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

Abstract

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-hν, and SO₂–MgO–O₃. In the three reaction systems, sulfate concentrations presented the order of hν > O₃ > 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-hν 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–O₃ 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 O₃ on sulfate heterogeneous formations were clearly higher than those of NO₂ and O₃. 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 O₃ 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 O₃, 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.