National Atmospheric Deposition Program/National Trends Network Research Articles

Inorganic chemical components in precipitation in the eastern U.S. and Eastern Canada during 1989–2016: Temporal and regional trends of wet concentration and wet deposition from the NADP and CAPMoN measurements

With the implementation of the Clean Air Act Amendments of 1990 in the U.S. and similar emission reduction measures in Canada, sulfur dioxide (SO2) and nitrogen oxides (NOx = NO + NO2) emissions in the eastern U.S. and Eastern Canada declined significantly during the period 1989–2016. Correspondingly, air quality over the region improved tremendously and sulfate, nitrate and acidity in precipitation decreased significantly. In this study, we analyzed measurement data in the eastern U.S. and Eastern Canada from the two national long-term wet deposition networks, the National Atmospheric Deposition Program/National Trends Network (NADP/NTN) in the U.S. and the Canadian Air and Precipitation Monitoring Network (CAPMoN) in Canada, to reveal the spatial-temporal trends of precipitation concentrations and wet deposition of sulfate, nitrate, ammonium and acid in the eastern part of North America (broken into 4 different regions) for the period of 1989–2016. Comparing 2014–2016 to 1989–1991, the results show that the acid, sulfate and nitrate wet deposition decreased by 78%, 69% and 46% for 1989–2016, and decreased annually at rates of −2.8~-3.6% yr−1, -2.5~-2.9% yr−1, and -1.6~-2.0% yr−1 over different regions, respectively. It also revealed that there were no statistically significant trends for the annual mean wet concentrations of ammonium (NH4+) in precipitation in the Midwest and the Mid-Atlantic of the U.S. (P>0.4 respectively). However, there were statistically significant increasing trends of annual wet deposition of NH4+ over these latter two regions (P = 0.01–0.13) with an annual rate of increase of 0.6% and 0.4%, respectively. The increasing trends of wet deposition of NH4+ in these regions were mostly due to increasing annual precipitation amounts, as indicated by positive correlation coefficients between them (R = 0.67–0.77), and a statistically significant increasing trend of the precipitation amount during the period. The annual mean wet concentration of chloride ion (Cl−) in precipitation in the above two regions had a clear decreasing trend of −1.5% annually (P < 0.001). In 20 years (2010–2014 vs. 1990–1994), the frequency of acidic rain with pH < 5 was reduced from 72% to 49% in the Northeast U.S. and Eastern Canada, 88% to 37% in the Midwest, 97% to 60% in the Mid-Atlantic and 79% to 49% in the Southeast. The imbalance between the measured cations and anions increased over the years from 2009 to 2016 with the anions dropping lower than the cations in all regions. This can be attributed to the increased dissociation of weak organic acids, i.e., the free H+ contributed from these weak acids increased as the acidity of the precipitation decreased, but the corresponding conjugate bases were not measured and not included in the calculation of total anions. This pH buffering effect needs to be taken into account in assessing the reductions in acid deposition following the implementation of the Clear Air Act Amendments (CAAA) of 1990 in the U.S. and measures to reduce the emissions of SO2 and NOx in Canada.

An R Package for Generating Covariance Matrices for Maximum-Entropy Sampling from Precipitation Chemistry Data

We present an open-source R package (MESgenCov v 0.1.0) for temporally fitting multivariate precipitation chemistry data and extracting a covariance matrix for use in the MESP (maximum-entropy sampling problem). We provide multiple functionalities for modeling and model assessment. The package is tightly coupled with NADP/NTN (National Atmospheric Deposition Program/National Trends Network) data from their set of 379 monitoring sites, 1978–present. The user specifies the sites, chemicals, and time period desired, fits an appropriate user-specified univariate model for each site and chemical selected, and the package produces a covariance matrix for use by MESP algorithms.

Inorganic nitrogen wet deposition gradients in the Denver-Boulder metropolitan area and Colorado Front Range - Preliminary implications for Rocky Mountain National Park and interpolated deposition maps.

