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墨西哥城大气污染
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墨西哥城(Mexico City)与空气污染的斗争持续了长达数十年之久，为世界各地城市如何改善空气质量展现出一些行之有效的策略。
自90年代以来，墨西哥政府一直采取一种称为“ProAire”的综合性方法，通过许多连续性项目，使二氧化碳和其他污染物得到显著减少。墨西哥城的努力为它赢得了C40 城市气候领袖群(C40 Cities Climate Leadership Group)颁发的2013年空气质量奖。C40 城市气候领袖群是一个由75个超大城市组成的国际网络，这些城市面临着类似的环境问题，因而一起协作寻求解决方案。
汽车和工业温室气体排放 ―― 一氧化碳、二氧化硫、氮氧化物和臭氧 ―― 还有微观颗粒物，是墨西哥城空气污染的主要来源。这些来自车辆和工业排放、燃烧以及烟尘的液体和固体微粒很难从大气中清除，并且对人体健康有致命影响。
地理环境进一步加剧了污染。墨西哥城大都会地区涵盖城市本身以及周边位于墨西哥谷(Mexico Valley)的一些州和城市。大约2100万人居住在这个海拔2240米的高原，四周群山环抱。这个由古代火山爆发而形成的环形盆地不利于排放物的消散。而且在这个海拔高度含氧量减少，导致车辆发动机效率降低，增加了污染物的释放。更糟糕的是，山谷上空被称为“逆温层”(inversion layer)的暖空气将一切笼罩起来。
科学家在墨西哥城空气质量监测中心检查空气过滤装置。(© AP Images)
哪些措施奏效
马里奥莫利纳中心(Centro Mario Molina)的环境政策研究论坛项目主任吉列尔莫·贝拉斯科(Guillermo Velasco)指出，由于车辆跨州运输，联邦和地方政府的合作非常关键。他说，“一个地区的排放当然会影响到另一地区，所以 [各政府]合作很重要。” ProAire项目的最大成就之一是促使不同辖区的官员开展合作。
扩大公共交通，引进催化转化器和无铅汽油以减少排放，强制执行一年两次的车辆排放检查，并关闭一家墨西哥国营石油公司(Pemex)的炼油厂，再加上精心监测，这些都是ProAire行之有效的策略。“No hoy circula” ―― 每周限制小轿车出行一天 ―― 是在车辆数量增加之前的另一个行之有效的做法。
贝拉斯科说，“墨西哥城迄今为止是一个成功的经历”。城市在改变人们驾驶习惯的同时，也改善了基础设施。但是，人口增长以及由此带来的汽车交通和工业活动的增长，意味着在不远的将来将需要更为积极的努力才可以保持空气的清洁。
现在的绿色基础设施包括会吃烟雾的建筑物。这家墨西哥城的医院使用的是二氧化钛涂覆的瓦片，当暴露在阳光下时，它可以产生一种化学反应使污染物变得没有危害。(Courtesy photo)
后续步骤
新的ProAire策略将实行到2020年，并包括进一步绿化城市运输车队，增添一个新的公交车(Metrobús)和“Ecobici”公用自行车项目。重新造林和创建绿地以及使用可再生能源也将有利于清洁空气。而且，对贝拉斯科来说，城市重建让人们可以“住得距离他们工作的地方更近一些”，这对减轻污染会有所帮助。
调查显示，人类是一个主要的污染源，仅次于车辆和工厂。贝拉斯科认为，通过教育让人们知道每个人如何能够减少空气污染也是最重要的举措之一。
贝拉斯科说，除非这些策略得到严格贯彻，“否则最有可能看到的实际情况是，我们的空气质量保持不变。它不会恶化，但也不会改善”。替代燃料之类的技术发展是解决方法之一，但是他说，要保持这一势头的话，“我们现在需要拿出非常大胆的举措。”
Mexico City’s air quality challengeMexico City’s decades-long, ongoing struggle with air pollution offers cities worldwide effective strategies to improve air quality.
Since the 1990s the Mexican government has pursued a comprehensive approach calledProAire, comprising successive programs that have reduced carbon dioxide and other pollutants significantly. Mexico City’s efforts earned it theC40 Cities Climate Leadership Group’s2013 air quality prize. The group is an international network of 75 megacities that face similar environmental problems and collaborate on solutions.
Automobile and industrial greenhouse gas emissions — carbon monoxide, sulfur dioxide, nitrogen oxides, ozone — are the chief sources of air pollution in Mexico City, along with microscopic particulate matter. These liquid and solid particles from vehicular and industrial emissions, fires, soot and dust are difficult to remove from the atmosphere and have a deadly impact on human health.
The pollution is intensified by geography. The Mexico City Metropolitan Area encompasses the city itself and surrounding states and cities in the Valley of Mexico. About 21 million people live at 2,240 meters elevation on a plain ringed by mountains, in the crater of an ancient volcano that traps emissions. Less oxygen at that altitude reduces the efficiency of vehicle engines, so they release more pollutants than they might elsewhere. Worse, warm air above the valley, called an “inversion layer,” seals in the pollution.
A scientist inspects an air filter at a Mexico City air quality monitoring center. (© AP Images)
What works
Guillermo Velasco, program director of the environmental policy research forum Centro Mario Molina, says the movement of vehicles across state lines makes federal and local government cooperation essential. “Of course, the emissions of one side go to the other, so it is important that [governments] work together,” he says. Getting officials from different jurisdictions to cooperate has been one of the ProAire program’s biggest accomplishments.
Expanding public transport, introducing catalytic converters and unleaded gasoline to cut emissions, mandating bi-annual vehicular emissions checks, and closing a Pemex refinery, plus careful monitoring, are ProAire strategies that work. “No hoy circula” — limiting car travel one day per week — is another strategy that worked well until car numbers increased.
“Mexico City has been, until now, a success story, ” says Velasco. The city has improved its infrastructure while changing people’s driving habits. But population growth, and the resulting increase in automobile traffic and industrial activity, mean more vigorous efforts will be needed in the near future to keep the air clean.
Green infrastructure now includes smog-eating buildings. This Mexico City hospital’s titanium dioxide–coated tiles create a chemical reaction when exposed to sunlight that makes pollutants more benign. (Courtesy photo)
Next steps
New ProAire strategies run up to 2020 and include further greening municipal transport fleets, with a new Metrobús and the “Ecobici” shared-bike program. Reforestation and creation of green areas will help clear the air, as will renewable energy. And, to Velasco, restructuring the city, so people “live closer to where they work,” will help.
Surveys show that people are big polluters, not far behind vehicle and industrial plants. Educating people about how they, as individuals, can reduce air pollution will also be a priority, according to Velasco.
Unless strategies are implemented strictly, “the most realistic thing you are going to see is that our air quality is going to stay as it is. It’s not going to deteriorate, but it’s not going to improve,” says Velasco. Technological developments such as alternative fuels are part of the solutionbut, he said, to keep up the momentum, “we need a very bold move now.”
