; RI = rural industrial; R = rural unspecified; CC = city
center; NP = National Park.
5-12
-------
co
o
O
o.
<
03
O
CD
O)
c
4
Q.
CM
O
co
seou9Jnooo jo
c:
O
"O VO
o oo
•.- 01
<4- »-l
O
01 03 I
to CD I
.—• c
co ^=
—• o
li-
es) eu
o —i
oo
^O ^?
C 1-
03 n_
> Q.
s-
O •!-
01
s_
O> i—
jaqtunN
5-13
-------
sites under the influence of anthropogenic sources, the vast majority of the
hourly S02 concentrations are at or near the minimum detectable level.
Distributions of hourly concentrations from source and background monitoring
sites in the Great Lakes states, Upper Midwest, and Southeast all indicate
infrequent occurrence of hourly ozone concentrations greater than 0.05 ppm.
With such infrequent occurrence of higher concentrations, the need for charac-
terizing S0£ concentrations regarding spatial and temporal occurrences is
apparent. Unfortunately, few published characterizations of S02 air quality
are available aside from those analyses conducted to investigate frequency of
co-occurrence with other gaseous pollutants (Lefohn and Tingey, 1984; Lefohn et^
al., 1986b). These analyses are similar to those described previously for
Icharacterization of ozone air quality. An example of the length of an episode
and the time between episodes for S02 air quality is given in Table V-5,
indicating an average duration of hourly values of 0.10 ppm or greater of 1
hour; only three of these events occurred from April-October with an average of
about 2000 hours respite time between events.
The diurnal pattern of S02 occurrences indicates the time during the 24-hr
period at which the maxima occur most frequently; however the preponderence of
concentrations near detectable limits may disguise this pattern. Two examples
are shown in Figure V-5. Both display peak concentrations occurring during
daylight hours.
Two patterns of seasonality are shown for the occurrence of the higher
concentrations of S02 in Figure V-6. In one, the highest hourly occurrences
over the year occurred in February, and in the other example no seasonality can
be discerned for peak occurrences. Both monitoring sites were source-oriented.
Nitrogen Dioxide
The need to understand the temporal and spatial dynamics of N02 occur-
rences in regional or site-specific air quality is the same as with ozone and
S02- At this time, however, very few of the hourly monitoring data have been
characterized in other than the annual mean format. Some indication of spatial
and temporal occurrence is given, at least for summer months, in the cumulative
frequency distributions for a few rural sites in Table V-6. Occurrences above
40 ug/m3 are infrequent, indicating a possible episodic nature of occurrences
of high concentrations, as pointed out with the two other gaseous pollutants.
A seasonality of N02 occurrence may be important also with indications that
concentrations may be 50-100% greater in the winter than in the summer
(Altshuller, 1984).
Summary
Analysis and characterization of air quality and the contributing roles of
the three gaseous pollutants indicate an episodic occurrence of the peaks or
higher concentrations over the growing season, at least, and distinct diurnal
temporal distribution of concentrations with ozone and S02. Any investigations
of the impact of the gaseous pollutants must include these spatial and temporal
characteristics of occurrence in exposure regimes designed to study impact of
chronic exposure on trees. The seasonality of ozone, S02, and N02 has also
been observed. This is an important element of experimental design to include
5-14
-------
Table V-5. Average duration of S02 events and respite times between events
indicated at Porter Co., IN (1982) (from Lefohn, personal communi-
cation).
Event (ppm)
Duration (hr) of events
Number of events
Respite time (hr) of respites
Number of respites
0.05
1.4
26
193
25
0.06
1.3
11
414
10
0.07
1.4
8
592
7
0.08
1.3
6
829
5
0.09
1.0
5
1037
4
0.10
1.0
3
2087
2
0.12
1.0
1
—
5-15
-------
§
o-
E o
a. -
Q. O-
- 8
t.
3
D
CO g
10 12 14
Hour
16 18
22 4
E o
a. -
a. o.
- S
O o.
° I
c. *
"I
— o-
CO g
B
6246
10 12 14 16 18 20 22 2
Hour
Figure V-5.
Average diurnal concentration pattern for SOg during the growing
season months. A. Columbis Co., WI (April-October 1978), and B.
Porter CO., IN (May-October 1982). From Lefohn (personal communi-
cation).
5-16
-------
0.00
JAN
FEB MAR APR
MAY
JUN JUL
MONTH
AU6 SEP OCT NOV
DEC
o.ta
0.16
O.H
0 0.12
2 0.10
p <>•<»
« 0.06
0.04
0.02
0.00
B
JAN
ftt MAR APR
MAY
JUN JU
MONTH
AU6 SEP OCT HOV DEC
Figure V-6. Monthly average concentrations of S02 ( ) with 12 highest hourly
maxima over the year ( ) to indicate the degree of seasonality
in S02 occurrence at two source-oriented rural monitoring sites
A. Columbia Co., WI; B. Porter CO., IN. From Lefohn (personal
communication).
5-17
-------
Table V-6. Cumulative frequency distribution of hourly concentrations of nitro-
gen dioxide at rural and suburban locations (ug/m3 x 5.32 x 10~4 =
ppm NOg). From Altshuller (1984).
Site/Reference
Measurement
Period 20
ug nr3 40 ug nr3 60 ug ttr3 80 ug nr3
DuBois, PA
Research Triangle
Institute 1975
Bradford, PA
Decker et al. 1976
McHenry, MD
Research Triangle
Institute 1975
Wooster, OH
Research Triangle
Institute 1975
McConnelsville, OH
Research Triangle
Institute 1975
Wilmington, OH
Research Triangle
Institute 1975
Creston, IA
Decker et al. 1976
Wolf Point, MT
Decker et al. 1976
De Ritter, LA
Decker et al. 1976
June-Aug. 1974 13.2
July-Sept. 1975 2.1
June-Aug. 1974
June-Aug. 1974
6.9
June-Aug. 1974 23.8
5.6
June-Aug. 1974 14.9
July-Sept. 1975 0.2
July-Sept. 1975 0.4
July-Sept. 1975 4.8
1.0
0.1
0.2
6.9
0.5
2.6
0.0
0.0
0.3
0.2
0.0
0.1
1.9
0.1
1.1
0.0
0.0
0.0
0.0
0.0
0.0
0.3
0.0
0.5
0.0
0.0
0.0
5-18
-------
in single pollutant studies, and even more so in pollutant combination studies.
Both S02 and N02 monthly means indicate greater concentrations in the winter
months, whereas ozone occurs almost exclusively March through October in the
temperate regions.
Air Quality Characterization:
Wet Deposition
The trends in wet deposition of the major chemical species have been
discussed in a number of reviews (e.g., Stensland, 1984) and consequently ar,e
not detailed here. Characterization of wet deposition as it relates to
exposure profiles must consider the dual components of a wet deposition event:
the chemical composition of the hydrometeor and the deposition rate. The large
scale monitoring projects, however, have focused on the annual deposition rates
and annual volume-weighted concentrations of principal ions, rather than the
dynamics of the event.
Lefohn and Krupa (1984) analyzed wet deposition data from three databases
for several sites over the years 1979-1981. Table V-7 and V-8 present data for
1981 as an example for annual deposition rates and volume-weighted concentra-
tions of the principal ions. The authors were looking for deposition patterns
across regions of the U.S. and changes in deposition and volume-weighted
concentration of ions with season. No consistent pattern in annual volume-
weighted concentration of the principal ions across the U.S. was observed;
however, the seasonal deposition of hydrogen and sulfate were highest during
the spring and summer and the annual deposition-was less in the Western states
than in the Northeast. Regional patterns of wet deposition of the important
ions are shown in Figures V-7 and V-8 for the Western and,Eastern states,
respectively. These data provide an indication of chemical composition and at ,
least the quantity of deposition on an annual basis for analysis of wet ,
deposition in constructing exposure scenarios. ...,-•
The frequency and duration of precipitation events and spatial and .•
temporal trends are poorly characterized for both rain and cloud water. The
most significant feature of precipitation events is the pronounced variability
in space and time. For example, patterns in monthly storm events across North
America are recognized as being region-specific (Hales, 1984). As a function
of time (e.g., season, month), precipitation rates vary little in the Northeast
from month-to-month but show pronounced variation in the Southwest and North-
west. Even more indicative of the degree of variation in incident precipita-
tion is the observation that the coefficient of variation in deposition for a
single event may be as much as 100% within ,a given watershed (Lindberg et a!.,
1977). An indication of duration and volume of events was reported by "Evan?
(1984) where for several nonurban sites in eastern North America 88% of all
rainfall events were less than 160 minutes in duration and 38-58% of all
rainfall events were less than 1 mm.
The type of precipitation is also important in considering ambient air
quality and impact on vegetation. For example, Rutter (1975) has described the
normal hydrology of a low elevation forest with about 40 inches of rain a year
5-19
-------
Table V-7. Annual deposition for principal ions (kg ha-*) from January 1,
1981, through December 31, 1981. From Lefohn and Krupa (1984).
t
*
*
*
*
*
t
*
*
Sitea
Caribou
Acadia NP
Greenville St.
Bridgton
Hubbard Brook
Turners Falls
Lewes
Huntington
Whiteface
Knob it
Stilwell lake
Brookhaven
Bennett Bridge
Aurora
Ithaca
Jasper
Chautauqua
Washington Cr.
Pennsylvania St.
Leading Ridge
kane
Caldwell
Wooster
Zanesville
Delaware
Oxford
Urbana
Spooner
Mead
Huron
Manitou
Hopland
Schmidt Farm
Olympic NP
State
RE
ME
ME
ME
NH
MA
DE
NY
' NY
NY
NY
NY
NY
NY
NY
NY
NY
NJ
PA
PA
PA
OH
OH
OH
OH
OH
IL
WI
NE
SD
CO
CA
OR
WA
Ca2+
1.60
0.94
1.08
0.57
1.95
0.35
1.05
2.00
1.13
1.46
1.42
1.00
3.20
1.90
1.39
0.92
2.10
0.89
2.08
1.81
1.04
1.54
1.64
2.23
1.36
1.39
0.41
1.73
2.58
1.37
1.38
0.52
0.96
2.22
Mg2+
0.40
0.83.
