Chemical Composition of Rainwater Near Two Historical Monuments The Thann Collegiate (Alsace, France) and the Tours Cathedral (Indre et Loire, France)

CHEMICAL COMPOSITION OF RAINWATER NEAR TWOHISTORICAL MONUMENTS: THE THANN COLLEGIATE

(ALSACE, FRANCE) AND THE TOURS CATHEDRAL(INDRE ET LOIRE, FRANCE)

MAURICE MILLET1∗, HENRI WORTHAM1, PHILIPPE MIRABEL1,JEAN-PAUL FLORI2, DOMINIQUE LAKKIS3 and MAURICE LEROY3

1 Equipe de Physico-chimie de l’Atmosphère du Centre de Géochimie de la Surface (UMR 7517) etDépartement de Chimie de l’Université Louis Pasteur, 1 rue Blessig, 67084 Strasbourg Cedex,

France; 2 Centre Scientifique et Technique du Batiment, Aérodynamique et EnvironnementClimatique – 11, rue Henri Picherit, 44323 Nantes Cedex 03, France; 3 Laboratoire de ChimieMinérale et Analytique de l’Ecole de Chimie, Polymères et Matériaux – 1, rue Becquerel, 67000

Strasbourg-Cronenbourg, France(∗ author for correspondence, e-mail: [email protected])

(Received 7 April 1999; accepted 11 October 2000)

Abstract. In the present study, included within the framework of a Franco-German Research Pro-gram for Conservation of Historical Monuments, the chemical composition of rainwater was investig-ated in Thann (Alsace, France) and in Tours (Indre et Loire, France) between 1992 and 1993. A totalof 78 and 24 samples, respectively, were collected, near the Thann collegiate and the Tours cathedralon a weekly basis and analysed for Cl−, NO−

3 , SO2−4 , Na+, NH+

4 , K+, Mg2+ and Ca2+. Resultsshow pH ranged from 3.60 to 6.58 and from 5.49 to 7.01 for Thann and Tours, respectively. In Thann,SO2−

4 is the major anion and rainwater acidity is neutralised by compounds of terrigenic origin which

come from the soil of the upper Rhine valley (Ca2+; ‘loess’) and the potash mines of Alsace. InTours, SO2−

4 is also the major anion and the acidity is neutralised partially by atmospheric ammoniaand partially by compounds of terrigenic origin and by dust from the erosion of the cathedral and theworks of restoration, in the form of CaCO3.

Keywords: historical monuments, major ions, neutralisation, rainwater, terrigenic influence

1. Introduction

Since the advent of the industrial civilisation, and especially during the last decadesatmospheric pollution has increased in considerable proportions in all industrial-ised countries. Besides, it has become a current subject of study in recent years,notably with the action of the CFCs in the destruction of the ozone layer, theaction of the CO2 in the modification of the Earth’s radiative budget balance bythe greenhouse effect or by the phenomenon named ‘acid rain’. These acid rainshave a share of responsibility in the decline of forest (Binns, 1985; Casado et al.,1989), the acidification of lakes and soils (Melack et al., 1985; Mosello et al.,1985) and on the deterioration of buildings and historical monuments (Camuffo,

Water, Air, and Soil Pollution 132: 105–126, 2001.© 2001 Kluwer Academic Publishers. Printed in the Netherlands.

106 M. MILLET ET AL.

1992). For this reason, in many European countries (Casado et al., 1989; Colin etal., 1989; Samara et al., 1992; Ghannouchi, 1993; Sanusi et al., 1996), in Canada(Chan et al., 1987), in the United States (Kessler et al., 1982; Saylor et al., 1992)and in Japan (Yamaguchi et al., 1991), acid rains were, and remain today the sub-ject of several research programs. In France for example, an important researchprogram known as the DEFORPA Program (‘DEpérissement FORestier attribué àla Pollution Atmosphérique’) has been carried out (Dubois et al., 1986; Derexel etal., 1989; Ghannouchi, 1991; Sanusi et al., 1996).

