Atmospheric Environment 38 (2004) 4215–4222 Fog and precipitation chemistry at Delhi, North India K. Ali*, G.A. Momin, S. Tiwari, P.D. Safai, D.M. Chate, P.S.P. Rao Indian Institute of Tropical Meteorology, Dr. Homi Bhabha Road, Pashan, Pune-8, India Received 18 July 2003; accepted 20 February 2004 Abstract 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 CO 2 equilibrated pure water. Neutralization of fog acidity by cations occurs in the order of NH 4 + >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 NO 3  , rain water collects an appreciable amount of NO 3  from aloft as well. Major contribution to the total lower tropospheric NH 4 + ion was made through human and animal excretion during winter season. Finally, natural source of cations like Ca 2+ ,K + and Mg 2+ 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. r 2004 Elsevier Ltd. All rights reserved. Keywords: Neutralization; Acidity; Fog; Chemical composition 1. Introduction Fogs play an important role in many environmental and ecological processes. The physico-chemical interac- tions among gases, particles and fog droplets can influence the composition of fog droplets (Jacob and Hoffmann, 1983; Munger et al., 1983; Pandis and Seinfeld 1989; Pandis et al., 1990). Fog droplets scavenge soluble gases like nitric acid and ammonia and act as a medium for various aqueous phase chemical reactions (Pandis et al., 1990). Fog droplets provide a better site for chemical reactions involving gaseous pollutants (e.g., aqueous phase conversion of SO 2 to SO 4 2 ). Cass (1981) has shown that the mean rate of conversion of SO 2 to SO 4 2 during July in Los Angeles was 6% h 1 , whereas Sander and Seinfeld (1976) have shown that the gas phase conversion rate of SO 2 to SO 4 2 ion could account for, at the most, 4.5% h 1 . Fog forms in the lower layer of the troposphere where aerosols and gases are most abundant. Since fog droplets are much smaller (nearly 100 times) than rain drops, they should be more concentrated than rain, and mass transfer should not limit the kinetics of fog droplet reactions (Lange et al., 2003; Babulal et al., 1981). The concentrated fog may occasionally result in potential hazards for human health, vegetation and buildings. Studies carried out in the United States of America, Europe and Japan have reported highly acidic fog (e.g., Jacob et al., 1984, 1985; Igawa et al., 2001; Mrose, 1966; Munger et al., 1983, 1990; Okita, 1968; Waldman et al., 1982, 1985; Manabu et al., 1998; Bridges et al., 2002) and have suggested that pre-existing aerosols are the major determinant of the chemical composition of fog water. ARTICLE IN PRESS AE International – Asia *Corresponding author. 1352-2310/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2004.02.055