BEIJING -- Beijing's density of hazardous fine particle matter PM2.5 rose to 84 micrograms per cubic meter in the first quarter, despite the general improvement of air quality, local authorities said Thursday.

Owing to unfavorable weather and the winter-heating season, it is higher than the annual average figure for 2016 of 73 micrograms per cubic meter, according to Beijing Municipal Environmental Protection Bureau.

In Q1, the capital had 51 days with good air quality and 15 seriously polluted days, up by four days compared with the same period of last year, it said.

The bureau also reported that the average level of sulfur dioxide decreased to 16 micrograms per cubic meter in Q1.

Beijing has made great efforts to deal with pollution in recent years. In 2017, it will decrease coal use by helping residents in 700 villages swap to clean energy, removed 300,000 vehicles off the roads, and shut down 500 factories.

Between January and March, Beijing investigated 422 instances of illegal activity related to air pollution and imposed fines totaling 14.15 million yuan (around $2 million), up 71 percent and 96 percent year-on-year, respectively.