United States
Environmental Protection
Agency
Atmospheric Research and Exposure^;
Assessment Laboratory -x
Research Triangle Park NC 27711 .,
Research and Development
EPA/600/S3-89/009 Aug. 1989
&EPA Project Summary
Mountain Cloud Chemistry
Project—Wet, Dry and Cloud
Water Deposition
Volker A. Mohnen
The spruce-fir forests in the higher
elevations of the Appalachian moun-
tains from North Carolina to Maine
are showing visible symptoms of
injury and increased mortality. Con-
cern has been raised that exposure
to and deposition of atmospheric
pollutants might play a role in this
decline. The Mountain Cloud Chem-
istry Project (MCCP) sponsored by
the U.S. Environmental Protection
Agency (EPA) and the National Acid
Precipitation Assessment Program
(NAPAP) is studying the exposure
and deposition of atmospheric con-
stituents to these forests.
Atmospheric pollution is deposited
to the forest a number of forms,
cloud water interception represents a
major deposition patterning and may
exceed deposition by precipitation
and gases. The full report provides
estimates of cloud, precipitation and
dry deposition to the spruce-fir for-
ests at six MCCP sites. Equally im-
portant is an understanding of the el-
evational gradients that exist in the
deposition of airborne consistents.
Comparisons are made at MCCP sites
at different elevations and between
MCCP sites at different latitudes and
longitudes.
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
The full report is the second in a series
of annual summaries of research on the
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 six high ele-
vation sites in the eastern United States.
The report is produced by the scientists
in the Mountain Cloud Chemistry Pro-
gram, a multi-year study of atmospheric
chemistry and physics sponsored by the
EPA.
One of the major objectives of this
research is the characterize geographical
and elevational variability in the amounts
of airborne chemicals transferred from
the atmosphere into the spruce-fir and
other forest ecosystems that cover high
elevation sites in the Appalachian Moun-
tains of the eastern part of the North
America. The reports in this series con-
stitute the principal linkage between the
Mountain Cloud Chemistry Program
(MCCP) and the Eastern Spruce-fir Re-
search Cooperative (ESFC). Both MCCP
and ESFC are important parts of the
Forest Response Program (FRP) which is
sponsored jointly by the U.S. Forest
Service, the U.S. Environmental Protec-
tion Agency, and the National Council of
the Paper Industry for Air and Stream
Improvement (NCASI). Both MCCP and
ESFC are contributors to the National
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Acid precipitation Assessment program
(NAPAP).
The MGCP 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.
The first two objectives are addressed
in the full report, the third is addressed in
a complimentary report.
The full report provides estimations of
deposition to the forests from
precipitation, wind blown clouds and by
dry deposition mechanisms. Measure-
ment methodology used to provide data
for these estimations, data sets and
models used for deposition estimates are
discussed in detail in the full report.
Comparisons of deposition are made
between southern and northern MGCP
sites. Elevation gradients in 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 also based
on Lovett's original model was developed
by Krovetz for use with the deciduous
canopy at the Shenandoah MGCP site.
The model used to estimate dry
deposition in 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 un-
certainties.
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 lim-
ited number of sites where proper ac-
cess and facilities are available. In order
to meet the needs of the project, five
high elevation sites have been selected
from 45N to 35N 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 For-
est, 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 con-
centration of pollutants and the associa-
ted 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 warm season at
the MGCP sites using standard
NADP/NTN measurements of rainfall
amounts and chemistry. The NADP sites
selected to represent MGCP sites are
Greenville,ME09, Whiteface,NY98, Hub-
bard Brook,NH02, Big Meadows,VA29,
Whitetop,VA28 and Clingsmans
Peak,NC45.
Deposition is reported only for the
warm season. This period is the longest
at Shenandoah and Howland sites,
extending from early April at both site
locations to mid-November and early
October, respectively. Mt. Moosilauke
and Whiteface have the shortest. For
comparison of wet deposition among the
northern and southern sites, the wet dep-
osition for the southern sites was
adjusted to correspond to the concurrent
warm season wet deposition for the
northern sites. This comparison is
shown in Table 1
Deposition of Pollutants in
Cloud Water
Wind driven cloud droplets together
with water-borne pollutants are deposit-
ed on the leaves of mountain trees. This
mechanism produces a flux of pollutants
at mountain tops over and above that
available in nearby low elevation sites.
To estimate deposition by this mechan-
ism, MGCP uses a combination of mea-
sured pollutant concentrations in droplets
and with cloud deposition rates.
Two models were used to estimate
cloud water and chemical deposition flux.
One model was developed by Lovett and
modified by Mueller This model was
designed for use with the spruce-fir
forest canopies The other model, is also
Table 1 Concurrent Warm Season
Ad/usted Wet Deposition
(kg/ha)
Location NH4+ SO4" NO4 +
Greenville, NE. .44 5.1 261
Whiteface, NY 1.30 9.10 3.80
Hubbard Beach, 1.13 n.34 5.84
NH.
