Biodiesel as a drop-in fuel increases marine-engine organic aerosol emissions by 1-2 orders of magnitude

Biodiesel fuels in marine engines offer a potential route to decarbonizing the marine shipping industry. The different chemical composition of biodiesels may lead to differences in combustion emissions, with potential effects on human health, air quality, and climate. While some of these differences can be addressed by engine tuning, there is strong interest in using biodiesel as a drop-in replacement to fossil fuels.
Here, as part of a collaborative field campaign, we used aerosol mass spectrometry to characterize the organic matter (OM) emitted by a 1920kW marine engine during fuel switching between conventional diesel and biodiesel under sea trial conditions. Measurements were performed as a function of engine load for two fuels, one 100% biodiesel and one blend containing mostly conventional diesel and a small amount of biodiesel. Comparisons with laboratory reference spectra as well as statistical analysis by positive matrix factorization were used to interpret the results.
When operating on fossil-diesel, OM emissions were dominated by lubricating oil, with ~15 mg OM/kg CO2 emitted at moderate load. This compares with a Black carbon (BC) emission factor of ~40 mg/kg CO2. When operating on biodiesel, organic emissions became dominated by unburned fuel and increased to ~500 mg OM/kg CO2,while BC emissions decreased to ~10 mg/ kg CO2 . At idle, OM emissions increased by a factor of ~8 for fossil-diesel and a factor of ~20 for biodiesel, while BC increased by a factor of 3-4 for both fuels. The OM emitted from fossil-diesel combustion at idle was enriched in aromatic hydrocarbons, whereas the biodiesel OM was not.
These results demonstrate that fuel type influences both the magnitude and chemistry of particulate emissions. Future work to understand the impact of PM on climate, air quality, and health must consider the effect of atmospheric dilution and atmospheric oxidation.