United States
Environmental Protection
Agency
Atmospheric Research and Exposure
Assessment Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/S3-90/058 Sept. 1990
&EPA Project Summary
An Assessment of Atmospheric
Exposure and Deposition to
High Elevation Forests in the
Eastern United States
Volker A Mohnen
The spruce-fir forests in the higher
elevations of the Appalachian
Mountains from North Carolina to
Maine are showing visible symptoms
of injury and increased mortality.
Concern has been raised that
exposure to and deposition of
atmospheric pollutants might play a
role in this decline. The Mountain
Cloud Chemistry Project (MGCP)
sponsored by the U.S. Environmental
Protection Agency (EPA) and the
National Acid Precipitation
Assessment Program (NAPAP) has
studied the exposure and deposition
of atmospheric constituents to these
forests.
Atmospheric pollution is deposited
to the forest in a number of forms,
cloud water interception represents a
major deposition pattern and may
exceed deposition by precipitation
and gases. The full report provides
estimates of cloud, precipitation and
dry deposition to the spruce-fir
forests at the MCCP sites.
This Project Summary was
developed by EPA's Atmospheric
Research and Exposure Assessment
Laboratory, Research Triangle Park,
NC, to announce key findings of the
research project that is fully
documented in a separate report of
the same title (see Project Report
ordering information at back).
Introduction
This report is the third in a series of
annual summaries of research on the
exposure and deposition of airborne
chemicals to forest canopies and the
forest floor in eastern North America. The
report is based on observations and
model estimates of atmospheric
deposition at the high elevation sites in
the eastern United States The report is
produced by the scientists in the
Mountain Cloud Chemistry Program, a
multi-year study of atmospheric
chemistry and physics sponsored by the
EPA.
The MCCP has three primary
objectives: (1) determine the elevational
gradients in wet and dry deposition of
pollutants and climate variables; (2)
determine the relative significance of
various deposition mechanisms to the
fluxes of chemical species into and
through forest canopies; (3) determine
the frequency distributions of chemical,
physical and climatic exposure.
This report provides estimations of
forest exposure to chemicals in air and
cloud water and deposition to the forests
from precipitation, wind blown clouds and
by dry deposition mechanisms.
Measurement methodology used provide
data for these estimations, data sets and
models used for deposition estimates are
discussed in detail. Comparisons of
deposition are made between southern
and northern MCCP sites. Elevation
gradients in exposure and deposition are
also discussed.
Two models are used to estimate cloud
water and chemical deposition flux. One
model was developed by Lovett and
modified by Mueller. This model is
designed for use with spruce-fir forest
canopies. The other model was
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developed by Krovetz for use with the
deciduous canopy at the Shenandoah
MGCP site. The model used to estimate
dry deposition is the inferential or "big
leaf" model. Since this model was
originally developed for flat terrain and
the model has not been fully
characterized for mountainous regions,
deposition estimates reported here reflect
these uncertainties.
Technical Approach
Resource and logistical considerations
dictate that measurements of inputs to
high elevation forests in eastern North
America can be performed at only a
limited number of sites where proper
access and facilities are available. In
order to meet the needs of the project,
five high elevation sites have been
selected from 45°N to 35°N to be
representative of the geographic and
meteorological variability in this large
region. This coverage has been
augmented by the addition of a low level
site (Howland, ME) to allow evaluation of
the impact of elevational gradient forest
types and enhance geographical
coverage. The research/monitoring sites
associated with MGCP are Howland
Forest, ME, Mt. Moosilauke, NH,
Whiteface Mtn., NY, Shenandoah, VA,
Whitetop, VA, and Mt. Mitchell, NC.
Site specific measurements of cloud
and rain water, of gaseous sulfur and
nitrogen compounds, and of ozone and
hydrogen peroxide are sampled hourly or
are directly converted into hourly
concentration values. In the case of
filterpack measurements, samples are
integrated over a week's time. These
concentration values then represent the
primary exposure parameters. The
concentration of pollutants and the
associated meteorological conditions are
needed to provide estimates of
deposition by precipitation,clouds, and
gases.
