Tropospheric Ozone Assessment Report, Phase I (TOAR-I, 2014-2019)

Archive page of the first phase of TOAR, 2014-2019



Tropospheric ozone is an important greenhouse gas, is detrimental to human health and crop and ecosystem productivity, and controls the oxidizing capacity of the troposphere. Tropospheric ozone is extremely difficult to monitor on the global scale due to its highly variable distribution and interannual variability, and the ongoing shift of ozone precursor emissions from high latitudes to low latitudes.  Despite these complexities there had been no effort, prior to TOAR, to produce an up-to-date scientific assessment of tropospheric ozone’s global distribution and trends from the surface to the tropopause. In contrast, stratospheric ozone has been regularly assessed since 1985 by the WMO/UNEP Scientific Assessments of Ozone Depletion.  To address the lack of an assessment of tropospheric ozone, the IGAC Scientific Steering Committee created the TOAR Activity on March 13, 2014.

TOAR-I's two stated goals were:

  1. Produce the first tropospheric ozone assessment report based on all available surface observations, the peer-reviewed literature and new analyses; and,
  2. Generate easily accessible, documented data on ozone exposure and dose metrics at thousands of measurement sites around the world, freely accessible for research on the global-scale impact of ozone on climate, human health and crop/ecosystem productivity.

These goals directly address IGAC’s Vision Statement, “IGAC’s intellectual leadership is guided by its vision to link emissions, atmospheric processes and atmospheric composition to global change and sustainability issues such as, human health, climate, ecosystems”.


TOAR-I was a science effort initiated by IGAC, and developed by an international team of experts. The first phase of TOAR received financial and logistical support from IGAC, the US National Oceanic and Atmospheric Administration, Forschungszentrum Jülich, and the World Meteorological Organization (an agency of the United Nations). TOAR-I was chaired by Owen R. Cooper (CIRES University of Colorado/NOAA Earth System Laboratory, USA) and steered by a group of 11 scientists from 8 countries (see list of committee members below and in the sidebar under "committee").

TOAR Diagram


TOAR delivered on its goals of producing the first tropospheric ozone assessment report and building the world’s largest database of surface ozone metrics, calculated consistently for over 9000 surface ozone time series worldwide (see below).  TOAR’s accomplishments were made possible by the collective efforts of the international research community, drawing on the expertise of 230 scientists and air quality specialists from 36 nations, representing research on all seven continents. For details, see TOAR Accomplishment Report, September 2019.

TOAR Database of Surface Ozone Metrics

TOAR has produced a relational database of global surface ozone observations, and generated a comprehensive suite of ozone metrics products including standard statistics, health and vegetation impact metrics, and trend information.  The metrics are made available through a common data portal and a web interface as described by Schultz et al. (2017) in the TOAR Special Feature described below.  Pre-compiled surface ozone metrics data sets can be downloaded from PANGAEA.

TOAR Assessment Report

The assessment report was published as a series of papers in the peer-reviewed journal, Elementa – Science of the Anthropocene, available through a Special Feature:

Tropospheric Ozone Assessment Report: A critical review of changes in the tropospheric ozone burden and budget from 1850 to 2100. Archibald, A. T., J. L. Neu, Y. Elshorbany, O. R. Cooper, P. J. Young, H. Akiyoshi, R. A. Cox, M. Coyle, R. Derwent, M. Deushi, A. Finco, G. J. Frost, I. E. Galbally, G. Gerosa, C. Granier, P.T. Griffiths, R. Hossaini, L. Hu, P. Jöckel, B. Josse, M. Y. Lin, M. Mertens, O. Morgenstern, M. Naja, V. Naik, S. Oltmans, D. A. Plummer, L.E. Revell, A. Saiz-Lopez, P. Saxena, