Satellites can see traces of air pollution from particular person ships

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All hands must be on deck if the world is to fight the deterioration, and one of the biggest emitters is also one of the least known – international shipping. A 2018 study estimated that cargo ship pollution resulted in 400,000 premature deaths from lung cancer and heart disease annually. Many of these deaths were due to the sulfur dioxide the ships belched into the air. Since the beginning of the year, sulfur dioxide has been limited to 0.5% of emissions, compared to 3.5% previously. While the long-term benefits of this emissions cap will take some time, there is another pollutant that could potentially be addressed in the near future: nitrogen dioxide.

Nitrogen dioxide (NO2) is one of the emissions from diesel engines and has been strictly limited in the automotive market for several years. While the shipping industry has so far escaped regulation, there is a high likelihood that restrictions will appear in the near future. Regulations in themselves are great, but they're useless if not enforced, and the open ocean is a notoriously difficult place to enforce. This daunting task may have just gotten easier when scientists from the European Space Agency realized they could use satellite data they had already collected to track nitrogen dioxide emissions from individual ships in the open ocean.

The satellite used by the scientists is called Copernicus Sentinel-5P and is mainly used to monitor air pollution. The company, which started in 2017, has, for example, monitored nitrous oxide emissions from Siberian gas pipelines and China's industrial cities. However, this is the first time it has turned its attention to the open ocean.

One reason for this is a particular difficulty in monitoring the ocean from space – sunlight. If you've been to an ocean or lake where a trail of sunlight goes straight to the sun, that's sunlight. In satellite images, this phenomenon shows up as the lightening of the water, which means that measurements of interesting atmospheric data points such as cloud cover and ship emissions are lost.

Example of sunlight from the perspective of an individual.
Image credit: ESA

Recently observers developed a way to solve this problem by correlating the image with height calculations. Originally, this technique was used to detect snow and ice, but the team modified it to easily differentiate ship emissions from both clouds and sun glare.

Satellite image showing sunlight from the perspective of a satellite.Example of sunlight from the perspective of a satellite – especially in the Mediterranean area.
Image credit: ESA

To do this, they need ship location data with which they can correlate their observations. While ships must use location transponders in the open ocean, some, including those trying to avoid emissions controls, can simply turn off their transponders. This lack of transparency has deeper roots than satellite technology can address, but could potentially pose a problem for emissions levels.

Data mapping shows ships traveling on their nitrogen dioxide emissions.Nitrogen dioxide concentrations in the Mediterranean area, including the representation of individual ship routes.
Image credit: ESA

Another potential problem is that while the satellites can track individual ships, they can only track the largest ships or convoys of smaller ships. Smaller ships can still evade detection due to their size. The researchers hope to overcome this technical challenge with future launches of more powerful satellites such as the Copernicus Anthropogenic Carbon Dioxide Monitoring Satellites. Developing these satellites and analyzing their data also requires hands on deck. However, this is another example of how space technology can help solve practical problems here on earth.

Learn more:
ESA – Detection of the pollution of individual ships from space
Ship technology – tracking and tracking polluting ships
NASA – A satellite's view of ship pollution

Lead Image Credit:
Nitrogen dioxide emissions over the Mediterranean
Image credit: ESA

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