MEXICO CITY — This megacity of more than 20 million rang in the New Year with a pleasant revelation: the region registered 248 days in 2012 in which the air quality considered good.

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The results, recorded by the federal district government, are in line with a trend of improvement. In a region once disparaged as “Mexsicko City,” the skies have cleared sufficiently to change perceptions of a place once considered one of the most contaminated in the world. “The city, without doubt, has stopped being the ugly duckling of the international community,” the previous mayor, Marcelo Ebrard, boasted in his final state-of-the-city address before leaving office on Dec. 5.

Mexico City has emerged as an aspiring environmental model citizen in recent years as the left-leaning local government has introduced everything from barter markets for recyclables to bicycle-sharing arrangements to zero emissions bus corridors.

But it is the clearing of the air that has burnished the local government’s environmental credentials the most, especially given that other Mexican cities like Guadalajara and Monterrey regularly register days with worse air quality than the capital’s.

Previously, people in the provinces viewed Mexico City’s air situation as apocalyptic. “We can say that Mexico City is improving, while in the rest of the cities, it’s getting worse,” says Guillermo Velasco Rodríguez, director of planning and projects at the the Centro Mario Molina in Mexico City.



It’s a sharp change from the early 1990s, when Mexico City’s problems seemed intractable – a product of unfavorable geography and poor urban planning, Mr. Velasco says. Mexico City covers a closed high-altitude valley that traps contamination. The metropolitan area, meanwhile, mushroomed as internal migration brought the nation’s downtrodden masses to the outskirts of Mexico City, where they built shanties that later became sprawling suburbs.

The federal government began taking some steps to fix the situation in the 1980s, Mr. Vesalco said. It began measuring air quality and listing scores on something known as the Metropolitan Air Quality Index. The initial scores were unflattering: Mexico City registered scores of less than 50 – air quality that was considered good and posing a low risk to human health – just eight times in 1992.

The state-run oil company Pemex started selling low-sulfur gasoline. Big industrial operations like a once-polluting Pemex refinery moved to more outlying areas or were closed. A program known as “Hoy No Circula,” loosely translated as “No Driving Today,” kept cars off the road one workday each week. The federal, Mexico City and Mexico state governments – the latter being responsible for the outlying suburbs, home to an estimated 11 million residents – also created a joint commission on improving air quality.

The commission brought some continuity to addressing the problem – something difficult in Mexico, where the federal and state governments change every six years and an absence of re-election discourages long-term planning.

The current federal district government is quick to credit the work of past administrations from the same party, the Democratic Party of the Revolution, which won power in 1997, the first year that mayoral elections were held. (The federal district was previously ruled by a regent appointed by the president.)

Tanya Müller García, the local environment secretary, presents posters charting the city’s air quality since 1986, when such record-keeping began, to underline how much the air has improved. Index scores topped 200 on a 500-point scale in the early years – bad enough that the local government could close schools and industry. No such scores have been registered since May 2003, while the number of good days steadily climbed from 181 days in 2008 to 211 days in 2011 to last year’s 248.

And Mexico City has been quite innovative in public policy, Ms. Müller said. She mentioned initiatives like the Metrobus, a system of clean-burning buses plying dedicated lanes that has encouraged commuters to leave their cars at home. A new bicycle-sharing service known as Ecobici proved so popular that it developed a six-week waiting list. Hoy No Circula was expanded to Saturdays, when older, out-of-state vehicles entered the city more frequently, and only vehicles manufactured since the 2005 model year may circulate daily without restrictions.

Vehicle emission verification was also toughened, and centralized monitoring of the computers used in the tests was introduced to combat corruption. “Verification centers used to be able to manipulate” the system easily, said Alejandro Ramos, an environmental reporter for the newspaper Reforma. “It’s harder now.”

Challenges remain, though. Mr. Ramos said that standards in Mexico State are less stringent than in Mexico City, prompting some with older cars to replate in the suburbs. Some poorer suburbs experienced an environmental precontingency over the Christmas holidays because of fireworks and the burning of tires and other trash in bonfires.

Measuring fine particulate matter is another problem. “Air quality has improved in terms of certain contaminants, but the monitoring service we have in Mexico City doesn’t measure contaminants such as 2.5″ — particles measuring less than 2.5 micrometers in diameter — from mobile sources, said Gabriel Niño, public policy coordinator at the Mexican Environmental Law Center. And generous gasoline subsidies keep the price of Mexican gasoline lower than in the United States and mostly benefit the rich, creating disincentives for motorists to stop driving, Mr. Velasco said.

The region also lacks urban density, he adds, with workers buying homes through government housing programs in far-off municipalities, which means a commute on clogged roads lasting more than two hours.

Still, there has been enormous progress over the last 25 years. “It’s like running a marathon,” Mr. Velasco said. “We’re halfway there.”