Size distributions of motor vehicle exhaust PM: A comparison between ELPI and SMPS measurements

Particle size measurements using the electrical low pressure impactor (ELPI ) and scanning mobility particle sizer (SMPS) are compared from the perspec tive of characterizing the particulate matter in motor vehicle exhaust. Bot h steady state vehicle operation and transient drive cycles are considered, and both gasoline and diesel fueled vehicle emissions are compared. Althou gh the ELPI and SMPS measure different physical properties, respectively, t he aerodynamic diameter and mobility diameter, the steady state particle si ze distributions are in close agreement, except for the 37 nm impactor stag e of the ELPI which may overestimate particle number by up to a factor of t wo relative to the SMPS. This has little effect on the volume, or mass, wei ghted distribution. These, too, are generally in good agreement, though dis crepancies appear at large particle size due to multiple charging effects i n the SMPS and to electrometer offsets and the small particle loss correcti on in the ELPI. Selecting particles based on their electrical mobility with the SMPS, and then measuring their aerodynamic diameter with the ELPI, rev eals that diesel particulate matter with well-specified mobility diameter e xhibits a wide range in aerodynamic diameter and, therefore, also in effect ive density. Over transient drive cycles, the ELPI provides second by secon d particle distributions, whereas the SMPS must be run in a fixed 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 ELP I and SMPS size distributions, however, remain in good agreement. The relat ive merits of the two instruments for steady state and transient tests are discussed.