United States
Environmental Protection
Agency
National Exposure
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/SR-98/027 September 1998
Project Summary
Clean Air Status and Trends
Network Deposition Summary
Report (1987-1995)
The National Dry Deposition Network
(NDDN) was established in 1986 to pro-
vide long-term estimates of dry acidic
deposition across the continental U.S.
In 1990, NDDN was incorporated into
the Clean Air Status and Trends Net-
work (CASTNet) which was created to
address the requirements of the Clean
Air Act Amendments (CAAA). Approxi-
mately 50 standard sites were opera-
tional from 1989 through 1995 with the
majority of the sites located in the rural
eastern U.S. Each site is equipped with
sensors for continuous measurements
of ozone (O3) and meteorological vari-
ables required for estimation of dry
deposition rates. Weekly average at-
mospheric concentrations of panicu-
late sulfate (SO42), paniculate nitrate
(NO3-), paniculate ammonium (NH4+),
sulfur dioxide (SO2), and nitric acid
(HNO3) were measured at all sites.
Precipitation samples were collected at
selected sites and analyzed for acidity
and related species in order to esti-
mate wet deposition. Under CASTNet,
a visibility monitoring network and a
Mountain Acid Deposition Program
(MADPro) were established. A mi-
crometeorological model has been ap-
plied to calculate deposition velocities
and dry deposition fluxes.
Atmospheric concentration data
showed species-dependent variability
in space and time. In general, the high-
est annual concentrations [>6.0 micro-
grams per cubic meter (ug/m3) for SO42 ]
were observed along the Ohio River
valley, and these were a factor of 5 to
10 times higher than concentrations
observed in the west. Significant con-
centration gradients were observed be-
tween the northeast and upper
northeast, and the Midwest and upper
Midwest. A comparison of 1989 and
1995 annual concentrations show 8O2
concentrations averaged over all east-
ern sites decreased by 43%. Sulfate
concentrations decreased by 23%. An-
nual average concentrations of nitro-
gen species showed little change
between 1989 and 1995. These results
do not account for the yearly variabil-
ity in concentrations.
Figure 1. A typical CASTNet monitoring station
(Site 161, Gothic, CO).
Dry deposition calculations for 1987
through 1995 showed that SO2 accounts
for more than 70% of dry sulfur deposi-
tion at eastern sites and more than
55% of dry sulfur deposition at west-
ern sites. HNO3 accounts for more than
65% of dry nitrogen deposition at all
sites. Data for all eastern sites showed
a 29% reduction in SO2 deposition and
a 6% reduction in SO42- deposition from
1989 to 1995. There is no apparent trend
for western sites. The dry deposition
calculations represent lower bound es-
timates of actual fluxes and do not ac-
count for quantified uncertainties of the
model.
Annual precipitation concentrations
of SO/- from 1989 to 1995 declined
significantly only in certain parts of the
eastern region. There were no statisti-
cally significant trends in precipitation
concentrations of NO3-. Wet depositions
Printed on Recycled Paper
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from 1989 to 1995 showed statistically
significant reductions of sulfur species
for the eastern region. No statistically
significant reductions were observed
for nitrogen species.
Total (wet plus dry) deposition esti-
mates from 1989 to 1995 showed that
dry deposition accounts for about 15
to 45% of total sulfur deposition, and
20 to 60% of total nitrogen deposition.
The total deposition data showed a 32%
reduction in atmospheric sulfur and no
trend in nitrogen from 1987 through
1995.
Ozone data indicated considerable
geographic variability in annual aver-
ages, but little yearly variability at indi-
vidual stations. There was no
discernible trend in annual averages.
Hourly concentrations above the 1-hour
National Ambient Air Quality Standard
(NAAQS) were limited to sites in the
Washington-New York corridor. Ozone
measurements show that about 80% of
the eastern sites would have recorded
violations of the new 8-hour O3 stan-
dard.
Data from MADPro for 1994 to 1996
showed that cloudwater can be the pri-
mary pathway for deposition of pollut-
ants to high elevation ecosystems.
An initial analysis of visual air qual-
ity measurements taken in 1994 shows
a strong relationship among atmo-
spheric light scattering, fine particle
concentrations, and visual quality. Fine
particle concentrations peaked in the
summer and were highly correlated with
fine SO42% which is declining in the east-
em U.S. Fine particle mass concentra-
tions were below the proposed national
standards of 15 ug/m3 annual average
and 65 ug/m3 24-hour average.
