BEIJING - Beijing's average PM2.5 density from Nov. 15 to Dec. 31 rose 75.9 percent year-on-year in 2015, despite the general improvement of air quality, the environmental regulator said Thursday.

Beijing witnessed continuous smoggy days this winter, due to high pollution and unfavorable weather. The local government issued red alerts, the highest, for heavy air pollution twice.

The Ministry of Environmental Protection (MEP) said the overall air quality improved last year, but it still did not meet public expectations.

The data monitored showed that PM2.5 density in Beijing, Tianjin and Hebei, the Yangtze River Delta and Pearl River Delta dropped by 10.4 percent, 11.7 percent and 19 percent respectively last year.

The PM10 density in 2015 in 338 cities dropped 7.4 percent on average year-on-year and PM 2.5 density dropped 11.3 percent on average in 161 cities.

The MEP urged all local environmental authorities to prepare for the upcoming Spring Festival, a time when Chinese set off firecrackers which could lead to more air pollution.

The ministry asked local departments to strictly monitor the sales and use of firecrackers and punish violators in a bid to alleviate pollution caused by firecrackers.