A Comparative Study on Fog and Dew Water Chemistry at New Delhi, India

Fog and dew, formed via different formation mechanisms, are suitable to study the liquid-gas-solid phase chemical interactions in the ambient atmosphere. A total of 24 fog and 19 dew samples were collected using Caltech Active Strand Cloud water Collector 2 and dew condensers, respectively, over New Delhi during winter months of 2014-15 and were characterised for pH and soluble inorganic ion using ion chromatograph. Dew samples were alkaline (pH = 6.26 +/- 0.37) in comparison to natural rainwater pH of 5.6 and fog collected at rooftop (pH = 5.38 +/- 1.3) and at ground level (pH = 5.96 +/- 0.3). The volume weighted mean equivalents of cations followed the order NH4+ > Ca2+ > Mg2+similar to K+ > Na+ and of anions as SO42- > NO3-similar to Cl- > HCO3- > F- > NO2- in fog whereas the order for dew was Ca2+ > NH4+ > Na+ > K+ > Mg2+ and SO42-similar to HCO3- > Cl- > NO2- > NO3- > F-. The Ca2+ ions were higher than NH4+ in dew while NH4+ was higher than Ca2+ in fog. Nitrite was higher in comparison to nitrate in dew while this was reverse in fog. Alkaline pH of dew samples might have played a role in the gas phase transfer and the base catalyzed transformation of NOx to HONO and subsequent dissolution of HONO in dew in comparison to fog. Acidity was caused more by sulphate ions (SO42-/NO3- ratio was 2.2 and 4.18 for fog and dew, respectively) but was effectively neutralised. Neutralisation factors were different in fog (NH4+ > Ca2+ > Mg2+) and dew (Ca2+ > NH4+ > Mg2+). The differences in the fog and dew composition are primarily linked to their formation processes. The agricultural fields and fossil fuel combustion were sources for ammonium, sulphates, nitrate and nitrite whereas locally resuspended crustal materials added calcium and magnesium carbonates. Vehicular and plant emissions, biomass burning and the oxidation of volatile organic compounds seems to be responsible for higher organic acids in dew and fog.