The research is a collaboration among researchers from San Diego State University, the University of California Irvine and the University of Utah. They drove specially equipped vans around each region, drawing in air and simultaneously analyzing it for methane.

The levels near La Jolla were elevated, but even higher were levels of the gas found near the Otay landfill in Chula Vista, with a plume leading west.

The flow of air west from the landfills may surprise Southern Californians accustomed to a prevailing breeze from the ocean. But in the cool of the night, the scientists say, that familiar ocean breeze actually reverses. Later, after the sun rises, it reasserts itself.

When he was gathering his air samples in the Ford Transit Connect van, Lai drove most of his research runs at night, the time he said when the air is calm.