Tracking maritime pollution from space

Maritime transport has a direct impact on air quality in many coastal cities. Now, new research is helping track emissions from individual ships in an effort to better monitor air pollution.

Commercial ships and vessels burn fuel for energy and emit several types of air pollution as a by-product, causing the degradation of air quality. A past study estimated that shipping emissions are globally responsible for around 400 000 premature deaths from lung cancer and cardiovascular disease, and 14 million childhood asthma cases each year, according to a statement from the European Space Agency.

Since January 2020, the maximum sulphur dioxide content of ship fuels was globally reduced to 0.5% (down from 3.5%) in an effort to reduce air pollution and to protect health and the environment. It is expected that the nitrogen dioxide emissions from shipping will also become restricted during the coming years.

Until recently, satellite measurements needed to be aggregated and averaged over months or even years to discover shipping lanes, limiting the use of satellite data for regulation control and enforcement. Only the combined effect of all ships could be seen, and only along the busiest shipping lanes.

Researchers have now discovered patterns in previously unused ‘sun glint’ satellite data over the ocean that strongly resemble ship emission plumes. Sun glint occurs when sunlight reflects off the surface of the ocean at the same angle that a satellite sensor views it. As water surfaces are irregular and uneven, the sunlight is scattered in different directions, leaving blurry streaks of light in the data.

In a study published last year, scientists were able to differentiate snow and ice from clouds by measuring the height of the cloud and comparing it with the surface elevation. When applying the same method for sun glint over oceans, the team were able to easily identify and attribute emissions from individual ships in daily Sentinel-5P measurements.

“For now, only the largest ships, or multiple ships travelling in convoy, are visible in the satellite measurements,” added Jos de Laat, from KNMI, one of the research partners. “Ship tracks from small ships never aligned with these emission plume structures, unless their tracks crossed the track of larger ships or large shipping lanes, or a small ship travelled in a busy shipping lane.”

Photo: watts_photos is licensed under CC BY 2.0, Flickr Creative Commons

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