Yu Shaocai believes he knows how to clean China's dirty air - wash it.

The academic at China's Zhejiang University and the US' North Carolina State University recently detailed his idea in Springer's journal Environmental Chemistry Letters. He proposes installing a sprinkler system to water China's atmosphere, essentially rinsing the smog away.

Yu's research suggests this could reduce the atmosphere's fine-particle load to 35 micrograms per cubic meter.

He says the idea came to him while watching a garden being watered.

"I immediately thought that we can clean air pollution by spraying water into the atmosphere - like watering a garden".

China's haze is caused by PM2.5 - particles smaller than 2.5 microns in diameter that the respiratory system can't filter out.

"It's well-known that precipitation scavenging is the single most efficient way of removing aerosol pollution in the atmosphere," he says.

Precipitation scavenging is the natural process by which atmospheric moisture - rain, fog or snow - removes substances from the air.

Yu cites research done at a Beijing urban atmospheric environmental monitoring station that shows a marked decrease in air pollution directly after heavy rainfalls - and increases in pollutants during dry spells.

"When we have rain, especially heavy rain, rainwater can clean the air pollution in a very short time period from a few minutes to hours or days, depending on the precipitation rates," Yu writes.

Spraying water into the atmosphere can cause rapid scavenging, he writes.

He believes this system alongside emission controls could provide a permanent solution to smog.

Yu says China already has much of the infrastructure - high buildings on which to place sprinklers, water and aircraft.

And he believes it would be cost-effective.

"The low-tech nature of this geo-engineering approach has led us to believe that it will cost much less than many other interventions, such as cutting emissions," Yu said in his paper.

Bonuses include removing harmful gases from the atmosphere and street cleaning. Drop size matters. If too small, they'll evaporate before reaching the ground, which would release the particles back into the atmosphere.

But it's easy to ensure the drops are large enough, Yu writes.

"The water could be collected and reused, meaning it would not exacerbate current water shortages."

"If the water collected has been polluted because of very polluted air, then we will need some treatment."

The sprinklers would need to be used daily to prevent accumulation. Yu is confident that this wouldn't be a problem for the public.

"If you can offer a half-hour watering your garden, then you can offer a half-hour watering your ambient atmosphere to keep air clean," he writes.

sundayed@chinadaily.com.cn

(China Daily 02/16/2014 page3)