Satellite used to detect marine pollution from individual ships

Recently and, for the first time, scientists were able to detect nitrogen dioxide (NO2) plumes from individual ships from space, by using data from the Copernicus Sentinel-5P satellite.

Since the beginning of 2020, the maximum sulfur oxide (mostly SO2) content of ship fuels was globally reduced to 0.5% (down from 3.5%) and to less than 0.01% inside ECAs (Emission Control Area) in an effort to reduce air pollution. The IMO also requires that ships constructed after 1999 use engines that are compliant with NOx (NOx=NO+ NO2) emission limits (IMO MARPOL ANNEX VI Reg 13, 2020).

Monitoring emissions is not an easy task. Local governments currently use drones or gas analyzer sensors located under bridges so that they can measure the emissions as close as possible to the ship’s smokestack. But what happens when ships are far away on open seas? In order to help with that, satellites, such as the Copernicus Sentinel-5P satellite, come in handy.

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

Scientists 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.

Satellite algorithms tend to mistake such bright surfaces for cloudiness, which is why, for a long time, sun glint was considered a nuisance in satellite measurements. Differentiating clouds from other bright reflective surfaces such as snow, clouds or even sun glint over the ocean surface has proven difficult—until now.

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. If the height of the cloud is found to be sufficiently close to the surface, it can be considered either snow or ice, rather than cloud coverage.

When applying the same method for sun glint over oceans, the team was able to easily identify and attribute emissions from individual ships in daily Sentinel-5P measurements.

These new results demonstrate exciting possibilities for the monitoring of ship emissions in support of environmental regulation from space. For now, however, only the largest ships, or multiple ships traveling in convoy, are visible in the satellite measurements.

Claus Zehner, ESA’s Sentinel-5P Mission Manager, commented, “We think that these new results demonstrate exciting possibilities for the monitoring of ship emissions in support of environmental regulation from space. Future planned satellite missions with improved spatial resolution, for example, the Copernicus Anthropogenic Carbon Dioxide Monitoring satellites, should allow for the better characterization of nitrogen dioxide ship emission plumes and, possibly, detection of smaller ship plumes.