Review of precipitation monitoring studies in India—a search for regional patterns

This study is a review of rain chemistry measurements reported in India. Data from nearly 100 stations were reviewed with regard to sampling location, sampling method and chemical analyses. Some characteristic differences were found between the concentrations in different environments in line with known distributions of emissions. This was most clearly seen for SO42− with median concentration increasing from rural and suburban to urban and industrial environments, with concentrations of HCO3− and H+ varying the opposite way. Ca2+ concentration was higher in suburban and industrial environments compared to rural and urban. Concentration of NH4+ was lower in rural than in the other locations. Sea salt concentration (Na+ and Cl− in approximately constant proportion) was higher in urban than in rural locations at the same distance from the sea. The reported data from rural and suburban locations were used to derive large-scale concentration fields over India. With knowledge of emission fields and with the aid of output from a regional transport model it was possible to see some limited spatial resemblance between modelled and observed concentrations in cases of NH4+, NO3- and SO42−. For soil dust, current models are very crude but here some resemblance could be seen: primarily limited spatial resemblance with Ca2+, pH and HCO3− with generally increasing values towards NW. Four sites, at least two of them located in forested environments, had a mean rainwater pH close to neutral (concentration of H+ and HCO3− about equal). Other sites had on average an excess of HCO3−, becoming very substantial in the direction of the Thar Desert in NW India. With this review we conclude that there is a need to better assure the quality of the data with regard to sampling methods, chemical analyses and spatial representativeness.