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Abstract

Heavy-duty marine engines can be operated on biodiesel instead of conventional petroleum diesel to reduce life-cycle carbon dioxide emissions. However, the use of biodiesel also alters the properties of the emitted particles. The objective of this study is to compare the particulate emissions from a marine diesel engine operating on biodiesel (B100, soy methyl ester) and baseline diesel (B5) at a range of engine loads. Particle emissions were analyzed for size distribution, effective density, mass concentration, chemical composition, optical properties, and morphology by Transmission Electron Microscopy (TEM). Regardless of fuel type or engine load, diesel particle emissions consist of semi-volatile organic compounds and non-volatile refractory black carbon. TEM images confirmed the presence of both collapsed and uncollapsed soot aggregates in both fuel operations, with collapsed aggregates being predominant. Additionally, a third category of non-volatile, sub-15 nm particles with very high effective density (> 2000 kg/m³) was identified and attributed to metal oxide particles. These particles were more prevalent in B100 operation. In terms of particle size distribution, biodiesel operation increases the number of particles in nucleation mode while decreasing the accumulation mode. Particle size distributions show a nucleation mode when the sample includes a mixture of organic carbon and black carbon, while removing the organic carbon by a catalytic stripper from the sample changed the mode to accumulation. At idle, a nucleation mode near 10 nm appeared, likely associated with metal-containing particles. The total PM mass concentration is higher when the engine is operating on biodiesel at all loads, with the maximum at idle condition, and is mainly composed of organic carbon. B5 fuel operation produces lower PM mass concentration, which is mainly made of elemental carbon. Organic carbon concentration is higher for B100 exhaust, and elemental carbon is higher for B5 at all engine loads. Optically, B100 particles were more scattering in nature (SSA > 0.5), while B5 emissions were more absorbing (SSA < 0.3). The mass absorption cross-section (MAC) at 870 nm for soot particles was 4.9 ± 0.50 m²/g for B100 and 5.5 ± 0.48 m²/g for B5.