Can artificial rainfall wash air clean?

Beijing is planning to use artificial precipitation to dissipate its persistent smog. Will this really work?

Zhuang Guoshun, atmospheric chemist at Shanghai's Fudan University and a leading figure in the study of PM 2.5, affirms that rainfall is the most effective way of eliminating smog. But he also warns that the effect is temporary because the pollutants will simply return either by travelling from their source or reemerging locally.

The major component of PM 2.5 air pollution in large Chinese cities is in the form of aerosol particles, says Professor Zhuang. When a large number of such fine particulates encounter water vapor in the air, they will soon expand and form smog.

Water also increases the weight of the particle matter and causes it to fall. Without interference, PM 2.5 air pollutants can remain suspended in the air for at least seven to ten days before they begin to fall. Artificial precipitation can accelerate this process and improve air quality.

However, we should not overestimate the impact of artificial rainfall, says Professor Zhuang Artificial precipitation is not always possible. First of all, there must be the presence of cumulonimbus clouds. And in any case the wind will bring new particles. For example, it only takes one or two days for fine particles to drift from Xinjiang to Beijing on the breeze. Even if the air is static, they can still spread. Thus rainfall will grant only a temporary breath of fresh air if a city is enveloped by a wide expanse of smog.

The biggest contribution to PM 2.5 comes from traffic exhaust, which emits twice as much as factories. A vehicle's pollution level will vary by brand and age. Therefore Chinese cities must commit themselves to improving the mandatory exhaust test and the scrapping of old cars, while tightening the quota of new vehicles, stresses Professor Zhuang.