National Atmospheric Deposition Program/National Trends Network Research Articles

Agricultural ammonia emissions and ammonium concentrations associated with aerosols and precipitation in the southeast United States

Temporal and spatial variations in ammonia (NH3) emissions and ammonium (NH4+) concentrations associated with aerosols and volume‐weighted NH4+ concentration in precipitation are investigated over the period 1990–1998 in the southeast United States (Alabama, Florida, Georgia, Kentucky, North Carolina, South Carolina, Mississippi, and Tennessee). These variations were analyzed using an NH3 emissions inventory developed for the southeast United States and ambient NH4+ data from the various Clean Air Status and Trends Network (CASTNet) and the National Atmospheric Deposition Program/National Trends Network (NADP/NTN). Results show that natural log‐transformed annual NH4+ concentration associated with aerosols increases with natural log‐transformed annual NH3 emission density within the same county (R2 = 0.86, p < 0.0001, N = 12). Natural log‐transformed annual volume‐weighted average NH4+ concentration in precipitation shows only a very weak positive correlation with natural log‐transformed annual NH3 emission densities within the corresponding county (R2 = 0.12, p = 0.04, N = 29). Analysis of NH4+ concentration associated with aerosols at CASTNet sites revealed that temperature, precipitation amount, and relative humidity are the most statistically significant (p < 0.05) parameters in predicting the weekly concentrations of NH4+ during the period 1990–1998. Wind speed and wind direction were also statistically significant (p < 0.05) at several CASTNet sites, but the results were less consistent. Investigation into wet NH4+ concentration in precipitation consistently yielded temperature as a statistically significant (p < 0.05) parameter at individual sites. Trends over the period 1990–1998 revealed a slight decrease in NH4+ concentration at CASTNet site SPD, Claiborne County, Tennessee (2.14–1.88 μg m−3), while positive trends in NH4+ concentration in precipitation were evident at NADP sites NC35, Sampson County, North Carolina (0.2–0.48 mg L−1) and KY35, Rowan County, Kentucky (0.2–0.35 mg L−1) over the period 1990–1998.

Atmospheric nitrogen in the Mississippi River Basin — emissions, deposition and transport

Atmospheric deposition of nitrogen has been cited as a major factor in the nitrogen saturation of forests in the north-eastern United States and as a contributor to the eutrophication of coastal waters, including the Gulf of Mexico near the mouth of the Mississippi River. Sources of nitrogen emissions and the resulting spatial patterns of nitrogen deposition within the Mississippi River Basin, however, have not been fully documented. An assessment of atmospheric nitrogen in the Mississippi River Basin was therefore conducted in 1998–1999 to: (1) evaluate the forms in which nitrogen is deposited from the atmosphere; (2) quantify the spatial distribution of atmospheric nitrogen deposition throughout the basin; and (3) relate locations of emission sources to spatial deposition patterns to evaluate atmospheric transport. Deposition data collected through the NADP/NTN (National Atmospheric Deposition Program/National Trends Network) and CASTNet (Clean Air Status and Trends Network) were used for this analysis. NO x Tier 1 emission data by county was obtained for 1992 from the US Environmental Protection Agency (Emissions Trends Viewer CD, 1985–1995, version 1.0, September 1996) and NH 3 emissions data was derived from the 1992 Census of Agriculture (US Department of Commerce. Census of Agriculture, US Summary and County Level Data, US Department of Commerce, Bureau of the Census. Geographic Area series, 1995:1b) or the National Agricultural Statistics Service (US Department of Agriculture. National Agricultural Statistics Service Historical Data. Accessed 7/98 at URL, 1998. http://www.usda.gov/nass/pubs/hisdata.htm). The highest rates of wet deposition of NO 3 − were in the north-eastern part of the basin, downwind of electric utility plants and urban areas, whereas the highest rates of wet deposition of NH 4 + were in Iowa, near the center of intensive agricultural activities in the Midwest. The lowest rates of atmospheric nitrogen deposition were on the western (windward) side of the basin, which suggests that most of the nitrogen deposited within the basin is derived from internal sources. Atmospheric transport eastward across the basin boundary is greater for NO 3 − than NH 4 + , but a significant amount of NH 4 + is likely to be transported out of the basin through the formation of (NH 4) 2SO 4 and NH 4NO 3 particles — a process that greatly increases the atmospheric residence time of NH 4 + . This process is also a likely factor in the atmospheric transport of nitrogen from the Midwest to upland forest regions in the North-East, such as the western Adirondack region of New York, where NH 4 + constitutes 38% of the total wet deposition of N.