For the first time in the 40-year history of the National Atmospheric Deposition Program/National Trends Network (NADP/NTN), a unique urban-to-rural transect of wet deposition monitoring stations was operated as part of the NTN in 2017 to quantify reactive inorganic nitrogen wet deposition for adjacent urban and rural, montane regions. The transect of NADP stations (sites) was used to collect continuous precipitation depth and weekly wet-deposition samples in the Denver - Boulder, Colorado, urban corridor. Gradients in reactive inorganic nitrogen (Nr) concentrations and wet deposition were identified along the transect, which included Rocky Mountain National Park. Back trajectory modeling and stable isotopes suggested contribution of agricultural ammonia (NH3) to urban Nr wet deposition in Denver, but apportionment of wet-deposited Nr to agricultural versus urban mobile sources was not possible for this study. The results demonstrate the importance of multiple monitoring sites across an urban area in defining fine-scale geographic patterns in atmospheric deposition and its sources. Data from new sites located within 50 km of the urban area demonstrate that the urban influence does not extend as far as the inverse distance weighting would have suggested without such empirical monitoring data. It is important to determine the radius of influence of urban emissions and associated deposition on the interpolated deposition raster, which is constrained by a paucity of monitoring sites east of Denver.

Spatial trends of precipitation chemistry in the Central Plains region of the United States

In the late 1970s, historically high concentrations of anthropogenic air pollutants in the United States prompted the development of the National Atmospheric Deposition Program/National Trends Network (NADP/NTN). While much has been learned since program inception, many long-term data sets have yet to be interpreted, the results of which will help guide changing policies. Given the lack of studies in the Central Plains of the Midwestern United States, spatial trends of the past 30 years are presented from ten NADP sites surrounding, and including, Missouri, USA. Concentrations of calcium were highest (0.40 mg/L) and lowest (0.14 mg/L) at north and southeast sites, respectively, with a westward trend of increasing concentration. Ammonium concentrations were highest (0.67 mg/L) at northern NADP locations, with all sites showing an average increase in concentration of 65 % from 1984 to 2014, including south central Iowa which showed an increase of 226 %. Sulfate concentrations were highest for eastern sites (1.70 mg/L), but showed an overall decrease of at least 50 % for all locations since the early 1980s. Nitrate concentrations decreased between 5 and 30 % for each site over the 30-year period. Alkalinity increased between 10 and 25 % from southern to northern sites, respectively. When grouped into north–south, and east–west regions, significant differences (p ≤ 0.05) were observed for all constituents, except potassium, between each region. Data syntheses, such as that presented here, advances understanding of long-term precipitation chemistry, thereby improving management strategies intended to mitigate atmospheric chemical deposition.

A novel hybrid approach for estimating total deposition in the United States

Atmospheric deposition of nitrogen and sulfur causes many deleterious effects on ecosystems including acidification and excess eutrophication. Assessments to support development of strategies to mitigate these effects require spatially and temporally continuous values of nitrogen and sulfur deposition. In the U.S., national monitoring networks exist that provide values of wet and dry deposition at discrete locations. While wet deposition can be interpolated between the monitoring locations, dry deposition cannot. Additionally, monitoring networks do not measure the complete suite of chemicals that contribute to total sulfur and nitrogen deposition. Regional air quality models provide spatially continuous values of deposition of monitored species as well as important unmeasured species. However, air quality modeling values are not generally available for an extended continuous time period. Air quality modeling results may also be biased for some chemical species. We developed a novel approach for estimating dry deposition using data from monitoring networks such as the Clean Air Status and Trends Network (CASTNET), the National Atmospheric Deposition Program (NADP) Ammonia Monitoring Network (AMoN), and the Southeastern Aerosol Research and Characterization (SEARCH) network and modeled data from the Community Multiscale Air Quality (CMAQ) model. These dry deposition values estimates are then combined with wet deposition values from the NADP National Trends Network (NTN) to develop values of total deposition of sulfur and nitrogen. Data developed using this method are made available via the CASTNET website.

