An interesting news story about the measurement of air quality on cruise ships appeared recently. Specifically, it dealt with the concentration of ultrafine particulate (UFP) matter in the air on four cruise ships, measured by a researcher from Johns Hopkins University. UFP is invisible matter with diameters of around 100 nanometres (nm), which is about 1,000 times smaller than a human hair, and it is implicated in airway inflammation and effects on other organs in the human body. Being interested in air quality, I looked up the actual study report which you can also read here. Here is my take on the work and meaning…
Technically, the work was reasonably well done as far as I can see from the report. It would have been nice to have additional sampling points and more data on the wind speeds and directions at the time of sampling. However, they were doing the sampling somewhat surreptitiously, so that seemed to limit things somewhat.
The results are quite interesting but not entirely surprising. They found UFP concentrations on the outdoor decks on the stern of the ships to be significantly higher than the concentrations on the bow. Since the engine emissions are coming out of the stacks and flowing backwards over the stern, this makes sense. They also found that the concentrations were similar to other reports for UFP concentrations in urban environments like Beijing, Taipei, Santiago (Chile), and Antwerp (Belgium). There are around 15,000 to 30,000 particles in every cubic centimetre in these environments, according to their results and others.
Why is this? The report doesn’t go into details but it is to be expected that there will be some downwash from these stacks (funnels) as the emissions are carried away. Downwash is a common phenomenon in air emissions, and is created by the interaction between the wind and the stack and surroundings, as illustrated in this picture.
(This picture is for a building, but the same concepts apply for wind flowing around a ship’s stack.) As the picture illustrates, swirls of wind can pull emissions from the top of the building down to the ground level. For a ship, the upwind (left) side in this picture would be the bow, and the right side is towards the stern.
It’s also interesting to compare the stacks of an old, coal-burning ship (the Titanic) with those of a modern oil-burning internal combustion engine ship (the MS Amsterdam, which was one of the ships in the study). Here are a couple of pictures to compare:
The Titanic’s stacks are relatively much higher above the deck, compared to the Amsterdam. Coal-fired ships would have a lot of soot and ash flying out the stacks, so they would have to keep them fairly tall to avoid downwash depositing ash onto the passengers and crew. Modern ships don’t have ash emissions, but as the research paper shows they do emit fine and ultrafine particulates which are not visible. With the shorter stack, there is some downwash effect on the stern deck, apparently.
Although the research shows that the air quality is not very good, we must remember that most people don’t hang around the stern decks 24/7, so it’s not really the same as living in a heavily polluted urban environment. When considering toxicology of air pollution, both concentration and time of exposure are important variables. Here the concentration is high, but the time is fairly short.
An interesting example of several topics that engineers touch on when dealing with air quality, air pollution control, fluid mechanics and contaminant transport.