United States
Environmental Protection
Agency
National bnvironmental
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/SR-95/086 August 1995
&EPA Project Summary
CASTNet
National Dry Deposition Network
1990-1992 Status Report
The National Dry Deposition Network
(NDDN) was established to provide
longterm estimates of dry acidic depo-
sition across the continental United
States. Fifty routine sites were opera-
tional from 1990 to 1992, including 41
sites in the eastern United States and 9
sites in the western United States. Each
site was equipped with sensors for con-
tinuous measurements of ozone (O3)
and meteorological variables required
for estimation of dry deposition rates.
Weekly average atmospheric concen-
trations of particulate sulfate (SO"), par-
ticulate nitrate (NO), particulate ammo-
nium (NH), sulfur dioxide (SO2), and
nitric acid (HNO3) were measured at all
sites and wet deposition of acidity and
related species were measured at se-
lected sites. Two methods development
sites were installed during 1991 to
evaluate: 1) comparability of United
States and Canadian air quality mea-
surements, and 2) effects of terrain on
pollutant concentration and deposition.
Routine application of an inferential
model for calculation of deposition ve-
locities and dry deposition fluxes was
also begun.
Atmospheric concentration data
show species-dependent variability in
space and time. In general, the highest
concentrations are observed in the
northeast and midwest, and these are
a factor of 5 to 10 times higher than
those observed in the west. Significant
concentration gradients are also ob-
served from the northeast through up-
per northeast and midwest through up-
per midwest. Annual average concen-
trations of most species decreased
from 1989 to 1992 in all subregions of
the network.
Dry deposition calculations for 1990,
1991, and 1992 show that SO2 and HNO3
dominate sulfur and nitratenitrogen
fluxes, respectively. In general, SO2 ac-
counts for more than 75 percent of dry
sulfur deposition at eastern sites and
more than 50 percent of dry sulfur
deposition at western sites. HNO3 ac-
counts for more than 90 percent of dry
nitratenitrogen deposition at all sites.
Total deposition estimates for approxi-
mately 15 sites show that dry deposi-
tion accounts for about 20 to 50 per-
cent of wet plus dry sulfur deposition
and 30 to 60 percent of wet plus dry
nitrate-nitrogen deposition.
Data from a pair of sites located in a
valley and on a nearby ridge show that
elevational gradients in concentration,
deposition velocity, and flux can be
significant. Reactive gas concentrations
and fluxes are 2 to 4 times higher at
the ridge than in the valley, suggesting
that deposition in areas of complex ter-
rain may be difficult to estimate.
Collocated data from the Canadian
Acid Precipitation Monitoring Network
(CAPMoN) and NDDN for a site in south-
ern Ontario indicate that annual aver-
age concentrations are within ±5 per-
cent, except for SO2. NDDN data for
SO2 are lower than CAPMoN by about
10 percent. These results indicate that
methodological differences between
networks should not interfere signifi-
cantly with pattern and trend analyses
across eastern North America.
This Project Summary was devel-
oped by the U.S. Environmental Pro-
tection Agency's (EPA's) National En-
vironmental Research Laboratory, Re-
search Triangle Park, NC, to announce
key findings of the research project
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that is fully documented in a separate
report of the same title (see Project
Report ordering information at back).
Background
Atmospheric deposition takes place via
two pathways: wet deposition and dry
deposition. Wet deposition is the result of
precipitation events (rain, snow, etc.) which
remove particles and gases from the at-
mosphere. Dry deposition is the transfer
of particles and gases to the landscape in
the absence of precipitation. Wet deposi-
tion rates of acidic species across the
United States have been well documented
over the last 10 to 15 years; however,
comparable information is unavailable for
dry deposition rates. This lack of informa-
tion increases the uncertainty in estimates
of interregional, national, and international
transport and confounds efforts to deter-
mine the overall impact of atmospheric
deposition. The direct measurement of dry
deposition is not straightforward, but a
number of investigations have shown that
it can be reasonably inferred by coupling
air concentration data with routine meteo-
rological measurements.
In 1986, EPA contracted with Environ-
mental Science & Engineering, Inc. (ESE)
to establish and operate the NDDN. The
objective of the NDDN is to obtain field
data at approximately 50 sites throughout
the United States to establish patterns
and trends of dry deposition. The approach
adopted by the NDDN is to calculate dry
deposition using measured air pollutant
concentrations and inferred deposition ve-
locities (Vds) estimated from meteorologi-
cal, land use, and site characteristic data.
