Unveiling the Role of Carbonate Radical Anions in Dust-Driven SO2 Oxidation

Carbonate radical anion ( C O 3 . − ${\mathrm{C}{\mathrm{O}}_{3}}^{.-}$ ) is generally overlooked in atmospheric chemistry. Our recent work emphasizes the important role of carbonate radicals produced on mineral dust surfaces in fast sulfate production under solar irradiation in the presence of CO 2 at specifically low RH and light intensity. Yet so far how C O 3 . − ${\mathrm{C}{\mathrm{O}}_{3}}^{.-}$ involves and affects secondary sulfate production under diverse RH, light intensity, and complex constituent matrix remains unknown, which essentially limits our comprehensive knowledge of C O 3 . − ${\mathrm{C}{\mathrm{O}}_{3}}^{.-}$ initiated SO 2 oxidation scheme in the atmosphere. Herein, we explored the heterogeneous SO 2 oxidation over both model and authentic dust and clays in the presence of CO 2 at atmospheric relevant RHs and light intensities. Interestingly, we observe that CO 2 promotes sulfate yield over authentic dust and clays at atmospheric-relevant RH and light intensity. This observation relates to the favorable kinetic between SO 2 oxidation and C O 3 . − ${\mathrm{C}{\mathrm{O}}_{3}}^{.-}$ while auto-quenching of these radical ions is largely minimized due to the sufficient sites of crustal constituents. Furthermore, employing a suite of authentic dust and machine learning strategies, we evaluated the relative importance of each constituent within airborne minerals or clays as well as environmental conditions including relative humidity, light intensity, and CO 2 concentration in affecting SO 2 uptake capability. On this basis, sulfate formation mediated by dust-driven pathway, accounting for nearly ∼20.9% of overall contribution by the end of this century during some pollution episodes, even higher than gas-phase · OH $\cdot \text{OH}$ (∼16.9%), will be increased by 163% if CO 2 -initiated SO 2 oxidation scheme is incorporated. Key Points Carbonate radical-initiated SO 2 heterogeneous oxidation is sensitive to RH, CO 2 concentration, light intensity, and dust constituents Carbonate radicals promote atmospheric sulfate production on authentic dust and clay particles at haze-relevant conditions This previously unrecognized scheme will account for ∼20.9% of overall atmospheric sulfate formation by the end of this century Plain Language Summary How carbonate radical ions involve and affect sulfate formation under different RHs, light intensities, and complex component matrices remains an open question. Secondary sulfate production is facilitated by C O 3 . − ${\mathrm{C}{\mathrm{O}}_{3}}^{.-}$ over authentic dust particles in atmospheric relevant RH and light intensity. More importantly, this research emphasizes that CO 2 is not an inert greenhouse gas that rarely participates in atmospheric chemistry but a strong oxidant precursor that will produce C O 3 . − ${\mathrm{C}{\mathrm{O}}_{3}}^{.-}$ and subsequently trigger quick SO 2 oxidation via chain reactions. This dust-driven reaction channel mediated by C O 3 . − ${\mathrm{C}{\mathrm{O}}_{3}}^{.-}$ potentially contributes to nearly ∼1/5 of secondary sulfate production during some pollution hours if the growing intensive anthropogenic activities continue.