Studies conducted on acid rains over the last few years increased the interestthat existed in the action of atmospheric pollution on the degradation of stone ingeneral and the degradation of buildings and historical monuments in particular.These degradations are induced by many kinds of pollutants and catalytic phe-nomena which act by synergy. For example, carbonated particles originating fromcombustion of fossil fuels play a very important role in the formation of blackcrusts on marble; this is due to their catalytic properties and to the adsorptionof gaseous atmospheric pollutants (Camuffo et al., 1982). In the past, when thepollution rates were weak, biological action was the major process of deterioration.Today, atmospheric pollution has destroyed lichens and algae but the deteriorationrate has increased due to both physical and chemical alterations which take place onweakened surfaces (Del Monte, 1989). Therefore, the problem of the degradationof historical monuments must not be attributed to one or several compounds butto a body of phenomena and their synergy. Thus, studies have been undertaken toevaluate the impact of dry deposition of compounds such as NOx , SO2 and HClor carbonaceous particles (Johnson et al., 1990; Haneef et al., 1992; Hutchinson etal., 1992; Kirkitsos and Sikiotis, 1995; Ausset et al., 1996; Sabbioni et al., 1996)on calcareous stone in laboratory chamber exposition and on marble (Delopoulouand Sikiotis, 1992; Sikiotis and Delopoulou, 1992). Researches on the impact ofwet deposition on these same materials have also been made (Cheng et al., 1987;Johnson et al., 1990; Del Monte and Rossi, 1992; Del Monte and Rossi, 1997).

Within the framework of the Franco-German multidisciplinary Research Pro-gram for the Conservation of Historical Monuments (Freiherr von Welck and Ro-biolla, 1990; Freiherr von Welck, 1991), we investigated the micrometeorologicalparameters and the chemical composition of rainwater near the Thann collegiate(Alsace, France) and the Tours cathedral (Indre et Loire, France), which are monu-ments chosen by the program. Between 1992 and 1993, we measured temperature,relative humidity, wind direction, and wind speed. During the same period wecollected rainwater and analysed them for major anions (Cl−, NO−

3 , SO2−4 ) and

cations (Na+, NH+4 , K+, Mg2+ and Ca2+). The aim of this study was to identify

the origin of the ionicspecies found in the rainwater fallen near the two historicalmonuments. This step was necessary for the evaluation of the potential role of wetdeposition in the degradation process of historical monuments.

CHEMICAL COMPOSITION OF RAINWATER 107

2. Materials and Methods

2.1. SAMPLING SITES

Rain events were collected at two sites some 800 km apart. The first site was locatedin Thann (Alsace North-East France), a small town (15 000 inhabitants) having inits vicinity an important titan oxide factory about 1 km to the East and seven potashmines about 3–7 km to the South-East and North-East. The rainwater collectorwas placed at ground level in an open field at the same site as the climatic stationof the C.S.T.B. (‘Centre Scientifique et Technique du Batiment’). This site wassituated near the railway station and behind the cemetery about 500 m to the Eastof the collegiate. This site was chosen because turbulence induced by buildings isnegligible.

The second site was situated in the Loire valley (Western France) in Tours(150 000 inhabitants). This medium size town is the industrial centre of its regionand is situated 150 km from the Atlantic ocean. The climatic station was placedin the campanile of the cathedral and the rain collector was set directly on the topof the nearby cloister. Here, turbulence induced by neighbouring buildings and thecathedral itself was minimised.

2.2. SAMPLING METHODS

At the two sites, rainwater samples were collected on a weekly basis by a wet onlyprecipitation collector (Précis Mécanique, France) in polyethylene sampling bottlespreviously washed in an acidic bath and rinsed with water of Milli-Q quality. Thiscollector is equipped by a rain sensor which permits, when rain occurs, the openingof the collection funnel and the closing of it when rain stops. The periodicity ofsampling was chosen because of the distance separating the sampling sites andour laboratory and because one week is the maximum duration compatible with aquantitative information on the chemical composition of rainwater (Galloway andLikens, 1978). Samplings were carried out from January 1992 to December 1993in Thann and from September 1992 to September 1993 in Tours.

2.3. MICROMETEOROLOGICAL PARAMETER MEASUREMENTS

In Thann and Tours, wind direction and velocity as well as ambient temperature,and relative humidity were measured. The direction and velocity of the wind weremeasured 15 m above ground level using a wind-propeller vane of Gill type by R.M. Young. Uncertainties were ±5% for wind speed and ±5◦ for wind direction.

Temperature was measured with a Pt-probe in an aerated shelter of meteorolo-gical type set at 1.5 m above ground level with an uncertainty of ±0.1%. Relativehumidity was measured in the same aerated shelter used for temperature measure-ment by means of an hair-hygrometer. The uncertainties for high humidity levels


were about 5% and tended to increase slowly as the humidity decreased. There-fore, a wet bulb thermometer was also used to improve the measurement of thehair-hygrometer for low humidity and to correct fluctuations observed with it.

Uncertainties of the wet bulb thermometer were about 5% but they increasedwith humidity and decreased with temperature (Flori, 1994).

All these parameters were recorded and stored in an acquisition station typemicro-mac (Analog Device) on an hourly basis. Periodically, data was transferredto a portable computer.

