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Nitrogen oxide pollution on traffic routes: Reduction by photocatalytic surfaces

Within a pilot program comprising several projects, the nitrogen oxide reduction potential of titanium dioxide (TiO2) was to be investigated under real conditions on various structures of highly frequented traffic routes.

Investigation of a noise protection wall

The investigations took place on the A1 autobahn between the Osnabrück-Nord and Osnabrück-Hafen junctions. The investigated area has a length of about two kilometres. In this section, the six-lane autobahn is bordered on both sides by a noise barrier. In September 2011, a TiO2-containing suspension was applied to both sides of this wall over a length of one kilometre using the airless process. The coated area covers approximately 25,000 square meters. A one kilometre long section without suspension served as a reference area in the course of the investigation.

The picture shows coating work on a noise protection wall on the A1 Coating work on a noise protection wall on the A1

Preliminary studies

Before using the photocatalytic materials, several preliminary investigations had to be carried out. These included, for example, model calculations and sound investigations to exclude the possibility that the TiO2 suspension selected for the noise barrier might influence the acoustic properties of the open-pored wall material.

In addition, various investigations were carried out during the project, such as soil analyses, weathering of test specimens or a nitrate balance of the discharge water at the noise barrier.

The picture shows weathering racks with test specimens Weathering racks with test specimens

Outcomes

The main investigations revealed NO2 reductions in the single-digit percentage range. The highest reduction rates were observed on the east side of the autobahn, where the pollutants could probably stay on the photoactive wall for longer periods of time due to the prevailing transverse flow than on the west side. This observation is supported by the model calculations accompanying the project.

The development of the reduction rates in connection with the weathering studies over the years of recording the measurement data suggests that photocatalytic suspensions sometimes take several months to burn free and let the active TiO2 particles emerge to the surface. The traffic-related impurities on the photoactive surfaces probably also have a counterproductive effect near roads.

Measurements with passive collectors at various distances from the autobahn also show that a noise barrier alone results in a significant reduction of the NO2 concentration in the immediate hinterland of the building. However, this is not - as in photocatalysis - a degradation or transformation of NO2, but only a transport of the pollutants into higher air layers.