Results
Deposition of Pollutant Ions in
Precipitation
Wet deposition of pollutants was
estimated for the 1987/88 growing
season at the MGCP sites using standard
National Acid Deposition
Program/National Trends Network
(NADP/NTN) measurements of rainfall
amounts and chemistry. The NADP sites
selected to represent MGCP sites are
Greenville, ME 09, Whiteface, NY 98,
Hubbard Brook, NH 02, Big Meadows, VA
29, Whitetop, VA 28 and Clingmans
Peak, NC 45. The growing season is
longest at the Shenandoah and Howland
sites, extending from early April at both
locations to mid-November and early
October, respectively. Mt. Mitchell and
Whitetop Mountain have the next longest
growing seasons, followed by Mt.
Moosilauke and Whiteface with the
shortest (June to early October).
The wet deposition data are shown in
Table 1 for S042- N03 and NH/ions in
precipitation (not cloud). The data
indicate that the most westerly sites in
the north, Whiteface, NY and Moosilauke,
NH, received greater wet deposition via
precipitation than did the more
northeasterly location in Maine. The
southern sites showed slightly higher
sulfate and nitrate wet deposition than the
northern sites.
Concentration and Deposition
of Pollutants in Cloud Water
The 1986-88 results for the five high
elevation MGCP sites are summarized in
Table 2 as overall chemical composition
of cloud water samples from precipitating
and non-precipitating clouds. The
concentrations in non-precipitating clouds
are significantly higher than in
precipitating clouds. The differences in
cloud water ion concentration between
the sites is a result of sample location
with regard to cloud base and synoptic
weather type. The southern sites
frequently are cloudy under the stable,
warm sector synoptic type, while the
northern sites experience cloudiness
associated more frequently with frontal
passage. Hence, any north-south
differences in cloud water concentrations
are likely related to cloud climatology
including cloud base height. The
importance of sample location in relation
to height above cloud base can be
demonstrated for Whiteface-1 (1483 m)
and Whiteface-2 (1245 m). For
simultaneously obtained cloud water
samples, Table 3 shows the differences
in the mean ion concentration observed
for the two vertically separated sites.The
substantial vertical gradient in cloud
water concentration is mainly explained
by increased dilution of precursor
substances as liquid water increase with
height above cloud base.
In estimating cloud deposition to the
forest canopy, MGCP uses an improved
version of the 1984 Lovett Cloud
Deposition Model (COM). Cloud
impaction events at each MGCP site are
classified according to meteorological
conditions that prevail during each hour
of the event A detailed analysis of cloud
chemistry and meteorological variables
(wind speed and liquid water content) has
demonstrated the usefulness of this
technique for uniquely characterizing
conditions during events. In addition to
synoptic classification, specific air
trajectory directions computed within a
given event type can further characterize
event conditions. This technique is able
to explain a major portion of the variance
in the chemistry and meteorological data
base, thereby allowing more complete
growing season estimates of cloud
deposition. Deposition estimates for
MGCP sites can now be made for periods
when data are incomplete as long as
cloud frequency and event type can be
determined. Event types are (1) pre-warm
front, (2) NW sector of cyclone, (3) post-
cold front, (4) warm sector of cyclone, (5)
stationary front, (6) marine flow off
Atlantic, (7) cutoff low in upper
atmosphere, and (9) cap cloud. Cloud
deposition estimates for each site are
made by computing, for each subclass,
the mean water deposition flux using the
improved CDM and subclass wind speed
and liquid water content. Best estimates
of canopy structure are then used to
calculate the gross (pre-evaporaiion)
cloud water flux to specific forest
canopies at each site.
Most of the site-differences in cloud
deposition can be explained on the basis
of differences in canopy structure. Cloud
deposition is found to be highly site
specific. Despite an almost 2:1 advantage
in cloud frequency, Whiteface Mountain
mean deposition estimates for the
growing season are generally lower than
those for Moosilauke because of lower
canopy surface area at the specific
Whiteface site. Differences in cloud water
deposition between the northern and
southern sites are significant and likely
caused by differences in such
parameters as canopy structure,
elevation above cloud base and synoptic
meteorology. It is also interesting to note
the annual changes in cloud water
deposition as a result of changing
meteorological conditions from year to
year.
