Nitrous gases and traffic

 Kfz-Verkehr - Abgaswolke
Image: Sergiy Serdyuk -

Harmful nitrogen dioxide comes mainly from motorised traffic on the roads and is created primarily by diesel engines. There are more and more of these vehicles on Berlin’s roads and they emit more nitrogen dioxide than their manufacturers promised. It is hardly surprising that the limit values are regularly exceeded – with the associated consequences for the health of the people of Berlin. Along a single main road, almost half of the NO2 comes from the cars on that road, whilst a further quarter comes from the emissions from all the other motorised traffic in Berlin. These emissions are carried across the city by the wind and, together with other sources like power stations (2%) or domestic heating systems (4%), constitute a general level of NO2 pollution. Approximately 14% of NO2 pollution comes from sources outside of Berlin, for example motorised traffic in areas around the city.

 Where does the NO2 pollution on a main road come from?
Image: SenUVK

Main cause of the problem – diesel cars

Today, diesel cars are the most important source of nitrous gases in traffic, followed by public buses, lightweight commercial vehicles, e.g. delivery vans, and lorries, i.e. heavy commercial vehicles weighing more than 3.5 tonnes. By contrast, petrol-driven cars only account for 6% of nitrous gas emissions.

Urban traffic in Berlin: diesel cars are the prime source of nitrous gases
Image: SenUVK

Diesel cars are at the heart of the debate about nitrous gas pollution because in real driving conditions they emit much more nitrogen dioxide than the laboratory readings, which are required before the cars can be approved, would indicate. On the road, a euro 5 diesel car has NOx emissions of 906 mg/km, which is around five times higher than the euro 5 limit value of 180 mg of NOx/km. When it comes to euro 6 diesel cars, the actual emissions are more than six times higher than the limit value (actual 507 mg NOx/km, limit value 80 mg NOx/km). One of the reasons for this is engine settings that are intended to lower fuel consumption but that, at the same time, cause NOx emissions to increase. In addition, it is often the case that the emission control systems of diesel cars don’t work particularly well at lower temperatures.

Measurements in the laboratory are taken at 20˚C to 30˚C, which ensures that high emissions temperatures can be achieved. Precautions for taking readings at lower temperatures are not part of the required test and so for the most part such readings are not taken. However, this means that NOx emissions rise markedly when the ambient temperature falls.

The weaknesses in this approval process for cars have now been recognised and the new emissions standards euro 6 temp and euro 6 d have been introduced. Before new vehicle types can obtain approval, measurements of actual emissions in normal traffic conditions have to be taken. The limit values now apply for the approval of vehicles from September 2017 or January 2020. Euro 6 temp applies for the sale of new vehicles from September 2019 and euro 6 d from January 2021. In the meantime, the first examples of these clean diesel vehicles are on sale. So, anyone buying a new diesel car should insist on the emissions standards euro 6 temp or euro 6 d.

It is interesting to note that for lorries, the vehicle’s adherence to the emissions limit levels has had to be proved under normal traffic conditions since as early as 2013. This means that a single, modern lorry weighing 40 tonnes does not emit more nitrous gases than a diesel car with euro 5 or euro 6.

It will probably take several years before there are sufficient numbers of the latest generation of vehicles with low emissions on the road. Consequently, an improvement in air quality can only be achieved if car manufacturers retrofit the diesel cars that are already on the road in such a way that they emit less pollution. With many cars this is technically possible.

In the long term, strengthening public transport and cycling will create attractive alternatives to car travel. The more people are able to and want to opt for the bus, the train or cycling, the better it will be for air quality in Berlin and for people’s health as well. Every car journey that is avoided makes the air cleaner and healthier. Whoever switches to alternative modes of transport, is making an immediate contribution to improving air quality.