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Institute of Energy and Climate Research

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Urban Air Quality

How does traffic exhaust lead to the ozone formation?

We have long been concerned with the complex factors regulating ozone formation in the lower layer (1-2 km) of our atmosphere. In order to understand how the traffic affects ozone production, we first consider the properties of gasoline and diesel vehicles.

The difference between petrol and diesel cars

We have determined with our measurements how much hydrocarbons (VOC) and nitric oxide concentrations in German city centers have declined over the last 20 years. With these results we have modeled the ozone production rate.

Hydrocarbon concentrations in inner cities have been lowered as a result of the catalysts. On the other hand, nitric oxide emissions have hardly been reduced by the many diesel engines. As a result, the nitrogen dioxide load in German inner cities remains unchanged too high.

Of course, nitrogen oxide concentrations have to be reduced. However, due to the lower ratio of hydrocarbons to nitrogen oxides, ozone is hardly produced in the cities anymore!

Both hydrocarbons and nitrogen oxides are involved in ozone formation. At low nitrogen oxide concentration levels, the more nitrogen oxides are present in the air, the more ozone is formed. In Germany's inner cities, however, the concentration of nitric oxide is so high that ozone formation is slowed down.

How has the ozone production rate changed over time?

Ozone formation in 1990 and now


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