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
Authors: Xu Wanyun , Xu Xiaobin , Lin Meiyun , Lin Weili , Tarasick David , Tang Jie , Ma Jianzhong , Zheng Xiangdong 
Published: January 2018

Abstract. Inter-annual variability and long-term trends in tropospheric ozone are both
environmental and climate concerns. Ozone measured at Mt Waliguan
Observatory (WLG, 3816 m a.s.l.) on the Tibetan Plateau over the period of
1994–2013 has increased significantly by 0.2–0.3 ppbv yr−1 during
spring and autumn but shows a much smaller trend in winter and no
significant trend in summer. Here we explore the factors driving the observed
ozone changes at WLG using backward trajectory analysis, chemistry–climate
model hindcast simulations (GFDL AM3), a trajectory-mapped ozonesonde data
set,
and several climate indices. A stratospheric ozone tracer implemented in
GFDL AM3 indicates that stratosphere-to-troposphere transport (STT) can
explain ∼ 60 % of the simulated springtime ozone increase at WLG,
consistent with an increase in the NW air-mass frequency inferred from the
trajectory analysis. Enhanced STT associated with the strengthening of the
mid-latitude jet stream contributes to the observed high ozone anomalies at
WLG during the springs of 1999 and 2012. During autumn, observations at WLG
are more heavily influenced by polluted air masses originating from South East
Asia than in the other seasons. Rising Asian anthropogenic emissions of ozone
precursors are the key driver of increasing autumnal ozone observed at WLG, as
supported by the GFDL AM3 model with time-varying emissions, which captures
the observed ozone increase (0.26 ± 0.11 ppbv yr−1). AM3
simulates a greater ozone increase of 0.38 ± 0.11 ppbv yr−1 at
WLG in autumn under conditions with strong transport from South East Asia and
shows no significant ozone trend in autumn when anthropogenic emissions are
held constant in time. During summer, WLG is mostly influenced by easterly
air masses, but these trajectories do not extend to the polluted regions of
eastern China and have decreased significantly over the last 2 decades,
which likely explains why summertime ozone measured at WLG shows no
significant trend despite ozone increases in eastern China. Analysis of the
Trajectory-mapped Ozonesonde data set for the Stratosphere and Troposphere
(TOST) and trajectory residence time reveals increases in direct ozone
transport from the eastern sector during autumn, which adds to the autumnal
ozone increase. We further examine the links of ozone variability at WLG to
the quasi-biennial oscillation (QBO), the East Asian summer monsoon (EASM), and the sunspot cycle. Our
results suggest that the 2–3-, 3–7-, and 11-year periodicities are linked
to the
QBO, EASM index, and sunspot cycle, respectively. A multivariate regression
analysis is performed to quantify the relative contributions of various
factors to surface ozone concentrations at WLG. Through an observational and
modelling analysis, this study demonstrates the complex relationships between
surface ozone at remote locations and its dynamical and chemical influencing
factors.