Chemical characterization of dew at a regional representative site of North-Central India

Dew and rainwater samples were collected during winter and monsoon periods in 1997–1998 at Rampur, India, a semiarid site with minimal anthropogenic activities. The study revealed that the decreasing order of ionic concentration in dew is Ca2+ > SO42− > Cl− > Mg2+ > NH4+ > Na+ > NO3− > K+ > HCO−3 > HCOO− > CH3COO−, while the range of pH was between 6.0 and 7.7 with a volume weighted mean pH value of 6.8. Rainwater pH varied from 5.9 and 7.4 with volume weighted mean pH of 6.6. The percentage contribution of alkaline components (Ca2+, Mg2+, Na+ and K+) was 43.9% and NH4+ 11.1%, while the contribution from acidic components was 45%. Similarly in rainwater the percentage contributions of alkaline, NH4+ and acidic components were 40.4%, 14.4% and 45.2%, respectively. The difference between the sum of cations and the sum of anions in dew was 233.8 μeq l− 1, while in rainwater it was 27.8 μeq l− 1 and their ratio was 1.2 in both dew and rainwater. A comparison of the ratios of Cl−/Ca2+, SO42−/Ca2+, Mg2+/Ca2+, Na+/Ca2+ and NO3−/Ca2+ in dew with soil indicated that 63 to 96 percentile samples in dew correspond to ratios in soil. Statistically significant correlation between Ca2+, Mg2+, K+, NO3−, SO42−, HCOO− and CH3COO− in dew indicate a significant contribution of NO3−, SO42−, HCOO− and CH3COO− from soil or they are associated with Ca2+, Mg2+ and K+ after neutralization. At the present site, in dew, NH4+ is 6 times, Ca2+ and Mg2+ more than 7 times, K+ and Na+ greater than 10 times and Cl−, NO3−, and SO42− are more than 9 times higher than those of rainwater. The steps governing dew composition are: (i) formation on dry deposition solids, (ii) dissolution of the soluble portion of the dry deposition by dew water, and (iii) sorption of gaseous NH3, formate and acetate into dew water.