US vehicle emissions: Creating a common currency to avoid model comparison problems

Emission inventory models are a key input to regional air quality plans, called State Implementation Plans (SIPs) in the US. These plans frequently include emissions caps, or “budgets,” to which transportation system emissions must be compared. When emissions models and vehicle fleet-related planning assumptions change substantially, but for a variety of reasons the State Implementation Plan and its emissions budgets do not, a mismatch arises. The SIP Currency approach addresses problems that arise when the latest emissions and transportation modeling tools and planning assumptions are used to estimate on-road vehicle emissions, but the resulting estimates are compared to outdated emissions targets. Translating emissions estimates into the currency of emissions estimates included in older air quality plans provides a way to ensure continued improvement in air quality while preventing unintended disruption of transportation planning. The article illustrates how carbon monoxide, volatile organic compound and oxides of nitrogen emissions may be translated into the currency of outdated emissions models.