The effect of photosensitization reaction of imidazole-2-carbaldehyde on the heterogeneous uptake of sulfur dioxide on hematite

The heterogeneous conversion of SO 2 plays a crucial role in atmospheric sulfate formation. In recent years, it has been recognized that light-absorbing brown carbon can generate reactive oxygen species and enhance the oxidative capacity of the atmosphere. However, the influence of atmospheric brown carbon on the heterogeneous conversion of SO 2 on mineral oxide surfaces has not yet achieved sufficient attention. In this study, the impact of imidazole-2-carboxaldehyde (2-IC), a ubiquitous atmospheric brown carbon, on the heterogeneous conversion of SO 2 on hematite particle surfaces was investigated in the absence and presence of simulated solar irradiation by using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Results indicated that 2-IC can significantly influence the heterogeneous reactivities of hematite. Under dry conditions, the heterogeneous reactivities decreased with the increase of the loaded 2-IC mass fraction in the absence of light, but in the presence of light, the higher reactivities were observed at an appropriate mass fraction range of 2-IC when compared to pure hematite, showing the promoting effects in this mass fraction range. Under the condition of 58% relative humidity, the heterogeneous reactivities in the absence of light decreased with the increasing mass fractions of 2-IC, and pure hematite showed the highest reactivity. However, in the presence of light, the heterogeneous reactivities increased first and then decreased with the increasing mass fractions of 2-IC, and the mixture containing 1×10 -4 % 2-IC showed the highest reactivity. Water molecules can renew surface reactive sites and facilitate the photosensitization reaction of 2-IC, and further enhance sulfate formation. A possible mechanism for the impact of 2-IC on the heterogeneous conversion of SO 2 on hematite is proposed. This study reveals a new pathway for brown carbon to enhance SO 2 conversion and sulfate formation in the atmosphere, providing new insights into its atmospheric impact.