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. Inter-annual variability and long-term trends in tropospheric ozone are bothenvironmental and climate concerns. Ozone measured at Mt WaliguanObservatory (WLG, 3816 m a.s.l.) on the Tibetan Plateau over the period of1994–2013 has increased significantly by 0.2–0.3 ppbv yr −1 duringspring and autumn but shows a much smaller trend in winter and nosignificant trend in summer. Here we explore the factors driving the observedozone changes at WLG using backward trajectory analysis, chemistry–climatemodel hindcast simulations (GFDL AM3), a trajectory-mapped ozonesonde dataset,and several climate indices. A stratospheric ozone tracer implemented inGFDL AM3 indicates that stratosphere-to-troposphere transport (STT) canexplain ∼  60 % of the simulated springtime ozone increase at WLG,consistent with an increase in the NW air-mass frequency inferred from thetrajectory analysis. Enhanced STT associated with the strengthening of themid-latitude jet stream contributes to the observed high ozone anomalies atWLG during the springs of 1999 and 2012. During autumn, observations at WLGare more heavily influenced by polluted air masses originating from South EastAsia than in the other seasons. Rising Asian anthropogenic emissions of ozoneprecursors are the key driver of increasing autumnal ozone observed at WLG, assupported by the GFDL AM3 model with time-varying emissions, which capturesthe observed ozone increase (0.26  ±  0.11 ppbv yr −1 ). AM3simulates a greater ozone increase of 0.38  ±  0.11 ppbv yr −1 atWLG in autumn under conditions with strong transport from South East Asia andshows no significant ozone trend in autumn when anthropogenic emissions areheld constant in time. During summer, WLG is mostly influenced by easterlyair masses, but these trajectories do not extend to the polluted regions ofeastern China and have decreased significantly over the last 2 decades,which likely explains why summertime ozone measured at WLG shows nosignificant trend despite ozone increases in eastern China. Analysis of theTrajectory-mapped Ozonesonde data set for the Stratosphere and Troposphere(TOST) and trajectory residence time reveals increases in direct ozonetransport from the eastern sector during autumn, which adds to the autumnalozone increase. We further examine the links of ozone variability at WLG tothe quasi-biennial oscillation (QBO), the East Asian summer monsoon (EASM), and the sunspot cycle. Ourresults suggest that the 2–3-, 3–7-, and 11-year periodicities are linkedto theQBO, EASM index, and sunspot cycle, respectively. A multivariate regressionanalysis is performed to quantify the relative contributions of variousfactors to surface ozone concentrations at WLG. Through an observational andmodelling analysis, this study demonstrates the complex relationships betweensurface ozone at remote locations and its dynamical and chemical influencingfactors.

atmospheric chemistry and physics

Long-term trends of surface ozone and its influencing factors at the Mt Waliguan GAW station, China – Part 2: The roles of anthropogenic emissions and climate variability