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Atmospheric Environment 44 (2010) 174e181

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Atmospheric Environment

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Chemical and strontium isotope characterization of rainwater in karst virginforest, Southwest China

Guilin Han a,*, Yang Tang a,b, Qixin Wu a,b, Qiu Tan c

a The State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, ChinabGraduate School of Chinese Academy of Sciences, Beijing 100039, Chinac School of Geographic and Environmental Sciences, Guizhou Normal University, Guiyang 550001,China

a r t i c l e i n f o

Article history:Received 12 April 2009Received in revised form13 October 2009Accepted 14 October 2009

Keywords:Major ionsRainwaterStrontium isotopesKarst virgin forest

* Corresponding author. Tel.: þ86 851 5891954; faxE-mail address: hanguilin@vip.skleg.cn (G. Han).

1352-2310/$ e see front matter � 2009 Elsevier Ltd.doi:10.1016/j.atmosenv.2009.10.019

a b s t r a c t

Strontium isotope ratios and concentrations of Ca2þ, NH4þ, Naþ, Kþ, Mg2þ, Cl�, SO4

2�, NO3� and Al3þ, Sr2þ

were measured for 52 rainwater samples collected in virgin forest in a rural region between May 2007and Dec. 2008. The rainwater pH values vary from 4.1 to 7.2 with a volume weight mean (VWM) value of5.40. 40 of 52 samples have pH value above 5.0, indicating that the regional rainwater was not acidic.Among anions and cations, sulphate concentration (40.4 meq l�1, VWM) is the highest in the rainwater,followed by ammonium and calcium (30.2 and 20.8 meq l�1, VWM). Rainwater quality is characterized bylow salinity and neutralized pH.

The chemical compositions and 87Sr/86Sr ratios of the rainwater samples vary considerably. Using Naþ

concentration as an indicator of marine origin, the proportions of sea salt and crustal elements wereestimated from elemental ratios. The 87Sr/86Sr ratios were used to characterize different sources base onthe data sets of this study and those from literatures. Such sources include weathering of limestone(87Sr/86Sr ¼ 0.7075), remote soil dust (87Sr/86Sr > 0.7135) and anthropogenic source (fertilizers:87Sr/86Sr ¼ 0.7079). The results of the present study suggest that one likely source for high ammoniumand calcium concentration is local soil. Due to a large contribution of these cations to the sulphateneutralization action, the rainwater in this region displays non-acidity, and thus has not significantenvironmental impact. The wet precipitation in the karst virgin forest in Guizhou province is stronglyinfluenced by natural sources rather than anthropogenic sources.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Acidic deposition, as a result of increasing worldwide industri-alization, has attracted more and more attention because of itsdirect adverse effects on ecosystems and indirect effects on humanhealth (Lara et al., 2001; Hu et al., 2003). Over the last few decades,China has undergone rapid and extensive industrial growthparticularly in the more densely populated. This has inevitably ledto dramatic increases in pollutant emissions to the atmosphere andconsequently the increases in deposition of these pollutants overwide areas. For example, emissions of SO2 associated with coalcombustion have considerably increased in China since the late1970s, leading to significant deposition of acid rain (Zhao et al.,1988). The estimated emissions of SO2 in China were about 22millionmetric tons in 2003 (Larssen et al., 2006). The investigations

: þ86 851 5891609.

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of precipitation chemistry from many areas in China reveal signif-icant effects of acid rain over the last three decades (Zhao et al.,1988; Wang and Wang, 1995; Yu et al., 1998; Feng et al., 2001;Larssen et al., 2006; Han and Liu, 2006; Aas et al., 2007; Huanget al., 2008; Xu and Han, 2009). Aas et al. (2007) have noticed thatthe monitoring sites of air pollutants were mainly in urban areas,and emphasized that more integrated approach is required toaddress the acid rain problem in China (Aas et al., 2007).

Chemical compositions of rainwaters can help distinguish themajor source types that contribute to rainwater chemistry andunderstand the dispersion (i.e. both local and regional scales) ofelements, whether pollutants or not, and their potential impact oneco-hydrosystems through deposition processes (Negrel et al.,2007). But chemical compositions are less sensitive to unravel thedetailed sources, which can be replenished by isotope studies(Andersson et al., 1990). Strontium isotopes (87Sr/86Sr) are expectedto provide insights into the sources of base cations in rainwaters(Herut et al., 1993; Aberg, 1995). However, integrated studies onthe chemistry and Sr isotope compositions of rainwaters are few,

G. Han et al. / Atmospheric Environment 44 (2010) 174e181 175

limiting the application of Sr isotopes in atmospheric geochemistry(Dupre et al., 1994; Nakano and Tanaka,1997; Negrel and Roy, 1998;Chabaux et al., 2005; Han and Liu, 2006; Negrel et al., 2007; Xu andHan, 2009).

In this study, we carried out comprehensive investigations, forthe first time in our knowledge, of the Sr isotope compositionsintegrated with major element compositions of rainwaterscollected between May 2007 and Dec. 2008 from remote coun-tryside (the Maolan National Nature Reserved Park) in southeastGuizhou province, Southwest China. The chemical and isotopiccharacterizations of those rainwaters in karst virgin forest areexpected to provide representative background data in the Chineseacid rain control zone.

