beijing china keep years-long battle smog ensure blue sky promote revolution clean energy president xi jinping say wednesday open ceremony key communist party congress improve notoriously toxic air across northern regions world second-largest economy cornerstone beijing economic social policy recent years china order factories cut output bid enforce big emission cut come months avoid repeat near-record level choke smog envelop key northern areas start year long term also launch series measure curb use coal nation favorite fuel boost use renewable power like wind solar government also take measure improve rural areas restore soil waterways xi add china move modernize vast agricultural sector china 's president xi say continue years-long war smog