Sources and processes governing rainwater chemistry in New Delhi,India

Rainwater plays an important role in scavenging of aerosols and gases from atmosphere, and its chemistry helps to understand the relative contributions of atmospheric pollution sources. The present work is aimed to understand and explain the sources, seasonal patterns and the processes thereof affecting rainwater chemistry in an urban environment of Delhi, India. Rainwater samples (n = 111) collected throughout the year in New Delhi showed alkalinity in general. Eight rainwater samples, collected in late monsoon and winter season, had pH less than 5.6 indicating that Delhi continues to face the prospects of acid rain despite the introduction of compressed natural gas as the clean fuel in city transport. Organic acids could be the possible contributors of acidity in rainwater samples having the fractional acidity (FA) value of 0.174, which is greater than the annual average FA (0.011) and the (Ca²⁺ + Mg²⁺ + NH₄ ⁺)/(SO₄ ^2? + NO₃ ^?) ration of more than one. Average acid neutralization factors of cations decrease in the order Ca²⁺ (1.01) > NH₄ ⁺ (0.77) > Mg²⁺ (0.10). However, neutralization by Ca²⁺ dominates only in summer season as cation-rich dust is transported from the Great Indian Thar Desert to this region by strong summer S–SW winds, while NH₄ ⁺ dominates in rainwater of other three sampling seasons. Identified dominant sources for soluble ions in rainwater are (1) non-silicate crustal source for carbonates and sulfates of Ca and Mg, (2) emissions from catalytic convertor-fitted vehicles and agriculture fields for NH₃ and (3) mixed anthropogenic sources for SO₄ ^2?, NO₃ ^? and Cl^?. Rainwater chemistry showed significant seasonal variations. This could be due to the changes in relative proportions of natural and anthropogenic sources of soluble ions to rainwater. Dominance of anthropogenic sources over crustal sources can result in acidic rains, which can adversely affect the environment and human health in this region.