2. Sampling site

The Maolan area in Guizhou Province, southwest China ischaracterized by dense virgin evergreen forests growing on peakcluster karst. The study site is in the Maolan National NaturalReserved Park (between 25�0902000and 25�2005000of latitude N and107�5201000 and 108�0504000of longitude E; Fig. 1). This area coversapproximately 200 km2, mostly mountains of jagged carbonaterock with a forest-covering rate of more than 90%. A karst forest ofsuch magnitude is rarely seen in the world. The study site is locatedin the middle part of ChongqingeGuiyangeLiuzhou acid raincontrol zone (Hao et al., 2001). Annual rainfall at the virgin forestsite is about 1750 mm; ca 80% of which is precipitated inmonsoonal rainy season from April to September (Zhou, 1987). Themean annual air temperature at the study site is about 17 �C withsummer (JuneeAugust) and winter (DecembereFebruary).

The rock exposures in the study area are represented by sedi-mentary formations of limestone, dolomite, sandstone and clay in

Fig. 1. Sketch map showing the lithology of Maolan National Nature Reserved Pa

middle and lower Carboniferous ages. The geological map of thestudy area is shown in Fig. 1.

3. Sampling and analytical procedure

The rainwater samples were collected manually at the begin-ning of each precipitation event by a 20-l Polypropylene bottleplaced approximately 150 cm above the building roofs. The Poly-propylene bottles were pre-cleaned by rinse using 2e3 N HCl fol-lowed by Milli-Q water (18.2 MU�cm) and then dried. In order tominimize contributions from dry fallout, the samplers were openedafter the onset of rainfall as quickly as possible. 52 rainwatersamples were collected fromMay 2007 to Dec. 2008. Most of themwere collected in the rainy season from June to August.

The pH values were measured instantaneously at the end of therain events at sampling sites with a portable pH meter. All the rain-water samples were filtered through 0.22 mm membrane filters(Millipore). A small aliquot of each sample was stored for anionmeasurements, another aliquot was acidified with ultra-purifiednitric acid to pH< 2 after collection formeasurements of cations andSr isotopic compositions. Major cations (Kþ, Naþ, Ca2þ and Mg2þ)were determined by inductively coupled plasma-optical emissionspectrometry (ICP-OESVarianVistaMPX)withaprecisionbetter than5%. NH4

þ concentrationwas determined by spectrophotometer usingthe Nessler method. The concentrations of Al3þ and Sr2þ weremeasured by ICP-MS (VG POEMS III) with a precision of 5%. Anions(Cl�, SO4

2� and NO3�) were measured by ionic chromatography (Dio-

nex DX-120) with a precision of 5%. Reagent and procedural blankswere determined in parallel to the sample treatments using identicalprocedures. Each calibration curvewas evaluated byanalyses of thesequality control (QC) standards before, during and after the analyses ofa set of samples. The analytical precision was better than �5%.

rk and sampling locations. 1. Limestone, 2. Dolomite, 3. Sandstone, 4. River.

G. Han et al. / Atmospheric Environment 44 (2010) 174e181176

Separation of Sr from other major elements for isotopic analysiswas carried out by a conventional ion-exchange technique usinga Dowex 50W-X8 200e400 mesh resin in HCl media. A total blanksfor Sr is less than 0.5 ng, negligible with regard to the total Sranalyzed in rainwaters. The isotopic compositions of Sr weredetermined by TIMS (IsoProbe T, GV Instrument) with nine Faradaycollectors in the Institute of Geochemistry, Chinese Academy ofSciences. The 87Sr/86Sr ratios were normalized to 86Sr/88Sr¼ 0.1194.The value 87Sr/86Sr for the standard NBS987 was 0.710235 �0.000018 (2s, n ¼ 50) during the measurement period of oursamples.

4. Results and discussions

The field data, concentrations of major ions, aluminum andstrontium, and 87Sr/86Sr ratios are given in Table 1.

Table 1Concentration of major ions (in meq l�1), Al and Sr (in mmol l�1), and 87Sr/86Sr ratios in r

