Experimental and field measurements have shown that, in the presence of both iron-containing aerosols and sunlight, oxidized mercury species such as HgCl2 and HgBr2 undergo reduction to elemental mercury (Hg degrees), which remains in the atmosphere longer than oxidized mercury species due to its higher volatility. We performed density functional theory (DFT, PW91 + U) calculations to elucidate the reduction mechanism for atmospheric HgCl2 and HgBr2 to Hg degrees on several iron-oxide aerosol surfaces relevant in the troposphere. On the OH-Fe-Rterminated alpha-Fe2O3 (0001) surface, predicted to be most prevalent under ambient conditions, we show that: (1) the first Hg-X bond is broken via either thermal or photolytic activation depending on the ambient temperature; (2) photons with an energy of 2.69 eV (461 nm) are required to break the second Hg-X bond; and (3) a photoinduced surface-to-adsorbate charge-transfer process can promote Hg degrees desorption with an excitation energy of 2.59 eV (479 nm). All the calculated excitation energies are below the threshold value of 3.9 eV (320 nm) for photons in the troposphere, suggesting that sunlight can facilitate mercury reduction on iron-oxide aerosol surfaces. In contrast, the gas-phase reduction of HgCl2 (HgBr2) involves photoexcitation requiring an energy of 4.98 (4.45) eV (249 (279) nm); therefore, the energy range of sunlight is not suitable for gas-phase reduction. Our computational results provide the first evidence on the detailed mechanism for the combined role of aerosols and photons in the reduction of HgCl2 and HgBr2. Our methodology can be adapted to study other photochemical heterogeneous processes in the atmosphere.