Air quality impacts of the trade-off between renewable diesel and sustainable aviation fuels in California

Renewable diesel (RD) and sustainable aviation fuel (SAF) are lower-carbon alternatives to conventional petroleum fuels that have begun to enter the U.S. market at commercial scale, largely driven by policy. The most widely deployed forms of RD and SAF are hydrotreated lipids, produced by similar processes, using vegetable or waste oil as the feedstock. Both RD and SAF typically result in lower air pollutant emissions than their petroleum equivalents, especially particulate matter and sulfur oxides (SOx), however the quantity and location of these reductions are different. The dependence of both RD and SAF on the same pool of feedstock and production capacity means that in the near term, increased SAF production may come at the expense of RD. This study investigates the possible air quality effects of such a shift, with a focus on potential impacts in disadvantaged communities. We develop two emissions scenarios in California, for which diesel and aviation emissions are scaled to reflect policies that favor SAF or RD. The air quality associated with each emissions scenario is then simulated using a chemical transport model. Health impacts from the air quality exposure fields are estimated for three major urban areas and exposure disparities are calculated based on historical socioeconomic data. The results of this show minimal air quality and public health changes between the high SAF and high RD scenarios for the California cities analyzed in the present study because California’s current diesel emission regulations limit the potential air quality benefit from RD. This suggests that air quality considerations should not be a dominant motivation when evaluating policy-driven shifts in the relative prevalence of RD and SAF in California. Other regions using less advanced diesel engine technology may see more significant air quality tradeoffs between SAF and RD.Implications: Hydrotreated alternative fuels like renewable diesel (RD) and sustainable aviation fuel (SAF) are entering the market at large scale, drawing from a limited global pool of feedstock (vegetable oil and non-fossil waste oils) and using similar production processes. Policymakers have explored expanding SAF consumption; likely near-term approaches to this would result in a commensurate reduction in RD. This paper evaluates whether this trade-off would cause significant impacts on air quality if it were to occur in California, where a SAF-focused policy change was considered in 2024. We find no significant regional air quality impact from such a shift.