Ammonium is one of the dominant inorganic water-soluble ions in fine particulate matter (PM2.5). In this study, source apportionment and thermodynamic equilibrium models were used to analyze the relationship between pH and the partitioning of ammonium (epsilon(NH4+)) using hourly ambient samples collected from Tianjin, China. We found a "Reversed-S curve" between pH and epsilon(NH4+) from the ambient hourly aerosol dataset when the theoretical epsilon(NO3-)* (an index identified in this work) was within specific ranges. A Boltzmann function was then used to fit the Reversed-S curve. For the summer data set, when epsilon(NO3-)* was between 0.7 and 0.8, the fitted R-2 was 0.88. Through thermodynamic analysis, we found that the values of k[H+](2) (k = 3.08 X 10(4) L-2 mol(-2)) and epsilon(NO3-)* can influence the pH- epsilon(NH4+) curve. Under certain situations, the values of k[H+](2) and epsilon(NO3-)* are similar to each other, and epsilon(NH4+) is sensitive to pH, suggesting that epsilon(NO3-)* plays an important role in affecting the epsilon(NH4+). During summer, winter, and spring seasons, when the relative humidity was greater than 0.36 and epsilon(NO3-)* was between 0.8 and 0.95, there was an obvious Reversed-S curve, with R-2 = 0.60. The theoretical k[H+](2) and epsilon(NO3-)* developed in this work can be used to analyze the gas-particle partitioning of ammonia-ammonium and nitrate-nitric acid in the ambient atmosphere. Also, it is the first time that we created the joint source-NH3 /HNO3 maps to integrate sources, aerosol pH and liquid water content, and ions (altogether in one map), which can provide useful information for designing effective strategies to control particulate matter pollution.