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

Supriya Nath ； Sudesh Yadav

《Aerosol and Air Quality Research》 18卷1期 (2018/01) Pp. 26-36

https://doi.org/10.4209/aaqr.2017.01.0033

摘要

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 NH_4^+ ＞ Ca^(2+) ＞ Mg^(2+)~K^+ ＞ Na+ and of anions as SO_4^(2-) ＞ NO_3^-~Cl^- ＞ HCO_3^- ＞ F^- ＞ NO_2^- in fog whereas the order for dew was Ca^(2+) ＞ NH_4^+ ＞ Na^+ ＞ K^+ ＞ Mg^(2+) and SO_4^(2-)~HCO_3 - ＞ Cl^- ＞ NO_2^- ＞ NO_3^- ＞ F^-. The Ca^(2+) ions were higher than NH_4^+ in dew while NH_4^+ was higher than Ca^(2+) 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 NO_x to HONO and subsequent dissolution of HONO in dew in comparison to fog. Acidity was caused more by sulphate ions (SO_4^(2-)/NO_3^- ratio was 2.2 and 4.18 for fog and dew, respectively) but was effectively neutralised. Neutralisation factors were different in fog (NH_4^+ ＞ Ca^(2+) ＞ Mg^(2+)) and dew (Ca^(2+) ＞ NH_4^+ ＞ Mg^(2+)). 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.

關鍵字

Neutralisation factors ； Nitrite ； Organic acids