Whitetop, VA. 123 12.78 5.29
Mt Mitchell. NC. 1.83 20 60 9.04
Big Meadows, VA 1.75 928 5.71
Shenandoah. VA 139 12.45 501
based on Lovett's original model and has
been modified by Krovetz for use with
the deciduous canopy at the Shenandoah
MGCP site.
Table 2 presents estimates for growing
season deposition of ions via the cloud
water mechanism for all sites. These
estimates are significant for all sites and
are comparable or larger than precipita-
tion deposition for most. The range of
deposition values is wide. It is clear that
there are some regional differences in
deposition of ions in cloud water
Dry Deposition of Gases and
Particles
MGCP uses current available
methodology to estimate dry deposition
flux. The model of choice to provide es-
timates of dry deposition velocities is the
inferential model under development by
the ATDL/NOAA. There is considerable
uncertainty associated with the applica-
tion of this type of model to mountain en-
vironments. Still, to obtain dry deposition
estimates it is necessary to apply a mo-
del to estimate dry deposition velocities
for use with MGCP measured con-
centration values.
Table 3 shows the dry deposition
values for different MGCP sites during
the warm season.
Conclusions
Wet deposition of pollution related ions
in rain summed over the growing season
(warm season) exhibited a significant
west to east gradient from Whiteface/-
Moosilauke to Howland, Maine. Sulfate
deposition decreased from about 12
kg/ha for the Adirondacks and Green
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Table 2. Total Cloud Deposition During the 1987 Growing Season
(ka/ha/growing season)
Location
NO3-
SO4 =
Whiteface
Moosilauke
Shenandoah
Whitetop
Mt. Mitchell
0.7
0.2
03
1.2
0.5
9.2
1.2
1.9
14.3
50
19.3
4.0
9.4
42.5
14.2
44.9
8.3
9.6
76.0
30.2
Mountains down to about 6 kg/ha in
central Maine. Combined nitrate and
ammonium deposition decreased from
about 8 kg/ha to 4 kg/ha.
Wet deposition for the southern MGCP
sites also exhibited a gradient with higher
values for Shenandoah and lesser values
for Whitetop and Mitchell. Sulfate
deposition summed over the growing
season decreased from about 21 kg/ha
for Shenandoah to about 8 kg/ha for
Mitchell. Combined nitrate and
ammonium deposition decreased from
ibout 12 kg/ha to 4 kg/ha.
Dry deposition of sulfur and nitrogen
compounds is low at all MCCP sites
except Shenandoah. Typical warm
season deposition fluxes are around or
below 3 kg/ha for the combination of sul-
fur dioxide and sulfate. Nitrate deposi-
tion is less than 1.5 kg/ha at all sites
except Shenandoah.
Deposition of pollution related ions in
cloud water (cloud interception)
represents a major input to montane
forest canopies. It can exceed the flux
from wet and dry deposition at mountain
sites frequently exposed to cloud.
Whiteface Mt. has an estimated warm
season sulfate deposition flux of 45
kg/ha, about a factor of five higher than
the measured wet deposition flux.
Whitetop Mt. exhibits equally high cloud
sulfate deposition with 76 kg/ha. Mt.
Mitchell has an estimated cloud water
sulfate deposition of 30 kg/ha. The
combined nitrate end ammonium
deposition from cloud interception are
also high for the high elevation MCCP
sites at Whiteface Mt. (28 kg/ha),
Whitetop Mt. (57 kg/ha) and Mt. Mitchell
(19 kg/ha). For the other MCCP sites,
wet and dry deposition equals or
exceeds cloud deposition flux for sulfate,
nitrate and ammonium ions, essentially
because of lower frequency of exposure
to clouds. Cloud interception therefore
has the potential to be the major and
sometimes dominant process for the
input of sulfur and nitrogen compounds
into montane forests. For MCCP sites
with elevations greater than 1200 m,
cloud water interception contributes more
sulfur and nitrogen to the forests than
does wet and dry deposition. However,
significant errors may be associated with
the deposition estimation procedures
available to, and used by, the MCCP for
the estimation of both dry deposition and
cloud water interception by mountain
forests. A major effort will be undertaken
by MCCP in the I989 field season to
define the accuracy of these estimation
procedures.
Table 3. MCCP Warm Season Dry Deposition (kg/ha/season)
Location Ozone SO2 HNO3
NO2
SO4
Howland Forest
Moosilauke
Whiteface
Shenandoah
Mt. Mitchell
Whitetop
33.8
31 5
26.5
105.9
42 1
29.1
0.85 1.76
1.38
22 4 6.04
2.13
.99
0.67
0.90
5.85
1.07
1.11
0.015
0.050
0.073
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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 "Mountain Cloud Chemistry Project—Wet, Dry and
Cloud Water Deposition," (Order No. PB 89-148 597/AS; Cost: $15.95, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
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/S3-89/009
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