The inferential model currently used for
dry deposition calculations is a multi-layer
version of the Big Leaf Model developed
by Hicks et al. (1985).
The full report summarizes results of
NDDN monitoring activities from 1990
through 1992. Annual concentration data
for atmospheric sulfur and nitrogen spe-
cies are presented, and temporal variabil-
ity is described. Results of dry deposition
calculations for 1990 through 1992 are
discussed, and the relative contribution of
gases versus aerosols are evaluated. Wet
deposition data for approximately 15
NDDN sites are presented and then used,
along with dry deposition calculations, to
estimate total depositions of sulfur and
nitrate-nitrogen. The relative magnitude of
wet and dry deposition are discussed.Ozone
concentrations and exposure statistics are
presented for 1990, 1991, and 1992.
Data are also presented from two com-
parability studies initiated in 1991. The
first of these involves investigation of mea-
surement biases between atmospheric
sampling methods used by United States
and Canadian acid deposition trends pro-
grams. Data are presented and analyzed
from a collocated site in Ontario, Canada.
The second study is an investigation of a
terrain-induced bias in concentration mea-
surements and dry deposition estimates.
Data are presented from a pair of sites in
southwestern North Carolina that are sepa-
rated horizontally by about 1,000 meters
(m) and in elevation by about 350 m.
Procedures
Ambient measurements for O3, SO2,
SO', NO, HNO3, NH, and meteorological
variables required for dry deposition cal-
culations were performed at each NDDN
site. Meteorological variables and O3 con-
centrations were recorded continuously
and reported as hourly averages consist-
ing of a minimum of nine valid 5-minute
averages. Atmospheric sampling for sul-
fur and nitrogen species was integrated
over weekly collection periods using a 3-
stage filter pack. In this approach, par-
ticles and selected gases are collected by
passing air at a controlled flow rate through
a sequence of Teflon®, nylon, and base-
impregnated cellulose filters. Filter packs
were prepared and shipped to the field
weekly and exchanged at each site every
Tuesday. Blank filter packs were collected
monthly to evaluate passive collection of
particles and gases as well as contamina-
tion during shipment and handling. At 16
sites located more than 50 km from Na-
tional Atmospheric Deposition Program/
National Trends Network (NADP/NTN)
sites, wet deposition samples were col-
lected weekly (according to NADP/NTN
protocols) and shipped to ESE for chemi-
cal analysis.
Filter pack sampling and O3 mea-
surements were performed at 10 m us-
ing a tilt-down aluminum tower (Aluma,
Inc.). Filter pack flow was maintained at
1.50 liters per minute (L/min) at eastern
sites and 3.00 L/min at western sites,
for standard conditions of 25 degrees
Celsius (°C) and 760 millimeters of mer-
cury (mmHg) with a Teledyne-Hastings
CST-10K mass flow controller (MFC).
Wet deposition samples were collected
in precleaned polyethylene buckets us-
ing an Andersen Model APS precipita-
tion sampler. Buckets were placed on
the sampler on Tuesday and removed,
whether or not rainfall had occurred, the
following Tuesday. Buckets were
weighed in the field, then sealed and
shipped to ESE for chemical analysis.
Precipitation amount (depth) was also
monitored at wet deposition sites.
Ozone was measured via ultraviolet
(UV) absorbance with a Thermo-Environ-
mental Model 49-103 analyzer operating
on the 0- to 500-ppb range. Ambient air
was drawn from the 10-m air quality tower
through a 3/8-inch TFE Teflon® sampling
line. Teflon® filters housed at the tower
inlet and the analyzer inlet prevented par-
ticle deposition within the system. Peri-
odic checks indicated that line losses
through the inlet system were consistently
less than 3 percent. Zero, precision
[60 parts per billion (ppb)], and span
(400 ppb) checks of the O3 analyzer were
performed every third day using an inter-
nal O3 generator.
