Fourier-transform infrared (FTIR) spectroscopy is a useful and nondestructive method for measuring the current atmospheric concentrations of inorganic compounds (sulfate, nitrate, and ammonium) and has been extensively used for environmental monitoring since the 1980s. In this study, we used FTIR spectroscopy to measure the inorganic compounds in particulate matter with a diameter of less than 2.5 µm and combined the data of gaseous pollutants (NO2 and SO2) to analyze the inorganic compounds in PM2.5 from January 24 to January 31, 2014, in Zhengzhou. The measurement period was divided into three pollution stages. During Stage 1 (January 24–26), the low-pollution stage, wind from the east of Zhengzhou caused the pollutants to rapidly disperse and the haze to clear. During Stage 2 (January 26 to the noon of January 30), transported emissions were the main contributor to the high sulfate concentration, as indicated by the poor correlation between the sulfur oxidation ratio (SOR) and the SO42– concentration (R2 = 0.45). Nitrate was formed through homogeneous gas-phase reactions of NO2 with OH or O3, resulting in HNO3 in the PM2.5, as indicated by the good correlation between the nitrogen oxidation ratio (NOR) and the NO3– concentration (R2 = 0.91). During Stage 3 (the afternoon of January 30 to January 31), the average concentration of the PM2.5 increased from approximately 140 µg m–3 to 260 µg m–3, and the concentrations of sulfate, nitrate, and ammonium decreased from 37.62 µg m–3, 56.63 µg m–3, and 34.63 µg m–3 to 32.14 µg m–3, 31.14 µg m–3, and 26.35 µg m–3, respectively. The high levels of PM2.5 during this stage may have been caused primarily by the hygroscopic growth of particles.