Inorganic PM_2.5 reduction in Kanto, Japan: The role of ammonia and its emission sources control strategies

Ammonia (NH₃) is attracting attention as a carbon-free energy source and a significant precursor to inorganic PM_2.5 (hereafter PM_2.5), aside from NO_x and SO_x. Since the emission of NH₃ has often been overlooked compared to NO_x and SO_x, this study aims to reveal the role of NH₃ and its emission control on PM_2.5 in Kanto, Japan. With the aid of gas ratio (GR) quantitatively defining the stoichiometry between the three precursors to PM_2.5, and the aid of atmospheric modeling software ADMER-PRO, coupled with thermodynamics calculations, the spatiotemporal distribution along with PM_2.5 reduction under different NH₃ emission cutoff strategies in Kanto had been revealed for the first time. The cutoff of NH₃ emission could effectively reduce the PM_2.5 concentration, with sources originated from agriculture, human/pet activities, and vehicle sources, overall giving a 93.32% PM_2.5 reduction. Different cutoff strategies lead to distinct reduction efficiencies of the overall and local PM_2.5 concentrations, with GR as a crucial factor. The regions with GR ∼1, are sensitive to the NH₃ concentration for forming PM_2.5, at which the NH₃ reduction strategies should be applied with high priority. On the other hand, installing a new NH₃ emission source should be avoided in the region with GR < 1, suppressing the so-yielded PM_2.5 pollution. The future PM_2.5 pollution control related to the NH₃ emission control strategies based on GR, which is stoichiometry-based and applicable to regions other than Kanto, has been discussed.