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

Abstract

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.