TROPOMI-based NOx emission estimates of individual ships

Maritime transportation is a substantial contributor to anthropogenic NOx emissions and coastal air pollution. Recognizing this, the International Maritime Organization (IMO) has implemented stricter emission standards for NOx and SO2 in recent years. However, monitoring emissions of sea-bound vessels possesses inherent challenges, prompting the exploration of satellite observations as a promising solution. This study presents a first-ever satellite-based NOx emission inversions for multiple individual ships based on TROPOMI-observed NO2 plumes and AIS data over the Mediterranean Sea in 2019. Our inversion approach accounts for the complex, high-resolution atmospheric dynamics and chemistry that drive the relationship between the NOx emissions and observable NO2 plume. Additionally, we test how an updated Air Mass Factor (AMF) scheme – using high-resolution local plume NO2 profiles from  PARANOX simulations – improves the inversion. For the inversion, we create a large library of pseudo-observations of NO2 plumes with the Gaussian Plume Model PARANOX and test these with the large eddy simulation model MicroHH, which was run with an atmospheric chemistry scheme. The plume dispersion of the two models shows good agreement and the simulated in-plume NO2 differs by only 6%, making PARANOX a suitable (and computational efficient) model choice, especially when considering the large uncertainties related to satellite retrievals above sea. The PARANOX library shows that background ozone and the effective wind speed determine the relationship between the ship NOx emission strength and the observable NO2 plume: Ozone drives the partitioning of the emitted NOx between NO and (observable) NO2 and the wind speed dictates the mixing and therefore lifetime of NOx in the plume. This explains the frequent occurrence of detectable ship NO2 plumes in the Eastern Mediterranean in summer, when background ozone is high and wind speeds are moderate. We study 130 NO2 plumes of individual ships found in TROPOMI data over the Mediterranean Sea in 2019, most of which are from container ships. In the early afternoon, the observed ships in the Mediterranean Sea emit on average 76g NO2 /s but reaching up to 240 g/s. Finally, these fluxes are compared against both the current IMO emission limits and the requirements for a future Emission Control Area in the Mediterranean Sea.