To explain the pattern of pollution, Retama, who’s the city’s director of atmospheric monitoring, opens his laptop to consult a camera positioned in the foothills southwest of the city. Punching dates and times into the URL, he culls a series of images that demonstrate the regular, noxious movement of dirty air down through the valley. In the morning, the view north is obscure. In the afternoon, the south clouds over. A wave of pollution washes across political boundaries, bridging an administrative gap between the outlying State of Mexico and the central Distrito Federal (DF), before pooling against the enormous mountains to the south of the city.
“The south, which has the fewest sources of pollution, is the most polluted, precisely because it’s being punished by the pollutants generated in the north,” Retama explains. “And the north, which produces the most pollutants, is cleaned by the wind, and by the afternoon, is one of the cleanest zones.”
Time-stamped photographs tell that story; so does a regional wind map. The 32 air monitoring stations perched on rooftops throughout the city offer confirmation.
Monitoring the air in the world’s fifth-largest metropolis requires both a planner’s knowledge of pollution sources and a meteorologist’s sense of atmospheric patterns. The former can be met head-on with policies and restrictions. The latter can only be observed and anticipated.
Over the past two decades, Mexico City has made tremendous progress fighting air pollution. Declared the world’s dirtiest city in 1992, the Western Hemisphere’s biggest agglomeration no longer places highly on the World Health Organization’s rankings of cities with serious particle pollution. The snow-capped volcanoes are once again visible; the sky is once again blue.
That doesn’t mean the district has recovered its mid-century reputation as a breezy subalpine utopia; on the contrary, there are signs that the transformative policies of the past decades may have reached an inflection point. But while Retama works with the city on strengthening anti-pollution measures, his primary responsibility is measuring, documenting and publicizing the city’s ongoing air pollution problem.
In a spare, central room down the hall from Retama’s office in downtown Mexico City, a wall of screens broadcasts all the city’s data on pollutants. It is more public display than real-life command center; all this information is available on the city’s air quality homepage, which, Retama boasts, gets 50,000 to 60,000 visitors a day. Under the driving restrictions of “contingency” days, which occur a half dozen times per year, road traffic falls while website traffic surges into the hundreds of thousands.
What good does all this outreach do? That depends on your perspective. For complacent, older chilangos, who remember the no-outdoor-exercise days of the ’80s and ’90s, the stats are a reminder that even a blue-skied city can be hazardous. For the city’s younger residents, who tend to be more suspicious of the government’s environmental back-patting, the data reinforces the claim that the city’s Air Quality Index often hovers in the same range as in American cities.
Either way, it’s the one issue that no Mexico City resident can escape. In a city with great contrasts between extreme wealth and desperate poverty, the air represents a great equalizer. Leafy enclaves in the city’s south and west inherit traffic exhaust from the center and industrial pollution from the north.
But air currents display significant variance at the local level too. The quantity of car pollutants like carbon monoxide in the middle of a busy street, Retama says, could be as much as three times higher as what a nearby monitor is reporting. At a distance of 50 meters from the source, pollutants decline by 70 percent. A student at MIT showed that the air quality here even improves if you walk away from the curb.
“There has always been an argument about whether we should we should measure the level in the street or at the monitoring stations,” Retama says. “Surely there are people recording in the streets, between cars, saying, ‘Much higher than what the monitoring stations are reporting!’ But the problem with trying to measure personal exposure is that we’d need hundreds or thousands of monitors to have with precision the levels of each contaminant.” Soon, his department plans to deploy smaller, cheaper monitors at street level and in other pedestrian locations, to compare with official data.
But the gap between street-level conditions and rooftop pollution monitors shouldn’t be cause for concern, he stresses. “The big advantage is that people don’t spend the whole day walking in the street. Those are relatively short trips,” he says. Between your office, your apartment, the subway, and your car, your respiratory mix probably closely resembles what is inhaled by the city’s measurement machines. (Counterpoint: If you work as a traffic cop or taxi driver, you might as well be living in New Delhi.)
Those geographic variations are all, to a greater or lesser extent, subordinate to seasonal shifts. The best time to breathe in Mexico City is between June and November. December and January produce temperature inversions, trapping a frisbee of dirty air above the city.
As the rest of Mexico City looked forward to warmer weather, Retama was anticipating the onset of the most difficult time of year, when pollutants cook in the heat, producing long strings of days with elevated ozone levels. Most call it spring; for Retama, it’s Ozone Season.