2.4. ANALYTICAL PROCEDURES

Upon receipt in the laboratory, each rain sample was filtered through a 0.45 µmpore diameter membrane filter and pH was measured using a combined glass elec-trode calibrated with pH = 4.00 and pH = 7.00. Then, 0.5% chloroform was addedto the samples in order to prevent biological decomposition. Finally, samples werestored in the dark at 4 ◦C before chemical analysis made within one month (Milletet al., 1995).

2.4.1. Anion AnalysisCl−, NO−

3 and SO2−4 were analysed by a Dionex 4500i Ion Chromatograph equipped

with a AG4ASC guard column, a AS4ASC column and an anion chemical sup-pressor AMMS. A CO2−

3 /HCO−3 buffer (1.7 mM NaHCO3/1.8 mM Na2CO3) at

2 mL min−1 was used as the eluent and 25 mM H2SO4 at 3–5 mL min−1 as theregenerant. Detection limits were 0.02, 0.05 and 0.05 mg L−1, respectively, whilethe analytical variability for all three anions was about 8%.

2.4.2. Cation AnalysisNa+, K+, Mg2+ and Ca2+ were analysed using two methods: Inductively CoupledPlasma/Atomic Emission Spectroscopy (ICP/AES) and Energy Dispersive X-RayFluorescence (EDXRF). Results obtained by the two methods were similar (F test)given a ±10% uncertainty. Detection limits were: 0.05 mg L−1 for Na+ and K+,0.15 mg L−1 for Mg2+ and Ca2+. The analytical variability for all cations was about10%.

2.4.3. Ammonium (NH+4 ) Analysis

NH+4 was analysed by Ion Chromatography (Dionex 4500i) equipped with a CG10

guard column, a CS10 column and a cationic chemical suppressor CMMSII.A HCl 40 mM/DAP 12 mM at 1 mL min−1 was used as the eluent and TBAOH

40 mM at 5–10 mL min−1 as the regenerant. The detection limit was 0.05 mg L−1

with about 5% of uncertainty.


2.5. STATISTICAL ANALYSIS AND VALIDATION OF RESULTS

In order to determine the possible links between components and to identify theirpossible sources, statistical analysis (correlation matrix) was applied to the datafrom both stations.

In order to validate the ion analysis, results were examined in terms of ion bal-ance. To be valid, the percentage difference of the ion balance (PDI), calculated byEquation (1), must be lower than 20% (Fuzzi et al., 1996). Only samples meetingthis criterion were included in further discussion.

PDI = �anions − �cations

�(anions + cations)× 100 . (1)

The regression line of ion balance (Figure 1) shows an average deviation fromperfect equivalence lower than 15%. Melack et al. (1985) and Joos and Baltensper-ger (1991) estimated that a quantitative balance was achieved when the deviationvaried between 0 and 15%. Since our deviation remained within this range limit,we considered all the major ions had been analysed. Nevertheless, the majorityof the samples presented an anion deficit, mainly due to the non analysis of somecompounds such as carbonate and short chain organic acids (Dikaiakos et al., 1990;Samara et al., 1992).

3. Results and Discussion

3.1. MICROMETEOROLOGICAL DATA

Mean values, minima and maxima of temperature, relative humidity and windspeed measured in Thann and Tours are presented in Tables Ia–c. Temperaturesmeasured in Thann in 1992 are comparable to those measured in 1993 and withdata collected during the same period by the meteorological station of Colmar-Meyenheim airport (Flori, 1994) situated 25 km North East of Thann (Figure 2).Relative humidity measured for the two years of measurements are also compar-able.

For wind direction, the predominance in Thann is from the West (59.3%) (seeFigure 3) because wind is tunneled by the mountain towards the valley (Figure 2).In the Alsace and Vosges regions these westerly winds induces a mechanism oflong distance transport of pollutants. However, easterly directions (14%) (North–East (15.3%), South–East (11.5%)) directions and calm (9%) periods were alsoobserved (Figure 3). In the Alsace and Vosges regions, easterly winds have previ-ously been described by several authors as polluted air mass (Colin et al., 1989;Probst et al., 1992; Sanusi et al., 1996). In Thann, these winds carry compoundssuch as ‘loess’ from the Rhine valley and potash mine dumps as discussed below.


TAB

LE

Ia

Mea

nva

lue,

min

ima

and

max

ima

ofte

mpe

ratu

re,r

elat

ive

hum

idit

yan

dw

ind

spee

dm

easu

red

inT

hann

(Fra

nce)

in19

92

Jan.

Feb.

