Chemical composition of rain water and influence of airmass trajectories at a rural site in an ecological sensitive area of Western Ghats (India)

Samples of rain water were collected during monsoon season (June to September) of 2006 and 2007 at Hudegadde, a rural site located in an ecological sensitive area of Western Ghats. The collected samples were analyzed for pH, conductivity and major ions. At this site, rainwater pH varied from 4.20 to 7.39 with 5.65 as volume weighed mean. The observed mean was slightly lower than the average pH reported at most of the Indian continental sites. Monthly variation showed that average pH of rain water was the lowest during September (end of monsoon) and the highest during July (peak of monsoon). Overall, marine sources had dominating influence at this site. However, significant influence of anthropogenic and crustal sources from local as well as inter-continental regions was also noticed. As compared to NO(3)(-), higher concentration of SO(4)(2-) was noticed which might be due to contribution from industrial activities responsible for SO(2) emission. At this site, influence of five types of airmass trajectories was noticed i.e. i) C.I.O. (Central part of Indian Ocean)-when air masses blown from Maldives and nearby region of central Indian ocean. These airmasses had higher concentrations of nss Ca(2+) which did not show any adverse impact on the pH; ii) N.W.I.O.(North-West Indian Ocean)- when airmasses travelled from oceanic region close to north-east Africa. These airmassses had higher concentrations of nss sulphate and nitrate and gave rise to acid rain; iii) S.W.I.O. (South -West Indian Ocean)when airmasses came from southern part of Indian ocean (close to Mauritius). During these airmasses, rain water samples had almost equal ratio of nss SO(4)(2-) and nss Ca(2+) similar to N.W.I.O but very low NO(3)(-); iv) Gulf-when airmasses were observed coming from Gulf region. Although these airmasses contributed only 2% of the total number of samples but carried high amount of nss SO(4)(2-) which gave rise to acid rain. The second lowest pH was observed during these airmasses which might be due to very high nssSO(4)(2-)/nssCa(2+) ratios; v) N.W.I.O. + S.W.I.C. (North-West Indian Ocean+South-West Indian Continental)- when airmasses originated from north-west Indian Ocean travelling towards south continental part of India and then arriving to the site. During these airmasses, samples showed typical influence of urban activities having high concentrations of nss SO(4)(2-) and NO(3)(-) leading to the lowest pH of rain water.