Three well-dated Sabina Przewalskii ring-width chronologies from Dulan, China, have been used to reconstruct annual precipitation (from prior July to current June) variations on the northeast Tibetan Plateau since 850 AD. The reconstructions account of the instrumentally recorded precipitation variance are: 54.7% for the period of 1385-2000AD; 50.5% for 1099-1384AD and 45.7% for 850-1098AD. On the millenary scale, the precipitation variation over this region displays "W" shape, which has three peaks and two valleys. The precipitation is low during 1571-1879 AD, and high during 1880-2000 AD. 1900-2000 AD is the century with the highest precipitation over the northeast Tibetan Plateau in the last 1000 years, and 1962-2000 is the period with the highest precipitation, and the highest variability of precipitation as well in the last 1000 years. The reconstructed series also reveals that the variability of annual precipitation is large when the precipitation is more, and contrarily, variability is small when the precipitation is low. With the temperature increasing obviously in the 20th century, the precipitation in the study region significantly increased too, the variability of precipitation became larger, and drought and flooding occurred more frequently.The yearly tree-ring width (high frequency signal) series in this region reflects the local annually precipitation variation. However, the series with 40-year moving average (low frequency signal) corresponds to the Northern Hemisphere temperature variations on the decadal to centurial scale. It correlates significantly with seven temperature curves of the Northern Hemisphere in the different time spans. For example, the correlation coefficients with the most temperature curves are around 0.9 during the period of 1852-1982 AD. In general, the temperature and the precipitation change synchronously in the Dulan region. It means that low temperature corresponds to low precipitation, and

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 O 3 concentrations over China. With anthropogenic emissions fixed at year 2005 levels, the model simulation for years shows that the changes in meteorological parameters alone lead to interannual variations in JJA surface-layer O 3 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 O 3 concentrations over the 21 yr period. Over the years 1986-2006, the O 3 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 O 3 concentrations are lower (or higher) in weaker (or stronger) EASM years. Relative to JJA surface-layer O 3 concentrations in the strongest EASM years (1990, 1994, 1997, 2002, and 2006), O 3 levels in the weakest EASM years (1988, 1989, 1996, 1998, and 2003) are lower over almost all of China with a national mean lower O 3 concentration by 2.0 ppbv (parts per billion by volume; or 4 %). Regionally, the largest percentage differences in O 3 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 O 3 is the most dominant factor that leads to lower-O 3 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 O 3 concentrations between the weakest and strongest EASM years. We also find that the impacts the EASM strength on JJA surface-layer O 3 can be particularly strong (comparable in magnitude to the impacts on O 3 by changes in anthropogenic emissions over years for certain years. The largest increases in O 3 by anthropogenic emissions are simulated over southeastern China, whereas the largest impacts of the EASM on O 3 are found over central and western China.