Mar

chA

pril

May

June

July

Aug

.Se

pt.

Oct

.N

ov.

Dec

.

Tem

pera

ture

(◦C

)

Mea

n0.

22.

86.

29.

116

.816

.419

.122

.514

.27.

86.

92.

3

Min

.–m

ax.

–9.6

–14

.2–9

.5–

15–0

.8–

16.2

–1.4

–24

.46.

8–

28.8

5.7

–28

.410

.8–

3310

.8–

33.8

3.1

–24

.8–1

.8–

18.4

–0.5

–16

.9–9

.8–

13.5

Rel

ativ

ehu

mid

ity(%

)

Mea

n87

8576

6868

7471

/78

8286

86

Min

.–m

ax.

42–

100

42–

100

30–

100

21–

9722

–10

031

–10

030

–97

/35

–97

41–

9940

–10

040

–10

0

Win

dsp

eed

(ms−

1)

Mea

n2.

52.

73.

32.

93.

22.

62.

83.

22.

92.

62.

62.

3


TAB

LE

Ib

Mea

nva

lue,

min

ima

and

max

ima

ofte

mpe

ratu

re,r

elat

ive

hum

idit

yan

dw

ind

spee

dm

easu

red

inT

hann

(Fra

nce)

in19

93

Jan.

Feb

.M

arch

Apr

ilM

ayJu

neJu

lyA

ug.

Sep

t.O

ct.

Nov

.D

ec.

Tem

pera

ture

(◦C

)

Mea

n4.

2–0

.25.

310

.914

.016

.4/

/12

.77.

91.

0/

Min

.–m

ax.

–12.

7–

15.6

–9.6

–13

.3–5

.3–

20.7

–0.7

–27

3–

29.2

7.1

–29

.2/

/3.

6–

25.7

–1.2

–20

–9.3

–11

/

Rel

ativ

ehu

mid

ity

(%)

Mea

n81

8465

6469

73/

/80

8692

/

Min

.–m

ax.

39–

100

35–

100

20–

100

30–

9230

–98

33–

96/

/40

–97

47–

100

61–

100

/

win

dsp

eed

(ms−

1)

Mea

n2.

12.

82.

92.

72.

4/

/2.

62.

01.

9/

/


TAB

LE

Ic

Mea

nva

lue,

min

ima

and

max

ima

ofte

mpe

ratu

rean

dre

lativ

ehu

mid

ity

mea

sure

din

Tour

s(F

ranc

e)in

1992

and

1993

June

1992

July

1992

Aug

.199

2S

ept.

1992

Oct

.199

2N

ov.1

992

Dec

.199

2Ja

n.19

93F

eb.1

993

Mar

ch19

93

Tem

pera

ture

(◦C

)

Mea

n17

.617

.718

.214

.49.

63.

76.

65.

95.

99.

6

Min

.–m

ax.

9.2

–28

.47.

9–

28.1

7.5

–30

.85.

6–

26.2

2.0

–19

.6–6

.8–

18.2

–2.8

–13

.4–5

.3–

13.5

–4.0

–18

.80.

7–

21.4

Rel

ativ

ehu

mid

ity

(%)

Mea

n73

7263

7783

8788

8987

80

Min

.–m

ax.

28–

100

29–

9626

–96

35–

9747

–97

49–

9763

–98

62–

9844

–98

38–

97


Figure 1. Ion balance for rainwater samples from Thann and Tours. The dashed line represents theregression slope (r = 0.96).

The moderate wind speed observed in Thann (Tables Ia and b) is induced by thelocalisation of this town, at the foot of the Vosges Mountains, which protects thissite from synoptic air mass movements (Figure 2).

Data for Tours are not complete because many technical problems occurredduring measurement campaigns. Wind speed and direction are mainly under theinfluence of South West Oceanic depression systems (Figure 3). Temperature andrelative humidity values obtained on the campanile are comparable to those dir-ectly measured indoors close to the stained-glass windows, the inertial effect of thebuilding itself is important on temperature and humidity fluctuations.


Figure 2. Localization map of studied area.

3.2. CHEMICAL COMPOSITION OF PRECIPITATION

In Table II, the median and the range of equivalent ionic concentrations measuredin rainwater collected in Thann and Tours from 1992 to 1993 are summarised. Fromanions, SO2−

4 clearly dominate in the mean and maximum concentrations at bothsites. For cations, the situation is less clear. In Thann, Ca2+ is the major elementfollowed by NH+

4 and Na+ which have about the same median concentrations.


Figure 3. Wind directions (in %) recorded in Thann (a) and Tours (b) during the sampling period.

