The researchers found the most striking neighborhood differences in the levels of ultrafine particles, the tiniest airborne particles (less than 0.1 microns in diameter), but the neighborhoods showed many of the same trends with other "tailpipe" pollutants. In contrast, there were not significant differences among neighborhoods in the amount of "traditional" pollutants routinely measured by air agencies, such as fine particulate matter known as "PM2.5" (less than 2.5 microns in diameter).

But tailpipe emissions are not the only source of tiny particles, Paulson noted.

"In Boyle Heights, and to a lesser degree downtown, there are additional ultrafine particles that are not freshly released from vehicles but instead form in sunlight-driven smog processes and are ultimately the result of pollution blown in from upwind areas to the west," she said.

Air quality is also an environmental justice issue, with poorer neighborhoods more likely to face greater pollution, said Winer.

"Despite substantial improvements in regional air quality, cleaner air remains a highly desirable amenity in Southern California," he said.

The researchers also uncovered a noticeable drop in ultrafine particle concentrations between 2008 and 2011 in West Los Angeles, which may be attributable to the region's wealth.

"The decrease in ultrafine particles between 2008 and 2011 in the West Los Angeles area was dramatic," Choi said. "Affluent West Los Angeles experiences rapid turnover of the vehicle fleet, resulting in a higher proportion of newer vehicles with cleaner engines and better fuel efficiency."