Stability of organic nitrogen in NADP wet deposition samples

Organic compounds represent an important yet largely uncharacterized component of atmospheric nitrogen deposition. Rapid progress in understanding the sources and spatiotemporal patterns of organic nitrogen (ON) deposition will require the use of existing large-scale monitoring infrastructure, such as the National Atmospheric Deposition Program's National Trends Network (NADP/NTN). The purpose of this study is to investigate the analytical and sampling requirements for adding ON measurements to the NTN, with specific interest in examining ON stability during sampling and storage. The analytical technique for total nitrogen (TN) used by the NADP's Central Analytical Laboratory (CAL) and associated quality assurance data are described. We then compare TN, inorganic nitrogen (IN = NH4+ + NO3−), and ON (ON = TN − IN) concentrations in a field study between standard weekly NADP/NTN samples (unrefrigerated during sampling and storage), daily event samples collected using the Atmospheric Integrated Research Monitoring Network protocol (AIRMoN, unrefrigerated during sampling but refrigerated during storage), and daily event samples that were preserved via refrigeration in the field upon collection (AIRMoN_Ref, refrigerated during sampling and storage). Using AIRMoN_Ref as the reference for comparison, total loss of ON in weekly NTN samples in the field and during laboratory storage is approximately 40%. This bias is likely dominated by losses of ON in the collection bucket. However, additional loss may occur during laboratory storage at room temperature prior to analysis. Loss of ON was also observed in AIRMoN samples, though differences relative to AIRMoN_Ref (10.8%) were less than weekly NTN samples. Biases in ON are more consistently negative at higher ambient temperatures. Storage experiments indicated that refrigeration at 4 °C at the CAL was sufficient to stabilize ON concentrations. We conclude that weekly sampling for ON is feasible if precipitation is refrigerated or frozen immediately upon collection. Samples should be kept refrigerated or frozen prior to analysis. Preliminary results indicate that NO2−, an additional inorganic species not currently measured by the CAL, makes a small contribution to TN (<1%), but if neglected may cause significant negative bias in ON determined as TN − IN. We recommend that CAL include NO2− quantification as a component of IN for bulk ON determination.

Improved mapping of National Atmospheric Deposition Program wet-deposition in complex terrain using PRISM-gridded data sets

High-elevation regions in the United States lack detailed atmospheric wet-deposition data. The National Atmospheric Deposition Program/National Trends Network (NADP/NTN) measures and reports precipitation amounts and chemical constituent concentration and deposition data for the United States on annual isopleth maps using inverse distance weighted (IDW) interpolation methods. This interpolation for unsampled areas does not account for topographic influences. Therefore, NADP/NTN isopleth maps lack detail and potentially underestimate wet deposition in high-elevation regions. The NADP/NTN wet-deposition maps may be improved using precipitation grids generated by other networks. The Parameter-elevation Regressions on Independent Slopes Model (PRISM) produces digital grids of precipitation estimates from many precipitation-monitoring networks and incorporates influences of topographical and geographical features. Because NADP/NTN ion concentrations do not vary with elevation as much as precipitation depths, PRISM is used with unadjusted NADP/NTN data in this paper to calculate ion wet deposition in complex terrain to yield more accurate and detailed isopleth deposition maps in complex terrain. PRISM precipitation estimates generally exceed NADP/NTN precipitation estimates for coastal and mountainous regions in the western United States. NADP/NTN precipitation estimates generally exceed PRISM precipitation estimates for leeward mountainous regions in Washington, Oregon, and Nevada, where abrupt changes in precipitation depths induced by topography are not depicted by IDW interpolation. PRISM-based deposition estimates for nitrate can exceed NADP/NTN estimates by more than 100% for mountainous regions in the western United States.

A 50-year record of NOx and SO2 sources in precipitation in the Northern Rocky Mountains, USA

Ice-core samples from Upper Fremont Glacier (UFG), Wyoming, were used as proxy records for the chemical composition of atmospheric deposition. Results of analysis of the ice-core samples for stable isotopes of nitrogen (δ15N, ) and sulfur (δ34S, ), as well as and deposition rates from the late-1940s thru the early-1990s, were used to enhance and extend existing National Atmospheric Deposition Program/National Trends Network (NADP/NTN) data in western Wyoming. The most enriched δ34S value in the UFG ice-core samples coincided with snow deposited during the 1980 eruption of Mt. St. Helens, Washington. The remaining δ34S values were similar to the isotopic composition of coal from southern Wyoming. The δ15N values in ice-core samples representing a similar period of snow deposition were negative, ranging from -5.9 to -3.2 ‰ and all fall within the δ15N values expected from vehicle emissions. Ice-core nitrate and sulfate deposition data reflect the sharply increasing U.S. emissions data from 1950 to the mid-1970s.

