Ø It is observed that upto 1980, pH values from Indian GAW stations were around 7.0 or even higher. As of about 1990 large number of pH values ranged between 5.0 and 6.0, a severe change has taken place. Analysis of two decades (1981-2000) data show that annual precipitation weighted mean pH values were between 3.38 and 8.20. All the GAW stations except Jodhpur show an increasing trend towards lowering of pH of rainwater. This can be attributed not only to increasing sulfate concentration but even more by increasing nitrate concentration.
Ø The concentration of H+ ions shows a major geographical gradient from west to east. At north-western station (Jodhpur) the range of annual average pH values is 6.6 to 7.8, whereas at Allahabad it is 5.2 to 7.6 and very low values 4.5 to 6.8 were found at the station Mohanbari from north-east.
Ø The station Kodaikanal shows the least mineralized samples of all. Except for the H+ values, the concentrations of all compounds either represent the minimum among all stations or else close to it. Strong influence of NO3- is observed over Kodaikanal, which could be the product of biogenic process from abundant vegetation around the station. Organic acids (formic and acetic mainly) can also play a significant role in lowering the rainwater pH values at densely vegetated areas like Mohanbari in north-east India and Kodaikanal in Tamilnadu. However, analyses of organic species are not being conducted presently but soon will be started, for which Ion Chromatograph has been procured by IMD. Low content of prime acid neutraliser in the precipitation is also one of the cause of low pH values at these stations
Ø The precipitation chemistry data show that rather than large-scale geographical influences, local effects seem to be more important for chemical wet deposition, e.g. the vicinity of a large urban and industrial area, major combustion sources, sea, arable fields or forest, etc.
Ø A somewhat positive feature emerging from the comparison of pH and other wet deposition data is that India is much better off compared to many other countries as far as acid rain situation is concerned.
Ø The TSPM values at Jodhpur exhibit significant positive correlation with pH indicating its alkaline behavior. The soil derived aerosols have pronounced effect on the precipitation acidity at a place.
Ø The observed atmospheric turbidity values at all the stations show systematic seasonal as well as long-term variation apart from random fluctuations. However, the nature of variation is station dependent and seasonal variation appears to be the strongest feature for all the stations because of regional sources and meteorology.
Ø Most of the turbidity values lie within the turbidity range of 0.10-0.35 for all the continental stations, and for islands and coastal stations the range is 0.10-0.25, while hill stations Kodaikanal and Srinagar found to have lower values in the range 0.05-0.15. Analysis of annual mean values of the turbidity coefficients for the two consecutive decades, 1976–85 and 1986–95, show a general increase of turbidity during the later decade at all the stations. The high altitude station Kodaikanal has remained almost unaffected by this trend. The increasing trend in turbidity at wavelength 500nm indicates that it is caused more by fine size range aerosol which are the product of primary and secondary production processes associated with anthropogenic sources of pollution.
Ø All of the GAW stations show anomalously high values of atmospheric turbidity in 1982-83 and 1991-92 following the eruptions of volcanoes El-Chichon, Mexico, in April, 1982 and Mt. Pinatubo, Philippines, in June, 1991. The increase in atmospheric turbidity values following these eruptions was discernible for 1.5-2 years after the eruption.
Ø TSPM values are significantly correlated with turbidity in all the months except in rainy season, the reason being the dust load in the atmosphere is reduced considerably by rainout and washout but the increased moisture content contributes to a rise in the turbidity values. During rainy months TSPM values decrease sharply as compared to turbidity values.