The quasi-stationary front is a significant weather system which influences East Asia in spring. The air quality deteriorated along with the moist circumstance when the quasi stationary front dominated the area. Surface meteorological parameters, air pollutants and PM_2.5 chemical species were observed during the air pollution episode. Liquid water content and aerosol acidity were calculated by thermodynamic model in order to investigate heterogeneous/aqueous reactions for secondary aerosol formation. The episode was divided into four stages based on quasi-stationary front influences. Hourly PM_2.5 concentrations were up to 150.2 μg·m^-3 while O₃ concentrations reached the minimum value of 1.27 μg·m^-3, indicating that the precursor gas NO_x participated in the different reactions during the episode. Nitrate proportion of water-soluble inorganic ions was 42.2%. High concentrations of secondary inorganic aerosol ions and the high sulfur oxidation ratio (SOR) and nitrogen oxidation ratio (NOR) indicated the increasing conversions from SO₂ and NOx to their corresponding particulate phases. Ratios of [NO₃^-]/[SO₄^2-] and [NH₄⁺]/[SO₄^2-] in the four stages declared that nitrate formation preferred heterogeneous conversions. A series of liquid water content (LWC) fitting equations between relative humidity and inorganic ions were conducted to verify heterogeneous aqueous reactions of NO₂ and secondary nitrate formation. The results of this study highlighted the significance of LWC and chemical reactions associated with acidity during the specific synoptic situation in South China.