Comparison of precipitation chemistry measurements obtained by the Canadian Air and Precipitation Monitoring Network and National Atmospheric Deposition Program for the period 1995–2004

Precipitation chemistry and depth measurements obtained by the Canadian Air and Precipitation Monitoring Network (CAPMoN) and the US National Atmospheric Deposition Program/National Trends Network (NADP/NTN) were compared for the 10-year period 1995-2004. Colocated sets of CAPMoN and NADP instrumentation, consisting of precipitation collectors and rain gages, were operated simultaneously per standard protocols for each network at Sutton, Ontario and Frelighsburg, Ontario, Canada and at State College, PA, USA. CAPMoN samples were collected daily, and NADP samples were collected weekly, and samples were analyzed exclusively by each network's laboratory for pH, H(+), Ca(2+), Mg(2+), Na(+), K(+), NH4(+), Cl(-), NO3(-), and SO4(2-). Weekly and annual precipitation-weighted mean concentrations for each network were compared. This study is a follow-up to an earlier internetwork comparison for the period 1986-1993, published by Alain Sirois, Robert Vet, and Dennis Lamb in 2000. Median weekly internetwork differences for 1995-2004 data were the same to slightly lower than for data for the previous study period (1986-1993) for all analytes except NO3(-), SO4(2-), and sample depth. A 1994 NADP sampling protocol change and a 1998 change in the types of filters used to process NADP samples reversed the previously identified negative bias in NADP data for hydrogen-ion and sodium concentrations. Statistically significant biases (alpha = 0.10) for sodium and hydrogen-ion concentrations observed in the 1986-1993 data were not significant for 1995-2004. Weekly CAPMoN measurements generally are higher than weekly NADP measurements due to differences in sample filtration and field instrumentation, not sample evaporation, contamination, or analytical laboratory differences.

Trends in agricultural ammonia emissions and ammonium concentrations in precipitation over the Southeast and Midwest United States

Emissions from agricultural activities, both crop and animal, are known to contain gaseous ammonia (NH 3) which through chemical reaction in rainwater changes into ammonium ion (NH 4 +). Using wet deposition data of ammonium from several National Atmospheric Deposition Program/National Trends Network (NADP/NTN) and ambient levels of ammonium from Clean Air Status Trends Network (CAST Net) sites as well as calculated NH 3 emissions from North Carolina, and the Southeast and Midwest regions of the United States, trends in ammonium concentrations in precipitation were analyzed for the period of 1983–2004. The beginning of 1997 coincides with the implementation of a swine population moratorium in the state of North Carolina. Results from the analysis in North Carolina indicate a lessening in the rate of increases in NH 4 + concentration in precipitation since the moratorium went into effect. Sampson County, NC, saw stable NH 4 + concentrations from 1983 to 1989, an average rise of 9.5% from 1989 to 1996, and an average increase of only 4% from 1997 to 2004. In addition, HYSPLIT back-trajectory model was used to determine that when ambient air in downwind sites arrived from the high NH 3 emissions source region, ammonium concentrations in precipitation were enhanced. For the Southeast United States domain, analysis shows that NH 4 + concentrations generally increased with increasing NH 3 emissions from within the same region. Similar analysis has been performed over the Midwest United States and compared to the results from the Southeast United States. Emissions from the Midwest are attributed to larger animals, including hogs and cattle, whereas the Southeast has a higher percentage of emissions coming from smaller livestock, such as chickens. In addition, the states of the Midwest United States have a much more uniform spatial distribution of emissions.

