On Sunday morning, Beijing issued a yellow alert, its third-highest, for the first time in the latter part of the year. The alert was in response to lingering smog covering the vast northern region during the holiday. The heavy smog is forecast to last until Tuesday.

The spell may herald the start of the coming smog season. Experts say the strong El Nino phenomenon may bring weather less likely to disperse air pollution in autumn and winter.

As of 2 pm, Beijing had seen tiny PM2.5 particles reach as high as 187 micrograms per cubic meter in southeastern districts like Tongzhou. That indicates air pollution fell into the second-highest level on the six-tier air quality index, the municipal environmental monitoring center said.

PM2.5 refers to fine particulate matter with a diameter of less than 2.5 microns, which is particularly hazardous to human health.

The yellow alert issued by the capital requires construction sites to suspend demolition and other work that could generate dust, and bans the practice of burning of corn straw after harvest, the government said.

The vast region including Hebei province and Tianjin also has seen severe air pollution since Friday, hindering traffic and tourism for the seven-day National Day holiday that started Saturday.

Many cities in Hebei like Shijiazhuang and Baoding have issued yellow alerts and strengthened pollution controls since Saturday, the Ministry of Environment Protection said on Sunday.

The ministry said the escalation of pollution levels in the Beijing-Tianjin-Hebei region is due to windless, stable weather and hard-to-disperse pollutants.

"Besides, the PM10 reading climbed fivefold over the year-on-year level, indicating that construction site controls did not work well, generating large quantities of dust in the region," said the ministry. PM10 particles are larger than those of PM2.5.

Residents and visitors in the region are expected to see cleaner air after winds arrive on Tuesday to disperse pollutants.

Though the smog is forecast to end quickly, it may mark the start of the northern smog season in the autumn and winter.

Several models indicate that the lasting influence of the El Nino atmospheric phenomenon may precipitate the season's start, with windless days and temperature inversions discouraging the dispersion of pollutants, said Wang Zifa, a researcher at the Institute of Atmospheric Physics with the Chinese Academy of Sciences. El Nino is a climate cycle that occurs when warm water in the western Pacific Ocean moves to the east, affecting weather patterns.

Northern regions also experienced several rounds of severe smog during the past winter, when PM2.5 readings at times exceeded 1,000.