BEIJING -- Beijing continued to see heavy air pollution on Saturday, which started Wednesday before the Chinese New Year.

Data registered at 10 am showed the city's air quality index had reached the level of heavy pollution, with the PM 2.5 concentration in the city's urban areas hitting 239 micrograms per cubic meter.

Local authorities issued a yellow alert for heavy air pollution on Thursday, the first day of China's Spring Festival holiday.

China has a four-tier color-coded warning system for air pollution, with red the most serious, followed by orange, yellow and blue.

Conditions for atmospheric diffusion during the first half of this year's Spring Festival holiday are the worst in recent years, making it hard for pollutants, including those from the burning of fireworks, to dissipate, the municipal ecology and environment bureau noted.

The conditions for atmospheric diffusion are expected to improve on Feb 15, according to a forecast by China National Environmental Monitoring Center.