0.40
0.46
0.65
0.09
0.71
0.46
0.22
0.55
0.86
0.70
0.71
0.41
0.23
0.22
'0.61
0.28
0.37
0.52
0.26
0.34
0.39
0.25
0.31
0.21
0..04
Cal
0.37
0.40
0.30
0.27
0.59
0.53
2.72
H+
0.32
0.40
0.26
0.31
0.70
0.44
0.39
0.42
0.42
0.44
0.59
0.44
0.76
0.62
0.81
0.29
0.63
0.15
0.58
0.66
0.36
0.45
0.42
0.88
0.36
0.41
0.16
ibration
0.07
0.05
0.01
0.07
0.04
0.02
0.12
NH4+
1.36
1.58
1.10
1.21
3.15
1.33
2.38
2.80
2.32
2.22
2.14
2.16
4.78
3.28
3.51
1.54
3.80
0.68
4.40
3.08
1.74
1.96
2.95
4.13
2.29
2.67
1.37
3.53
3.71
2.27
0.91
0.73
0.94
0.61
Na
0.
4.
1.
2.
1.
0.
6.
0.
0.
0.
4.
5.
0.
0.
0.
0.
1.
0.
1.
1.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
3.
:2.
22.
+
93
83
45
60
78
62
27
69
35
88
54
93
95
64
57
60
18
65
39
16
35
45
53
90
51
77
14
59
72
26
34
99
89
56
so42-
19.82
20.44
15.80
15.20
35.57
14.22
22.54
25.02
22.05
22.33
27.82
25.22
41.09
31.52
40.38
15.63
35.00
10.46
33.08
32.99
17.06
24.14
24.16
35.97
20.47
22.58
10.56
12.11
11.59
4.62
' 6.24
3.41
4.18
12.87
N03-
7.94
11.02
7.91
7.89
19.82
10.72
11.33
15.77
13.12
13.04
16.47
14.01
26.64
17.75
21.67
8.88
20.11
4.78
18.16
18.44
12.25
11.55
12.43
19.46
10.85
11.62
5.22
8.59
8.11
4.64
5.80
2.61
1.75
2.74
cr
1.68
8.58
2.13
4.50
2.89
1.16
11.57
1.09
1.21
1.68
8.51
10.87
1.98
1.31
2.11
0.78
1.89
1.10
3.17
2.30
1.14
1.11
1.19
2.05
0.93
1.82
0.39
0.68
0.97
0.50
0.46
7.42
5.08
42.03
a Database from NADP except those marked (*) are from MAP3S and (t) are from
EPRI.
5-20
-------
Table V-8. Annual volume weighted concentration values for principal ions
(ueq I-1) from January 1, 1981, through December 31, 1981. From
Lefohn and Krupa (1984).
t
*
*
*
*
*
t
*
*
Site3
Caribou
Ac ad i a NP
Greenville St.
Bridgton
Hubbard Brook
Turners Falls
Lewes
Huntington
Whiteface
Knobit
Stilwell lake
Brookhaven
Bennett Bridge
Aurora
Ithaca
Jasper
Chautauqua
Washington Cr.
Pennsylvania St.
Leading Ridge
kane
Caldwell
Wooster
Zanesville
Delaware
Oxford
Urbana
Spooner
Mead
Huron
Manitou
Hopland
Schmidt Farm
Olympic NP
State
ME
ME
ME
ME
NH
MA
DE-
NY
NY
NY
NY
NY
NY
NY
NY
NY
NY
NJ
PA
PA
PA
OH
OH
OH
OH
OH
IL
WI *>,
NE
SO
CO
CA
OR
WA
Ca
7
5
4
4
6
6
6
8
5
8
8
5
11
11
6
10
10
9
10
9
8
14
12
14
10
9
5
13
19
18
17
2
5
4
2+
.30
.17
.93
.01
.29
.16
.19
.16
.03
.58
.21
.17
.50
.66
.14
.06
.81
.66
.46
.02
.83
.07
.37
.14
.76
.21
.94,
.40
.05
.71
.01
.46
.84
.02
Mg2+
3.08
7.51
2.91
4.34
3.47
2.67
6.91
3.33
1.64
5.36
7.15
5.82
4.4l
4.12
1.69
3.83
4.47
5.21
3.10
4.29
3.41
4.71
4.87
3.24
3.87
2.31
0.91
4.80
4.98
6.91
5.82
4.73
5.56
7.70
H+
31.06
43.89
23.54
41.13
45.66
82.25
45.99
41.22
37.63
51.70
68.65
45.54
61.04
80.27
71.50
50.63
69.15
38.72
58.35
68.55
58.88
80.57
59.45
79.94
60.19
54.87
47.81
10.64
6.66
2.07
17.33
3.82
2.72
4.19
NH4+
7.21
9.53
5.51
9.11
11.56
15.77
15.54
13.08
11.40
14.26
14.21
12.53
20.24
22.80
17.19
15.22
21.73
9.78
24.65
17.79
16.07
19.22
23.71
24.37
20.34
19.55
22.20
30.48
30.94
40.07
13.20
3.90
6.10
1.18
Na+
3.62
22.78
5.45
11.79
5.05
7.60
32.02
2.81
1.34
4.57
17.74
25.71
3.09
3.23
2.21
4.43
4.73
7.61
6.11
5.15
3.25
5.51
3.68
6.93
3.64
4.41
1.77
3.77
4.53
3.28
3.69
16.84
16.55
33.79
so42-
39.81
46.68
29.73
41.83
48.44
54.54
55.12
48.80
40.71
54.89
67.51
54.26
67.13
84.57
74.26
60.24
78.49
53.22
69.50
71.13
60.16
91.74
72.60
71,18
69.60
62.10
64.13
40.09
26.49
29.83
33.83
6.73
10.68
9.60
N03-
11.97
19.13
11.50
17.71
20.44
39.61
21.45
22.83
18.75
25.57
30.87
23.48
33.70
35.79
30.85
25.93
33.14
18.77
29.53
30.28
31.86
31.66
29.27
34.00
28.68
24.73
24.55
21.07
19.55
23.21
24.36
3.92
3.36
1.66
cr
4.38
26.14
5.04
12. 96
5.31
8.90
38.25
2.82
3.01
5.39
21.43
30.36
4.24
4.41
5.24
3.80
5.07
8.51
9.00
6.62
5.05
6.01
5.20
7.42
4.15
6.75
3.20
2.64
3.85
4.27
3.37
20.35
19.18
41.18
a Database from NADP except those.marked (*) are from MAP3S and (t) are from
EPRI.
5-21
-------
Figure V-7.
Mean annual wet deposition at western NADP sites. A. Precipita-
tion (cm), B. pH (volume weighted mean), C. H (g m~^), D. SO^
-2 + -2
(g m ), E. N03 -N + NH4 -N (g m ). = 1981, = 1982,
1981-82. = data not quality assured by NADP. From Olsen and
Watson
-------
1.0
2.0
2.0
Figure V-8. Wet deposition in the eastern U.S. for 1982. All values g nr2
A. N03", B. NH4 , C. S04^". From NADP (1985).
5-23
-------
and approximately 20 inches evaporation. For this type of area, mist and fog
make up less than 2% of the water input. In contrast, Lovett et aK (1982)
suggested from models that fog water deposition accounts for almost half of the
water input to high elevation fir forest in New Hampshire. In the Green
Mountains of Vermont, Vogelman et aH. (1968) estimated that 70% of the precipi-
tation is via fog water. Analysis of existing data would suggest that the
deposition in fog water for mountaintops could equal the deposition in rainfall
and dry deposition combined (R. Bradow, personal communication); thus analysis
of ambient air quality for mountainous forests should include considerations of
the dynamics and chemical composition of fog. Monitoring of fog deposition has
been limited, but programs are beginning (R. Bradow, personal communication,
S. E. Lindberg, personal communication). In addition to chemical analysis of
fog water for principal ion concentration, the temporal and spatial nature of
the events, particle size distribution, wind speed, etc. are necessary to
collect in monitoring programs (Bradow, personnel communication). An example
of fog water chemistry is shown in Table V-9, and indicates the much greater
deposition of sulfate, nitrate, and ammonium ions by fog than by bulk precipi-
tation in the White Mountain area.
Air quality in a given region may have elements of both wet and dry
deposition as well as more than one important component within the overall
general description of wet or dry deposition. Characterization of air quality
for pollutant combinations has received only limited attention and yet is
undoubtedly important in evaluating air quality impact on forests.
Co-Occurrence of Pollutants
Most studies concerning the impact of pollutant combinations on plants
have assummed that there are frequent periods of pollutant co-occurrence.
However, this assumption is not supported by analyses of actual air quality
data. Only a few studies have attempted to described the co-occurrence of
pollutants in the atmosphere and related this information to a biological
viewpoint. This lack of characterization data is a consequence of both limited
ambient monitoring for multiple .pollutants at a given site and also of the
conception that the air must contain multiple pollutants just by its very
nature and therefore it is not necessary to resort to actual air quality data
to decribe the joint occurrence of multiple pollutants.
Although air pollutants have been co-monitored at only a few sites, these
provide sufficient indication of pollutant co-occurrence. For example, in a
study using data from SAROAD (1981), T.VA (1978 to 1982) and EPRI SURE/ERAQS
(1978 and 1979), Lefohn and Tingey (1984) found 91, 124, and 147 site-years of
data available in which the pollutant pairs, S02/N02, 03/802, and Os/N02,
respectively, were monitored simultaneously. The authors defined pollutant
co-occurrence as the simultaneous occurrence of two pollutants with hourly mean
concentrations of 0.05 ppm or greater of each gas. Based on this criterion,
episodes of pollutant co-occurrence usually lasted only a few hours per week
and the interval between episodes was generally large (weeks to sometimes
months). Most monitoring sites experienced fewer than 9 hours of pollutant
co-occurrence, for any of the three pairs of pollutants during the 5-month
growing season (May to September).