A comparison of the precipitation chemistry measurements obtained by the CAPMoN and NADP/NTN networks.

Precipitation chemistry measurementsobtained by the Canadian Air and PrecipitationMonitoring Network (CAPMoN) and the U.S. NationalAtmospheric Deposition Program/National Trends Network(NADP/NTN) have been examined using more than 7 yrof collocated data from two sites, namely, Sutton,Quebec, Canada and State College, Pennsylvania, U.S.A.In the case of the CAPMoN data, weeklyprecipitation-weighted mean concentrations, totalsample depths and total standard gauge depths werecomputed from daily data and compared to thecorresponding weekly sampling data of the NADP/NTNnetwork. Seasonal and annual precipitation-weightedmean concentrations and deposition values were alsocomputed for both networks and compared. Statisticallysignificant between-network biases were found to existin the weekly results for most of the measuredvariables, particularly standard gauge depth, sampledepth, pH, H+, NO3-,NH4+,Na+; the NADP/NTN values were consistently lowerthan those of CAPMoN with the exception of pH andNa+. The magnitude of the biases was less than35% of the median CAPMoN weekly value for the 7 yr. For most of the measured parameters, thevariability of the between-network differencesrepresented less than 20% of the median CAPMoN weeklyvalue. Both the between-network biases andvariabilities were functions of several physicalparameters, the most dominant being the sample depthand the ionic concentration. For seasonal and annualdeposition values, statistically significantbetween-network biases were found for H+,SO42-, NO3-,Ca2+,NH4+ for both periods; for Mg2+ andK+ for seasonal data; and Cl- for yearlydata, with the NADP/NTN deposition values being lowerthan those of CAPMoN. The relative biases ranged from7 to 37%. Part of the between-network bias in thedeposition estimates was directly attributable to astrong bias in the standard gauge depths of the two networks.

Atmospheric transport and wet deposition of ammonium in North Carolina

Wet deposition and transport analysis has been performed for ammonium (NH 4 +) in North Carolina, USA. Multiple regression analysis is employed to model the temporal trend and seasonality in monthly volume-weighted mean NH 4 + concentrations in precipitation from 1983 to 1996 at six National Atmospheric Deposition Program/National Trends Network (NADP/NTN) sites. A significant ( p<0.01) increasing trend beginning in 1990, which corresponds to an annual concentration increase of approximately 9.5%, is detected at the rural Sampson County site (NC35), which is located within a densely populated network of swine and poultry operations. This trend is positively correlated with increasing ammonia (NH 3) emissions related to the vigorous growth of North Carolina's swine population since 1990, particularly in the state's Coastal Plain region. A source–receptor regression model, which utilizes weekly NH 4 + concentrations in precipitation in conjunction with boundary layer air mass back trajectories, is developed to statistically test for the influence of a particular NH 3 source region on NH 4 + concentrations at surrounding NADP/NTN sites for the years 1995–1996. NH 3 emissions from this source region, primarily evolving from swine and poultry operations, are found to increase NH 4 + concentration in precipitation at sites up to ≈80 km away. At the Scotland County (NC36) and Wake County (NC41) sites, mean NH 4 + concentrations show increases of at least 44% for weeks during which 25% or more back trajectories are influenced by this source region.

