My pollutants spewed per mile are lower than a petrol engine.
Research particulates from a gasoline engine vs a diesel engine. Diesels emit much more -- if they were popular we would have extreme smog issues.
Another easy way to tell: Look at the bumper of diesel cars around the exhaust -- sooty! That soot is being released into the air and is a carcinogen.
With recent EPA requirements changing, manufacturers have had to combat this. One low tech technique is to put a paper filter on the exhaust to catch the particulates. Another is to change the fuel/air ratio to ensure combustion of the soot -- however that results in increased NOx transmissions. To solve this, newer diesel cars/SUVs (BMW, Mercedes, Jeep, Chevy) come with a urea tank. The urea is injected into the exhaust which then allows a catalytic converter to break down NOx. The engine computer is required to prohibit you from starting the car if the urea tank is empty. (Usually there is a small grace period -- 10 starts or so.)
I am looking forward to clean (and affordable!) diesels but we are still not quite there yet.