Chemistry of Monsoon and Post-Monsoon Rains at a High Altitude Location, Sinhagad, India

Rainwater samples collected at Sinhagad on 65 rain occasions during monsoon season (June – September 2005) and on 23 rain occasions during post-monsoon season (October-November 2005) with standard rain collection instruments, i.e. wet-only (WO) and bulk collectors (BC), were considered for the present study. Sinhagad is a hill station on a mountaintop in the Western Ghats, located about 40 km southwest of Pune. The following ionic components were determined: H, NH4, Ca, Mg, K, Na, SO4, NO3, Cl and F. The pH analyses showed that rainwater in both the seasons were alkaline. The major neutralizing component was associated with Ca. In monsoon rains, the major anion was Cl and the major cation was Na; whereas in post-monsoon the major anion was SO4 and the major cation was Ca. The concentrations of nss SO4, NO3 and NH4 were found to be higher during post-monsoon than monsoon. Surprisingly high concentrations of Ca and SO4 were found during the monsoon season. Since no large upwind sources of these compounds are expected, one is led to conclude that long-range transport may be involved, possibly even including African sources. Since the local sources didn’t seem to influence the samples significantly, this rural site is useful for obtaining regionally representative precipitation data.


INTRODUCTION
changes in precipitation chemistry are useful indicators of trends in air chemistry.
Furthermore, the chemicals carried by the rain can affect ecosystems, sometimes positively but usually negatively.For these reasons, measurements of precipitation chemistry have been a standard feature of many national monitoring programs.It is well documented that precipitation in coastal areas is strongly affected by sea salt (Ozeki Rainfall is one of the most effective ways of removing atmospheric pollutants.The chemical composition of rainfall reflects the quality of the air through which it falls (Hideaki et al., 2008).Consequently, et al., 2006), while in inland areas it contains proportionately more substances originating from soil (Mouli et al., 2005).Past studies have shown very high concentration of anthropogenic substances like NO 3 -and SO 4 2-in urban or industrial areas compared to other areas (Tu et al., 2005).
Europe and North America have been adversely affected by acidic deposition (Balasubramanian et al., 1999;Rodhe et al., 2002;Hu et al., 2003).Emission of air pollutants is increasing rapidly in many southeastern Asian countries (Granat et al., 1996) and China (Rodhe et al., 2002), due to faster growth in population and the consequent upward trend in agricultural production, industrialization, energy consumption, transport, housing, etc.
Systematic observations on the chemical composition of precipitation has been carried out for several years in Europe and North America (Rodhe et al., 2002) but such studies are limited in the rest of the world, especially under non-urban conditions in the tropical region (Granat et al., 1996;Whelpdale et al., 1996;Norman et al., 2001;Rodhe et al., 2002).Rainwater composition data are useful for the validation of Global Chemistry Transport Models (GCTMs) (Langner and Rodhe, 1991;Rodhe et al., 1995) et al., 1989;Rao, 1997;Pillai et al., 2001;Safai et al, 2004;Momin et al., 2005) and the Indo-Swedish collaboration on atmospheric chemistry (Parashar et al., 1996;Granat et al., 2001;Norman et al., 2001;Kulshrestha et al., 1996;Satsangi et al., 1998;Srinivas et al., 1999;Jain et al., 2000;Kumar et al., 2002;Kulshrestha et al., 2003).These studies have generally highlighted the alkaline nature of rainwater in India, possibly due to the contribution of soil-derived particles in the atmosphere, which are found to buffer its acidity during a below cloud scavenging process.
Furthermore, variation in composition reflects local or regional characteristics.
Researchers have monitored and collected rainwater samples in many urban and rural areas to determine the composition of the rainwater which may subsequently help in understanding the relative importance of the different contributing sources to rainwater chemistry.As the studies on seasonal variation in precipitation chemistry are sparse, especially at high altitude locations in India, the present study will fill this gap to some extent.

Location of Sampling Site
Rainwater samples were collected at Sinhagad, a hill station about 40 km

Chemical Analysis of Rainwater
Subsequent to the collection of samples,  et al., 1996).

Quality Control Methods
Proper care was taken in collection and preservation of these samples until the completion of all the analyses.Samples that were found to be contaminated with dust or bird droppings were removed.The data obtained by the chemical analyses were then subjected to the quality check which was done by employing the ion balance method.
It was found that the ratio between the sums of cations and anions of individual samples varied between 0.8 and 1.2.A significant correlation was observed between the sum of anions and sum of cations as shown in

Acidic and Neutralization Potentials
The difference between acidic potential

Enrichment Factors
Enrichment factors (EF) are normally applied to identify the origin of an element in precipitation (Zhang et al., 2006).This calculation is based on the elemental ratio found between ions collected in the atmosphere or in precipitation, as compared to a similar ratio for a reference material.
The EF values may imply for information about the source of the element.Commonly, Na is taken as the best reference element for seawater since it is assumed to be of purely marine origin.The EFs were calculated for K, Ca, Mg, Cl and SO 4 by using the following equation and given in Table 1.
Where, X is the concentration of the ion of interest.All the ionic concentrations were taken in µeq/L.In both the seasons, the EFs were found to be high for Ca, SO 4 and K indicating other-than-sea sources; i.e., soil or anthropogenic.

