Beijing's average PM2.5 density of small deadly particles increased 76% year-on-year between November 15 and December 31, a period during which China sounded its first red alert over pollution triggering a host of emergency measures.

The Chinese capital - a city of more than 22 million - witnessed continuous smoggy days this winter, due to high pollution and unfavourable weather.

The local government twice issued red alerts, the highest in a four-tier warning system, for heavy air pollution in December. The lesser hazardous levels are categorised under orange, yellow and blue.

Red warrants closure of schools and construction, implementation of odd-even number plate rule for all vehicles.

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

It said Beijing's average PM2.5 density between November 15 and December 31 rose 75.9% year-on-year in 2015.

The data monitored showed that PM2.5 particles (of size less than 2.5 microns) --the smallest and deadliest form of airborne particulate matterdensity -- in Beijing, Tianjin and Hebei, the Yangtze River Delta and Pearl River Delta dropped by 10.4%, 11.7% and 19% respectively last year, state-run Xinhua news agency reported.

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

The MEP urged all local environmental authorities to prepare for the upcoming Spring Festival in February, when millions of Chinese set off firecrackers which could lead to more 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.