deposition of sulphur, nitrogen and acidity in precipitation over ireland: chemistry, spatial...

Atmospheric Environment 36 (2002) 1379–1389

Deposition of sulphur, nitrogen and acidity in precipitationover Ireland: chemistry, spatial distribution and long-term

trends

J. Aherne*, E.P. Farrell

Department of Environmental Resource Management, University College, Belfield, Dublin 4, Ireland

Received 10 September 2000; received in revised form 21 June 2001; accepted 1 October 2001

Abstract

The chemical composition of pollutant species in precipitation sampled daily or weekly at 10 sites in Ireland for the

five-year period, 1994–1998, is presented. Sea salts accounted for 81% of the total ionic concentration. Approximately

50% of the SO42� in precipitation was from sea-salt sources. The proportion of sea salts in precipitation decreased

sharply eastwards. In contrast, the concentration of NO3� and the proportion of non-sea-salt SO4

2� increased eastwards

reflecting the closer proximity to major emission sources. The mean (molc) ratio of SO42�:NO3

� was 1.6 for all sites,

indicating that SO42� was the major acid anion.

The spatial correlation between SO42�, NO3

� and NH4+ concentrations in precipitation was statistically significant.

The regional trend in NO3� concentration was best described by linear regression against easting. SO4

2� concentration

followed a similar pattern. However, the regression was improved by inclusion of elevation. Inclusion of northing in the

regression did not significantly improve any of the relationships except for NH4+, indicating a significant increase in

concentrations from northwest to southeast.

The spatial distribution of deposition fluxes showed similar gradients increasing from west and southwest to east and

northeast. However, the pattern of deposition shows the influence of precipitation volume in determining the overall

input. Mean depositions of sulphur and nitrogen in precipitation were E30 ktonnes S yr�1 and 48 ktonnes N yr�1 over

the five-year period, 1994–1998, for Ireland.

Least-squares linear regression analysis indicated a slight decreasing trend in precipitation concentrations for SO42�

(20%), NO3� (13%) and H+ (24%) and a slight increasing trend for NH4

+ (15%), over the period 1991–1998. r 2002

Elsevier Science Ltd. All rights reserved.

Keywords: Acid deposition; Sulphate; Nitrate; Ammonium

1. Introduction

Long-range transboundary air pollution and its

potential effects on the environment have been a major

concern since the late 1960s. Acid deposition, originat-

ing largely from man-made emissions of three gaseous

pollutants, sulphur dioxide (SO2), nitrogen oxides (NOx)

and ammonia (NH3), continues to damage acid-sensitive

freshwater systems, forests, soils and natural ecosystems

in large areas of Europe (European Environment

Agency, 1998). As a result of the concern over acid

rain, many research groups and national agencies

throughout Europe and North America established

extensive programmes of precipitation chemistry

*Corresponding author. Present address: Environmental and

Resource Studies, Trent University, 1600 West Bank Drive,

Peterborough, ON, Canada K9J 7B8. Tel.: +1-705-748-

1011x1348; fax: +1-705-748-1569.

E-mail address: [email protected] (J. Aherne).

1352-2310/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.

PII: S 1 3 5 2 - 2 3 1 0 ( 0 1 ) 0 0 5 0 7 - 6

monitoring in order to determine the extent of the

problem.

Ireland’s exposure to transboundary air pollution is

limited due to the predominantly southwesterly or

westerly airflow and its favourable location on the

western periphery of Europe. As such, it has been

suggested that Ireland can be used as an unpolluted

reference for European studies (Beltman et al., 1993).

The chemical composition of rainfall in Ireland has been

the subject of scientific investigations for over a century.

The earliest recorded measurements were carried out at

Valentia Observatory, Co. Kerry, during the late 1800s

(Smith, 1872). Since 1958, the Irish meteorological

service (Met !Eireann) has operated a programme of

monthly sampling and analysis of precipitation at

several stations throughout Ireland (Mathews et al.,

1981). Data from this network have been used in a

number of studies and deposition mapping assessments

(Stevenson, 1968; Fisher, 1982; Jordan, 1997). However,

the Met !Eireann network was not designed specifically

to provide accurate information for detailed assessment

of acid deposition over the country (Mathews et al.,

1981). The locations of the sites are not well suited for

this purpose (three sites are at airports and most of the

others are on the coast) and the analysis of precipitation

chemistry is undertaken largely on the basis of monthly

samples, which is not suited to quantitative assessment

of atmospheric deposition. More recently, a number of

measurement sites, operated by a range of agencies, have

been established with daily and weekly sampling

regimes. Most of these sites contribute data to European

monitoring programmes.

