Trends in ozone, its precursors, and related secondary oxidation products in Los Angeles, California: A synthesis of measurements from 1960 to 2010

Decreases in ozone (O-3) observed in California's South Coast Air Basin (SoCAB) over the past five decades have resulted from decreases in local emissions of its precursors, nitrogen oxides (NOx=NO+NO2) and volatile organic compounds (VOCs). Ozone precursors have been characterized in the SoCAB with measurements dating back to 1960. Here we compile an extensive historical data set using measurements in the SoCAB between 1960 and 2010. Faster rates of decrease have occurred in abundances of VOCs (-7.30.7%yr(-1)) than in NOx (-2.60.3%yr(-1)), which have resulted in a decrease in VOC/NOx ratio (-4.80.9%yr(-1)) over time. Trends in the NOx oxidation products peroxyacetyl nitrate (PAN) and nitric acid (HNO3), measured in the SoCAB since 1973, show changes in ozone production chemistry resulting from changes in precursor emissions. Decreases in abundances of PAN (-9.31.1%yr(-1)) and HNO3 (-3.0 +/- 0.8%yr(-1)) reflect trends in VOC and NOx precursors. Enhancement ratios of O-3 to (PAN+HNO3) show no detectable trend in ozone production efficiency, while a positive trend in the oxidized fraction of total reactive nitrogen (+2.2 +/- 0.5%yr(-1)) suggests that atmospheric oxidation rates of NOx have increased over time as a result of the emissions changes. Changes in NOx oxidation pathways have increasingly favored production of HNO3, a radical termination product associated with quenching the ozone formation cycle.