5-24
-------
Table V-9. Annual deposition in cloud capture and bulk precipitation. Adapted
from Lovett et al. (1982) (Bradow, personal communication).
Ion
H+
NH4+ (as N)
Na+
K+
S04" (as S)
N0o~~ (as N)
Cloud Deposition
kg/ha-yr
2.4
13.4
5.8
3.3
91.9
22.9
Bulk Precipitation
kg/ha-yr
1.5
3.4
1.7
2.1
21.5
5.3
Percentage
from Clouds
62
80
77
61.
81
81
5-25
-------
Analysis of data from a portion of the Ohio River Valley that contained
four coal-fired power plants supports the conclusion that the co-occurrence of
S02 and NOX was small (Jacobson and McManus, 1985). Using minimum concentra-
tions of 0.05 ppm and 58 ug m~6 for S02 and NOX, respectively, the authors
showed that the two gases occurred jointly for less than 1% of the total hours
monitored. Analysis of air quality data from central London, during the
heating season (January through March), also found that the co-occurrence of
S02 and N0£ was small, less than 1% of the time (Lane and Bell, 1984a). The
occurrences of S02 and NO were positively correlated; simultaneous occurrences
were measured between 2 and 4% of the monitoring period when a minimum concen-
tration of 0.05 ppm for each gas was used.
The analyses of Lefohn and Tingey were critized because they: (1) used
only one year (1981) of SAROAD data; (2) included a large number of non-rural
sites; (3) used a relatively high (0.05 ppm) concentration threshold to
determine pollutant co-occurrence; and (4) failed to consider the importance of
possible sequential pollutant occurrence within the same day. In response to
these issues, Lefohn £t jjl_. (1986b) have extended the analysis of pollutant
co-occurrences for rural sites to consider both simultaneous and sequential
occurrences using a lower (0.03 ppm) concentration threshold and several years
of data. Even using the lower concentration to define a simultaneous pollutant
co-occurrence did not change the conclusion that simultaneous occurrences were
rare. For any of the two pollutant pairs, the majority of sites experienced
fewer than 10 to 20 hours of simultaneous occurrence during a 5-month period
(May to September; 3672 hours). In addition, sequential occurrences, when both
pollutants exceeded the minimum concentration (0.03 ppm) within the same day,
were also rare events during the 5-month (153 days) period. The majority of
the sites experienced fewer than 10 to 20 days of pollutant co-occurrence. The
low numbers of simultaneous and sequential occurrences were observed in all
five years (1978 to 1982) of data analyzed. Although not well characterized
from a biological perspective, seasonal patterns of co-occurrence should be
considered, especially in relation to possible effects on long-lived perennial
species. For example, ozone tends to be elevated during the second and third
quarters of the year while in many areas sulfur dioxide would display its
highest concentrations during the first and/or fourth quarters of the year
(e.g., Figure V-6a), but at other sites (Figure V-6b) there is no apparent
seasonality in the occurrence of high concentrations of sulfur dioxide.
While the temporal and chemical dynamics of individual dry or wet deposi-
tion events are recognized as important parameters to characterize in evaluat-
ing the influence of changes in air chemistry on the productivity of vegeta-
tion, the temporal and chemical relationships of the combination of sequential
dry and wet deposition events is unclear. It is hypothesized that the duration
and chemistry of the dry deposition period preceding a wet deposition event are
important because: (1) the potential for interactive effects of sequential
pollutant exposures (e.g., dry deposition of ozone followed by the interception
of acidic cloud water; Krause et jfL, 1984; Shriner et^ a]_., 1986); and (2) the
potential hydration on the leaT~surface of previously dry-deposited pollutants
(e.g., sulfur dioxide) which would enhance their mobility and phytotoxicity
(Hoffman et j^l_., 1980). In a reciprocal fashion, a wet deposition event
immediately prior to a period of elevated levels of a gaseous pollutant might
predispose the plant to injury through a modification of the plant's water
5-26
-------
relations. It is likely that the more ephemeral the precipitation event (e.g.,
brief rain events, cloud water interception), the more concentrated (in the
liquid phase on the leaf surface) will be the solubilized dry-deposited
pollutants (Evans, 1984). Consequently, on a theoretical basis, the temporal
and chemical features of sequential periods of wet and dry deposition are
likely to be important features of air quality characterization. However, the
temporal association of dry and wet deposition events is not well character-
ized. This lack is, in part, the result of different'temporal measurement
scales for measuring dry and wet deposition and also the lack of co-located
monitors for both types of deposition. Also, scientists have only recently
become, aware of the importance of conducting studies that include both wet and
dry deposition. Using the SAROAD air quality data base and wet .deposition data
from NADP and EPRI, Lefohn and Benedict (1983) concluded there was a potential
for periods of elevated ozone and acidic precipitation to occur over the same
geographic area during the growing season. Similarly, from an analysis of
ozone concentrations and wet deposition events, Taylor and Norby (1985)
concluded that the region of the eastern United States experiencing the highest
annual rainfall acidity (pH 4.2) also experienced elevated ozone exposures.
An example of the potential for sequential exposures to elevated levels of
ozone and Wet deposition events is provided by a review of data for a deciduous
forest in Tennessee. In this area there are generally five cycles of "defined
periods of dry and wet deposition" per month during the growing, season, with a
dry period defined as being >_2 days in duration (G. M. Lovett, personal
communication). During the same time interval, periods of elevated ozone
levels commonly average two-to-three per month with each event averaging
two-to-three days (Taylor and Norby, 1985). Thus, at a maximum, only three of
the wet deposition periods per month are likely to be preceded by a dry
deposition period of elevated ozone. This sequential occurrence is likely to
be even less frequent both earlier and later in the growing season as the
frequency of sustained elevated levels of ozone declines. Because this
analysis is based upon site- and year-specific data, the pattern should not be
generalized indiscriminately, but it does indicate that the frequency of
sequential exposures involving wet and dry deposition is not likely to be much
greater than three or four events per month.
5-27
-------
C. Exposure Regimes for Determining the Effects of Deposition on Plants
The methods for determing the effects of wet and dry deposition on
vegetation are as diverse as the disciplines participating in the investiga-
tions. The assessment of effects involves dry and wet deposition monitoring,
exposure methodologies, and statistics as well as various biological and
physical sciences which provide the tools for the diagnosis and evaluation of
air pollution impacts. It is obvious that different experimental objectives
require different experimental and exposure methodologies.
To establish and determine the effects of air pollutants on plants several
basic criteria must be satisfied: (1) the conditions (climatic, edaphic,
exposure, etc.) must approximate or'be representative of those in the area of
interest; (2) a relationship must be established between the cause and the
resultant effect; (3) the exposure-response relationships must be quantified.
To satisfy these criteria, a range of experimental approaches with several
levels of environmental control is required.
Exposure Regimes
In studying the effects of dry and wet deposition on plants a number of
exposure regimes has been used, ranging from artificial "square wave" exposures
to highly sophisticated simulations of ambient conditions. The types of
exposure regimes selected for use in plant response studies have been influ-
enced by the'specific objectives of the individual investigators. For example,
"square wave" exposure regimes frequently have been used to conduct initial or
exploratory studies of plant response to a new pollutant or for screening a
large number of plants for their relative sensitivity. Sophisticated simula-
tions of ambient air quality distributions have been used to investigate the
components of an exposure that are most important for influencing plant
response and to provide a realistic respresentation of the exposure so that the
results are readily extrapolated to ambient conditions.
In addition to changing experimental objectives, exposure regimes have
become more sophisticated as technology and equipment have improved, resulting
in decreased cost over time. These advanced control systems are easier to
operate and the expertise to manage them is readily available. As scientists
better understand the relationship between plant response and the temporal and
spatial patterns of pollutant exposure, there is the companion need to under-
stand the characteristics of an exposure that impairs plant performance. This
awareness has fostered development of exposure regimes representative of
ambient air quality and that can also be modified to test specific biological
hypotheses.
The types of exposure regimes that have been used in dry and wet deposi-
tion studies will be reviewed separately. The review is not comprehensive, but
briefly describes general types of exposure regimes, listing some of the
strengths and limitations of each. References are not provided for regimes
with historic use, but are given for some of the hewer protocols or ones that
are not widely cited in the literature.
5-28
-------
1. Dry (Gaseous) Deposition
The types of exposure regimes used in dry deposition studies can generally
be separated into two classes, artificial and episodic. The distinguishing
factor between the two is whether there was an overt attempt to represent the
temporal patterns of ambient pollutant concentrations.
a. Artificial Exposure Regimes
Although there are many types of artificial regimes they all have the same
characteristic: the regime was not developed to represent the temporal
variation of ambient exposures but to meet other specific objectives; and the
concentrations and exposure durations are frequently much higher and longer
than ambient. Many studies have used "square wave" exposure regimes, of either
short or long durations, i.e., the concentration was approximately constant for
the duration of the study. Most investigation into the effects of pollutant
combinations have also used artificial exposure regimes because the frequency
of pollutant co-occurrence and the duration of exposure were much larger than
those found in the ambient air.
