The city's infamous brown haze is created by sunlight triggering organic compounds and nitrogen oxides from car exhaust to form ozone and particulates, which build up as mountains surrounding the region trap and hold the pollution.
After the smog problem peaked in the 1950s, regulations to reduce smog formation by cutting emissions were introduced and were largely effective, Carsten Warneke of the National Atmospheric and Oceanographic Administration's Earth Systems Research Laboratory in Boulder, Colo., said.
However, while emissions of volatile hydrocarbons are down by a factor of 50 from the 1960s, the nitrogen oxides that also contribute to the formation of ozone and particulates have not seen as dramatic a decrease, officials said.
Peak levels of ozone were still 143 parts per billion in 2010, down from 710 ppb in 1966.
"Things have improved greatly in the last 20 to 30 years, but progress has slowed in the last decade," Sam Atwood, a spokesman for the South Coast Air Quality Management District, told NewScientist.com
Ozone levels were reported to be above federal maximums on 102 days in 2010, and those levels remain the worst in the United States, the American Lung Association said.
The findings were reported in the Journal of Geophysical Research: Atmospheres.