2009 News & Events

A new study published in February in the Journal of Geophysical Research by CSD and CIRES scientist Dan Lack and colleagues finds that globally, commercial ships emit as much particulate matter (PM) pollutants into the air as nearly half the cars on the road. Their study is the first to provide a global estimate of shipping's total contribution to air particle pollution based on direct measurements of emissions. An earlier (2008) study by Lack and colleagues focused exclusively on soot.

Since more than 70% of shipping traffic takes place within 250 miles of the coastline, this is a significant health concern for coastal communities, notes Lack. These particles also affect climate. Particles from shipping have a global cooling effect and mask, by at least five times, the global warming resulting from the carbon dioxide from ships, which makes up roughly 3% of human-emitted carbon dioxide. Shipping also contributes almost 30% of smog-forming nitrogen oxide gases.

During the summer of 2006, Lack and colleagues, aboard the NOAA research vessel Ronald H. Brown, analyzed the exhaust from over 200 commercial shipping vessels – including cargo ships, tankers, and cruise ships – in the Gulf of Mexico, Galveston Bay, and the Houston Shipping Channel. They estimated that globally ships emit 0.9 teragrams, or about 2.2 million pounds, of particle pollution each year.

The researchers also examined the chemistry of particles in ship exhaust, in order to understand what makes ships such hefty polluters. They found that emissions of sulfates – the same particles associated with diesel-engine cars and trucks, which motivated improvements in on-road vehicle fuel standards – vary with the concentration of sulfur in ship fuel. Globally, fuel sulfur content is capped under the International Convention for the Prevention of Pollution From Ships. As a result of the cap, some ships use "cleaner," low-sulfur fuels, while others continue to use the high-sulfur counterparts.

Yet, sulfates make up just under half of shipping's total particle emissions, according to the study. Organic pollutants and sooty, black carbon, which make up the other half of emissions, are not directly targeted by today's regulations. Emissions of these particles, the study found, 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. Engines burning 'cleaner,' low-sulfur fuels tend to require less lubrication. So fuel regulations have an indirect effect on the organic particles emitted.

One surprising result of burning low-sulfur fuels: while total particle emissions shrink, the time the particles spend airborne – and pose a risk to human health and affect climate– appears to increase. Lack and colleagues found that the organic and black carbon portion of ship exhaust is 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.

Particles from shipping are thought to mask, by at least 5 times, the global warming by ship carbon dioxide, which makes up roughly 3% of human-emitted carbon dioxide. Shipping also contributes almost 30% of smog-forming nitrogen oxide gases.