Long-term artifical exposure regimes are not well suited for studies
designed to determine the effects of air pollutant(s) on plant growth or yield
because the exposures are not representative of ambient. Consequently, it is
difficult to relate the results to ambient conditions. However, for some
physiological and biochemical studies, preliminary work on a problem or for
cases where there is insufficient data to develop a more realistic regime, they
may prove useful.
b. Episodic Exposure Regimes
This class of regimes range from actual ambient to those that simulate
ambient. The distinguishing characteristic is an overt attempt by the scien-
tist to represent the actual temporal concentration and frequency variations
that exist in ambient air quality data. Some types of episodic exposures have
been used for years while others are recently developed. The episodic regimes
can be segregated into three classes-: ambient, modified ambient, and simulated
ambient.
. . (1) Ambient Profiles
These regimes include comparisons of filtered and unfiltered air in
various types of chambers or air exclusion systems as well as placement of
plots at varying locations along a pollution gradient, usually near a point
source of pollution.
The comparison of the effects of filtered and unfiltered air on plants
permits the determination of the potential impact of the current pollution load
on plant performance as a consequence of the actual air quality at the location
of the study. However, because only a control and one treatment level are used
5-29
-------
it is not possible to construct exposure-response functions; inferences cannot
be made concerning what would happen if the air quality either improved or
deteriorated. Also the data are site-specific preventing inference to the
region or beyond.
Placement of plots along a pollution gradient permits the assessment of
plant performance under the actual air quality and environmental conditions
that exist. But unlike the filtered-unfiltered comparisons, a range of
pollutant(s) concentrations is used and thus exposure-response functions can be
generated. An ambient gradient has been used to determine the effects of
sulfur dioxide (e.g., Guderian and Stratmann, 1962a,b) and ozone (e.g., Oshima
et a!., 1976) on plant growth and yield. However, unless the pollution
gradTent is steep or exists over an area of uniform climatic and edaphic
conditions, the influence of these .factors can confound the results. Other
factors may also differ along the gradient which may cause additional problems.
(2) Modified Ambient Regimes
These approaches are, in reality, extensions of the ambient exposure
methodologies using filtered- and unfiltered-air but with an added design
element, attempting to create a range of exposure conditions so that exposure-
response functions can be developed. This approach is directly coupled to the
current ambient concentrations and consequently the same exposures cannot be
repeated at different locations or under different environmental conditions.
The modified ambient exposures have been used extensively to assess the impact
of air pollutants on plant growth and yield.
There are two basic approaches to creating a range of conditions so that
exposure-response functions can be developed. In the first, additional ozone
is added, as either a range of fixed concentrations or proportions of ambient
concentration, to create a range of exposure levels (e.g., Heck et^ jiJL, 1982;
Temple et ail_., 1985) as was done in the National Crop Loss Assessment Network
(NCLAN)Trogram. The ozone-addition approach can create exposures at the
ambient level and higher, but not for exposures below the current ambient. No
studies have been published to determine if the pollutant frequency distri-
butions resulting from the ozone-addition approach preserve a frequency
distribution similar to that found in the ambient air. A second approach has
used differential air filtration in which varying proportions of charcoal-
filtered air and unfiltered air are mixed to produce a range of pollutant
concentrations (e.g., Oshima, 1978; McCool et jiK, 1986). This approach can
only produce concentrations up to the current ambient level, and is most
appropriate for areas that have a relatively high natural pollution load. To
develop a range of exposure conditions below and above the current levels using
the modified ambient approach, it would be necessary to combine both the
pollutant-addition and the differential-filtration methods.
5-30
-------
(3) Simulated Ambient Regimes
Although various methods have been used, all of the simulated ambient
approaches take actual ambient air quality data to create exposure regimes that
simulate the observed temporal concentration changes.
The simplest approach is to obtain air quality data for a given site and
time period and then reproduce that exposure under defined experimental
conditions (e.g., Mclaughlin et aH., 1976). This method works well to deter-
mine if a specific exposure condition is phytotoxic, but the resultant data are
source- or site-specific, limiting extrapolation.
The statistical approaches have analyzed and mathematically characterized
ambient air quality. The resultant mathematical parameters were used in a
statistical model to construct exposure regimes. The log-normal statistical
parameters that characterized ambient sulfur dioxide concentrations were used
in a computer program to stochastically create a range of exposure concentra-
tions to study the effects of sulfur dioxide on crops (Male, 1982; Male et_ jH.,
i. y 00 /«
Pragmatic approaches have also been used which mathematically characterize
the ambient air to develop regimes that simulate temporal variations of ambient
exposures. Some use actual air monitoring data from one to several sites for
characterization (Lane and Bell, 1984a, 1984b; Hogsett et _al_., 1985; Lefohn et
jH., 1986a, 1986b), while others have used predicted hourly concentrations
generated by pollutant dispersion models (Laurence and Kohut, 1984). Rather
than attending to determine the specific statistical parameters of the distri-
bution, the pragmatic approaches use statistical correlations, frequencies of
occurrence, duration of and between events and other "rules of thumb" to
generate regimes that maintain the same statistical properties as the original
air quality data used to develop them. The air quality that is characterized
in the pragmatic approach can represent a site or a range of sites making up a
region of interest for one or more years. The broader data set allows develop-
ment of average characteristics and variations thus providing opportunity for
wider extrapolation of the data.
The simulated ambient approaches can be used to develop exposure-response
functions for plants exposed to various air pollutants. The effects of
pollutant combinations on plants can be determined using realistic frequencies
of pollutant co-occurrence. Because the regimes are mathematical representa-
tions of ambient conditions they can be used to investigate the components of a
pollutant exposure that influence plant response. Simulated ambient exposures
provide the experimental flexibility necessary to adapt to changing biological
objectives. Despite their obvious realism and experimental advantages, the
simulated ambient exposures require sufficient ambient air Quality data to
characterize the ambient concentrations and their temporal variations before
exposure regimes can be developed.
The simulated exposures are realistic representations of the ambient air
because they maintain the temporal concentration variation and the statistical
properties of actual exposures. These characteristics permit the resultant
data to be extrapolated to ambient conditions, especially if the environmental
5-31
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conditions are realistic. Both the statistical and pragmatic approaches permit
the generation of exposure regimes that are characteristic of a given area, not
just a specific site. These approaches are well-suited for creating a range of
exposure concentrations necessary for the development of exposure-response
functions and for long-term studies on plant growth. The simulated ambient
regimes can be reproduced over time, at several locations and under a range of
environmental and edaphic conditions to yield the required replication of a
response over time and space. This replication is necessary to establish
cause-effect relationships required to determine if airborne pollutants are
impacting forests.
2. Wet Deposition
Exposure regimes for wet deposition are potentially more complicated than
those for dry deposition studies. They must'recognize the dual nature of a wet
deposition event which includes both the chemical composition of the hydro-
meteor (which can take several forms) and its deposition rate. Experimental
protocols used to investigate the impact of wet deposition on plants have
attempted to (1) determine relative sensitivity using various biological
indicators; (2) determine the mode of action; and (3) develop exposure-response
functions which mathematically relate plant response to wet deposition expo-
sure. The term "exposure-response" suggests a univariate association between
exposure and plant response. However, a wet deposition event includes several
different parameters; consequently complex factorial or multivariate analyses
may be required to define the "exposure-response" relationships adequately.
Studies of wet deposition impacts on vegetation have been conducted in
both controlled environments (e.g., greenhouse chambers, controlled environment
chambers) and under field conditions. Controlled environment studies are
useful indicators of potential impacts and permit the study of effects not
readily measured under less controlled field conditions. Plant variability in
controlled environments is frequently less, allowing the detection of smaller
differences; these type studies are well suited for physiological and biochem-
ical studies as well as short-term (a few days) screening studies.- However,
because the cultural and environmental conditions are substantially different
from those in the ambient, it is difficult to extrapolate the results to the
field, especially for long term growth and yield studies. Plants grown in'
controlled environments are more sensitive to acid rain injury than field grown
plants (Irving, 1984), In most controlled environment studies the total
deposition of H+, S042", and N03~ is greater than in field studies because of a
larger number of exposures and/or a higher deposition rate.
In field studies, the environmental conditions are the same as or similar
to ambient conditions. Consequently the data are site-, or at most, region-
specific, and it is difficult to extrapolate the data to other climatic or
regional conditions. Because the conditions are less controlled, there is
usually greater plant variability than in controlled environment studies., To
manage for this variation and obtain reliable data requires large numbers of
plants as well as studies conducted over several years. Although labor and
cost intensive, field studies are often the only way to obtain the necessary
type of data. Another frequent problem with field studies is the lack of a
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comparable unpolluted control. In many regions it is necessary not only to
control or exclude the wet deposition .events but also to control other pollut-
ants (gaseous or dry) that are not a part of the study so they do not confound
the experimental results.
In the field, wet deposition exposure regimes can be divided into two
classes: those that receive wet deposition in addition to ambient rain events
and those in which the ambient wet deposition is excluded and a simulated wet
deposition is applied.
The addition of wet deposition to ambient rain requires relatively simple
equipment and usually induces little or no change in the environment around the
plants. The plants are exposed to the ambient array of wet and dry deposition
types. The approach cannot determine vegetation effects at or below the
current ambient deposition level; also the crops receive an unusually large
amount of water and larger depositions of H+, S04Z~, and N03~ than would be
typical for the area.
The exclusion of ambient wet deposition and the subsequent simulation of
wet deposition permits the development of exposure-response functions and the
determination of potential impacts both above and below the current ambient wet
deposition level. However, for the data to be most useful, the study must
simulate not only the wet deposition chemistry and hydrometeor characteristics,
but also ensure that the environmental conditions during a wet deposition event
are similar to ambient conditions. The methods for excluding and simulating
wet deposition are relatively expensive and may alter the environmental
conditions from the ambient. Because wet deposition is excluded, the method
must provide additional water to the vegetation to compensate for the loss of
ambient rainfall.
