Diesel vs. gasoline emissions: Does PM from diesel or gasoline vehicles dominate in the US?

In the US, the majority of the on-road fleet and vehicle miles travelled are attributed to light-duty vehicles, which are fuelled almost entirely by gasoline. However, due to their significantly higher PM emission rates, emissions inventories have tended to attribute the majority of the mobile source PM to contributions from heavy-duty diesel vehicles and strategies to reduce mobile source PM have focused on the contribution from this source. A limited number of source attribution studies have implied that PM emission inventories over-estimate the diesel contribution and emissions from gasoline vehicles may be greater than previously believed. Other receptor-modelling studies have found diesel vehicles to be the dominant source of motor vehicle PM. The former conclusion is supported by recent on-road PM emission rate results obtained in a highway tunnel and a series of crossroad experiments. This paper describes the often-conflicting results obtained from receptor modelling studies and emission inventories and uses on-road emission factor results to estimate the relative contributions from the diesel and gasoline sectors of the fleet.