Particulate air pollution has been suggested as the cause of the recently observed decreasing trends of 10 to 25% in the ratio between hilly and upwind lowland precipitation, downwind of urban and industrial areas. We quantified the dependence of this ratio of the orographic-precipitation enhancement factor on the amounts of aerosols composed mostly of pollution in the free troposphere, based on measurements at Mt. Hua near Xi’an, in central China. The hilly precipitation can be decreased by 30 to 50% during hazy conditions, with visibility of less than 8 kilometers at the mountaintop. This trend shows the role of air pollution in the loss of significant water resources in hilly areas, which is a major problem in China and many other areas of the world.
SciDev.Net reports on their work:
In this kind of high-altitude rainfall, known as orographic precipitation, moist air is deflected upwards by the mountain. This cools the air and causes the moisture in clouds to condense and form droplets, which then merge to create raindrops.
Cloud droplets form around aerosols. According to Rosenfeld, the higher number of aerosols in polluted air divide cloud droplets into smaller ones, which slows the formation and fall of rain.
“This is the first time a direct link between increasing pollution and decreasing precipitation has been observed,” he said. “The finding is important since precipitation is one of the main sources of water in northern China.”
Yao Zhanyu, co-author of the paper, told SciDev.Net that of all the natural disasters in China, droughts are the most serious. “In the western region, the annual average precipitation is about a fourth that of the world’s average,” he said.