I'm in central London where there were problems with that and the new diesels are hugely better. Used to be black smoke and layers of the stuff. Now not noticable. I'm not sure the figures but 10x or 100x?

Google AI has: Modern diesel engines are dramatically cleaner than older vehicles, emitting up to 95-99% fewer nitrogen oxides (NOx) and capturing up to 99% of harmful particulate matter (soot).

I quite cycling around 2006 because I got smokers cough from the clouds of smoke coming out of the busses and similar. Now it's fine. Both the new diesels being better and things going electric.

Buses can carry 100 people at a time so have a far larger social utility. Besides, most are going electric.

If those buses use DEF, have a anti particulate filter and are well maintained, their emissions should in theory be extremely low, with no heavy compounds. I hope local municipal authorities don't remove filters from their buses. Even then, most if not the new buses in my town are either electric or CNG/electric hybrids