Exposure protocols used to investigate the influence of wet deposition on
vegetation are defined by the combination of precipitation chemistry and the
multiple aspects of the hydrometeor1s deposition rate which include deposition
rate (cm/h), frequency and duration of events. The temporal dynamics of these
dual features are important to consider in developing exposure regimes. One
conspicuous feature of wet deposition studies is the extreme diversity among
the experimental designs in the selection of chemical and, depositional proper-
ties. The chief explanations for this diversity are the pronounced spatial
(e.g., continent, region, elevation) and temporal (e.g., season, year) varia-
tion in precipitation chemistry and climatology and the multiple forms in which
precipitation can occur.
To simulate the chemistry of the hydrometeor, most studies have used
published records of ambient rainfall or cloud water chemistry to develop their
exposure regimes. However, the extent of the mimicry is commonly restricted to
selected aspects of the ambient precipitation. For example, the most common
simulant specification is acidity, achieved through the addition of sulfuric
and/or nitric acids. While many designs maintain a realistic sulfuric-to-
nitric acid ratio in achieving the desired hydrogen ion concentration, this
ratio specification is not always appreciated. In many of the regimes, the
chemical characterization has considered only the sulfuric/nitric ratio or the
hydrogen ion concentration without taking into account other cations and anions
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that are common components of precipitation (e.g., geologic or anthropogenic
cations and anions, including trace elements, heavy metals, organics, and free
radical species). This aspect is essential when considering cloud water/mist
because concentrations of multiple cations and anions tend to be enriched in
cloud water relative to rainfall in the same region.
Most of the exposure protocols have maintained a constant ("square wave")
chemical profile (based on growing season average concentrations for the
individual ions for the area), for the duration of the wet deposition event
with the same event repeated throughout the duration of the study. However, in
most natural rain events the hydrometeors show an initial enrichment in
sulfates, nitrates, and hydrogen ions compared to droplets occurring later in
the event. Although no studies have investigated the importance of "peak"
events versus total deposition, studies have shown that plant response is
different when the rain chemistry varies from event to event rather than being
the same all the time (Irving, 1984). Only a few studies have attempted to
increase the pH during a rain event (e.g., Kelly £t ^1_., 1984) as occurs in
nature. Johnston et a/L (1982) reported that the growth of bush beans was
reduced more when exposed to acidic rain (pH 3.2) in which the hydrogen ion
concentration varied during the event than in plants exposed to a constant
concentration of the same average pH.
The most common method of varying the exposure regime is through changes
in the depositional features of the hydrometeor. The physical parameters used
to characterize the hydrometeor encompass the rate/intensity of the event
(cm/h), event duration (h), number of events/week, duration of the time between
precipitation events, and the periodicity of the events. Most protocols
establish some combination of rate, duration, and number of events per week to
achieve a predetermined hydrometeor or chemical deposition rate which is
commonly selected based on ambient climatological or chemical data. This
predetermined specification is typically a multiple year or seasonal average
and thus may not convey many of the stochastic or rhythmic features of ambient
wet deposition. As with the chemical aspects of wet deposition protocols, most
of the physical features can be characterized as being "square wave". Thus,
the duration, intensity, number of events per week, and the periodicity in most
exposure protocols are invariant, with the only variable being the duration of
the inter-event period. This feature is descriptive of most controlled and
field exposure protocols.
Exposure protocols with respect to cloud water/mist interception have
additional features that separate them from typical rain deposition studies.
The concentrations of various ions in cloud water/mist are typically enriched
with the same concentrations of multiple cations and anions as the region's
rainfall. Also, cloud water deposition occurs predominately to the foliage
rather than the soil. Most experimental' protocols for cloud water/mist studies
qualitatively (rather than quantitatively) characterize deposition according to
the degree of surface wetness or immersions by the event. Commonly, the degree
of wetness is invariant in repeated exposures so that the chemical deposition
rate is constant within and among events.
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Studies should also consider the area being simulated. For example, it is
estimated that high elevation forests are immersed in advective clouds as much
as 50% of the time (annual average). With a decrease in elevation, the percent
of time that the foliage is wetted by clouds declines to near zero.
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VI. Recommendations
A. Workshop Guidelines
The workshop was structured to obtain a collective judgment on recommenda-
tions for standardizing pollutant exposure systems (both wet and dry) and
exposure protocols for the Forest Response'Program of the National Acid
Precipitation Assessment Program. The participants were working either
directly with pollutant exposures as biologists or engineers, or indirectly in
the forest sciences, contributing the perspective of tree growth and assess-
ment. The workshop was informal and interactive; the objective was to achieve
a consensus on the recommended level of standardization of exposure studies
with forest species. Small, interactive groups explored and discussed recom-
mendations for standardization within the three categories: (1) dry deposition
exposure systems; (2) wet deposition exposure systems; and (3) deposition
exposure regimes. Participants were asked to answer:
1. What is the minimum level of standardization of exposure studies {wet and
dry deposition) to pursue?
2. Should that level of standardization be applied to specific equipment/
facility or should the level of standardization be applied to performance
characteristic(s)?
3. What is the recommended standard of exposure system and protocol for
specific research objectives?
The preceding chapters reviewed existing exposure systems and evaluated
them with criteria designed to determine the system's ability to provide
reproducible experimental conditions; discussed the strengths and limitations
of the various types of exposure systems; and identified approaches used to
develop exposure regimes for fumigation studies. Decisions recommending a
standardized system and/or standard performance characteristic, emphasize the
obvious need to consider the research or biological objective and the practices
required to work with tree species. This caveat pertains to the previous draft
document provided the participants and to the recommendations offered here.
The objective of the workshop was a consensus recommendations for stan-,
dardizing exposure systems and protocols in forestry species studies. As a
clear guideline to achieve this end and recognizing the need to consider the
biological objectives, the participants focused their discussion on the follow-
ing generic biological objectives, tempered with concern for the specific
requirements of forest species:
1. Multi-year (2-5 years) growth/physiological studies
A. Seasonal exposures
B. Year-round exposures
C. Climatic and biotic interaction with exposure (e.g., drought stress,
nutrient stress, pathogens)
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2. Short-term (< 2 years) growth/physiological studies
3. Gas-exchange/pollutant uptake studies
In the above categories, the age or size of plant material, the species
(deciduous or coniferous), and pollutant type will also influence the recommen-
dation(s).
For each of these broad objectives and for the type of pollutant, the
participants were instructed to answer the following questions for each of the
generic objectives:
1. Chambered versus non-chambered systems?
2. Type of chambered system? or performance characteristic?
3. Type of non-chambered system? or performance characteristic(s)?
4. Standardization of the monitoring needs from equipment perspective as well
as sampling frequency.
5. For each generic research objective, address the appropriateness of
standardized exposure regimes. If appropriate, what would the standard-
ized exposure regime consist of and how should it be developed.
It was important to acknowledge that there might not be unanimity for
certain standardization recommendations, and that was acceptable. Dissenting
views would be incorporated into the final recommendations if there were valid
scientific reasons.
B. General Recommendations for Exposure System
There was workshop consensus that certain elements of exposure studies
should be standardized and these are presented in Table VI-1. The first five
items in Table VI-1 were identified as those to be standardized within and
between programs; the details should be specifically determined in the indi-
vidual quality assurance programs. Certain points, however, were emphasized:
(1) the need to use EPA-approved monitoring equipment; (2) ensure that sampling
frequency is sufficient to characterize hourly mean concentration values,
perhaps utilizing time-series models to determine the appropriate sample size;
and (3) ensure that gaps in monitoring data be handled the same at all research
sites. In developing quality assurance programs, the primary objective of
useful air quality data from all research sites must be paramount. Acquisition
of micrometerology should be of high priority in both chambered and non-
chambered exposure systems, and the techniques employed similar and standard-
ized throughout the programs. The importance of this micrometerology data is
obvious in characterizing the growth and vigor of trees within the exposure
systems, and this characterization of tree growth is imperative to document in
future studies with these exposure systems. Standardizing the units for
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Table VI-1. Components of pollutant exposure studies which should be standard-
ized.
**
**
**
**
**
**
**
POLLUTANT MONITORING
Equipment/Precision
Sampling frequency (dependent on deposition)
Handling missing data
MICROMETEOROLOGY DATA
Temperature
Solar radiation
Relative humidity
Wind speed
FORMATS FOR COMPUTER DATA BASES
UNITS
PERFORMANCE CHARACTERISTICS FOR DOCUMENTING SYSTEMS
EXPOSURE SYSTEMS
Dry deposition
Wet deposition
Wet/dry deposition
Particulate (need more information)
EXPOSURE REGIMES
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various measurements and employing consistent formats for tolerance across all
sites are obvious requirement for large programs where data from many sites
will be integrated.
The desire to have researchers characterize their exposure systems and to
maximize comparability of studies among research sites highlights the need to
select performance characteristics that can document system performance. An
extensive list of performance criteria are given in Table III-l with three
primary categories: (1) physical and chemical nature of the pollutant in the
atmosphere of the exposure system; (2) physical and chemical features of the
environment; and (3) biological features of the exposure environment. Docu-
menting the biology of the trees might include whole plant carbon economy
(i.e.., carbon acquisition and allocation to various "sinks" and maintenance
costs) as a measure of plant vigor in response to the exposure environment.
This performance characteristic becomes critical because most exposure systems
have been used with annual agricultural crops, not perennial, woody species,
and not primarily in year-round studies of growth response. During the
workshop, many considerations of exposure systems were tempered by the lack of
plant (tree) performance data for the system. The atmospheric environment of .
the exposure systems is usually characterized, at least to some degree, by
physical and chemical features; however, only minimal attention has been paid
to plant performance, and there has been no substantial characterization of
tree growth and tree culture needs.
Recommendations for exposure systems by categories of dry and wet deposi-
tion were made for the various types of systems; they were not directed toward
a specific brand name or "official" equipment. Examples of the systems are
given in references where appropriate, and potential users are charged to
duplicate performance characteristics of the various type systems, not neces-
sarily the specific equipment (brand-names, etc.) that comprise the system.
