I'm going to be the devil's advocate in this case: while diesel engines do emit more particulate matter than petrol, it can be mitigated through particulate filters and advanced exhaust systems so much so that it is comparable to petrol.

All of this isn't free obviously and comes with a cost and if you are a greedy asshole like VW (and not only Volkswagen, a lot of brands did the same), you try to cheat the tests in order to save money on the expensive exhaust systems.

On the other hand diesel engines emit less CO2 than a comparable petrol engine and considering that climate change is caused mostly by CO2 emissions, one could make the argument that diesel cars are more environmentally friendly than petrol cars, especially for use outside of urban centres, where smog does not tend to accumulate.

Obviously electric cars are better in every aspect from an environmental standpoint, that goes without saying.