United States
Environmental Protection
Agency
Atmospheric Research and Exposure
Assessment Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-91/018 June 1991
EPA Project Summary
National Dry Deposition Network:
Third Annual Progress Report
(1989)
Eric S. Edgerton, Thomas F. Lavery, and Hugh S. Prentice
The National Dry Deposition Network
(NDDN) Is designed to provide long-
term estimates of dry acidic deposition
across the continental United States.
Fifty NDDN sites were operational dur-
ing 1989, Including 41 sites in the east-
ern United States and 9 sites in the
western United States. Each site was
equipped with sensors for continuous
measurements of ozone and meteoro-
logical variables required for estima-
tion of dry deposition rates. Weekly
average atmospheric concentrations of
SO/-, NO •, NH4*, SO2, and HNO3 were
measured (using 3-stage filter packs)
throughout the year, while Na*, K*,
Ca2*, and Mg2* were measured from
January through September. Separate
day/night samples were analyzed from
January through September and
around-the-clock samples were ana-
lyzed for the remainder of 1989.
Results showed species-dependent
variability in atmospheric concentra-
tions from site to site, season to sea-
son, and day to night. In general, SO4S-,
NH4*, SO2 , and HNO3 concentrations
were much higher (factor 5-10) at east-
ern sites than at western sites. On the
other hand, NO-, Na*, K*, Ca2*, and
Mg ** concentrations were frequently
comparable at eastern and western
sites. Average SO42-, NH4*, and HNO3
concentrations were typically highest
during summer and lowest during fall.
In contrast, SO2 and NO3~ were highest
in winter and lowest In summer. Day/
night variability was low for aerosols,
but frequently pronounced for SO2 and
HNO3 , especially during the summer
and at sites located In complex terrain.
Comparison of O3 data for 1988 and
1989 showed marked differences be-
tween years and a distinct tendency
for higher concentrations in 1988.
Ninety-eight exceedances of the MAAQS
were observed at 18 sites In 1988, while
only 15 exceedances were observed at
43 sites In 1989. Approximations of an-
nual dry deposition rates for SO4% SO2 ,
NO3-, and HNO3 suggest that gaseous
deposition greatly exceeds aerosol
deposition and that dry fluxes are simi-
lar to wet deposition at numerous sites
in the eastern United States. Applica-
tion of site-specific dry deposition
models are needed to refine these esti-
mates.
This Project Summary was developed
by EPA's Atmospheric Research and
Exposure Assessment Laboratory, Re-
search 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
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
Printed on Recycled Paper
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over the last 5 to 10 years; however, due
to measurement difficulties, comparable
information is unavailable for dry deposi-
tion rates.
In 1986, the U.S. Environmental Pro-
tection Agency (EPA) contracted with En-
vironmental Science & Engineering, Inc.
(ESE) to establish and operate the National
Dry Deposition Network (NDDN). The ob-
jective of NDDN is to obtain field data at
50 to 100 sites throughout the United
States to establish patterns and trends of
dry deposition. Ultimately, dry deposition
fluxes will be calculated using measured
air pollutant concentrations and inferred
deposition velocities estimated from me-
teorological, land use, and site character-
istic data.
This report describes progress on the
NDDN during calendar year 1989. The
purpose of this report is to familiarize the
reader with the general approach of NDDN
and the various types of data which are
being produced and reported to EPA. It
describes the network configuration and
deployment schedule as well as proce-
dures developed for field operations,
laboratory operations, database manage-
ment, and quality control (QC). An over-
view of air quality data for 1989, including
QC results, is also presented. Finally, dry
deposition rates are estimated for selected
sites and compared with wet deposition
rates. These estimates were prepared us-
ing literature values for deposition veloci-
ties and, therefore, are intended only to
illustrate likely ranges of deposition.
Deployment of all currently planned
NDDN sites was completed during 1989
(see Figure 1). Forty-one primarily rural
monitoring sites were operated through-
out the year in the eastern United States.