Sampling number Date pH NH4þ Kþ Naþ Ca2þ

LB-01 2007-5-12 5.23 37.78 10.72 10.65 71.50LB-02 2007-5-20 5.80 241.17 3.44 9.50 56.70LB-03 2007-5-24 4.41 43.56 7.15 2.83 67.20LB-04 2007-5-29 5.11 45.50 3.44 2.86 27.70LB-05 2007-6-1 5.37 30.00 2.35 2.06 12.80LB-06 2007-6-6 4.95 31.94 3.82 3.35 13.60LB-07 2007-6-12 5.04 30.00 2.00 1.49 14.35LB-08 2007-7-1 5.06 55.22 5.65 6.30 20.85LB-09 2007-7-2 5.54 68.78 7.69 10.76 19.65LB-10 2007-7-6 5.21 57.83 6.05 9.26 58.10LB-11 2007-7-12 5.34 19.89 5.70 6.75 25.10LB-12 2007-7-24 7.20 49.33 5.47 8.81 21.50LB-13 2007-7-26 5.12 69.22 6.42 33.20 39.05LB-14 2007-8-21 5.26 53.11 5.82 6.00 69.02LB-15 2007-8-22 4.56 56.89 4.92 3.35 16.45LB-16 2007-8-23 4.59 35.11 3.95 3.87 10.90LB-17 2007-8-23 4.96 27.50 2.03 1.49 5.30LB-18 2007-8-24 5.21 37.00 3.74 2.52 9.85LB-19 2007-8-26 5.43 22.78 0.68 0.80 7.90LB-20 2007-9-8 4.81 42.67 5.49 1.91 12.50LB-21 2007-12-21 4.90 63.56 23.69 2.17 53.95LB-22 2007-12-22 5.36 67.33 15.90 6.96 32.25LB-23 2008-3-9 4.41 126.11 16.77 6.87 34.20LB-24 2008-3-21 4.51 64.44 9.54 2.52 11.20LB-25 2008-4-24 4.10 155.00 17.92 6.26 54.60LB-26 2008-4-27 6.09 204.44 16.18 3.43 153.30LB-27 2008-5-4 4.89 40.56 6.08 1.09 10.55LB-28 2008-5-18 4.88 43.33 5.71 0.79 29.75LB-29 2008-5-24 5.29 44.75 4.40 0.84 31.05LB-30 2008-5-27 5.25 42.67 2.93 2.41 27.11LB-31 2008-6-8 5.83 11.47 1.17 1.57 16.87LB-32 2008-6-8 5.98 14.24 0.59 0.50 14.51LB-33 2008-6-11 5.46 44.75 1.32 0.67 19.23LB-34 2008-6-12 5.97 12.16 0.73 0.56 19.23LB-35 2008-7-5 5.54 10.78 0.88 0.45 11.36LB-36 2008-7-8 6.31 45.44 5.13 4.60 29.47LB-37 2008-7-14 5.16 37.12 2.64 1.29 22.38LB-38 2008-7-16 5.76 35.74 7.03 8.36 24.75LB-39 2008-7-21 6.24 105.76 4.25 5.95 46.79LB-40 2008-7-22 5.73 16.32 1.32 0.67 16.09LB-41 2008-7-24 5.81 26.03 4.84 4.43 21.60LB-42 2008-8-3 6.58 24.64 1.03 1.12 18.45LB-43 2008-8-8 6.85 14.24 1.17 1.51 19.23LB-44 2008-8-18 6.72 12.16 1.17 1.40 23.96LB-45 2008-8-26 6.32 6.62 0.44 0.34 14.51LB-46 2008-9-3 5.19 23.26 0.88 0.39 41.91LB-47 2008-9-24 5.67 20.48 0.59 0.50 12.94LB-48 2008-9-26 5.08 30.19 1.03 2.13 20.02LB-49 2008-10-25 5.36 35.04 0.88 0.95 20.02LB-50 2008-10-31 5.36 15.63 3.66 1.51 18.45LB-51 2008-11-2 5.43 10.08 1.32 0.73 18.45LB-52 2008-11-7 5.25 8.70 1.90 1.57 19.23

u.d. stands for under detection limit.

4.1. pH

The pH values of rain samples range between 4.1 and 7.2. Mostsamples show pH value from 5.0 to 5.6, while 12 rainwater sampleshave pH values smaller than 5.0. The mean pH value observed inMaolan was 5.4. The highest acidity was observed on August 24th,2008 with a pH of 4.1, and the lowest acidity was on July 24th, 2007with a pH of 7.2. According to previous studies, the naturallyexisting CO2, NOx and SO2 can dissolve into the clouds and droplets,resulting in pH values of the rain in the clean atmosphere to bebetween 5.0 and 5.6 (Charlson and Rodhe, 1982; Galloway et al.,1993). Rainwater with pH value below 5.0 is due to the presence ofnatural H2SO4, weak organic acids, or anthropogenic emission ofH2SO4 and/or HNO3. The samples with pH values above 6.0 maysuggest certain inputs of alkaline species into the precipitation inthe study area. HCO3

� can be significant in the higher ranges of pH

ainwaters from Maolan areas.

Mg2þ F� Cl� NO3� SO4

2� Al3þ Sr2þ 87Sr/86Sr

15.23 3.91 37.96 3.40 100.74 1.46 0.039 0.70838511.19 5.07 14.20 9.19 271.44 0.57 0.220 0.7087689.17 1.66 5.04 4.70 134.10 0.15 0.304 0.708580

11.60 1.26 6.06 2.45 80.98 0.14 0.137 0.7085641.65 0.10 2.64 2.06 31.59 0.13 0.058 0.7081452.39 1.02 8.51 2.13 36.35 0.16 0.136 0.7083462.55 0.35 3.47 1.07 37.45 0.11 0.077 0.7083584.12 1.19 7.22 2.08 64.77 0.30 0.116 0.7084185.02 2.82 12.97 2.49 72.73 0.24 0.082 0.7091088.48 9.27 10.21 3.83 119.14 1.27 0.169 0.7088816.34 0.62 10.64 0.80 19.89 0.55 0.224 0.7081122.55 1.43 10.04 1.77 27.47 0.09 0.066 0.708902

10.70 2.04 40.82 2.92 80.57 3.61 0.140 0.7090435.69 1.04 6.27 2.75 76.75 0.32 0.171 0.7086632.22 0.61 3.75 2.96 109.10 0.06 0.046 0.7086021.83 0.33 3.06 1.24 69.27 0.08 0.018 0.7084730.74 0.07 2.18 0.85 31.15 0.07 0.012 0.7086481.56 0.55 2.74 1.20 33.49 0.08 0.025 0.7082160.41 0.68 3.55 0.55 12.06 0.05 0.023 0.7088591.32 1.11 4.88 2.85 71.88 0.13 0.024 0.7092828.48 0.84 9.70 3.68 123.13 0.09 0.039 0.7101914.12 0.74 10.21 2.55 83.96 0.31 0.074 0.7105286.09 1.16 16.36 9.50 187.98 0.23 0.065 0.7098791.73 0.63 5.08 2.81 56.25 0.13 0.024 0.710564

