Behaviour of self-cleaning glass in urban atmosphere

In order to analyse the self-cleaning mechanism of TiO2-coated window float glass, a field experiment was conducted in Paris, at a site representative of background urban pollution. Two hydrophilic photocatalytic TiO2-coated industrial glasses and a traditional float glass (reference) were exposed in unsheltered and sheltered condition for various time duration. The sheltered exposure allowed to study the soiling and self-cleaning mechanism without the disturbing effect of rain washing. In parallel, air filtrations were carried out in order to assess the chemical composition of airborne particulate. Analytical measurements were performed on the deposit and on the weathered glass layer. The analyses of the deposit on glass and of atmospheric particles on filters consisted in the determination of the organic fraction (organic carbon, particulate organic matter, water-soluble organic carbon), elemental carbon and the inorganic soluble fractions. The impairment of glass optical properties was followed by haze measurements. Investigation of the weathered surfaces was performed using EPMA and scanning electron microscopy (SEM)/EDX. Only a moderate weathering of the glass surfaces after 24 months of exposure was detected. The self-cleaning mechanism was evidenced by a significant decrease of the organic fraction in the deposit on the TiO2-coated glasses.