Source identification and variation in the chemicalcomposition of rainwater at coastal and industrialareas of India

P. R. Salve & T. Gobre & H. Lohkare & R. J. Krupadam &

A. Bansiwal & D. S. Ramteke & S. R. Wate

Received: 18 July 2011 /Accepted: 31 January 2012 /Published online: 22 February 2012# Springer Science+Business Media B.V. 2012

Abstract Studies on precipitation chemistry were carried out to understand the nature andsources of rainwater at Rameswaram and NTPC Dadri, India representing coastal andindustrial region during 2010. The rainwater samples, collected at these locations, wereanalyzed for major ions and pH. The data were assessed for its quality. The pH of rainwaterat two locations varied from 5.53 to 6.73 and 5.1–6.6, indicating alkaline nature except a fewacidic events. The dominance of Cl− and Na+ were observed in coastal environmentwhereas dominance of Ca2+ was seen in industrial environment. The nss-SO42- (37.9%)at Rameswaram is less influenced by anthropogenic activities whereas nss-SO42- (72.5%) atNTPC Dadri is influenced by pollutants emitted by anthropogenic sources. The ratio of H+/(NO3- + SO42-) was observed as 0.04 and 0.008 for Rameswaram and NTPC Dadri whichis close to zero, indicate 99.99% of acidity was neutralized in precipitation. Ca2+, NH4+ andMg2+ play an important role in neutralization of acidic ions in rainwater. For sourceidentification, correlation matrix analysis was established, which showed that in general,at both sites; correlation between the acidic ions SO42- and NO3- indicating their originfrom similar sources, because of the similarity in their behavior in precipitation and the co-emissions of their precursors SO2 and NOX. The correlation coefficient of (SO42- + NO3-)vs (Ca2+) at Rameswaram and NTPC Dadri were 0.77 and 0.87 indicates that CaCO3 is themajor neutralizing agent for both the region. Overall, the influence of marine, terrestrial andanthropogenic sources was observed in the rain events.

Keywords Coastal . Industrial . Major ions . Rainwater chemistry

1 Introduction

Studies on rainwater chemistry was initially developed in response to the adverse effects ofacidic rain phenomenon on vegetation, human beings, animals, soil, water, fish and

J Atmos Chem (2011) 68:183–198DOI 10.1007/s10874-012-9217-6

P. R. Salve (*) : T. Gobre : H. Lohkare : R. J. Krupadam : A. Bansiwal :D. S. Ramteke : S. R. WateEnvironmental Impact & Risk Assessment Division, National Environmental Engineering ResearchInstitute, Nehru Marg, Nagpur 440 020 MS, Indiae-mail: pr_salve@neeri.res.in

buildings (Raddum and Skjelkvale 2001). Precipitation chemistry provides important infor-mation by which the deposition of water-soluble gases and aerosol-bound constituents on theearth’s surface can be estimated. Such estimates are fundamental in many studies ofterrestrial and marine ecology. These also provide the data for testing of regional and globaltransport models (Rodhe et al. 1995). Further, the study of rainwater chemistry enables us tounderstand the effects of urban, industrial and agricultural emissions and the biogeochemicalfactors. These factors regulate the chemistry of rainwater and provide an understanding ofthe processes controlling the chemical composition of the atmosphere in remote areas andnatural biogeochemical cycle (Galloway and Gaudry 1984; Tiwari et al. 2007).

In industrial environment, thermal power plant emits fly ash and large amounts of gaseswhich are considered as acid precursors, such as SO2 and NOx, into the atmosphere, andcontribute to the acid precipitation in the region. The scavenging of the atmospheric pollutantsaffects the chemical composition and the pH of rainwater. The degree of acidity depends on theneutralization by certain rainwater components such as ammonia and calcium carbonate and/orhydroxide. The chemicals present in the atmosphere also influence the water cycle of theatmosphere whereas the material deposited by the rain affects soil, superficial water andvegetation. The composition of the rainwater depends on the local emission, on the pollutantstransport, on the sea level elevation and on the drop size, which in turn influences the rainout(in-cloud scavenging) and the washout (below-cloud scavenging). The different climate con-ditions also influence the trace substances concentration (Flues et al. 2002). In the coastalenvironment, SO4

2− concentration frequently presents an excess, whose origin is different fromsea salt. Moreover SO2 levels in a marine atmosphere increase with primary productivity insurface waters (probably due to production of dimethyl sulfide), suggesting that biologicalprocesses can be important to explain sulfate excess in rainwater of marine and coastalenvironment. In remote areas, it has been found that the presence of a sulfate fraction doesnot depend on sea salt. The levels of SO4

2− are mostly low and represent background level fromremote marine sites reported elsewhere (Galloway et al. 1983). Background concentrations areuseful to establish a reference point for comparisonwith urban and industrial areas (Vong 1990).When SO4

2− levels exceed this background value, there are three probable sources attributed tolocal production of acids, natural emission from biogenic activity in Surface Ocean, and long-range transport (Casimiro et al. 2003; Ceron et al. 2005).

2 Materials and methods

2.1 Study area

Rameswaram is a town in Ramanathapuram district in the Indian state of Tamil Nadu. It islocated on Pamban Island also known as Rameswaram Island separated from mainland Indiaby the Pamban bridge channel. It is bordered by both the Gulf of Mannar and Bay of Bengal

184 J Atmos Chem (2011) 68:183–198

India is a rapidly developing country where pollution levels are steadily rising inindustrial, urban, suburban, and coastal environment. The quality of rainwater varies fromsite to site and season to season due to variations in chemical composition which is highlydependent on topography, climate, natural and anthropogenic sources (Tiwari et al. 2006).The present investigation is the first effort to collect rainwater samples in a pristine tropicalcoastal site Rameswaram (Tamilnadu) and industrial site NTPC Dadri (Uttar Pradesh) India,to determine the chemical composition of rainwater on a set of quality-checked data, identifythe potential natural and anthropogenic sources of the different chemical content of rainwatercontributing to rainwater chemistry.

on the southern and the northern sides, respectively. Rameswaram has dry tropical climate,experiences both North-East and South-West monsoons with average annual rainfall of94 cm. Temperature is around 30°C to 35°C. The South-West monsoon extends from Juneto September over a period of 4 months, whereas the North-East monsoon commences inNovember and ceases in February or early March. With the onset of the South-Westmonsoon the Gulf of Mannar becomes rough and choppy, while the Palk Bay is calm.During the North-East monsoon these conditions are reversed.

