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

Christopher M. B. Lehmann & Van C. Bowersox & Robert S. Larson & Susan M. Larson

Received: 17 June 2005 /Revised: 23 February 2006 /Accepted: 12 March 2006 / Published online: 5 January 2007 # Springer Science + Business Media B.V. 2007

Abstract 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 stat istically 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 halfof the continental U.S. on a precipitation-weightedmean 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.

Keywords chemical climate

. precipitation chemistry

. trend analysis

1 Introduction

Trends in precipitation chemistry indicate changes in air quality and the chemical climate of the Earth ’ s atmosphere. Since precipitation scavenges airbornegases and particles, trends in air quality and precipitation chemistry are related. Such trends represent the combined effects of natural and human-inducedpollutant emissions, physical and chemical transformations, and climate. Long-term precipitation chemistry measurements are a resource for examining these changes. The National Atmospheric Deposition Program/National Trends Network (NADP/NTN) has measured the concentrations of acids, nutrients, and base cations in weekly United States (US) precipitation samples for more than two decades to characterize the chemical climate of the US and its temporal and spatial trends (Lamb & Bowersox, 2000). NADP/ Water Air Soil Pollut: Focus (2007) 7:59

– 66 DOI 10.1007/s11267-006-9100-z

C. M. B. Lehmann ( * ) 

### : V. C. Bowersox 

### : R. S. Larson National Atmospheric Deposition Program, Illinois State Water Survey, 2204 Griffith Dr., Champaign, IL 61820-7495, USA e-mail: clehmann@uiuc.edu

V. C. Bowersox e-mail: sox@sws.uiuc.edu

R. S. Larson e-mail: blarson@uiuc.edu

S. M. Larson Department of Civil and Environmental Engineering, University of Illinois, 206 Engineering Hall, MC-272, 1308 W. Green St., Urbana, IL 61801, USA e-mail: smlarson@uiuc.eduNTN reported measurements include concentrations of sulfate, nitrate, chloride, ammonium, potassium, sodium, calcium, and magnesium, as well as pH andspecific conductivity (National Atmospheric Deposition Program (NADP), 2005). This study focuses on trends in ammonium and sulfate concentrations in precipitation and their resulting influence on precipitation pH. Ammonium deposition from precipitation is of interest because ammonium is a nutrient that stimulates plant growth in nitrogen-limited systems (Walker, Aneja, & Dickey, 2000). While ammonium deposition can promote crop growth, it also can alter the structure and diversity of native plant communities (Krupa, 2003). Sulfate contributes to acidic deposition (i.e., precipitation pH<5.0), leading to adverse impacts in acid-sensitive ecosystems (Sisterson, Bowersox, & Olsen, 1990; Stoddard et al., 2003), although sulfate deposition contributes to the sulfur requirements of some agricultural crops (Knights, Zhao, Spiro, & McGrath, 2000). The chemical equivalent ratios of ammonium to sulfate in precipitation ([NH 4 eq.]/[SO 4 eq.]) are anindicator of which ammonium/sulfur species dominates in the atmospheric aerosol phase. Considering an aerosol system where the dominant reactive species are ammonium and sulfate, ammonium will preferentially react with sulfate to form (in turn) ammonium bisulfate (NH 4 )HSO 4 ([NH 4 eq.]/[SO 4 eq.]=0.50), letovicite (NH 4 ) 3 H(SO 4 ) 2 ([NH 4 eq.]/ [SO 4 eq.]=0.75), and ammonium sulfate (NH 4 ) 2 SO 4 ([NH 4 eq.]/[SO 4 eq.]=1.00), depending on the relative amount of ammonium available (Seinfeld & Pandis, 1998). Under typical atmospheric conditions, these sub-micrometer aerosol species are relatively stable, and can be transported long distances. When the ammonium in the atmosphere exceeds that required to form ammonium sulfate (i.e., [NH 4 eq.]/[SO 4 eq.] >1.00), there exists an “ ammonia rich ” condition in which ammonium is available to react with other species. For example, ammonium can react with nitrate to form ammonium nitrate, NH 4NO 3 (Seinfeld & Pandis, 1998). Ammonium nitrate is a labile species that partitions between gas and aerosol phasesdepending on temperature and gas-phase concentrations of nitric acid and ammonia, and because of its reactivity, is more likely to be scavenged and deposited locally. Modeling results indicate that when NO x and SO 2 emissions are reduced in relation to ammonia emissions, NH 3 remains in the reactive gas phase, significantly reducing the transport distance, leading to local deposition (Hov, Hjollo, & Eliassen, 1994). The present study evaluates trends in ammonium and sulfate concentrations in precipitation using the Seasonal Kendall Trend (SKT) test. The SKT test is a non-parametric hypothesis test based on Kendall ’ s tau statistic, and is useful for evaluating trends in environmental data where the underlying distribution is unknown (Gilbert, 1987; Helsel & Hirsch, 1992). The SKT is an extension of the Kendall Trend test and accounts for seasonal variations (seasonality) in data by comparing the same period of the year over time; i.e., trends in spring concentrations and summer concentrations are treated separately (Gilbert, 1987; Helsel & Hirsch, 1992).