Quantitative information on the deposition of pollu-

tant species in rainfall is essential for understanding

regional variations and investigating the efficiency of

emissions reduction policies. Furthermore, detailed

spatial description of deposition patterns is a key

requirement in the application of the critical load

approach to emissions control. The quality of rain and

its spatial and temporal evolution over Ireland are

examined in this paper. The objective is to provide a

scientifically sound, up-to-date analysis of the phenom-

enon of acid deposition. The chemical composition of

rainfall, regional variation in deposition, variation

throughout the year and long-term trends are described.

Only stations sampling and analysing precipitation on a

daily or weekly basis during the eight-year period, 1991–

1998 were used in the study.

2. Methods

2.1. Sampling sites

During the period of investigation there were 10

stations sampling precipitation chemistry in Ireland on a

daily or weekly basis; eight sites in the Republic of

Ireland and two in Northern Ireland (Fig. 1). The

majority of sites, except two, used bulk precipitation

collectors as opposed to wet-only samplers (Fig. 1). The

Forest Ecosystem Research Group (FERG), sample

precipitation at four sites in the Republic of Ireland

(Roundwood, Brackloon, Cloosh and Ballyhooly), three

of which are part of an EU-wide network of forest

health monitoring plots (Farrell et al., 1996; Boyle et al.,

2000). The four remaining sites in the Republic of

Ireland all contribute data to the co-operative pro-

gramme for monitoring and evaluation of the long-

range transmission of air pollutants in Europe (EMEP).

Valentia Observatory is operated by Met !Eireann, The

Burren and Cappard Ridge by the Electricity Supply

Board (ESB) and Turlough Hill by the ESB on behalf of

the Irish Environmental Protection Agency (EPA). The

two sites in Northern Ireland (NI), Lough Navar and

Hillsborough, are part of the official United Kingdom

(UK) acid deposition network administered by AEA

Technology plc, on behalf of the UK Department of the

Environment, Transport and the Regions (DETR).

Lough Navar is operated by the Forestry Service NI

(FSNI) and Hillsborough by the Department of

Brackloon

Ballyhooly

Cloosh

Roundwood

Valentia Observatory

Turlough Hill

Lough NavarHillsborough

The Burren

Capard Ridge

Bulk collector

Wet-only collector

Fig. 1. Location of sampling sites, and type of precipitation

collector, for the 10 monitoring stations in the Republic of

Ireland and Northern Ireland.

J. Aherne, E.P. Farrell / Atmospheric Environment 36 (2002) 1379–13891380

Agriculture and Rural Development NI (DARDNI) on

behalf of AEA Technology plc. Lough Navar also

contributes data to EMEP.

All sites were remote from local sources of pollution

and thus provided data representative of the region in

which they were located. Due to the short sampling

frequency and the relatively low pollution levels it was

assumed that the differences between bulk concentra-

tions and wet-only collectors were negligible. Cape et al.

(1984) state that the location of rain collectors in rural

areas, where there are only small concentrations of

pollutant gases, minimises the relative size of this

systematic error which is unlikely to influence strongly

the observed regional pattern. Total (wet plus dry)

depositions were not estimated in the current study.

Bulk collectors generally underestimate total deposition

depending on land cover and proximity to emission

sources. Lindberg et al. (1986, 1990) measured differ-

ences of up to 40% between bulk and total deposition to

forests.

All monitoring sites were sampled and analysed on a

daily or weekly basis (Table 1). Precipitation volume,

pH, major cations (ammonium, calcium, magnesium,

sodium and potassium) and anions (sulphate, nitrate

and chloride) were measured at all sites. Sampling and

chemical analyses were carried out by the responsible

agencies. Methods for sampling and chemical analysis

between all stations were not standardised, but most

laboratories followed a manual of recommended proce-

dures due to their involvement in national and European

networks. In addition, rigorous quality control and

detailed data checking were carried out by the data

source agencies. Non-sea-salt ion concentrations were

calculated using theoretical sea-salt ratios assuming that

all the sodium in precipitation originated from sea-salt

(e.g. (molc) ratio for SO42�:Na+ is 0.12:1: Werner and

Spranger, 1996).