NTPC Dadri Township lies in Gautam Buddha district, Uttar Pradesh State in NorthernIndia. It is also a part of National Capital Region (NCR) and about 24 km from Ghaziabadand around 45 km from New Delhi towards Hapur. The district is bounded by GhaziabadDistrict to the north, Bulandshahar District to the east, and Aligarh District to the south. Theclimatic condition is tropical monsoon. About 90% of the rainfall occurs during thesouthwest monsoon season (June to September). With most of the rainfall concentratedduring this 4-month period, floods become a recurring problem and cause heavy damage tocrops, life, and property. Periodic failure of monsoons results in drought conditions and cropfailure. The state receives 85% of average annual rainfall of 99 cm.

2.2 Sampling

Rainwater samples were collected at Rameswaram (09°17′12.3″N, 79°19′5.6″E) at the roofterrace of Hare Rama Hare Krishna Hotel which is commercial-residential area in the townin Southeast (SE) direction and about 0.3 km from the coastline. Similarly, at NTPC Dadri(28°36′50″N, 77°36′4.2″), the rain collector was installed at the roof top of building ofresidential area in the colony. The aerial distance from the NTPC thermal power plant isabout 1.8 km in North-North West (NNW) direction. The predominant wind direction duringthe monsoon season was South West (SW) and speed varied from 3 to 5 m/s. The sampleswere collected on event basis using 1 l polyethylene bottle kept under funnel (diameter14 cm). They were placed at a height of 1-m to counter the contamination by dropletsplashes. The bottle as well as the funnel has been cleaned both in the morning and eveningusing triple-distilled water to avoid any dry deposition of gaseous and particulate species.Collected samples have been stored in small polythene bottles also cleaned by triple-distilledwater. Thymol (5 mg) was added to these samples for preventing any biological degradation.All samples were refrigerated at 40 C in the laboratory till the analysis was over (Tiwari et al.2007). Prior to their use, the bottles were cleaned repeatedly using distilled water and soakedfor ∼72 h. The sampling was conducted during September 2010–February 2011 at Rames-waram representing coastal environment. This short duration period was considered forrainwater samples collection because north east monsoon starts from November to Februarywhere about 95% of intense rainfall continuously occurs in the region whereas south westmonsoon for remaining months which bring very less and scanty rainfall. Hence studyduration was too short. At NTPC Dadri, the sampling period was January–December 2010representing industrial environment. A total of 27 samples were collected at Rameswaramand 45 rainwater samples at NTPC Dadri which had sufficient volume for chemical analysis,which represents most of the rain events during the study period.

2.3 Analysis

Immediately after collection, the pH of samples was measured. Measurements of sample pHwere made using pH electrode with a combination glass electrode. Calibration was per-formed using buffer solutions at pH 4 and 7. The major ions were analysed on Dionex ion

J Atmos Chem (2011) 68:183–198 185

chromatograph (ICS 3000) using anions AS12A/AG12 columns coupled to an anion self-regenerating suppressor (ASRS) in recycled mode. A combination of Na2CO3 and NaHCO3

was used as eluent maintained at a flow rate of 1 ml/min. Major cations were also measuredby using cation column (CS12A/CS12G) and cation self-regenerating suppressor (CSRS) inrecycled mode. All solutions were prepared with deionized water supplied by Millipore QMillipore system and with standard solutions were prepared by dissolution of 99% analyticalgrade salts. The limits of detection for cations and anions were 1 ppb in Dionex ionchromatograph (Salve et al. 2006, 2008; Gobre et al. 2010).In order to check the instrumen-tal errors, a quality standard of cations and anions, as well as blanks samples were preparedwith different concentrations and given a random number among the real samples. Thespecific QA and QC procedure included the routine running of blanks and control samples aswell as replicates samples. The analytical precision was maintained by running knownstandard after every 15 samples to check the peak response; if deviation was more than2%, recalibration was done. The levels of all the cations and anions in the blanks and inrinses of plastic ware were below detection limit. An overall precision expressed as%relative standard deviation (RSD) was obtained for the entire samples. Detection limits ofthe major ions, concentrations corresponding to three times the standard deviation ofreplicate blank level measurements. Analytical precision for both cations and anions werewithin 10% (Ramirez et al. 2010; Singh and Mondal 2008; Wang et al. 2007). Certifiedstandards were used for the calibration of instruments for cations and anions. Dionex fiveanions standard and Dionex five cations standard were used for the calibration of the ionchromatograph for determination of anions and cations respectively.

