Slinging Dirt on the High Seas
Diesel-powered cargo ships are among the most serious air polluters. Taken together, they emit almost twice as much nitrogen as the whole of Germany and blow more than one and a half times the amount of sulfur compounds into the air. Emissions from cargo ships explain certain spatial patterns in global air pollution, say two Carnegie Mellon University researchers, James Corbett and Paul Fischbeck (Science 31 October). So far, sources on the mainland have been held responsible for the pollutants. According to the authors, the International Maritime Organization (IMO) will soon issue a multinational regulation that addresses this problem. However, a new regulation for the emission of nitrogen oxides will only apply to new ships or major ship conversions from 1 January 2020. January 2000.
Although the calculations were made using rather conservative assumptions, the authors conclude that "worldwide nitrogen emissions from ships account for nearly half of total United States emissions, 42 percent of North American nitrogen emissions, 74 percent of emissions European OECD countries (Organization for Economic Cooperation and Development) and 190 percent of nitrogen emissions in Germany.”
They add that “shipping sulfur emissions account for 43 percent of total United States sulfur emissions, 35 percent of North America sulfur emissions, 53 percent of European OECD emissions, and 178 percent of Germany sulfur emissions. Most land-based sulfur emissions come from stationary sources.”
According to Bryan Wood-Thomas of the U. S. Environmental Protection Agency (EPA) “The research conducted by Corbett and Fischbeck is the most thorough investigation of the sulfur oxides and nitrogen oxides emissions caused by international shipping. While many studies have modeled emissions at local and regional scales, this study provides an accurate analysis and estimate of emissions on a global scale.” The scientists believe that these emissions have an impact on the level of environmental pollution worldwide. For example, the air over the North Pacific is more polluted than that over the South Pacific.
Corbett and Fischbeck point out that the concentration of pollutants such as sulphur, ash-asph alts and metals in the residues (the fuels of ships) has increased in recent decades, particularly since the 1973 oil crisis. Manufacture fuels from residues using secondary refining technologies in order to extract the maximum amount of refined products (distillates) from crude oil. Other types of marine fuel are higher quality distillate oils, but these are often mixed with residues. Seventy to eighty percent of merchant shipowners prefer the cheaper residue fuels.
Data from three reputable sources were used to estimate global ship emissions: test data on ship exhaust including fuel-dependent emission rates for sulfur and nitrogen, information on international use of ship fuel, and the engine power of world-registered merchant ships weighing 100 gross tons or more, and the engine power of naval vessels.
In September, the IMO passed global emission limit values. These will ultimately lead to a multinational agreement on the principles of monitoring ship emissions. However, the authors report that "a measurable decrease in nitrogen emissions will be many years away," assuming that 1.5 percent of the fleet is replaced each year. “Also,” they add, “the IMO limits the sulfur content in fuel to 4.5 percent. This leads to only a small reduction, if any, and practically does not change the status quo at all, since the International Organization for Standardization (ISO) limited the sulfur content in fuel to 5 percent in 1987.”
Overall, air emissions of this type may well play a role in global climate change.
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