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.

Mobile Laboratory

The MIT team began its work in 1999 by conducting some preliminary studies to assess various strategies for air pollution control. The researchers tried to use their computer model to simulate atmospheric chemistry, but they hit a wall: a lack of reliable data about pollution sources. Without figuring out where pollution was coming from, when pollutants were at their worst throughout the day and year, and what regions of the city suffered the worst pollution, among other factors, they had difficulty evaluating the “what if” scenarios of pollution control.

This need for accurate data led to a comprehensive, five-week field measurement campaign aimed at pinpointing pollution levels and sources with a degree of specificity that had never before been attained. The campaign was completed last spring. More than 30 institutions in the United States, Mexico, Germany, Switzerland, and Sweden participated. The effort required a great deal of logistical coordination and a mass of sophisticated equipment.

Foremost among the research tools was a van carrying $1.5 million worth of state-of-the-art instruments to identify gases and particles and measure their concentrations in real time. (Conventional methods of measurement, in which paper filters are exposed to pollution and later analyzed in a laboratory, usually take hours or even days.) This mobile laboratory, developed by Billerica, MA-based Aerodyne Research, chased cars, cabs, trucks, buses, and “colectivos”-22-seat microbuses-to measure their emissions. The van was also equipped with a Global Positioning System receiver, a camera, and a speedometer, so researchers could correlate the measured emissions with each vehicle’s characteristics and with driving conditions.

“It’s like a hunt,” says Linsey Marr, a postdoc who participated in the field campaign. “You drive around, and when you see a vehicle you want to chase, you try to stay right behind it, following it close enough so that other vehicles don’t get in between.” The researchers in the van-one beside the driver, and three in the back-kept their eyes on several screens, cheering when they got behind a big, dirty diesel truck and a spike appeared in the graph monitoring carbon dioxide concentration. This meant they were right in the truck’s plume.

But it wasn’t always easy to chase the polluters. Some truck drivers were suspicious of being followed by a van with strange instruments sticking out of it. They pulled to the side of the road and stopped. And following intrepid taxi drivers around Mexico City was almost hopeless. In the end, one of the researchers had to hail a cab and ask the driver to, well, drive slowly.

Sometimes the researchers parked the van for one or two days in a particular region of the city to obtain more accurate measurements for that area. They thus constructed a detailed map of the pollution in the city. The field campaign also involved taking measurements at the project’s headquarters, which was purposely located in a heavily polluted area. The data collected there and in the van will ultimately contribute to better predictions of atmospheric chemistry.

By John MacNeill.

Early Results

Thus far, the measurements have revealed extremely high concentrations of pollutants such as ozone and so-called particulate matter. In the stratosphere, ozone blocks the sun’s harmful ultraviolet radiation, but at ground level, it is a toxic gas that irritates lungs. Particulate matter-microscopic particles of carbon and other compounds-is even more dangerous, because it penetrates deep into the lungs and can cause severe respiratory diseases. A 10 percent reduction in the concentration of fine particulate matter would represent a health benefit on the order of $2 billion per year in reduced hospital costs and productivity losses, according to the Harvard School of Public Health group.

In addition, the researchers detected certain chemicals in Mexico City’s air for the first time. These toxic substances, formed when emissions from motor vehicles decompose in the atmosphere in the presence of sunlight, had previously only been detected in lab studies. They also observed some unusual chemical activity, rarely seen in other urban areas, in a group of carcinogenic compounds called polycyclic aromatic hydrocarbons.

Group members are only now beginning to analyze the hundreds of gigabytes of data they collected. With accurate information, they hope to improve their pollution models and project scenarios for different kinds of pollution control strategies in Mexico City. For instance, what is the impact of, say, improving public transportation? Or enforcing regulations on the maximum age of taxis and microbuses? “We’re not doing this just because of the science,” says Luisa Molina, who coordinated the field campaign. “Many of these experiments are policy-driven questions that we want to know.”

Ultimately, the researchers hope their work in Mexico City will lead to a solution that can be adapted to other polluted megacities, particularly those in developing countries. Many of these cities, such as So Paulo, Bangkok, and Beijing, are still growing and can’t afford expensive antipollution measures or research. The MIT-led project could give them an understanding of the problem that they could never gain alone.

It might take several years before all the results from the Mexico project become available and the population feels the real effects. But Mario Molina is confident that the work will yield benefits. “It’s already happening to some extent,” he says. “We’re just accelerating the process, making sure that it works all right.” Perhaps within a few years, when Molina flies again to his hometown, the thick, brown cloud that covers the city will have disappeared.