Concentrations of a number of air pollutants, including ozone and its precursors, have fallen dramatically in Los Angeles in recent decades despite a tripling of the population and of gasoline sales during that time. Regulations on vehicle emissions get most of the credit, according to a report in a forthcoming issue of the Journal of Geophysical Research: Atmospheres. Scientists looked at data gathered in and around the Los Angeles metropolitan area—a region that scientists call the South Coast Air Basin—between 1960 and 2010 by research aircraft, roadside sensors, and other ground-based instruments (image shows a smoggy downtown Los Angeles in September 1973). One of the largest declines measured is the drop in the concentration of peroxyacetyl nitrate, the organic compound most often associated with eye irritation due to smog. Levels of that eye-stinging gas decreased 9.3% per year during the 5-decade study period, the researchers say. Also, ozone precursors such as volatile organic chemicals and nitrogen oxides declined during the same period by about 7.3% per year and 2.6% per year, respectively. Detailed analyses of atmospheric chemistry in the region demonstrate that significant reductions in pollutants have been the direct result of reduced emissions, with the largest changes stemming from tighter emissions standards and improved technology in motor vehicles.
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