In addition, nine sites in the western United
States were established and operational
by midyear.
Each NDDN site was equipped with
sensors and sampling apparatus for con-
tinuous measurement of ozone (O3) and
meteorological variables required to esti-
mate dry deposition. Weekly average at-
mospheric concentrations of paniculate
sulfate (SO42'), paniculate nitrate (NO3"),
particulate ammonium (NH4*), SO, , and
HNO3 were determined throughout the year
so weekly dry deposition loadings could
be calculated. Particulate sodium (Na*),
particulate potassium (K*), particulate cal-
cium (Ca2*), and particulate magnesium
(Mg2*) were determined for part of the
year to evaluate the presence of atmo-
spheric base cations.
Field Operations
Each site was equipped with a shelter
(complete with telephone and 100-amp
electrical service), two 10-meter (m) tow-
ers, a meteorological system, an O3 and
air-quality monitoring system, and a data
acquisition system (DAS). Windspeed and
wind direction were measured at 10m,
temperature was measured at 9 m and
2 m, and relative humidity was measured
at 9 m. Precipitation and solar radiation
were measured on 1-m platforms located
outside the rain and sun shadows of the
shelter and towers. Surface wetness was
measured at a height of approximately 3
to 6 inches above the surrounding low-
lying vegetation (typically grass).
Filter pack sampling and O3 measure-
ments were performed at 10 m using a
tilt-down aluminum tower. Filter pack flow
was maintained at 1.50 liters per minute
(L/min) at eastern sites and 3.00 L/min at
western sites with a mass flow controller
and recorded as hourly averages. O3 was
measured via ultraviolet (UV) absorbance
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Ftguro 1. Status of NDDN Monitoring Sites — December 1989
0 200 400km
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Scale
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using an all-Teflon* sampling line. Periodic
checks indicated that line losses through
the inlet system were consistently less
than 3 percent.
All field equipment was subjected to
inspections and multipoint calibrations by
ESE personnel on a quarterly basis. In
addition, independent equipment audits
were performed semiannually by ERC En-
vironmental and Energy Services, Inc., and
randomly by EPA or its designee.
Laboratory Operations
Filter pack samples were loaded,
shipped, received, extracted, and analyzed
by ESE personnel at the Gainesville,
Florida laboratory. Filter packs contained
three types of filters in sequence: a Te-
flon8 filter for collection of aerosols, a ny-
lon filter for collection of HNO3, and dual
potassium carbonate (K2CO3) impreg-
nated cellulose filters for collection of SO2.
Following receipt from the field, exposed
filters and blanks were placed in color-
coded bottles and extracted in 25 millili-
ters (mL) of deionized water (Teflon*),
25 ml of 0.003 N NaOH (nylon), or 50 ml
of 0.05-percent H£O2 (cellulose). Extracts
were then analyzed for SO42- and NO3- by
micromembrane suppressed ion chroma-
tography (1C) using a Dionex Model 40001
1C equipped with an Autoion 1000 Con-
troller. Analysis of Na*. Mg2*, and Ca2*
was performed with a Perkin-Elmer P-2
inductively coupled argon plasma (ICAP)
emission spectrometer. Analysis of NH4*
was by the automated indophenol method
using a Technicon II or TRAACS-800
AutoAnalyzer system. Analysis of K* was
via atomic emission on a Perkin-Elmer
5100 atomic absorption spectrophbtom-
eter. Various QC samples were routinely
analyzed to track the accuracy and preci-
sion of laboratory data.
Data Management
Data Management Center activities
consisted of three major operations: data
acquisition, validation, and transmittal to
EPA. The data acquisition process
stressed multiple levels of redundancy to
minimize data loss. The primary mode of
data acquisition from the field was via
telephone modem. If daily polling resulted
in incomplete data capture from any site,
then diskettes of data from the primary
and backup DAS were read into the data-
base management system. If the database
was still incomplete, then missing data
were entered manually from site printouts.
