Impact of adsorbed nitrate on the heterogeneous conversion of SO on α-FeO in the absence and presence of simulated solar irradiation.

Adsorbed nitrate is ubiquitous in the atmosphere, and it can undergo photolysis to produce oxidizing active radicals. Nitrate photolysis may be coupled with the oxidation conversions of atmospheric gaseous pollutants. However, the processes involved remain poorly understood. In this study, the impact of adsorbed nitrate on the heterogeneous oxidation of SO on α-FeO was investigated in the absence and presence of simulated solar irradiation by using in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). The results indicate that for α-FeO particles with no adsorbed nitrate, the formation of adsorbed sulfate on humid particles is stronger than that on dry particles. Meanwhile, light can also promote the heterogeneous conversion of SO and the formation of sulfate on dry particles because α-FeO is a typical photocatalyst. However, the heterogeneous conversion of SO on humid α-FeO particles is somewhat suppressed under light, suggesting the occurrence of photoinduced reductive dissolution. For the heterogeneous conversion of SO on α-FeO particles with adsorbed nitrate, the formation of sulfate on humid particles is still higher than that on dry particles. For the dry α-FeO particles with adsorbed nitrate, light promotes the formation of adsorbed sulfate. For the humid α-FeO particles with adsorbed nitrate, the heterogeneous conversion of SO under light is stronger than that under no light, indicating that the photolysis of adsorbed nitrate is coupled with the oxidation of SO and the formation of sulfate. The consumption of adsorbed nitrate and the formation of adsorbed NO are observed during the introduction of SO. A possible mechanism for the impact of adsorbed nitrate on the heterogeneous conversion of SO on α-FeO particles is proposed, and atmospheric implications based on these results are discussed.