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January 26, 2014

More details about Skyscraper water spraying to mitigate air pollution

Nextbigfuture covered the concept of spraying water from skyscrapers to reduce air pollution in cities in China.

Spraying water from skyscrapers 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. Air pollution is a big problem in China and this is approach to pollution mitigation is being developed there.

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.

Tests will be performed at Zhejiang University campus first and then Hangzhou city if everything goes well. 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."

Background

Environmental Chemistry Letters - Water spray geoengineering to clean air pollution for mitigating haze in China’s cities

All 74 of China's big cities fail to meet air quality standards. It will take decades to radically improve the emissions of China's factories, powerplants and cars.

China produces 55 billion tons of rain per year from massive cloud seeding programs. China plans to increase this to 280 billion tons of rain per year from cloud seeding.

China is predicted to reach 4.8 billion metric tons per year by 2020, up from 3.65 billion tons in 2013. The prediction was made by China National Coal Association vice president Liang Jiakun. China uses about half of the world total in coal. 1 billion tonnes is used in Europe and another 1 billion tons in the United States.

How much is 4.8 billion tons ? That is 600,000 110 car trains full of coal. The 110 cars each hold 100 tons of coal but about 25% of it is water weight. So only 8000 tons of coal per train which coal plants need every 1 to 3 days. Coal is burnable dirt. The world is burning a literal mountain of it every year.

China can achieve rapid improvement in lower particulate emissions by increasing the fines and turning on the emission control systems at coal plants. The coal plant operators follow what the central government orders. The central government had given a wink to turning off the emissions control equipment in order to get lower electricity prices. The cost benefit has clearly changed. The central government needs to improve the air for the people in urban centers. The emission control systems are being turned on. However, a lot of the emissions are construction dust and pollution from cars. It will take a lot more to make the air in the cities better. This is why the water spraying systems will help bridge the 2-4 decade gap to better air quality.

The US geological service has some information on rainfall. The water spraying system would be creating about 30 minutes of artificial rain on days that have no rain and high pollution levels. China has another large water project that quoted 1 kilowatt hour of energy to raise one ton of water by 200 meters. If there was 100 hours of artificial rain needed per year, then we would just need to know the rate or amount of artificial rain for the air pollution cleaning effect. The energy times about 0.05 dollars per kilowatt hour would approximate the operating cost.


There is an online rainfall calculator.

SOURCES - South China Morning Post, Environmental Chemistry Letters, US Geological Service, IBITimes

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