Nanoscale insights into morphological and structural characteristics of soot particles emitted from general aviation engines burning sustainable aviation fuel

With the aviation industry committed to achieving net-zero emissions by 2050, sustainable aviation fuels (SAFs)—notably hydroprocessed esters and fatty acids (HEFA)—have emerged as a pivotal decarburization pathway due to their drop-in capability. However, limited understanding of morphological and nanostructure characteristics of soot particles derived from SAF hinders a comprehensive assessment of its environmental and health impacts. This study investigates the microscopic characteristics of soot particles emitted from two general aviation engines operated under low (7%), medium (50% or 60%), and high (100%) loads burning 100% HEFA-SAF and RP-3 fuels. The results indicate that the D_p¯ of HEFA-SAF soot particles remain smaller than RP-3 across all loads, and HEFA-SAF soot particles feature the morphology of smaller "core" part (amorphous carbon) and thicker "shell" part (concentric graphite layers). Additionally, the findings reveal a close correlation between the elemental ratio of carbon-to-oxygen, internal nanostructures, and the degree of graphitization of soot particles and engine load, while the influence of fuel composition on these parameters demonstrated non-monotonic characteristics, which can be attributed to the kinetic time scales competition between fuel-air mixing and fuel pyrolysis kinetics. Therefore, this study highlights that morphological and nanostructure of soot particles are influenced by coupled effects of fuel composition and operational load. Importantly, while HEFA-SAF effectively reduces soot particle emission concentrations, its derived soot particles microscopic characteristics remain statistically indistinguishable from those of soot particles derived from RP-3. This finding implies that HEFA-SAF implementation can mitigate aviation's particulate climate forcing without enhancing per-particle radiative efficiency, thereby decoupling emission reduction from amplified climate impacts and accelerating aviation industry decarbonization.