C. Dry Deposition
The participants recognized that the "ideal" system for multi-year growth
studies with both seasonal and year-round exposure to different pollutant types
would be structureless, i.e., not introducing artifacts of growth with hardware
and shelter. There are, however, necessary tradeoffs. One is the need to
exclude ambient pollutants (wet and dry deposition) and a chamber or enclosure
is a means to accomplish this. Another is the need for a wide range of
treatments in exposure-response studies and enclosures are a less costly means
to accomplish this. The automated exclusion system, in use for some acid-rain
deposition studies, represents an attempt to minimize the structural artifacts
except during the ambient event (i.e., rain event). The cost of such systems,
however, is not trivial, particularly when a range of treatments and replica-
tions are required.
Since there is no exposure system well-characterized for tree growth and
pollutant delivery, participants recognized the value in using a variety of the
available exposure systems. Thus avoiding the undesirable introduction of
consistent bias into the programs yet the opportunity for cross-comparibility
between research sites with some systems is still possible. The workshop
64
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recognized the utility of a variety of exposure systems for multi-year growth
studies including both chambered and non-chambered types. Actually, since the
exposure systems are all poorly characterized regarding use with trees,
especially the larger sizes, a multi-system approach is appropriate, with
characterization of the systems being conducted concurrently. If possible,
developmental work on exposure systems should be funded. Recommendations for
dry deposition exposure systems are given in Table VI-2. The chamber design
recommended, based on years of experience in crop exposure studies and the
attendant characterization of these chambers, was the open-top type. More
important, it was recognized that the open-top chambers have the ability to
exclude certain ambient pollutants, thus allowing a wide range of defined
pollutant treatments with a moderate modification of the environment. Ambient
pollutant are excluded with filters, although they are not available for all
pollutant types. The charcoal filter used for ozone exclusion is rather
non-specific, and the other ambient gases and particles excluded or not should
be characterized. The availability of filtration systems for other gaseous
pollutants and particulates in dry deposition studies is a recognized limita-
tion of the open-top system, and filtration efficiencies and characteristics
should be documented. The environmental modifications imposed by the chamber
are relatively slight compared to other types of field chambers, and have been
documented; consequently chamber effect on plant (annual crops) responses can
be factored into the response evaluations. Documentation for tree species is
not available. There are several acceptable open-top designs — for example,
the Heagle et jil_. (1973) chamber (Appendix C) has been used for a large variety
of plant species with no consistent effect on annual crop growth during the
growing season; the ITE chamber (Appendix C) has the most acceptable air flow
characteristics of any current design, but is too small for many plant and tree
species (D. Fowler, personal communication); the Kats et al. (1985) chamber
(Appendix C) has proven useful for studies with small "cTtrUs trees. The much
larger open-top chambers of the Boyce Thompson design (Laurence et jil_., 1985)
(Appendix C) may be useful for larger tree seedlings and saplings. Other
chamber designs are appropriate for specific uses, such as those with umbrella
tops for acidic rain and gaseous exposures (Hogsett et _aj_., 1985; Krause,
personal communication) (Appendix C).
A limitation of the open-top chamber is the alteration of the microclimate,
especially in the cooler months and during those particularly critical times of
entering dormancy and bud break. The concern is one of uncertainty as to tree
growth behavior with microclimate change. The chamber temperature increase of
1-3 C over ambient might alter metabolic activity in the trees and result in
aberrant reaction to stress treatment. The possiblity of winter desiccation
exacerbated by continual operation of blowers for pollutant dispersal was a
concern for proposed year-round exposure studies. These are all unknowns, and
without data to predict the outcome it is necessary to encourage caution.
Where possible, corroborative studies should be pursued to characterize these
concerns. A corollary of the microclimate alteration is the uncertainty
regarding the length of time trees should be held in the chambers or the
duration of the exposure studies in open-top chambers. The altered micro-
climate might lead to cumulative effects on the tree growth/metabolism over
time, and the nature of this effect is unknown. A third limitation of the
open-top chambers is the size of the trees to be used. The maximum functional
chamber size has not yet been determined, but the selection of the experimental
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Table VI-2. Recommendations for dry deposition exposure systems.
Multi-year (2+ year) Growth/Physiological Studies:
Year-Round and Seasonal Exposures
OUTDOOR CHAMBERS (open-top design)
Strengths
* Operational
* Defined pollutant
Limitations
* Age/size of trees
* Duration limit
* Moderate environmental modification * Chamber 'microclimate (e.g.,
* Large range of treatments temperature difference)
* Filtration and gradients defined * Filtration not available for all
pollutants
NON-CHAMBERED SYSTEMS (plume and exclusion)
Strengths
* Minimum environmental
modification
* Supplementary
* Large trees
* Best for homogeneous canopy
CUVETTES
Strengths
* Supplementary to chamber or
non-chamber studies
Limitations
* Needs development
* Exclusion — air flow across
canopy atypical
* Plume -- needs "clean" environment
* Large space requirement for range
of treatments
Limitations
* Only a portion of tree exposed
Short-Term Growth/Physiological Studies:
OUTDOOR CHAMBERS (open-top types)
NON-CHAMBERED (plume and exclusion)
INDOOR CHAMBERS (all types)
CUVETTES (suppementary to outdoor systems)
Gas-Exchange/Pollutant Uptake:
INDOOR CHAMBERS (CSTR's)
CUVETTES
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plants should ensure that they fit well within the chambers. Rapidly growing
species will fully occcupy present chambers within a few growing seasons. With
more slowly growing species, there may be accumulated effects of the slight
differences in microclimate that exist between chambers and the surrounding
environment. There was concern and caution urged regarding size of trees to be
used in chamber studies, however no guideline was established. Until documen-
tation is available, caution is urged in using chambered systems for multi-year
growth/physiological studies, and in particular year-round exposures. Addi-
tional concerns which might also present limitations in the use of open-top
chambers for particular studies include: (1) suspected non-specific filtration
since current systems (charcoal filters) may remove ambient gases and particles
from the air in addition to the gas and pollutant under study; (2) the recogni-
tion that open-top chambers do not have the equivalent of ambient vertical
gradients of pollutant concentration; and (3) the structural issue of chambers
withstanding severe winter conditions for year-round exposures.
Non-chambered exposure systems were also recommended for multi-year
studies. Although even less is known regarding the use of these systems with
trees, the lack of structure-induced artifacts in the exposure studies is
attractive and the minimum alteration of the environment is important for both
year-round and seasonal exposures. Both types of non-chambered systems, the
plume and air exclusion systems could be useful, depending on the specific
research objective. The non-chambered systems can provide a supplementary
approach to tree exposure studies using open-top chambers. The large circular
or square plume systems which encircle the entire exposure plot are recommended
for large-scale exposure of trees with no environmental modification due to the
exposure system. The dispersion characteristics for the systems have been well
designed based on computer modeling and field testing. The distance between
emitters and receptor plants has been determined to provide uniformity in
pollutant concentrations under ambient wind conditions. Development of
computer-based pollutant dispensing, control and monitoring systems, and
computer-based environmental monitoring permits continuous use of the systems
and characterization of the pollutant exposures within experimental plots.
Examples of this recommended plume exposure system are those of McLeod et al.
(1985) and Greenwood £t _al_. (1980). Aside from the lack of experience with the
plume-type exposure systems for trees, a major limitation is the fact that only
pollutant addition is possible. Consequently, a "clean" environment is
required to achieve a full range of treatments or the effects of ambient
pollutant conditions cannot be ascertained. Replication of treatments requires
a large investment in space, not a trivial cost to consider in designing tree
exposure studies. Homogeneous canopies are desired to avoid the air pollutant
sampling discrepancies which could occur with heterogeneous canopies. The
other non-chambered system recommended is the air exclusion design. With a ,
high air flow and well-defined exclusion characteristics, it is recommended for
both ambient air and pollutant addition type studies. Experience with these
systems is limited to low-growing crops; performance with mid-size and larger
trees is not available. These systems impose only minor environmental modifi-
cation during the periods of exclusion with duct inflation. Examples of this
type system include Kuja et al. (1985) and Thompson and Olszyk (1985). An
important concern in use "bT tfh~e air exclusion system, like most chamber designs
as well, is the atypical air flow across canopies.
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The cuvette exposure systems for leaf, needle, or branch measurements were
recommended for supplementary studies within both chambered and non-chambered
systems. The cuvette systems facilitate more closely controlled pollutant
exposures of leaves, needles, or branches in chambered and non-chambered
systems, and would be especially useful with physiological studies of gas-
exchange (photosynthesis, transpiration, pollutant flux rates) in larger trees,
supplementing biomass studies. The pollutant exposures should be controlled to
reflect experimental needs. It was suggested that additional work be conducted
to evaluate the gas mixing and boundary layer characteristics of cuvettes, a
more detailed analysis of the water vapor surrogate method of estimating S02
absorption by leaves, validation of this approach as a method for assessing the
fluxes of other pollutants into leaves, and evaluation of the potential for
using the water vapor surrogate approach (among others) to quantify the air
pollutants adsorbed by plants in cuvettes, chambers, and chamberless plots.
Selection of the appropriate cuvette type for the research objectives is
important and is aided by reference to Table IV-5. Cuvette exposure systems
are limited by having only a portion of the plant tissue being exposed to
pollutant while the remainder of the plant is influenced by ambient conditions.
This limitation is important in interpretation of these type studies.
The recommendations of the workshop participants for exposure systems to
be used in short-term growth/physiology studies were the same as for multi-year
growth studies, with the addition of controlled-environment or greenhouse
exposure chambers. The modification of the environment inherent in indoor
chambers is less of an issue for short-term studies of a few months, and is
desirable for environmental or climatic interaction type studies.
