To investigate haze-fog (HF) formation mechanisms and transport, trace gases and aerosols in the aged air masses during regional haze episodes were measured at a regional background site in the North China Plain during 4–19 July, 2011. Mixing state of individual particles, soluble ions of PM2.5, and particle number concentrations were studied using transmission electron microscope, ambient ion monitoring, and wide-range particle spectrometer, respectively. Average mass concentration of PM2.5 was 3 times higher on HF days (70 μg/m3) than on clear days (22 μg/m3). The major soluble ionic components (SO42 −, NO3−, and NH4+) in PM2.5 were over 4 times higher on HF days (40.6 μg/m3) than on clear days (9.1 μg/m3). The high sulfur oxidation ratios (SOR) and nitrogen oxidation ratios (NOR) values during HF days suggest that polluted weather favored transformation of SO2 and NOx into sulfates and nitrates compared to clear days. Particle number fraction of the accumulation mode increases from 11% on clear days up to 26% on HF days. Individual particle analysis shows that secondary inorganic particles (e.g., sulfate and nitrate) as the most abundant species likely determine internal mixing of individual particles and almost half of them mixed refractory particles (e.g., metal, fly ash, soot, and mineral) on HF days. These fine refractory particles were likely emitted from coal fired power plants, heavy industries, and urban city in Shandong and Hebei provinces. Our results suggest that aged air masses mostly contain aged particles of long-range transport and some from new particle formation and growth in the regional background atmosphere.