Chemistry of fogs at Agra, India: Influence of soil particulates and atmospheric gases

Fog water samples were collected in the months of December and January during 1998-2000 at Agra, India. The samples were analyzed for pH, major anions (F-, Cl-, SO42-, NO3-, HCOO- and CH3COO-), major cations (Ca2+, Mg2+, Na+ and K+) and NH4+ using ion chromatography, ICP-AES and spectrophotometer methods, respectively. pH of fog water samples ranged between 7.0 and 7.6 with a volume weighted mean of 7.2, indicating its alkaline characteristic. NH4+ contributed 40%, SO42- and NO3- accounted for 28%, while Ca2+, Mg2+, Na+ and K+ accounted for 16% of the total ionic concentration. The ratios of Mg2+/Ca2+ and Na+/Ca2+ in fog water indicates that 50-75% of fog water samples correspond to the respective ratios in local soil. Significant correlation between Ca2+, Mg2+, Na+ and K+ suggests their soil origin. The order of neutralization, NH4+ (1.4) > Ca2+ (0.28) > Mg2+ (0.12), indicates that NH4+ is the major neutralizing species. Fog water and atmospheric alkalinity were also computed and were found to be 873 and 903 neqm(-3), respectively. Both of these values are higher than values reported from temperate sites and thus indicate that at the present level of pollutants, there is no risk of acid fog problem. The study also shows that the alkaline nature of fog water is due to dissolution of ammonia gas and partly due to interaction of fog water with soil derived aerosols.