首页 / 雜誌文章 / Blown away一吹而散

Blown away一吹而散

退役的喷气发动机或许有助清除笼罩大城市的雾霾

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。

他的想法是把一台喷气发动机放在印度一座污染严重的燃煤发电厂旁,将排气喷嘴指向天空,然后开启发动机。他希望发动机排出的气流能干扰所谓的“逆温”过程,该气象现象指的是一层温暖气流沉降在较冷的空气上,将其覆裹。而发动机排出的上升气流会把构成雾霾的微小颗粒带走。

逆温现象不光加剧了德里的空气污染问题,从洛杉矶到德黑兰,许多城市都深受其害。尤为严重的一个例子就是1952年的伦敦大烟雾——持续四天的空气污染导致了12,000人死亡。阿勒马洛认为,喷气发动机排出的气流可以穿透逆温层,创造一道“虚拟烟囱”,把困在空气中的污染物往上吹,使其消散于更广阔的大气层中。据其计算,使用喷速为每秒460米的单台喷气发动机便可将容量一千兆瓦的燃煤发电厂所有的排放物吹散。不过,他还没有计算清楚一台喷气发动机是否可以干扰逆温层并把污染物吹离城市上空,所以他正打算测试这一假设。

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。

一些气象学家则持怀疑态度。他们提出,目前可用的发动机并不具备足够的力量把污染物质轰出德里的逆温层,尤其是在白天,那时冷暖空气的锋面处于海拔1000米左右的高度。他们还认为阿勒马洛的“虚拟烟囱”构想过于简单。排气流在爬升时遭遇的大气湍流和摩擦作用将削弱其力量。此外,日内瓦世界气象组织研究员亚历山大·巴克拉诺夫(Alexander Baklanov)表示,即使这一技术果真能奏效,对抗覆盖全城的逆温层需要众多喷气发动机及大量燃料,成本将极其高昂且难以承受。

阿勒马洛当然不同意,而且假如测试能按其时间表推进,孰对孰错不久便可见分晓。即便他的“全城虚拟烟囱阵”构想确实野心过大,在极端污染的情况下也许还是可以派上用场。温哥华英属哥伦比亚大学的城市气象学研究员安德烈亚斯·克里斯滕(Andreas Christen)指出,最严重的污染出现在空气变冷的时候,比如在晚上。因为温度下降时空气体积收缩,上方的暖空气得以向下膨胀。这使得空气中的灰霾更为集中,但也令逆温层更接近阿勒马洛的排气机射程。正如克里斯滕注意到的那样,富裕国家的一些农民已开始运用直升飞机来干扰自家农田上空的逆温层,保护作物免受霜冻。阿勒马洛的排气机也许能成为一个替代方案。

英文对照


Retired jet engines could help clear the smog that smothers big cities

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.

发表评论

切换注册

登录

忘记密码 ?

您也可以使用第三方帐号快捷登录

Q Q 登 录
微 博 登 录
切换登录

注册