Occurrence of acid rain over Delhi.

Precipitation samples were collected as wet-fall only and primarily on event basis in Delhi during the monsoon period of 1995. Concentrations of major anions (SO4(2-), NO3- and Cl-) and cations (Ca2+, Mg2+, Na+ and K+) were determined. The pH of the rain water was found to be more than 5.6, showing alkalinity during the early phase of monsoon, but during the late phase of monsoon pH tendency was towards acidity due to lack of proper neutralization of acidic ions. Neutralization is not only due to the local process but also due to the pre-monsoon 'Andhi' which brings Suspended Particulate Matter (SPM) containing Ca2+, Mg2+, Na+ and K+ as well as the local emission of NH3. In the late monsoon the concentration of cations gets reduced because of heavy rainfall and relatively unfavourable condition for their transport from the adjoining areas, whereas the anion concentrations remain unchanged owing to their continuous emission.