Fog and precipitation chemistry at Delhi, North India

Fog water samples were collected during three consecutive winters from the year 2000 to 2003 and their chemical compositions were studied to describe and assess the air quality in Delhi. It was found that all the samples were alkaline in nature in comparison to the neutrality of atmospheric CO2 equilibrated pure water. Neutralization of fog acidity by cations occurs in the order of NH4+ > Ca-2(+) > Mg-2(+). Comparison of the chemical composition of fog water and that of rain water, which were collected during the same period, indicates that nearly all the chemical constituents were higher in fog water than those in rain water except for one case of rain water which occurred on 25 December 2003. On account of much less amount of rainfall on the very occasion, all the ionic species showed very high concentration. Concentration of nitrate both in fog and temporally nearest rain water samples was nearly the same except for the rain sample of 25 December 2003, which indicates that while fog water drags only the lower tropospheric NO3-, rain water collects an appreciable amount of NO3- from aloft as well. Major contribution to the total lower tropospheric NH4+ ion was made through human and animal excretion during winter season. Finally, natural source of cations like Ca-2(+) K+ and Mg-2divided by. dominated over the anthropogenically produced acidic anions. Correlation analysis between different chemical species was computed, but no concrete conclusion is drawn on the basis of this result, because the data set is statistically very small. Nevertheless, it gives an idea about the source of different ions and their chemical composition. (C) 2004 Elsevier Ltd. All rights reserved.