HCO3− exists in rainwater through dissolution of atmospheric CO2 and other particulate

carbonate species. HCO3− concentration would be significant at the alkaline pH values

encountered in most of the rain samples. As HCO3− was seldom determined experimentally,

it was calculated using a theoretical relation between the pH and HCO3− . According to this,

when the pH of a sample is above 5.60 and the sample is in equilibrium with atmosphericcarbon dioxide, the concentration of HCO3

− (μeq/l) is given as

HCO3�½ � ¼ 10�11:2þpH:

2.4 Data quality

Ion balance is an important parameter for data quality assessment. Data quality of eachrainwater event was checked by ionic balance. The data was rejected if it did not meet thequality criteria, which allowed for 15% deviation of ion balance ratio (sum of cations/sum ofanions). Based on the principle of electro-neutrality, the quality of chemical analysis, tosome extent, may be assessed by the ionic balance considering all the major ions in a singlesample. The percent error (E%) was calculated by using equation reported elsewhere and theerror was obtained as 3.5% and 4.5% for Rameswaram and Dadri which was found to bewithin ±5% (Singh et al. 2007a, b). The data has been subjected to the linear regressionanalysis on the basis of the ratio value of the sum of anions to the sum of cations. A strongcorrelation coefficient of 0.92 and 0.88 at Rameswaram and NTPC Dadri was observedbetween sum of anions and sum of cations indicates that most of the ions have been analyzed(Fig. 1). However, less than unity ratio points out that some of the anions are missing. Thesemay be some organic acids that have not been measured in the present study.

The statistical analysis, Oneway ANOVAwas applied to estimate the measurement uncer-tainty in analysis. The analytical variance for the data should comprise not more than, say, 20%

186 J Atmos Chem (2011) 68:183–198

of the total variance. The statistics were performed within and between batches using SPSSsoftware and F test results of Rameswaram and NTPC Dadri which are presented in Table 1.

Rameswaram

0

20

40

60

80

100

120

140

160

0 20 40 60 80 100 120 140 160

Total Cations

To

tal A

nio

ns

NTPC Dadri

0

50

100

150

200

250

300

0 50 100 150 200 250 300

Total Cations

To

tal A

nio

ns

Fig. 1 Linear regression analysis of total cations and anions in rainwater

Table 1 Oneway ANOVA test

Ions Variation df Mean square F df Mean square F

Rameswaram NTPC Dadri

F− Between groups 1 1.77×10−2 0.016 1 0.178 0.020

Within groups 52 1.09 88 9.02

Total 53 89

Cl− Between groups 1 0.719 0.004 1 71.11 0.571

Within groups 52 189.03 88 124.6

Total 53 89

SO42− between groups 1 4.156 0.184 1 92.01 0.565

Within groups 52 22.628 88 162.89

Total 53 89

NO3− Between groups 1 0.581 1.047 1 547.6 11.72

Within groups 52 0.555 88 46.69

Total 53 89

Na+ Between groups 1 9.55 0.077 1 139.37 1.503

Within groups 52 123.5 88 92.76

Total 53 89

K+ Between groups 1 1.48 1.28 1 16.9 9.778

Within groups 52 1.15 88 1.728

Total 53 89

Ca2+ Between groups 1 8.42 0.234 1 567.51 2.55

Within groups 52 35.99 88 222.47

Total 53 89

Mg2+ Between groups 1 1.12 0.394 1 26.67 2.103

Within groups 52 2.86 88 12.68

Total 53 89

NH4+ Between groups 1 0.689 0.126 1 12.844 0.207

Within groups 52 5.46 88 62.061

Total 53 89

J Atmos Chem (2011) 68:183–198 187

The F test statistic was applied to test the null hypothesis. The calculated Fvalue at Rameswaramwere observed in the range of 0.004–1.28 is less than Fcritical of 3.16 for a stated level ofconfidence (typically 95%) means that the difference being tested is statistically significant at95% confidence level. The calculated Fvalue statistics at NTPCDadri were observed in the rangeof 0.020–11.7 is also less than Fcritical of 3.09 except NO3

− and K+ for a stated level ofconfidence (typically 95%) means that the difference being tested is statistically significant at95% confidence level. To reiterate the interpretation of ANOVA results, a calculated F test thatis greater than Fcritical for a stated level of confidence (typically 95%) means that the differencebeing tested is statistically significant at that level for remaining major ions. In this application,the measurement variance contributed 5.51% of the total variance, which is well within 20%target.

3 Results and discussions

3.1 Chemical composition of major ionic content

The statistical summary of volume weighted mean (vwm) concentration of major ions,together with pH in rainwater at Rameswaram and NTPC Dadri are presented in Table 2and depicted in Fig. 2. The average and standard deviation of major ionic content atRameswaram was found to be 218±41μeql−1.The major ions in rainwater showed thegeneral trend Cl−>Na+>Ca2+>HCO3

−>NH4+>SO4

2−>Mg2+>K+>NO3−>F−>H+. The

dominance of Cl− and Na+ indicates that site is nearer to coastal environment. Due to closeproximity to sea, Cl− seems to be dominating species followed by Na+ in the coastalenvironment and bordered by both the Gulf of Mannar and Bay of Bengal on the southernand the northern sides, respectively. The high concentrations of sea salt (C1− and Na+) maybe due to the monsoon currents which carry these particles from the sea extensively due tothe low-pressure system around Bay of Bengal or Arabian Sea, which are highly rich regionsfor sea salt concentrations. These sea salt rich air masses are brought in by monsoon clouds.The contribution of gaseous chloride could have dominant influence on the rainwatercomposition (Rastogi and Sarin 2005). Among the total anions, the sequence observed as

Table 2 Summary of major ionsin precipitation (μeq/l)

All values of major ions inμeq/l except pH

Major ions Rameswaram NTPC Dadri

N027 N045

pH 5.53–6.73 (6.16±0.37) 5.1–6.6 (6.10±29)

F− 1.12–5.34 (2.94±1.05) 3.24–16.0 (10.1±3.0)

Cl− 39.12–85.24 (65.56±13.65) 39–90 (66.4.±11.3)

SO42− 10.11–27.23 (21.22±4.73) 51–109.2 (69.4±12.4)

NO3− 2.12–4.62 (3.66±0.76) 29.0–61 (40.09±7.06)

Na+ 29.02–71.24 (52.68±11.17) 21–69 (51.3±9.6)

