Marine Fuel Regulations and Engine Emissions: Impacts on Physicochemical Properties, Cloud Activity and Emission Factors

Marine regulations aim to reduce sulfur and nitrogen exhaust emissions from maritime shipping. Here, two compliance pathways for reducing sulfur dioxide emissions, fuel sulfur content reduction and exhaust wet scrubbing, are studied for their effects on physicochemical properties and cloud forming abilities of engine exhaust particles. A test-bed diesel engine was utilized to study fresh exhaust emissions from combustion of non-compliant, high sulfur content fuel with (WS) and without (HiS) the usage of a wet scrubber as well as a regulatory compliant, low sulfur content fuel (LoS). Particle number emissions are decreased by approximate to 99% when switching to LoS due to absence of 20-30 nm sulfate rich particles. While number emissions for WS are also decreased, a shift in the sulfate mode toward larger sizes was found to increase particle mass emission factors by at least 31%. Changes in the mixing state induced by the compliance measures are reflected in the hygroscopicity of the exhaust particles. Fuel sulfur reduction decreased cloud condensation nuclei emissions by at least 97% due to emissions of primarily hydrophobic soot particles. Wet scrubbing increased those emissions, mainly driven by changes in particle size distributions. Our results indicate that both compliance alternatives have no obvious impact on the ice forming abilities of 200 nm exhaust particles. These detailed results are relevant for atmospheric processes and might be useful input parameters for cloud-resolving models to investigate ship aerosol-cloud interactions and to quantify the impact of shipping on radiative budgets from local to global scales. We investigate how two pathways to comply with international regulations, aiming to reduce emissions of atmospheric pollutants from ships, alter properties of exhaust particles. Both investigated compliance measures (i.e., combustion of cleaner, low sulfur content fuels and aftertreatment of exhaust from a high sulfur content fuel via wet scrubbing) have substantial impacts on the chemical and physical properties of these particles. We find that, while both alternatives reduce the total number of emitted particles substantially, the effect on emissions of cloud forming particles is path dependent. While fuel sulfur content reduction decreased the number of cloud forming particles by about 97%, wet scrubbing led to a strong increase in emissions, suggesting that the measures can have substantial and opposing impacts on local cloud formation and evolution. Low sulfur content fuels can reduce emissions of ultrafine particulate matter significantly Exhaust wet scrubbing can lead to shifts in particle size distributions and reduce soot emissions Usage of low sulfur content fuels or exhaust wet scrubbing have opposing effects on CCN activity of ship exhaust particles