Variation in the ionic composition in convective showers in Pune, India

Rain water samples of equal volume (100 ml) were collected sequentially from 5 convective shower events for a period of 2 years during the summer monsoon seasons of 1988 and 1989 at Pune. A marked decrease in the concentrations of soil-orientated elements Na ⁺, K⁺, Ca2⁺ and Mg2⁺ was observed in the first few samples in all the events followed by a decrease in pH of the subsequent samples. Surprisingly, SO₄ ²- also showed similar variation, indicating that soil is contributing some SO₄ ²- in rain water. Washout process appears to be an important mechanism for the removal of the major ions from the atmosphere. However, rainout processes seem to be important for the removal of NO₃ ^- ions. The study points out that sequential sampling during a single shower appears to give better information about the relationship among H⁺, Na⁺, K⁺, Ca2⁺, Mg2⁺, SO₄ ^2- and NO^3- as compared to bulk sampling for longer duration.