Relation between fog formation and the number concentration of aerosol for Delhi, India

Aerosol number concentrations and size distributions have been measured in the Capital of India, Delhi, from 3rd to 9th January, 2004 and 6th to 13th January, 2006 in order to have an improved understanding of their role in fog formation in this mega-city. Due to the strong link between aerosol characteristics and fog microphysics, the overall life cycle of a fog layer is influenced by the aerosols. The studies to understand the effect of aerosols/pollution on fog formation are lacking for metropolitan-city like Delhi. In the present work, measurements were performed in the particle size range of 0.3 ìm to 20 ìm using an aerosol spectrometer. Most of these aerosols being of anthropogenic origin need greater attention. The analysis in terms of number density and meteorological factors such as relative humidity (RH), dew point temperature and wind speed is carried out for the entire period, specifically, before the fog sets up, during and afterwards. A critical number concentration is needed for fog formation which further depends on the origin of the pollution. The first sampling site was located in an industrial area in the proximity of a busy traffic road whereas the second sampling site was located in IIT, Delhi, an urban residential area. The distribution of number concentration with particle size is fitted to log normal function, which shows the typical Gaussian nature as expected. Analysis covers a wide range of concentrations in order to identify whether there is a possibility of a threshold number concentration which would favor fog episodes in comparison to others. The result shows that the number concentration density was high without leading to the fog formation even though meteorological conditions were favorable. Explanation for the same based on Kohler curves is also attempted.