Aerial drones have uncovered illegal emissions by some of the largest industrial companies in the north.
A unit of Hebei Iron & Steel Group, the mainland's top steelmaker; Shanxi Huaze Aluminium & Power; and Inner Mongolia Yihua Chemical were found to have "serious environmental problems", the Ministry of Environmental Protection said on its website.
Data gathered by the drones indicate a quarter of the 254 businesses targeted might be involved in illegal practices, it said.
This month's use of drones to enforce pollution laws followed trial monitoring from November to February in Hebei province, the ministry said.
Drones equipped with thermal infrared cameras covered 1,000 square kilometres across the regions of Hebei, Shanxi and Inner Mongolia from June 16 to Friday, according to its statement.
Premier Li Keqiang declared war on pollution in March after swathes of the north were blanketed in smog and the government's climate-change adviser said the situation had reached intolerable levels.
Poor air quality in the world's biggest carbon emitter is a regional health issue and damaging foreign investment and talent retention, World Health Organisation chief Dr Margaret Chan Fung Fu-chun warned the same month. The ministry said it would follow up with on-the-ground inspections to ensure companies halted illegal emissions.
The use of unmanned aircraft means that companies secretly discharging pollutants at night or at weekends would find no place to hide, Xinhua reported, citing Chen Shanrong, the deputy head of environmental supervision at the ministry.