Effects of missing seasonal data on estimates of period means of dry and wet deposition

The current study uses resampling to investigate the impacts of cyclic seasonal behavior on 1- and 5-year period means composed from seasonal mean values in the presence of missing data. This is an empirical study using complete years of seasonal monitoring data collected in the eastern US and extracted from the clean air status and trends network (CASTNET) dry and the National Atmospheric Deposition Program/National Trends Network (NADP/NTN) wet deposition data archives. Estimators of period means with missing seasonal data are determined using means of the non-missing values as estimates of the missing data. Estimates are evaluated in terms of 95% inclusion intervals (e.g., estimates are within ± X% of the true value ⩾95% of the time). For dry deposition, missing transition seasons (i.e., spring or fall) usually yield estimates of annual means that are within ±20% of the true annual mean ⩾95% of the time. Missing summers or winters usually have larger impacts on estimates of annual means of dry deposited species than missing transition seasons. A missing summer has the largest impact on estimates of annual means of dry deposition for all constituents, except SO 2, where winter is especially important. For wet deposition, a missing season yields estimates of annual means that are within ±30% of the true annual mean ⩾95% of the time. A missing summer has the largest impact on estimates of annual means of wet deposition for all constituents, except NH 4 +, where spring and fall are important. A strategy requiring at least 3 years of seasonal representation for three seasons with the fourth season having at least two seasonal values, yields estimates of wet deposition that are within ±17% of the true 5-year means ⩾95% of the time for all species. Corresponding confidence statements for dry deposition results are considerably stronger, with estimates that are within ±10% of the true 5-year mean ⩾95% of the time.

Precipitation chloride at West Point, NY: Seasonal patterns and possible contributions from non-seawater sources

Chloride derived from the atmosphere can be a valuable tracer in ecosystem and watershed processes. For these purposes and other environmental studies, it is important to establish temporal patterns and sources for Cl - in wet deposition. Weekly composite precipitation samples have been analyzed by the National Atmospheric Deposition Program/National Trends Network (NADP/NTN) at West Point, NY during 1981–2003, although systematic contamination of precipitation Na + significantly perturbed data for Na + prior to 1998. Chloride and sodium ion seasonal wet deposition were highest in winter and lowest in summer through most of the record, probably as a result of more frequent marine-trajectory storms in winter. During 1998–2003, the period of highest quality Na + data, the ratio of [ Cl - ] / [ Na + ] was significantly higher than average in summer and lower in winter. Higher summer [ Cl - ] / [ Na + ] occurred consistently throughout the record, often reaching values four times the seawater ratio. Based on the ratio of [ Cl - ] / [ Na + ] in seawater ( 1.16 ) ∼ 16 % of annual wet deposition of Cl - during 1998–2003 was in excess of that for surface seawater. Additionally, a minor terrestrial dust Na + component was approximated, which had the net effect of increasing annual excess Cl - wet deposition to ∼ 22 % ( 2.56 mEq m - 2 or 0.90 kg ha - 1 ) of the mean annual Cl - wet deposition at West Point ( 11.9 mEq m - 2 or 4.2 kg ha - 1 ). Consistent with plausible sources of non-seawater Cl - , we attribute excess Cl - wet deposition to HCl emission from coal fired generating stations, HCl emissions from domestic and industrial waste incineration and to HCl formation in the regional atmosphere from reactions of sea-salt aerosols with S and N acidic gases.

Monitoring Long-term Trends in Sulfate and Ammonium in US Precipitation: Results from the National Atmospheric Deposition Program/National Trends Network

Data from the National Atmospheric Deposition Program/National Trends Network (NADP/NTN) indicate significant changes have occurred in precipitation chemistry and the chemical climate in the United States (US). A Seasonal Kendall Trend (SKT) analysis shows statistically significant increases in precipitation ammonium concentrations at 64% of 159 continental US NADP/NTN sites evaluated from Winter 1985 to Fall 2004 (Dec. 1984 – Nov. 2004). Sulfate decreases were widespread, with an SKT analysis indicating statistically significant decreases at 89% of sites evaluated. Ratios of chemical equivalent concentrations of ammonium to sulfate in precipitation have risen to the extent that ammonium now exceeds sulfate over more than half of the continental U.S. on a precipitation-weighted-mean annual basis. These trends in the concentrations of ammonium, sulfate, and other species have been accompanied by significant decreases in the frequency of acidic precipitation (pH < 5.0) in the last decade.

Introduction of Asian Soybean Rust Urediniospores into the Midwestern United States-A Case Study.

