Inhibition Effect of HO on the Heterogeneous Reaction between Isoprene and Fe-Substituted Cryptomelane.

The transportation and transformation of biogenic isoprene are vital for the organic carbon cycle in the troposphere. As a typical mineral with high oxidation potential, Fe-substituted cryptomelane oxidizes the surface monolayer of isoprene into formic and acetic acids, and simultaneously, the Mn ions in the structure are reduced to Mn and Mn. The flow of HO in isoprene decreases the adsorption and oxidation of isoprene significantly, even at low relative humidity (10%). As physisorbed HO retains Fe-substituted cryptomelane's crystal structure and oxidation ability, the adsorption and oxidation capacity recovers when HO is absent in the isoprene flow. Theoretical calculations on (001) surfaces show that isoprene prefers to be adsorbed by the Fe site and HO tends to form hydrogen bonds. Due to the decrease in total adsorption energy of HO and isoprene, Fe-substituted cryptomelane favors the adsorption of HO in the flow of humid isoprene. The low oxidation performance at ambient relative humidity suggests that direct oxidation by aerosols of mineral dust might not be the transformation pathway of biogenic isoprene at night.