Black carbon from ships: a review of the effects of ship speed, fuel quality and exhaust gas scrubbing

The International Maritime Organization (IMO) has moved to address the health and climate impact of the emissions from the combustion of low-quality residual fuels within the commercial shipping industry. Fuel sulfur content (FS) limits and an efficiency design index for future ships are examples of such IMO actions. The impacts of black carbon (BC) emissions from shipping are now under review by the IMO, with a particular focus on the potential impacts of future Arctic shipping. Recognizing that associating impacts with BC emissions requires both ambient and onboard observations, we provide recommendations for the measurement of BC. We also evaluate current insights regarding the effect of ship speed (engine load), fuel quality and exhaust gas scrubbing on BC emissions from ships. Observations demonstrate that BC emission factors (EFBC) increases 3 to 6 times at very low engine loads (<25 % compared to EFBC at 85–100 % load); absolute BC emissions (per nautical mile of travel) also increase up to 100 % depending on engine load, even with reduced load fuel savings. If fleets were required to operate at lower maximum engine loads, presumably associated with reduced speeds, then engines could be re-tuned, which would reduce BC emissions. Ships operating in the Arctic are likely running at highly variable engine loads (25–100 %) depending on ice conditions and ice breaking requirements. The ships operating at low load may be emitting up to 50 % more BC than they would at their rated load. Such variable load conditions make it difficult to assess the likely emissions rate of BC. Current fuel sulfur regulations have the effect of reducing EFBC by an average of 30 % and potentially up to 80 % regardless of engine load; a removal rate similar to that of scrubbers. Uncertainties among current observations demonstrate there is a need for more information on a) the impact of fuel quality on EFBC using robust measurement methods and b) the efficacy of scrubbers for the removal of particulate matter by size and composition.