In addition to the above, various obser-
vations were periodically made at the
NDDN sites to support model calculations
of dry deposition. Site operators recorded
surface conditions (e.g., dew, frost, snow)
and vegetation status weekly. Surface in-
formation was collected to determine the
frequency of conditions that could influ-
ence deposition rates for gases (espe-
cially SO2) and particles. Vegetation data
were obtained to track evolution of the
dominant plant canopy, from leaf emer-
gence (or germination) to senescence (or
harvesting). Once a year, site operators
also provided information on major plant
species and land-use classifications within
1.0 km of the site. Additional land-use
data was obtained by digitization and
analysis of aerial photographs obtained
from the U.S. Geological Survey (USGS)
National Cartographic Information Center
in Reston, VA. Leaf area index (LAI) mea-
surements were conducted at all NDDN
sites during the summers of 1991 and
1992. LAI was measured using an LAI-
2000 Plant Canopy Analyzer manufactured
by Li-Cor (Lincoln, NE).
Filter packs contained three types of
filters in sequence: a Teflon® filter for
collection of aerosols, a nylon filter for
collection of HNO3, and dual potassium
carbonate (K2CO3) impregnated cellulose
filters for collection of SO2. Following re-
ceipt from the field, exposed filters and
blanks were extracted and then analyzed
for SO- and NO by micromembrane-sup-
pressed ion chromatography (1C). Teflon®
filter extracts were also analyzed for NH
by the automated indophenol method. Wet
deposition samples were filtered and then
analyzed for pH, conductivity, acidity, so-
dium (Na+), potassium (K+), NH, calcium
(Ca2+), magnesium (Mg2+), chloride (Cr),
nitrite (NO), NO, and SO.
Atmospheric concentrations of particu-
late SO', NO, and NH were calculated
based on the analysis of Teflon® filter
extracts; HNO3 was calculated based on
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the NO found in nylon filter extracts; and
Atmospheric concentration data for
SO2 was calculated based on the sum of HNO, and NO also show substantial vari-
SQ- found in nylon and cellulose filter ex-
tracts.
Dry deposition calculations for 1990
through 1992 were made using the multi-
layer version of the National Oceanic and
Atmospheric Administration (NOAA) infer-
ential model. The model calculates fluxes,
F, as the product of measured concentra-
tions and inferred Vds. Deposition velocity,
in turn, is calculated as the inverse sum of
three separate resistances: atmospheric
resistance (Ra), boundary layer resistance
(Rb), and canopy resistance (Rc).
The three resistance terms are calcu-
lated for each chemical species and veg-
etation/surface type for every hour of avail-
able meteorological input data. Hourly val-
ues of Vd are averaged over a week and
multiplied by the weekly integrated con-
centrations to produce weekly fluxes of
HNO3, SO2, and particles. Ozone flux is
calculated using hourly measurements of
O3 and hourly values of Vd. Weekly flux
calculations for all chemical species are
considered valid only if >70 percent of
hourly Vd values are available for that
week. Annual values are considered valid
only if Vd and flux data for all four seasons
are available.
Conclusions
The full report presents quantitative
information on dry deposition fluxes and
atmospheric concentrations for the net-
work from 1990 through 1992. Annual
concentration data for sulfur and nitro-
gen species show fairly consistent spa-
tial patterns. In the eastern United
States, the highest average concentra-
tions of SO- [6 to 7 micrograms per
cubic meter (ug/m3)] and SO2 (15 to 20
ug/m3) occur in an area surrounding the
Ohio River Valley. Average SO" de-
creases gradually toward the periphery
of the network to about 2.0 ug/m3 in
northern Maine and 3 to 4 ug/m3 in
Florida, Arkansas, and Wisconsin. Av-
erage SO2 exhibits much more local vari-
ability than SO" and decreases more
rapidly towards the periphery of the net-
work. For western sites, SO" and SO2
concentrations are generally well below
1.0 ug/m3 and show no strong evidence
of a pattern, except for somewhat el-
evated values at two Arizona sites.
ability across the network; however, dif-
ferences between eastern and western
sites are not as great as for SO' and SO2.
In general, concentration patterns for HNO3
and NO appear to be strongly influenced
by land-use and topographic features.
Maximum HNO3 concentrations (3 to 4
ug/m3) are observed at scattered sites in
New York, Pennsylvania, Virginia, and
Ohio, while minimum concentrations
(<1.0ug/m3) are observed in Maine, Ar-
kansas, Florida, Kentucky, and North Caro-
lina. Substantial variability in HNO3 con-
centrations (factors of 2 to 3) occurs over
fairly short distances in and around the
southern Appalachian Mountains.
