Using High-Temporal-Resolution Ambient Data to Investigate Gas-Particle Partitioning of Ammonium over Different...

Ammonium is one of the dominant inorganic water-soluble ions in fine particulate matter (PM_2.5). In this study, source apportionment and thermodynamic equilibrium models were used to analyze the relationship between pH and the partitioning of ammonium (ε(NH₄⁺)) using hourly ambient samples collected from Tianjin, China. We found a "Reversed-S curve" between pH and ε(NH₄⁺) from the ambient hourly aerosol dataset when the theoretical ε(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 ε(NO₃^-)^* was between 0.7 and 0.8, the fitted R² was 0.88. Through thermodynamic analysis, we found that the values of k[H⁺]² (k = 3.08 × 10⁴ L² mol^-2) and ε(NO₃^-)^* can influence the pH-ε(NH₄⁺) curve. Under certain situations, the values of k[H⁺]² and ε(NO₃^-)^* are similar to each other, and ε(NH₄⁺) is sensitive to pH, suggesting that ε(NO₃^-)^* plays an important role in affecting the ε(NH₄⁺). During summer, winter, and spring seasons, when the relative humidity was greater than 0.36 and ε(NO₃^-)^* was between 0.8 and 0.95, there was an obvious Reversed-S curve, with R² = 0.60. The theoretical k[H⁺]² and ε(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.