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L.A. smog still a threat

The infamous Los Angeles smog may have thinned in the last 50 years, but not enough to quash surface ozone levels, which remain the worst in the U.S.
Emissions regulations introduced in the 1960s have paid off, says Carsten Warneke, of the NOAA Earth Systems Research Laboratory, Boulder, Colo. Typical emissions of volatile hydrocarbons are down by a factor of 50 from the 1960s (Journal of Geophysical Research: Atmospheres).
However, the rules have not led to such dramatic reductions in the nitrogen oxides that also contribute to the formation of ozone and particulates. Peak levels of ozone were 143 parts per billion in 2010, only marginally 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,” says Sam Atwood, a spokesman for the South Coast Air Quality Management District, which blames air pollution for 5,000 premature deaths a year in the area, where the total population is 17 million.
Despite the regulations, ozone levels exceeded federal standards on 102 days in 2010.

New Scientist Magazine