Laser sizes up dispersed oil.

When tankers or off-shore wells unleash thousands of barrels worth of oil at sea cleanup crews immediately look for help from chemical dispersants - agents that work together with natural wave action to break large bodies of oil into droplets that resist coalescing. While dispersants do not remove oil from the water, they do foster a dilution that can minimize hydrocarbons' toxicity to aquatic life.

Until recently, scientists studying the dispersal of oil droplets were limited to laboratory experiments. Chemists were unsure exactly how oil and water mix at sea, with or without chemical dispersants, asserts oceanographer Tim Lunel of Warren Spring Laboratory in Stevenage, England. Now, Lunel and his co-workers have developed a laser device that not only distinguishes oil droplets from other particles below an oil slick on the basis of features such as symmetry and refractive index, but also gauges the size of those droplets.

The researchers tested their "phase Doppler particle analyzer" on crude oil experimentally released in the North Sea. The results provide "the first measurements of oil droplet sizes at sea," Lunel says. One major surprise was the small size of the dispersed oil particles, which averaged only about 20 microns in diameter. "There had been expectations that particle diameters would vary more," particularly on the basis of the type of oil spilled or the relative effectiveness of the dispersants used, Lunel says. But neither those factors nor winds, which largely determine wave turbulence, played a major role in the size of dispersed oil droplets. Again and again, Lunel's team found, roughly 90 percent of dispersed oil breaks into droplets smaller than 45 microns -- accounting for half of the dispersed-oil volume - and 99 percent breaks into droplets smaller than 70 microns.

The investigators also compared spills of medium fuel oil (MFO) treated with a highly efficient dispersant called Slick-gone NS to spills treated with a less efficient dispersant called 1100X. These experiments indicated that dispersants affect only the amount of oil sheared into small particles (see diagram), not the proportion of sheared particles that end up small.

Plugging these data into computer programs that model oil spills should yield "slightly different predictions of the shape of slicks and very different predictions of the amounts of oil that will be dispersed," Lunel says.