As Delhi's air quality plunges ahead of Diwali, a graded response plan kicked in today that lets authorities respond to a spike in toxic air with restrictions. As the first step, the powerful Supreme Court-appointed pollution watchdog ordered the coal-fired Badarpur power station in south Delhi to be shut and banned diesel-based power generating sets. It was the concern over Delhi's air that prompted the Supreme Court to temporarily ban on firecrackers for Diwali that have sent pollution levels rocketing in previous years.