Annual patterns of NO aerosol show
peak concentrations (3 to 4 ug/m3) in agri-
cultural areas of the midwest and mini-
mum concentrations (<0.5 ug/m3) in the
forested northeast. Intermediate concen-
trations occur at sites in pastured land or
near limited agricultural activities. These
observations suggest that land use (spe-
cifically agricultural activity) affects the par-
titioning of gas and aerosol nitrogen spe-
cies.
Dry deposition calculations show that
annual sulfur (S) fluxes (SO- plus SO2)
range from 1 kg-S/ha to about 10 kg-S/ha
in the eastern United States and from 0.2
kg-S/ha to 1.5 kg-S/ha in the western
United States. Deposition of SO2 accounts
for the majority of dry sulfur flux at all
NDDN sites and for at least 75 percent
and 50 percent of dry sulfur deposition at
eastern and western sites, respectively.
Although the physical and chemical phe-
nomena involved are complex, results sug-
gest that the spatial pattern of SO2 depo-
sition is controlled primarily by variability
in SO2 concentrations, rather than vari-
ability in deposition velocity.
Dry deposition of nitrate-nitrogen (HNO3
plus NO) ranges from <1 kg-N/ha to 5 kg-
N/ha in the eastern United States and
from 0.3 kg-N/ha to 1.3 kg-N/ha in the
western United States. Deposition of HNO3
accounted for at least 90 percent, and
usually greater than 95 percent, of dry
nitrate-nitrogen fluxes at all sites. Inspec-
tion of deposition data suggests that spa-
tial variability is a function of both concen-
tration and deposition velocity (Vds). These
results are uncertain by ±20 percent to
±40 percent and are biased low for a
number of reasons.
Total deposition estimates (i.e., mea-
sured wet deposition plus calculated dry
deposition) for 15 sites shows that dry
deposition accounts for 20 to 50 percent
and 30 to 60 percent of sulfur and nitrate-
nitrogen inputs, respectively. Dry sulfur
deposition appears to be roughly equiva-
lent to wet sulfur deposition near sources
of SO2 (e.g., the midwest and northeast).
Dry nitrate-nitrogen deposition appears to
be roughly equivalent to wet deposition at
sites located on ridges and in substantial
clearings.
Data from sites in and around the Ap-
palachian Mountains indicate that concen-
trations and fluxes of SO2 and HNO3 can
vary by as much as factors of 2 to 5 within
only a few hundred kilometers. Data from
two closely spaced sites located on a ridge
and in a valley in the southern Appala-
chian Mountains may explain this phe-
nomenon. The ridge site exhibits signifi-
cantly higher concentrations and Vds than
the valley site. Together, these factors
give rise to dry deposition rates that are
factors of 1.6 (for SQ-) to 4.4 (for HNO3)
higher at the ridge site than the valley
site. Ridgetop sites may provide upper
limit estimates of regional deposition, while
valley and intermediate elevation sites may
provide reasonable estimates of average
deposition to specific ecosystems. Further
work has been initiated to determine if
elevation is important elsewhere, and, if
so, how data at a single site can be scaled
over areas of interest.
Comparison of atmospheric concentra-
tion data from the United States and Ca-
nadian trends networks [NDDN and Ca-
nadian Acid Precipitation Monitoring Net-
work (CAPMoN)] shows that annual aver-
ages of most species agree within 5 per-
cent. Data for SO2 differ by about 10 per-
cent, on average, between networks, with
NDDN concentrations almost invariably
lower than those of CAPMoN. In general,
these findings suggest that data from the
two networks can be readily combined for
analysis of spatial patterns and long-term
trends across eastern North America. Ad-
ditional work has been initiated to eluci-
date the disparity in SO2 concentrations
between networks and to compare United
States and Canadian approaches for esti-
mating dry deposition fluxes.
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Ralph Baumgardner is the EPA Project Officer (see below).
The complete report, entitled "CASTNet National Dry Deposition Program 1990-
1992 Status Report," (Order No. PB95-234506; Cost $27.00, 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:
National Environmental Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection Agency
Technology Transfer and Support Division (CERI)
Cincinnati, OH 45268
Official Business
Penalty for Private Use $300
EPA/600/SR-95/086
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