The output curbs coincide with the celebrations of the 70th anniversary of the founding of the People's Republic of China in Beijing. Industrial firms in the province of Hebei, which surrounds the capital, are under pressure to try to prevent emissions from drifting across the city as ceremonies get underway.
Authorities have so far decided against a special campaign to force factories to close before National Day, relying instead on routine air quality procedures.
Three local government notices reviewed by Reuters do not mention National Day specifically, saying only that Hebei faces a sustained period of "unfavourable" weather that makes it harder to disperse smog, triggering more stringent industry curbs starting from Sunday.
According to a document dated Sept. 21 and authenticated by two industry sources, the city of Tangshan, which produced nearly 100 million tonnes of steel a year, ordered some steel mills to cut sintering output by no less than 50% from Sept. 22 to the end of Sept. 27 and impose stricter emissions curbs over the period.
Sintering is a process of condensing iron ore ahead of smelting into steel.
The city's producers of coke, used as fuel for the smelting process, will have to extend the amount of time they take to produce the fuel and also halt all wet coke quenching, which has higher rates of carbon dioxide emissions and energy losses, the document showed.
Coal-fired power plants in the city will also make further emissions cuts, while cement manufacturers will cut output by half, the document showed.
Tangshan has already imposed routine production curbs this month in response to pollution build-ups.
A separate document, also dated Sept. 21, from the district of Fengrun in Tangshan orders mills to cut production until Sept. 27.
The Tangshan government did not immediately reply to requests for comment.
Separately, the city of Handan in Hebei ordered steel mills, cement plants and coke producers in the district of Yongnian to impose deeper production cuts until Oct. 10, amid forecasts of "unfavourable weather," according to a Sept. 22 document.
In Shandong province, which also falls under the Beijing-Tianjin-Hebei pollution control zone, 13 cities, including the provincial capital of Jinan, have issued orange pollution alerts from Sept. 25-29, triggering industry restrictions, the official Qilu Evening News reported on Monday. That is the second-highest warning level behind red in China's four-tier system.
(Reporting by David Stanway in Shanghai and Min Zhang in Beijing; editing by Christian Schmollinger)
By David Stanway and Min Zhang