Aerosol pollution may affect rainfall pattern

By Archita Bhatta
Published: Wednesday 28 February 2007

-- the brown haze over north-India's sky could portend a dry future. Caused by liquid pollutants suspended in air, the aerosol cloud, infamous for its effects on crops and human health, may result in localised decrease of rainfall.

Aerosols reduce speed of winds near the earth's surface, thus leading to reduction of rainfall, say scientists. A study conducted jointly by researchers of Stanford University, usa, and nasa shows a significant decrease in rainfall over California at certain times in a year when aerosol-content was high, says the paper published online on December 27 in Geophysical Research Letters. Wind speed measurements over South Coast Basin in California showed an average 8 per cent decrease in rainfall.

"The more the aerosol pollution, the greater the reduction in rainfall.This may also explain the reduction of Asian seasonal monsoons," says researcher Mark Z Jacobson, associate professor at Stanford University. And although no such correlation has been established in India, studies show significant decrease of rainfall over north and central India.

A paper presented by V N Sharda of the Central Soil and Water Conservation and Training Institute at the Science Congress held recently in Tamil Nadu projected a 16-25 per cent decrease in annual rainfall in the Garhwal hills and Gujarat. Meanwhile, the Asian brown haze due to aerosol pollution in these parts has been well mapped by nasa.
How does it happen? Particles of aerosols floating in the atmosphere absorb sunlight and prevent it from reaching the earth's surface. This cools down the earth's surface and prevents daytime vertical convention that mixes slower winds near the earth's surface with faster winds at higher altitudes.

Slower winds cause less evaporation from oceans, rivers and lakes, and hence formation of rain-clouds is reduced. Besides, cooling of the ground provoked by the aerosol particles, reduces the evaporation of soil water. Also, accumulation of aerosol particles in the atmosphere makes clouds last longer without releasing rain.

Atmospheric water forms deposits on naturally occurring particles, like dust, to form clouds. But if there is pollution in the atmosphere, the water is bound to deposit on more particles. Spread thin, the water forms smaller droplets, which 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, explains Jacobson. Thus, aerosol pollution can result in drought.
Indian scenario Indian scientists feel that aerosol pollution needs to be taken into account while predicting rainfall trends. "Though this paper does not mention about monsoons, we can expect such effects in case of monsoons. We need to conduct detailed analysis on whether wind speed has decreased over India and whether it is due to aerosol content," says M Rajeevan of the National Climate Centre of the Indian Meteorological Department (imd). "Overall changes in rainfall need to account for the combined effect of aerosols and green-house gases together," adds Rajeevan. Jacobson, however, says that greenhouse gases and aerosol pollution impact differently on climate. "Greenhouse gases may cause an increase in precipitation over land but a decrease over the ocean due to increased stability over the ocean. Greenhouse gases also warm ocean surface temperatures increasing tropical cyclone activity. Both these effects are separate from the aerosol effect," says Jacobson.

But referring to the system of predicting rainfall as complex, Rajeevan says that there are several factors that determine it and hence there has to be something more than just attributing some change to just aerosols. Talking about different studies carried out on the subject, he says "While one analysis suggests that intense rainfall events over India have increased, the other showed wind speed has decreased over Central India (paper published in Current Science)." According to him, with the arrival of monsoons the aerosol content falls drastically. "We need to study the pre-monsoon scenario as well and see how it is affected," says Rajeevan.

In all probability, for a realistic prediction of rainfall pattern over the country, aerosol pollution needs to be factored into studies.

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