The Effects of Local Particulate Airborne Pollution. (March 9, 2007)

    The argument is still heard around the traps that Australia is such a small country that even if per head of population we are one of the planets biggest polluters, it does really matter.

Rainy days in the mountains are fewer near polluted cities. Credit: Punchstock/Nature

    Well think on this: Quirin Schiermeier writing in News@nature.com reports, "Air pollution is severely diminishing rainfall in Chinese mountains, researchers have found. The same effect is probably causing water shortages in many other highly polluted areas that depend on the nearby hills for their water... Small particles known as aerosols are known to alter the size and properties of the water droplets that form in clouds, and can affect the weather in several ways. In some mountain ranges in the western United States, rainfall has decreased by 10-25% over the past 50 years. Tiny particles in air pollution are suspected to be the main cause, but a lack of data has hindered any robust testing of this notion.

"Daniel Rosenfeld, a meteorologist at the Hebrew University of Jerusalem, Israel, and his team used a 50-year record of continuous measurements made on top of Mt. Hua Shan, near Xi'an in central China, to show that orographic precipitation decreased by 30-50% when conditions were hazy. Overall, annual precipitation at Mt. Hua Shan decreased by around a third between 1970 and 2005, as pollution rose. The data is reported in Science (Rosenfeld D., et al. Science, 315 . 1396 - 1398 (2007))."

Ulrike Lohmann, an atmospheric physicist at the ETH Zurich in Switzerland told Nature, "Extracting the human influence on precipitation is extremely interesting, but also extraordinarily difficult. Given strong pollution, China is probably the best place in the world to study this relation."

In addition Mr Schiermeier points out "the fine particles generated by diesel engines, biomass burning and fertilizers used in agriculture — as seen in this Chinese study — have different properties than large particles such as mineral dust or sea salt. Larger aerosols contain more soluble material, which generally attracts water. This can help to coalesce raindrops, thus enhancing rainfall.

And Professor Rosenfeld says that changes in rainfall due to manmade influences are just as important to take note of as the change in temperature caused by rising emissions of carbon dioxide. "Hydrology and precipitation are at least equally important."