With just two weeks left in the official California smog season, the data are in: After years of air-quality improvements, the state experienced a sudden downturn this year, enduring a shroud of haze and ozone pollution even in areas unaccustomed to smog. The usual suspects — urban communities like San Francisco and the famously smoggy Los Angeles — experienced cleaner air than other parts of the state, but inland areas got notably smoggier. No one’s really sure if the bad smog season was an aberration or the beginning of a trend. V. John White of the Sierra Club says population growth and sprawl may be eating up any gains made in pollution prevention But many air quality officials say long-term indicators show that California is unmistakably cleaning up its act. Most of that improvement is due to strict regulations imposed on industry; for example, Californians drive the world’s cleanest cars powered by the world’s cleanest fuels.