Off Belgium's coast, a small twin-propeller plane swoops above giant freight ships, picking up traces of their exhausts and subjecting them to on-the-spot analyses of sulphur and nitrogen levels.

Ships risk a fine if the reading comes out too high.

The Coast Guard aircraft is fitted with an atmospheric pollution sensor that crews call the "sniffer."

Developed by researchers at a Swedish university, it is a point of pride for Belgian authorities.

"We're the only country to use this system at the moment," one of the aerial operators, Ward Van Roy, told AFP.

"It's really effective because we can check up to 10 to 15 ships per hour, whereas a port inspector has to spend a whole day on just one ship," said the young Flemish technician.

The device sits behind the cockpit of the light plane -- a 49-year-old British-made Britten-Norman Islander -- flown by two military pilots used to low-altitude operations.

A computer screen feeds the operator real-time data of the samples being measured.

The task of the "pollution-sniffer" is to see whether the nitrogen oxides and sulphur emitted by the sea vessels meet European norms.

The ship's age is a factor in determining acceptable levels of nitrogen oxides.

Meanwhile any aerial detection has to be followed up in port to test the ship's fuel before any fine can be levied.

Van Roy explained that the financial blow can be as much as 300,000 euros ($350,000), on top of the cost to the operating company if its ship is stuck in port.

That hefty risk may account for a decline in fines handed out over the past six years.

- Busy shipping route -

But there is also the fact that port authorities at the ships' destinations have to cooperate.

"Since 2015, out of 9,000 ships that were checked, around 400 were in violation, but only 150 of them had a Belgian port as their destination," Van Roy said.

The maritime corridor off Belgium's 65-kilometer (40-mile) stretch of coast is part of the busy shipping route between the North Sea and the Channel.

It is one of the world's busiest, plied by tankers, freighters, industrial trawlers and other vessels -- up to 400 per day, according to the Belgian Coast Guard, which sees a lot of traffic into and out of Europe's massive cargo ports of Rotterdam, Antwerp and Hamburg.

Clamping down on illegal levels of emissions is thus very important, Van Roy said, adding that around 15 percent of all sulphur and nitrogen oxide pollution worldwide comes from ships.

Beyond that specific task, the sniffer plane crew can spot fuel slicks and fishing violations, "or simply a container or a mussel cage lost at sea," said Benjamin Van Roozendael, from the Royal Belgian Institute of Natural Sciences which is a partner in the "sniffer" plane initiative.