Ca2+ 20.76–42.51 (34.43±5.96) 79–149 (103.1±14.9)

K+ 2.78–6.64 (5.08±1.06) 5–11 (7.3.±1.2)

NH4+ 6.02–15.02 (11.22±2.41) 26–64.4 (37.1±7.7)

Mg2+ 4.02–10.89 (8.30±1.71) 14–31 (19.08±3.4)

H+ 0.18–2.95 (1.00±0.92) 0.83–1.03 (0.92±1.01)

HCO3− 2.00–32.78 (11.56±8.30) 0.87–26.8 (10.3±6.3)

188 J Atmos Chem (2011) 68:183–198

Cl−>SO42−>HCO3

−>NO3−>F− whereas for total cations the sequence observed as Na+>

Ca2+>NH4+>Mg2+>K+>H+ respectively. The total anions and cations contribute 48.2%

and 51.3%, respectively indicating that all the most of the ionic species are measured.The average and standard deviation of major ionic content at NTPC Dadri was found to

be 415±49 μeql−1. The major ions in rainwater showed the general trend Ca2+>SO42−>Cl−>

Na+>NO3>NH4+>Mg2+>HCO3

−>F−>K+>H+. The dominance of Ca2+ in rainwater ismainly due to calcareous soil blown up by wind. The interference of soil derived cationsindicates that soil dust significantly alter the nature of rainwater in India (Kulshrestha et al.2001; Tiwari et al. 2007). The total anions and cations contribute 47.47% and 52.43%respectively. A slight cations excess may be due to unaccountable anions formate, acetate etc.In attempting to achieve ion balance with the major ions, the deficit is largely found to lie withanions. Some escaped ions like organic acid; acetate, formate etc. were not measured and notaccounted in the study. The charge balance between cations and anions suggests that the organicacids had a negligible effect on the rainwater composition (Saxena et al. 1996).

The average concentration of SO42−at NTPC Dadri observed as 69.4 μeq−1 indicates

industrialized location whereas average concentration at Rameswaram reported as 21.2 μeq−1

represent coastal environment. Low concentration of SO42− at coastal site indicates that the

location is relatively free from pollution. Sulphate in rain arises mainly from anthropogenicsource like thermal power plant and vehicular activities. The sulphate from sea spray is neutraland does not increase the acidity of rainwater. The high sulphate concentration indicates that thesite is largely affected by the coal plant emission. However, SO4

2− from anthropogenic sourcesis considered as a major component which increases the H-ion concentration in rainwater whichcauses decrease in pH. The average concentration of NO3

− at Rameswaram and NTPC Dadriare 3.66 and 40.09 μeq−1 respectively. The presence of NO3

− in rainwater is also considered tobe responsible for the increase of H-ion concentration (Naik et al. 2002).

There is a difference between concentration of NO3− and SO4

2− at Rameswaram and NTPCDadri sampling sites representing coastal and industrial environment. At Rameswaram, thelower concentration of SO4

2− and NO3−were observed as compared to NTPC Dadri site which

may be due to absence of anthropogenic activities in the region except for vehicular activities.Among the SO4

2− and NO3− concentration, high SO4

2− concentration in the region suggestinfluence of sources whose origin is different frommarine aerosols (Ceron et al. 2008).Over theIndian Ocean, most of the nss- SO4

2− is anthropogenic while at continental sites it is soil derivedcontribution. This is possible due to northeast prevailing winds transported SO2 and anthropo-genic SO4

2− aerosol from land to the ocean which is subsequently scavenged by clouds(Kulshrestha et al. 2001). The high concentration of SO4

2− and NO3− was observed at NTPC

RameswaramF1%

Cl31%

SO410%NO3

2%HCO3

5%

Na24%

K2%

Ca16%

Mg4%

NH45%

NTPC DadriF2% Cl

16%

SO417%

NO310%HCO3

2%Na

12%

NH45%

K2%

Mg9%

Ca25%

Fig. 2 Composition of major ions in rainwater at Rameswaram and NTPC Dadri, India

J Atmos Chem (2011) 68:183–198 189

Dadri as the site is located near the coal based thermal power plant. The burning process of thelocal coal, with a high content of pyrite results in the emission of large amounts of SO2 into theatmosphere and at high temperature oxygen and nitrogen reaction takes place to form oxides ofnitrogen which also release into the atmosphere. In general, the contribution of SO4

2− and NO3−

are higher in dry season compared to wet season, due to enhanced photochemistry duringsummer months (Al-Momani et al. 1995). In the data set although acidifying compounds havehigher concentrations of SO4

2− and NO3−, the higher concentrations of neutralizing agents like

Ca2+, Mg2+and NH4+ does not allow pH to decrease to the values expected from SO4

2− andNO3

− levels. This indicates that pH in the rain water is not only under the control of acidifyingcompounds but also existing of neutralizing species. The SO4

2− andNO3− concentrations imply

the relative contribution of H2SO4 and HNO3 to the rain water acidity which changes from onelocation to another. It is also known that coarse particles are scavenged out from atmospheremore efficiently than fine particles, and a significant fraction of NO3

− (approximately 70%) isassociated with coarse particles, whereas almost all SO4

2− is associated with fine particles in theatmosphere. Consequently, during the wet season, NO3

− is removed faster from the atmosphereby wet deposition than SO4

2−, resulting in higher SO42−/NO3

− ratio in particles. Since particlecomposition reflects in rain water composition, similar ratio is also observed in rain water(Ozlem 2006; Tuncel and Ungor 1996).

