The impact of high-particulate-emission vehicles on the total light-duty vehicle particulate emission inventory in the South Coast Air Quality Management District (SCAQMD) has been estimated by an analysis of the SCAQMD's CUT-SMOG database, a field survey of smoking vehicles, and particulate emissions testing of representative smoking vehicles. The field study, carried out at 61 sites throughout the SCAQMD, showed that 1.11%-1.75% of the vehicles in the light-duty fleet emit visible smoke. This fleet of smoking vehicles is composed mainly of older vehicles, 8 to 18 years of age. Particulate emission testing of 23 smoking vehicles gave an average Federal Test Procedure (FTP) weighted emission rate of 399 mg/mi with a range of 64-2,323 mg/mi, while the average IM240 emission rate was 346 mg/mi, with a range from 13-1,807 mg/mi. The contribution of light-duty smoking vehicles to the IMPLICATIONS Understanding the population density and emission characteristics of high-emitting vehicles has been found to be an important element in improving mobile source emissions inventories. For gaseous emissions, much progress has been made in this area through the use of remote sensing and follow-up dynamometer studies. To date, no attempt has been reported to define the population of highparticulate-emission vehicles, their characteristics, and emission rates. This paper reports results on the population and characteristics of high-particulate-emission lightduty vehicles as determined by a field study to identify visibly smoking vehicles in the South Coast Air Quality Management District. Particulate emission rates for a representative fleet of these vehicles were determined, and estimates were made of the contribution of these vehicles to the particulate emission inventory in the district. This updating and improvement of the particulate emissions inventory for the district will allow improved air quality management.particulate emissions inventory in the SCAQMD is estimated to be 0.51-0.81 tons per day.