How aerosols affect the when, where and how much of rainfall

Berlin, September 10 : A team of scientists from the Max Planck Institute for Chemistry in Mainz, Germany, has discovered how aerosols affect the when, where and how much of rainfall, basically their effects on precipitation.

The approach used by the scientists is observing how aerosols change the flow of energy in the atmosphere and thus air circulation, the way drops form and the way they fall. ecause the role of aerosols has to date been very much a subject of dispute and a source of great uncertainty in climate predictions made by researchers, this work removes one of the largest obstacles to the development of more accurate climate forecasts.

Human beings blow vast quantities of aerosols into the air with their cars, power plants and heating systems. Fires set to clear forests also release these floating particles, which, in some cases, measure just a few thousandths of a millimetre, or even less.

But, the effect that aerosols have on clouds has been largely debated.

"The number of aerosols controls how the energy, which ultimately originates from the sun, is distributed in the atmosphere," said the main author of the study, Daniel Rosenfeld, from the Hebrew University in Jerusalem.

Aerosols act in two ways: firstly, like a sun umbrella, they reduce the amount of solar energy reaching the ground: less water evaporates.

Furthermore, the ground does not heat up as much and less of the warm, wet air necessary for cloud formation rises.

Dark particles of soot from forest fires or coal burning have a similar effect, in that they absorb solar energy. They heat up the air around them, so that the cloud droplets evaporate instead of falling as rain.

Secondly, rain drops cannot form without aerosols. They start the clouds off by providing the moisture with points around which to collect, called condensation nuclei. Every single raindrop needs one of these tiny particles, which have diameters of less than one thousandth of a millimetre, as a starting point.

"The effects of the aerosols on the energy on the ground and on droplet formation at altitude have been considered separately up to now. Consequently, the results were so contradictory that the subject was often sidelined," said Meinrat Andreae, director at the Max Planck Institute for Chemistry in Mainz.

The common thread that the team has now followed through the labyrinth of conflicting effects is the flow of energy. With this approach, the team has linked the two processes.

"Now, for the first time, we can estimate by how many watts the energy available for the circulation in the atmosphere changes when the number of aerosols changes," explained Andreae.

ANI