The graph illustrates the different levels of nitrogen oxide emitted by four types of vehicles, namely diesel cars, petrol cars, lorries and buses.The nitrogen oxide emission is measured in gram per kilometer (g/km) and speed in kilometer per hour (kph).
Overall, diesel cars contribute the most to the nitrogen oxide emission, followed by petrol cars. On the other hand, lorries and buses release more or less the same level of nitrogen oxide regardless of the speed.
At 10 kmh, a diesel car emits nitrogen oxide at 40 gram per kilometer and drops the lowest at 60 kmh. Then, it dramatically increase again and its emission peaks at about 42 gram at the speed of 130 kmh, indicating the lowest speed and highest speed release the highest levels of emission. However, the emission of pertoal car consistently drops as the speed increases until 90 kph. After that, it gradually rises again.
The nitrogen oxide emissions of lorries and buses stay more or less the same regardless of the speed although the emission of the former is slightly higher than the latter. It is evident that the emission of lorries is at the constant rate of 10 g/km whereas that of buses is 5 g/km.