2.2. Deposition mapping

Concentration fields were determined by ordinary

kriging, a linear estimation technique that provides an

unbiased estimator with a minimum estimation variance

(Whelpdale and Kaiser, 1996), using five-year mean

annual precipitation-weighted concentration data. No

allowance was made for the bearing of wind direction.

However, the effects of wind direction on chemistry are

irrelevant since the data are annual means. In general,

concentrations are spatially coherent and have regional

patterns that vary in a smooth and systematic way over

the course of a season or year. On the other hand, the

variation in precipitation quantity occurs over a much

smaller scale than the variation in concentration

(Whelpdale and Kaiser, 1996). Thus, in regions with

dense precipitation-monitoring networks, such as those

operated by meteorological services, regional wet Table

1

Sitename,

location(Irish

gridco-ordinate

system

),five-yearmeanannualconcentrations(1994–1998),data

sourceandsamplingfrequency

forprecipitationmonitoringstationsa

Sitename

Elevation

(m)

Co-ordinates(m

)Rainfall

(mm)

pH

NH

4+NO

3�SO

42�

Ca2+

Mg2+

K+

Na+

Cl�

nss

SO

42�

Data

source

Sampling

frequency

Easting

Northing

(mmol cl�

1)

Brackloon

75

97300

279900

1271

4.93

14.6

8.1

46.2

17.9

52.2

7.4

249

284

16.0

FERG

Weekly

Cloosh

102

110400

234600

1622

4.80

10.2

5.6

40.0

14.8

41.7

5.3

200

230

15.8

FERG

Weekly

Roundwood

395

318000

207300

1500

4.62

27.3

20.2

38.5

11.7

20.0

3.4

98

127

26.6

FERG

Weekly

Ballyhooly

75

172300

98000

1174

4.97

27.4

9.7

31.2

11.3

21.8

4.4

109

136

17.9

FERG

Weekly

TurloughHill

420

307500

198500

1415

4.90

23.9

16.3

29.9

12.9

18.6

2.8

74

85

21.0

EMEP

Daily

TheBurren

90

126200

195000

1029

5.14

17.9

9.9

45.6

17.7

55.2

2.4

245

274

15.9

EMEP

Daily

Cappard

Ridge

340

236800

206900

1231

5.14

31.3

13.6

28.5

9.8

18.7

1.3

84

88

18.4

EMEP

Daily

ValentiaObserv.

945700

78700

1582

5.02

12.7

5.2

54.8

20.7

89.4

9.2

416

442

7.7

EMEP

Daily

Hillsborough

120

324300

357700

681

5.26

46.1

21.2

47.4

23.3

30.9

2.5

101

116

35.1

AEA

Weekly

LoughNavar

130

206500

354500

1520

5.25

12.6

11.0

31.7

25.4

41.0

3.2

138

156

15.1

AEA

Weekly

Weightedmean

176

1302

4.98

20.7

11.6

39.1

16.4

40.0

4.4

178

201

18.0

aBallyhooly

nodata

for1995;ValentiaObservatory

nodata

for1998;nss=

nonseasalt.

J. Aherne, E.P. Farrell / Atmospheric Environment 36 (2002) 1379–1389 1381

deposition can be more accurately estimated by combin-

ing grided fields of concentration with finer scale

precipitation data.

Mapped ion concentration fields were combined with

rainfall volume measurements from the Met !Eireann

gauge network. The 30-year mean annual rainfall data

(E620 sites: Fitzgerald, 1984) were used to describe the

spatial pattern of precipitation volume. To produce a

reliable precipitation field these measurements were

complemented with mean annual estimates or ‘dummy

values’, derived from nearby measured values and

various climatological normals (E40 values: Hamilton

et al., 1988), for mountainous areas which did not have

measured data. This procedure of combining dense

rainfall volume measurements with more sparse rainfall

chemistry to produce national ion deposition maps is

similar to that used in the UK (e.g., E4000 volume

measurements combined with 40 chemistry: Campbell

et al., 1994; Vincent et al., 1996).

