Up in smoke

Chemists at UC San Diego have measured the impact that dirty smoke from ships cruising at sea and generating electricity in port can have on the air quality of coastal cities.

The scientists say that the impact of dirty smoke from ships burning high-sulphur fuel can be substantial, on some days accounting for almost half of the fine, sulphur-rich particulate matter in the air known to be hazardous to human health.

Most of the sulphur emitted by ships burning a cheap, sulphur-rich fuel called ’bunker oil’ is released as sulphur dioxide, or SO₂, a gaseous pollutant which is eventually converted to sulphate in the atmosphere.

But SO₄ is also released, and although it may represent a fraction of the total emissions from a ship, these so-called primary sulphate particulates are particularly harmful to humans, because they are especially fine - less than 1.5 microns in size. As a result, they can travel extremely long distances because they stay in the atmosphere for longer periods. Worse still, unlike bigger dust grains and particles that are removed by the body when inhaled, they remain in the lungs.

Sampling air at the end of the pier at the UC San Diego’s Scripps Institution of Oceanography in La Jolla, California, Mark Thiemens, a professor of chemistry and biochemistry at UCSD, and Gerardo Dominguez, a postdoctoral researcher, found that the smoke from ships contributed as much as 44 per cent of the sulphate found in fine particulate matter in the atmosphere of coastal California.

The researchers said the chemical fingerprinting techniques they developed to actually measure the primary sulphur emissions from ships should assist the California Air Resources Board as well as regulators in other states and countries monitor the impact of ships off their coasts.

Their results have particular significance for the state of California, which, from next July, will require that all tankers, cargo and cruise ships sailing into a California port switch to more expensive, cleaner-burning fuels when they come within 24 miles of the coast. Similar international rules requiring clean-burning ship fuels are set to take effect in 2015.

Thiemens said: ’This will tell us whether California’s new regulation requiring cleaner burning fuel 24 miles off the coast is having the effect it is intended to have.’