2 Methodology

Data from the NADP/NTN are available on theInternet at http://nadp.sws.uiuc.edu/NTN. Only samples flagged as valid by the NADP were used in the trend analysis. Valid samples are those collected following standard procedures and not flagged as being significantly contaminated (NADP, 2005). For January 1984

–December 2004, data from 159 continental US NADP/NTN sites met or exceeded defined data completeness criteria (Lehmann, Bowersox, & Larson, 2005). For the SKT analyses, statistical seasons were based on meteorological seasons (December

–February = winter, etc.) and divided into high and low sample volume classes, resulting in eight statistical seasons per year (Lehmann et al., 2005). The high sample volume class includes samples with volumes at or above the seasonal median sample volume. The low sample volume class includes samples with volumes below the seasonal median sample volume. Applying this approach identified more statistically significant trends in precipitation chemistry when compared to evaluating trends on an annual or a meteorological season basis (Lehmann et al., 2005). The SKT analyses were performed for the period Winter 1985

–Fall 2004 (Dec. 1984

– Nov. 2004) usingthe EnvironmentalStats version 2.0 package of SPLUS 6.1 (Millard & Neerchal, 2000). The null hypotheses were that the trend is zero (Kendall ’ s tau 60 Water Air Soil Pollut: Focus (2007) 7:59

– 66statistic=0) and that the seasonal taus were homogeneous (differences in Kendall ’ s tau values were statistically insignificant) over all statistical seasons. The statistical significance level was set at 90% for evaluation of trend ( p ≤ 0.10) and seasonal homogeneity ( p >0.10). The magnitude of the trend slope was determined by taking the Sen ’ s median estimator of the natural log of the concentration data in units of
μeq/l. The Sen ’ s median estimator of log-transformed concentrations provides a non-parametric estimate of the percent change of concentration over the period of interest (Gilbert, 1987; Helsel & Hirsch, 1992; Millard & Neerchal, 2000). The median trend for all 159 sites evaluated was calculated to represent the overall national trend. In addition to the SKT analysis, changes in equivalent ratios of ammonium to sulfate and the frequency of acidic precipitation were determined. Equivalent ratios of precipitation ammonium tosulfate concentrations were calculated on a threeyear precipitation-weighted-mean basis. The frequency of acidic precipitation was determined from weekly events over this three year period, with acidic precipitation defined as samples having a pH< 5.0 (Seinfeld & Pandis, 1998). All isopleth contourmaps in this study were created using an InverseDistance-Weighted (IDW) algorithm based on all 2.5 km grid cells within 500 km of NADP/NTN sites (Lehmann et al., 2005). A linear IDW fit was used for trend maps, and a cubic IDW fit was used forammonium to sulfate ratio maps and acidic precipitation frequency maps. The relative continental U.S. area contained in each isopleth contour class was calculated from the number of grid cells contained in each class. 3 Results and Discussion

3.1 Trends in Ammonium and Sulfate Concentrations in Precipitation

Significant increases in ammonium concentrations in precipitation were obse rved across most of the continental US over the 20-year period from Winter 1985 to Fall 2004 (Table 1, Fig. 1a). Trends were found to be increasing and statistically significant at 101 (64%) of the 159 sites evaluated, with most of these trends (95 sites, 60%) being statisticallysignificant and homogeneous across seasons. Statistically significant decreasing trends were found at only three sites (2%), with two of these trends (1%) being statistically significant and homogeneous. The few sites with decreasing trends were predominately along the coasts, and trends decreasing by more than 10% in magnitude represented less than 1% of the area of the continental US. Trends increasing by more than 50% in magnitude represented 30% of the US and were spread over a large area in the central part of the country. The largest magnitude increase (+123%) occurred in southeastern North Carolina, where Walker et al. (2000) have reported large ammonium increases related to swine population growth. The median ammonium trend across the 159 sites evaluated was +28.5% (Table 1). In the US, estimates of ammonia emissions are limited (and existing emissions inventories from the US EPA do not begin until 1990 (United States Environmental Protection Agency (U.S. EPA), 2005)), so it is difficult to compare trends in ammonium precipitation with ammonia emissions. However data from the US EPA indicate that ammonia emissions may have