This Project Summary was developed
by the U.S. Environmental Protection
Agency's (EPA's) National Exposure Re-
search Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering infor-
mation at the back).
Introduction
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
U.S. have been well documented over the
last 10 to 15 years; however, comparable
information is unavailable for dry deposi-
tion rates. This lack of information in-
creases the uncertainty in estimates of
interregional, national, and international
transport and confounds efforts to deter-
mine the overall impact of atmospheric
deposition.
In 1986, the EPA contracted with Envi-
ronmental Science and Engineering, Inc.
(ESE) [now QST Environmental Inc.
(QST)] to establish and operate the Na-
tional Dry Deposition Network (NDDN).
The objective of the NDDN was to obtain
field data at approximately 50 sites
throughout the U.S. to establish patterns
and trends of dry deposition. The approach
adopted by the NDDN was to estimate
dry deposition using measured air pollut-
ant concentrations and modeled deposi-
tion velocities estimated from
meteorological, land use, and site charac-
teristic data. The model currently used for
dry deposition calculations is a multi-layer
version of the Big Leaf Model.
The complete report summarizes results
of NDDN and CASTNet monitoring activi-
ties from 1987 through 1995. Concentra-
tion and deposition data for atmospheric
sulfur and nitrogen species are presented
and analyzed. Annual and seasonal aver-
ages for each year are discussed region-
ally. Relative contribution of gases versus
aerosols are evaluated. Wet deposition
data for 21 CASTNet and 38 National
Atmospheric Deposition Program (NADP)
sites are presented and then used, along
with dry deposition calculations, to esti-
mate total depositions of sulfur and nitro-
gen. The relative magnitude of wet and
dry deposition is discussed. Ozone con-
centrations and exposure statistics are
analyzed and discussed in terms of exist-
ing and proposed national air quality stan-
dards.
Data and results from the visibility and
mountain acid deposition programs are
briefly discussed. A mobile system for the
direct measurement of dry deposition is
described along with field studies in which
the system was successfully utilized. Ini-
tial results of the comparison between fil-
ter packs and annular denuders are also
given.
Procedures
Measurements of ambient O3, SO2,
SO/-, NO?-, HNO3, NH/, and meteoro-
logical variables required for dry deposi-
tion calculations are taken at each
CASTNet site. Meteorological variables
and O3 concentrations are recorded con-
tinuously and reported as hourly averages
consisting of a minimum of nine valid 5-
minute averages. Atmospheric sampling
for sulfur and nitrogen species is inte-
grated over weekly collection periods us-
ing a 3-stage filter pack. In this approach,
particles and selected gases are collected
by passing air at a controlled flow rate
through a sequence of Teflon®, nylon,
and base-impregnated cellulose
(Whatman) filters. The Teflon® filter re-
moves particulate SO/-, NO3-, and NH4+;
the nylon filter removes HNO3; and the
cellulose filter, which is impregnated by
potassium carbonate (K2CO3), is used for
removal of SO2. In practice, a fraction
(usually <20%) of ambient SO2 is cap-
tured on the nylon filter. The nylon filter
SO2 and Whatman filter SO2 are therefore
summed to provide weekly average con-
centrations. The nylon filter HNO3 is con-
verted to NO3- and added to the Teflon®
filter NO3- to provide weekly total NO3-
concentrations.
Filter packs are prepared and shipped
to the field weekly and exchanged at each
site every Tuesday. Blank filter packs are
collected monthly to evaluate passive col-
lection of particles and gases as well as
contamination during shipment and han-
dling. At 21 sites located more than 50
kilometers (km) from NADP sites, precipi-
tation samples are collected weekly (ac-
cording to NADP protocols) and shipped
to QST for chemical analysis.
Filter pack samples and O3 measure-
ments are taken at 10 meters (m) using a
tilt-down aluminum tower (Aluma, Inc.).
