Aerosol pollution slows winds and reduces rainfall

Pollution slows winds worldwide, lessening the power produced by wind farms such as this one.
Date:30 January 2007 Tags:, ,

The winds that blow near the surface of the Earth have two beneficial effects: They provide a renewable source of clean energy and they evaporate water, helping rain clouds to build up. But aerosolised particles created from vehicle exhaust and other contaminants can accumulate in the atmosphere and reduce the speed of winds closer to the Earth’s surface, which results in less wind power available for wind-turbine electricity and also in reduced precipitation.

These aerosol particles are having an effect worldwide on the wind speeds over land; there’s a slowing down of the wind, feeding back to the rainfall too, says civil and environmental engineering Associate Professor Mark Z Jacobson, co-author of the study. “We’re finding a reduction of rain, and that can lead to droughts and reduction of water supply.”

The study, conducted by Jacobson along with his colleague Yoram J Kaufman, was based on Nasa satellite data of aerosol accumulation, measurements of wind speeds over the South Coast Basin in California and in China, and computer model simulations over California as a whole and the South Coast Basin. The researchers used both the model and data to study the effects of aerosol particles on wind speed and rainfall.

Aerosol particles floating in the atmosphere absorb or scatter solar radiation, and prevent it from getting to the ground. This cools the Earth’s surface and reduces daytime vertical convection that mixes the slower winds found near the ground with the faster winds at higher altitudes. The overall effect is a reduction in the speed of near-surface winds, which Jacobson has calculated to be up to 8 per cent slower in California.

Clean and renewable, wind power made up 1,5 percent of the Golden State’s energy portfolio in 2005, according to the California Energy Commission. But slower gusts may reduce wind’s economic competitiveness compared to other energy sources, such as fossil fuels.

“The more pollution, the greater the reduction of wind speed,” Jacobson says. Aerosol particles may be responsible for the slowing down of winds worldwide. Wind supplies about 1 per cent of global electric power, according to Jacobson. Slow winds may hinder development of wind power in China, where it’s a needed alternative to dirty coal-fired plants. Aerosols’ reduction of the wind also may explain the reduction in the Asian seasonal monsoon and “disappearing winds” in China, observations found in other studies. Moreover, slack air currents may hurt energy efficiency in Europe, where countries like Denmark and Germany have made major wind-power investments.

Slower winds evaporate less water from oceans, rivers and lakes. Furthermore, the cooling of the ground provoked by the aerosol particles reduces the evaporation of soil water.

What’s more, the accumulation of aerosol particles in the atmosphere makes clouds last longer without releasing rain. Here’s why: Atmospheric water forms deposits on naturally occurring particles, like dust, to form clouds. But if there is pollution in the atmosphere, the water has to deposit on more particles. Spread thin, the water forms smaller droplets. Smaller droplets in turn take longer to coalesce and form raindrops. In fact, rain may not ever happen, because if the clouds last longer they can end up moving to drier air zones and evaporating.

Jacobson advocates replacing existing motor vehicles with cleaner ones, such as renewable-energy powered battery-electric and hydrogen-fuelled vehicles, and substituting contaminating power plants with networked wind farms. These actions would reduce particle emissions practically to zero, he says. The second-best option would be adding particle traps to existing vehicles and other sources of pollution.

“If we want to solve the global warming problem, we have to replace most of the existing energetic infrastructure with wind and other renewable-based energy,” says Jacobson, whose next step will be to study the effect of greenhouse gases on winds.

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