United States National Environmental

Environmental Protection Research Laboratory

Agency Research Triangle Park, NC 27711

EPA Project Summary

Research and Development EPA/600/SR-95/086 August 1995

The National Dry Deposition Network

(NDDN) was established to provide

longterm estimates of dry acidic deposition across the continental United

States. Fifty routine sites were operational 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 continuous measurements of ozone (O3

)

and meteorological variables required

for estimation of dry deposition rates.

Weekly average atmospheric concentrations of particulate sulfate (SO-

), particulate nitrate (NO), particulate ammonium (NH), sulfur dioxide (SO2

), and

nitric acid (HNO3) were measured at all

sites and wet deposition of acidity and

related species were measured at selected sites. Two methods development

sites were installed during 1991 to

evaluate: 1) comparability of United

States and Canadian air quality measurements, and 2) effects of terrain on

pollutant concentration and deposition.

Routine application of an inferential

model for calculation of deposition velocities 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 observed from the northeast through upper northeast and midwest through upper midwest. Annual average concentrations of most species decreased

from 1989 to 1992 in all subregions of

the network.

CASTNet

National Dry Deposition Network

1990-1992 Status Report

 Dry deposition calculations for 1990,

1991, and 1992 show that SO2 and HNO3

dominate sulfur and nitratenitrogen

fluxes, respectively. In general, SO2 accounts 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 accounts for more than 90 percent of dry

nitratenitrogen deposition at all sites.

Total deposition estimates for approximately 15 sites show that dry deposition accounts for about 20 to 50 percent 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 terrain may be difficult to estimate.

 Collocated data from the Canadian

Acid Precipitation Monitoring Network

(CAPMoN) and NDDN for a site in southern Ontario indicate that annual average concentrations are within +5 percent, 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 significantly with pattern and trend analyses

across eastern North America.

This Project Summary was developed by the U.S. Environmental Protection Agency’s (EPA’s) National Environmental Research Laboratory, Research Triangle Park, NC, to announce

key findings of the research project
2

that is fully documented in a separate

report of the same title (see Project

Report ordering information at back).

Background

 Atmospheric deposition takes place via

two pathways: wet deposition and dry

deposition. Wet deposition is the result of

precipitation events (rain, snow, etc.) which

remove particles and gases from the atmosphere. Dry deposition is the transfer

of particles and gases to the landscape in

the absence of precipitation. Wet deposition 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 information increases the uncertainty in estimates

of interregional, national, and international

transport and confounds efforts to determine 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 meteorological measurements.

 In 1986, EPA contracted with Environmental 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 velocities (Vd

s) estimated from meteorological, 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 species are presented, and temporal variability 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 comparability studies initiated in 1991. The

first of these involves investigation of measurement biases between atmospheric

sampling methods used by United States

and Canadian acid deposition trends programs. 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 measurements and dry deposition estimates.

Data are presented from a pair of sites in

southwestern North Carolina that are separated horizontally by about 1,000 meters

(m) and in elevation by about 350 m.

Procedures

 Ambient measurements for O3, SO2,

SO-, NO, HNO3, NH, and meteorological

variables required for dry deposition calculations were performed at each NDDN

site. Meteorological variables and O3 concentrations were recorded continuously

and reported as hourly averages consisting of a minimum of nine valid 5-minute

averages. Atmospheric sampling for sulfur and nitrogen species was integrated

over weekly collection periods using 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 baseimpregnated 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 contamination during shipment and handling. At 16

sites located more than 50 km from National Atmospheric Deposition Program/

National Trends Network (NADP/NTN)

sites, wet deposition samples were collected weekly (according to NADP/NTN

protocols) and shipped to ESE for chemical analysis.

 Filter pack sampling and O3 measurements were performed at 10 m using 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 mercury (mmHg) with a Teledyne-Hastings

CST-10K mass flow controller (MFC).

Wet deposition samples were collected

in precleaned polyethylene buckets using an Andersen Model APS precipitation 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-Environmental 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 particle deposition within the system. Periodic 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 internal O3

 generator.

 In addition to the above, various observations 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 information was collected to determine the

frequency of conditions that could influence deposition rates for gases (especially SO2

) and particles. Vegetation data

were obtained to track evolution of the

dominant plant canopy, from leaf emergence (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) measurements were conducted at all NDDN

sites during the summers of 1991 and

1992. LAI was measured using an LAI2000 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 receipt from the field, exposed filters and

blanks were extracted and then analyzed

for SO- and NO by micromembrane-suppressed ion chromatography (IC). 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, sodium (Na+

), potassium (K+), NH, calcium

(Ca2+), magnesium (Mg2+), chloride (Cl-),

nitrite (NO), NO, and SO-.

