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)

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

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 the C40 Cities Climate Leadership Group’s 2013 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.

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.

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 solution but, he said, to keep up the momentum, “we need a very bold move now.”