Table 1 Seasonal Kendall trend test of NTN concentrations, Winter 1985

–Fall 2004

Median national trend (Sen ’ s estimator) % Increasing precipitation concentration trend Decreasing precipitation concentration trend
Total sites Statistically significant trend ( p ≤ 0.10) Statistically significant & homogeneous trend ( p >0.10) Total sites Statistically significant trend ( p ≤ 0.10) Statistically significant & homogeneous trend ( p >0.10)

Number % Number % Number % Number % Number % Number %

NH 4 +28.5 142 89% 101 64% 95 60% 17 11% 3 2% 2 1% SO 4 − 45.7 5 3% 0 0% 0 0% 154 97% 141 89% 124 78% Water Air Soil Pollut: Focus (2007) 7:59

– 66 61dropped since 2000 (United States Environmental Protection Agency (U.S. EPA), 2005), in contrast to the measured ammonium trends in precipitation. Sulfate concentrations decreased over nearly the entire continental US from Winter 1985 to Fall 2004 (Table 1, Fig. 1b). Statistically significant decreases occurred at 141 of the 159 sites in this study (89%), with the majority of decreasing trends (124 sites, 78%) being statistically significant and homogeneous. Sulfate concentration increases were confined to Texas and southern Florida, representing less than 1% of the continental US, and none of the increases was statistically signifi cant. Elsewhere, sulfate decreases were 10% or more with 25 to >50% reductions throughout most of the western and northeastern US (The decrease of >100% in northern California is an aberration of the Sen ’ s median estimator. The 90th percentile confidence interval at this site extends from

− 130 to

− 86%.) The median sulfate trend across the 159 sites evaluated was

− 45.7% (Table 1). Other researchers have reported sulfate decreases similar to those observed in this study, particularly in the northeastern U.S. where sulfur dioxide emissions reductions have occurred (Civerolo & Rao, 2001; Lynch, Bowersox, & Grimm, 2000; Nilles & Conley, 2001). Other researchers (Malm, Schichtel, Ames, & Gebhart, 2002) also have observed sulfate increases in Texas, Fig. 1 Trend significance ( p ≤ 0.10), trend homogeneity ( p >0.10), and percent change (Sen ’ s estimator) from Winter 1985 to Fall 2004 for a ammonium concentration and b sulfate concentration. Numeric values indicated at sites with a significant trend 62 Water Air Soil Pollut: Focus (2007) 7:59

– 66Fig. 2 Three-year precipitation-weighted-mean ammonium to sulfate chemical equivalent ratios for a 1984

– 1986, b 1994

– 1996, and c 2002

–2004 Water Air Soil Pollut: Focus (2007) 7:59

– 66 63which they attributed to sulfur dioxide emission increases from 1988 to 1998 in the southwestern US Transport of pollutants from outside the US may also be a factor. However, sulfur dioxide emissions trends data are not available to confirm this supposition.

3.2 Changes in the Ammonium to Sulfate Equivalent Ratio

The resulting impact of increasing ammonium trendsaccompanied by decreasing sulfate trends is demonstrated by the ammonium to sulfate equivalent ratio. Three-year precipitation-weighted-mean ammonium to sulfate equivalent ratios were calculated for Jan. 1984

–Dec. 1986 (Fig. 2a), Jan. 1994

–Dec. 1996 (Fig. 2b), and Jan. 2002

–Dec. 2004 (Fig. 2c). From 1984 to 1986, ammonium to sulfate ratios were less than 0.5 for more than half of the continental US (Fig. 2a). Ammonia-rich regions ([NH 4 eq.]/[SO 4 eq.]>1.00) were less than 10% of the US. For the period 1994

– 1996, regions with an ammonium to sulfate ratio less than 0.5 constituted approximately 24% of the US, and ammonium to sulfate ratios exceeded 1.0 in approximately 34% of the US (Fig. 2b). For the period 2002

–2004, ammonium to sulfate ratios were less than 0.5 in only 16% of the US. In these regions, the influence of sea salt sulfate is a likely contributor. Ammonium to sulfate ratios exceeding 1.0 were found in over half of the U.S., corresponding to regions with significant ammonium Fig. 3 Three-year frequencyof occurrence of acidic precipitation (pH<5.0) for a 1994

– 1996 and b 2002

– 2004 64 Water Air Soil Pollut: Focus (2007) 7:59

– 66concentration increases (Fig. 1a). In these ammoniarich regions, it is likely that ammonium nitrate andammonia gas scavenged by precipitation have contributed to the upward trends in ammonium concentrations in precipitation, leading to local deposition of ammonium.

