Annual and decadal variation in chemical composition of rain water at all the ten GAW stations in India

Spatial variation of long term annual precipitation volume weighted concentrations of major chemical constituents (SO4 (-2), NO3 (-), Cl-, NH4 (+), Ca+2, Mg+2, Na+ and K+ ) at all the ten Global Atmospheric Watch (GAW) stations in India for the period from 1981 to 2012 is studied in this paper. Ionic abundance and balance is studied as well. The range of long term annual mean pH at ten stations was 5.25 +/- 0.82 to 6.91 +/- 0.76, lowest at Mohanbari and highest at Jodhpur. The long term annual mean pH for the period 1981-2012 showed decreasing trend at all the stations (significant at 5 % level). Decadal mean pH among ten stations for 1981-1990, 1991-2000 and 2001-2012 ranged between 7.31 to 5.76, 7.45 to 4.92 and 6.16 to 4.77 respectively and showed decreasing trend at all the stations during 1981-1990 to 2001-12. The percentage occurrence of acidic pH (< 5.65) at ten stations ranged from 3 to 72 %, lowest at Jodhpur and highest at Mohanbari and it increased from 1981-1990 to 2001-2012 almost at all the stations. Temporal variation of annual mean values of nssSO(4) (-2), NO3 (-), Ca+2 and pH for the study period were attempted. Variation of nss K (non sea salt Potassium) at all the stations was studied to assess the biomass burning contribution in different regions. Non-marine (terrestrial) contribution dominated for majority of ionic constituents at most of the stations. However marine contribution was found to be dominant for Mg at Port Blair and Minicoy. Also sea salt fraction of SO4 was higher than terrestrial at Minicoy. Sources of measured ionic constituents in rain water are assessed through correlation analysis. The concentrations of all the ionic species were lowest at Kodaikanal, a high altitude hill top station and the total ionic mass was 136.0 mu eq/l. Jodhpur, an arid station not only had highest concentrations of Ca+2, SO4 (-2) and K+ but also had highest total ionic content (1051.8 mu eq/l) among all the stations. At Srinagar, Jodhpur, Allahabad, Nagpur and Pune stations Ca+2 was the dominant cation while dominant anion was NO3 (-) for Srinagar, Allahabad, and Nagpur and Cl- for Jodhpur and Pune; at Mohanbari NO3 (-) and Ca+2; at Visakhapatnam, Port Blair and Minicoy Na+ and Cl- were abundant. Temporal variation had shown an increasing trend for nssSO(4) (-2) and NO3 (-) and obviously decreasing trend for pH at all the stations. However, Ca+2 showed a decreasing trend at all the stations except at Port Blair. With the exception of Pune and Jodhpur stations, nssK showed a decreasing trend at all the stations revealing decreasing influence of soil/biomass burning over Indian GAW stations. Negative correlation of pH with SO4 (-2) was found to be weak compared to NO3 (-).