3. Results and discussion

3.1. Precipitation volume

The mean annual precipitation volume for all sites

was E1300mmyr�1 for the period 1994–1998 (Table 1).

This is slightly greater than the 30-year mean for Ireland

(1200mmyr�1: Fitzgerald, 1984), due to the higher

mean site elevation (105m a.s.l. compared to 175m a.s.l.

for the current study). The 10 sampling stations showed

marked differences in annual precipitation volume

(Table 1); the lowest mean was recorded at Hillsborough

(681mmyr�1), while the maximum was measured at

Cloosh (1622mmyr�1). The pattern reflects the higher

precipitation volume at the western coastal sites or those

located at higher altitudes. Three factors control much

of the spatial variability of Ireland’s weather: prevailing

westerly winds, proximity to the sea, and elevation

(Rohan, 1986).

Monthly precipitation volumes at four sampling sites

are shown in Fig. 2. The sites represent a transect from

east to west across Ireland, with significant differences in

elevation and mean rainfall (Table 1). Nevertheless, all

stations had similar and clear monthly patterns in

precipitation volume. All stations had lowest rainfall in

April and September and highest in January–February

and October.

3.2. Ionic concentrations and spatial variation

Precipitation-weighted mean annual concentrations of

the principal ions in deposition for the period 1994–1998

at each station are given in Table 1. The quality of the

analytical data was checked by a cation–anion balance

(Fig. 3a). The ionic balance revealed an anion deficit of

3.5%, probably due to the presence of short chain

organic acids and bicarbonate. It was assumed that all

major ions had been analysed since deviation was

o10%. Weighted mean pH for all sites was 4.98 (range

4.62–5.25), slightly higher than values observed in rural

areas in Austria (Puxbaum et al., 1998), England (Raper

and Lee, 1996), France (Sanusi et al., 1996), Italy

(Balestrini et al., 2000) and Wales (Reynolds et al.,

1999). The major cations were sodium (Na+) and

magnesium (Mg2+); and anions were chloride (Cl�)

and sulphate (SO42�). The dominant cations and anions

were of marine origin. Sea salts accounted for 81% of

the total ionic concentration. Approximately 50% of the

SO42� in precipitation was from sea-salt sources; the

proportion varied from >70% at the western coastal

sites to o15% in the east. The overall mean Na+: Cl�

ratio (molc) was 0.87:1 close to the theoretical value for

seawater of 0.86:1 (Werner and Spranger, 1996)

although values ranged from 0.77 to 0.95.

The precipitation chemistry was spatially variable due

mostly to the eastward decline in the marine influence

and eastward increase in pollutant concentrations. The

proportion of sea salts in precipitation decreased sharply

eastwards (Fig. 3b). In contrast, the concentration of

nitrate (NO3�) and the proportion of non-sea-salt SO4

2�

increased eastwards reflecting the closer proximity to

major emission sources (Figs. 3c and d). The higher

pollutant concentrations in the east of the country are

due to the atmospheric deposition of anthropogenic

pollutants from easterly air-streams. Bowman and

McGettigan (1994) observed that non-sea-salt SO42�

concentrations on the east coast of Ireland were 4–7

times greater when associated with air masses of an

easterly direction compared with those from the west.

The remaining discussion is restricted to non-sea-salt

SO42�, NO3

� and ammonium (NH4+), the chemical

variables most relevant to acid deposition. In addition,

all further references to SO42� refer to non-sea-salt SO4

2�,

unless otherwise stated.

The precipitation-weighted mean annual concentra-

tions of NO3� in rain were lower than those of SO4

2�

100

200

300

400

500

600

700

800

J J JM M S O N D

Turlough HillCapard Ridge

The BurrenValentia Observatory

Pre

cip

ita

tio

n (

mm

)

F A A

Fig. 2. Mean monthly values of precipitation volume (mm) for

the period 1995–1998.


(Table 1). The mean SO42�:NO3

� ratio (molc) for all sites

was 1.6, indicating that SO42� was the major acid anion.

The ratio varied from 2.8 in the west to 1.3 in the east.

