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New surveys confirm that large amounts of nitrogen dioxide are being emitted by shipping into the atmosphere and that it is generally more persistent than had previously been thought. It also shows how the effects of NO2 generated by ships can combine with high local-concentrations of the gas in and around metropolitan port areas.

The new information comes from a satellite called Sentinel-5P that was launched last October. It can “see” the trail of fumes left behind by each ship and also how these trails combine in busy shipping routes to create a dense and persistent cloud of this, and other pollutants.

The photo below shows a scan of the Straits of Gibraltar with the land masses of Spain and North Africa (Morocco) in pink. The lines of NO2 clearly show the shipping lane, along which only some 20 ships are present. Given that a medium-sized tanker can burn between 75 and 120 tonnes of bunker fuel a day, the extent of the problem does not come as a total surprise and the shipping industry has been trying to reduce the amounts of noxious gas emissions for some time. Apart from anything else, a reduction in fuel-consumption can make a lot of difference to the overall profitability of a transit.

Image copyright: ESA/NASA – SOHO/LASCO

It begs the question as to what is going on in an area closer to home, such as the Straits of Dover where, each day, some 400 ships pass though the approximately 20-mile-wide passage between Dover and Calais. Local concentrations of pollutants such as NO2 and CO2 can peak as a result of a combination of land shapes, weather (especially wind patterns), as well as thermal or convection currents that are created by solar heating as well as large urban areas.

In the latter case, a “doughnut system” can be created in which pollutants are lifted up and away from cities to circle back down to the ground some 50 or more miles away, creating a large doughnut-shaped air system. Where the downward cycle of air meets slopes that face the city, the concentrations can exceed anything experienced in the urban areas themselves. This can, for example, be the case at the foot of the South Downs in Sussex where levels of some London-sourced pollutants can be dangerously high, and especially for the very young and old. Port areas are less likely to suffer from the doughnut-effect given that they are open on one side to the sea. However, where a port is situated on an inlet, the same effects may be felt to an extent.

As well as offering unprecedented detail, the Sentinel-5P mission has a swath width of 2600 km, which allows the whole planet to be mapped every 24 hours. Josef Aschbacher, ESA’s Director of Earth Observation Programmes, said:

“Sentinel-5P is the sixth satellite for the EC Copernicus environmental monitoring programme but the first dedicated to monitoring our atmosphere. These first images offer a tantalising glimpse of what’s in store and are not only an important milestone for the Sentinel-5P mission, but also an important milestone for Europe. Data such as we see here will soon underpin the Copernicus Atmosphere Monitoring Service, and will be used to issue forecasts, and will ultimately be valuable for helping to put appropriate mitigation policies in place.”

Sentinel-5P carries the most advanced sensor of its type to date: Tropomi. This state-of-the-art instrument can map pollutants such as nitrogen dioxide, methane, carbon monoxide and aerosols, all of which affect the air we breathe and our climate.

From a defence viewpoint, it will be interesting to see how this might impact warship signature-management considerations. Current efforts cover a range of signatures, with the emphasis being upon reducing radar cross-sections and thermal outputs. The latter includes cooling the plume of a warship’s exhaust gases and related superstructure such as funnels etc.

However, TMT is not currently aware of consideration being given to the reduction or elimination of tell-tale gases such as NO2 which might, in time, become a practical method of tracking fleet movements, or even for real-time location of individual vessels. This will be an especially interesting technology if a “finger-print” profile for specific warship engines can be developed.


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