The idea came to Yu Shaocai while he was watching a garden being watered: to curb air pollution, why not spray water into the atmosphere from sprinklers atop skyscrapers and towers?
Yu, a former research physical scientist with the US Environmental Protection Agency, says his system "can be one of the permanent solutions to air pollution in megacities globally, in addition to other solutions such as emission controls".
Yu, who is also a professor at Zhejiang University and North Carolina State University, has studied air pollution and wet deposition - the processes by which aerosols are scavenged by cloud and fog drops, rain and snow - for more than 30 years.
In an article published in the journal Environmental Chemistry Letters this month, Yu explains the mechanism of his geoengineering scheme and highlights its benefits: it could help to reduce the concentration of PM2.5 pollution - tiny particles in the air which are especially hazardous to health - efficiently to a safer level of 35 micrograms per cubic metre, and in as quick as 30 minutes.
In addition, the process is natural, technologically feasible, efficient and low cost. All the necessary technologies and materials required to make it work are already available, Yu says, from high buildings, towers and aircraft, to weather modification technology and automatic sprinkler heads.
Yu says his study report is based on theory, but he plans to begin tests on the mainland soon when he returns from the US. Experiments are under way to design a suitable water-delivery system to successfully implement the scheme.
"We will do some tests in Zhejiang University campus first and then Hangzhou city if everything goes well," he says. "If we are successful, our work can be followed by the other cities in China and around the world."
Air pollution in China has progressively worsened over the past 30 years, particularly in its megacities, due to rapid economic growth and expansion of industrial activity. According to a Greenpeace report released last week, in 2013, 92 per cent of Chinese cities failed to reach the national standard of a PM2.5 density of no greater than 35 micrograms per cubic metre. Thirty-two cities were double the standard, while the top 10 cities were three times the standard.
The six most polluted cities are in Hebei province, led by the industrial cities of Xingtai and Shijiazhuang. Among China's international business centres, Beijing was the worst at No 13, with an average PM2.5 index of 89.5 micrograms per cubic metre, followed by Qingdao (No 47) and Shanghai (No 48).
Natural precipitation is effective at cleaning air pollution - just think how much clearer the Hong Kong skyline is after a rainy day. In Beijing, an urban atmospheric environmental monitoring station showed that PM2.5 concentrations decreased from about 220 to 30 micrograms per cubic metre on September 26, 2011 because of heavy rain. Precipitation can also efficiently reduce gaseous air pollutants such and nitric acid and sulphur dioxide.
Yu's system is designed to spray raindrops of specific sizes and rain intensity, and at different heights, for the most efficient pollution reduction depending on the conditions.
Water should be sprayed into the atmosphere from at least 100 metres high, he says, because most air pollution is below this height. For areas with no tall buildings, towers of 100 to 200 metres high can be built.
The spraying would need to be done daily to avoid the accumulation of air pollution. Ideally, the water will be obtained from rivers and lakes to keep costs low, he says, and can be collected and reused, thereby preventing any exacerbation of existing water shortages. Although there are potential problems - such as flooding, humidification of the low atmosphere, and slippery grounds - Yu says these are outweighed by the benefits.
Dr Chan Chak-keung, a professor at Hong Kong University of Science and Technology's division of environment, says Yu's proposal is "interesting" but is concerned about the scheme's water usage.
"Where will we find that much water? You could recycle the water, but that itself is a challenging task," says Chan. "If I spray water from the roof, what about pollution above the roof? Assuming his team can find a system that works, and they've done enough economic analysis and considered the handling of water resources, this could be a viable option.
"I would also recommend he considers spraying water right at the street level, especially along heavy traffic roads."