Each datum was automatically given a
source flag that could be used to trace its
mode of entry into the system. Data vali-
dation consisted of a thorough review of
'Mention of trade names or commercial products does
not constitute endorsement or recommendation for use.
INS = Insufficient Samples for the Period Covered (<75%)
Figure 2. Annual average SO/- concentrations fag/m3) for the eastern United States during 1989.
operator logs, onsite reasonableness
checks, results of field calibrations and
audits, and a variety of parameter-specific
range and consistency checks. Validated
data were submitted to EPA on a quar-
terly basis.
Results And Discussion
Filter Pack Measurements
Annual arithmetic mean concentrations
of SO42' for 1989 ranged from 7.9 micro-
grams per cubic meter (ug/m3) in western
Ohio to 2.7 ug/m3 in northern Maine (see
Figure 2). Annual averages of 5.0 ug/m3
covered nearly the entire eastern United
States, from New York and Michigan to
northern Mississippi and Alabama. Only
sites from northern New York to Maine,
northern Michigan, Wisconsin, and Florida
exhibited concentrations below 5.0 u.g/m3.
Quarterly data for SO42' showed dramatic
differences from season to season, but
reasonably consistent locations of peak
concentrations. The highest concentrations
were observed during the third quarter
(i.e., summer) and the lowest concentra-
tions in the first quarter (i.e., winter). The
seasonal progression of SO42- concentra-
tions, therefore, appears to follow tem-
perature and solar radiation, which also
exhibit maxima and minima in summer
and winter, respectively.
Semiannual average SO42~ concentra-
tions for the nine western sites ranged
from 1.65 ug/m3 in southern Arizona to
0.65 ug/m3 in southern Idaho and were
invariably lower than those observed
across the eastern United States.
Annual average concentrations for NO3~
(see Figure 3) exhibited much more vari-
ability than SO42' and a definite pattern of
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INS - Insufficient Samples for the Period Covered (<75%)
Figure 3. Annual average NO3m concentrations fag/rrP) lor the eastern United States during 1989.
higher concentrations in the midwest than
elsewhere. The lowest concentrations were
observed at forested sites in New En-
gland and the southern Appalachian
Mountains, while the highest concentra-
tions (i.e., greater than 2.0 u.g/m3) were
observed in agricultural areas of the
midwest. Intermediate values (i.e., 1.0 to
2.0 ng/m3) appear to be associated with
agricultural sites anywhere in the eastern
United States. Quarterly data for NO3" ex-
hibit a seasonal cycle that runs counter to
that of SO4*-. That is, the highest concen-
trations occurred during the first quarter
and the lowest in the third quarter. This
cycle is consistent with the temperature-
dependent equilibrium between participate
NH4NO3 and gaseous NH3 and HNO3. Re-
sults for the western sites indicated semi-
annual concentrations similar to the low-
est values observed at eastern sites.
Annual average concentrations of NH4*
(see Figure 4) ranged from 3.2u.g/m3 in
northern Indiana to 0.69 ng/m3 in Maine
and, in general, exhibited higher values at
agricultural sites than at forested sites.
Since the midwest is primarily agricultural,
this results in regional differences between
the midwest, northeast, and southeast.
Concentrations above 2.0 u.g/m3 were
found at all sites within Illinois, Indiana,
and Ohio, plus the two southernmost sites
in Michigan. Third and fourth quarter re-
sults for the western sites showed about a
factor of 2 range in concentrations. The
highest value (0.57 u.g/m3) occurred in
southern Arizona and the lowest average
value (0.28 mj/m3) occurred in Montana,
Wyoming, and Nevada.
