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Nitrogen Oxides in German Inner Cities

From our research to date, the problem of the high ozone production rate in cities is, at present, solved. Now, the problems of nitric oxide emissions and fine dust loads must also be tackled. It should be ensured that old problems are not brought back to the back door or create new problems - therefore, air-chemical investigations are essential!

Particularly high particulate matter and pollutant loads occur when the exhaust fumes cannot be drained away from the city. This is especially an issue in cities with basin or valley topography or in the case of stable high-pressure weather conditions.

Problem of basin topography ... comparable to stable high-preassure conditions Please click to enlarge.

For towns that are surrounded by mountains (for example, Stuttgart), the city air cannot easily travel sideways towards fields and forests. If there is slight inversion or unfavorable wind conditions, an ascent of the air is also suppressed. The air stays trapped in the city for a long time and is continuously enriched with exhaust fumes.

A similar effect is produced by a so-called stable high-pressure weather situation: sinking air masses prevent the city air from ascending and push fumes back into the city. During such special weather conditions, alarms for fine dust and nitrogen oxides often occur. Thanks to modern catalyst technology, the earlier peak values for ozone are no longer reached, but particles and especially nitrogen oxides remain problematic.
What is done about it?

EU regulations have already addressed this problem and have set limits - this forces the automotive industry to react. However, when a new technology is developed, it usually takes about a decade until a large part of the car fleet is equipped with it.

Environmental zones are another measure to reduce the pollutant concentrations in the inner city. This can lead to an exchange of the vehicle fleet in the long term if residents have to buy a car with modern exhaust gas cleaning technology. However, there is also the risk of a shifting of the problem to the surrounding area. Sooner or later, pollutants have to be worked on themselves.

SCR catalytic converters for diesel engines are designed to meet the EU limit values for nitrogen oxides. The use ammonia, which reacts with nitrogen oxides to produce nitrogen and water. For the supply of ammonia, urea is used, is additionally fueled and split into ammonia at high temperature.

Where do we get into the game?

New research questions are coming up with new technologies. In the case of the SCR catalysts, for example, the question arises as to whether and how much ammonia can leak from the catalyst. In addition, the extremely toxic formed as the product of the decay of urea along with ammonia. Although isocyanic acid is believed to be removed by reaction with water, it is questionable whether this process happens completely.

In the future, we will continue to support the development of traffic emissions with our measurements. In order to avoid unwanted side-effects of new technologies, air-chemical investigations are useful already in the run-up to market launch.


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