3.2 pH in rainwater

The pH of natural rainwater is controlled by the dissolved CO2 and it is greatly influenced by theaddition of acidic components (SO2 and NOx) generated by industrial thermal power activitiesand long range transport over sea. CaCO3 present is soil dust, and NH3 of both natural andanthropogenic origin are the main proton acceptor. Thus acidity in rainwater is the results ofcomplex reactions due to presence of many of these compounds. The% frequency distribution ofpH at Rameswaram and NTPC Dadri are depicted in Fig. 3. At Rameswaram, pH was found tobe in the range of 5.53–6.73 (6.16±0.37). In this study, out of 27 rain events, in 96% cases the pHwas higher than 5.6 which were due to high loading of particulate matter in the atmosphere,which is mostly found in Indian climatic conditions (Charlson and Rodhe 1982). The contribu-tion of SO4

2− is relatively significant in the events characterized by low pH (at or less than 5.6).

Rameswaram

0

20

40

60

80

100

5.0-5.6 5.61-6.0 6.01-7.0

pH values

% F

requ

ency

NTPC Dadri

5.0-5.6 5.61-6.0 6.01-7.0

pH values

Fig. 3 % Frequency distribution of pH in rainwater at Rameswaram and NTPC Dadri, India

190 J Atmos Chem (2011) 68:183–198

The difference in free acidity between the lowest sample (pH 5.53, [H+] 2.95 μeq/l) and naturalrainwater in equilibriumwith atmospheric CO2 (pH 5.6, [H+] 2.51 μeq/l) is only 0.44 μeq/l. Thissuggested that most of the SO4

2− is present as neutral salts ((NH4)2SO4) and not as dissociatedacids (Rastogi and Sarin 2005). At NTPC Dadri, pH was found to be in the range of 5.1–6.6(6.10±0.19) which is in the alkaline range. In this study, out of 45 rain events, in 98% cases thepHwas higher than 5.6. However, one samples of acidic rain (pH<5.6) has been noticed which isattributed to heavy and prolonged rainfall, which reduced the buffering potential of rainwater to aminimum by washing out the dust particles from the atmosphere (Kulshrestha et al. 2003). Theacidic rain event was due to high concentrations of SO4

2− and NO3− which is produced by

combustion of fossil fuels and gets dissolved in rainwater. The difference in free acidity betweenthe lowest sample (pH 5.17, [H+]06.76 μeq/l) and natural rainwater in equilibrium withatmospheric CO2 (pH 5.6, [H+]02.51 μeq/l) is 4.25 μeq/l. The acidic pH reveal the presenceof strong acids in rainwater, while neutral or alkaline pH values are attributed to the effect of soildust, predominantly CaCO3, MgCO3, and ammonia. In Indian condition, acid rain event werenot observed at industrial location is attributed to the characteristics of aerosol which seems to bealkaline in nature and neutralizes the acidity in rainwater. Moreover, the soil in northern India isalkaline and high proportion of carbonates (CO3

−) in the dust clouds act to neutralize the SO2

emissions in the area. (Khemani et al. 1995).

3.3 Sea-salt (SS) and non-sea-salt (nss) contribution

In order to estimate the sea salt and non-sea salt contributions to rainwater, equivalent ratioswere calculated usingNa+ as the reference element and assuming that all Na+ is ofmarine origin(Al-Khashman 2005; Keene et al. 1986). The enrichment factor of Cl−, Ca2+, Mg2+ and SO4

2−

was higher than one indicating significant influence of local and coastal sources (Table 3). AtRameswaram, the observed ratio of Cl−Na+ was 1.24 which is higher than 1.16 is due to stronginfluence of Northeast monsoon wind brought over by Indian Ocean. Sea salt was derived fromthe local source that was not transported directly from the sea. Alternately, it could be of indirectmarine origin, lying underneath for a long time and resuspension of sea salt from soil orenrichment of Na+ ions (Parashar et al. 2001; Kulshrestha et al. 2001). The nss-SO4

2−

contribution was very less (37.9%), indicating remaining major contribution was from sea.At NTPCDadri, the observed rainwater ratio of Cl−/Na+ (1.19) is nearer to seawater ratio (1.16)

Table 3 Enrichment factor (EF),sea salt (ss) and non sea salt(nss) fractions of major ionsin precipitation at Rameswaramand NTPC Dadri

Major ions Cl−/Na+ K+/Na+ Mg2+/Na+ Ca2+/Na+ SO42−/Na+

Rameswaram

Sea water 1.16 0.037 0.038 0.12 0.25

Rainwater 1.24 0.097 0.157 0.653 0.403

SS (%) 93.5 38.3 24.2 18.4 62.1

NSS (%) 5.5 61.7 75.8 81.6 37.9

EF 1.07 2.61 4.14 5.44 1.61

NTPC Dadri

Sea water 1.16 0.037 0.038 0.12 0.25

Rainwater 1.19 0.096 0.25 1.35 0.91

SS (%) 96.8 38.4 15.2 8.88 27.47

NSS (%) 3.2 61.6 84.8 91.12 72.53

EF 1.026 2.604 6.5 11.26 3.64

J Atmos Chem (2011) 68:183–198 191

indicates contribution of sodium in fly ash after burning of coal in thermal power plant, influx ofmarine spray from Bay of Bengal during south west monsoon period and from soil dust in theregion (Tiwari et al. 2006). The non sea salt (nss) contributions of major ions, viz. nss-Ca2+ wasobserved as 91.1% suggesting their crustal origin whereas nss-Mg2+ and nss-K+ showed theircontribution as 84.8 and 61.6% indicating influence of soil sources. The major ions like K+, Ca2+

and Mg2+ are potentially basic in nature and their presence in high concentrations in rainwaterhelps in neutralizing the acidic effects of anthropogenic source (SO4

2− and NO3−) and in

maintaining the pH of rainwater in the alkaline range (Naik et al. 2002). The nss-SO42−minimum

(37.9%) in rainwater at Rameswaram indicates that coastal site, is less influenced by anthropo-genic activities whereas nss-SO4

2− contribution at NTPC Dadri was significant (72.5%), sug-gested that rainwater is influenced by pollutants emitted by anthropogenic sources.