Annual averages for HNO3 (see Figure
5) exhibited a maximum concentration of
3.6 u.g/m3 in southeastern Pennsylvania
and a minimum of 0.7ug/m3 in northern
Maine. Concentratbns in excess of 3.0 p,g/m3
occurred at six sites, while concentrations
of 1.0 (xg/m3 or less occurred at another
six sites. The overall pattern of HNO3 might
be influenced by terrain as much or more
than other factors (e.g., emissions).
Quarterly data showed that HNO3 con-
centrations are relatively constant. For the
41, eastern sites, averages ranged from
roughly 1.9u.g/m3 during the fourth quar-
ter to 2.5 ug/m3 during the second quar-
ter. Thus, seasonal variability is much less
than and slightly out of phase with SO/-
(i.e., another secondary pollutant). Results
for the western region indicate concentra-
tions that are somewhat lower than the
lowest values observed in the east.
Annual average data for SO2 ranged
from 23.2 (ig/m3 in southwestern Pennsyl-
vania to 2.4 u.g/m3 in Maine (see Figure 6).
Concentrations of 15 ug/m3 or greater oc-
curred in a small area encompassing
Pennsylvania and Maryland, as well as at
isolated sites in northern Illinois, southern
Indiana, and western Ohio. A much larger
area extending from Kentucky and Indi-
ana eastward to New York exhibited con-
centrations in the range of 10 to 15ug/m3.
Quarterly averages show dramatic
changes in concentrations from season to
season. Mean concentrations are nearly a
factor of 3 higher in the first quarter
(16.2u.g/m3) than in the third quarter
(6.2 ug/m3). Despite large relative changes
in concentrations, the locus of peak con-
centrations remains more or less station-
ary from season to season. Concentration
data for the western sites ranged from
1.8 ug/m3 in southern Arizona to 0.4 u.g/m3
in central Colorado and northern Nevada
and southern Idaho.
Day Versus Night
Concentration Data
Results for SO2- showed appreciable
seasonality in day/night concentration dif-
ferences. Results for fourth quarter 1988
and first quarter 1989 showed nearly
identical averages for day and night at
virtually all sites. During second and third
quarter 1989, differences became increas-
ingly more pronounced and were usually
statistically significant. Results for the pre-
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sumptive aerosol species NO3- and NH/
also showed similar day/night variability.
Data for HNO3 and SO, showed more
frequent and pronounced day/night differ-
ences than the aerosol species. Concen-
trations of HNO3 were lower at night for
nearly every she-season combination. This
was especially apparent during summer,
when all sites showed statistically signifi-
cant nocturnal reductions ranging from 35
to 80 percent. The day/night behavior of
SO2 is similar to that of HNO3. Statistically
significant differences occurred at most
sites for most seasons, and the magni-
tude of the differences increased from
winter to summer. Coupled with shallow
nocturnal boundary layers, dew formation
could be responsible for essentially com-
plete depletion of SO2 and HNO3 at night.
Aerosol Ion Balances
Ion balances, in nanoequivalents per
cubic meter (neq/m3), from Teflon® filter.
extracts at three NDDN sites are illus-
trated in Figure 7. The sites presented
appear to be representative of the eastern
seaboard (Site 108), forested northeast
(Site 117), and agricultural midwest
(Site 133). In general, the data for these
sites indicated that SO/ and NH4* are the
dominant anion and cation species, re-
spectively, at both forested and agricul-
tural sites in the eastern United States,
and that the nature of the ion balances
differs between forested and agricultural
sites. The forested sites exhibited fairly
minor ionic contribution for NO3~ and the
base metal cations and a clear excess of
anions over cations. The agricultural
midwestern site, in contrast, exhibited sig-
nificant ionic contributions from NO3~ ,
Mg2*, and Ca2* and an apparent excess
of cations. Results for the western sites
indicated a clear excess of cations in the
aerosol samples, with NH4* comprising only
about 50 percent of the total cations; NO3,
is also a minor contributor to the overall
ion balance.
