Blue-sky thinking: how cities can keep air clean after coronavirus

Many urban dwellers have gazed in awe at their newly clean cities over the last few months. But three – Copenhagen, Oakland and Mexico City – are leading the way in making such transformations permanent. How?For those not directly affected, the ability to breathe more easily and see further has perhaps been the greatest consolation amid the trauma of the coronavirus pandemic.As city after city begins to emerge from lockdown, urban planners and environmental campaigners are grappling