China's top environment watchdog disclosed violations of smog-easing measures yesterday as heavy air pollution in the north is set to persist.

The Ministry of Environmental Protection punished more than 500 enterprises and construction sites for breaching haze response plans, and 10,000 vehicles were caught breaching regulations.

The move came as 72 cities in the Beijing-Tianjin-Hebei region and Shaanxi Province have issued or maintained yellow or more severe smog alerts.

China has a four-tier warning system for severe weather, with red being the most serious, followed by orange, yellow and blue. When a high alert is in force, heavy polluting vehicles and trucks carrying construction waste are banned from roads and some manufacturing firms have to cut production. Inspectors sent by the ministry identified several metallurgy, agricultural chemicals and steel plants in the region that failed to follow the alert-triggered bans.

The government of heavy-industry Tangshan City in Hebei Province has been warned for failing to strictly abide by a truck ban on its roads. Meanwhile, an unlicensed quarry in the city was found to have continued operation even after one of its officials had been detained, the ministry said, citing it as an example of foot-dragging over rectification orders.

The ministry urges local governments to take prompt remedial action and severely punish wrongdoings.

As of last night, China’s national meteorological center renewed orange smog alerts for large swathes of the northern region. The continuing heavy smog in large parts of China is disrupting schools and causing holiday travel problems.

The smog is predicted to dissipate from Sunday with the arrival of a cold front.

Yesterday was the last day of the three-day New Year holiday in the country, but authorities in Zhengzhou, capital of central China’s Henan Province, ordered students of primary and high schools as well as kindergartens to take today off as the city is on red alert for smog.

According to an urgent notice issued by the city’s educational bureau, primary and high schools may use weekends or holidays to make up the missing classes.

In neighboring Shandong Province, the provincial weather station yesterday issued orange alerts for fog and smog that has led to visibility of less than 200 meters in 11 cities in the province. Around 50 flights were affected in the provincial capital Jinan as of 10am. Nearly 300 toll gates along 30 expressways have been closed, according to the provincial transport department.

Heavy fog was also seen in Chengdu, capital of southwest China’s Sichuan Province, stranding more than 8,000 passengers at its airport. A runway at Chengdu Shuangliu International Airport was closed for four hours, leaving over 80 flights delayed and seven canceled. .

Beijing Capital International Airport said flights there were returning to normal. Smog on Sunday caused hundreds of flights to be canceled and highways to shut.

Weather forecasts yesterday showed the smog would return to Beijing and nearby Tianjin City from today. It was expected to persist until Thursday in Hebei, the heavily industrialized province that surrounds the capital, and Henan and Shandong provinces as the region battles freezing temperatures.

Authorities have been issuing smog alerts across the north since mid-December, prompting orders for hundreds of factories to scale back production or close outright and for restrictions on motorists to cut emissions.

A pollution index that measures the average concentration of small breathable particles, known as PM2.5, dropped to just over 100 micrograms per cubic metre in Beijing early yesterday from over 500 late on Sunday. The safe recommended level of PM2.5 is 10 micrograms per cubic metre, according to the World Health Organization.

Yesterday, the Beijing government maintained its orange alert for heavy pollution and a ban on heavy-duty construction trucks from using the roads.