The Underestimated Danger: Ship exhaust gases

Thick air cannot only be found in cities but also where we would not expect it at all: on rivers and seas.  An extremely harmful mixture of sulphur dioxide, nitrogen oxides and soot oozes from the chimneys of cruise ships and container ships.  This not only pollutes the air on coasts but also far inland.  
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The Helmholtz Centre regularly measure the pollution levels on the banks of the Elbe in Hamburg.  Every passing ship brings the sensitive instruments of the measuring station to the limit.  And this despite the fact that since 2015 ships on their way to the port of Hamburg have only been allowed to burn comparatively clean fuel.  It may contain a maximum of 0.1% sulphur.  Sounds clean, but it is still 100 times more than in normal car diesel.

The ocean liners then become dirt slingshots on the high seas. Because there they are allowed to burn the cheapest and most toxic fuel there.  The viscous mass contains high levels of sulphur, ash and heavy metals.  When it burns, all of this ends up in our atmosphere.
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Ship exhaust fumes do not only travel far inland - up to 500 kilometres inland.  They become even more dangerous there, as nitrogen oxides from ships' diesel engines react with air pollutants from agriculture and together they cause the formation of fine dust.

Catalysts or other propulsion technology, such as liquid gas, could help.

These measures could contribute significantly to the reduction of nitrogen oxides.

Cruise Passengers Also Travel Dangerously
Incidentally, the exhaust gases are a huge burden not only for the people living nearby but also for the people on the ships themselves.  A camera team took air samples on a cruise ship and had the quality measured.  The frightening result: the pollution with soot and other fine dust particles was four to ten times higher there than on a busy city crossing!

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Alternative LNG 

The ferry "MS Helgoland", for example, uses such liquid gas.  It is the first German seagoing vessel newbuilding ever to be powered by liquefied petroleum gas or LNG.  The natural gas, which is liquefied by extreme cooling, is slowly heated before combustion and then injected directly into the propulsion engines.  Compared to conventional fuels, LNG propulsion reduces nitrogen oxide emissions by around 80 %, and particulate matter emissions are virtually zero.  Once a week the ferry has to bunker fuel.  The comparatively small amount of frozen gas can still be delivered by tanker.  However, a comprehensive infrastructure for refuelling large container or cruise ships would first have to be established.