TROPOMI-based NOx emission estimates of individual ships

Maritime transportation is a substantial contributor to anthropogenic NO_x emissions and coastal air pollution. Recognizing this, the International Maritime Organization (IMO) has implemented stricter emission standards for NO_x 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 NO_x 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 NO_x 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 NO_x emission strength and the observable NO₂ plume: Ozone drives the partitioning of the emitted NO_x between NO and (observable) NO₂ and the wind speed dictates the mixing and therefore lifetime of NO_x 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.