For a long time, Los Angeles was synonymous with eye-watering, lung-blackening smog. This have gotten better over the years, though the air quality in the city is nowhere near ideal. Scientists who look at the various chemicals floating around during bad air-quality days have pinpointed the reduction in peroxyacetyl nitrate (PAN), a "secondary pollutant present in photochemical smog", to explain why LA's pollution isn't quite as irritant to the eyes as before. There's also been a reduction in ground-level ozone - another common pollutant:
Both ozone and PAN are major components of smog in Los Angeles, [researcher Ilana Pollack] said. PAN is formed in a series of reactions that involve compounds found in sources like tailpipe emissions, sunlight and molecules with different combinations of nitrogen and oxygen. PAN serves as a store of the nitrogen-oxygen compounds that can be transported over long distances. [...]
Los Angeles is particularly vulnerable to the effects of ozone because it lies in a basin, Pollack said."Precursor emissions and the secondary pollutants formed from them often get trapped in the 'bowl-like' basin of air that is created by the surrounding mountains" (source)
Most of the progress can be attributed to stricter emission regulations for vehicles, though they remain the primary source of air pollution in the city. The reduction is even more impressive when you consider that since the 1960s, the number of vehicles in LA has grown about 3x. Can you imagine how bad things would be if all regulations that forced emission improvements had been blocked and cars and trucks still spewed as much pollutants as back in the era of bell bottom pants (and that time was better than the 1940s...)?
Further improvements in emission controls for cars and truck can help, as will the transition to plug-in vehicles and a cleaner power grid, but improving transit in the city and making it more bikable should also be priorities, as a lot of vehicle trips are unnecessary and should be eliminated altogether.