The greater relative contribution of NO3� at eastern sites

suggests additional sources of NO3�. The mean concen-

trations of SO42�, NO3

� and NH4+ were lower than those

at rural sites in other European countries (Raper and

Lee, 1996; Sanusi et al., 1996; Puxbaum et al., 1998;

Reynolds et al., 1999; Balestrini et al., 2000). The lowest

pollutant concentrations were measured in the west of

Ireland as expected. The correlation coefficients between

important species were determined with the aim of

identifying potential precursors of ions in precipitation

(Table 2). Hillsborough was excluded from this analysis

because, unlike other stations, it had high concentra-

tions of SO42�, NO3

� and NH4+ in combination with high

pH (Table 1), suggesting that alkalinity due to NH4+

dominated the chemistry. The spatial correlation for the

remaining sites was significant indicating a common

origin for SO42�, NO3

� and NH4+ in precipitation.

10

20

30

40

50

60

70

80

Non

-sea

salt

sulp

hate

(%

of

sulp

hate

)

West East

60

65

70

75

80

85

90

95

100

Sea

salt

(% o

f to

tal

ion

conc

entr

atio

n)

West East100

200

300

400

500

600

100 200 300 400 500 600

Ani

ons

(µm

olc l–1

)

Cations (µmolc l –1)

Nitr

ate

(µm

olc l

–1)

0

5

10

15

20

25

West East

(a)

(c) (d)

(b)

Fig. 3. (a) Ion balance of the major cations and anions in precipitation (mmolc l�1); (b) west to east variation in the proportion of sea

salts that contribute to the total ionic concentration; (c) west to east variation in the proportion of non sea salt SO42� that contribute to

total measured SO42�; and (d) west to east variation in NO3

� concentrations (mmolc l�1).

Table 2

Spatial correlation (pearson-product) between ion concentra-

tions at all sitesa, 1994–1998

SO42� NO3

� NH4+ H+

SO42� 1

NO3� 0.886** 1

NH4+ 0.665* 0.723* 1

H+ 0.636 0.449 0.170 1

aHillsborough excluded from correlation.

Levels of significance * 0.05, ** 0.01 and *** o0.001.


According to Raper and Lee (1996), such a matrix of

correlations between ions is a common finding for a

number of reasons: some of the pollutants have common

sources, for example NOx and SO2 from power stations

and manufacturing industries; atmospheric chemistry

may result in a correlation between ions originating

from different sources, for example NO3� and SO4

2�

commonly correlate with NH4+ because of gaseous

reaction of ammonia with sulphuric and nitric acids; and

meteorology provides a mechanism whereby ions may

be interrelated because of the trajectory of an air mass

and its history. Similar correlations have been found by

a number of researchers (Cape et al., 1984; Raper and

Lee, 1996; Balestrini et al., 2000).

The regional gradient in ion concentrations, which

was relatively simple, can be described by location and

elevation. Using multiple regression analysis, empirical

equations describing the spatial variation in concentra-

tions of SO42�, NO3

� and NH4+ are given in Table 3. All

equations are of the form:

Concentration ¼C1 � eastingsþC2 � northings

þ C3 � elevationþC4;

where C124 are the estimated coefficients (Table 3). The

regional trend in NO3� concentration was best described

by linear regression against easting. This represents a

significant increase in NO3� from west to east (see

Fig. 2d). SO42� concentration followed a similar pattern.

However, the regression was improved by inclusion of

elevation. An increase in elevation corresponded to a

decrease in concentration; this can be explained by

dilution due to higher precipitation volume. Inclusion of

northing in the regression did not significantly improve

any of the relationships except that for NH4+, indicating

a significant increase in concentrations from northwest

to southeast. The higher concentrations in the south and

southeast of Ireland are associated with high livestock

densities on intensively managed grasslands, which are a

major source of atmospheric ammonia.

3.3. Deposition maps

The spatial depositions for SO42�, NO3

� and NH4+ are

shown in Figs. 4a–c. Similar to concentration, the

highest depositions were in the east and southeast of

Ireland. However, the pattern of deposition shows the

influence of precipitation volume in determining the

overall input. Mean deposition of SO42� was

22mmolcm�2 yr�1, which corresponded to

30 ktonnes S yr�1 during the period 1994–1998

(23 ktonnes for the Republic of Ireland). This is

significantly smaller than previous assessments based

on data from sites which were sampled on a bi-weekly or

monthly basis (E60 ktonnes S yr�1: Jordan, 1997). The

suitability of the monthly sampled data for detailed

assessments of acid deposition over the country have

previously been questioned (Mathews et al., 1981).