In Tours, Ca2+ is strongly dominant followed by Na+ while NH+4 is the least

concentrated. The dominance of Ca2+ and NH+4 depends on the location of the

station. In urban areas, Ca2+ clearly dominates whereas in rural areas NH+4 is the

more abundant cation (Sanusi et al., 1996).On the other hand, except for the specific case of Ca2+ discussed below and

for Na+ and Mg2+ which present similar concentrations at the two sites, we notethat other elements (Cl−, NO−

3 , SO2−4 , NH+

4 , K+) are more concentrated in Thannthan in Tours. This result is not in accordance with previous work which show that


TABLE II

Dissolved major ions (in µeq L−1) and pH of wet only deposition sampled in Thann andTours (France) between 1992 and 1993 and comparison with other studies

pH Cl− NO−3 SO2−

4 Na+ NH+4 K+ Mg2+ Ca2+

Thann (Alsace, France), January 1992 – December 1993 (N = 78)

Median 5.16 33 34 92 39 49 22 14 66

Minimum 3.60 1 3 25 1 5 7 1 14

Maximum 6.58 1720 254 1177 1825 2915 343 86 608

Sewen (Haut-Rhin, France), October 1991 – March 1992 (N = 9) Sanusi et al. (1996)

Median 4.90 17 26 30 15 38 4 3 5

Minimum 4.10 4 6 9 3 5 1 1 1

Maximum 6.00 54 84 93 64 184 41 8 21

Colmar (Haut-Rhin, France), October 1991 – March 1992 (N = 19) Sanusi et al. (1996)

Median 5.70 167 78 147 70 140 83 16 166

Minimum 4.40 6 4 23 2 13 2 2 6

Maximum 7.90 956 404 926 674 696 553 97 1623

Strasbourg (Bas-Rhin, France), October 1991 – March 1992 (N = 20) Sanusi et al. (1996)

Median 5.00 71 68 128 36 70 24 19 101

Minimum 3.80 0 9 28 4 12 1 3 10

Maximum 6.80 315 374 617 211 381 196 86 370

Tours (Indre et Loire, France), September 1992 – September 1993 (N = 24)

Median 6.29 24 12 68 45 8 15 13 232

Minimum 5.49 8 n.d. 30 26 n.d. 4 4 45

Maximum 7.01 156 89 235 351 150 149 88 1557

generally, ionic concentrations are correlated to human activity and to the size ofthe town (Sanusi et al., 1996). Nevertheless, in the present work, results obtainedare a consequence of two phenomena:

• First: Thann is a small town but there is a high level of human activity withtwo major polluting industries (see above).

• Second: The meteorological situations of the two towns are totally different.Thann is situated in a narrow valley where the wind velocity is low. Moreover,


Thann is far from the ocean (about 800 km) and the air masses reaching this sitehave already passed over polluted regions. On the other hand, Tours is situatedin a large valley with higher wind velocity. Finally, the prevailing winds comefrom the West and bring clear air masses from the ocean.

3.3. COMPARISON WITH OTHER STUDIES

Concentrations obtained for Thann can be compared with the data of an otherstudy (Table II) conducted in the same area (Sanusi et al., 1996) in 1991 and1992, especially data from Sewen (situated in the south of Thann at 500 m height),Colmar (situated in the North of Thann; 200 m height) and Strasbourg (situated inthe North of Colmar; 140 m height). The two last sites are situated in the UpperRhine Valley while Sewen is situated in the Vosges mountains. As shown, medianconcentration of all ions in rain follow the order: Colmar > Strasbourg > Sewen.In contrast, the acidity of rain is highest in Sewen and lowest in Colmar. Rain inSewen presents lower concentrations especially for Ca and K in comparison withother sites. The high concentrations obtained in Colmar, Strasbourg and Thannmight be explained by the high level of human activities and industries aroundthe towns and by the relatively low amount of rainfall in urban areas which doesnot favour dilution of pollutants. Higher levels of Ca and K observed in Colmarand Strasbourg are the result of the localisation of these towns in the Rhine UpperValley, which are delivered through wind conditions high quantities of compoundsof terrigenic (natural and anthropic) origin (see after). Indeed, even if west winds(Derexel, 1991) are predominant, North East (see Section 3.1) and South Eastwinds were also frequently observed. These easterly directions favour dispersionof terrigenic compounds in the urban areas situated in the valley while Vosgesmountains constitute a barrier for them.

