The photooxidation of isoprene (C5H8) dominates the source of formaldehyde (CH2O) in the atmosphere. The isoprene degradation initiated by atmospheric radicals has been well understood. However, the potential role of metal-oxide particles, which are important components of mineral dust, has not been well studied. Herein, the generation of CH2O through photooxidation of isoprene on iron oxide clusters (Fe x O y +, 3 ≤ x ≤ 23, 3 ≤ y ≤ 35) up to a nanosize has been identified successfully by mass spectrometric experiments. Theoretical studies indicate that the production of CH2O from the C5H8 adsorption complex has to overcome a significant barrier and the photo-irradiation can accelerate this process. The product selectivity of CH2O can be enhanced by visible light irradiation with respect to ultraviolet irradiation. The large-sized clusters can be more efficient to generate CH2O in a gas-particle interaction system. This study can provide a new source of formaldehyde from the photooxidation of isoprene on iron oxide-based mineral dust in the atmosphere.

Titania (TiO 2 ) nanoparticles are active photocatalysts, and isoprene (C 5 H 8 ) is a biogenic volatile organic compound that contributes crucially to global particulate matter generation. Herein, the direct photooxidation of isoprene by titanium oxide cluster anions with dimensions up to a nanosize by both ultraviolet (UV) and visible (Vis) light excitations has been successfully identified through mass spectrometric experiments combined with quantum chemistry calculations. The potential role of "dry" titania in atmospheric isoprene oxidation has been revealed, and a clear picture of the photooxidation mechanism on titanium oxide nanoparticles has been provided explicitly at the molecular level. The adsorption of isoprene on the atomic oxygen radicals (O •− ) of titanium oxide clusters leads to the formation of the crucial interfacial state (IS) within the band gap of titanium oxides. This IS is demonstrated to be the significant factor in delivering the electron from the π orbital of C 5 H 8 to the Ti 3d orbital in the photooxidation process (C 5 H 8 + Ti 4+ −O •− → C 5 H 8 O + Ti 3+ ) and creating photoactivity in the Vis region. It is revealed that after the photogeneration of the O •− radicals by UV excitation on the TiO 2 particle surface, the subsequent reactions can be induced by Vis excitation through the IS. This multicolor strategy in both the UV and Vis regions can enhance the efficiency of solar energy harvesting and improve the product yield of the photocatalysis on TiO 2 nanoparticles. New insights have been provided into both the atmospheric chemistry of isoprene and the photochemistry of TiO 2 nanoparticles.