**1. Introduction**

Emission from the transport sector has a particular impact on the overall air quality because of its rapid rate of growth: goods transport by road in Europe (EU-27) has increased by 31% (period 1995–2009), while passenger transport by road in the EU-27 has gone up by 21% and passenger transport in air by 51% in the same period [1]. The main emissions caused by motor traffic are nitrogen oxides (NO*x*), hydrocarbons (HC) and carbon monoxide (CO), accounting for respectively 46%, 50% and 36% of all such emissions in Europe in 2008 [2].

These pollutants have an increasing impact on the urban air quality. In addition, photochemical reactions resulting from the action of sunlight on NO2 and VOC's (volatile organic compounds) lead to the formation of "photochemical smog" and ozone, a secondary long-range pollutant, which impacts in rural areas often far from the original emission site. Acid rain is another long-range pollutant influenced by vehicle NO*x* emissions and resulting from the transport of NO*x*, oxidation in the air into HNO3 and finally, precipitation of (acid) NO– 3 with harmful consequences for building materials (corrosion of the surface) and vegetation.

The European Directives [3] impose a limit to the NO2 concentration in ambient air of maximum 40 μg/m³ NO2 (21 ppbV) averaged over 1 year and 200 μg/m³ (106 ppbV) averaged over 1 h. These limit values gradually decreased from 50 and 250 in 2005 to the final limit in 2010.

Heterogeneous photocatalysis is a promising method for NO*x* abatement. In the presence of UV-light, the photocatalytically active form of TiO2 present at the surface of the material is activated, enabling the abatement of pollutants in the air. The translation from laboratory results to real cases is starting. Different applications are implemented in Belgium in order to see the influence of the photocatalytic materials on real scale and to determine the durability of the air purifying capacity over time.

In the first part of this paper, the principle of photocatalytic concrete will be elaborated, followed by a description of the past laboratory research indicating important influencing factors for the purifying process. Next, an overview of the results regarding the first pilot project in Antwerp [2] is given, and finally, the different applications in Belgium that have recently been finished, will be discussed.
