Growing cities, featuring more people with higher incomes who live and work in the suburbs and commute by private vehicle, should be a recipe for increased air pollution. Instead, California's major polluted urban areas have experienced sharp improvements in air quality. Technological advance has helped to "green" the average vehicle. Such quality effects have offset the rising quantity of miles driven. This paper uses several vehicle data sets to investigate how California's major cities have enjoyed air pollution gains over the last 20 years.