Dry Deposition of Gases and
Particles
The Atmospheric Turbulence Diffusion
Laboratory program (DRY DEPOSITION)
used in the MGCP, calculates the
deposition velocities of sulfur dioxide,
ozone, nitric acid vapor, and sulfate
particles from meteorological and site
specific biological information. Site
information includes: major and minor
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Table 1. Concurrent Growing Season Wet Deposition Via Precipitation
kg ion/ha/mo
Location
Greenville,
ME(Howland)
Whiteface, NY
Hubbard Brook, NY
(Moosilauke)
Whitetop, VA
Clingmans Peak
(Mt. Mitchell, NC)
Big Meadows, VA
(Shenandoah)
NH4 +
0.11
0.33
1.11
0.28
0.11
0.31
0.14
0.46
0.12
0.48
0.92
so42-
1.28
2.27
1.18
2.84
2.16
3.20
2.84
5.15
2.69
2.32
2.90
Table 2. Average Ion Concentrations in 1986-1988 Cloud
H +
Whiteface-1 171
Whiteface-2 225
Moosilauke 263
Shenandoah 171
Whitetop 1 74
Mt. Mitchell 398
so42-
205
352
257
176
321
489
A/03-
73
92
732
94
144
174
N03"
0.65
0.95
0.93
1.46
1.10
1.32
1.17
2.26
1.19
1.43
1.34
Water (jieg/L)
NH4 +
97
157
107
93
152
184
Year Elevation
1987 322 m
1987 622 m
1988
1987 250 m
1988
1987 1689m
1988
1987 1987m
1988
1987 1074 m
1988
Elevation Cloud
(m) Frequency*
1483 37%
1245
1000 19%
1015 11%
1689 30%
1950 29%
*% of hours in cloud.
Table 3. Mean Ion Concentrations for 108
Simultaneously Collected Cloud
Water Samples 1987/88 (fj.equiv/L)
H + SO42- NO3- W/V
Whiteface-1
Whiteface-2
122
218
79
151
48
85
74
142
plant species type, leaf area index for
plant species, and site location. All
available meteorological data including
canopy wetness and rainfall are also
used.
The growing season dry deposition flux
is presented in Table 5 for 1987/88,
derived by multiplying weekly deposition
velocities (model calculation) by the
appropriate weekly averaged concen-
trations at each site.
As is the case with cloud deposition,
the northern sites receive generally less
dry deposition than the southern sites
due to mainly differences in canopy
structure. This is particularly obvious for
the Shenandoah site which is the only
non-coniferous site within MCCP.
With the exception of ozone, all other
pollutants showed very low ambient
concentrations. Therefore, only doses for
ozone are calculated in MCCP and
presented in Table 6. In addition to the
MCCP sites, other nearby ozone
monitoring stations have been included
for the characterization of forest
exposure. In order to provide a
biologically relevant value, the sum of
season dose (ppm/hr) is calculated by
summing up all ozone values above 70
ppb occurring during daylight hours (7
am - 6 pm) of the growing season. The
exposure data (dose) in Table 6 suggests
a significant north-south gradient and an
elevational gradient within a region. The
data also show a pronounced year to
year change in ozone dose for most of
the stations with the 1988 exposure
higher by about a factor of two. Howland,
the most northeasterly site received the
lowest ozone dose, while the most
southern sites, Mt. Mitchell and Whitetop,
experienced highest exposure.
Summary
The average monthly sulfur and acidic
nitrogen deposition fluxes determined to
date (October 1989) for the MCCP sites
and for the 1987-88 growing seasons are
summarized in Table 7. From these data
it is possible to estimate the importance
of cloud deposition to the overall flux of
pollutants received by the forest
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Table 4. Calculated Growing Season Cloud Deposition Flux (kg ion/ha)
Moosilauke
Whiteface-2'
Whitetop A
Whitetop B~
Mt. Mitchell
1986
1987
1988
1986
1987
1988
1986
1987
1988
1986
1987
1988
1986
1987
1988
Cloud Water
Flux (cm)
12.9
10.1
3.5
11.6
10.5
6.0
53.1
39.4
34.3
77.3
61.2
54.3
33.5
27.5
21.8
so42-
21
15
6
11
10
6
83
64
56
135
108
95
52
51
76
A/03-
76
11
5
3
4
2
41
32
29
65
53
48
25
25
18
NH4 +
3.4
2.4
1.0
1.9
1.7
1.2
14.0
10.8
9.5
22.1
17.7
15.6
6.9
6.9
5.0
"There is no forest on top of Whiteface Summit (WF-1). Therefore, deposition has been
calculated on a canopy structure slightly above (WF-2).
