Dust is a very serious environmental issue in the metropolitan cities. Minute particles in air known as aerosols, which come from combustion of fossil fuels, wildfires, agricultural waste burning and other sources, are important in shaping weather and climate.
Apart from causing a numerous health-related problems such as asthma, these particles, which are smaller than one-thousand the width of human hair (smaller than 50 nanometres), act as nuclei for condensation of water vapour in the atmosphere at 1-2 kms above the ground (troposphere). Now the latest research suggests that ultrafine aerosols can cause storms to intensify, clouds to grow and more rain to fall. Meteorologists can now blame dust pollution for flooding at many places.
Scientists have investigated the role of substantial convection and precipitation enhancements by ultrafine aerosol particles, which add to our knowledge of the interactions between aerosols, clouds and precipitation. Until now the role of smaller particles below 50 nanometres, such as those produced by vehicles and industrial processes, was uncertain. The new study published in Journal Science on 26 January 2018, revealed that these particles, whose effects on clouds have been mostly neglected in studies, can invigorate clouds in a much more powerful way than their larger counterparts (greater than 50 nanometres).
Scientists carried out measurements of aerosols in Manaus metropolis in the Amazon rainforest area with 2 million people to mimic the aerosol concentrations to preindustrial times (hundreds per cubic centimetre). The study capitalized on data gathered from ground-based and air-borne measurements during 2014-15. They showed how particles below 50 nanometres can invigorate clouds in a much more powerful way than their larger counterparts when specific conditions are present. In a warm and humid environment with no large particles to attract airborne moisture, water vapour can build up to extreme levels, causing relative humidity to rise well beyond cent per cent.
While ultrafine particles are small in size, they can reach large numbers to form pollution plume which is generated by the converging wind patterns. These particles act as condensation nuclei which quickly and efficiently draw excess water vapour from the atmosphere. This enhanced condensation liberates more latent heat of condensation in the lower atmosphere, which enhances the upward movement of air (cold-phase invigoration). The additional condensation from this process increases the amounts of both warm rain and supercooled cloud water. Cumulatively, these enhanced ice-related processes at middle and upper levels of the atmosphere (between 3 and 9 kms above the ground) further enhance convection (turbulent air movement in the vertical), but to a much lesser degree relative to enhanced condensation at low levels. As more warm air is pulled into the clouds, more droplets are launched aloft, producing a runaway effect that results in more ice and snow pellets, lightning, and rain.
Even under clean and humid conditions, like those that exist over the ocean and some land in the tropics, tiny aerosols have a big impact on weather and climate and can intensify storms in a great way. The results suggest that from pre-industrial to the present day, human activity possibly may have changed storms in these regions in powerful ways. The recent cyclone Ockhi that devastated the coastal region from Kerala to Gujarat may have been driven by aerosols forcing.