10.95 1.26 11.59 9.89 151.29 0.31 0.015 0.71163332.59 1.95 10.10 8.40 216.21 0.13 0.013 0.7086731.98 0.16 2.12 0.60 28.19 0.03 0.028 0.7076073.65 0.61 7.33 6.00 73.33 0.09 0.098 0.7085265.04 1.90 4.37 u.d. 71.92 0.06 0.057 0.7127523.32 1.89 5.41 u.d. 57.86 0.09 0.010 0.7090593.85 0.78 4.03 u.d. 19.13 0.06 0.183 0.7086433.19 0.19 2.44 2.39 20.89 0.02 0.038 0.7117381.33 0.06 2.99 u.d. 67.12 0.08 0.011 0.7080261.46 0.52 5.08 3.61 21.95 0.02 0.009 0.7086741.00 0.57 2.23 2.20 11.47 0.02 0.015 0.7085602.26 0.28 6.82 9.04 41.70 0.08 0.015 0.7083522.06 0.92 3.69 10.77 47.82 0.14 0.058 0.7084363.32 0.91 14.77 7.29 46.73 0.20 0.053 0.7087315.31 1.25 11.78 15.66 86.03 0.18 0.020 0.7085441.26 0.76 1.50 3.99 20.09 0.02 0.038 0.7085422.72 1.31 6.21 7.67 46.20 0.14 0.053 0.7100961.66 0.36 2.91 7.32 35.99 0.02 0.008 0.7074631.99 0.97 2.94 7.67 14.39 0.05 0.010 0.7100482.85 0.49 5.21 2.29 14.86 0.09 0.012 0.7106541.26 0.56 0.71 3.56 7.12 0.05 0.057 0.7110624.71 0.66 u.d. 6.88 31.82 0.02 0.039 0.7088060.86 0.98 1.39 3.28 21.66 0.02 0.060 0.7086542.32 0.79 5.28 9.24 39.56 0.03 0.076 0.7087122.46 0.87 2.64 9.19 33.27 0.04 0.110 0.7087191.59 1.24 7.79 6.03 35.11 0.10 0.165 0.7102431.26 0.89 2.33 6.06 25.95 0.03 0.195 0.7082461.46 0.74 3.88 3.41 25.19 0.09 0.065 0.708324

100

90

80

70

60

50

40

30

20

10100

90

80

70

60

50

40

30

20

10

100 90 80 70 60 50 40 30 20 10

Ca2+ +Mg2+

NH4+ K++Na+

100

90

80

70

60

50

40

30

20

10100

90

80

70

60

50

40

30

20

10

100 90 80 70 60 50 40 30 20 10

NO3-

SO42- C l-

South of ChinaBeijing, ChinaGuiyang, ChinaMaolan, ChinaSoutheastern coast of China

Fig. 2. Ternary diagrams showing cation and anion compositions. The data of rainwaters collected from other areas in China are also shown here for comparison. Data sources: Aaset al. (2007) for South of China, Xu and Han (2009) for Beijing, Han and Liu (2006) for Guiyang, Yu et al. (1998) for southeastern coast of China.

G. Han et al. / Atmospheric Environment 44 (2010) 174e181 177

(>5.5), but negligible in the lower ranges of pH (<5.5; Noguchiet al., 1995). The higher pH values reported here could be theresult of dissolution of windblown dust with a high CaCO3 content,so the high pH value does not correspond with the high HCO3

�

concentration. Even for the rainwater samples with pH > 5.5, theconcentrations of HCO3

� are still under the detection limit.

4.2. Compositional variations of major ions

Most of the rainwater samples have total cation concentrations(TZþ ¼ Kþ þ Naþ þ Ca2þ þ Mg2þ þ Hþ) larger than total anionconcentrations (TZ� ¼ SO4

2� þ NO3� þ Cl�þF�). This imbalance is

usually attributed to unmeasured organic anionic species, such asacetate, formate, HCOO� and CH3COO�, etc. (Losno et al., 1991;Negrel and Roy, 1998; Mouli et al., 2005).

Variations of major ion compositions are shown in the anionand cation ternary diagrams (Fig. 2). It is clear that the Maolanrainwaters have more NH4

þ, Ca2þ, Mg2þ and SO42� in comparison

with Kþ, Naþ and Cl�, NO3�. NH4

þ is the most abundant cation,ranging from 6.6 to 241.2 meq l�1 with a mean value of 47.5 meq l�1.Ca2þ is the second cation of the rainwater samples. The concen-trations of Ca2þ vary from 5.3 to 153.3 meq l�1, with a mean value of28.6 meq l�1. Ca2þ and NH4

þ together account for 68e95% of the totalcations measured. SO4

2� and NO3� are conventional acidic ions in

precipitation. SO42� is the dominant anion and its concentration

ranges from 7.1 to 135.7 meq l�1, with a mean value of 62.4 meq l�1.SO4

2� accounts for 55e96% of total anions. The second abundantanion is Cl�, with concentration varying from 0.7 to 40.8 meq l�1

with mean of 7.4 meq l�1. The sum of SO42� and Cl� account for

66e99% of the total anions measured.The Maolan rainwater samples have chemical compositions

similar to those of the rainwaters collected from Guiyang and south

Table 2Chemical composition (meq l�1) of rainwater in Maolan area.