Ozone
Annual average O, concentrations at
eastern sites ranged from 22.1 parts per
billion (ppb) in eastern Kentucky to
45.3 ppb in northern Virginia. The highest
annual averages occurred at mountaintop
sites along the Blue Ridge and Appala-
chian Mountains, while the bwest annual
averages occurred in sites located in sharp
valleys and in semiurban areas. Hourly
average concentrations equal to or greater
than the National Ambient Air Quality
Standards (NAAQS) were relatively rare
during 1989. Eight sites exhibited one or
more hourly values greater than or equal
to 120 ppb, and only 15 days of
exceedances occurred at the 43 sites op-
erational throughout the year.
Estimated Dry Deposition
Estimated dry deposition for SO4* plus
SO2 appeared to be highest in western
Pennsylvania and lowest in southwestern
North Carolina, northern Maine, and
northern Florida. This pattern reflects the
annual average SO, concentration, since
it appears unlikely that SO42' contributes
more than 30 percent of the estimated dry
deposition at any site. Relatively high
deposition in northern Illinois, southern In-
diana, and eastern Tennessee may be
the result of local SO2 emissions.
Estimated dry deposition of NO3" plus
HNO, showed similar values over much of
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deposition velocity of SO2 is approached.
Results for nitrogen suggest regional dif-
ferences in the comparability of wet and
dry deposition. For the northeast, dry
deposition would be comparable to wet
deposition only if the upper limit deposi-
tion velocity for HNO3 is attained. Data for
the midwest and parts of the southeast, in
contrast, suggest that wet and dry deposi-
tion are comparable even if the lower limit
deposition velocity for HNO3 is attained.
Conclusions And
Recommendations
The following conclusions summarize
results of a preliminary analysis of the
1989 database:
1. Results of filter pack analyses
throughout the year at 41 eastern
sites show species-dependent vari-
ability from site to site, season to
season, and day to night. Annual
average concentrations of atmo-
spheric SO.,2' exhibited peak values
of approximately 7.8ug/m3 in west-
ern Ohio and central Kentucky and
minimum values around 2 to 4ug/
m3 on the periphery of the network
(i.e., Maine, Wisconsin, and Florida).
Spatial variability for SO/- was rela-
tively low as compared to other spe-
cies. Data for nine western sites op-
erated over the last half of 1989
showed appreciably lower concen-
trations than eastern sites (i.e., 0.7
to 1.7ug/m3}. Among western sites,
the highest SO/' concentrations
were consistently observed in north-
ern and southern Arizona.
2. Annual SO2 concentrations for east-
ern sites showed a maximum of
23.2 ug/m3 in western Pennsylvania
and an ellipse of values above 10 ug/
m3 extending eastward from Illinois
and Kentucky to the eastern sea-
board. As for SO42', the lowest con-
centrations of SO2 among eastern
sites (i.e., 2.4 to 3.0 ug/m3) were ob-
served in Maine, Wisconsin, and
Florida. Western sites exhibited dra-
matically lower SO2 than eastern
sites (i.e., 0.4 to 1.3 ug/m3) and high-
est concentrations in southern Ari-
zona.
3. Annual average HNO3 ranged from
3.6 ug/m3 in southeastern Pennsyl-
vania to 0.7 ug/m3 in Maine. Con-
centrations above 2.0 ug/m3 covered
a broad region (excluding a few iso-
lated sites) from the Great Lakes to
northern Alabama and Georgia. Av-
erage HNO3 for the western sites is
typically about 50 percent of the low-
est values observed in the east. Ari-
INS = Insufficient Samples for the Period Covered (<75%)
Figure 5. Annual average HNO3 concentrations (ng/rr?) for the eastern United States during 1989.
zona sites, however, exhibited HNO3
concentrations similar to those re-
ported for Florida, Wisconsin, and
Maine.
