Photoinduced Production of Chlorine Molecules from Titanium Dioxide Surfaces Containing Chloride

<div>Titanium dioxide (TiO<sub>2</sub>) is extensively used with the process of urbanization and potentially inflfluences atmospheric chemistry, which is yet unclear. In this work, we demonstrated strong production of Cl<sub>2</sub> from illuminated KCl-coated TiO<sub>2</sub> membranes and suggest an important daytime source of chlorine radicals. We found that water and oxygen were required for the reactions to proceed, and Cl<sub>2</sub> production increased linearly with the amount of coated KCl, humidity of the carrier gas, and light intensity. These results suggested that water promotes the reactivity of coated KCl via interaction with the crystal lattice to release free chloride ions (Cl<sup>−</sup>). The free Cl<sup>−</sup> transfer charges to O<sub>2</sub> via photoactivated TiO<sub>2</sub> to form Cl<sub>2</sub> and probably the O<sub>2</sub><sup>−</sup> radical. In addition to Cl<sub>2</sub>, ClO and HOCl were also observed via the complex reactions between Cl/Cl<sub>2</sub> and HOx. An intensive campaign was conducted in Shanghai, during which evident daytime peaks of Cl<sub>2</sub> were observed. Estimated Cl<sub>2</sub> production from TiO<sub>2</sub> photocatalysis can be up to 0.2 ppb/h when the TiO<sub>2</sub>-containing surface reaches 20% of the urban surface, and this is highly correlated to the observed Cl<sub>2</sub>. Our results suggest a non-negligible role of TiO<sub>2</sub> in atmospheric photochemistry via altering the radical budget.</div>