China confirmed this week that the number of its citizens living in cities has surpassed the rural population for the first time in its history. That massive urbanization — 690.79 million people are now city-dwellers according to the National Bureau of Statistics — has brought huge benefits, chief among them lifting hundreds of millions out of poverty. But it has also led to serious problems, perhaps none more so than the increasingly foul air in these heaving metropolises that are growing bigger, busier and dirtier by the day.