Comparison of precipitation chemistry in the Central Rocky Mountains, Colorado, USA

Volume-weighted mean concentrations of nitrate (NO 3 −), ammonium (NH 4 +), and sulfate (SO 4 2−) in precipitation were compared at high-elevation sites in Colorado from 1992 to 1997 to evaluate emission source areas to the east and west of the Rocky Mountains. Precipitation chemistry was measured by two sampling methods, the National Atmospheric Deposition Program/National Trends Network (NADP/NTN) and snowpack surveys at maximum accumulation. Concentrations of NO 3 − and SO 4 2− in winter precipitation were greater on the western slope of the Rockies, and concentrations of NO 3 − and NH 4 + in summer precipitation were greater on the eastern slope. Summer concentrations in general were almost twice as high as winter concentrations. Seasonal weather patterns in combination with emission source areas help to explain these differences. This comparison shows that high-elevation ecosystems in Colorado are influenced by air pollution emission sources located on both sides of the Continental Divide. It also suggests that sources of nitrogen and sulfur located east of the Divide have a greater influence on precipitation chemistry in the Colorado Rockies.

Trends in precipitation chemistry in the United States: A national perspective, 1980–1992

Thirteen years (1980–1992) of precipitation chemistry data from 58 National Atmospheric Deposition Program/National Trends Network (NADP/NTN) sites are examined for trends using a general linear least squares model. SO 4 2- concentrations are decreasing throughout the United States; significant ( p < 0.05) trends are evident at 42 of the 58 sites included in this analysis and the average decrease is 12.026 μeq ℓ −1. The largest concentration of sites with significant decreasing SO 4 2− trends is located in the north central and western regions of the country. Eleven sites exhibit significant NO 3 − trends, nine of which are decreasing. Ca 2+ and Mg 2+ concentrations show the most widespread decline (44 and 52 sites, respectively) of all the major cations and anions in precipitation. Ca 2+ concentrations have decreased 4.587 μeq ℓ −1 since 1980; Mg 2+ concentrations have decreased 2.034 μeq ℓ −1. The most consistent and statistically most significant ( p < 0.001) Ca 2+ and Mg 2+ trends occur in the northeast. Decreasing trends are also evident at 28 sites for Na + and at 35 sites for K +. NH 4 + concentrations exhibit very little change over this 13-year period. The widespread decline in base cations, particularly Ca 2+ and Mg 2+, appears to have offset the effects that decreasing SO 4 2− concentrations should have on free acidity in precipitation. Only 17 sites exhibit both significant decreasing H + and SO 4 2− concentration trends, most of which are located in the north central portion of the United States. The average decrease in H + at the 17 sites is 9.991 μeq ℓ −1. Hawaii is the only site in the NADP/NTN network to exhibit a significant increasing H + trend.

The standard error of a weighted mean concentration—II. Estimating confidence intervals

One motivation for estimating the standard error, SEM w, of a weighted mean concentration, M w, of an ion in precipitation is to use it to compute a confidence interval for M w. Typically this is done by multiplying the standard error by a factor that depends on the degree of confidence one wishes to express, on the assumption that the weighted mean has a normal distribution. This paper compares confidence intervals of M w concentrations of ions in precipitation, as computed using the assumption of a normal distribution, with those estimated from distributions produced by bootstrapping. The hypothesis that M w was normally distributed was rejected about half the time (at the 5% significance level) in tests involving nine major ions measured at ten diverse sites in the National Atmospheric Deposition Program/National Trends Network (NADP/NTN). Most of these rejections occurred at sites with fewer than 100 samples, in agreement with previous results. Nevertheless, the hypothesis was often rejected at sites with more than 100 samples as well. The maximum error (relative to M w) in the 95% confidence limits made by assuming a normal distribution of the M w at the ten sites examined was about 27%. Most such errors were less than 10%, and errors were smaller at sampling sites with > 100 samples than at those with < 100 samples.