Filter pack flow is maintained at 1.50 liters
per minute (Lpm) at eastern sites and
3.00 Lpm at western sites, for standard
conditions of 25 degrees Celsius (°C) and
760 millimeters of mercury (mmHg) with a
mass flow controller (MFC). Precipitation
samples are collected in precleaned poly-
ethylene buckets using an Andersen Model
APS precipitation sampler. Buckets are
placed on the sampler on Tuesday and
removed, whether or not rainfall has oc-
curred, the following Tuesday. Buckets are
weighed in the field, decanted to a 1-liter
(L) polyethylene bottle, sealed, and
shipped to QST for chemical analysis. Pre-
cipitation amount (depth) is also moni-
tored at wet deposition sites.
O3 is measured via ultraviolet (UV) ab-
sorbance with a Thermo-Environmental
Model 49-103 analyzer operating on the
0- to 500-part per billion (ppb) range. Am-
bient air is 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 pre-
vent particle deposition within the system.
Periodic checks indicate that online losses
through the inlet system are consistently
less than 3%. Zero, precision (60 ppb),
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•142
Legend
o Dry deposition only
• Dry and wet deposition
Figure 2. CASTNet monitoring sites (Source: QST).
and span (400 ppb) checks of the O3
analyzer are performed every third day
using an internal O3 generator.
In addition, various observations are
periodically made at the CASTNet sites to
support model calculations of dry deposi-
tion. Site operators record surface condi-
tions (e.g., dew, frost, snow) and
vegetation status weekly. Vegetation data
are obtained to track evolution of the domi-
nant plant canopy, from leaf emergence
(or germination) to senescence (or har-
vesting). Once a year, site operators also
provide information on major plant spe-
cies and land-use classifications within 1.0
km of the site. Additional land-use data
were 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) measurements
were taken at all CASTNet sites during
the summers of 1991 and 1992. LAI was
measured using an LAI-2000 Plant Canopy
Analyzer manufactured by Li-Cor (Lincoln,
NE).
All field equipment is subjected to quar-
terly inspections and multipoint calibra-
tions, using standards traceable to the
National Institute of Standards and Tech-
nology (NIST). In addition, independent
equipment audits were performed annu-
ally by Ogden Environmental and Energy
Services, Inc. (Ogden), and randomly by
EPA or its designee. Results of field cali-
brations are used to assess sensor accu-
racy and flag, adjust, or invalidate field
data.
Following receipt from the field, exposed
filters and blanks are extracted and then
analyzed for SO42- and NO3- by
micromembrane-suppressed ion chroma-
tography (1C). Teflon® filter extracts are
also analyzed for NH4+ by the automated
indophenol method using a Technicon II
or TRAACS-800 Autoanalyzer system.
Wet deposition samples are filtered and
then analyzed for pH, conductivity, acid-
ity, sodium (Na*), potassium (K*), NH/,
calcium (Ca2+), magnesium (Mg2+), chlo-
ride (CI-), nitrite (NO2-), NO3-, and SO42-.
A micrometeorological model called the
multilayer model (MLM) was used to simu-
late deposition velocities for the measured
ambient species as the inverse sum of
three separate resistances: atmospheric
resistance (Ra), boundary layer resistance
(Rb), and canopy resistance (Re). Dry
depositions were then calculated as the
product of concentrations and deposition
velocities. An analysis of the uncertainties
in simulated deposition velocities suggest
that the MLM underestimates observed
deposition velocities and consequently dry
depositions. Since the uncertainties in the
model have not been quantified, calcu-
lated dry depositions represent lower
bound estimates only.
Results and Discussion
The CASTNet measurements show sta-
tistically significant reductions in annual
ambient SO2, SO42-, and HNO3 concentra-
tions averaged over all eastern sites. The
data for the eastern sites combined show
a 23% reduction in SO42- and a 43% re-
duction in SO2 between 1989 and 1995
annual concentrations. Changes in sulfur
species are more pronounced in the north-
east and smaller, but still substantial, in
the southeast. There is no apparent trend
in the data from the western sites. The
eastern data indicate about 70% of ambi-
ent sulfur is in the form of SO2. These
comparisons do not account for year-to-
year variations in concentrations. Analy-
ses that address these variations may yield
slightly different results.
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SOj concentrations show significant re-
ductions in summer and winter, even
though concentrations are much lower in
summer when SO2 emissions are involved
in photochemical reactions.
