Chemical characteristics of rainwater in Central India

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Air quality in India is depleting tremendously due to a huge vehicular and industrial emissions. Rain is formed due to condensation of the cloud droplets in the upper troposphere and scavenges the atmospheric air pollution. The rain chemistry is indicator of the atmospheric pollution in a particular area. The rain chemistry in the various locations of the World has been reported [1-30]. The Asian rainwater was contaminated with higher levels of ions due to increased human and industrial activities [8-30]. The rain chemistry of some locations of India i.e. Delhi, Mumbai, Dhanbad, Osansol, Bhubaneswar, Tirupati, etc. has been investigated [22-30]. air rain emission pollution

The rain chemistry of the most industrialized location of central India has been not reported. In the present work, the rainwater chemistry of Raipur city (capital, Chhattisgarh state, central India) during period, May - October, 2008 is described. The content, variations, sources, enrichment and fluxes of ions i.e. Cl-, NO3-, SO42-, NH4+, Na+, K+, Mg2+ and Ca2+ are discussed.

Materials and methods.

Site Description.

Raipur city (21° 13? 48? N, 81° 37? 48? E) capital, Chhattisgarh state, central India was selected for the proposed studies due to running of several industries and thermal power plants, Figure 1. The population of city is 2 million and exposed with severe air pollution [31].

The rain gauze was used for collection of rain samples on the event basis. The collector was installed at roof of building, ? 10 m from the ground level in Dagania, Rohinipuram, Raipur. The collector was washed with deionized double distilled water after collection of each event samples. Total amount of rain precipitated in Raipur during period, May - October, 2008 was 67 cm in 42 events. Of them, 31 event samples was collected, filtered and transferred into 1-lit cleaned polyethylene bottle. They were refrigerated at 4 oC to avoid the degradation.

Analysis.

The pH, conductivity and TDS values of the samples were measured immediately after collection by using the Checkmate (E - 58902) analyzer. The content of ions i.e. Cl-, NO3-, SO42-, NH4+, Na+, K+, Mg2+ and Ca2+ was analyzed by using technique i.e. ion chromatography (Dionex Corporation, Sunnyvale, CA, USA) equipped with anion (AS9-HC, 250x4 mm) and cation (CS12A, 250x4 mm) equipped with separation columns and conductivity detector.

The E. Merck standard was used for preparation of the calibration curves. The laboratory blank was used to assess the contamination.

Figure 1 - Location of Raipur in India and sampling description

Results and discussion.

Meteorology.

The monthly mean values of meteorology i.e. rainfall (RF), ambient temperature (T), relative humidity (RH), vapor pressure (VP) and wind speed (WS) of Raipur during the study period are summarized in Figure 2. The mean value of T, RH and VP during the rainy season, July - September was found to 28 oC, 83% and 23 mm of Hg, respectively. The 98% (66 cm) of the total rain was precipitated in four months, June - September, 2008 with the highest precipitation in month of September, 2008.

The value of wind speed was decreased sharply from June to September, 2008. The correlation matrix of the meteorology with the rain composition is shown in Table 1. The meteorology remarkably influenced the rain content of three ions i.e. Na+, Mg2+ and Ca2+. The RF value had negative correlation with the content of ions i.e. Cl-, NO3-, SO42-, Na+ or Ca2+, and they become diluted when the precipitation of rain amount was increased.

The T value had good positive correlation with the content of two ions i.e. Na+ and Ca2+ unlikely to other ions i.e. Cl-, NO3-, SO42- and NH4+. Increase in value of RH and VP remarkably decreased the rain content of Na+ unlikely to Mg2+. The rain content of the most of the ions was diluted with increase in WS value, may be due to their large emission by the local emission sources.

Figure 2 - Meteorology 2008 for study period

Table 1 - Correlation matrix of ions with meteorology

Physical characteristics.

The volume weighted mean value (n=31) of pH, conductivity and TDS of the summer monsoon rain at Raipur was 6.42, 483 µS and 237 mg l-1, respectively. The lowest pH value was observed in month of July, probably due to the highest wind speed, Figure 3. However, the value of conductivity and TDS was found to be increased from July to September, 2008, Figure 4.