3.4. PRECIPITATION ACIDITY

The minimum pH values recorded in Thann and Tours are respectively 3.60 and5.49 while the median pH values are 5.16 and 6.29 (Table II). These values areclose to or higher than the reference value of 5.60 (Charlson and Rodhe, 1982)despite the SO2−

4 and NO−3 which originate among others from the dissociation of

sulphuric and nitric acids. Considering the presence of these acids, the recorded pHcan only be obtained by neutralisation with basic components such as NH3, basicaeolian dust, and fly ash components (CaCO3, silicates, ...) (Sequeira, 1981–1987).

Indeed it has been shown that pH values recorded in rural areas with low terri-genic influence are acidic while highest pH values were observed in urban areaswhere human activities generate pollution and therefore acidity (Sanusi et al.,1996). This remarkable result was attributed to high influence of neutralised com-ponents in urban areas. For Tours, alkaline pH values might be explained by thehigh concentrations of neutralised components (Ca2+) in opposition with the mod-erate level of acidic components (SO2−

4 , ...).


TABLE III

Ion pair correlations between major ions analysed in rainwater collected in Thann in 1992 and1993 (N = 78). Significant correlations are bold faced (p < 0.05)

Cl− NO−3 SO2−

4 H+ Na+ NH+4 K+ Mg2+ Ca2+

Cl− 1.000

NO−3 0.510 1.000

SO2−4 0.614 0.610 1.000

H+ –0.349 –0.164 –0.315 1.000

Na+ 0.907 0.461 0.609 –0.469 1.000

NH+4 0.608 0.740 0.186 –0.072 0.631 1.000

K+ 0.579 0.130 0.608 –0.077 0.538 0.416 1.000

Mg2+ 0.877 0.774 0.472 –0.091 0.715 –0.267 0.179 1.000

Ca2+ 0.710 0.806 0.592 –0.172 0.705 0.639 0.617 0.835 1.000

TABLE IV

Ion pair correlations between major ions analysed in rainwater collected in Tours in 1992 and 1993(N = 24). Significant correlations are bold faced (p < 0.05)

Cl− NO−3 SO2−

4 H+ Na+ NH+4 K+ Mg2+ Ca2+

Cl− 1.000

NO−3 0.587 1.000

SO2−4 0.868 0.868 1.000

H+ –0.433 –0.433 –0.420 1.000

Na+ 0.915 0.615 0.512 –0.432 1.000

NH4+ 0.612 0.712 0.745 –0.363 0.668 1.000

K+ 0.406 0.406 0.395 –0.462 0.621 –0.091 1.000

Mg2+ 0.935 0.935 0.945 –0.290 0.782 0.453 0.237 1.000

Ca2+ 0.529 0.529 0.861 –0.324 0.863 0.758 0.351 0.687 1.000

We note for both sites, that no any ion is significantly correlated with H+ con-centrations (Tables III and IV) which seems to indicate that the acid-base balancein our samples is the result of complex neutralisation processes.

3.5. ORIGIN OF Ca2+

We note that the highest concentrations of Ca2+ are found in Tours. A first explan-ation is that Ca2+ may be of anthropogenic origin. For example it could be emittedby human activities such as automobile traffic, cement work, etc. (Sequeira, 1993).


The large turbulence generated by buildings enhance the spreading of aerosols inthe atmosphere.

A second possibility is revealed by the study of the soil composition in the Rhineand Loire valleys. This revelation indicates that the upper Rhine valley is composedof ‘loess’ which is rich in Ca2+. Because of its formation by freezing-defreezingprocesses, the loess consists of many very fine dust particles easily spread by thewind in the Alsatian plains where Thann is situated. A similar phenomenon oc-curred in the lower Loire valley because the soil consists of ‘Tuffeau’ which is alsorich in Ca2+. The consequence of loessical effect and calcareous aerosol on rainconcentrations has previously been studied by several authors (Meszaros, 1966;Sequeira, 1993).

The sum of these two first phenomena (human activities and soil composition)can explain the moderate concentrations observed in Thann (mean of 85 µeq L−1)but seems insufficient to explain those measured in Tours (mean of 358 µeq L−1).A third source of Ca2+ in Tours could be the cathedral itself because as presentedpreviously, the rain collector was set directly on the cathedral which is made ofTuffeau. So, dust generated by the erosion of this building and by the restorationwork can be integrated into the rainwater and increased the Ca2+ concentrations.A similar hypothesis has been put forward in Mechelen (Belgium) to explain thehigh Ca2+ concentrations (Roekens et al., 1988).

