Swooping above giant freight ships off the Belgium coast, a small twin-propeller plane picks up traces of the vessel's emissions.
The coastguard aircraft is checking and analysing the sulfur and nitrogen levels with an atmospheric pollution sensor called a "sniffer" developed by researchers at a Swedish University.
"In the plane, we have installed a sensor called a sniffer. With this sniffer, we can monitor the emissions from ships. In fact, we monitor two regulations on ship emissions: one for SOx, sulfur, and one for NOx, nitrogen," said Ward Von Roy, 35, an Aerial Surveillance Operator for the Royal Belgian Institute of Natural Sciences.
"It's very efficient because we can monitor up to ten to fifteen ships per hour. To compare, an inspector in the port can only do one ship per day. So we can do many more ships than any other system."
If the levels are too high, the ship is tested again in port and could expect a fine of around three hundred thousand euros.
The ship's age is a factor in determining acceptable levels of nitrogen oxides.
Around 15 percent of all sulfur and nitrogen oxide pollution globally comes from ships according to experts.
The stretch of water patrolled by the Belgian coast guard between the Channel and the North Sea is one of Europe's busiest shipping routes.