Estimating NO_x emissions of individual ships from TROPOMI NO₂ plumes

Maritime transportation is a substantial contributor to anthropogenic NO_x emissions and coastal air pollution. Recognizing this, the International Maritime Organization (IMO) has steadily implemented stepwise stricter emission standards for ships in recent years. However, monitoring emissions from sea-bound vessels poses inherent challenges, prompting the exploration of satellite observations as a promising solution. Here we use TROPOMI measurements of NO₂ plumes together with information on ship position and identity, and atmospheric models to quantify the NO_x emissions of 130 plumes from individual ships in the eastern Mediterranean Sea in 2019. Because most of the emitted NO_x is in the form of NO, which is not immediately converted into detectable NO₂, plumes show their NO₂ maximum some 15-30 km downwind of the ship's stack. Further downwind NO₂ decreases because of plume dispersion and photochemical oxidation. Background ozone and wind speed play a significant role both in detectability of the NO₂ plume and the relationship between NO_x emissions and observed NO₂, explaining the good detection conditions in the eastern Mediterranean summertime, where ozone levels are high. Taking such effects of emissions, dispersion, entrainment, and in-plume chemistry in full account, we find emission strengths of 10-317 g (NO₂) s⁻¹. We then calculate emission factors of the detected ship plumes using AIS and ship specific data and find that newer Tier II ships have higher emission factors compared to older Tier I ships. This is especially the case when running at lower engine loads, which is the most frequently observed mode of operation in our ensemble. Additionally, at the time of detection around half of the emission factors detected for Tier II ships lie above the IMO weighted average limits. The presented method sets the stage for automated ship emission monitoring at sea, contributing to better air quality management.