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October 29, 2013; 110 (44)

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Cover image: Pictured is the ornamental South Pole, located near the geographic South Pole in Antarctica. Robina Shaheen et al. measured snow samples collected near the South Pole that contain a 25-year record of atmospheric sulfate aerosols. Stable isotopes of oxygen in the aerosols revealed that isotopic ratios in upper troposphere ozone rose in connection with El Ni?o events. See the article by Shaheen et al. on pages 17662–17667, a part of the Special Feature on Chemistry and Applications in Nature of Mass Independent Isotope Effects. Image courtesy of Justin McCabe.

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