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m in which airborne particulate matter is present. The lower the ceilin
g is, the higher the density of pollutants which then mix with the cleaner air.
With a low mixing layer, Taihang mountain range to the west and the Yanshan mountai
n range to the north serve as walls that form a semi-closed “room” in the Beijing-Tianjin-Hebei cluster.
The high level of emissions is the major reason for air pollution in th
e greater Beijing area, and meteorological conditions also play a key role as an exte
rnal factor, said Xu Xiangde, a researcher at the Chinese Academy of Meteorological Sciences.
Zhang Xiaoye, Xu’s colleague, said the “marked effect” of continuous pollutant accu
mulation on meteorological conditions could make overall pollution even worse.
“After the accumulation reaches a certain level, it will further wor
sen the meteorological conditions, which then will further affect air quality, causing explosive