A month or two ago, the South Coast Air Quality Management District began dispatching vans to freeway on-ramps – that is where motorists accelerate, their cars and trucks tossing out a good sampling of their emissions.

Using beams of light and mirrors, sensors can determine the amount of light absorbed through emissions, a setup that is "pretty accurate'' in figuring out if the emissions surpass legal limits, says Sam Atwood, an AQMD spokesman. "This is technology that has been around for 10, 15 years.''

The ADMD is after vehicles between smog checks that wouldn't pass; the vans are to get samples over the next year or so from 10 percent of the vehicles in Orange, Los Angeles, San Bernardino and Riverside counties – one million in all.

The owners of excessive emissions-passers get a letter (license plates are used to track down suspects) notifying them of their problem. And then they get offered a heck of a deal.

First, a free smog check. If the vehicle fails, the owner can receive up to $500 toward repairs, or up to $1,000 to scrap the vehicle ($2,000 for low-income residents).

The $4-million program is voluntary and might be extended after a year.

Atwood says each van will stay on a particular on-ramp for a week or two.

"We are not advertising the locations or schedule,'' he says.