Direct measurement of atmospheric dry deposition to natural surfaces in a semiarid region of north central India

Abstract

Dry deposition flux of major ions (Na+, K+, Ca2+, Mg2+, NH4+, F−, Cl−, NO3− and SO42−) to natural surfaces e.g., leaf of Ashok (Polyalthia longifolia) and Cassia (Cassia siamea), were determined at Dayalbagh, a suburban site of semiarid region. Dry deposition flux was highest for Ca2+ on both Ashok and Cassia while lowest for Na+ on Ashok leaf and F− on Cassia leaf. Overall average dry deposition fluxes varied from 0.27 mg m−2 d−1 to 3.98 mg m−2 d−1. The deposition was comparatively higher to Cassia leaf than Ashok. The high deposition flux to Cassia leaf may be due to surface roughness as shown by scanning electron micrographs. The sum of major cations (Na+, K+, Ca2+, Mg2+) and NH4+ was higher than sum of major anions (F−, Cl−, NO3− and SO42−). It indicates that dry deposition at present site is alkaline in nature and difference in the sum of major cations and major anions might be due to unmeasured carbonate and bicarbonate ions as well as organic anions (formate and acetate). Deposition flux of SO42− and NO3− is relatively lower than Ca2+ and Mg2+. Seasonally dry deposition fluxes were higher in monsoon or summer for cationic species while for anionic species fluxes were higher in winter or monsoon. Average dry deposition velocity of major ions on Ashok and Cassia leaf varied from 0.41 to 2.36 cm s−1, while seasonally values varied from 0.32 to 4.04 cm s−1. The variation in deposition velocity may be due to meteorological conditions and surface characteristics.