It's part of the exhaust treatment. They developped a new exhaust system that's supposed to be green. This additional tube is here to enhance the effectiveness of the test by putting the exhaust gaz just in the face of the pedestrians.

It's an IRL test of the system:

they drive slowly in the downtown, and if the people around the car turn green and fall to the ground making weird noises, it means the system is not working. If they turn green and start to cough insanely, it's not very effective. If they look disgusted, it still stinks. If they laugh and show the car with their fingers, the system is perfectly effective. (NB: a mask is part of this equipment.)

The second phase of the test is that they drive back where they were. If the place is filled with emergency vehicles, they disappear quickly and write in the report that the system is rubbish. If not, they can go back home and remove their masks.

Third and last phase, they watch the news. if it is spoken about a mysterious and deadly polluting agent who destroyed peoples' lungs, the system will be removed with a STIHL and its engineers will have to eat it without ketchup. If not, BMW will sell the car like that and Bangle will use very long and impressive words to explain how good it looks.