Mean deposition of NO3�-N and NH4

+-N for Ireland

was E16 and 32 ktonnesN yr�1, respectively (13 and

25 ktonnes for the Republic of Ireland, respectively).

Therefore, mean deposition of nitrogen in precipitation

was E48 ktonnesN yr�1 (38 ktonnes for the Republic of

Ireland), significantly larger than sulphur deposition.

The spatial deposition pattern of NH4+ differs to that

of SO42� and NO3

� (see Figs. 4a–c) with greater deposi-

tions in the southeast of Ireland, again this is due to

ammonia emissions associated with high livestock

densities in this region. The deposition of NH4+ may

influence the effects of acid deposition on ecosystems, as

NH4+ is rapidly oxidised in soil and water, either

through plant uptake or microbial transformation,

producing further acidity. The effects of acid deposition

in areas receiving high amounts of NH4+ deposition may

be better described by potential acidity (Mosello and

Marchetto, 1996):

Potential acidity ¼ non-sea-salt SO2�4 þNO�

3 þNHþ4

� non-sea-salt ðCaþ2 þMgþ2 þKþÞ:

The deposition of potential acidity is shown in Fig. 4d.

The influence of NH4+ can clearly be seen in the spatial

deposition pattern. Potential acidity can be translated

into a potential pH, which corresponds to a pH of 4.2 in

the east and northeast increasing to pH>4.6 in the west.

3.4. Seasonal variation

The monthly mean concentrations and depositions of

SO42�, NO3

�, NH4+ and H+, based on four stations

Table 3

Regression equationsa describing regional patterns of ion concentrations in precipitation (mmolc l�1) based on location (m) and

elevation (m)

Ion Coefficients r p

C1 C2 C3 C4

SO42� 9.31� 10�5 0.00 �0.027 5.625 0.92 0.002

NO3� 5.41� 10�5 0.00 0.000 1.570 0.96 o0.0001

NH4+ 1.58� 10�4 �3.78� 10�5 �0.053 9.452 0.87 0.03

a Ion concentration=C1 � easting+C2 �northing+C3 � elevation+C4:


(Turlough Hill, The Burren, Cappard Ridge and

Valentia Observatory), are shown in Figs. 5a and b.

Both ion concentrations and depositions show great

seasonal variation, however, most ions followed the

same trend. The variation in concentration from month

to month was inversely related to the precipitation

volume (see Fig. 2). Nevertheless, there was a maximum

in late spring–early summer and in autumn. This

increase of strong acid anions was counteracted by a

marked increase in NH4+ concentration, so that H+ ion

concentrations were lower in summer; this also occurred

in autumn (see April and September in Fig. 5a).

The seasonal variation in the flux of ions was strongly

influenced by volume of precipitation. When the effects

of precipitation volume on concentration were removed

by considering the monthly deposition, the variation

(a)

(c) (d)

(b)

Fig. 4. Five-year mean deposition of (a) SO42�, (b) NO3

�, (c) NH4+, and (d) potential acidity (mmolcm

�2 yr�1) in precipitation (1994–

1998).


was reduced (Fig. 5b). The highest depositions occurred

between spring and summer even though the volume of

precipitation was low during this period. In contrast, the

high precipitation volumes during Autumn (October)

did not produce high ion depositions.

3.5. Long-term trends

European emissions of sulphur have followed a steep

decline since the 1980s (Mylona, 1993). The effectiveness

of emission controls can to some degree be evaluated

from long-term trends in precipitation chemistry. It was

assumed that the reductions in sulphur should be

evident in the depositions of SO42� and H+ over Ireland.

Annual volumes of precipitation and concentration of

SO42�, NO3

�, NH4+ and H+, for the eight-year period

1991–1998, are shown in Table 4. The first four years of

data (1991–1994) were initially compared with the last

four years (1995–1998). The percentage change between

the two periods is shown in Fig. 6, with the 100% line

corresponding to the relative depositions of the first

period. The mean SO42� deposition for the second period

was 12% lower than that for the first. The drop in

concentration was the same as the precipitation volume

for the two periods was identical (Fig. 6). The mean

NO3� deposition showed a slight decrease (7%) between

the two periods. Conversely, mean NH4+ deposition

showed an increase of 13% compared to the first period.

