Low irradiance pollutant photodegration activity in air

Photocatalytic materials for air depollution are usually studied at UV-A irradiance levels comparable to full sunlight exposure (typically adopted value 10 W m-2). However, the naturally available UV irradiation in outdoor surfaces can be extremely variable with daily mean values significantly lower than the peak. Indoor surfaces are also subjected to high UV radiation constraints and the practical available value is a fraction of the direct solar one. It is therefore of great importance to determine of the relationship between UV irradiation and the photocatalytic activity in the characterization of air depolluting materials both for indoor and outdoor applications. The toluene and nitric oxide degradation activity in air of cementitious photocatalitic materials was studied using a wide irradiation level range with a specifically developed experimental system operating at constant pollutant concentration in typical ambient conditions. A non-linear photocatalytic activity-irradiance relationship was found for both pollutants, with the reaction rate measured at low irradiance level higher than the value expected from the measurements at high irradiances. These results suggest that the photoactivity at low irradiance can be underestimated if extrapolated from high irradiance measurements and, in case of performance testing, the irradiance at the sample surface should be determined according to the final application.