3.4 Neutralization factor (NF)

Neutralization factors were used to evaluate the neutralization of major ions by crustalcomponents and ammonia. The neutralization of acids by base cations were evaluated bycalculating the neutralization factors using the formula:

NF xð Þ ¼ nssXð Þ= NO3� þ nssSO4

2�� �

where X is the component of interest.The NF was observed as NFCa02.40, NFMg00.54, NFNH400.0.96, and NFK00.27 respec-

tively at Rameswaram revealing below cloud process in which crustal components are respon-sible for neutralization of anions (Table 4). The% contribution of Ca2+, NH4

+, Mg2+, and K+ are58, 23, 13 and 6% respectively. Similar studies were reported at Bhubaneswar, Ahmadabad,Goa, and Comba showed dominance of calcium in rainwater (Das et al. 2010; Rastogi and Sarin2005; Parashar et al. 2001; Gobre et al. 2010). The NF was observed as NFCa00.93, NFNH400.41, NFMg00.18 and NFK00.05 respectively at NTPC Dadri reveals that calcium is thedominant neutralization species in the region. The% contribution of Ca2+, NH4

+, Mg2+, andK+ are 59, 26, 11 and 3% respectively. Firstly, Ca2+ has strong buffering capacity in rainwaterwhich greatly reduced the acidity of rain. Secondly NH4

+ is also a major neutralizer in rainwaterwas related to the introduction of gaseous ammonia into the air, which was mainly from plants,fertilizer use and cattle breeding and industrial emissions (Rastogi and Sarin 2005).

3.5 Ionic ratios

The ratio of different ionic constituents was used as an index for evaluating the relativesignificance of these ions to the acidity, alkalinity and the degree of terrestrial/anthropogenic

Table 4 Neutralization factor for major ions in precipitation

Locations Ca2+ Mg2+ NH4+ K+ References

Rameswaram 2.40 0.54 0.96 0.27 Present study

NTPC Dadri 0.93 0.18 0.41 0.05

Comba 1.22 0.42 0.37 0.14 Gobre et al. 2010

Ahmedabad 1.77 0.11 0.73 – Rastogi and Sarin 2005

Bhubaneswar 0.70 0.17 0.64 0.06 Das et al. 2010

Goa 1.26 0.12 0.27 – Parashar et al. 2001

192 J Atmos Chem (2011) 68:183–198

sources. The acidic/alkaline nature of the rainwater is the result of balance between acidicand alkaline species. The average SO4

2−/NO3− ratio of rainwater at Rameswaram and NTPCDadri are 5.8 and 1.73 (Table 5) indicating the high concentration of SO4

2− at both thelocations. Further the contribution of NO3

− and SO42− to rainwater acidity was obtained

from average ionic equivalent ratios of NO3−/H+ (3.6) and SO4

2−/H+ (21.1), SO42− contrib-

uted 85% to the acidity and NO3− contributed 15% at Rameswaram whereas average ionic

equivalent ratios of NO3−/H+ (43.5) and SO4

2−/H+ (76.4), SO42− contributed 63.7% to the

acidity and NO3− 36.3% at NTPC Dadri respectively. The higher equivalent ratio of free

acidity in rainwater is due to SO42− which contributed acidity in a greater proportion than

NO3− reflects dominance of industrial sources. The acidity is primarily buffered by Ca2+ and

NH4+ ions in the region. The coarse mode Ca2+ aerosols tend to occur at a lower height while

submicron mode NH4+ aerosols, mainly in the form of ammonium sulphate have relatively

higher concentration at higher altitude (Kulshrestha et al. 1996). The ratio of H+/(NO3

−+SO42−) is also an indicator to determine the extent of neutralization process in

rainwater. The ratio close to zero indicates extensive neutralization whereas values closeto one indicate lack of neutralization in rainwater. The ratio was observed as 0.04 and 0.008for Rameswaram and NTPC Dadri which is close to zero, indicates 99.99% of acidity wasneutralized in rainwater (Seinfield 1986).

Similarly equivalent ratio of (SO42−+NO3

−)/(NH4++Ca2+) is also used as an index for

evaluating the degree of anthropogenic activities. The presence of NO3− and SO4

2− contentin rainwater are through HNO3 and H2SO4 derived from anthropogenic sources of SO2 andNOx and their secondary aerosols (Galloway 1995; Mphepya et al. 2004; Singh and Mondal2008). The scatter plot of (SO4

2−+NO3−) vs (NH4

++ Ca2+) at Rameswaram and NTPC Dadriare depicted in Fig. 4. The correlation coefficient between (SO4

2−+NO3−) vs (NH4

++ Ca2+)at Rameswaram and NTPC Dadri was 0.86 and 0.92, indicating that the acidity of rainwaterwas neutralized by either CaCO3 or NH3 or both. However, when the sum of (SO4

2−+NO3−)

is fitted against (Ca2+), the correlation coefficient was 0.77 and 0.87 at Rameswaram andNTPC Dadri respectively, indicating that CaCO3 is the major neutralizing agent for both theregion (Al-Momani 2003; Khemani et al. 1994).

To assess the balance between acidity and alkalinity, ratio of neutralization potential (NP)to acidifying potential (AP) was computed as, NP/AP02[nss-Ca2+]+[NH4

+]/2[nss-SO42−]+

[NO3−]. At Rameswaram, average observed values of NP is 74.4 (ueq/l) and AP is 34.5 (ueq/l)

respectively. The relationship NP>AP indicates that value of NP was much higher (NP/AP02.15) and indicates that Ca2+, Mg2+ and ammonium ions are sufficient to neutralize the acidityin the region. At NTPC Dadri, average NP values observed as 207 (ueq/l) and AP value as140.7 (ueq/l) respectively. The relationship NP>AP indicates that although value of AP is highhowever value of NP was much higher (NP/AP01.47).