Percentage Contribution of Ionic Species
The percentage contributions of various ionic species to total ionic content in bulk  indicates that all the ionic species, both soiloriented and anthropogenic were found to be higher at Pune, which is an urban location.in this region is smaller by a factor of ten.
3. The major anions in most of the samples were Cl -and SO 4 2-and the major cations were Ca 2+ and Na + .4. The difference between composition of bulk and wet-only rainwater samples was quite small.It implies that the local sources (within the nearby surroundings) didn't seem to influence the Sinhagad samples significantly.This shows that this rural site is useful for obtaining regionally representative precipitation chemistry data.

Fig. 1 .
Fig. 1.Location of observational measuring site in India.
their pH and conductivity were measured as immediately as possible.The pH was measured with a digital pH meter using reference and glass electrodes standardized at pH 4.0 and 9.2.Conductivity was measured with a digital conductivity meter calibrated against a reference KCl solution of analytical grade, E-merk make.The conductivity was measured at actual sample temperature.The time delay between the conductivity measurements and chemical analyses was about two weeks.Thymol was then added to the samples to prevent biodegradation and the samples were kept in a refrigerator at 4℃ until the completion of all the analyses.Anions F -, Cl -analyzed with an Ion Chromatograph.Cations Na + , K + , Ca 2+ and Mg 2+ were measured with an Atomic Absorption Spectrophotometer

Fig. 2 .Fig. 2 .Fig. 3 .Fig. 4 .
Fig. 2.This indicates the completeness of the analyses; i.e., all the major ionic components were analyzed.The second check was made through comparison between the measured and calculated (EMEP/CCC-Report 1/95, 1996) conductivities of the individual samples as shown in Fig. 3.A good agreement between the measured and calculated conductivities (R = 0.98 and 0.99) further confirms the good quality data of the chemical analyses.The higher total ionic concentrations in monsoon samples than in post-monsoon samples (Fig. 2) are due to high concentrations of all the ionic components, except NO 3 and NH 4 , in monsoon season than in post-monsoon season.In monsoon season the higher rain fall intensities, sea Figs. 5 and 6 indicate the composition of bulk and wet-only rainwater samples respectively, collected during monsoon and post-monsoon seasons at Sinhagad.It can be seen that except for NO 3 2-, all the chemical constituents were found to be higher in bulk samples than in wet only samples.This is because the bulk collector remains open all the time and the large particles falling under gravity (dust-fall) get deposited even if there is no rain.Particles are also deposited through impaction and by sorption of gases on already deposited material in the open collector (Pillai et al., 2001).During the monsoon season Cl -showed the highest concentration among all.Concentrations of Na + and Cl -were higher (almost two times more) during monsoon than during post-monsoon, which is not surprising with the Arabian Sea as the origin.However, concentrations of NO 3 2-and NH 4 2were found to be higher during postmonsoon than monsoon.In the case of SO 4 2-, even though the total SO 4 2-was higher in monsoon season, the non-sea salt sulfate (nss SO 4 2-) was found to be higher in postmonsoon period.The higher concentrations

(Fig. 5 .
Fig. 5. Comparison between concentrations of chemical ions measured in bulk rainwater samples collected at Sinhagad in 2005.

Fig. 6 .
Fig. 6.Comparison between concentrations of chemical ions measured in wet-only rainwater samples collected at Sinhagad in 2005.
and wet-only rainwater during monsoon season and post-monsoon season are shown in Fig. 8.During the monsoon season, among the cations, Na + was dominant, and among the anions, Cl -was dominant; whereas, during the post-monsoon season, Ca 2+ was dominant among the cations and SO 4 2-was dominant among anions.This indicates that the sea salt contribution is more during monsoon season, during which the southwest monsoon winds bring sea salt from the Arabian Sea to the sampling location.The percentage contributions Mg 2+ ) and HCO 3 -were found to be less in post-monsoon season than those in monsoon.These differences are due to the change in prevailing wind directions during both seasons.The back trajectories at 1400 amsl obtained from Hysplit, NOAA, USA for the monsoon and post-monsoon seasons are shown in Fig 7. It can be seen that during the monsoon season, winds originate from the African countries before passing through Arabian Sea and reaching the sampling location.During the post-monsoon season, the winds were from either the north or northwest and traveled completely over land before reaching the sampling location.Hence, there is a possibility of long-range transport of aerosols over the Sinhagad region when air masses come from as far as the East African Gulf Coast during monsoon season and from northern and northwestern regions of India during post-monsoon.Factor Analysis Source identification of different chemical ions present in precipitation samples at Sinhagad was further carried out through Varimax rotated factor analysis (SPSS, 1983), which are shown in 2+ and SO4 2-were measured during the southwest monsoon season.Since no large upwind sources of these compounds are expected on the Indian subcontinent, it is concluded that long-range transport may be involved, possibly from African sources, which is also verified by trajectory analyses.nss K + in post-monsoon season than those in monsoon is due to the differences in the air masses reaching the sampling location from different regions due to change in wind directions.

Table 1 .
Comparison of sea water ratios with rainwater ratios with respect to Na and enrichment

Table 2 .
Factor loadings matrix obtained from rainwater samples at Sinhagad during 2005.

Table 3 .
Comparison between the chemical composition (μeq/L) of wet-only and bulk rainwater samples for monsoon seasons at Pune (2003) and Sinhagad (2005).