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In situ measurements of ozone (O3),carbon monoxide (CO) and meteorological parameters were made at the Shangri-La regional atmosphere background station (28.006°N,99.726°E,3580 m a.s.l.) in southwest China from December 2007 to November 2009.It is found that both O3 and CO peak in spring while the valley of O3 and CO is respectively in summer and in winter.An indicator (marked as ‘Y) of the downward transport on the basis of the monthly normalized O3,CO,and water vapor,is proposed to evaluate the occurrence of O3 downward transport from the upper O3-rich atmosphere.It is shown that the most frequent downward transport events happened in winter and they might play a significant contribution to surface O3 at Shangri-La.A case of strong O3 downward transport on the synoptic condition of a deep westerly trough is studied by the combination of the Y indicator,potential vorticity,total column ozone,and trajectory analysis.An estimation of 9.6 ppb increase (21.0%) of surface ozone is deduced with impacts of deep downward transports in winter.