Day and night PM2.5 samples were collected from two typical urban and suburban communities in Tianjin. The major chemical components in PM2.5, including the metal elements, polycyclic aromatic hydrocarbons (PAHs), and inorganic water-soluble ions, were monitored. A positive matrix factorization (PMF) model was used to apportion the potential sources of PM2.5 at the two sites in the daytime and nighttime. The results indicated that the PM2.5 concentration was higher in the suburban area than in the urban area during the daytime in winter. The daytime and nighttime PAHs concentrations at both sites were both generally higher in winter than in summer. The concentrations of some of the metal elements were higher in summer than in winter. Regional differences and day and night differences in the metals and water-soluble ions commonly existed. The PMF analysis indicated that coal combustion and transportation-related sources were the predominant sources in the urban and suburban areas in the daytime in winter, and secondary aerosols were the most important source for the suburban area in the nighttime in winter. There were more pollution sources of PM2.5 during the daytime in summer, especially in the suburban area. In the nighttime in summer, the pollution sources of PM2.5 in the urban and suburbs areas were basically the same, but the source apportionment was quite different.