~ Higher canopy density than Whitetop A.
Table 5. Monthly Mean Growing Season Dry Deposition (kg species/ha/mo)
Howland
Moosilauke
Whiteface
Shenandoah
Whitetop
Mt. Mitchell
1987
1988
1987
1988
1987
1988
1987
1988
1988
1987
1988
S02
0.22
0.28
0.30
0.51
3.06
0.75
0.39
1.48
SO/- HNO3 O3
0.11 0.29 5.48
0.15 0.37 5.39
5.96
0.31 1.61 6.57
0.36 6.46
0.28 6.09
0.80 14.06
14.83
11.95
1.64 7.7
12.7
canopy .Although it must be kept in mind
that cloud interception is highly
dependent upon canopy structure and
location above cloud base, it can be
nevertheless concluded, that cloud
deposition can deliver to the canopy the
same and up to four times the amount of
pollutants as precipitation does (see
Table 8).At the cloud free Howland site,
dry deposition appears to account for
less than one third of the total acidic
substances deposited. The Shenandoah
site has a very low cloud impaction
frequency due to its relatively low
elevation and dry deposition appears to
be of greater relative importance
(deciduous trees). The estimates for
Whiteface suggest that dry
sulfurdeposition accounts for less than a
quarter of the total sulfur deposition flux.
Based on these research results, it can
be concluded that any assessment of
forest damage at high elevations must
take all delivery mechanisms into
account, in particular, cloud deposition.
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Table 6 Ozone Exposure - Total Season April 15 - October 15
(Daylight Hours 7 AM - 6 PM)
Sum of Season Dose (ppm'hr)
> 0.07 ppm
Site Name/State
How/and Forest, ME
Moosilauke, NH
Whiteface Mountain-1, NY
Whiteface Mountain-3, NY
Whiteface Mountain-4, NY
Huntington, NY
Hampshire Co., MA
Beaver Co., PA
Shenandoah-i , VA
Shenandoah-2, VA
Shenandoah-3, VA
Big Meadow, VA
Dickey Ridge, VA
Sawmill Run, VA
Whitetop, TN
Giles Co., TN
Marion, VA
Mt. Mitchell -1, NC
Ml Mitchell-2, NC
1986
ND.
N D
2 29
ND
N D.
509
9 14
N D.
N.D
ND.
ND.
5 56
3 21
11.39
ND.
1638
411
8.34
ND
1987
0.82
7.81
9.68
9.41
3.47
5.74
9.26
13.06
9.49
9.01
6.07
28.50
31.07
26.80
38.54
16.73
9.27
5.14
6.68
1988
4.16
12.51
20.85
16.5
N.D
11.36
34.93
31.70
23.27
39.44
20.88
31.89
40.25
30.16
37.68
28.91
26.92
45.17
19.49
Elevation
(m)
250
1000
1483
1026
604
500
312
1300
1015
716
524
1071
631
453
1689
244
710
1950
1750
Table 7. Estimated 1987-88 Deposition Budgets at MCCP Sites Growing Season Mean
Sulfur and Nitrogen Deposition (kg S or N/ha-mo)
Wet Cloud Dry
Howland
Moosilauke
White face -2
Shenandoah
Whitetop
Mt. Mitchell
S
0.43
0.84
0.58
0.99
1.01
1.31
N
0.15
0.29
0.21
0.28
0.28
0.39
S N
no clouds
0.69 0.35
0.65 0.17
insufficient data
3.58 1.23
2.65 0.87
S
0.17
0.31
1.80
N
0.08
0.23
Table 8. Estimated Cloud-to-Wet Depsition Flux
Ratios for 1987-88 Growing Season
Sulfate
Nitrate
Moosilauke
Whiteface -2
Whitetop
Mt. Mitchell
0.8
1.1
3.5
2.0
1.2
0.8
4.4
2.2
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Volker A. Mohnen is with the State University of New York at Albany, Albany, NY
12222.
Ralph Baumgardner is the EPA Project Officer (see below).
The complete report, entitled "An Assessment of Atmospheric Exposure and
Deposition to High Elevation Forests in the Eastern United States," (Order
No. PB 91-100 1641 AS; Cost: $31.00 subject to change) will be available
only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Atmospheric Research and Exposure Assessment Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States Center for Environmental Research
Environmental Protection Information
Agency Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S3-90/058
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