Joggers trot through parks in the morning, and cyclists increasingly take to the streets. On many days, residents can gaze southeast at the snowy 17,802-foot volcano with the hard-to-pronounce name - Popocatepetl.
A haze still covers Mexico City, and ozone levels are often unhealthy. But the capital is no longer the smog-choked city of two decades ago, when birds were said to fall from the sky dead. It's been years since teachers kept kids off playgrounds to prevent respiratory illness.
Mexico City's air pollution reached its nadir in 1991, when the city chalked up only eight days with air quality below hazardous levels.
In contrast, this year is setting a record - 193 days with adequate to good air quality through Thursday.
"For many years, we were considered the city with the most contaminated air in Latin America. Today, we aren't even the most polluted city in Mexico," said Martha Delgado, the city's secretary of the environment. "There's a clear trend toward a dramatic decline in air pollution."
Delgado said the capital is reaping the rewards of two decades of pollution-fighting policies. Private cars, which must pass emissions tests every six months, are kept off city roads at least one day a week. Authorities have mandated a reduction of lead and sulfur in fuels. And the heaviest polluting of the city's 50,000 factories have been relocated.
The cleanup of the capital gives Mexico cachet as thousands of global climate change negotiators haggle in the resort of Cancun through Dec. 10. The summit is aimed at slashing the greenhouse gas emissions contributing to warming and extreme weather.
"The improvement is definitely visible," said Luisa T. Molina, an air quality expert who is president of Molina Center for Strategic Studies in Energy and the Environment in La Jolla, Calif. "Mexico City has become cleaner. But it's not over. ... You can see the haze from time to time."
The emissions from some 4.5 million vehicles are the primary source of air pollution - and a minority of those vehicles are major culprits.
"A surprisingly high amount of total pollution comes from a strikingly small amount of cars - super emitters," said Lucas W. Davis, an economist at the University of California, Berkeley, who has studied air quality in Mexico City.
Mexico's capital, which sits in a dried lakebed 7,350 feet above sea level, faces particular problems from vehicle exhaust. Intense solar radiation at such altitude worsens air pollution, which includes a noxious mix of carbon monoxide, nitrogen dioxide, sulfur dioxide, ozone and tiny suspended particulates.
When air pollution grew acute in the late 1980s, both the city and federal governments imposed measures, including the removal of lead from gasoline, obligatory use of catalytic converters, and substitution of fuel oil in factories and power plants with natural gas.
The boldest move may have been a 1989 measure, titled Hoy No Circula, or "No Driving Day," that bans most drivers from using their vehicles one day per week.
The policy was copied and imitated in cities elsewhere in Latin America, most notably the capitals of Chile and Colombia, and in Sao Paulo, Brazil, even as experts said some commuters bought second cars to get around the plan, limiting its effectiveness.
As the capital's population swelled beyond 20 million people, new measures included expansion of the city's subway system, one of the world's largest. It now has a 12th line under construction. A system of low-emission articulated buses along special corridors began in 2005 and is slated to have six lines by the end of 2012.
Recent projects have burnished the environmental credentials of Mayor Marcelo Ebrard, who says Mexico City's climate action program is the first of its kind in Latin America. Ebrard is expected to run for president in 2012.
Ebrard earlier this year placed bikes-for-hire stations in central areas of the city with more than 1,000 red two-wheelers and has announced that 500 Nissan Leaf electric taxis will be on city roads next year, a step toward getting gas-guzzling vehicles off the roads. Electric buses are also in the works.
The high-profile push for electric vehicles brings scorn from one of Mexico's top air quality experts, Humberto Bravo Alvarez of the Atmospheric Sciences Center at the National Autonomous University.
"Marcelo wants to use electric taxis. Maybe they haven't explained to him how much electricity these taxis will use," Bravo said. "Mexico's power-generating capacity isn't enough."
Bravo says cultural factors also impede anti-smog policies: Corruption among employees at emissions testing stations allows some exhaust-spewing older vehicles to stay on roads.
Some municipal policies are contradictory. City Hall is building second levels to highways to ease traffic congestion but potentially pulling more cars onto roads, said Sandra Guzman of the Mexican Center for Environmental Law.
"There are no incentives for people to get out of their autos," Guzman said.
Instead, city planners should spend more on aggressive extension of the rapid-transit Metrobus articulated vehicles, she added.
While skies are clearing, the presence of colorless ozone is still harmful, giving rise to bronchial complaints, eye irritation and fatigue. Last year, ozone exceeded healthy levels on 180 days, only a 2 percent improvement from the previous year.
"When it is high, you feel tired. Children get more allergies," Bravo said.
Armando Retama, director of atmospheric monitoring for the city, took a visitor to the roof of his office and gazed at the city horizon. "We can see a light haze today caused by tiny particulates that limit visibility," he said.
The remaining pollution still affects the health of residents, he said.
"We have chronic symptoms that we aren't aware of. Our bodies adapt," he said. When he leaves the capital for a week or more, "I can breathe better. I'm not all dry. My eyes aren't irritated. My skin doesn't crack."
A few floors below, engineers watch large screens giving readouts from 34 air-quality monitoring stations scattered about the metropolitan area.
"The city has one of the best monitoring systems in the world - comparable to London or Los Angeles," said Delgado, the city environmental chief. "We are in the lead in Latin America and on a par with any megacity in the world."
