Mechanism for the promotional formation of NH4+ by SO2 on different mineral dust surfaces

Ammonium is an important atmospheric particulate component that dictates many environmental processes. The promotion of the heterogeneous conversion of NH₃ to NH₄⁺ by SO₂ on different mineral dust surfaces displays remarkable discrepancies, especially on MgO and α-Fe₂O₃ surfaces, however, the underlying mechanisms are not well known. Here, using periodic density functional theory (DFT) calculation and Born-Oppenheimer molecular dynamics (BOMD) simulation, we explored the heterogeneous adsorption of NH₃ on MgO (110) and α-Fe₂O₃ (001) surfaces in the presence and absence of SO₂. The results show that on MgO (110) surface, hydrogen-bonding interactions of NH₃ on both adsorbed hydroxyl or bisulfite/bisulfate sites are observed no matter whether SO₂ is present or not. While, on the α-Fe₂O₃ (001) surface, significant conversion of NH₃ to NH₄⁺ occurs with the coexistence of SO₂, which is due to the hydrogen transfer reaction from surface HSO₄ to N in NH₃. The fundamental reason may be that the stronger electron affinity of Fe³⁺ than Mg²⁺ results in adsorbed bisulfate and/or bisulfite with greater acidity on α-Fe₂O₃ surface than MgO surface. Our results give a molecular-level explanation for the heterogeneous conversion of NH₃ to NH₄⁺ on different mineral dust surfaces under complex air pollution conditions. Considering the fact that ammonium is abundant in secondary particulates, this work would help in understanding the rapid conversion of ammonia to ammonium and in developing classification governance policies for the key precursor pollutants in China.