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Cleaning Up Commercial Shipping to reduce Air Pollution

In an attempt to reduce air pollution from commercial shipping, the state of California now requires ships to switch to cleaner burning fuels as they near the coastline.

When container ships reach California's busy ports, they spew pollutants such as sulfur dioxide and particulate matter into the air over nearby cities. These compounds contribute to respiratory illness in humans. To reduce these emissions, California limits the sulfur content of marine gas oil to 1.5% within 44.5 km of the state's coast. Fuels with higher sulfur contents produce more sulfur dioxide and particulate matter during combustion.

Image of Air pollution by hipping

Daniel Lack, an atmospheric chemist at the National Oceanic and Atmospheric Administration's Earth System Research Laboratory in Boulder, Colo., wanted to see if California's regulations were actually improving air quality. "Because low-sulfur fuel is costlier than high-sulfur fuel, the shipping industry wants to know that California's regulations are having the effect the regulators said they would have," Lack says. So he and his colleagues focused on a single container vessel that switched from a fuel containing 3.15% sulfur to one containing 0.07% sulfur as the ship neared the Port of Los Angeles. The container vessel also participated in a voluntary state slowdown program intended to further reduce air pollution, slowing from 22 knots to 12 knots as it approached the coast.

The researchers used an airplane and a ship to collect air and particle samples from the vessel's exhaust plume before, during, and after the 60-minute fuel switch. When they measured air pollutants in the samples using mass spectrometry and other techniques, they found that the levels of several pollutants, including sulfur dioxide, sulfate, and particulate matter, plummeted by at least 88% over the regulated waters. Both the fuel switch and slowdown contributed to these effects, Lack says.

In addition, Lack and his colleagues observed a 75% decrease in black carbon after the fuel switch, a compound that they hadn't expected to change significantly. "We were able to show that these regulations may have an even better effect on human health than intended," Lack says. The study has important global implications, he says, because a United Nations ban on the use of high-sulfur fuel near coastlines will take effect in 2012.

The combination of low-sulfur fuel and slower vessel speed also decreased by 99% the emission of cloud condensation nuclei, small particle pollutants that promote cloud formation and contribute to climate cooling. But James Corbett, an expert on marine policy at the University of Delaware, points out that the benefits of improved air quality will likely overshadow any small-scale, negative climate effects caused by fewer cloud condensation nuclei. "The global implications for climate change are likely to be dominated by the fuel that's used during the many days of voyage between ports, not by the few hours that ships are within regulated waters," he says.

Corbett adds that this detailed study of a single vessel is "the first of a number of studies that need to be done for other vessel types, locations, and operating conditions" to determine the full impact of the regulations.

Reference: Laura Cassiday, www.pubs.acs.org