3.3 Frequency of Acidic Precipitation, 1994

– 1996 vs. 2002

–2004

The frequency of acidic precipitation was evaluated for two 3-year periods, Jan. 1994

–Dec. 1996 (Fig. 3a) and Jan. 2002

–Dec. 2004 (Fig. 3b). The 1984

– 1986 trend maps were not included in this part of the study, because the NADP/NTN instituted a sample protocol change in 1994 to eliminate a sampling artifact inherent in prior determinations of samplepH in certain regions (National Atmospheric Deposition Program (NADP), 1995). (Precipitation concentrations presented in Fig. 2a, were not affected by the sampling artifact.) For the period 1994

– 1996, acidic precipitation occurred in more than half of precipitation samples measured over approximately 40% of the continental US. At least 1% of samples at all sites had a pH<5.0. The highest incidence of acidic precipitation occurred in the northeastern US,with areas in eastern Ohio, Pennsylvania, and surrounding regions having acidic precipitation in virtually all samples. For the years 2002

–2004, the area where more than half of the samples with pH<5.0 decreased to 35%. In this period, three sites in thewestern US experienced virtually no acidic precipitation. The highest incidence of acidic precipitation remained centered in the northeast, although the incidence of acidic precipitation decreased overall. This change in acidic precipitation frequency is due tothe combined effects of increased ammonium, decreased sulfate, and trends in other precipitation species noted in our previously published work (Lehmann et al., 2005), including decreased nitrate and increased buffering capacity from earth crustal cations. The greatest decrease in precipitation acidity occurred in states surrounding Lakes Superior and Michigan. Only six sites studied experienced an increase in the frequency of acidic precipitation between the two time periods. Although the reduced frequency of acidic precipitation mitigates the chronic effects of ecosystem exposure, the majority of the US continues to experience acidic precipitation to some
extent.

4 Summary

Data from the National Atmospheric DepositionProgram/National Trends Network indicate that statistically significant trends have occurred over the twenty-year period from Winter 1985 to Fall 2004 forammonium and sulfate concentrations in precipitation. Trends in ammonium, sulfate, and other species have led to a decrease in the frequency of acidicprecipitation (pH<5.0) in the continental US, although the majority of the US continues to experience acidic precipitation at some frequency.Increases in ammonium concentrations in precipitation and ammonium wet deposition were observed across most of the continental United States, with a median trend across the 159 sites evaluated of +28.5%. Increases were particularly notable in the central and upper Midwest. Only limited ammonia emissions trends data are available from the US EPA for comparison (indicating a need for more detailed ammonia emissions inventories), but those available indicate that changes in emissions alone cannot account for observed ammonium increases. These findings point to the need for routine air concentrationmeasurements of gas-phase ammonia, a gas constituent not routinely measured in the US, so that we can better understand the relationships between ammonia emissions, gaseous ammonia, particulate ammonium, and ammonium in precipitation. Sulfate concentrations in precipitation and wet deposition decreased markedly in nearly the entire continental US over the trend analysis period. Sulfate concentration increases were confined to Texas and southern Florida, although no increasing trends were statistically significant. The national trend in precipitation sulfate was

− 45.7%, consistent with decreasing sulfur dioxide emissions trends reported in the literature. Trends in ammonium and sulfate have resulted in ammonium to sulfateequivalent ratios exceeding 1.0, creating an ammonia-rich environment for more than half of the US from 2002

–2004. This change has likely reduced the transport distance of ammonium, indicating that trends in ammonium may be most influenced by local emissions sources. Water Air Soil Pollut: Focus (2007) 7:59

– 66 65Trends observed in precipitation chemistry and wet deposition are a manifestation of changes in the chemical climate of the atmosphere in the US, only some of which appear to be attributable to changes in emissions. Because trends in pollutant species are linked, there is a need for continued research into multi-constituent emission relationships. There is alsoa continued need for long-term monitoring of precipitation chemistry to evaluate the impact of emissions regulations on the chemical climate of the atmosphere.

Acknowledgements The NADP is National Research Support Project-3: A Long-term Monitoring Program in Support of Research on the Effects of Atmospheric Chemical Deposition. More than 240 sponsors support the NADP, including StateAgricultural Experiment Stations; universities; private companies and other non-governmental organizations; Canadian government agencies; state, local, and tribal governmentorganizations; and federal agencies, including the US Department of Agriculture-Cooperative State Research, Education, and Extension Service (under agreement no. 2002-39138- 11964). Any findings or conclusions in this article do not necessarily reflect the views of the US Department of Agriculture or other sponsors.

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