sentences:  In 2012 stricter air pollution monitoring of ozone and PM2.5 were ordered to be gradually implemented from large cities and key areas to all prefecture-level cities, and from 2015 all prefecture-level or higher cities were included.State media acknowledged the role of environmental campaigners in causing this change.On one micro-blog service, more than a million mostly positive comments were posted in less than 24 hours although some wondered if the standards would be effectively enforced.