Mobile Source Emission Reductions for Meeting Environmental Goals

Alberto Ayala

California Environmental Protection Agency - Air Resources Board

     Abstract Number: 486
     Last modified: January 29, 2010

Abstract
Motor vehicles are vital to the fabric of modern society; we depend on them for personal mobility, services, and for the movement of goods. But motor vehicles are energy-intensive and represent a major source of emissions that adversely impact air quality, health, and the local, regional, and global environment. In California, the transportation sector is the single largest contributor to greenhouse gas (GHG) emissions, for which efforts are underway to achieve significant reductions. Mobile sources also tax our air with more CO and NOx than any other sector. These factors highlight our increasing need for the environmental reconciliation between a growing reliance on fossil fuels and reductions in air and climate-active pollutants.

Mobile source technology is advancing rapidly. The most prominent example is hybridization, a transformational luminary on the path toward electrification and zero emissions. But transforming a fleet like California’s will take time. In the near-term, environmental and climate protection efforts will also necessitate advances in low-carbon fuels, gasoline, and clean diesel engines.

Motor vehicle air pollution control started in California half a century ago, and has spread all over the world. Future policies in California will force all new passenger vehicles to be as clean as the best super-ultra-low emission vehicles in use today. The adoption of more efficient technology like gasoline direct injection will help meet GHG emission reduction targets. New diesel engines will also result in significant emission reductions. For example, we have high expectations for significant reductions of NOx emissions from mobile sources with the imminent widespread use of selective catalytic reduction in the heavy-duty diesel sector.

We now have extensive evidence that order-of-magnitude reductions in PM mass emissions are possible with enabling technology like the diesel particle filter (DPF). However, in some cases, reductions in PM mass from DPFs have led to an increase in the number of particles emitted. This increase in number is due primarily to volatile ultrafine particles formed by nucleation under some operating conditions. But the composition of these emissions, often at or near background levels, is dominated by relatively non-toxic ions like sulfate and ammonium.

With the expanding recognition of the associations between adverse health outcomes and exposure to mobile source emissions, the health effects community has a strong interest in those emissions. But estimates of source-receptor relationships used to inform source-specific health assessments are only as good as the representation of the factors that influence emissions from a given source. Fortunately, mobile-source research is keeping pace with the evolution of emission-reduction technology. Recent and on-going studies are advancing our understanding of the nature of vehicle emissions and their precursors, and the ways that advances in clean fuels, lubricants, engines, and aftertreatment are changing those emissions and their health-relevant characteristics. This will enable health scientists to have the most useful source-specific inputs for their research.