The H+ depositions closely followed those of SO42�,

with a decrease of 15%. Furthermore, the mean annual

H+ concentrations over the eight-year period were

better correlated with SO42� than with NO3

� (Table 5),

suggesting that the decrease in sulphur emissions

Table 4

Annual average precipitation and SO42�, NO3

�, NH4+ and H+

concentrations for all sites during the period 1991–1998

Year Rainfall

(mm)

SO42� NO3

� NH4+ H+

(mmolc l�1)

1991 1133 21.90 14.77 20.53 13.98

1992 1274 19.19 8.79 18.43 10.42

1993 1288 23.20 13.64 22.52 14.35

1994 1413 18.97 12.28 17.19 10.89

1995 1216 17.36 10.81 20.25 10.10

1996 1162 22.20 13.85 25.38 14.06

1997 1261 15.48 12.49 23.13 7.92

1998 1451 18.23 9.37 20.32 10.48

Mean 1275 19.57 12.00 20.97 11.53

0

20

40

60

80

100

J J JF M MA A S O N D

J J JF M MA A S O N D

Sulphate

Nitrate

Ammonium

Hydrogen

Co

nce

ntr

atio

ns (µ

mo

l c

l–1)

0

5

10

15

20

25

30Sulphate

Nitrate

Ammonium

Hydrogen

De

po

sitio

n (m

mo

l c m

–2

yr–

1)

(a)

(b)

Fig. 5. Mean monthly (a) concentration (mmolc l�1) and (b)

deposition (mmolcm�2 yr�1) for SO4

2�, NO3�, NH4

+ and H+.

Table 5

Temporal correlation (pearson-product) between annual ion

concentrations, 1991–1998a

SO42� NO3

� NH4+ H+

SO42� 1

NO3� 0.559 1

NH4+ 0.267 0.509 1

H+ 0.984*** 0.627 0.317 1

aLevels of significance * 0.05, ** 0.01 and *** o0.001.

0

20

40

60

80

100

120

Precipitation Sulphate Nitrate Ammonium Hydrogen

100

Re

lative

ch

an

ge

(%

)

8893

113

85

Fig. 6. Relative change in precipitation volume and deposition

of SO42�, NO3

�, NH4+ and H+ (mmolcm

�2 yr�1) between 1991–

1994 and 1995–1998.


resulted in a decrease in H+ concentration of precipita-

tion.

The change in concentration as a function of time

using least-squares linear regression analysis was in-

vestigated (Table 6). As expected, the results indicate a

decreasing trend for SO42�, NO3

� and H+ and an

increasing trend for NH4+. However, the trends are

not statistically significant. This may be a result of the

limited data set used or the opposing trends at individual

sites; increasing trends for SO42� and H+ were found at

some sites on the west coast. Further trend analysis was

carried out on groupings of east coast (Hillsborough,

Roundwood and Turlough Hill) and west coast (Brack-

loon, Cloosh and Valentia Observatory) sites. A

significant decreasing trend in H+ concentrations

(r ¼ 0:73; p ¼ 0:03) was observed at east coast sites

(Fig. 7). In contrast, an increasing trend was observed at

the west coast sites (r ¼ 0:59; p ¼ 0:1; Fig. 7). Similar

opposing trends were observed for SO42� concentrations.

It is probable that H+ concentrations at east coast sites

reflect reductions in European emissions, whereas

increasing concentrations at west coast sites reflect the

increased economic growth in rural areas of Ireland over

the last five years. However, in general the largest

declines in emissions of sulphur and nitrogen in Europe

and North America occurred prior to the period 1991–

1998 and current changes in atmospheric concentrations

over Ireland are unlikely to be significant considering the

historically lower pollutant concentrations.

The average decrease in SO42� concentration for all

sites was 0.58mmolc l�1 yr�1, which is comparable to the

decrease for H+ (0.46 mmolc l�1 yr�1). Based on the

linear regression model (Table 6), concentrations of

SO42� decreased from 20.73 to 16.65mmolc l

�1 (E20%);

H+ decreased from 13.73 to 10.51 mmolc l�1, or pH

increased from 4.86 to 4.98 (E24%); NO3� decreased

from 13.35 to 11.68mmolc l�1 (E13%); and NH4

+

increased from 19.30 to 22.11 mmolc l�1 (E15%).

