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Long-term trend of tropospheric ozone

Ozone (O3) in the troposphere affects atmospheric oxidative capacity; it is greenhouse gas in the upper troposphere and a pollutant at ground level. Therefore long-term changes in ozone concentrations and underlying causes have been a focus of both scientific community and the governments. Assessment of changes in ozone in China was hindered by lacking continuous monitoring data in regionally represented sites. PolyU has made a pioneering effort by establishing a background air monitoring station in the early 1990s in Hong Kong, and has been measuring surface ozone (and CO) since 1994. We analyzed record for 1994-2007 and showed upward changes in surface ozone in the background atmosphere (Wang et al., ACP, 2009), this increase is attributed to concurrent increase in NOx emissions in eastern China, suggesting importance of super-regional pollution transport to local ozone pollution. The ozone trend result was adopted in the UNEP and WMO joint report “Integrated Assessment of Black Carbon and Tropospheric Ozone” in 2011 ( and the IPCC AR5 in 2014. The more result (up to 2015) have been included into the Troposperic Ozone Research Report (TOAR) (see Lefohn et a., AE).

A follow-up study examined ozone, NOx and organics data in Hong Kong’s urban stations in 2002-2013 and revealed that while local emission-reduction efforts reduced the ozone produced from HK, increased cross-border contributions negated the local efforts (Xue at al., ES&T, 2014). The findings was adopted by the Hong Kong Government’s Environment Bureau in “Clean Air Plan for Hong Kong – 2013-2017 progress report” (

In northern China where there were no long-term ozone measurements prior to 2005, we analyzed ozone data obtained from MOZAIC aircraft platform for the Beijing area and derived ozone changes during 1995-2005 at different levels of the troposphere and in different reasons (Ding et al., 2008). Boundary-layer ozone showed sharp increase during summer, which is attributed to the rapid rise in precursor emissions. At Mount Tai, our earlier measurements of ozone during 2002-3 (Gao et al., AE, 2005) provided starting point for analysis of trend of summer ozone, which indicated rapid rise in tropospheric ozone in the north China plains (Sun et al., ACP, 2016).

In the Yangtze River delta, our measurements of ozone during 1999-2001 (Wang et al, GRL, 2001) have been used to obtain the trend during 1995-2005 (Xu et al., ACP, 2006).


Figure 1. Surface ozone at background (HT) and urban (CW) station in Hong Kong and air mass transport during 1994-2007 (Wang et al., 2009)


Figure 2. Theil-Sen trend (%/year) (1994–2015) at Hok Tsui, Hong Kong, China for (a) O3 concentrations in each bin and (b) 6 human health and 8 vegetation O3 metrics (Lefohn et al., 2017).


Figure 3. Local and regional contributions to surface ozone trend in urban Hong Kong during 2002-2013 (Xue et al., 2014).


Figure 4 (a) Comparisons of mean O3 profiles over Beijing between 1995–1999 and 2000–2005. The solid lines are annual mean values, and the dashed lines on the right hand side represent the data collected in summer afternoons (at LT 15:00–16:00 in MJJ) (Ding et al., 2008)


Figure 5 (a)  Monthly average 1-hour ozone at Mt Tai in summer months of 2003-2015 (Sun et al., 2016) and (b) Long-term changes in averages of the monthly highest 5% and lowest 5% of surface ozone at Linan (Xu et al., 2008).

Related References:

  1. Lefohn, A. S. *, C. S. Malley, H. Simon, B. Wells, X. B. Xu, L. Zhang, and T. Wang. "Responses of Human Health and Vegetation Exposure Metrics to Changes in Ozone Concentration Distributions in the European Union, United States, and China." Atmospheric Environment 152 (Mar 2017): 123-45.

  2. Sun, L., L. K. Xue*, T. Wang, J. Gao, A. J. Ding, O. R. Cooper, M. Y. Lin, P. J. Xu, Z. Wang, X. F. Wang, L. Wen, Y. H. Zhu, T. S. Chen, L. X. Yang, Y. Wang, J. M. Chen, and W. X. Wang. "Significant Increase of Summertime Ozone at Mount Tai in Central Eastern China." Atmospheric Chemistry and Physics 16, no. 16 (Aug 2016): 10637-50.

  3. Xue, L. K. *, T. Wang*, P. K. K. Louie, C. W. Y. Luk, D. R. Blake, and Z. Xu. "Increasing External Effects Negate Local Efforts to Control Ozone Air Pollution: A Case Study of Hong Kong and Implications for Other Chinese Cities." Environmental Science & Technology 48, no. 18 (Sep 2014): 10769-75.

  4. Wang, T. *, X. L. Wei, A. J. Ding, C. N. Poon, K. S. Lam, Y. S. Li, L. Y. Chan, and M. Anson. "Increasing Surface Ozone Concentrations in the Background Atmosphere of Southern China, 1994-2007." Atmospheric Chemistry and Physics 9, no. 16 (2009): 6217-27.

  5. Xu, X. *, W. Lin, T. Wang, P. Yan, J. Tang, Z. Meng, and Y. Wang. "Long-Term Trend of Surface Ozone at a Regional Background Station in Eastern China 1991-2006: Enhanced Variability." Atmospheric Chemistry and Physics 8, no. 10 (2008): 2595-607.

  6. Ding, A. J., T. Wang *, V. Thouret, J. P. Cammas, and P. Nedelec. "Tropospheric Ozone Climatology over Beijing: Analysis of Aircraft Data from the Mozaic Program." Atmospheric Chemistry and Physics 8, no. 1 (2008): 1-13.

  7. Xu XB.*; Lin WL.; Wang T.; Meng ZY.; Wang Y.; Long-term Trend of Tropospheric Ozone over the Yangtze Delta Region of China, Adv. Clim. Change Res., (in Chinese), 2006, 2 (5): 211-216.

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