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We apply a global three-dimensional Goddard Earth Observing System (GEOS)chemical transport model (GEOS-Chem) driven by the NASA/GEOS-4 assimilatedmeteorological fields to quantify the impacts of the East Asian summermonsoon (EASM) on interannual variations of June-July-August (JJA)surface-layer O3 concentrations over China. With anthropogenicemissions fixed at year 2005 levels, the model simulation for years 1986–2006shows that the changes in meteorological parameters alone lead tointerannual variations in JJA surface-layer O3 concentrations by2–5% over central eastern China, 1–3% in northwestern China, and5–10% over the Tibetan Plateau as well as the border and coastal areasof southern China, as the interannual variations are relative to the averageO3 concentrations over the 21 yr period. Over the years 1986–2006, the O3concentration averaged over all of China is found to correlate positivelywith the EASM index with a large correlation coefficient of +0.75,indicating that JJA O3 concentrations are lower (or higher) in weaker(or stronger) EASM years. Relative to JJA surface-layer O3concentrations in the strongest EASM years (1990, 1994, 1997, 2002, and2006), O3 levels in the weakest EASM years (1988, 1989, 1996, 1998, and2003) are lower over almost all of China with a national mean lower O3concentration by 2.0 ppbv (parts per billion by volume; or 4%). Regionally, the largest percentagedifferences in O3 concentration between the weakest and strongest EASMyears are found to exceed 6% in northeastern China, southwestern China,and over the Tibetan Plateau. Sensitivity studies show that the differencein transboundary transport of O3 is the most dominant factor that leadsto lower-O3 concentrations in the weakest EASM years than in thestrongest EASM years, which, together with the enhanced vertical convectionsin the weakest EASM years, explain about 80% of the differences insurface-layer O3 concentrations between the weakest and strongest EASMyears. We also find that the impacts the EASM strength on JJA surface-layerO3 can be particularly strong (comparable in magnitude to the impactson O3 by changes in anthropogenic emissions over years 1986–2006) forcertain years. The largest increases in O3 by anthropogenic emissionsare simulated over southeastern China, whereas the largest impacts of theEASM on O3 are found over central and western China

Impacts of the East Asian summer monsoon on interannual variations of summertime surface-layer ozone concentrations over China