Ozone near the Earth’s surface is a greenhouse gas and a health-damaging air pollutant regulated by the U.S. Environmental Protection Agency (EPA). A new study, led by Cooperative Institute for Climate Science – Princeton (CICS-P) Scientist Meiyun Lin, discovers that climate shifts have caused Asian ozone pollution reaching Hawaii to rise unexpectedly in autumn since the mid-1990s. The findings, published in Nature Geoscience, indicate that variability in airflow patterns must be considered when attributing observed ozone changes to human-induced trends in precursor emissions.
The longest record of ozone measurements in the U.S. dates back to 1974 at Mauna Loa Observatory, a premier research facility in Hawaii operated by NOAA. Tropospheric ozone levels in the remote atmosphere have increased most in the spring due to powerful springtime winds and tripling emissions of ozone precursors from Asia over the past few decades. However, the 40-year Hawaiian record reveals little change in ozone levels during spring, but a sudden rise in autumn. Using chemistry-climate models and observations, Lin and her colleagues identified decadal shifts in atmospheric circulation patterns that “hide” the increase in pollution from Asia in the spring, but amplify the change in the autumn. They show that the flow of ozone-rich air from Eurasia towards Hawaii during spring weakened in the 2000s as a result of La-Niña-like decadal cooling in the equatorial Pacific Ocean. During autumn, in contrast, transport of Asian pollution to Hawaii has increased since the mid-1990s due to a positive phase of atmospheric circulation variability known as the Pacific-North American pattern.
This study not only solves the mystery of Hawaiian ozone changes since 1974, but also has broad implications for attribution of tropospheric ozone trends globally. Characterizing shifts in atmospheric circulation patterns is of paramount importance for understanding the response of tropospheric ozone levels to a changing climate and evolving global emissions of ozone precursors.
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Contact Information: Meiyun Lin at (609) 452-6551 or Meiyun.Lin@noaa.gov