United States
                    Environmental Protection
National bnvironmental
Research Laboratory
Research Triangle Park, NC 27711
                    Research and Development
EPA/600/SR-95/086     August 1995
&EPA      Project Summary
                    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

that is fully documented in a separate
report of the same  title  (see  Project
Report ordering information at back).

   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.

   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

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-
   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.
   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-
   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
   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.

   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

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