In 2005, weekly rain samples collected at 124 National Atmospheric Deposition Program/National Trends Network (NADP/NTN) sites in the eastern and central United States were screened for Asian soybean rust (ASR; Phakopsora pachyrhizi) urediniospores. Application of a quantitative polymerase chain reaction method detected P. pachyrhizi DNA in the filter residue of rain samples collected during the week of 19 to 26 July 2005 in Minnesota, Missouri, and South Dakota. To determine the geographic origin of ASR urediniospores in those weekly composite samples, back air trajectories of the lifted condensation and mixed boundary layers were calculated for each rain event within the week, by sampling site. The calculations, based on the hybrid single-particle lagrangian integrated trajectory model, pointed to source areas in eastern and southern Texas. In a separate case, DNA of P. pachyrhizi was detected in a 28 June to 5 July 2005 rain sample from an eastern Texas site. Back trajectories pointed to southern Texas and the Yucatan Peninsula in Mexico as potential source areas of ASR urediniospores. Vertical motions of those back trajectories indicated a ventilation of the boundary layer in the upwind areas, suggesting the possible injection of urediniospores into the free troposphere where they can be transported for long distances before wet deposition.

The Effect of Nitrogen Depostition and Edaphic Conditions on Microbial Activity in the Alpine Zone of the Grand Teton National Park

Atmospheric nitrogen (N) deposition rates show increasing N loadings since the 1980s in the western U.S. associated with increasing N emissions from industrial, urban, and agricultural sources (Fenn et al., 2003). Compared to the eastern U.S., the number of NADP/NTN (National Atmospheric Deposition Program/ National Trends Network) and CASTNet (Clean Air Status Network) monitoring sites is much more limited in the west, and they are rarely located in the highest-elevations, where ecosystems are likely to be more sensitive (Bums 2004). Although N deposition tends to increase with elevation in this region (Williams and Tonnesen 2000), there are considerable uncertainties about the actual N deposition levels in the Rocky Mountains. Model-simulations indicate a "hotspot" of N deposition near Grand Teton National Park (GRTE) (Fenn et al., 2003; Nanus et al. 2003), with feedlot and fertilizer N emissions in Southern Idaho, as potential sources impacting the alpine communities in GRTE. However, little data is currently available on the actual atmospheric N inputs to alpine ecosystems in GRTE, either as snow during winter or wet and dry deposition during the short snow-free period.

Improved daily precipitation nitrate and ammonium concentration models for the Chesapeake Bay Watershed

Daily precipitation nitrate and ammonium concentration models were developed for the Chesapeake Bay Watershed (USA) using a linear least-squares regression approach and precipitation chemistry data from 29 National Atmospheric Deposition Program/National Trends Network (NADP/NTN) sites. Only weekly samples that comprised a single precipitation event were used in model development. The most significant variables in both ammonium and nitrate models included: precipitation volume, the number of days since the last event, a measure of seasonality, latitude, and the proportion of land within 8 km covered by forest or devoted to industry and transportation. Additional variables included in the nitrate model were the proportion of land within 0.8 km covered by water and/or forest. Local and regional ammonia and nitrogen oxide emissions were not as well correlated as land cover. Modeled concentrations compared very well with event chemistry data collected at six NADP/AirMoN sites within the Chesapeake Bay Watershed. Wet deposition estimates were also consistent with observed deposition at selected sites. Accurately describing the spatial distribution of precipitation volume throughout the watershed is important in providing critical estimates of wet-fall deposition of ammonium and nitrate.

Spatial and temporal trends of precipitation chemistry in the United States, 1985–2002

