Satellite identifies ships emitting excessive nitrogen

The nitrogen issue dominates agricultural debates, at least in the Netherlands. Industry and land and air transport are also discussed, though to a lesser extent. However, international shipping largely escapes scrutiny in the public debate despite its significant nitrogen compound emissions. Ships worldwide emit 24.4 billion kilograms of NO₂ annually, according to Christoph Riess, who earned his doctorate at Wageningen University & Research last autumn. Although emissions from ships do not directly contribute to nitrogen deposition in Dutch nature (see Figure 1), these nitrogen emissions do cause air pollution.

24 billion kilograms account for 20% of total nitrogen emissions, though it may be underestimated. "We simply don’t know precisely because the measurement methods are far from optimal," says Riess. "Moreover, this share could grow as international shipping expands and other sectors improve in reducing nitrogen emissions." Finnish and American researchers estimate that 800,000 people die prematurely each year due to shipping-related pollution.

The newer the ship, the stricter the rules

Countries face the challenge of reducing nitrogen emissions from ships. The IMO is responsible for setting regulations alongside its original safety-focused mandate. The most recent international regulations concerning nitrogen oxide emissions date back to 2011, although more recent standards have been introduced for the North Sea. In essence, newer ships are allowed to emit significantly less than older ones. "Enforcement of such international regulations is complex, but it starts with accurate measurements," says Riess.

Jasper van Vliet, Chief Data Scientist at the Inspectie Leefomgeving en Transport (ILT), explains how measurements and inspections are currently conducted in the Netherlands. The ILT inspects ships docking in the Netherlands, including their nitrogen emissions. However, these onboard inspections only reveal how the engine performs "on paper". Additionally, "sniffer poles" on docks measure emissions from passing ships, and drones are deployed. "However, with these techniques, we can only monitor a fraction of the tens of thousands of ships that visit Dutch ports, and we also lack insight into how ships behave on the open sea," says Van Vliet. As a result, whether the regulations have the intended effect remained unclear .

Satellite captures daily images of nitrogen plumes

The ILT contacted Wageningen’s Meteorology and Air Quality Group and researchers from Leiden University to address this issue. Together, and with funding from the ILT, they initiated a study to measure nitrogen emissions from ships worldwide. The instrument they used, TROPOMI, operates at an altitude of 824 kilometres. This Dutch-developed instrument is attached to a European Space Agency satellite. TROPOMI maps the entire Earth's atmosphere daily, detecting key atmospheric components such as ozone, nitrogen dioxide (NO₂), sulphur dioxide (SO₂), carbon monoxide (CO), methane (CH₄), formaldehyde (HCHO), and particulate matter.

The satellite data proved capable of detecting the plumes emitted by moving ships. (also see Figure 2) An aircraft study was used to validate the satellite data over sea. In this way, the researchers confirmed the accuracy of the satellite’s data, also enabling calculations of actual emissions closer to the ships.

Van Vliet anticipates that satellite data could become a supplementary measurement method. "These measurements could help pre-select ships for inspection before they enter the port. We’d know which ships to board to check their documentation."

Newer ships perform no better

A notable finding was the difference between newer and older ships. Surprisingly, newer ships emit no less nitrogen than older ones, Riess discovered. "That’s remarkable because the latest ships are required to have engines that, according to factory tests, emit less nitrogen than older models." These tests measure emissions at various speeds and calculate averages. The averages at higher speeds are weighted more than emissions at lower speeds. Riess explains: "Such calculated emissions don’t align with real-world conditions, as ships often travel slowly to save fuel and costs. However, slower speeds actually increase nitrogen emissions."

Thus, guidelines for new ships seem to contribute little to reducing nitrogen emissions. How could this be improved? Riess suggests: "It would be better to adjust the average calculation to reflect real-world speeds. Emissions at lower speeds should carry more weight in determining nitrogen emissions for new ship engines. Another option is to set an upper limit regardless of speed, allowing assessments based on actual measurements."

International legislation under review

Van Vliet’s colleagues submitted the findings to the IMO. Based on this study and others from countries like Germany, Belgium, and Canada, the IMO decided to review the 2011 regulations in October. Van Vliet expects new regulations will come into effect in five years at the earliest. "Unfortunately, this doesn’t solve everything, as enforcement remains challenging. Fines must be imposed by the country where the ship is registered, and enforcement varies widely by country. However, if we and other countries can contribute better evidence, that’s a step forward."