ADN by Sean Cockerham.  The Obama administration has cleared another hurdle for Shell to drill in Alaska's Arctic waters -- the second in as many days -- changing the company's air pollution limits so its drill ship can operate in the Chukchi Sea.  Shell told the Environmental Protection Agency in June that the company was able to meet overall air quality standards. But it said a set of generators on the drilling rig Noble Discoverer fell short of the specific requirements for nitrous oxide and ammonia emissions.  The EPA now [...]