A Seasonal Kendall Trend (SKT) test was applied to precipitation-weighted concentration data from 164 National Atmospheric Deposition Program National Trends Network (NADP/NTN) sites operational from 1985 to 2002. Analyses were performed on concentrations of ammonium, sulfate, nitrate, dissolved inorganic nitrogen (DIN, sum of nitrogen from nitrate and ammonium), and earth crustal cations (ECC, sum of calcium, magnesium, and potassium). Over the 18-year period, statistically significant ( p ≤ 0.10) increases in ammonium concentrations occurred at 93 sites (58%), while just three sites had statistically significant ammonium decreases. Central and northern Midwestern states had the largest ammonium increases. The generally higher ammonium concentrations were accompanied by significant sulfate decreases (139 sites, 85%), and only one significant increase which occured in Texas. In the west central United States there were significant nitrate increases (45 sites, 27%), while in the northeastern United States there were significant decreases (25 sites, 15%). Significant DIN decreases were observed in the northeastern United States (11 sites, 7%); elsewhere there were significant increases at 75 sites (46%). ECC decreased significantly at 65 sites (40%), predominately in the central and southern United States, and increased at 11 sites (7%). The concentrations of sulfate, nitrate, and ammonium in precipitation have changed markedly over the time period studied. Such trends indicate changes in the mix of gases and particles scavenged by precipitation, possibly reflecting changes in emissions, atmospheric chemical transformations, and weather patterns.

Spatial and temporal variability of the overall error of National Atmospheric Deposition Program measurements determined by the USGS collocated-sampler program, water years 1989–2001

Data from the U.S. Geological Survey (USGS) collocated-sampler program for the National Atmospheric Deposition Program/National Trends Network (NADP/NTN) are used to estimate the overall error of NADP/NTN measurements. Absolute errors are estimated by comparison of paired measurements from collocated instruments. Spatial and temporal differences in absolute error were identified and are consistent with longitudinal distributions of NADP/NTN measurements and spatial differences in precipitation characteristics. The magnitude of error for calcium, magnesium, ammonium, nitrate, and sulfate concentrations, specific conductance, and sample volume is of minor environmental significance to data users. Data collected after a 1994 sample-handling protocol change are prone to less absolute error than data collected prior to 1994. Absolute errors are smaller during non-winter months than during winter months for selected constituents at sites where frozen precipitation is common. Minimum resolvable differences are estimated for different regions of the USA to aid spatial and temporal watershed analyses.

Complementary co-kriging: spatial prediction using data combined from several environmental monitoring networks

We consider the problem of optimal spatial prediction of an environmental variable using data from more than one sampling network. A model incorporating spatial dependence and measurement errors with network-specific biases and variances serves as the basis for the analysis of the combined data from all networks. We develop the associated optimal prediction methodology, which we call complementary co-kriging because (a) data from each network complements the other, and (b) the solutions to several prediction problems of interest are co-kriging predictors. A hypothetical example illustrates how much better the complementary co-kriging predictor can be, when compared to the ordinary kriging predictors from each network alone and to a ‘naive’ combined predictor. We use the methodology to obtain optimal predictions of wet nitrate concentration data over the eastern U.S. using data combined from the National Atmospheric Deposition Program/National Trends Network (NADP/NTN) and the Clean Air Status and Trends Network (CASTNet). Copyright © 2005 John Wiley & Sons, Ltd.

Estimates of sulfate aerosol wet scavenging coefficient for locations in the Eastern United States

Abstract Scavenging of atmospheric aerosols by falling precipitation is a major removal mechanism for airborne particles. The process can be described by a wet scavenging coefficient (WSC), denoted L , that is dependent on the rainfall rate, R , and the collision efficiency between raindrops and aerosol particles, E . We report bulk average L values for location in the Eastern United States, estimated based on sulfate mass balance in the atmospheric domain of interest. Data used are taken from several observational networks: (a) the Atmospheric Integrated Research Monitoring Network (AIRMoN) which is part of the National Atmospheric Deposition Program/National Trends Network (NADP/NTN); (b) the Interagency Monitoring of Protected Visibility Environments (IMPROVE); and (c) the National Climatic Data Center (NCDC). The results are fitted relatively well by L values computed using a microphysical representation of the WSC process based on collision efficiency and precipitation size distribution. Such representation leads to a simple expression L = f ( R ) for soluble aerosols, suitable for WSC description in regional scale models. The agreement between the bulk method and the microphysical representation is due in part to the predominant widespread precipitation, well represented by Marshall and Palmer raindrop distribution, and in part due to assumptions made in the bulk model. Results indicate that high-resolution rainfall rates and realistic vertical cloud structure information are needed to improve the accuracy of aerosol wet scavenging modeling for pollution studies.