Influence of air mass trajectories on the chemical composition of precipitation in India

Chemical composition of precipitation was measured with wet-only samplers at a rural site at Bhubaneswar in eastern India during 1997–1998. All rain events were compared with trajectories and precipitation fields from the ECMWF. The pH and ionic concentrations were found to vary systematically with the origin of air and the amount of rainfall along the trajectory. A seasonal cycle for pH was found with a monthly median pH below 5.0 during October–December. The highest monthly median concentration of Ca²⁺ was found in May with 20 μmol l⁻¹ and for SO₄²⁻ in January with 52 μmol l⁻¹. Samples with trajectories within 400 km from Bhubaneswar during the last 5 days were found to have a median pH slightly below 5.0 as an average. These samples also had the highest concentration for all measured ions, indicating large pollution sources within the region. Samples with continental origin showed a decrease of ∼70% in concentration if there had been rain during >50% of the last 5 days compared to rain during <50% of the last 5 days. High concentrations of Na⁺ and Cl⁻ were also found in continental samples. Resuspension of previously deposited sea salt is believed to be the reason. The data were compared with data from three other sites in western India and higher concentrations of almost all ions (NH₄⁺ being the exception) compared to Bhubaneswar were found at the west coast in monsoon samples.