As I understand it there are two key kinds of air pollution: ozone and fine particles.
Are there any long-term data series that chart average ozone levels and/or fine particulates over time going back to 1970?
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The air quality in Los Angeles does not meet federal air quality standards. That being said, the air quality has improved immensely since the 1970s.
From the California Air Resource Board, in 1965 the maximum one-hour ozone concentration was 0.58 ppm. Since then, that number has come down to about 0.12 ppm. You can obtain the latest ozone data from EPA. See a 30 year ozone trend image below from the South Coast Air Quality Management District:
Particulate matter is a bit tougher to quantify. PM10 (coarse and fine particles together) has been tracked since the late 80s, but PM2.5 (fine particles) has only been tracked since 1999. You can read a bit about it here.
The reduction in particulate matter has not been as dramatic as ozone. This makes sense, though, since ozone is a secondary pollutant whose precursors (NOx and VOCs) largely come from vehicle tailpipes. NOx and VOC emissions from vehicles has decreased significantly in recent years, especially with fleet turnover. In contrast, coarse and fine particulate matter come from many different sources, e.g. road dust, agriculture, cooking, etc. which is difficult to control since rain is the major control method.
A figure downloaded from this site.
It represent the history trends of the pollutants related to the photochemical pollution from 1960s to 2000s
We can find that $O_3$, $PAN$, $VOCs$, $NO_x$ have all declined. And O3 variation trends were strongly correlated to $NO_x$ which can be explained by the VOC-NOx chemistry.
Another material I have searched out was a class lecture downloaded here.
It introduced the contents below:
- Air Pollution Regulation: Pre Clean Air Act 1970 - Air Pollution: Definition, Concentration, Standards - Clean Air Act 1970 - Local Agencies (California & Bay Area)
The lecture has depict the air pollution trends and regulation situation from 1950s till 2000s.
May it help!