The local ship speed reduction effect on black carbon emissions measured at a remote marine station

Abstract. Speed restrictions for ships have been introduced locally to reduce the waves and turbulence causing erosion, and safety hazards, and to mitigate the air and underwater noise emissions. Ship speed restrictions could be used to minimize the climate impact of maritime transport since many air pollutants in ship exhaust gas are reduced when travelling at lower speeds. However, for example, methane and black carbon emissions do not linearly correlate with the load of internal combustion engines. Therefore, the effect of speed restrictions may not be trivial. Black carbon concentrations from ship plumes were examined at a remote marine site in the southwestern Finnish archipelago. Ships with service speeds over 15 kn and equipped with an exhaust gas cleaning system were analysed for black carbon emissions as a function of speed. Both unadjusted and weather-adjusted main engine loads were modelled to determine load-based emission factors. Black carbon concentration per kilogram of fuel decreased as a function of engine load. However, calculated per hour, the black carbon emission increased as a function of ship speed, reaching peak values at around 15–20 kn and decreasing thereafter. In terms of local air quality, total black carbon emission per nautical mile was the highest around the halved speeds, 10–13 kn, or when the speed was higher than 20–23 kn. From a climate warming perspective, the CO2 emissions dominated the exhaust gas, and reducing the speed decreased the global warming potential in CO2 equivalent, both per hour and per nautical mile.