BEIJING — The average density of PM 2.5, fine particulate matter that causes smog, in the Beijing-Tianjin-Hebei region dropped by 26.5 micrograms per cubic meter, or 30 percent, from 2013 to 2016, according to satellite data released by the Chinese Academy of Sciences (CAS) on March 1.

CAS disclosed findings of its research on the causes, early warning, forecasting and surveillance of smog as well as clean coal technology, at a press conference in Beijing.

According to remote-sensing satellite data, despite the overall lowering of smog density in the region, air quality had worsened in Beijing during winter.

The average density of PM 2.5 in Beijing during winter jumped from 70 micrograms per cubic meter in 2014, to 89 micrograms per cubic meter in 2015, and to 110 micrograms per cubic meter in 2016.

Gu Xingfa, head of the Institute of Remote Sensing and Digital Earth of CAS, said the remote sensing satellite data proved that Beijing did make headway in smog control, but that public complaints about worsening pollution since the winter of 2016 were also valid.

According to CAS research on smog, the average density of industrial and coal burning emissions in Beijing-Tianjin-Hebei region accounted for over one-third of the sources of PM 2.5 in the region.

Wang Yuesi, researcher with the Institute of Atmospheric Physics at CAS, said that when industrial emissions and smoke from straw burning drifted to Beijing from neighboring areas, they tended to accumulate in certain weather conditions and become a “lid” over the city’s skyline.

Pollutants from within the city such as vehicle exhausts then accumulate under the “lid,” resulting in secondary pollution, according to Wang.

In terms of meterological conditions, CAS research found that the last four decades saw slower winds in the Beijing-Tianjin-Hebei region, with average wind velocity dropping by 37 percent. Particularly, the frequency and velocity of the northerly wind, which helps disperse pollutants in the region, decreased markedly.

Wang Yuesi said that as smog control in China involved complex issues including the industry mix, energy mix and social economic cost, and that a major adjustment of the industrial and energy mix was beyond reach in a short time frame, there was a long way to go to drastically improve China’s air quality.