of ethanol-blended RFG might increase evaporative emissions to a greater extent than would a fuel having a lower RVP. The significance of malfunctioning evaporative systems to total VOC emissions from the vehicle has not been studied extensively. The contribution of high emitters is expected to increase in the coming decade (Sawyer et al. 1998).

Regulatory control of emissions from LDVs has become ever more stringent since the passage of the Clean Air Act. Manufacturers have responded with appropriate control technologies that have become more effective as they mature.

The contribution of ozone precursors to the national emissions inventory by on-road vehicles has been trending downward, despite a substantial increase in vehicle miles traveled.

Evidence indicates that the proportion of driving time spent in transient driving maneuvers that depart significantly from those accounted for by the current FTP is small, but those departures can contribute a disproportionate share of tailpipe emissions. Of particular concern are emissions arising from cold starts and trips with multiple stopovers. Changes to the certification procedure to account for shortcomings of the FTP are forthcoming. However, ongoing monitoring of driving profiles is warranted to ensure realistic integration of tailpipe emissions measurement with the presently uncoupled measurement of evaporative emissions.

The relatively small proportion of high-emitting vehicles present in the fleet can add disproportionately to total fleet emissions. Effectively repairing such vehicles or removing them from the fleet could be the single most effective ozone-precursor reduction strategy in the mobile-source control arsenal. The new generation of on-board diagnostics is expected to decrease the incidence of high emitters. Meanwhile, an ongoing program that diagnoses the specific emissions-component malfunctions in high-emitting vehicles could help to avoid high emitters in the future.