Smog lowered visibility to less than 50 meters (yards) in parts of northeast China, leading to the weekend closure of highways and an airport.
Seven expressways were partly or wholly shut on Sunday, according to the transport authority in Liaoning province.
Farther north in Heilongjiang province, which borders Russia, more roads and an airport shut Saturday, including Taiping International Airport in Harbin city, which canceled all 209 inbound and outbound flights.
The airport said that flights were operating again Sunday as visibility improved to a few hundred meters (yards).
Last month, Harbin practically ground to a halt with a particularly bad bout of pollution that followed the switching on the city's heating systems, which require the burning of coal. The smog stopped flights and buses and closed schools.
China's national meteorological center said a cold front and rain approaching from the west would gradually reduce the haze.