CARB: I have heard that particulate matter (PM) n a diesel exhaust must be below five percent. What does this mean and how is this verified?

The PM in a vehicle’s exhaust is measured during a smoke opacity test. This is done by shining a light through the exhaust stream leaving the tailpipe and onto a sensor. If more than five percent of the light is blocked by PM during idling or during snap accelerations, the vehicle fails the test. For reference, the human eye cannot see PM at five percent, so if there is black soot being discharged from the vehicle exhaust, it is likely over five percent.