The thick smog that often hovers over Los Angeles certainly looks like an ashtray in the sky and, scientists say, to some extent it is just that. Cigarette smoke accounts for 1 to 1.3 percent of the fine particle mass in the atmosphere there, researchers have found.

The report on cigarette smoke, published in the July issue of the journal Environmental Science and Technology, was prepared by a team of scientists headed by Dr. Glen R. Cass, an environmental engineer at the California Institute of Technology in Pasadena.To find out just how much pollution smokers produce, the researchers had to find a tracer for cigarette smoke that would stand out among a vast number of emission sources.

The tracer had to be a particle chemically stable enough to survive from its source to test sites in the atmosphere. This disqualified nicotine, a common tobacco-smoke tracer for indoor studies, because it tends to break down between the gas and particle phases.

Of more than 100 chemicals found in cigarette smoke, the researchers decided to use iso-alkanes and anteisoalkanes, hydrocarbon compounds found in the surface waxes of tobacco leaves.

"About one-third of the particulate material in the atmosphere is organic," said Dr. Lynn M. Hildemann, a civil engineering professor at Stanford University. "We're interested in seeing where it is coming from as the groundwork for control strategies."

Motor vehicle exhaust causes substantially more pollution than cigarettes. "But it is kind of amazing when you think of how small cigarettes are compared to fireplaces or tailpipe emissions," Hildemann said.

The group's previous studies looked at charbroilers, automobiles, road dust, vegetation and home appliances fueled by natural gas as sources of air pollution.