In the 1950s and 1960s the U.S. (particularly the Los Angeles urban area) experienced extremely poor air quality; measured ambient ozone concentrations in those decades have not been equaled anywhere else in the world. Although today many U.S. urban areas still exceed the ozone National Ambient Air Quality Standard (NAAQS), air quality has been greatly improved; however this improvement required more than 4 decades of concerted effort. Here we present a simple quantitative relationship that provides an excellent fit for the temporal evolution of ambient ozone concentrations, not only in the Los Angeles urban area (i.e., the South Coast Air Basin), but also all southern California air basins. Background ozone transported into the U.S. makes substantial contributions to ambient concentrations. The quantitative fit to the temporal evolution of ambient ozone concentrations allows observationally-based estimates of the magnitude of U.S. background ozone concentrations (i.e., the ambient ozone concentration that would be present in the absence of U.S. anthropogenic emissions of ozone precursors) in the respective air basins. Projection of the past temporal evolution into the future, suggests that reducing ozone to the 2015 NAAQS may be more difficult than currently expected. Comparisons of observationally-based estimates of U.S. background ozone concentrations to those from model calculations find significant disagreements, whose causes will be investigated.
David Parrish is a Senior Research Scientist with the Cooperative Institute for Research in Environmental Sciences (CIRES) at CSD, since his retirement as a CSD Research Chemist and Tropospheric Chemistry Program lead in 2014. He is a consultant for all atmospheric chemistry issues related to air quality and climate change.
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