Visibility degradation during foggy period due to anthropogenic urban aerosol at Delhi, India

Fog which is defined as an obscurity near the surface layer of the atmosphere and is caused by a suspension of water droplets associated with visibility and its frequencies dominate in urban areas other than rural areas. It may occur due to increased air pollutant concentrations emanating from variety of sources in the urban areas. The increased pollution levels may lead to the atmospheric reactions resulting into the formation of secondary pollutants that may also lead to the needed cloud condensation nuclei. Northern part of India experience severe fog conditions (warm fog) during the winter period (November-January) every year. In this study, we have simultaneously measured the particulate mass concentration (0.23 mum to 20 mum in 16 size channels), meteorological parameters and atmospheric visibility in mega city Delhi during winter months of the years 2007-2008 in order to have an improved understanding of their role in fog formation. The effects of aerosols on fog formation are discussed through an analysis of trends in fog frequency and comparison with meteorological parameters especially visibility which is an indicator of aerosol loading in the lower atmosphere. The existing dataset is used to find linear regression amongst various parameters with due consideration of significance value. This satisfies the precondition for using these relations even in limited data set. The association between the meteorological parameter (visibility, depression temperature) and air pollutants are examined. The Windows software SPSS (version 17.0) is used to fit a linear regression model. The model explained the variation in visibility due to depression temperature and aerosols loading