飞机要降落到英迪拉甘地机场（Indira Gandhi Airport），就会从如洗碧空落入棕色天际。德里的空气具有毒性。根据世界卫生组织的研究，印度首都的大气污染水平位居世界大城市之首。政府正试图推出法规来遏制排放，例如只允许私家车隔天上路，并对所有汽车实施更严格的排放标准。但即便这些想法能够付诸实施，改善也将非常缓慢。最好能有个速效的方法。麻省理工学院的研究员莫舍·阿勒马洛（Moshe Alamaro）认为自己就有一个。
阿勒马洛计划在未来八个月内将其“上升气流风机”置于一家燃煤发电厂旁，并用一组无人机监测其运作。他正与拥有发电企业的塔塔集团（Tata Group）商议在其一座发电站旁做试验。另一处不错的候选地点是印度巴达尔普尔（Badarpur）的一家国有电厂，距离德里市中心不到50公里。据德里的研究及游说团体“科学与环境中心”（Centre for Science and the Environment）称，巴达尔普尔电厂是印度污染最严重的电厂之一。本月早些时候，因空气污染异常严重，政府采取了一系列紧急应对措施，该电厂被关闭了十天。
阿勒马洛已找到一些退役喷气发动机来打造他的第一台“上升气流风机”。印度及美国空军都乐于出手相助。印度空军无偿提供了六台退役发动机，而美国空军也计划从亚利桑那州戴维斯-蒙森空军基地（Davis-Monthan Air Force Base）的“飞机坟场”（Boneyard）调出另外四台发动机供阿勒马洛使用，目前正在审批。他们只要求对每个发动机收取5000美元，以支付整备发动机所需的劳动力成本外加运输费用。
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.