Shanghai residents breathed the most polluted air they had seen in two months on Wednesday as weak cold air from the north brought pollutants to the Yangtze River Delta.
But clean air is expected on Thursday thanks to another round of cold air, forecasters said.
Seasonal factors played an important role as winter is the high pollution season, and straw burning in nearby provinces also contributed to the pollution, experts said.
Shanghai's air quality index, a new air quality reporting system that monitors sulfur dioxide, nitrogen dioxide, carbon monoxide, ozone, PM10 and PM2.5, reached 254 by 7 pm on Wednesday.
The figure, which indicated the air had reached the level of heavy pollution, was the highest seen since the index was introduced two months ago, according to the Shanghai Environmental Monitoring Center.
Shanghai's hourly density of PM2.5, air particles smaller than 2.5 microns, also reached 250 micrograms per cubic meter by 10 am, while the reading was only 60 micrograms per cubic meter early Tuesday morning.
The heavy pollution in the eastern metropolis followed thick air pollution in Beijing over the weekend. Beijing's density of PM2.5 broke the record since the municipality began publishing the data in early 2012 as its figure went higher than 900 micrograms per cubic meter in several districts of the city on Saturday.
Lin Chenyan, a forecaster with Shanghai Environmental Monitoring Center, said the cold front brought in airborne pollutants from the north.
"The cold snap is like a broom sweeping down all the way to Shanghai, and it brings the dust here," said Lin, adding that atmospheric motion sometimes causes trouble such as that.
Before arriving in Shanghai, the pollutants had left some of Shanghai's neighboring provinces shrouded in smog and fog.
Fifty out of the 72 monitoring stations in Jiangsu province reported medium to heavy pollution on Monday evening, according to Zhang Xiangzhi, deputy director of the Jiangsu Environmental Monitoring Center.
Yang Xin, a professor with Fudan University's department of environmental science and engineering who specializes in atmospheric particulates, said seasonal influences explain the record-breaking data in both Beijing and Shanghai.
"Take Shanghai as an example. It just celebrated its cleanest summer in 2012 - sea breeze from the southeast is helpful in diluting pollutants in the air. However, the monsoons coming from the northwest in the winter usually bring dirty air from the north," Yang said.
Agricultural straw burning in nearby areas also contributed to part of Shanghai's PM2.5 reading, according to Wang Lin, a researcher at Fudan University's Department of Environmental Science and Engineering.
"But it's hard for the Shanghai government to prohibit farmers in the neighboring regions from doing so," Wang said.
According to Lin, the forecaster, Shanghai's air was set to improve on Wednesday night and early Thursday morning. The next wave of cold air is much cleaner because of the clearing of air pollution in the north.
Authorities from the Shanghai Environmental Protection Bureau said measures such as an emergency pollution-reduction plan have been applied to coal-fired power plants and other relevant chemical industries. Construction sites in Shanghai were also asked to take dust-control measures.
But an employee with Baosteel who wouldn't give a name told China Daily that no reduction measure had been applied because "the company already met the city's emission standards".
Meanwhile, Jiangsu province's environmental protection authority also drafted an emergency warning plan for air pollution.
The draft said when air quality index readings of more than half of the province's monitoring stations reached 201 to 300, outdoor activities of its primary school and middle school students would be suspended and students can take a vacation from school when the readings are more than 300.
Outdoor barbecues and fireworks and crackers will be banned in both situations.