True, I'm not sure what the black dirt is from, it might not be from the diesel as you suggest. However most of the air pollution on high traffic roads is from diesel - they are not really clean at all. That is a complete myth propagated by companies like VW - the whole notion of "clean diesel" is completely bogus, like "clean coal". Even Euro 6 tests are completely unrealistic and don't reflect urban driving - hence the newer 6d test, but I am highly doubtful at this point after many years of outright lies. DPFs for instance are used to pass these tests, but all they do is save the pollutants, which they then periodically burp out in high concentrations into the local area. For urban driving diesel is by far the worst possible type of car and should simply be banned, the evidence is overwhelming that this is the main cause of urban pollution in most EU cities right now.