Mexico Air Quality
Mexico City sits at 2,240m in a basin surrounded by mountains on three sides. With over 5 million vehicles and the world's first vehicle restriction program (1989), it remains a case study in urban air quality management — and its limits.
Monthly AQI — Mexico City
March–May pre-monsoon peak: dry, stagnant air + ozone
The Basin Problem
Mexico City sits in the Valley of Mexico — a basin at 2,240m elevation enclosed by mountains and volcanoes on three sides (Popocatépetl, Iztaccíhuatl, Sierra de las Cruces). The only natural air drainage is north through a gap toward Pachuca.
During the dry season (November–May), thermal inversions prevent vertical mixing. The mountain walls prevent horizontal dispersion except northward. All vehicle and industrial emissions from one of the world's largest metropolitan areas (21.9 million people) accumulate in this basin day after day.
High altitude amplifies ozone formation — UV radiation is ~25% more intense at 2,240m, accelerating the photochemical reactions that convert vehicle NOx and VOCs into ground-level ozone. The March–May period combines maximum UV, dry air, and stagnant conditions to produce Mexico City's worst ozone days.
Hoy No Circula: World's First Vehicle Ban
In 1989, Mexico City launched Hoy No Circula (HNC) — “today no driving” — the world's first urban vehicle restriction program. Each vehicle is banned from driving one weekday based on its last license plate digit.
The program initially showed promise — ozone concentrations fell in the early 1990s. But by 1998, researchers found a rebound effect: middle-class families bought old, cheaper second cars to bypass the restriction, actually increasing total vehicle emissions and fleet age.
HNC has evolved since then. Modern versions target older (pre-2000) vehicles more stringently, with pre-1993 cars restricted 2 days per week. SEDEMA (Mexico City's Environment Secretariat) activates “contingencias ambientales” (environmental emergencies) during worst pollution days, restricting even newer vehicles.