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The region along the Taihang Mountains in the North China Plain (NCP) is characterized by serious fine particle pollution. To clarify the formation mechanism and controlling factors, an observational study was conducted to investigate the physical and chemical properties of the fine particulate matter in Jiaozuo city, China. Mass concentrations of the water-soluble ions (WSIs) in PM 2.5 and gaseous pollutant precursors were measured on an hourly basis from December 1, 2017, to February 27, 2018. The positive matrix factorization (PMF) method and the FLEXible PARTicle (FLEXPART) model were employed to identify the sources of PM 2.5 . The results showed that the average mass concentration of PM 2.5 was 111 μg/m 3 during the observation period. Among the major WSIs, sulfate, nitrate, and ammonium (SNA) constituted 62% of the total PM 2.5 mass, and NO 3 - ranked the highest with an average contribution of 24.6%. NH 4 + was abundant in most cases in Jiaozuo. According to chemical balance analysis, SO 4 2- , NO 3 - , and Cl - might be present in the form of (NH 4 ) 2 SO 4 , NH 4 NO 3 , NH 4 Cl, and KCl. The liquid-phase oxidation of SO 2 and NO 2 was severe during the haze period. The relative humidity and pH were the key factors influencing SO 4 2- formation. We found that NO 3 - mainly stemmed from homogeneous gas-phase reactions in the daytime and originated from the hydrolysis of N 2 O 5 in the nighttime, which was inconsistent with previous studies. The PMF model identified five sources of PM 2.5 : secondary origin (37.8%), vehicular emissions (34.7%), biomass burning (11.5%), coal combustion (9.4%), and crustal dust (6.6%)., (Copyright © 2020. Published by Elsevier B.V.)