United States
Environmental Protection
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
  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

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
  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-
  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).

  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
  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-
   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

   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-
  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),

                     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,
  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
   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.

  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
  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-
  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-



               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
   Slope       -0.629
   p             0.01
   r - squared  0.779


oo  -2
22   co
20  m
              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).

 varying terrain settings and land uses to
 better understand deposition  processes
 and to improve the MLM and other mod-
   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

 Conclusions and
   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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