Component VWM Median Mean SD Range

pH 5.40 5.32 5.41 0.64 4.10e7.20F� 0.90 0.86 1.20 1.45 0.06e9.27Cl� 5.14 5.08 7.35 7.58 0.71e40.82NO3

� 3.01 3.41 4.63 3.36 0.55e15.66SO4

2� 40.35 43.95 62.41 53.59 7.12e135.72NH4

þ 30.21 37.06 47.47 45.48 6.62e241.17Kþ 3.50 3.78 4.99 5.04 0.44e23.69Naþ 2.35 2.15 3.89 5.10 0.34e33.20Ca2þ 20.84 20.02 28.6 24.21 5.30e153.30Mg2þ 3.03 2.55 4.48 5.20 0.41e32.59

VWM ¼ Volume weighted mean; S.D. ¼ standard deviation.

of China, but clearly different to those from Beijing and south-eastern of China (Fig. 2). The dominance of NH4

þ and Ca2þ as well asSO4

2� and low levels of NO3� and Kþ þ Naþ represent the typical

characteristics of the rainwater collected from the Maolan virginforest.

4.3. Statistical analysis

Table 2 shows the volume weighted mean (VWM) of the ioniccompositions of rainwater and related statistical analyses. TheVWM values in rainwater are able to account for the effect ofprecipitation amount on ion concentration of rainwater. The orderof cation abundance is NH4

þ> Ca2þ > Mg2þ > Kþ. NH4

þ and Ca2þ

show VWM concentrations of 30.2 and 20.8 meq l�1, respectively.The NH4

þ concentrations vary over a 36.5-fold range (241.2 meq l�1

in May 2007e6.6 meq l�1 in August 2008) and the Ca2þ concen-trations vary over a 29-fold range (153.3 meq l�1 in April2008e5.3 meq l�1 in August 2007). The order of anion abundance isSO4

2� > Cl� > NO3� with SO4

2�VWM concentration of 40.4 meq l�1,being 7.9 times of that of Cl� (5.1 meq l�1) and 13.5 times of that ofNO3

� (3.0 meq l�1). The VWM values of rainwater samples arecommonly less than the arithmetic means, indicating that the highconcentrations of ions are usually associated with low precipita-tion. The data of the ion concentrations show a high relativestandard deviation (from 0.64 to 53.59%), indicating a large vari-ability in the cation and anion concentrations in each rain events.

In order to disclose possible association between ions inprecipitation and consequently, the likely sources of pollutions,correlation coefficients (R) between ions were calculated and pre-sented in Table 3. Good correlation between Ca2þ and Mg2þ

(R ¼ 0.92), as being expected because of their similarity, is consis-tent with the wide distribution of carbonate in sampling site.

Table 3Correlation coefficients (R) of ionic concentrations (in meq l�1) in rainwater samplesfrom Maolan.

Ions NH4þ Kþ Naþ Ca2þ Mg2þ F� Cl� NO3

� SO42�

NH4þ 1

Kþ 0.56 1Naþ 0.34 0.30 1Ca2þ 0.66 0.55 0.26 1Mg2þ 0.69 0.55 0.36 0.92 1F� 0.39 0.17 0.41 0.43 0.42 1Cl� 0.35 0.41 0.85 0.41 0.50 0.41 1NO3

� 0.43 0.12 0.00 0.35 0.22 0.11 0.16 1SO4

2� 0.91 0.61 0.33 0.71 0.72 0.51 0.41 0.36 1

0.1

1

10

100

0.1 1 10 100

Cl-/Na+ (equivalent ratio)

SO

42-/N

a+(e

quiv

alen

t ra

tio)

seawaterCl- /N

a+=1

.17

Fig. 3. Covariation of SO42�/Naþ with Cl�/Naþ ratios in the rainwater samples collected

from Maolan area and some other areas in China (data source and symbols are thesame as those in Fig. 2).

G. Han et al. / Atmospheric Environment 44 (2010) 174e181178

SO42� shows good correlation with NH4

þ, Ca2þ and Mg2þ, withcorrelation coefficients of 0.91, 0.71 and 0.72, respectively. Thisindicates that NH4

þ and Ca2þ would play a major role in neutralizingacidic sulfur gas. Table 3 suggests that NaCl, NH4HSO4, (NH4)2SO4,CaSO4 and MgSO4 are the predominant species combination. Theymay be formed in the atmospheric water droplets by aerosolscavenging and subsequent reaction of gaseous species.

