SO2 and NOx emissions from European shipping: a measurement study

The shipping industry is a significant source of both SO₂ and NO_x, two air pollutants which have neg-active implications for climate and air quality. SO2 is heavily regulated such that January 2020 marked a global reduction of the maximum permitted sulfur fuel content (SFC) in shipping fuel from 3.5% to 0.5% by mass. There are also various limits on NO_x in coastal and inland waterways with more widespread NO_x emissions limits likely to be implemented in the future. The anticipated effect of the new regulations is an improvement of coastal air quality, but there is a potential drawback in terms of reducing the climate cooling effect of SO₂ caused by a change in cloud properties. However, the difficulty in measuring emissions from ships means that the overall impact and level of compliance with new and future regulations is unclear. Therefore, monitoring strategies capable of providing regular and long-term measurements of air pollutant emissions from the shipping industry are essential for fundamental research and compliance monitoring. Here we present top-down methodologies for calculating the SFC, along with emission ratios of ∆NO_x/∆CO₂ from individual ships. First, we demonstrate the application of an airborne platform to perform targeted measurements of ship plumes in the English Channel and Atlantic shipping lanes, allowing the comparison of emissions inside and out of sulfur emission control areas (SECAs). Second, we implement a stationary, point-sampling approach to measure ships arriving and departing two European ports.

Our results show there has been a significant reduction in the SFC of ships in the open ocean shipping lanes due to the new emission regulations, with most ships now well below the 0.5% limit imposed in 2020. Our measurements also show good agreement with a comprehensive ship specific emissions inventory (the Ship Traffic Emission Assessment Model – STEAM). This confirms the validity of models that use the new sulfur limit for radiative forcing calculations. In terms of NO_x, the measured ∆NO_x/∆CO₂ ratios were generally greater than those from diesel vehicles operating within a typical European fleet, suggesting ships could be a significant source of NO_x to cities, especially where ports are close to populated areas. It is envisaged that the presented methodologies could be implemented to facilitate widespread monitoring of ship emissions, which could provide the basis for policy formulation and validation.