Ships pollute half as much as world’s cars

Since most shipping traffic takes place close to the coastline, ship emissions are a significant health concern.

Globally, commercial ships emit almost half as much particulate pollution into the air as the total amount released by cars, according to a new study1. Ship pollutants affect the Earth’s climate, ecosystems, and the health of people.
   Based on direct measurements of emissions, it is estimated that worldwide, ships emit 900,000 tonnes of particulate matter (PM) pollution each year. Shipping also contributes up to 30 per cent of smogforming nitrogen oxides (NOx), and 5-8 per cent of global man-made sulphur dioxide (SO2) emissions.
  “Since more than 70 percent of shipping traffic takes place within 250 miles (400 kilometres) of the coastline, this is a significant health concern,” says lead author Daniel Lack, a researcher at the National Oceanic and Atmospheric Administration (NOAA) in Boulder, Colorado.
   Lack and colleagues have analyzed the exhaust from over 200 commercial vessels, including cargo ships, tankers and cruise ships. They have also examined the chemistry of particles in ship exhaust gases.

Ships emit sulphates − the same polluting particles associated with diesel-engine cars and trucks that prompted improvements in on-road vehicle fuel standards. Sulphate emissions from ships vary with the concentration of sulphur in ship fuel. Globally, ship fuel sulphur content is regulated under the International Maritime Organisation (IMO). Some ships use cleaner low-sulphur fuel oils, while most others continue to use high-sulphur heavy fuel oils.
   According to the new study, sulphates make up just under half (46%) of shipping’s total particle emissions. The other half is composed of organic particles (39%) and sooty, black carbon particles (15%).
   Both the sulphate particles and the organic particles appear to be linearly correlated with fuel sulphur content, Ships pollute half as much as world’s cars Jeff Clow /Fotol ia and fuels with higher sulphur contents produce more small particles than fuels containing less sulphur.
   An earlier study by Lack’s team found that emissions of the non-sulphate particles depend on the operating speed of the engine and the amount of lubricating oil needed to deal with wear and tear from burning less-refined fuels.

One surprising result of switching to low-sulphur fuels is that, although total particle emissions diminish significantly, the time that remaining emitted particles spend in the air appears to increase. The organic and black carbon particles are less likely to form cloud droplets. As a result, these particles remain suspended for longer periods of time before being washed to the ground through precipitation.

1 Particulate emissions from commercial shipping: Chemical, physical, and optical properties. By Lack, D. A., et al. (February 2009). Journal of Geophysical Research, 114, D00F04, doi:10.1029/2008JD011300

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