A slight reduction in HNO3 concentra-
tions results from the decline in summer
averages. The downward trend is observed
in most of the network, except for the
southern peripheral sites and the western
sites. No trends are observed in annual
concentrations of NO3- aerosol and total
N9r
The geographic distributions of annual
dry sulfur depositions (fluxes) are similar
from year to year and correspond well
with distributions of SO2 and SO42- con-
centrations and reflect the locations of SOX
sources. Annual fluxes of total nitrogen
show a fairly uniform pattern of deposi-
tions above 2.0 kg/ha (as N) from year to
year. Lower depositions were calculated
for the CASTNet sites in the Appalachian
chain, similar to the results for HNO3 con-
centrations.
Time series of weekly fluxes show con-
siderable site-by-site and weekly variabil-
ity. The seasonal cycles of fluxes of
individual species are similar to the sea-
sonal behavior of concentrations.
Scattergrams and correlation statistics
show a strong relationship between con-
centrations and depositions of SO2, SO/-,
and HNO3.
Results of linear regressions of dry
depositions versus year for all eastern sites
combined show downward trends for the
sulfur species although the trend lines are
not considered statistically significant. The
downward slopes for SO2 and total sulfur
are greater than for SO/-, consistent with
the results for concentrations. The data
show a 29% reduction in dry deposition of
SO2 (as S) and only a 6% reduction in
deposition of SO/- aerosol. These per-
centage reductions in depositions are lower
than the comparable reductions in con-
centrations. The linear regressions show
no significant trends in depositions of the
nitrogen species. Depositions of sulfur and
nitrogen species calculated for the west-
ern sites exhibit no trends.
Concentrations of anions and cations
measured in precipitation samples col-
lected at CASTNet sites and concentra-
tions measured at those NADP sites
approximately collocated with CASTNet
sites that do not collect precipitation
samples were combined to form one con-
centration and wet deposition database.
Annual SO/- concentrations exhibited sta-
tistically significant downward trends for
the northeastern and southern peripheral
sites. Although the eastern region did not
exhibit a statistically significant reduction,
a downward trend is indicated by the data
and analyses.
Linear regressions of annual wet depo-
sitions of SO42- (as S) show statistically
significant downward trends for all the east-
ern sites combined. The results show an
overall decrease in wet SO/- deposition
of approximately 35% over the period 1989
to 1995. No significant trends were indi-
cated for wet NO3- depositions. However,
the measurements still show a reduction
of about 20% for the eastern data com-
bined over the 7-year period.
Dry and wet depositions were summed
to obtain total depositions of sulfur and
nitrogen species. A preliminary trends
analysis of total deposition data, which
does not correct for meteorological varia-
tions or model uncertainties, shows statis-
tically significant reductions in annual
deposition of sulfur over the period 1989
through 1995. The downward trends are
considered significant throughout the east-
ern network. Figure 3 depicts the linear
regression analysis of total sulfur deposi-
tion versus year. The figure also shows
nationwide annual SO2 emissions. No trend
is apparent in the western data. Estimates
of total deposition of nitrogen species ex-
hibit no trend.
The CASTNet O3 data provide estimates
of exposure statistics and allow gauging
compliance with the existing and proposed
NAAQS for O3. After 1988, violations of
the 1-hour standard were limited to subur-
14
12
10
8
o
a.
to
Q
Total S
all eastern sites
ban sites in the Washington-New York
corridor. Concentrations above the pro-
posed 8-hour standard of 85 ppb were
measured throughout the midwest and
northeast subregions. The measure
SUM06 has been suggested as a second-
ary standard for O3. From 1989 through
1995, many CASTNet sites show SUM06
values above 25 ppm-hr, the proposed
numerical limit. Measurements from the
CASTNet visibility network for 1994 show
a strong relationship between fine particle
mass (FPM) and fine SO/- concentrations.
Fine SO/- contributes more than 85% of
the mass of total particulate SO/-. The
rural CASTNet measurements show com-
pliance with the proposed 24-hour and
annual FPM standards of 65 u,g/m3 24-
hour average and 15 |ig/m3 annual aver-
age. Furthermore, the CASTNet data show
a downward trend in ambient SO/-, which
is a major contributor to FPM in the east.
MADPro is a component of CASTNet
designed to study the deposition of air
pollutants to high elevation forests.
MADPro objectives are to measure cloud
chemistry, determine total deposition and
define source regions which impact high
elevation ecosystems in the eastern U.S.
The results to date show that cloudwater
can be the primary pathway for deposition
of air pollutants.
