LAUSR

High frequencies of haze in China, especially in the Beijing–Tianjin–Hebei region, have received widespread attention in recent years. In this study, samples of filtered atmospheric fine particulate matter (PM 2.5 ) were collected synchronously at three urban sites (Beijing, Tianjin, and Shijiazhuang) and at a regional background site (Xinglong) for one month during each season from June 2014 to April 2015. Chemical composition determination/analysis, chemical mass closure, positive matrix factorization (PMF) and backward trajectory clustering were employed to investigate the chemical speciation, haze formation mechanism, emission sources, and influences of regional transport in North China. Our results reported that the aerosol chemical compositions were very similar at the urban sites and the background site and mainly comprised organic matter (16.0 %–25.0 %), sulfate (14.4 %–20.5 %), nitrate (15.1 %–19.5 %), ammonium (11.6 %–13.1 %) and mineral dust (14.7 %–20.8 %). Sources apportionment of PM 2.5 by PMF model revealed that secondary aerosols (background) and secondary inorganic aerosols (urban) were the dominant sources, which accounted for 29.2 %–45.1 % of PM 2.5 throughout the entire study and played a vital role in the formation and development of haze pollution. Emissions of motor vehicle exhaust exerted a significant impact on haze formation at urban sites, particularly at Beijing; and coal combustion also played an dominant role in winter, especially at Shijiazhuang. Backward trajectory analysis revealed that haze pollution has remarkable regional characteristics and usually occurs when air masses originated from polluted industrial regions of the south prevailed, which accompanied by high PM 2.5 loadings with high contributions of secondary aerosols. This study suggests that the control strategies to mitigate the haze formation in BTH region should be focused on the emission reduction of gaseous precursors from fossil fuel combustion, particularly from motor vehicles by improving the quality of oil products.