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As his airplane lands, MIT professor and Nobel laureate Mario Molina peers through the window, watching the sky change from blue to brown. A thick cloud of smoke and dust menaces the city below, and Molina can see scores of cars, trucks, and buses belching exhaust into the sky. Once on the ground, he steps outside and immediately feels his lungs burn. He can hardly make out the ring of mountains on the hazy horizon. Then Molina knows he is in Mexico City, his hometown and one of the most polluted cities in the world.

This scene is repeated many times a year. Molina makes frequent trips to Mexico City to study effective air pollution control strategies. He’s one of the leaders of a research team drawn from MIT’s Integrated Program on Urban, Regional, and Global Air Pollution-which is directed by his wife, Luisa-the Harvard School of Public Health, and other U.S., Mexican, and European institutions. With funding from MIT, the Mexican government, and the National Science Foundation, the researchers are measuring pollution from different sources and in different areas of the city and using this information to help craft pollution control approaches. Their goal is to find workable solutions for Mexico City and use them as a model for combatting air pollution in other “megacities.”

The Mess in Mexico City

With almost 20 million inhabitants, Mexico City is the second-largest urban area in the world (after Tokyo). More than 3.5 million vehicles and about 35,000 industries coexist there, producing thousands of tons of pollutants every day. Because of the warm climate and high altitude, pollutants collect in the valley-trapped by the surrounding mountains-and sunlight chemically transforms them into other, more dangerous compounds that accumulate in the atmosphere and form smog. In contrast to large U.S. cities, in which concentrations of certain pollutants exceed acceptable limits only a few times a year, Mexico City exceeds those limits almost 300 days a year. The city is suffocating itself.

A few decades ago, the problem was even worse. To fight it, the Mexican government conducted an aggressive air quality management campaign during the 1990s, significantly reducing pollution and levels of toxic compounds, such as the lead used in fuel. However, some of the proposed control measures were too expensive to implement. As a result, the population is still exposed to dangerous concentrations of airborne pollutants.

The problem has no single, immediate solution. Effective strategies have to balance economic, social, and technological factors, since measures such as restrictions on vehicle circulation have a direct impact on the economy and on people’s mobility. “The challenge is to make the pollution go down while the city is still growing,” says Molina, who won the 1995 Nobel Prize in chemistry for helping to reveal that chlorofluorocarbon gases-formerly used as propellants in spray cans and as refrigerants in air conditioners-were destroying the earth’s ozone layer.

The smog research conducted by the Molinas’ group is the basis for the strategic-planning aspect of a 10-year air quality management program for the city. The group is helping Mexican policymakers answer questions such as, What improvements are monetarily feasible and won’t affect productivity and mobility? Which pollutants and sources should be targeted? Should the government impose restrictions on vehicle age, change the composition of fuels, or close industrial plants near the city? To help answer these questions, the team will use data it has collected about pollutants to improve its atmospheric computer model, which simulates the chemistry of the air and shows what would result if different variables were altered. “We need a tool to assess what the different proposed strategies will do. It’s a tool to answer what-if questions,” says Molina, now an Institute Professor. “That way we can prioritize the measures, find the most efficient ones.”

Mexico has never conducted an atmospheric assessment like this before, according to Adrin Fernndez, general director of research on urban, regional, and global pollution at the National Institute of Ecology, the research arm of Mexico’s Ministry of the Environment. “It’s the first time there are so many people from different areas working in a coordinated way to go deep into the roots of the problem,” Fernndez says.

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