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Tropospheric ozone (O₃) is a trace gas playing important roles in atmospheric chemistry, air quality and climate change. In contrast to North America and Europe, long-term measurements of surface O₃ are very limited in China. We compile available O₃ observations at Mt. Tai – the highest mountain over the North China Plain – during 2003–2015 and analyze the decadal change of O₃ and its sources. A linear regression analysis shows that summertime O₃ measured at Mt. Tai has increased significantly by 1.7 ppbv yr⁻¹ for June and 2.1 ppbv yr⁻¹ for the July–August average. The observed increase is supported by a global chemistry-climate model hindcast (GFDL-AM3) with O₃ precursor emissions varying from year to year over 1980–2014. Analysis of satellite data indicates that the O₃ increase was mainly due to the increased emissions of O₃ precursors, in particular volatile organic compounds (VOCs). An important finding is that the emissions of nitrogen oxides (NO_<i>x</i>) have diminished since 2011, but the increase of VOCs appears to have enhanced the ozone production efficiency and contributed to the observed O₃ increase in central eastern China. We present evidence that controlling NO_<i>x</i> alone, in the absence of VOC controls, is not sufficient to reduce regional O₃ levels in North China in a short period.