Table 5 Ratios of different ionicconstituents in rainwater Ratios of major ions Rameswaram NTPC Dadri

Ratio values

SO42−/NO3 5.8 1.73

NO3−/H+ 3.6 43.5

SO42−/H+ 21.1 76.4

NP/AP02[nss-Ca2+]+[NH4+]/2

[nss-SO42−]+[NO3

−]2.15 1.47

H+/(NO3−+SO4) 0.04 0.008

J Atmos Chem (2011) 68:183–198 193

3.6 Correlation matrix

A correlation matrix between major ions for Rameswaram and NTPC Dadri are presented inTables 6 and 7. At Rameswaram and NTPC Dadri, a significant correlation was noticed betweenthe acidic ions SO4

2− and NO3− (r00.83) and SO4

2− and NO3− (r00.93) indicating their origin

from similar sources, because of the similarity in their behavior in rainwater and the co-emissionsof their precursors SO2 and NOX (Kumar et al. 2002). Similarly, highly significant correlation forNH4

+ and SO42− (r00.78) and NH4

+ and NO3− (r00.70) at Rameswaram indicates that

(NH4)2SO4 and NH4NO3 form by the reaction of H2SO4 and HNO3 with NH3 on moist soilsurface and partly because of formation of un-dissociated NH4NO3 and (NH4)2SO4 when relativehumidity is more than 62%. The correlation between NH4

+ and SO42− (r00.88) followed by

NH4+ and NO3

− (r00.79) at NTPC Dadri suggesting their association due to contribution ofanthropogenic activities and they occurred in the form of (NH4)2SO4 and NH4NO3 (Seinfield1986). The relationship between Na+ and Cl− (r00.84) indicated predominantly associated seasalt ions derived from oceanic circulation; therefore contribution of sea-salt decreases with theincreasing from seacoast environment whereas Na+ and Cl− (r00.91) indicates contribution ofsodium is fly ash, after the coal burning process in thermal power plant. Similarly, a strongcorrelation was seen between Ca2+ and Mg2+ suggesting the common source of these ions fromnatural source (crustal origin) probably in the form of carbonate/bicarbonate and/or oxide salts (asCaCO3/CaO, MgCO3). This was due to calcareous nature of soil in the region. Other relativelygood correlations were observed between K+ and SO4, K

+ and NO3−, Ca2+ and SO4

2−, Ca2+ andCl−, Mg2+ and SO4

2− andMg2+ and NO3− showed common occurrence in rainwater as a result of

atmospheric chemical reactions probably from the mineral acids in the atmosphere such as HNO3

Rameswaram

0

15

30

45

60

75

0 10 20 30 40

(SO4+NO3)

(Ca+

NH

4)Rameswaram

0

25

50

75

100

0 5 10 15 20 25 30

(SO4)

(Cl+

NO

3)

NTPC Dadri

0

30

60

90

120

150

180

0 30 60 90 120

(SO4)

(Cl+

NO

3)

NTPC Dadri

0

50

100

150

200

250

0 50 100 150 200

(SO4+NO3)

(Ca+

NH

4)

Fig. 4 Scatter plot between SO42− vs (NO3

−+Cl−) and (SO42−+NO3−) vs (Ca2++NH4

+)

194 J Atmos Chem (2011) 68:183–198

and H2SO4 with alkaline compounds rich in carbonate materials, carried into the atmosphere bysuspension and resuspension of wind-blown dust in the coastal environment (Rastogi and Sarin2005). At Rameswaram, correlation between Ca2+ and Cl− (r00.66), and Ca2+ and Na+ (r00.60)indicating origin from soil and sea. A strong correlation between NH4

+ and Cl− (r00.77) and K+

and Cl− (r00.67) indicates significant presence of NH4Cl and KCl in the region.

3.7 Geographical comparision

The dominance of major ions in rainwater geographically differs from coastal and industriallocations are presented in Table 8. Our present study at Rameswaram showed a dominanceof Cl− in the region. Similar studies were also reported which showed a dominance of Cl− atComba, [Cl−>Na+>Ca2+>SO4

