Significant impacts of NO2 and NH3 on the sulfate formations on the surface of nano MgO particles in a smog chamber

Sulfate is one of the most important secondary components in atmospheric fine particles, especially in haze days, exerting a significant impact on haze formations. In this study a photochemical smog chamber was developed to investigate SO2 heterogeneous reactions on the surface of nano MgO particles under different initial conditions in the three reaction systems of SO2-MgO-dark, SO2-MgO-h nu, and SO2-MgO-O-3. In the three reaction systems, sulfate concentrations presented the order of h nu > O-3 > dark. In the reaction system of SO2-MgO-dark, sulfate heterogeneous formations could be positively affected by the factors following the order of NH3 > RH > NO2 > MgO particles. In the reaction system of SO2-MgO-h nu with NO2 or NH3, the concentrations of produced sulfate, nitrate or ammonium presented different changes with NO2 or NH3 levels, and the synergistic effects of NH3 and UV light on sulfate heterogeneous formations were obviously higher than those of NO2 and UV light. In the reaction system of SO2-MgO-O-3 with NO2 or NH3, the produced sulfate, nitrate or ammonium could increase with increasing NO2 or NH3 levels, respectively, and the synergistic effects of NH3 and O-3 on sulfate heterogeneous formations were clearly higher than those of NO2 and O-3. Note that NH3 was an important factor influencing sulfate heterogeneous formations, and the synergistic impacts of NH3 and UV light were larger than those of NH3 and O-3 being larger than those of NH3 and dark. NO2 was also a main factor influencing sulfate heterogeneous formations, and the synergistic impacts of NO2 and UV light were larger than those of NO2 and O-3, and far higher than those of NO2 and dark. The size distributions of sulfate produced in the three systems with NO2 all presented a single-peak profile. However, the size distributions of sulfate produced in the three reaction systems with NH3 mostly presented a multi-peak profile.