Cloud view clears up

Origin of aerosols dictates cloud shape

By Smriti Sharma
Published: Sunday 31 October 2010

imageHOW cloudy is it outside? Will it rain? The answers may depend on the level of atmospheric pollution in one’s region. Cloud-forming microscopic particles, called aerosols, absorb and reflect solar radiation.

These particles have the ability to modify cloud formation and encourage or suppress precipitation. They can be released from manmade sources like vehicles, industry, agriculture, and natural sources like sea salt, volcanic dust, sulphates from biogenic gases.

Scientists are not fully aware of the role of aerosols in affecting climate because of lack of information on their distribution and characteristics. Two scientific papers, published in the September 17 issue of Science, fill this gap.

In the first study, Antony Clarke and Vladimir Kapustin, from School of Ocean and Earth Science and Technology, University of Hawaii, USA, analysed more than 1,000 atmospheric aerosol profiles over the Pacific Ocean. The data included regions with very little atmospheric pollution from anthropogenic (or manmade) sources and regions with heavy levels of such pollution.

Their results demonstrated that aerosol concentration is higher in areas of human perturbation as compared to pristine regions. The study concluded that human intervention leads to aerosols with enhanced light scattering properties and radioactive effects, and increase in the number of particles available to form cloud droplets.

In the second study, Ulrich Pöschl from the Max Planck Institute of Chemistry, Germany, and his colleagues studied aerosol and cloud formation over the Amazon rainforests during the wet season, when the air is at its most pristine. The research team built an observation tower in the forest north of Manus in Brazil, and collected air samples from the atmosphere over a period of 10 days to determine the size, concentration and origin of aerosols. They found the forest produced its own rain, that is, clouds and rain were formed by volatile gases emitted by plants. The data also showed that the number of cloud droplets above the Amazonian forests depended directly on the number of aerosols. This is in contrast to more polluted areas, where the number of cloud droplets depends on how quickly hot particles from burning fires or fossil fuels ascend into the atmosphere.

More research into how aerosols influence climate is needed to accurately predict their long-term effects on climate change. “It is crucial to quantify the relative roles of different drivers of recent climate changes,” said Leon Rotstayn, an atmospheric scientist at Commonwealth Scientific and Industrial Research Organisation in Australia.

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