Model calculations for the degradation of nitrogen oxides by means of photocatalysis

Over the course of the corona pandemic, a considerable reduction in road traffic has occurred and, as a result, nitrogen dioxide (NO2) emissions have fallen sharply. Since the end of the first lockdown in mid-April 2020, significant increases have already been recorded. Looking back to the year 2019, the annual average limit value for NO2 of 40 µg/m³ air was exceeded at around 20 percent of the measuring stations in Germany which are located close to traffic – despite the fact that diesel driving bans were already in force and environmental zones had been established. In comparison, in 2018 this figure was still 42 percent of the stations. The Fraunhofer IST has extensive experience in the development of surfaces which, when equipped with photocatalytic activity, can contribute to the reduction of air pollutants. Particularly in urban environments, large built-up areas are available for this purpose. In collaboration with the Ostfalia University of Applied Sciences in Wolfenbüttel, a study was therefore initiated within the framework of a master's thesis with the aim of applying numerical simulation to determine the degradation potential of photocatalytically equipped surfaces for nitrogen oxide reduction. For this purpose, the model of a real street canyon on the premises of the Fraunhofer IST was selected, as, firstly, the pollutants are poorly removed here under certain wind conditions and, secondly, sufficient surfaces are available which can potentially be equipped with photocatalytic properties.

Model canyon on the Fraunhofer IST site in Braunschweig.
© Fraunhofer IST, Inga Ziemer
Model canyon on the Fraunhofer IST site in Braunschweig.
ArcMap model sketch of the evaluated area.
© Fraunhofer IST, Inga Ziemer
ArcMap model sketch of the evaluated area.
Percentage reduction of NO<sub>x</sub> emissions in the photocatalytically active model canyon.
© Fraunhofer IST, Inga Ziemer
Percentage reduction of NOx emissions in the photocatalytically active model canyon.

The procedure

As an evaluation parameter for the photocatalytic activity, the photocatalytic deposition rate of nitrogen monoxide (NO) was determined in the laboratory in accordance with prEN 16980-1:2020 for widely differing commercially available products such as glass, concrete paving slabs and rough plaster. This value was then used to calculate the photocatalytic resistance, which in turn is an essential parameter for atmospheric dispersion calculations.

These values served as input variables in the investigation during which the programs LASAT (Lagrange Simulation of Aerosol Transport for the simulation of the dispersion of pollutants in the atmosphere) and WinMISKAM (prognostic microscale flow and dispersion model for Windows) were utilized in order to simulate the reduction of NOx concentrations using the example of the street canyon on the premises of the Fraunhofer IST, taking into account meteorological data on wind flow and solar radiation, using the year 2018 as an example (see adjacent Figures top and middle).
 

Improvement of air quality through photocatalysis

The results showed that, assuming complete photocatalytic equipping of the road, façade and roof, the annual average NOx concentrations can be reduced by 1 to 2 percent, even at low deposition rates of 0.14 cm/s. By using high-performance photocatalytic building materials with average deposition rates of up to 1.50 cm/s, such as those developed in the BMBF joint project “PureBau”, the NOx emissions can be reduced by an average of 10 to 12 percent (see adjacent bottom Figure). The degradation rates thereby fluctuate between 2 and 4 percent in winter and between 16 and 18.5 percent on typical summer days. Photocatalysis can therefore verifiably provide a decisive contribution towards the improvement of air quality in our city centers, even in areas which experience less sunshine.
 

Outlook

Within the scope of an expansion of the laboratory capacities, the measurement technology available at the Fraunhofer IST is currently being expanded to include the possibility of determining the photocatalytic deposition rates of nitrogen dioxide (NO2) and ozone (O3) in order to be able to determine the atmospheric input variables even more precisely. Parallel to this, the IST, as a member of the DIN standards committee Photocatalysis, supports the development of future test standards in this field and will therefore be able to offer its customers a comprehensive range of services in the future for the evaluation of their photocatalytically active products - from the material through to the application.