Scavenging of urban air emissions by Fog at Delhi, India

Abstract:

The present study focuses upon the understanding of fog water chemistry in Delhi city. Total seventy fog water samples were collected at two different sites in Delhi during December 2014 to March 2015. Selected parameters such as pH, major anions (Cl^-, F^-, NO₃^- and SO₄^2-) and major cations (NH₄⁺, K⁺, Ca²⁺, Mg²⁺, Na⁺⁾ were determined in the samples. The pH of the fog water collected during the monitoring period at Site I (traffic intersection) varied from 4.68 to 5.58 indicating the acidic nature of fog water while at the site II (green cover area), it ranged from 6.11 to 6.88 having slightly lower acidity. At the Site I, the average concentration of Cl^-, Na⁺, SO₄^2-, NH₄⁺ was recorded as 1.5 X 10^-2, 8 X 10^-3, 4 X 10^-3 and 1 X 10^-2 µEqu/L respectively. Such values of ionic species may be attributed to the local sources, including factories, motor vehicle emissions and civil construction etc. However, non-local sources such as moderate- and long-range transport of sea salt also had significant influence on ionic content of fog water. In general the Na⁺ ratio values were found to be higher side suggesting the influence of non-marine sources. Extremely high values of Cl^-/ Na⁺ ratios indicated the contribution from combustion of organochlorine compounds. Hence, the higher ratios of inorganic ions and acidic pH revealed that fog is an effective mechanism for the scavenging of various pollutants emitted by different sources in the city.