 Atmospheric concentrations of particulate SO-

, NO, and NH were calculated

based on the analysis of Teflon® filter

extracts; HNO3 was calculated based on
3

the NO found in nylon filter extracts; and

SO2 was calculated based on the sum of

SO- found in nylon and cellulose filter extracts.

 Dry deposition calculations for 1990

through 1992 were made using the multilayer version of the National Oceanic and

Atmospheric Administration (NOAA) inferential model. The model calculates fluxes,

F, as the product of measured concentrations and inferred Vd

s. 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 calculated for each chemical species and vegetation/surface type for every hour of available meteorological input data. Hourly values of Vd

 are averaged over a week and

multiplied by the weekly integrated concentrations to produce weekly fluxes of

HNO3, SO2, and particles. Ozone flux is

calculated using hourly measurements of

O3 and hourly values of Vd. Weekly flux

calculations for all chemical species are

considered valid only if >70 percent of

hourly Vd values are available for that

week. Annual values are considered valid

only if Vd and flux data for all four seasons

are available.

Conclusions

 The full report presents quantitative

information on dry deposition fluxes and

atmospheric concentrations for the network from 1990 through 1992. Annual

concentration data for sulfur and nitrogen species show fairly consistent spatial patterns. In the eastern United

States, the highest average concentrations of SO-

 [6 to 7 micrograms per

cubic meter (μg/m3)] and SO2 (15 to 20

μg/m3) occur in an area surrounding the

Ohio River Valley. Average SO- decreases gradually toward the periphery

of the network to about 2.0 μg/m3 in

northern Maine and 3 to 4 μg/m3 in

Florida, Arkansas, and Wisconsin. Average SO2

 exhibits much more local variability than SO-

 and decreases more

rapidly towards the periphery of the network. For western sites, SO-

 and SO2

concentrations are generally well below

1.0 μg/m3 and show no strong evidence

of a pattern, except for somewhat elevated values at two Arizona sites.

 Atmospheric concentration data for

HNO3 and NO also show substantial variability across the network; however, differences 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

μg/m3) are observed at scattered sites in

New York, Pennsylvania, Virginia, and

Ohio, while minimum concentrations

(<1.0 μg/m3) are observed in Maine, Arkansas, Florida, Kentucky, and North Carolina. Substantial variability in HNO3

 concentrations (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 μg/m3) in agricultural areas of the midwest and minimum concentrations (<0.5 μg/m3

) in the

forested northeast. Intermediate concentrations occur at sites in pastured land or

near limited agricultural activities. These

observations suggest that land use (specifically agricultural activity) affects the partitioning of gas and aerosol nitrogen species.

 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 phenomena involved are complex, results suggest that the spatial pattern of SO2

 deposition is controlled primarily by variability

in SO2 concentrations, rather than variability in deposition velocity.

 Dry deposition of nitrate-nitrogen (HNO3

plus NO) ranges from <1 kg-N/ha to 5 kgN/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. Inspection of deposition data suggests that spatial variability is a function of both concentration and deposition velocity (Vd

s). These

results are uncertain by ±20 percent to

±40 percent and are biased low for a

number of reasons.

 Total deposition estimates (i.e., measured 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 nitratenitrogen inputs, respectively. Dry sulfur

deposition appears to be roughly equivalent to wet sulfur deposition near sources

of SO2 (e.g., the midwest and northeast).

Dry nitrate-nitrogen deposition appears to

be roughly equivalent to wet deposition at

sites located on ridges and in substantial

clearings.

 Data from sites in and around the Appalachian Mountains indicate that concentrations 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 Appalachian Mountains may explain this phenomenon. The ridge site exhibits significantly higher concentrations and Vd

s than

the valley site. Together, these factors

give rise to dry deposition rates that are

factors of 1.6 (for SO-) 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 concentration data from the United States and Canadian trends networks [NDDN and Canadian Acid Precipitation Monitoring Network (CAPMoN)] shows that annual averages of most species agree within 5 percent. Data for SO2

 differ by about 10 percent, 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. Additional work has been initiated to elucidate the disparity in SO2

 concentrations

between networks and to compare United

States and Canadian approaches for estimating dry deposition fluxes.
4

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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