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ABSTRACT. Particle size measurements using the electrical low pressure impactor (ELPI) and scanning mobility particle sizer (SMPS) are compared from the perspective of characterizing the particulate matter in motor vehicle exhaust. Both steady state vehicle operation and transient drive cycles are considered, and both gasoline and diesel fueled vehicle emissions are compared. Although the ELPI and SMPS measure different physical properties, respectively, the aerodynamic diameter and mobility diameter, the steady state particle size distributions are in close agreement, except for the 37 nm impactor stage of the ELPI which may overestimate particle number by up to a factor of two relative to the SMPS. This has little effect on the volume, or mass, weighted distribution. These, too, are generally in good agreement, though discrepancies appear at large particle size due to multiple charging effects in the SMPS and to electrometer offsets and the small particle loss correction in the ELPI. Selecting particles based on their electrical mobility with the SMPS, and then measuring their aerodynamic diameter with the ELPI, reveals that diesel particulate matter with well-speci ed mobility diameter exhibits a wide range in aerodynamic diameter and, therefore, also in effective density. Over transient drive cycles, the ELPI provides second by second particle distributions, whereas the SMPS must be run in a xed particle size mode and size distributions constructed from repeated tests. The ELPI registers higher instantaneous PM emission rates during transients than the SMPS due to the faster time responses of the ELPI. The time integrated ELPI and SMPS size distributions, however, remain in good agreement. The relative merits of the two instruments for steady state and transient tests are discussed.

This paper reports mass measurements, size distributions, and the transient response of tailpipe particulate emissions from 21 recent model gasoline vehicles. Transient measurements are made for the FTP drive cycle (and limited ECE tests) using a scanning mobility particle sizer and an electrical low-pressure impactor. The particles emitted in vehicle exhaust have diameters in the 10-300 nm diameter range, with a mean diameter of about 60 nm. Particle emissions during the drive cycles occur as narrow peaks that correlate with vehicle acceleration. Cold start emissions generally outweigh those from a hot start by more than a factor of 3. Particulate mass deduced from the transient distributions agrees semiquantitatively with gravimetric measurements. Tailpipe particulate emissions from the recent model gasoline vehicles tested are very low, with mass emission rates ranging downward from 7 mg/ mi for a light-duty truck during the cold start phase of the FTP drive cycle to e0.1 mg/mi during phase 2 for nearly half of the test vehicles. Three high-mileage (>100 K mi) test vehicles exhibited similarly low particulate emission rates. The FTP-weighted 3-bag average is under 2 mg/mi for all the conventional gasoline vehicles tested.

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Factors influencing the number and size of tailpipe particles from port injection, spark ignition vehicles are examined by comparing emissions recorded during steady-state operation and those obtained from FTP and US06 drive cycles. Size distributions are measured using the scanning mobility particle sizer (SMPS) and the electrical low-pressure impactor (ELPI). Steady-state particulate emissions are examined as a function of vehicle speed and air to fuel ratio. The emission rates increase moderately with increasing speed but climb steeply with decreasing A/F. This is consistent with the observations from transient drive cycle measurements where particulate emissions occur predominantly during periods of heavy acceleration. As expected from the more aggressive speed and acceleration of the US06 cycle, the per mile particulate emission rates are higher than for phases 2 and 3 of the FTP. For the eight vehicles tested, the US06 mass emissions range from 1.2 to 9.6 mg/mi. Use of a US06-compliant calibration leads to a factor of 2 reduction of particulate emissions, in both number and mass, over the drive cycle.