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Increasing springtime ozone mixing ratios in the free troposphere over western North America


In the lowermost layer of the atmosphere—the troposphere—ozone is an important source of the hydroxyl radical, an oxidant that breaks down most pollutants and some greenhouse gases1. High concentrations of tropospheric ozone are toxic, however, and have a detrimental effect on human health and ecosystem productivity1. Moreover, tropospheric ozone itself acts as an effective greenhouse gas2. Much of the present tropospheric ozone burden is a consequence of anthropogenic emissions of ozone precursors3 resulting in widespread increases in ozone concentrations since the late 1800s3,4,5,6,7. At present, east Asia has the fastest-growing ozone precursor emissions8. Much of the springtime east Asian pollution is exported eastwards towards western North America9. Despite evidence that the exported Asian pollution produces ozone10, no previous study has found a significant increase in free tropospheric ozone concentrations above the western USA since measurements began in the late 1970s5,11,12. Here we compile springtime ozone measurements from many different platforms across western North America. We show a strong increase in springtime ozone mixing ratios during 1995–2008 and we have some additional evidence that a similar rate of increase in ozone mixing ratio has occurred since 1984. We find that the rate of increase in ozone mixing ratio is greatest when measurements are more heavily influenced by direct transport from Asia. Our result agrees with previous modelling studies, which indicate that global ozone concentrations should be increasing during the early part of the twenty-first century as a result of increasing precursor emissions, especially at northern mid-latitudes13, with western North America being particularly sensitive to rising Asian emissions14. We suggest that the observed increase in springtime background ozone mixing ratio may hinder the USA’s compliance with its ozone air quality standard.

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Figure 1: Springtime ozone distributions for 1984, 1995–2008 in the mid-troposphere (3.0–8.0 km), and air mass source regions.
Figure 2: Average 1995–2008 FLEXPART retroplume residence times.


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This work was supported in part by NOAA’s Climate Goal Program. We acknowledge the support of MOZAIC by the European Communities, EADS, Airbus and the airlines (Lufthansa, Austrian, Air France) who have carried MOZAIC equipment free of charge since 1994. B. Ridley, NCAR (retired), measured ozone from the NCAR C-130 during TOPSE. G. L. Gregory, NASA (retired), measured ozone from the Convair CV-990 during CITE-1C. M. Proffitt, NOAA (retired) measured ozone from the NASA ER2 during STRAT and POLARIS, and measured ozone from the NASA WB57 during WAM. PACDEX ozone data were provided by NCAR/EOL under sponsorship of the National Science Foundation ( EDGAR ( is a product of the National Institute for Public Health and the Netherlands Organisation for Applied Scientific Research and is part of the Global Emissions Inventory Activity of IGBP/IGAC. Finally we thank R. Dickerson for providing the mean ozone values for June 1985 and May–June 1986.

Author Contributions O.R.C., D.D.P., A.S. and M.T conceived the study and guided its development. O.R.C. merged the ozone datasets and ran the PDM with assistance from A.S. O.R.C. analysed the data and wrote the text. P.N., V.T., J.P.C., S.J.O., B.J.J., D.T., T.L., I.S.M., D.J., R.G., J.S., T.R., K.A., T.C., A.W. and M.A.A. made the ozone measurements and guided interpretation of the datasets.

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Correspondence to O. R. Cooper.

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Cooper, O., Parrish, D., Stohl, A. et al. Increasing springtime ozone mixing ratios in the free troposphere over western North America. Nature 463, 344–348 (2010).

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