4.4. Origins of major ions in the rainwater

Atmosphere aerosols including sea salts, crustal dust, volcanicdust, biogenic material and anthropogenic emissions are the mainsources of chemical elements in rainwater as the chemicalcomposition of rainfall is strongly affected by the chemicalcomposition of the atmosphere (Roy and Negrel, 2001; Chetelatet al., 2005; Negrel et al., 2007). The most usual method to evaluatethe contribution of sea salt (SS) to ion contents in precipitation is tocompare the Cl�/Naþ ratio in rainwater to that of seawater. SS isconsidered to be the major source of both ions, although othernatural and industrial sources are also possible (Rastogi and Sarin,2005). To calculate the SS and Non-Sea Salt (NSS) contributions fora given element, Na is used as a marine tracer in rainwater (Negreland Roy, 1998; Negrel et al., 2007). However, the selection of thiselement as reference must be validated, particularly with regard topossible terrestrial influence. Since aluminum comes almostexclusively from terrestrial material, the presence of terrestrial Na(NaT) can be estimated according to the approach proposed byNegrel and Roy (1998):

NaT ¼ AlRWðNa=AlÞT (1)

Hofmann et al. (1977) and Church et al. (1984) have shown thatshales are more representative of average soils than rocks of theupper continental crust. Therefore, the correction of crustal Na hasbeen applied using the Na/Al ratio of shale (0.11) as a terrestrial dustreference (Negrel and Roy, 1998). The terrestrial Na estimated bythis method accounts for less than 1.5% of total Na in our samples,indicating a minor and negligible contribution of the terrestrialcomponent in Na content. The proportions of SS and NSS end-members were therefore calculated using Na as the marine refer-ence species:

Naref ¼ NaRW � 0:11� AlRW (2)

The contribution of NSS component is as bellows:

XNSS ¼ XRW � Naref ðX=NaÞSW (3)

Where X represents Kþ, Ca2þ, Mg2þ, SO42� and Sr2þ.

The results show that NSS sources contribute substantially tothe observed concentrations of Kþ, Ca2þ, Mg2þ, SO4

2� and Sr2þ

(see Table 4).

4.4.1. Origins of the anions in the rainwaterNaþ and Cl� in rainwater are generally assumed to originate

from seawater. Fig. 3 shows the correlation of Cl� and SO42�. As

compared with seawater, the Cl�/Naþ equivalent ratios of rainwa-ters from the Maolan virgin forest are larger than that of seawater

Table 4Mean, Minimum andMaximumvalues for the NSS component in the rainwater fromMaolan.

Ca2þ NSS(%) Mg2þ NSS(%) Kþ NSS(%) SO42� NSS(%) Sr2þ NSS(%)

Min 96.3 21.0 88.8 95.1 99.2Max 99.9 98.1 99.8 99.9 99.9Mean 99.3 76.5 98.0 99.1 99.8

(Cl�/Naþ ¼ 1.17; Berner and Berner, 1987). This suggests that thehigh content of Cl� relative to seawater is mainly contributed byanthropogenic sources. Only one rain event (sample LB-16) haslower Cl�/Naþ ratios (0.79), which can be explained by the presenceof anthropogenic Naþ, or replacement of Cl� by SO4

2� or NO3� (Keene

et al., 1990; Clegg and Brimblecombe, 1985, 1986). In fact, bothmechanisms for producing low Cl�/Naþ ratio are possible, since therainwaters fromMaolan have high SO4

2� concentration, and the Naþ

in crustal aerosol could be easily washed out.All of the rain samples lies on the right of the seawater line

(Fig. 3), indicating a significant anthropogenic Cl� source. It couldcome from the stack gases of refuse incineration. In addition, thecombustion/decomposition of organochlorine compounds, such aspolyvinyl chloride, produces HCl in gas phase (Johnson et al., 1987;Sigg et al., 1987; Sanusi et al., 1996). It may also be produced byautomobile exhaust since gasoline contains lead bromochloride asan additive (Friedlander, 1973; Fuzzi et al., 1984). Because of the lowpH (pH ¼ 4.1), we prefer that the excess Cl� came from hydrogenchloride emission. However, since HCl is a very soluble gas inwater,it cannot be transported over long distances in conditions of rain(Johnson et al., 1987). The Hydrogen chloride probably originatesfrom a local source near the sampling site. As described earlier,rainwater site is in the rural virgin forest, where there is no auto-mobile, no industry, no refuse incineration. So we think the highCl� could be from agricultural activities. SO4

2� and NO3� are

conventional acidic ions in precipitation. The NSS input of SO42�

ranges from 99.2 to 99.9% in our samples. In China, coal accounts forabout 70% of the commercial energy production, this leads to largeemission of SO2 (Aas et al., 2007), particularly in Guizhou province,where coal resources are rich and coal combustion is common. Thechemical compositions of rainwater indicate that the combustion ofcoal still show large impact on the atmospheric environment, atleast in the studied area.

4.4.2. Origins of the cations in the rainwaterNH4

þ is a dominant neutralizing cation and thus of importance toneutralize acid rains (Nakano and Tanaka, 1997; Larssen and Car-michael, 2000). The presence of NH3 is intimately related to the soilfeature in the area of study. Volatilization of NH3 increases as thesoil pH increases (e.g., the pH values of soils in the Maolan regionare around 7.1e8.1; Zhou, 1987). The high level of NH4

þ in rainwatercoincides with the fact that the emission of ammonia in the Asian

0707.0

0807.0

0907.0

0017.0

0117.0

0217.0

0317.0

0417.0

00001000100101

aC +2 rS/ +2

87Sr

/86S

r

0707.0

0807.0

0907.0

0017.0

0117.0

0217.0

0317.0

0417.0

000010001001011

gM +2 rS/ +2

87Sr

/86S

r

0707.0

0807.0

0907.0

0017.0

0117.0

0217.0

0317.0

0417.0

0001001011

aN + rS/ +2

87Sr

/86S

r

0707.0

0807.0

0907.0

0017.0

0117.0

0217.0

0317.0

0417.0

0001001011

K+ rS/ +2

87 Sr

/86S

r

Fig. 4. Correlations between 87Sr/86Sr and element ratios (Ca2þ/Sr2þ, Mg2þ/Sr2þ, Naþ/Sr2þ and Kþ/Sr2þ) in the rainwater samples from Maolan.