Figure 3 - Volume weighted monthly pH and WS (km hr-1) for year 2008

Figure 4 - Volume weighted monthly conductivity and TDS value for year 2008

Chemical characteristics.

The volume weighted mean content of Cl-, NO3-, SO42-, NH4+, Na+, K+, Mg2+ and Ca2+, was 2.83, 2.71, 7.64, 0.73, 1.00, 0.92, 1.1 and 2.53, mg l-1, respectively. The relative abundance of ions i.e., NH4+, Na+, K+, Mg2+, Ca2+, NO3-, Cl- and SO42- was 4, 5, 5, 6 , 13, 13, 14 and 39%, respectively. The sum of ratio of equivalent content of the [anion]/ [cation] at Raipur was found to be 1.02 with coefficient (r) value of 0.35.

Scavenging ratio.

The annual mean content of species i.e. Cl-, NO3-, SO42-, NH4+, Na+, K+, Mg2+ and Ca2+ associated to the PM10 in the air of Raipur was 4.8, 6.4, 9.5, 1.6, 2.2, 2.1, 0.7 and 7.1 µg m-3, respectively. The scavenging (SR) value for
Cl-, NO3-, SO42-, NH4+, Na+, K+, Mg2+ and Ca2+ was found to be 590, 423, 804, 456, 455, 438, 1571 and 356, respectively. Among them, Mg2+ and SO42- exhibited relatively higher SR value, due to the most effectively scavenging out [29].

Variations.

The volume weighted monthly mean content of the ions in rain of Raipur is shown in Figure 5. The highest content of almost all species during month of June, 2008 at Raipur was observed due to first flush out of monsoon water. No exact seasonal variations in other three months: July, August and September was noticed. The highest content of SO42- was observed in the rain water.

Figure 5 - Volume weighted monthly of ions for year 2008

Correlation and Sources.

The correlation matrix of ions is summarized in Table 2. Anions (i.e. Cl-, NO3- and SO42-) and cations (i.e. Na+, K+, Mg2+ and Ca2+) among themselves have good correlation, suggesting their common origin in the rain water. All ions among themselves (except Cl- and NO3-) had fair to good correlation. They are contributed by the anthropogenic and natural sources. The following approach [33] is used to estimate

the marine contribution of ions in the rain.

EFmarine = ([x]/ [Na])rain/ ([x]/ [Na]marine

Where, x denotes the concentration of the species of interest in the rain and sea. The EFmarine values for ions i.e. Cl-, SO42-, K+, Mg2+ and Ca2+ was ranged 1.6, 31, 25, 29 and 21, respectively when Na was used as marine indicator. It means that ions i.e. SO42-, K+, Mg2+ and Ca2+ in the rain was largely contributed by anthropogenic and crustal sources.

Table 2 - Correlation matrix of ions

Comparison of rain composition.

The ionic composition of rain of various locations of the World is presented in Table 3. The highest rain content of the ions i.e. Na+, Cl- and Ca2+ were observed in the locations (i.e. Mumbai, Nanjing, Eshidiya, Shanghai, etc.) lie near sea due to marine contribution. Whereas, the highest rain content of ions i.e. SO42-, NO3- and Mg2+ were seen in the industrial and mega cities i.e. Tokyo, Nanjing, Shanghai, Esidiya, Mumbai, Delhi, Asansol, Raipur, etc., due to anthropogenic contribution. Significant rain content of NH4+ was observed in the Chinese cities i.e. Nanjing, Zhejiang, etc. The rain content of ions of Raipur is found to be higher than the rain ion content value of many locations, Table 3.

Table 3 - Comparison of rain acidity and ionic contents, µeq

Conclusion.

The acid rain is halting in Asia due to increased industrial and vehicular emission of acidic oxides i.e. NOx and SOx. The two acids i.e. H2SO4 and HCl have major contribution in the Asian rain. Increase in rain amount and wind speed influence adversely the rain content of ions i.e. Cl-, NO3-, SO42-, Na+ and Ca2+. The atmospheric humidity and vapor pressure has positive correlation with the atmospheric acidity. In central India, the rain ions are largely contributed by the anthropogenic sources.

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