Story of the Week  

Aerosols and Clouds: modification of Earth's Climate

 

Aerosols, suspended particles in the atmosphere emitted either naturally or anthropogenically, affect the Earth's climate by perturbing the radiation budget through scattering and absorption of incoming solar and outgoing thermal infrared radiation (so-called direct effect). Most aerosol species like sulfate, nitrate, sea-salt, dust (partly absorbing also) etc. scatter the radiation, whereas black carbon (BC) aerosols absorb the radiation. In general, aerosols cool the surface and warm the atmosphere

In IIT Kanpur, we focus on the issues related to the climatic effects of aerosols and clouds over India , with emphasis on the Gangetic basin (GB). The altitude profiles of aerosols are very important, as they affect the atmospheric heating rate. We have measured composite aerosol and BC concentration altitude profile over Kanpur by instruments onboard aircraft in different seasons. During winter, enhanced layer of absorbing aerosols were observed over Kanpur , which persists during summer but at higher altitude. The enhanced BC layer increases the atmospheric absorption, which suppresses the convection and evaporation processes in the boundary layer and affects rainfall. In summer, dusts transported from the Thar Desert and Gulf regions to the GB, add to the pollution load. Presence of aerosols in the GB reduces the surface-reaching solar radiation by ~20% in winter and ~15% in summer. This is significant in terms of long-term climate change in the region.

Aerosols also act as cloud condensation nuclei, hence increase in aerosol concentration increases cloud droplet concentration and in turn, reduce the effective radius of cloud droplets and modify the cloud properties (so-called indirect effect). The effect is different in the clean and polluted clouds as schematically shown below. An existing aerosol microphysical model is being extended to simulate mixed-phase cloud microphysical processes such as growth, collision, coagulation and precipitation. Airborne measurements will be carried out to collect data, which will be used in this integrated aerosol-cloud model to understand the aerosol indirect effect on the clouds in the GB.

 

Dr. S. N. Tripathi
Department of Civil Engineering
Indian Institute of Technology Kanpur
    eMail: snt@iitk.ac.in
    Web: http://home.iitk.ac.in/~snt/

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