0 7 0 7 . 0

0 8 0 7 . 0

0 9 0 7 . 0

0 0 1 7 . 0

0 1 1 7 . 0

0 2 1 7 . 0

0 3 1 7 . 0

0 4 1 7 . 0

0 1 8 6 4 2 0

l C - a N / + ) o i t a r r a l o m (

87 Sr

/ 86 Sr

gnirehtaewetaciliS

tupnicinegoporhtnAgnirehtaewetanobraC

Fig. 5. Variation of Sr isotope ratios against Cl�/Naþ ratios in the rainwater samplesfrom Maolan area.

G. Han et al. / Atmospheric Environment 44 (2010) 174e181 179

region is several times higher than those in North America andEurope (Galloway, 1995). We can attribute the presence of nitrateand ammonium ions in the samples to a direct input of nitrogenousfertilizer as well as to an input of nitrate and ammonia containingaerosols during sample collection (Sanusi et al., 1996).

Almost all Ca2þ in the Maolan area (mean value is 99.3%) was ofNSS origin. The NSS Ca2þ may come from the dissolution of CaCO3dusts (Schmitt and Stille, 2005; Rastogi and Sarin, 2005). Identi-fying the provenance of Ca2þ in rainwater has great importance tothe study of acid rain problem, because this element is dominantcation that neutralizes acid and is indispensable for plant growth.There are three main origins for the Ca2þ in rainwater. The first oneis anthropogenic Ca2þ emitted by human activities such as traffic,cement working, etc. The large turbulences generated by buildingcan enhance the spreading of aerosols in the atmosphere (Sanusiet al., 1996). The second one is from carbonate weathering. Thethird one is the long-range transport of soil dust. Since Maolan issurrounded by virgin forest with scattered carbonate distribution,we think that the first possibility is less likely but the secondpossibility is more likely to explain the high level of Ca2þ in theprecipitations in Maolan.

Some of rainwater samples are enriched in Kþ relative to Naþ.The NSS Kþ can be from (1) soil dust with a local or remote originand (2) agricultural soil dust with fertilizer. Because the lack ofcorrelation between NSS Kþ and NSS SO4

2� indicates that Kþ fromanthropogenic sources does not have the same origin as SO4

2�. Sowe think the NSS Kþ mainly come from agricultural activities.

4.5. Sr and its isotopic compositions

Strontium isotope ratios vary between different sources becausethe contents of 87Rb, the radiogenic mother of 87Sr, vary between

sources (Faure, 1986). When coupled with Sr concentrations, Srisotope system can be used to investigate the mixing of different Srsources (Herut et al., 1993; Negrel and Roy, 1998; Negrel et al., 2001,2007; Aubert et al., 2002; Chabaux et al., 2005; Han and Liu, 2006).The Maolan rainwater samples contain Sr from 0.01 to 0.3 mmol l�1

with 87Sr/86Sr ratios from 0.70746 to 0.71275. The nonlinear rela-tionship between 87Sr/86Sr ratios and Ca2þ/Sr2þ, Mg2þ/Sr2þ, Naþ/Sr2þ, Kþ/Sr2þ ratios (Fig. 4) suggest that the origins of cations aremore complicated than simple binary mixing of two differentpopulations of aerosols (Dupre et al., 1994). Numerous sources,both local and remote, natural and anthropogenic inputs, must beconsidered.

Table 5Comparison of the major ions concentration (in meq l�1) in Maolan with other areas in China and worldwide.

Region pH NH4þ Kþ Naþ Ca2þ Mg2þ Cl� NO3

� SO42� References

Chengdu (VWM) 4.4 250.7 20.8 22.6 192 33.2 42.3 30.4 431.5 Lei et al., 1997Guiyang (Mean) 4.53 11 4 113.2 25.5 21.2 48.2 188 Han and Liu, 2006South of China (Mean) 4.41 61.7 10.8 11.3 82.1 21.5 16.3 28.2 166.3 Aas et al., 2007Shanghai (VWM) 4.49 80.9 14.9 50.1 204 29.6 58.3 49.8 199.6 Huang et al., 2008Beijing (VWM) 5.12 185.6 17.6 25 607.2 40.4 104 109 315.8 Xu and Han, 2009Massif (France) mean 5.22 5.7 14.4 14.6 3.4 19.6 36.2 22.3 Negrel and Roy, 1998Mexico (VWM) 5.08 92.4 2.2 7 26.4 2.5 9.6 42.6 61.9 Baez et al., 2007Maolan (VWM) 5.45 30.3 3.6 2.4 21.1 3.1 5.2 3.1 40.6 this study

G. Han et al. / Atmospheric Environment 44 (2010) 174e181180

Most of the rainwater samples collected in Maolan have87Sr/86Sr lower than that of seawater, which reflects a contributionfrom at least one non-radiogenic Sr source. The most likely candi-date would be the Sr from the weathering of middle and lowerCarboniferous pure limestone and dolomite rock around thesampling site. The isotopic compositions of Sr in dusts can beinferred by 87Sr/86Sr ratios of river water. The 87Sr/86Sr ratios of0.7075e0.7080 have been reported from rivers around Guizhouprovince in previous studies (Han and Liu, 2004).

