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This paper explores the extent to which standard dilution tunnel measurements of motor vehicle exhaust particulate matter modify particle number and size. Steady state size distributions made directly at the tailpipe, using an ejector pump, are compared to dilution tunnel measurements for three configurations of transfer hose used to transport exhaust from the vehicle tailpipe to the dilution tunnel. For gasoline vehicles run at a steady 50 - 70 mph, ejector pump and dilution tunnel measurements give consistent results of particle size and number when using an uninsulated stainless steel transfer hose. Both methods show particles in the 10 - 100 nm range at tailpipe concentrations of the order of 104 particles/cm3.

Particle number emissions are measured with two instruments according to the upcoming European emission regulations for light-duty diesel passenger vehicles and compared to data from other methods, including the current regulatory total particulate matter (PM) mass, photo-acoustic soot sensor (PASS) and engine exhaust particle sizer (EEPS). At the very low emission levels of diesel particulate filter (DPF) equipped vehicles, the solid particle number data correlate well with soot mass and with particle number measured by EEPS, if only those particles belonging to the accumulation mode are considered in the latter case. PN differences of ≻100% between tests of the same vehicle are observed. Comparison of the two PN instruments and the photoacoustic soot sensor show that these are systematic differences which originate primarily with the vehicle and not from instrument uncertainties.