Tropospheric ozone assessment report: Global ozone metrics for climate change, human health, and crop/ecosystem research

Allen S. Lefohn, Christopher S. Malley, Luther Smith, Benjamin Wells, Milan Hazucha, Heather Simon, Vaishali Naik, Gina Mills, Martin G. Schultz, Elena Paoletti, Alessandra De Marco, Xiaobin Xu, Li Zhang, Tao Wang, Howard S. Neufeld, Robert C. Musselman, David Tarasick, Michael Brauer, Zhaozhong Feng, Haoye TangKazuhiko Kobayashi, Pierre Sicard, Sverre Solberg, Giacomo Gerosa

Research output: Journal article publicationJournal articleAcademic researchpeer-review

248 Citations (Scopus)


Assessment of spatial and temporal variation in the impacts of ozone on human health, vegetation, and climate requires appropriate metrics. A key component of the Tropospheric Ozone Assessment Report (TOAR) is the consistent calculation of these metrics at thousands of monitoring sites globally. Investigating temporal trends in these metrics required that the same statistical methods be applied across these ozone monitoring sites. The nonparametric Mann-Kendall test (for significant trends) and the Theil-Sen estimator (for estimating the magnitude of trend) were selected to provide robust methods across all sites. This paper provides the scientific underpinnings necessary to better understand the implications of and rationale for selecting a specific TOAR metric for assessing spatial and temporal variation in ozone for a particular impact. The rationale and underlying research evidence that influence the derivation of specific metrics are given. The form of 25 metrics (4 for model-measurement comparison, 5 for characterization of ozone in the free troposphere, 11 for human health impacts, and 5 for vegetation impacts) are described. Finally, this study categorizes health and vegetation exposure metrics based on the extent to which they are determined only by the highest hourly ozone levels, or by a wider range of values. The magnitude of the metrics is influenced by both the distribution of hourly average ozone concentrations at a site location, and the extent to which a particular metric is determined by relatively low, moderate, and high hourly ozone levels. Hence, for the same ozone time series, changes in the distribution of ozone concentrations can result in different changes in the magnitude and direction of trends for different metrics. Thus, dissimilar conclusions about the effect of changes in the drivers of ozone variability (e.g., precursor emissions) on health and vegetation exposure can result from the selection of different metrics.

Original languageEnglish
Article number28
Publication statusPublished - 1 Jan 2018


  • Ground-level ozone
  • Metrics
  • Ozone distributions
  • Shifting ozone concentrations
  • Trends
  • Tropospheric ozone

ASJC Scopus subject areas

  • Oceanography
  • Environmental Engineering
  • Ecology
  • Geotechnical Engineering and Engineering Geology
  • Geology
  • Atmospheric Science


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