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

Abstract. We apply a global three-dimensional Goddard Earth Observing System (GEOS) chemical transport model (GEOS-Chem) driven by the NASA/GEOS-4 assimilated meteorological fields to quantify the impacts of the East Asian summer monsoon (EASM) on interannual variations of June-July-August (JJA) surface-layer O3 concentrations over China. With anthropogenic emissions fixed at year 2005 levels, the model simulation for years 1986–2006 shows that the changes in meteorological parameters alone lead to interannual variations in JJA surface-layer O3 concentrations by 2–5% over central eastern China, 1–3% in northwestern China, and 5–10% over the Tibetan Plateau as well as the border and coastal areas of southern China, as the interannual variations are relative to the average O3 concentrations over the 21 yr period. Over the years 1986–2006, the O3 concentration averaged over all of China is found to correlate positively with 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 O3 concentrations in the strongest EASM years (1990, 1994, 1997, 2002, and 2006), O3 levels in the weakest EASM years (1988, 1989, 1996, 1998, and 2003) are lower over almost all of China with a national mean lower O3 concentration by 2.0 ppbv (parts per billion by volume; or 4%). Regionally, the largest percentage differences in O3 concentration between the weakest and strongest EASM years are found to exceed 6% in northeastern China, southwestern China, and over the Tibetan Plateau. Sensitivity studies show that the difference in transboundary transport of O3 is the most dominant factor that leads to lower-O3 concentrations in the weakest EASM years than in the strongest EASM years, which, together with the enhanced vertical convections in the weakest EASM years, explain about 80% of the differences in surface-layer O3 concentrations between the weakest and strongest EASM years. We also find that the impacts the EASM strength on JJA surface-layer O3 can be particularly strong (comparable in magnitude to the impacts on O3 by changes in anthropogenic emissions over years 1986–2006) for certain years. The largest increases in O3 by anthropogenic emissions are simulated over southeastern China, whereas the largest impacts of the EASM on O3 are found over central and western China.