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Tropospheric ozone is an important atmospheric oxidant, greenhouse gas andatmospheric pollutant at the same time. The oxidation capacity of theatmosphere, climate, human and vegetation health can be impacted by the increase of the ozone level. Therefore, long-term determination of trendsof baseline ozone is highly needed information for environmental and climatechange assessment. So far, studies on the long-term trends of ozone atrepresentative sites are mainly available for European and North Americansites. Similar studies are lacking for China and many other developingcountries. Measurements of surface ozone were carried out at a baseline Global Atmospheric Watch (GAW) station in the north-eastern TibetanPlateau region (Mt Waliguan, 36°17′ N, 100°54′ E,3816 m a.s.l.) for the period of 1994 to 2013. To uncover the variationcharacteristics, long-term trends and influencing factors of surface ozone atthis remote site in western China, a two-part study has been carried out,with this part focusing on the overall characteristics of diurnal, seasonaland long-term variations and the trends of surface ozone. To obtain reliableozone trends, we performed the Mann–Kendall trend test and the Hilbert–Huangtransform (HHT) analysis on the ozone data. Our results confirm that themountain-valley breeze plays an important role in the diurnal cycle ofsurface ozone at Waliguan, resulting in higher ozone values during the nightand lower ones during the day, as was previously reported. Systematic diurnaland seasonal variations were found in mountain-valley breezes at the site,which were used in defining season-dependent daytime and nighttime periodsfor trend calculations. Significant positive trends in surface ozone weredetected for both daytime (0.24 ± 0.16 ppbv year−1) and nighttime(0.28 ± 0.17 ppbv year−1). The largest nighttime increasing rateoccurred in autumn (0.29 ± 0.11 ppbv year−1), followed by spring(0.24 ± 0.12 ppbv year−1), summer(0.22 ± 0.20 ppbv year−1) and winter(0.13 ± 0.10 ppbv year−1), respectively. The HHT spectral analysisidentified four different stages with different positive trends, with thelargest increase occurring around May 2000 and October 2010. The HHT resultssuggest that there were 2–4a, 7a and 11a periodicities in the time series ofsurface ozone at Waliguan. The results of this study can be used forassessments of climate and environment change and in the validation ofchemistry–climate models

Long-term trends of surface ozone and its influencing factors at the Mt Waliguan GAW station, China – Part 1: Overall trends and characteristics