2−>Mg2+>NH4+>NO3

−>HCO3−>F−>H+] Alibagh [Cl−>

Na+>Mg2+>Ca2+>SO42−>NO3

−>NH4+>K+], Goa [Cl−>Na+>Ca2+>Mg2+>SO4

2−>NO3

−>K+], and Bombay [Cl−>Na+>Ca2+>Mg2+>SO42−>NO3

−>K+] showed dominance

Table 6 Correlation matrix of major ions at Rameswaram, India

Ca2+ Cl- F− H+ HCO3 K+ NH4 Na+ Mg2+ NO3− SO4

2−

Ca2+ 1.0

Cl− 0.66 1.0

F− 0.03 0.13 1.0

H+ −0.33 −0.27 −0.01 1.0

HCO3 0.33 0.27 0.01 −1.0 1.0

K+ 0.53 0.67 −0.10 −0.11 0.11 1.0

NH4+ 0.66 0.77 −0.07 −0.01 0.01 0.78 1.0

Na+ 0.60 0.84 0.18 −0.45 0.45 0.69 0.76 1.0

Mg2+ 0.64 0.81 0.03 −0.17 0.17 0.84 0.78 0.59 1.0

NO3− 0.33 0.49 0.13 −0.07 0.07 0.67 0.70 0.53 0.64 1.0

SO42− 0.79 0.67 0.02 −0.15 0.15 0.78 0.78 0.70 0.77 0.83 1.0

Bold value indicates r0>0.53 are considered as significant

Table 7 Correlation matrix of major ions at NTPC Dadri, India

Ca2+ Cl− F− H+ HCO3− K+ NH4

+ Na+ Mg2+ NO3− SO4

2−

Ca2+ 1.0

Cl 0.16 1.0

F −0.40 −0.09 1.0

H+ −0.21 −0.08 −0.10 1.0

HCO3 0.23 0.01 −0.21 −0.57 1.0

K+ 0.75** 0.09 −0.24 −0.21 0.28 1.0

NH4+ 0.73** −0.08 −0.43 −0.09 0.32 0.77 1.0

Na+ 0.10 0.91** 0.06 −0.05 0.02 0.10 −0.07 1.0

Mg2+ 0.83** 0.03 −0.37 −0.11 0.29 0.73** 0.80** 0.07 1.0

NO3 0.81** −0.09 −0.42 −0.04 0.27 0.73** 0.79** −0.09 0.89** 1.0

SO42− 0.88** −0.02 −0.39 −0.18 0.31 0.83** 0.88** −0.02 0.93** 0.93** 1.0

Bold value indicates r0>0.73 are considered as significant

J Atmos Chem (2011) 68:183–198 195

of Cl− (Naik et al. 2002; Parashar et al. 2001; Sequeria 1976) whereas at Colaba [Na+>Cl−>Ca2+>Mg2+>SO4

2−>NO3−>NH4

+>K+], Silent Valley [Na+>Ca2+>Cl−>NO3−>SO4

2−>Mg2+>K+>NH4

+], Chembur [Na+>Cl−>Ca2+>NO3−>NH4

+>Mg2+>SO42−>K+], there is

a dominance of Na+ as these sites are nearer to sea coast (Naik et al. 2002; Rao et al. 1995;Khemani et al. 1994). The present study along with Asansol and Bhubaneswar showed adominance of Ca2+ which is the common feature of northern Indian region with terrestrialand industrial sources (Banerjee 2008; Das et al. 2010). In Panipat industrial area, domi-nance of sulfate concentration signifies the anthropogenic contribution from fossil fuelburning, vehicular and industrial emission (Tiwari et al. 2007). In Ballia, there is a domi-nance of ammonium, attributed to agriculture and human and animal excretion activities inthe region (Tiwari et al. 2006).

4 Conclusion

The investigations of chemical composition of major ions in rainwater at Rameswaram andNTPC Dadri were assessed with an aim to characterize the chemical composition ofrainwater and identify possible sources. The major conclusions drawn from the presentstudy are:

& The pH values were observed at Rameswaram and NTPC Dadri are of alkaline naturewhich may be due to influence of cations that originated from the soil

& Dominance of Cl− was observed in coastal environment whereas dominance of Ca2+ atNTPC Dadri in industrial environment

Table 8 Comparision of major ions in different parts of India (μeql−1)

Site Cl− NO3− SO4

2− Na+ Ca2+ K+ Mg2+ NH4+ References

Cl− dominance sites

Rameswaram 65.5 3.6 21.2 52.6 34.4 5.08 8.3 11.2 Present study

Comba 109.8 12.1 23.3 86.8 43.4 4.8 15.1 13.2 Gobre et al. 2010

Alibagh 236 9 36 220 46 5 64 8 Naik et al. 2002

Goa 113.4 5.5 27.4 97.2 41.5 2.5 24.5 5.5 Parashar et al. 2001

Bombay 138 – 10 115 36 3.6 24 – Sequeria 1976

Na+ dominance sites

Colaba 171 34 52 179 133 6 59 12 Naik et al. 2002

Silent Valley 43.0 21.0 20.0 46.0 43.0 4.0 14.0 3.0 Rao et al. 1995

Chembur 164.5 29.5 70.4 168.2 89.5 6.9 36.5 41.1 Khemani et al. 1994

Ca2+ dominance sites

NTPC Dadri 66.4 40.0 69.4 51.3 103.1 7.3 19.08 37.1 Present study

Bhubaneswar 18 10 19.1 15 20.2 1.8 5.2 18.7 Das et al. 2010

Asansol 63 16 44 20 107 4 37 33 Banerjee 2008

SO42− dominance site

Panipat 25.5 30.9 79.6 21.3 76.5 16.5 24 37.3 Tiwari et al. 2007

NH4+ dominance site

Ballia 8 16 37 6 40 3 5 58 Tiwari et al. 2006

196 J Atmos Chem (2011) 68:183–198

& The acidic rain event observed at NTPC Dadri is due to emissions of sulphur andnitrogen from combustion of fossil fuel thermal power plant and other industries

& The major ions at coastal site showed minimal anthropogenic influence representbackground values in the region whereas industrial site showed anthropogenic influencein the region

& The relationship NP>AP was higher at both the sites indicates that Ca2+, Mg2+ andNH4

+ are sufficient to neutralize the acidity& The acidity due to SO4

2− was observed at Rameswaram whereas SO42− and NO3

− atNTPC Dadri

& The correlation coefficients of (SO42−+NO3

-) vs (Ca2+) at Rameswaram and NTPCDadri were 0.77 and 0.87 indicates that CaCO3 is the major neutralizing agent for boththe region

& Alkaline species were influenced mostly by the natural process which neutralizes theacidic species generated by manmade sources effectively.

Acknowledgments We thank the Director, National Environmental Engineering Research Institute (NEERI),Nehru Marg, Nagpur for continuous encouragement and keen interest in publishing this work. The R & Dwork was carried out from Project No. C-1-(1756). Thanks to Environmental Material Division, for extendingion chromatography facilities for major ions measurement in rainwater samples. Assistance provided by Mr.Senthil Kumar and Mr. Ranjit Kumar in collection of rainwater samples at Rameswaram and NTPC Dadri isalso acknowledged. Mr. C.W. Francis also acknowledged for editing the manuscript for proper English styleand grammar as native English speaker.

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