Highway motor vehicle emissions contribute to the total atmospheric particulate burden. The possible health and welfare effects of these emissions depend upon their composition and concentration in the atmosphere, the exposure of man and materials, and in some instances the length of time of the exposure. The characteristics of long-term national and mesoscale (city-wide) exposures and short-term localized (congested freeway, city street canyon) exposures to mobile source particles were examined for 1977, 1988, and the year 2000. Because of interest in the possible impact of passenger car dieselization on atmospheric particulate levels, three degrees of diesel penetration (5, 10, and 20% of light-duty sales by 2000) were examined with and without an assumed scenario of particulate emissions regulation. The compositional characteristics of mobile source particles will change with the outyear fraction of lead decreasing and the fraction of elemental and organic carbon increasing.