It’s nearly impossible to escape air pollution in today’s industrialized world. Even far out at sea, where there are almost no other sources of pollution, massive ships burn through vast quantities of oil and diesel fuels. A new analysis shows how brown carbon (a general term for complex mixtures of light-absorbing organic molecules) in pollution from marine vessel engines operated on heavy fuel oil may potentially be a contributor to climate change, particularly in regions such as the Arctic, where the use of these fuels is common and where shipping activities are expected to significantly increase in the near future.
Marine engine exhaust contains black carbon: sooty, black particles produced by the incomplete combustion of fossil fuels. This particulate exhaust absorbs sunlight directly as aerosol and after deposition on snow and ice as well, such as in the Arctic. Depending on the types of fuels used, such exhaust can also be rich in brown carbon, which has also recently been recognized as an important contributor to climate impacts of aerosols.
To better understand how air pollution from ships affects Earth’s atmosphere, Corbin et al. carefully analyzed emissions from a four-stroke, single-cylinder research ship engine at the University of Rostock in Germany. The engine was similar to those used on many smaller ships as a main power supply and on larger ships for extra power or as a backup.
The team ran the engine on several different types of fuel, including light fuels such as diesel and marine gasoline oil, and heavy fuel oil, a black, viscous by-product left over from distilling lighter, more transparent fuels. They measured the light absorption—and related properties—of pollutant particles in situ, as well as particles that were collected on a filter. They then combined a suite of instrumentation to retrieve the effective refractive index of brown carbon in the particles.
Whereas burning lighter distillate fuels produced no brown carbon, the team found that burning heavy fuel oil produced large quantities of particulates, especially brown carbon. The additional brown carbon increased the warming influence that heavy fuel oil exhaust would have over snow surfaces by 18% compared to that of the lighter fuels and by even larger amounts at the low engine loads used in icy waters—a major difference that needs to be considered in future climate models, the authors say. (Journal of Geophysical Research: Atmospheres, https://doi.org/10.1029/2017JD027818, 2018)
—Emily Underwood, Freelance Writer