Your comments highlight the crux of the problem. Back in the day, inefficient (read truck like) diesel were shooting out black smoke. That particulate is large in size (10 or 100 of microns) that you actually "see". Improvements in efficiencies (both in combustion and trapping) made modern "clean engines" reduced the size of particulate to few microns. Those are much more difficult to see. Yet they are far more dangerous. Large particulate is trapped in your upper respiratory tract, the fine stuff gets deep in your lungs, often bioaccumulaating like abspestos does. You know how the stoey goes. Not because you don't see it it means it's not there... Next time stick a paper towel on the exhaust of your cold diesel and leave it there for a few minutes. Look at the color. Now you have somerhing to "see".