Aerosols aloft lift, thicken winter fog over North India: IIT-M study

Winter fog is a familiar hazard in the Indo-Gangetic Plain, reducing visibility for hours at a time. Fog often forms in polluted air near the ground, and polluted events tend to last longer. Forecasters are trying to understand the vertical structure of the fog, that is, how thick the fog layer is, because thickness helps determine how long the fog will persist.

New research from IIT-Madras, based on 15 years of CALIPSO satellite data, reported that aerosol loading on fog over the Plain thickens fog layers. While the base remains close to the ground, the top rises and the droplets near the top grow larger. The findings were published at: Science Developments On January 9th.

Researchers created a number called AODFOG to estimate how much dust and smoke is in the air above a layer. They then looked at a part of the Plain where dense fog often occurs and compared days with low AODFOG (less pollution load) to days with high AODFOG. On days with more pollution, the layer was about 17% thicker because the top rose higher.

The researchers then used MODIS satellite data to estimate the size of water droplets near the top. On days when AODFOG was high, droplets were slightly larger on average.

Finally, the team used a weather model to recreate the massive fog event of January 2014. The model suggested a self-reinforcing cycle: When there were more pollutants in the air, there were more “seeds” for water vapor to adhere to, thus creating more smog droplets. As the steam condensed, some heat was released. Because so many droplets are formed, heat can agitate the fog and help it mix upwards.

At the same time, a fog layer containing many droplets can lose heat more efficiently by emitting infrared radiation, keeping the air at the top cool and moist, allowing more water vapor to condense there.

“North India’s winter haze is a vicious cycle: aerosols fuel fog, fog traps affect pollution, air quality, aviation and daily life. Tackling air pollution can clear skies, boost health and energize the economy,” said Chandan Sarangi, IIT-Madras earth system scientist and author of the study. Hindu.

The team also said soot can absorb sunlight and warm air near or above the fog, a “semi-direct” effect they were unable to isolate because the aerosol properties above the fog are poorly known and observations are too sparse to constrain the model. This is a limitation.