TO LAND at Indira Gandhi Airport is to descend from clear skies to brown ones. Delhi’s air is toxic. According to the World Health Organisation, India’s capital has the most polluted atmosphere of all the world’s big cities. The government is trying to introduce rules that will curb emissions—allowing private cars to be driven only on alternate days, for example, and enforcing better emissions standards for all vehicles. But implementing these ideas, even if that can be done successfully, will change things only slowly. A quick fix would help. And Moshe Alamaro, a researcher at the Massachusetts Institute of Technology, thinks he has one.

His idea is to take a jet engine, put it next to one of India’s dirty coal-fired power plants, point its exhaust nozzle at the sky and then switch it on. His hope is that the jet’s exhaust will disrupt a meteorological phenomenon known as “inversion”, in which a layer of warm air settles over cooler air, trapping it, and that the rising stream of exhaust will carry off the tiny particles of matter that smog is composed of.

Inversion exacerbates air pollution in Delhi and in many other cities, from Los Angeles to Tehran. A particularly intense example caused the Great Smog of London in 1952, when four days of air pollution contributed to 12,000 deaths. Dr Alamaro thinks a jet engine could punch through the inversion layer to create a “virtual chimney” which would carry the trapped pollution above it, so that it could be dispersed in the wider atmosphere. He calculates that all the emissions from a gigawatt coal-fired power plant could be lifted away using a single engine with a nozzle speed of 460 metres a second. However, he has not calculated whether a jet engine could disrupt the inversion layer and allow the pollution to escape the city—so he is now going to test that hypothesis.

Within eight months, Dr Alamaro plans to put one of his updrafters next to a coal-fired power plant and monitor what happens using a fleet of drones. He is in discussions with Tata Group, a conglomerate with an electricity-generating arm, to run it next to one of the firm’s power stations. Another good candidate would be a government-run plant at Badarpur, less than 50km from the middle of Delhi. According to the Centre for Science and the Environment, a research and lobbying group based in the Indian capital, Badarpur is one of the most polluting power plants in the country. Earlier this month the government shut it down for ten days as part of a set of emergency measures intended to curb a particularly intense bout of air pollution.

Dr Alamaro has already found some of the decommissioned jet engines he needs to build his first updrafter. Both the Indian and the American air forces have been forthcoming. The Indians have offered six retired engines for nothing and the Americans are in the process of approving a further four engines from the Boneyard, an aircraft-storage facility located on Davis-Monthan Air Force Base in Arizona. They are asking for just $5,000 per jet to cover the labour needed to prepare the engines, plus shipping.

Some meteorologists are sceptical. They suggest that the engines on offer will not have the oomph to push material through Delhi’s inversion layer, especially during daylight hours, when the boundary between warm and cool air sits at an altitude of around a kilometre. They also say that Dr Alamaro’s notion of a virtual chimney is too simple. Turbulence and friction will weaken the exhaust stream as it climbs. Moreover, even if the technique does work, using it to attack a citywide inversion layer would require so many jets and so much fuel as to be prohibitively expensive, says Alexander Baklanov, a researcher at the World Meteorological Organisation, in Geneva.

Dr Alamaro, naturally, disagrees—and if he can keep to his timetable it will not be long before it is clear who is right. Even if his ambitions for citywide arrays of virtual chimneys prove too ambitious, they may still work in some of the worst cases of pollution. Andreas Christen, who studies urban meteorology at the University of British Columbia, in Vancouver, notes that the direst episodes of pollution happen when air is cold—at night, for example. This is because the air contracts into a smaller volume at low temperatures, giving warm air above it room to expand downwards. That concentrates airborne gunk, but it also brings the inversion layer within closer range of Dr Alamaro’s jets. As Dr Christen observes, some farmers in rich countries already use helicopters to disrupt inversion layers above their fields and thus protect their crops from frost. Dr Alamaro’s jets may offer an alternative.


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