United States Environmental Protection Agency National bnvironmental Research Laboratory Research Triangle Park, NC 27711 Research and Development EPA/600/SR-95/086 August 1995 &EPA Project Summary CASTNet 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). 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 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. Procedures 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- tracts. 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. Conclusions 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- cies. 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 clearings. 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 Official Business Penalty for Private Use $300 EPA/600/SR-95/086 BULK RATE POSTAGE & FEES PAID EPA PERMIT No. G-35 ------- |