SOx

SOx is the collective term for sulphur oxides, principally sulphur dioxide, that are released when a ship burns fuel containing sulphur. Unlike nitrogen oxides, sulphur oxides come directly from the sulphur present in the fuel, so the amount emitted is governed by the quality of the fuel burned: the more sulphur in the fuel, the more sulphur oxide in the exhaust. Traditional heavy fuel oil could contain several percent sulphur, making shipping a significant source of these emissions. Sulphur oxides are damaging pollutants that contribute to acid rain, which harms vegetation, soils and waters, and to fine-particle air pollution that affects human health, with the greatest impact near busy shipping lanes and ports.

Control of sulphur oxides is therefore based on limiting the sulphur that enters the exhaust in the first place, through MARPOL Annex VI. Since the IMO 2020 rule, the sulphur content of marine fuel used generally is capped at 0.50 percent by mass, a sharp reduction from the previous 3.50 percent limit. Inside designated emission control areas the limit is far lower, at 0.10 percent, because those coastal regions are especially sensitive. Ships comply either by burning compliant low-sulphur fuels (very low sulphur fuel oil or distillates such as marine gas oil) or by fitting an exhaust gas cleaning system, a scrubber, that removes the sulphur oxides from the exhaust and so allows continued use of higher-sulphur fuel.

The practical effects of the sulphur rules on ship operation are considerable. Vessels must manage their fuel carefully, often carrying more than one grade and switching to compliant fuel before entering an emission control area, logging the changeover, and ensuring the engines and fuel systems handle the different fuels safely. The rules reshaped the marine fuel market, spurred investment in scrubbers and cleaner fuels, and demonstrably reduced sulphur emissions from shipping. Sulphur oxide control sits alongside the measures on nitrogen oxides and greenhouse gases as part of the tightening framework governing the environmental impact of ships' exhaust.