Large commercial vessels use poorly refined heavy fuel containing sulfur compounds. These boats release large amounts of sulfur dioxide SO2 which is a source of acidification for the environments (oceans, forests ...). Due to local regulations on the US coasts, the Baltic Sea, the North Sea and the English Channel, these vessels must switch locally to a more refined fuel with a limited content of sulfur compounds, similar to the diesel fuel. Elsewhere they can use this heavy fuel, even in the Mediterranean where there is a lot of traffic.
In addition to SO2, these ships produce a lot of nitrogen oxides, such as NO2, to the point that coastal areas may become highly polluted areas. One can imagine pollution in cities with car traffic or near certain industrial sites. But at sea, people think that the air is clean, which is not the case everywhere.
The Windy maps that present these pollutants, SO2 and recently NO2, speak for themselves: they make it possible to highlight and monitor this pollution along the trade routes of the Atlantic, the Mediterranean and the Indian Ocean ... The comparison of these traces of pollutants at sea with the main shipping routes shows an obvious correlation.
However, these big boats remain the least expensive and even the least polluting means per ton of freight transported. If it were necessary to transport the same quantities of goods by truck, plane or even diesel train, the pollution would be worse!