*5.1. Life+ Project PhotoPAQ*

The European Life+ funded project PhotoPAQ [18] was aimed at demonstrating the usefulness of photocatalytic construction materials for air purification purposes in an urban environment. Eight partners from five different European countries participated in the project.

In this framework, an extensive three-step field campaign was organized in the Leopold II tunnel in Brussels, from June 2011 till January 2013 [26,27]. A photocatalytic cementitious coating material (TX Active® white Skim Coat from CTG Italcementi Group) was applied on the side walls and roof (total area of about 2700 m²) of a tunnel section of about 160 m in length in one of the tunnel tubes directing to the city center. The air-purifying product was activated by a dedicated UV lighting system (including Supratec "HTC 241 R7s" light bulbs from Osram, see Figure 7). More details can be found in [26,27].

**Figure 7.** Application of the photocatalytic product and installation of the UV lamps in the Leopold II tunnel in Brussels, in the framework of PhotoPAQ.

Possible advantages of purifying the tunnel air may be, obviously, cleaner air to breathe, with a potentially reduced need of ventilation, but also (and maybe mainly) a reduction of the pollution impact of tunnel exhaust on the city air quality. During the field campaigns, the effect of the photocatalytic coating on the air pollution (including NO*x*, VOC's, particulate matter, CO, *etc.*) inside the tunnel section was rigorously assessed.

The PhotoPAQ consortium mobilized a large panel of up-to-date instrumentations, installed in the tunnel for several weeks, aiming at characterizing the levels of pollution in this section of the Leopold II tunnel, with and without the air-purifying product (Figure 8).

**Figure 8.** Full characterization of the air quality inside the tunnel test section during the PhotoPAQ campaigns.

However, in contrast to first estimations based on laboratory studies, the results indicated no observable reduction of the pollution level, *i.e.*, the reduction of nitrogen oxides (NO*x*, one of the major traffic related air pollutants) is below 2%, which is the experimental uncertainty of the measurements.

A severe de-activation of the photocatalytic material was observed inside the highly trafficked and strongly polluted Leopold II tunnel. In conjunction, final UV lighting intensity (only 2 W/m² UV-A) was below the targeted values (above 4 W/m²), which led to too low levels for proper activation inside the polluted tunnel environment. Another negative condition was the high wind speed (up to 3 m/s) inside the tunnel, limiting the contact time between pollutants and the active surface. Finally, January 2013 turned out to be an unusually wintry period causing cold and humid conditions inside the tunnel, with relative humidity ranging from 70% to 90%, which also reduces the activity of the photocatalytic material as shown before. Thus, all these issues together resulted in a reduction of the activity of the photocatalytic surfaces inside the harsh environment of the Leopold II tunnel, by a factor of 10 compared to the theoretical expectations. More details about the set-up and results of these extensive field campaigns inside the Leopold II tunnel are presented elsewhere [26,27].

Nevertheless, combining the knowledge gained during these campaigns and the laboratory based investigations performed by the PhotoPAQ consortium, numerical simulations (with the commercially available general purpose Computational Fluid Dynamics code ANSYS CFX®) were performed in order to estimate the possible best-case abatement of pollutants.

These calculations indicate that, under the best case scenario (proper level of UV light intensity higher than 4 W/m², relative humidity below 50%, and limited pollution to avoid passivation), the reduction of the NO*x* concentration may be expected to attain:


Despite the fact that the results were not as expected, the Leopold II field campaigns conducted by the PhotoPAQ team proved to be a unique real world and fully comprehensive assessment of the effect of photocatalytic air-purifying (road) construction materials on air pollution inside a tunnel environment. Based on the extensive experimental data set gathered and numerical model calculations, a valuable tool for extrapolation can be provided to estimate the expected pollution reduction in other urban tunnel sites, also for use by non-experts [18].
