Abstract. Aerosol acidity is one of the most important parameters influencing atmospheric chemistry and physics. Based on continuous field observations from January 2005 to May 2006 and thermodynamic modeling, we investigated the spatial and seasonal variations in PM_2.5 acidity in two megacities in China, Beijing and Chongqing. Spatially, PM_2.5 was generally more acidic in Chongqing than in Beijing, but a reverse spatial pattern was found within the two cities, with more acidic PM_2.5 at the urban site in Beijing whereas the rural site in Chongqing. Ionic compositions of PM_2.5 revealed that it was the higher concentrations of NO₃⁻ at the urban site in Beijing and the lower concentrations of Ca²⁺ within the rural site in Chongqing that made their PM_2.5 more acidic. Temporally, PM_2.5 was more acidic in summer and fall than in winter, while in the spring of 2006, the acidity of PM_2.5 was higher in Beijing but lower in Chongqing than that in 2005. These were attributed to the more efficient formation of nitrate relative to sulfate as a result of the influence of Asian desert dust in 2006 in Beijing and the greater wet deposition of ammonium compared to sulfate and nitrate in 2005 in Chongqing. Furthermore, simultaneous increase of PM_2.5 acidity was observed from spring to early summer of 2005 in both cities. This synoptic-scale evolution of PM_2.5 acidity was accompanied by the changes in air masses origins, which were influenced by the movements of a subtropical high over the northwestern Pacific in early summer. Finally, the correlations between [NO₃⁻]/[SO₄²⁻] and [NH₄⁺]/[SO₄²⁻] suggests that under conditions of high aerosol acidity, heterogeneous reactions became one of the major pathways for the formation of nitrate at both cities. These findings provided new insights in our understanding of the spatial and temporal variations in aerosol acidity in Beijing and Chongqing, as well as those reported in other cities in China.