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 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-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 airborne

gases and particles, trends in air quality and precipitation chemistry are related. Such trends represent the

combined effects of natural and human-induced

pollutant 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.edu
NTN reported measurements include concentrations

of sulfate, nitrate, chloride, ammonium, potassium,

sodium, calcium, and magnesium, as well as pH and

specific 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 ([NH4 eq.]/[SO4 eq.]) are an

indicator 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 (NH4)HSO4 ([NH4 eq.]/[SO4

eq.]=0.50), letovicite (NH4)3H(SO4)2 ([NH4 eq.]/

[SO4 eq.]=0.75), and ammonium sulfate (NH4)2SO4

([NH4 eq.]/[SO4 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., [NH4 eq.]/[SO4 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, NH4NO3 (Seinfeld

& Pandis, 1998). Ammonium nitrate is a labile

species that partitions between gas and aerosol phases

depending 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

NOx and SO2 emissions are reduced in relation to

ammonia emissions, NH3 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 the

Internet 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) using

the 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) 7:59–66
statistic=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 to

sulfate 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 contour

maps 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 for

ammonium 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 observed 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 statistically

significant 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 %

NH4 +28.5 142 89% 101 64% 95 60% 17 11% 3 2% 2 1%

SO4 −45.7 5 3% 0 0% 0 0% 154 97% 141 89% 124 78%

Water Air Soil Pollut: Focus (2007) 7:59–66 61
dropped 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 significant. 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) 7:59–66
Fig. 2 Three-year precipitation-weighted-mean ammonium to sulfate chemical

equivalent ratios fora

1984–1986,b 1994–1996,

andc 2002–2004

Water Air Soil Pollut: Focus (2007) 7:59–66 63
which 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 trends

accompanied 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 ([NH4 eq.]/[SO4

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 frequency

of occurrence of acidic precipitation (pH<5.0) for a

1994–1996 and b 2002–

2004

64 Water Air Soil Pollut: Focus (2007) 7:59–66
concentration increases (Fig. 1a). In these ammoniarich regions, it is likely that ammonium nitrate and

ammonia 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 sample

pH 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 the

western 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 to

the 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 Deposition

Program/National Trends Network indicate that statistically significant trends have occurred over the

twenty-year period from Winter 1985 to Fall 2004 for

ammonium and sulfate concentrations in precipitation. Trends in ammonium, sulfate, and other species

have led to a decrease in the frequency of acidic

precipitation (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 concentration

measurements 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 sulfate

equivalent 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 65
Trends 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 also

a 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 State

Agricultural Experiment Stations; universities; private companies and other non-governmental organizations; Canadian

government agencies; state, local, and tribal government

organizations; 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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