Air pollution 'causes more rain - but also less rain'

It is an argument that has divided experts, but now scientists believe they have the answer to the question of whether air pollution increases or decreases rainfall.

Do large amounts of aerosols help or hinder cloud formation?

Do large amounts of aerosols help or hinder cloud formation?

The good news is that both camps appear to be right, according to an international team of scientists.

Writing in the journal Science, the scientists said pollution can both increase and decrease rainfall depending on local environmental conditions.

"Both camps are right", said Professor Meinrat O. Andreae, director of the Max Planck Institute for Chemistry in Germany, a co-author of the study.

"But you have to consider how many aerosol particles there are."

Clouds and rain occur when moist, warm air rises from ground level and water condenses or freezes on aerosol particles in the air.

Aerosols can have two effects on this process - on the one hand they act like a sunscreen reducing the amount of sun energy reaching the ground, which reduces the amount of water evaporating to form rainclouds, but on the other hand, clouds cannot form without aerosol particles.

But the scientists found that if there is a surplus of aerosols, the droplets never reach the critical mass needed to fall to earth as rain, as there is not enough water to share between all the aerosol particles.

With rising pollution, the amount of rain at first rises, then maxes out, and finally falls off sharply at very high aerosol concentrations, they concluded.

Professor Andreae added: "With these results we can finally improve our understanding of aerosol effects on precipitation and climate, since the direct contradiction of the different aerosol effects has seriously hindered us from giving more accurate predictions for the future of our climate, and especially for the availability of water."

Professor Daniel Rosenfield, of the Hebrew University of Jerusalem, who headed the study, said: "These results have great significance for countries like Israel where rainfall is scarce and can be easily affected by over-production of aerosols.

"Our study should act as a red light to all of those responsible for controlling the amounts of pollution we release into the atmosphere."

Kate Martin



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