Rare earths: oil's signature in the air.

Tracing pollutants back to the oil refineries and oil-fired power plants from which they spewed should become less of a forensic puzzle now that trackers know what to look for. Chemists at the University of Maryland in College Park have identified a distinctive metallic signature for these emissions in air.

Because rare-earth oxides contained in most crude-oil products resist chemical transformation, pollutant plumes can carry a recognizable oil signature over long distances. For example, the University of Maryland's Glen Gordon notes, the rare-earth signature should permit the identification of air masses coming from areas that have a high density of oil refineries -- like Houston and the Texas Gulf Coast. On a regional scale, he says, "we can use these tracers to associate air masses that have high acid-rain content with their source."

Most products refined from crude oil have been subjected to caalysts made from synthetic zeolites. Formed from a mix of oxides and rare earths, these zeolites apparently impart to oil products a telltale sign of their encounter--unusual proportions of such rare-earth metals as lanthanum, neodymium, samarium ytterbium, lutetium and vanadium.

The proportion of one rare-earth element to another in any sample of dirt or rock tends to be fairly constant, explains Gordon. However, in bastnasite and monazite -- minerals that serve as the two major commercial rare-earth sources -- the relative ratios of rare-earth metals are dramatically skewed from that otherwise constant pattern, which geochemists call the "crustal abundance."

For example, the ratio of lanthanum to samarium in dirt is usually about 5 to 1. However, among particles 2.5 microns in diameter or smaller emitted from the smokestacks of oil refineries and oil-fired power plants, that ratio is typically between 20 to 1 and 41 to 1, Gordon says. The difference appears to represent the influence of the zeolite catalysts, according to Gordon and Ilhan Olmez, writing in the Sept. 6 SCIENCE.

Arsenic and selenium provide similar tracers for coal-derived pollutants. Still missing, Gordon says, are comparable signatures for carbonaceous polluters, such as diesel engines and wood stoves. The goal, he says, is to identify what proportion of the pollution mix in any setting is attributable to particular sources--even distant ones.