The Nitty-Gritty Details of Air Pollution

Hoping to clear up one of the murky mysteries of dirty urban air, scientists have devised a mathematical model to predict how hydrocarbons from gasoline are transformed into tiny, and potentially unhealthy, particles. The model, described in tomorrow's issue of Science*, should help regulators get a better handle on how to reduce this public health threat.

Prompted by population studies linking fine particles to thousands of premature deaths from respiratory illnesses each year, the Environmental Protection Agency (EPA) hopes this summer to put forth its first-ever limits for very fine particulate matter in air, called PM2.5. Most people think of particulate pollution as soot from smokestacks and diesel buses. But up to 20% of it can come from hydrocarbon vapors, mainly from evaporated and partially burned gasoline, solvents used in paints and manufacturing, and natural sources like trees. In sunlight, these gases react and condense onto bits of dust to form very fine particles.

A team led by atmospheric chemist John Seinfeld at the California Institute of Technology (Caltech) in Pasadena devised a model that is the first to predict accurately how this kind of airborne particle forms. In tests with 12 kinds of gasoline in a smog chamber--a gigantic Teflon bag the size of a two-car garage--the team showed that a gasoline's aromatic content determined whether it formed particles, and that the contributions of the myriad different aromatics in the gasolines were additive. "This is a clear breakthrough in our understanding," says Steven Japar, an atmospheric chemist at Ford Motor Co.

Removing aromatics from gasoline helps cuts down on ground-level ozone, a big component of smog. EPA plans to regulate ozone more tightly along with fine particles, and being able to describe accurately the links between gasoline and the formation of both these pollutants will be of great help in developing cost-effective control strategies, says EPA chemist John Bachmann. "These are the kinds of advances we really need," he says. Japar cautions, however, that the Caltech group still must show that its model will work with the whole gamut of hydrocarbons in urban air--especially because in many eastern cities, gases from trees are the biggest source of particle-forming hydrocarbons.

* For more details, Science Online subscribers can link to the full text of the Report.