Methane and carbon monoxide emissions originating from LNG-powered ships measured at a remote marine station

The global transport of liquefied natural gas (LNG) is expanding, with LNG comprising 6% of the fuel used in international shipping in 2023 and 722-1120 LNG powered vessels in operation. LNG combustion emits less CO₂ than oil-based fuels, but its well-to-tank carbon intensity is higher, and the escape of unburned methane (CH₄) into the atmosphere adds a significant greenhouse gas impact. Fifty four exhaust plumes from LNG powered ships were measured at a remote marine station in Finland, resulting in median CH₄ slip of 3.0% for low-pressure four-stroke (LP4S) engines and 0.3% for low-pressure and high-pressure two-stroke (LP2S, HP2S) engines. The slip was negatively correlated with the load of the modeled main engine. The values were close to the default values of the EU Directive 2023/1805 for LP4S and HP2S but lower for LP2S. The integration and regression methods of the plume area gave similar medians, although the former detected additional non-exhaust CH₄ sources. The results underscore the influence of the load, the engine design and the measurement method, and the need to account for fugitive emissions in regulation and emissions inventories. The observed median emission factor for CO in grams per kg fuel was 5.4 for LP2S engines, 19.4 for LP4S engines, and 10.0 for HP2S engines.