Airborne Pollutant inside a Highway Tunnel Coated with a Photocatalytic Mortar

Air pollution measurements were carried out at the Cavallo highway tunnel in Ancona, Italy. The tunnel is used daily by an average of 20,000 vehicles, of which 25% are gasoline fueled passenger cars, 25% diesel cars, and 50% trucks. Tunnel air concentrations of carbon dioxide, carbon monoxide, volatiles organic compounds (VOC), aromatic hydrocarbons and Nitrogen Oxides (NOx) are presented. An integrated sampling-analysis strategy has been implemented in order to get evidence on photocatalytic processes involved inside a highway tunnel freshly coated with mortar containing photocatalytic TiO2. In situ measurements, rather than laboratory material testing, has offered an efficient way for evaluating the occurrence of photocatalytic reactions, in spite of the presence of many conflicting meteorological and traffic parameters. The CO2 level inside the tunnel after the photocatalytic mortar treatment resulted initially 22% lower than before treatment but it reduced to a 14% within 6 months. The consistent decreases of acetates, nitrites, and sulphates, together with the sensible increase of oxalates observed on carbon adsorbing panels, point to the occurrence of oxidative reactions occurring inside the coated tunnel. Further study is required to elucidate the effect of NOx adsorption, in order to be able to make predictions of the time-dependence on the de-polluting effect, or how the mortar is influenced by the relative humidity, as well as its composition and exposure history.