Some of the rainwater samples from Maolan have Sr valueshigher than that of seawater, indicating a contribution of at leastone more radiogenic Sr source. The most likely candidate would bethe Sr in soil dust from distant continental bedrocks and soils,which are produced obviously by silicate weathering. The Srisotope ratios of this silicate component can be approachedthrough the dissolved load of rivers draining silicate basement. Forgranite and gneiss basements, the 87Sr/86Sr ratios range between0.7135 and 0.717 (Negrel et al., 2007). We adopt the Sr isotopicratios (>0.713) as the remote soil dust source for the rainwater.

Another Sr source could be mixture of various anthropogenicinputs. Since the Sr isotope ratios of typical contaminant sources forthe rainwaters in Maolan have not been characterized, the valuesfrom literatures can be used as the reference. The 87Sr/86Sr ratiosrange between 0.7079 and 0.7087 for fertilizer, between 0.7077 and0.7083 for automobile exhaust, between 0.7083 and 0.703335 forurban heating, between 0.7097 and 0.7100 for incinerators in theParis atmosphere (Negrel et al., 2007). Because theMaolan NationalNature Reserved Park in a rural site and there is no industry. Theanthropogenic input is mostly from agricultural activities. Althoughit is very difficult to identify the sources for Sr based on these data,it can be reasonably postulate that the anthropogenic componentsin the Maolan area are characterized by moderate 87Sr/86Sr ratio.

Fig. 5 shows the variation of 87Sr/86Sr with Cl�/Naþ ratios in therainwaters from the Maolan area. Cl�/Naþ ratio can be used asindicator of sources: marine source has Cl�/Naþ ratios of 1.17 andanthropogenic sources should have higher ratios. As seen on Fig. 5,at least three-component mixing is needed to interpret the data.The first source is probably form carbonate weathering, which ischaracterized by lowest 87Sr/86Sr and Cl�/Naþ ratio as well as thelowest Naþ concentration. In contrast, another source of anthro-pogenic origins is characterized by high Cl�/Naþ ratio. The lastsource is silicate weathering, which is characterized by highest87Sr/86Sr and lowest Cl�/Naþ and higher Na concentration.

4.6. Neutralization of acidity

Although pH is a direct parameter to evaluate whether rain-water is acidic or not, the pH unit alone gives very limited infor-mation on the acidity of rainwater. Reactions between the acidicand alkaline constituents and water droplets determine the finalpH of the rainwater.

Compared with other sites worldwide, the precipitation acidityis heavily governed by the relative contribution of NH4

þ, Ca2þ, SO42�

and NO3�. As mentioned above, rainwater from Maolan has lowest

SO42� concentration and the NO3

� concentration is also found to belower than the data achieved from worldwide cities. Comparedwith these acid-rain areas in the southeast China and south China,the problem of acid rain is not exist in the Maolan area. This isconsistent with the fact that Maolan is located in carbonated areasand there are a few human activities. From Table 5, it can beobserved that the concentrations of ions of rainwater in Maolan arelower than those reported for other areas. In comparison to thosedata of rainwater collected in southwest and south of China(Table 5), the data fromMaolan show the lowest ionic compositionand highest pH value. It is noteworthy that the tendency of rainacidification in Maolan is not obvious although the Maolan rain siteis located in the middle part of ChongqingeGuiyangeLiuzhou acidrain control zone (Hao et al., 2001).

5. Summary

The chemical and Sr isotope compositions of rainwaters inMaolan, the most important karst virgin forest in China, wereanalyzed on the basis of samples collected over one and a half years.Integrated statistic analyses and strontium isotope systematicswere employed to identify the origins of major ions. The cation andanion analyses for the rainwater samples show that sulphate isthe highest ion, followed by ammonium and calcium. The highsulphate concentration is likely due to the presence of SO2 in thelocal atmosphere.

The rainwater samples contain Sr from 0.01 to 0.3 mmol l�1 witha small range (0.70746e0.71275) of 87Sr/86Sr ratios. Covariation of87Sr/86Sr ratio with major-element concentration ratios in therainwater suggests at least three sources for rainwater Sr, includingcarbonate soil dust originated locally, silicate soil dust from remoteareas and anthropogenic sources (fertilizers).

Some rainwater samples (12 samples) have pH < 5.0, indicatingthat the regional rainwater is slightly acidic. The high sulphateconcentration in the rainwater is potentially responsible for its highacid level. It was clear that the cations, such as NH4

þ and Ca2þ, actas sulphate neutralizers in each rainwater sample. Due to thesubstantial contribution of these cations to the sulphate neutrali-zation action, the rainwater in the study region is not acidic andshows no much environmental impact.

Acknowledgments

This workwas supported jointly by the Chinese National NaturalScience Foundation (No. 40721002, 40673010), National BasicResearch Program of China (973 Program) (No. 2006CB403206) andthe Innovation Program of Chinese Academy of Sciences (No. KZCX-YW-306). The authors gratefully acknowledge executive editorH. B. Singh and anonymous reviewers for significant improvementson the manuscript.

G. Han et al. / Atmospheric Environment 44 (2010) 174e181 181

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