Current technology will determine the capability to build the best
possible indoor facilities that meet the basic requirements: (1) minimum
fluctuation in temperature, humidity, and. light conditions; (2) minimum
gradients of pollutant concentration within chambers; (3) capability to create
dynamic patterns of pollutants; (4) capability to determine pollutant flux
rates; (5) capability to determine physiological parameters such as photosyn-
thesis and transpiration rates; and (6) air movement for minimum leaf boundary
layer resistance without wind injury.
Additional'features recommended by Heagle and Philbeck (1978) (Appendix G)
should be included: nonreactive surfaces; not a closed air flow system;
negative pressure; easily portable (for some applications); and transparent
covering for chambers within greenhouses or growth rooms).
All of these requirements can be met reasonably well by any of the four
types of indoor chambers (Table IV-3), Chambers of the CSTR design (Heck et
al., 1978; Rogers et a^., 1977) are recommended for studies in which plant
growth responses and gas exchanges are to be measured in well-mixed, spatially
uniform gaseous environments. Growth room locations for CSTRs will be prefer-
able when environment definition and repeatability are major factors in the
research program; greenhouse locations willbe adequate when the inevitable
seasonal fluctuations in light and temperature environment are deemed accept-
able in the specific experimental design. CSTR chambers should not be used
when gradients of pollutant concentrations as environmental factors are
desired. Non-CSTR chambers such as those described by Heck et al. (1968),
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Jensen and Bender (1977), and Lockyer et al. (1976) appear to be as satisfac-
tory as CSTR chambers for comparative studies of plant sensitivities to gaseous
pollutants and are preferred over the CSTR chamber for studies involving
gradients. Recently constructed self-contained units such as those of Aiga et
al. (1984) and Payer et jiV. (1985) and chambers within rooms such as those of
"Payer et aK (1985) appear to establish the current state-of-the-art technology
for inCfoor gaseous exposure chamber systems.
Gas-exchange/pollutant uptake studies should be conducted with indoor
chambers of the CSTR-type (Appendix E) and cuvette exposure systems. Size
considerations will limit the scope of such studies.
Screen ing-type studies can fall within both multi-year and short-term
growth/physiological studies, and are essential to the Forest Response Program,
both for screening sensitive species and/or clones, and screening effective
pollutant type. The participants recognized that these screening studies could
be conducted in both chambered (open-top) and non-chambered systems. Indoor
chambers would also be useful, but only for short exposure periods of less than
one year. Screening studies of growth response must measure current season
growth and that following spring bud break, thus covering two growing seasons.
Where physiological responses are used as a surrogate for growth there must be
experimental evidence that establishes the relationship of the measured physio-
logical process and growth (e.g., instantaneous measurements of net photosyn-
thesis rate may not relate well to long-term growth).
D. Wet Deposition
The workshop participants identified certain elements in the monitoring of
wet deposition studies and exposure systems that should be standardized; these
are listed in Table VI-3. The frequency of these chemical analyses and measure-
ments should be determined within the context of the quality assurance program
development; more substantive characterization may be warranted with some
experimental objectives.
Routine chemical characterization of the incident hydrometeor was strongly
encouraged, and the characterization should include a complete cation and anion
analysis. Similarly, regular characterization of the throughfall/stemflow was
recommended, particularly if in the objective or design of the study a large
percentage of the deposition is modified in passage through a canopy to the
soil. Both the deposition rate and chemistry should reflect ambient condi-
tions, depicting the full cation and anion features in addition to pH, nitrate,
and sulfate concentrations. The dissimilar chemistry and deposition rate of
cloud/mist versus rainfall should be recognized, in particular the enrichment
of multiple cations, anions, and particles in cloud water relative to rainfall.
When a soil component is included in the study, participants recommended that
the porosity and solution infiltration rates be reasonable to allow for near
normal chemical processing of the wet deposition within the soil and rhizo-
phere, and that the characteristics be documented.
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Table VI-3. General recommendations for wet deposition exposure systems,
oo
oo
oo
oo
oo
oo
Chemical analysis of the incident hydrometeor
Chemical analysis of the throughfall
Measurement of wet deposition rates
Determination of wet deposition chemistry
Inclusion of dynamic component in wet deposition exposure
Suitable physical and chemical properties of the soil
6-10
-------
Recommendation of wet deposition exposure systems were a function of the
experimental objective, tree or canopy height, site or location, and statis-
tical design. Many of the same concerns raised with dry deposition were
discussed, and the participants recognized that the "ideal" system would be one
which introduces no aritifacts from the physical structure of the system.
Consequently, as with the dry deposition studies, no single system was recom-
mended to accommodate all research plans. Experimental requirements in some
instances dictate the need for exclusion of ambient dry and/or wet deposition
and the ability to create a range of treatments for experimental design. In
addition, available wet deposition exposure systems, like those used in dry
deposition studies, have not been characterized regarding tree growth and
vigor; consequently size/developmental stage of material, and duration of
experimental time period is an unknown. Because of these and other uncertain-
ties, it was recommended that a variety of systems be used until better
characterization of the exposure systems with trees has been accomplished,
perhaps in conjunction with planned experiments. As was suggested for dry
deposition exposure systems, developmental work should be funded for various
wet deposition exposure systems, especially cloud water deposition.
1. Rainfall Simulation
The workshop participants recommended the systems indicated in Table VI-4;
each have merit for a particular application. The recommendations are made for
either year-round or seasonal exposures with multi-year growth studies. The
most flexible, in the sense of being able to address a variety of experimental
objectives, is the automated exclusion system with modified scheduled addition
of rain. The "ideal" system of no shelter (shelterless exclusion) is useful
only in regions where the rain is not acidic, and where the volume of added
rain would not exceed the normal range of rainfall for that location (i.e., the
trees would not be overwatered). The automated exclusion system with the
shelter coming into place only during ambient rain events is at this time the
best compromise between chambered and non-chambered systems. There was the
concern that since only rain is excluded in most of these type systems, there
is the confounding influence of ambient dry deposition. It was recommended
that continuous monitoring of ambient dry deposition be conducted for proper
characterization of these exposure systems. The scheduled rather than auto-
mated addition of rain is recommended to allow for experimental flexibility.
Scheduled additions should occur, however, during times when evaporation is low
or there is not rapid evapotranspiration. The addition should simulate ambient
rain events as recommended by the workshop (see exposure regimes below).
The shelterless and chambered system with permanent or manual exclusion of
ambient wet deposition (Table VI-4) were preferred for certain experimental
objectives and to address the limitations of the automated exclusion systems in
multi-year growth studies. Rain addition was recommended as discussed above.
The automated exclusion systems represent a significant financial investment in
facility and equipment. They require some type of system to prevent inter-
action with dry deposition (e.g., chambered or duct-type air exclusion); they
may not be capable of exclusion and addition with larger trees that have mature
canopies. The shelterless systems have the advantage of minimum hardware, thus
the least impact of structure on experimental design and would be the preferred
•6-11
-------
Table VI-4. Recommendations for rainfall simulation exposure systems.1
Multi-Year Growth/Physiological Studies
o Automated exclusion/modified scheduled addition (ambient simulation)
Strengths
* Reduces the structural artifacts of chamber on growth
* Suitable for all experimental objectives
Limitations
* Shelter must be in place during the "event"
* No exclusion of ambient dry deposition
* Significant financial investment
* Questionable use with mature canopies
* Questionable operation at high elevations, steep slopes, or high wind
areas
o Shelterless exclusion/modified scheduled addition
Strengths
* Minimum hardware, least impact of structure on growth
* Suitable for mature canopies
* Ideal for use in areas where rainfall not acidic
Limitations
* Treatments have rainfall in excess of controls
* No characterization of snow
* Requires considerable water and mixing capabilities
o Shelter (permanent or manual) exclusion/modified scheduled addition
Strengths
* Excludes some types of ambient dry deposition
* Provides total exclusion of wet deposition
* Useful in remote sites
* Suitable for species and rain .chemistry screening, climatic inter-
* action
Limitations
* Modification of growing environment with chamber
~~~~~(continued)
6-12
-------
Table VI-4 (continued)
Short-Term Growth/Physiology Studies
o Same as for multi-year studies
o Indoor chambers — controlled environment or greenhouse
Strengths
* Low cost
* Variety of manipulation studies possible
* Suitable for screening studies, physiology studies, climatic inter-
action
Limitation
* Duplicate/terminal velocity of droplet.
1 No priority implied in order of listing.
6-13
-------
system for areas where rain chemistry is not seriously affected by anthropo-
genic inputs. The shelterless or non-chambered system is perhaps the most
feasible for mature forest canopies and is suitable for species screening
and/or study of rainfall chemistry. Caution was urged, however, regarding the
addition of rain in excess of ambient to avoid complicating the interpretation
of a stress response. With these systems, characterization of snow chemistry
becomes a necessity. The shelterless plots, if large, require considerable
water supplies and conditioning capabilities. The chambered systems (sheltered
exclusion), in contrast, are most useful for excluding ambient dry deposition,
and provide for total exclusion of any wet deposition. The permanent shelter
can be used for species screening and testing various aspects of rainfall
chemistry in areas with acidic rainfall. It is also valuable for testing
interactive effects of wet deposition and natural climatic stress (e.g.,
drought, frost tolerance). In some locations, such as high elevation, slopes,
or high wind areas, the chambered systems with permanent wet deposition
exclusion might be the prudent choice. The primary limitation of the chambered
system is the modification of the growing environment, and the degree to which
this is crucial to interpretation of the experimental results has not yet been
characterized. These same concerns relating to climatic modifications with
chambered systems were recognized for dry deposition exposure systems.
Supplementary studies are needed to answer the questions on how these systems
affect tree growth and vigor. For year-round exposures, the issue of snow
within the automated exclusion systems and the shelterless systems was dis-
cussed, but there was no resolution on how to handle this type of wet deposi-
tion.
