| Summary: | The formation of inorganic fine particulate matter (i.e., iPM<sub>2.5</sub>) is controlled by the thermodynamic equilibrium partitioning of NH<sub>3</sub>-NH<sub>4</sub><sup>+</sup>. To develop effective control strategies of PM<sub>2.5</sub>, we aim to understand the impacts of changes in different precursor gases on iPM<sub>2.5</sub> concentrations and partitioning of NH<sub>3</sub>-NH<sub>4</sub><sup>+</sup>. To understand partitioning of NH<sub>3</sub>-NH<sub>4</sub><sup>+</sup> in the southeastern U.S., responses of iPM<sub>2.5</sub> to precursor gases in four seasons were investigated using field measurements of iPM<sub>2.5</sub>, precursor gases, and meteorological conditions. The ISORROPIA II model was used to examine the effects of changes in total ammonia (gas + aerosol), total sulfuric acid (aerosol), and total nitric acid (gas + aerosol) on iPM<sub>2.5</sub> concentrations and partitioning of NH<sub>3</sub>-NH<sub>4</sub><sup>+</sup>. The results indicate that reduction in total H<sub>2</sub>SO<sub>4</sub> is more effective than reduction in total HNO<sub>3</sub> and total NH<sub>3</sub> to reduce iPM<sub>2.5</sub> especially under NH<sub>3</sub>-rich condition. The reduction in total H<sub>2</sub>SO<sub>4</sub> may change partitioning of NH<sub>3</sub>-NH<sub>4</sub><sup>+</sup> towards gas-phase and may also lead to an increase in NO<sub>3</sub><sup>−</sup> under NH<sub>3</sub>-rich conditions, which does not necessarily lead to full neutralization of acidic gases (pH < 7). Thus, future reduction in iPM<sub>2.5</sub> may necessitate the coordinated reduction in both H<sub>2</sub>SO<sub>4</sub> and HNO<sub>3</sub> in the southeastern U.S. It is also found that the response of iPM<sub>2.5</sub> to the change in total H<sub>2</sub>SO<sub>4</sub> is more sensitive in summer than winter due to the dominance of SO<sub>4</sub><sup>2−</sup> salts in iPM<sub>2.5</sub> and the high temperature in summer. The NH<sub>3</sub> emissions from Animal Feeding Operations (AFOs) at an agricultural rural site (YRK) had great impacts on partitioning of NH<sub>3</sub>-NH<sub>4</sub><sup>+</sup>. The Multiple Linear Regression (MLR) model revealed a strong positive correlation between cation-NH<sub>4</sub><sup>+</sup> and anions-SO<sub>4</sub><sup>2−</sup> and NO<sub>3</sub><sup>−</sup>. This research provides an insight into iPM<sub>2.5</sub> formation mechanism for the advancement of PM<sub>2.5</sub> control and regulation in the southeastern U.S.
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