3.6. ORIGIN OF Na+, Cl− AND Mg2+

Na+ in rain is generally considered a tracer for the marine source. However, asmall contribution of terrigenic origin may arise due to the scavenging of crustalaerosols. This contribution can be estimated using a relationship between the so-dium concentration and a tracer element of purely crustal origin such as Al. But,the enrichment factors are frequently low and so negligible (Colin et al., 1989;Sanusi et al., 1996). Assuming that all Na+ in rain originates from the sea, we canevaluate the marine contribution to an element X by calculating the [X]/[Na] ratio.For sea water, the ratio [Cl−]/[Na+] is 1.17 with Cl− and Na+ expressed in µeq L−1

(Riley and Chester, 1971). In atmospheric water, a ratio of 1.17 is generally onlyobtained in rainwater collected close to the sea. Far inland and in the absenceof Cl− emission, the ratio is generally less than 1.17 which can be explained bythe presence of sodium of terrigenic origin. However, as discussed previously thisphenomenon is generally of minor importance. Another possibility would be toconsider the replacement of Cl− by NO−

3 or SO2−4 (Robbins et al., 1959; Eriksson,

1960). If this mechanism took place in Tours, then a significant association betweenNa+ and NO−

3 or SO2−4 would be expected due to the potential formation of NaNO3

and Na2SO4. However, correlation coefficients found in Table IV do not show suchassociation. By this means, it is not possible to explain the Cl−/Na+ ratio of lessthan 1.17 observed in the majority of rainwater samples in Tours (Figure 4) anda non seasalt origin for Na+ could be hypothesized but not determined actually.


Nevertheless, the high statistical correlation obtained in Tours between Cl− andNa+ (Table IV) indicates that Cl− comes essentially from the ocean.

In Thann which is far from the ocean the Cl−/Na ratio is very close to andsometimes even higher than 1.17 (Figure 5). Considering that the importance ofthe two previous phenomena increases with the distance from the coast, the Na+and Cl− concentrations seem to be the result of local emissions and, because of thehigh correlation between these two ions (Table III), they must have the same origin.These sources could be the waste dumps of the potash mines situated in the vicinityof our station (about 5 km away) composed of over 80% of NaCl plus a few percentof other salts such as KCl (Kuzio, 1998). This local NaCl source has a Cl−/Na+ratio close to 1.17 which explains the Cl−/Na+ ratio observed in rainwater despitethe distance from the ocean. The presence of NaCl in the atmosphere due to potashmine waste dumps is important for our study on the evaluation of the potential roleof wet deposition in the degradation of old monuments. Indeed, it has been shownrecently that aerosol of marine origin increase the degree of corrosion in stonebuildings due to wet and dry deposition processes (Moropoulou et al., 1994; Zezza,1989, 1994; Zezza and Macrì, 1995). Thus, it would be interesting in the futureto undertake a more complete study of the atmosphere near the dumps and thecollegiate, especially aerosol characterization, in order to confirm this observation.

A similar approach can be used for the study of Mg2+ because as Cl−, Mg2+ isgenerally of sea origin. Mg2+ has a weak crustal origin which increases slightlythe Mg2+/Na+ ratio in rainwater (Crawley and Sievering, 1986). In Tours, theMg2+/Na+ ratio is close to that found in sea water (Figure 4). This result is notsurprising for a station 150 km from the ocean because as discussed previously theenrichment of Na+ of crustal origin is of minor importance. Moreover, replacementof Mg2+ by other cations does not occur. This result is corroborated by a highcorrelation of Mg2+ with Na+ (0.782) and Cl− (0.935) which have previously beenidentified as compounds of sea origin.

In Thann, the Mg2+/Na+ ratio in rainwater is close to the one of the ocean(Figure 5). Nevertheless, it has been demonstrated for this site that the waste dumpsof the potash mines emitted a large amount of Na+. So, the Mg2+ found in rain-water comes not only from the ocean. Since the Na+ and Mg2+ concentrations areproportional in 76 of the 78 rain samples and exhibit a high correlation (0.715),these two cations have the same origin and Mg2+ comes both from the ocean andthe dumps of the potash mines. This result is confirmed by the high correlationbetween Mg2+ and Cl− (0.877) which is another compound identified as com-ing both from the potash mines and the ocean. Unfortunately, on account of thelack of knowledge concerning the contribution of the dumps of the potash mineson the Na+ concentration, we have no way to compute the importance of eachcontribution on Mg2+ concentration.


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3.7. ORIGIN OF NO−3 , SO2−

4 AND NH+4

We can attribute the presence of nitrate and ammonium ions in the rain samplesto a direct input of gaseous nitric acid and ammonia as well as to an input ofnitrate and ammonia containing aerosols such as NH4NO3, Mg(NO3)2, etc. Withthe available data, we have no way to compute the relative importance of the twoinputs but the high correlation of NO−

3 with NH+4 and Mg2+ obtained for both

stations (Tables III and IV) indicates a significative contribution of these aerosols.On the other hand, we can note that NO−

3 is significantly correlated to Ca2+ inThann while the correlation is not significant in Tours (Tables III and IV). Fromthis observation we can hypothesize that neutralization of HNO3 dissolved in rainwith CaCO3 is likely to occur in Thann.