A mobile system to measure direct dry
deposition fluxes via eddy correlation has
been in operation since 1994. The system
has been deployed at several sites with
23 p
OL
Slope -0.629
p 0.01
r - squared 0.779
24
o
CO
oo -2
22 co
CO
"E
20 m
18
CO
CD
c
o
•
1989 1990 1991 1992 1993
Note: Emissions are indicated by a *.
1994 1995
Figure 3. Linear regressions of total sulfur depositions versus year for all eastern sites combined
(Source: QST).
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varying terrain settings and land uses to
better understand deposition processes
and to improve the MLM and other mod-
els.
Preliminary results of the CASTNet
intercomparison study between annular
denuders and filter packs show a high
correlation between SO42- measurements
using the two different methods, and simi-
lar results for SO2. Filter pack HNO3 mea-
surements were higher than denuder
measurements.
Conclusions and
Recommendations
The CASTNet database presents the
opportunity for the first time to investigate
trends in total (wet plus dry) deposition of
sulfur and nitrogen species and contrast
the results with trends in emissions. Total
deposition of atmospheric sulfur (Figure
3) averaged over all CASTNet eastern
sites has decreased by 32% over the pe-
riod 1989 through 1995. Nationwide SO2
emissions have declined by 22% over the
period 1985 through 1995. Electric utility
SO2 emissions have dropped by 24% over
that same period. A dramatic drop in SO2
emissions has been reported from 1994
to 1995. Nationwide SO2 emissions have
dropped 13% and utility emissions 17% in
one year. NOX emissions have been rela-
tively flat since 1970. However, an 8%
reduction in overall NOX emissions and
21% reduction in electric utility NOX emis-
sions was reported between 1994 and
1995. Despite these reported recent NOX
emission reductions, the CASTNet data
show no change in total deposition of ni-
trogen over the period 1989 through 1995.
Advanced statistical analyses, beyond
the scope of this report, are performed
independently on the CASTNet data to
further elucidate the apparent trends, and
to decipher trends not detectable by simple
linear regressions. Statistical analyses
should investigate trends at individual sites
as well as subregional averages to better
understand the response to changes in
emissions and meteorological fluctuations.
The full report presents quantitative in-
formation on dry deposition fluxes and
atmospheric concentrations for the net-
work from 1987 through 1995. A descrip-
tion of the network's quality assurance/
quality control (QA/QC) program and key
indicators of data quality such as preci-
sion and accuracy statistics are presented
as well. The results of the QA/QC pro-
gram demonstrate conclusively that the
observed changes in concentrations and
depositions are real and not the result of
network modifications or of data impreci-
sion or inaccuracy.
Concentration and deposition changes
are the result of changes in emissions
and of meteorological fluctuations. In short,
the CASTNet data constitute an excep-
tional database for the purpose of dis-
cerning status and trends in air quality
and of supporting other scientific activi-
ties. To preserve the integrity of and to
continue building upon the existing data-
base, CASTNet should be operated with
minimum disruptions. The number of sites
and locations should be reviewed to opti-
mize site locations for improvement in the
detection of trends. Evaluation of the MLM
should continue in order to reduce the
uncertainties of the accuracy and preci-
sion of deposition estimates. Model ac-
ceptance criteria should be fine-tuned.
Therefore, the mobile dry deposition field
studies should continue until the MLM is
fully evaluated and flux calculations are
representative of a wide variety of land
use and terrain settings.
The visibility network should be contin-
ued and perhaps expanded to better de-
tect trends in visibility related air quality
parameters and fine particle concentra-
tions. The MADPro sites should also be
continued so that investigation of the im-
pact of cloud deposition to sensitive eco-
systems may continue.
CASTNet has produced an exceptional
database that satisfies many of the re-
quirements of the CAAA of 1990. The
network will help assess compliance with
the proposed NAAQS for O3 and fine par-
ticles. It will continue to measure improve-
ments in air quality and depositions
associated with CAAA-mandated reduc-
tions in SOX, NOX> and VOC emissions
over the next 10 years.
Ralph Baumgardner is the EPA Project Officer (see below).
The complete report, entitled "Clean Air Status and Trends Network Deposition
Summary Report (1987-1995)," (Order No. PB98-150097; Cost: $71.50, subject
to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-605-6000
The EPA Project Officer can be contacted at:
National Exposure Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC27711
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United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
Penally for Private Use
$300
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