4. Annual NO3' concentrations showed
marked regional character, with
maxima above 2.0 ug/m3 throughout
the midwest and minima below
0.5 ug/m3 at scattered locations from
New England to Florida. Examina-
tion of land use characteristics sug-
gests a link between NO3' and land
use. The highest overall concentra-
tions correlate strongly with agricul-
tural areas (in any region), and the
lowest concentrations correlate with
forested areas. Results for the west-
ern sites indicate comparable con-
centrations to forested areas of the
eastern United States.
5. Annual average NH4* data ranged
from 3.2 ug/m3 in northern Indiana
to 0.7 ug/m3 in Maine and exhibited
spatial variability similar to NO3\ Con-
centrations above 2.0 ug/m3 were
observed throughout the midwest
and at sites near agricultural activity
in the southeast and northeast. Only
sites in extreme northern New York,
New Hampshire, Maine, and Florida
exhibited annual averages below
1.0 ug/m3. Data for western sites
showed the majority of NH4* con-
centrations in the range of 02 to 0.4 ug/
m3. Consistent with other measure-
ments, the highest concentration
among western sites occurred in
southern Arizona.
6. Estimated dry deposition of sulfur
species ( i.e., SO42- and SO2) ranged
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from about 100 equivalents per hect-
are (eq/ha) in Maine to about 750
eq/ha in western Pennsylvania. Esti-
mated dry deposition of nitrogen spe-
cies (i.e., HNO3 and NO3") ranged
from about 55 eq/ha in Maine to
about 290 eq/ha in eastern Pennsyl-
vania. Due to faster deposition ve-
locities, the gaseous species HNO3
and SO2 represent a large fraction
of dry nitrogen and dry sulfur depo-
sition, respectively. Comparison of
wet and dry deposition of sulfur and
nitrogen at 28 eastern sites suggests
that the two are of similar magnitude
over large areas.
7. Inspection of O3 data for 1989 shows
that there were relatively few epi-
sodes of elevated concentrations.
Comparison of O3 data for 1988 and
1989 shows that these two years
differ significantly by virtually any
measure. For example, the 18 sites
operational during 1988 reported 98
exceedances of NAAQS for O3 of
120 ppb, while the 43 sites opera-
tional during 1989 reported only 15.
8. Calculations of aerosol ion balances
for selected sites indicate general
differences between regions and land
use categories. Forested northeast-
ern and southeastern sites exhibited
an excess of measured anions (SO42-
and NO") over measured cations
(NH4*, Na% K*, Ca2*, and Mg2*); agri-
cultural midwestern sites exhibited a
slight excess of cations over anions;
and western sites exhibited a sub-
stantial excess of cations over an-
ions.
9. Operation of dual, side-by-side air
samplers at four sites indicates that
filter pack measurements can be very
precise. For three eastern sites,
analyses of SO,2', SO2, and NH4+
exhibited precision estimates of 5
percent, or better; NO3~ and HNO3
exhibited precision of 10 percent, or
better. For a single site in the west-
ern United States (i.e., 167), mea-
surements of the above species uni-
formly exhibited precision within 5
percent.
INS ~ Insufficient Samples for the Period Covered (<75%)
Figure ft Annual average SO^onfentrations fag/rrf) for the eastern United States swing 1989.
7
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250
225
200
175
150
125
100
75
50
25
Legend
108
117
Site Number
Figure 7. Aerosol ton balances for Sites 108, 117, and 133.
if U.S. GOVERNMENT PRINTING OFFICE: 1991 - 548-028/40019
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E.S. Edgerton is with Environmental Science & Engineering, Inc. (ESE), Durham, NC
27713. T.F. Lavery and H.S. Prentice are with ESE, Gainesville, FL 32607.
Rudolph P. Boksteltner is the EPA Project Officer (see below).
The complete report, entitled "National Dry Deposition Network: Third Annual
Progress Report (1989)," (Order No. PB91-181784; Cost: $23.00, subject to
change) will be available only from:
National Technical Information Service
3285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Atmospheric Research and Exposure Assessment Laboratory
US. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati, OH 45268
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