Short-term growth/physiological studies should use the same systems as for
multi-year studies; in addition, indoor chambers, either controlled-environment
or greenhouse exposure chamber systems (Appendix E), are acceptable and in some
instances desirable for screening studies, physiological studies, exposure-
response, and climatic interaction studies. Indoor chambers are relatively low
cost and there are many well-documented examples in the literature offering a
wide breadth of possible manipulative studies involving climate, soil, species
and rain chemistry. Caution is urged to control and characterize the degree
and duration of foliage surface wetness by carefully maintaining appropriate
temperature, humidity/evaporation conditions, and the realization that dupli-
cation of the terminal velocity of incident rain.is probably not possible.
Mimicking the physical characteristics of rain were discussed, but since there
is no clear indication of the importance in wet deposition studies, recommenda-
tions for standardization were not made.
2. Cloud Water/Radiation Fog Simulation
Cloud water simulation studies introduce an even more complex element into
exposure systems. The canopy impact of cloud water is an important component
in this type of exposure. Thus, there is the need for delivery of the deposi-
tion with inertia! velocity. The appropriate method was not resolved in the
workshop; it was urged, however, that some effort toward development of expo-
sure systems in this area be pursued, since the potential impact of cloud water
chemistry, especially in high elevation forests, was recognized as crucial to
an understanding of forest decline in these areas.
6-14
-------
Recommendations were made regarding exposure systems that simulate radia-
tion fog, where impact velocity is not a factor (Table VI-5). For multi-year
studies, the open-top chambers were recommended primarily for the needed
permanent exclusion, since no fog filtration is available and it is necessary
to encapsulate experimental material for efficient delivery of a range of fog
treatments. The chamber would be used with permanent or manually-movable tops
in place during fog generation. The disadvantage is the modified environment
and the extent of that modification has not been characterized for tree growth.
There is also the need to control leaf surface wetness (evaporation). For
short-term growth studies, controlled-environment exposure systems such as
CSTR, as well as the greenhouse box-type chambers (Appendix E) were found
acceptable. Wind tunnels (Appendix E) as exposure facilities for fog permit
the characterizing of deposition, but have the disadvantage of limiting
experiment duration since they are not constructed to optimize seedling growth.
Also, the wind tunnel restrict plant material size to only seedlings. For
cloud water simulation, the wind tunnel might be the optimum system for
delivering inertia! fog. Irregardless of exposure system, there is also the
need to characterize the site of deposition within the canopy, especially with
fog, in order to conduct appropriate monitoring of incident deposition.
E. Wet and Dry Combinations
The summary of workshop recommendations for combined wet and dry deposi-
tion exposure studies is presented in Table VI-6, and reflects the same
strengths and limitations identified for the individual exposure systems. The
recommendation of automated exclusion with scheduled additions for wet deposi-
tion, and a chamber less system for dry deposition offers the least structural
influence on the exposure study. However, particular experimental objectives
may be best served with one of the other two recommended system combinations;
or the expense of the automated exclusion system may dictate an alternative
approach. Lacking clear-cut characterization of the tree growth/vigor with
these exposure systems, corroborative studies should be pursued with all
systems. The open-top chambers are recommended for exclusion of ambient
pollutant and because they offer the ability to deliver a wide range of
experimental treatments. Scrubbing or excluding air pollutants limits the
chamberless systems. The type of dry deposition pollutant under consideration
in the interaction-type study would also affect the choice. For example, with
ozone as the interacting pollutant, the open-top chambers with charcoal-filters
are preferred; with SC>2 the non-chambered systems would be feasible, based on
current knowledge of background SOg and frequency of occurrence. The short-
term exposure studies using indoor chambers, both CSTR and greenhouse types,
are recommended for screening studies and their ability to control evaporation
more readily.
F. Aerosols
No recommendations were given for this type of exposure system since there
has been little development based on experience of the participants and in
literature reviews. Instead, the workshop participants recommended that
monitoring for aerosols be included in the research plans. Monitoring is
6-15
-------
Table VI-5. Recommendations for cloud water/fog simulation exposure systems,
Radiation Fog:
Multi-Year Growth/Physiology Studies
o Open-top chambers (permanent or manual exclusion tops)
Strengths
* Permanent exclusion (no fog filtration)
* Encapsulates experimental material
Limitations
* Modifies ambient environment
* Control of evaporation
Short-Term Growth/Physiology Studies
o Open-top chambers
o Indoor chambers (CSTR, greenhouse, or wind tunnel)
Strengths
* Control of evaporation
* Suitable for species screening or deposition features
* Climatic interaction
Limitations
* Limited duration of experiment
* Restricted to seedling size
Cloud Water Simulation:
Multi-Year
o Nothing available
Short-term
o Wind tunnel
Need developmental studies to characterize
6-16
-------
Table VI-6. Recommended exposure system for combinations of wet and dry
deposition.
Multi-Year Growth/Physiology Studies
o Wet deposition -- automated or manual exclusion/scheduled addition
Dry deposition — chamberless exposure system
o Wet deposition -- automated or manual exclusion/scheduled addition
Dry deposition — open-top chamber technology
o Wet deposition — permanent exclusion/scheduled addition
Dry deposition — open-top chamber technology
Short-Term Growth/Physiology Studies
o Same as multi-year plus indoor chambers
6-17
-------
needed to examine the extent of occurrence and chemistry of aerosols. A
similar recommendation was given for participates in dry deposition since these
two components of air quality may play important but as yet misunderstood roles
in impact on forest growth, especially in high elevation forests and trees
exposed to the vagaries of environment.
Summary
Workshop participants recognized the importance of heterogeneity in
experimental approaches. In past, this recommendation responds to the lack of
experience in documenting tree growth and physiology within the existing
exposure systems. The workshop agreed that the "ideal" system would be one
that introduces no artifacts on' growth through structure of the system. In
reality, however, there are particular research objectives that could best be
addressed at this time with chambered systems which introduce the artifact of a
structure in order to attain exclusion of ambient pollutants, create a range of
pollutant treatments, or allow climatic interaction studies. In a number of
instances, the issue of exclusion in experimental design, whether dry (gaseous)
or wet, is best resolved with chambered systems. Documentation of tree growth
in the open-top chamber and length of time for a growth study inside chambers
is needed. Also, the experience factor with open-top chamber systems is
important. The exclusion systems, either manual or automated, reduce the
imposition of structure at least during an exposure event, and thus provide an
advantage. However, the expense and the location requirements of the automated
exclusion systems limit their use. Non-chambered systems (plume and exclusion)
can also be quite costly, with extensive space requirements for treatments and
replications.
The recommendation to employ both chamber and non-chambered systems for
multi-year growth studies of wet and dry deposition also addressed the issue of
introducing a consistent bias across all sites. Without knowledge of tree
culture and exposure system types, the participants agreed that such a bias was
not desirable. Cross-comparability would still be possible and supplementary
studies should be designed within the National Forest Response Program four
research cooperatives to address this issue by using similar systems and
designs at different sites.
6. Exposure Regimes
Recommendations
The workshop participants addressed the question "Should exposure regimes
be standardized?" They concluded were: (1) no, but there should be guidelines
for developing exposure regimes; (2) exposure regimes should be region/site
specific; and (3) exposure regimes should be selected to test experimental
hypotheses.
The participants made eight specific recommendations regarding guidelines
for regimes to be used in exposure studies within the Forest Response Program
(Table VI-7). In addition, certain limitations were also recognized in current
6-18
-------
Table VI-7. Recommendations for exposure regimes to be used in wet and dry
deposition exposure studies.
1. The exposure regimes should reflect the episodic (seasonal, diurnal)
nature of pollutant occurrence.
2. The exposure regimes should be representative (frequency of occurrences)
of the study area or the area where the study species is indigenous.
3. The exposure regimes should consider the daily physiological function of
the plant but the exposure should follow ambient air quality and not be
arbitrarily terminated.
4. The pollutant regimes should yield a range of pollutant(s) concentrations
above and below the current ambient concentration(s).
5. Exposure regimes should use realistic frequency and concentrations of
pollutant co-occurrence.
6. Data for the development of realistic exposure regimes for combinations of
dry and wet deposition are limited.
7. Data for the development of realistic exposure regimes for particles and
aerosols are limited.
*
8. Additional monitoring data are needed to characterize the concentration
distributions for the dry deposition of particles and aerosols.
6-19
-------
air quality monitoring for dry and wet deposition pollutants, and the need for
additional monitoring data was noted as a priority within the exposure program
(Table VI-7).
Participants recognized that the exposure regimes used in current gaseous
pollutant studies could also be used to classify exposure regimes for other
pollutant sources, i.e., wet deposition. The regimes are identified as either
"artificial" or "episodic", with three variations on the episodic type. The
definitions, recommended uses, and recognized limitations of each exposure
regime type are summarized in Table VI-8 and discussed in Chapter 5. As with
exposure systems, the choices of regime is dependent on the biological or
research objective. Consequently, the participants made recommendations for
the acceptable type of exposure regime based on the same generic biological
objectives used in making the exposure system recommendations. The recommenda-
tions are summarized in Table VI-9 for the four different exposure regime
types.
Recommendations for particular techniques to use in development and
construction of exposure regimes were not offered. Some methods have been
described in the literature for gaseous exposures, but this is a relatively new
area of emphasis in experimental design and would benefit from new ideas. The
methods currently used include both the characterization of ambient air quality
by site and region, as well as the method for constructing experimental fumiga-
tion regimes (Male, 1982; Laurence and Kohut, 1984; Garsed and Rutter, 1984;
Lefohn et al., 1986a,b).
6-20
-------
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6-21
-------
Table VI-9. Summary of exposure regime recommendations for generic biological
objectives.1
Exposure Regimes
Generic Objectives Artificial
Long-Term Growth/
Physiology
Year-Round ?
Seasonal ?
Climatic ?
Short-Term
Growth Y
Physiology Y
Gas-Exchange Y
1 Y = regime is appropriate.
N = regime not recommended.
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6-22
-------
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