Cause of PM pollution during the 2016-2017 heating season in Beijing, Tianjin, and Langfang, China.

To investigate the cause of fine particulate matter (particles with an aerodynamic diameter less than 2.5 µm, PM) pollution in the heating season in the North China Plain (specifically Beijing, Tianjin, and Langfang), water-soluble ions and carbonaceous components in PM were simultaneously measured by online instruments with 1-hr resolution, from November 15, 2016 to March 15, 2017. The results showed extreme severity of PM pollution on a regional scale. Secondary inorganic ions (SNA, i.e., NO+SO+ NH) dominated the water-soluble ions, accounting for 30%-40% of PM, while the total carbon (TC, i.e., OC + EC) contributed to 26.5%-30.1% of PM in the three cities. SNA were mainly responsible for the increasing PM pollution compared with organic matter (OM). NO was the most abundant species among water-soluble ions, but SO played a much more important role in driving the elevated PM concentrations. The relative humidity (RH) and its precursor SO were the key factors affecting the formation of sulfate. Homogeneous reactions dominated the formation of nitrate which was mainly limited by HNO in ammonia-rich conditions. Secondary formation and regional transport from the heavily polluted region promoted the growth of PM concentrations in the formation stage of PM pollution in Beijing and Langfang. Regional transport or local emissions, along with secondary formation, made great contributions to the PM pollution in the evolution stage of PM 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.