Investigating the vital influence of multi-factor synergies on the formation of nitrate on the surface of nano MgO particles†

Abstract

With the rapidly increasing number of motor vehicles in megacities in recent years, nitrate has become a crucial component in atmospheric fine particulate matter, which has significant impacts on the formation of haze. In this study, a photochemical smog chamber was used to investigate the effect of atmospheric pollutants such as O₃, NH₃ and SO₂ on the heterogeneous formation of nitrate on the surface of nano MgO particles in the two reaction systems of NO₂–MgO-dark and NO₂–MgO-hν and their related formation mechanisms. In the dark, similar to nighttime, O₃, NH₃ and SO₂ played a key role in promoting the heterogeneous formation of nitrate on the surface of nano MgO particles, following the order O₃ > SO₂ > NH₃. It was noted that the synergy between O₃ and SO₂ or NH₃ was more favorable for the heterogeneous formation of nitrate. Besides, the heterogeneous formation of nitrite was greatly impacted by NH₃ and SO₂. Alternatively, under UV light, similar to daytime, the synergy between UV light and O₃ greatly promoted the heterogeneous formation of nitrate, whereas NH₃ and SO₂ exerted an inhibitory effect. The mechanisms for the heterogeneous formation of nitrate on the surface of nano MgO particles were strongly affected by UV light, O₃, NH₃ and SO₂, presenting different reaction mechanisms. The synergistic effects of O₃, NH₃ and SO₂ on the heterogeneous formation of nitrate at nighttime were obviously higher than that of UV light, O₃, NH₃ and SO₂ at daytime, implying that nighttime is more favorable for the heterogeneous formation of nitrate.