However, the problem is slightly more complicated for sulphate since it also hasa partial marine origin. This marine contribution can be calculated as previouslydescribed using Na as tracer of the marine source. For sea water, the SO2−

4 /Na+is 0.12 with SO2−

4 and Na+ expressed in µeq L−1 (Riley and Chester, 1971). Inwhichever rainwater sample, from Tours or Thann, the SO2−

4 /Na+ ratio is alwayslarger than 0.12 (Figures 4 and 5). Nevertheless, on account of terrestrial emissionof Na+ in Thann, the explanation of these phenomena cannot be identical for bothsites.

(a) Tours: Na+ is mainly of marine origin. Thus, the SO2−4 /Na+ ratio larger than

0.12 indicates that the marine contribution to SO2−4 is either weak or negligible.

Indeed, in mean, from the 87 µeq L−1 measured in rainwater only 9 µeq L−1

come from the ocean. This result is corroborated by the absence of a significantcorrelation between SO2−

4 and Na+ (Table IV). SO2−4 is, therefore, essentially

of anthropogenic origin. The significant correlation of SO2−4 with NH+

4 sug-gests neutralization reactions involving atmospheric ammonia. In addition, thestrong association between SO2−

4 and Ca2+ suggests reaction of SO2 with Ca-containing particles in the atmosphere, as previously reported by other authors(Sequeira, 1982; Seinfield, 1986; Saxena et al., 1996), or direct emission ofCaSO4 particles from erosive processes, industrial activities, ...

(b) Thann: The results are less clear. Assuming that Na+ has a local origin wecannot compute the SO2−

4 marine contribution. Despite this difficulty, we notea significant correlation of SO2−

4 with Na+, K+ and Ca2+, and a lack of cor-relation with NH+

4 (Table III). Nevertheless, ammonium sulphate is generallyconsidered as the predominant form of sulphate aerosol in the atmosphere(Junge, 1963; Seinfield, 1986). If we consider that SO2−

4 in rain comes mainlyfrom an input of sulphuric acid, possibly emitted by the titan oxide factorywhich uses H2SO4 in the fist step of the production process to oxidise theore, an association of SO2−

4 and NH+4 would have been observed since H2SO4

is rapidly neutralized by atmospheric NH3. In addition a reverse associationbetween SO2−

4 and H+ would be expected. This is not the case as shownin Table IV and the significant association of SO2−

4 and Na+, K+ and Ca2+


could suggest direct sulphate salt emission from the factory (e.g. as a result ofdesulphurisation) sulphation processes occuring on soil dust or particles whichare emitted from the dumps of the potash mines.

4. Conclusions

In the framework of the Franco-German Research Program for the Conservation ofHistorical Monuments, a study of rainwater samples collected near two monuments(the Thann collegiate and the Tours cathedral, France) was conducted between1992 and 1993 in order to determine the chemical composition of rainwater in con-tinental and oceanic areas and to determine the pollution origins. The mechanismsof acid neutralisation in these two kinds of region seem to be quite different. Neut-ralisation of SO2−

4 with NH3 is important for the rain in Tours but not for the rainin Thann, although the latter has a higher NH+

4 content. In both sites, calcareousparticles of anthropogenic origin seem to be major bases. In Tours neutralisationis obtained from soil particles but especially from dust generated by the erosionof the cathedral and by the restoration works. This latter kind of neutralisation isprobably a result of the location of the collector.

We have also shown that at both sites the Na+ concentrations are well correlatedwith Cl− and Mg2+ and that the X/Na+ ratios of these two species are close tothose observed in sea. These results were expected for an oceanic site such asTours. However, the results in Thann are more surprising for such a continentalstation and they can be explained by a local co-emission of Na+, Cl− and Mg2+probably from the dumps of the potash mines. Finally, we have demonstrated thatSO2−

4 could come mainly from neutralization and erosive processes in Tours whileit could have an industrial origin in Thann.

Acknowledgements

Authors want to thank the ‘PFARMH’ for its financial help and Mr. Plum and Mr.Ramboz from Thann and Tours for their help in the collection of rainwater samples.

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Documents

Chemical Composition of Rainwater Near Two Historical Monuments The Thann Collegiate (Alsace, France) and the Tours Cathedral (Indre et Loire, France)