Changes in the deposition rate of SO42�, NO3

�, NH4+

and H+ with time followed the same pattern as the

changes in ion concentrations (Figs. 8a and b). During

1994–1995 there was a crossover in concentration and

deposition between SO42� and NH4

+, with NH4+ becom-

ing the dominant ion after that period. The opposing

trend observed for NH4+ reflects the increase in

emissions from local agricultural activities. As such,

NH4+ is the parameter of most concern as the emissions

responsible are unlikely to decrease for some time.

A number of researchers have investigated long-term

trends in the precipitation chemistry in different

countries from central and western Europe and from

North America (Dillon et al., 1988; Lynch et al., 1995;

Avila, 1996; Puxbaum et al., 1998). In general, most

authors detected decreasing trends in the SO42� concen-

trations as well as an increase in pH over the

investigation period. For Spain, Avila (1996) found a

19% decrease in SO42� concentrations over a five-year

period; in Austria, Puxbaum et al. (1998) found a 33%

decrease over a 10-year period; in Canada, Dillon et al.

(1988) found a 29% decrease over a 10 year period; and

in the USA, Lynch et al. (1995) found a 28% decrease

over a 13-year period. Most studies, in general, found no

change or a decrease in NO3� concentrations, and no

change or an increase in NH4+ concentrations over the

period of investigation. The results for Ireland are not as

significant but nevertheless indicate a decrease in SO42�,

NO3� and H+ concentrations and an increase in NH4

+

concentrations over the period 1991–1998.

Despite the relatively small changes in atmospheric

concentrations of sulphur and nitrogen, current atmo-

spheric depositions of pollutants to Ireland are low.

Comparison of deposition fluxes with critical loads of

sulphur and nitrogen estimated on a 50 km grid basis

(Posch et al., 1999; Aherne and Farrell, 2001) indicated

that critical loads are not exceeded at any of the

monitoring sites. However, accurate quantification of

5

10

15

20

25

30

1990 1992 1994 1996 1998 2000

y = –1605 + 0.81xy = +3348 – 1.67xEast

West

Hyd

roge

n (µ

mol

c l–1

)

Time (years)

Fig. 7. Long-term trends in H+ concentration (mmolc l�1) at

east and west coast sites, 1991–1998.

Table 6

Relationshipa between annual concentrations of SO42�, NO3

�,

NH4+ and H+ (mmolc l

�1) and time (yr) using least-squares

linear regression

Ion Slope se Intercept r p

SO42� �0.58 0.37 20.70 0.54 0.17

NO3� �0.24 0.35 12.37 0.27 0.52

NH4+ +0.40 0.41 18.71 0.37 0.36

H+ �0.46 0.34 13.61 0.48 0.23

a Ion concentration=slope� year+constant.


exceedance requires total (wet plus dry) deposition

fluxes.

4. Conclusions

The level of anthropogenic acidity in precipitation

over Ireland was low compared to levels for other

European countries. The prevailing wind is south-

westerly originating over the Atlantic Ocean, and the

associated precipitation contains concentrations of

pollutants at background levels. Therefore, the domi-

nant ions in precipitation were of marine origin.

Nevertheless, pollutant concentrations on the east coast

were considerable. There was a significant correlation

between SO42�, NO3

� and NH4+ in precipitation. Mean

depositions of sulphur and nitrogen in precipitation

were E30 ktonnes S yr�1 and 48 ktonnes N yr�1 during

the five-year period 1994–1998. Nitrogen input is

dominated by NH4+ deposition, which was approxi-

mately double that of NO3�. Both ion concentrations

and depositions showed great seasonal variation. The

variation in concentration was inversely related to the

precipitation volume. The highest depositions occurred

between spring and summer even though the volume of

precipitation was low during this period. In contrast, the

high precipitation volumes during autumn (October) did

not produce high ion depositions. Trend analysis of

concentrations in precipitation indicate a decrease in

SO42�, NO3

� and H+ concentrations and an increase in

NH4+ concentrations over the eight-year period 1991–

1998.

Acknowledgements

The authors acknowledge the Environmental Protec-

tion Agency, Electricity Supply Board, Met !Eireann,

AEA Technology plc, Department of Agriculture

and Rural Development Northern Ireland and the

Chemical Co-ordinating Centre of EMEP for providing

data.

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