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To investigate the cause of fine particulate matter (particles with an aerodynamic diameter less than 2.5 μm,PM2.5) pollution in the heating season in the North China Plain (specifically Beijing,Tianjin,and Langfang),water-soluble ions and carbonaceous components in PM2.5 were simultaneously measured by online instruments with 1-hr resolution,from November 15,2016 to March 15,2017.The results showed extreme severity of PM2.5 pollution on a regional scale.Secondary inorganic ions (SNA,i.e.,NO3-+SO42+ NH4+) dominated the water-soluble ions,accounting for 30％-40％ of PM2.5,while the total carbon (TC,i.e.,OC +EC) contributed to 26.5％-30.1％ of PM2.5 in the three cities.SNA were mainly responsible for the increasing PM2.5 pollution compared with organic matter (OM).NO3-was the most abundant species among water-soluble ions,but SO42-played a much more important role in driving the elevated PM2.5 concentrations.The relative humidity (RH) and its precursor SO2 were the key factors affecting the formation of sulfate.Homogeneous reactions dominated the formation of nitrate which was mainly limited by HNO3 in ammonia-rich conditions.Secondary formation and regional transport from the heavily polluted region promoted the growth of PM2.5 concentrations in the formation stage of PM2.5 pollution in Beijing and Lang-fang.Regional transport or local emissions,along with secondary formation,made great contributions to the PM2.5 pollution in the evolution stage of PM2.5 pollution in Beijing and Langfang.The favourable meteorological conditions and regional transport from a relatively clean region both favored the diffusion of pollutants in all three cities.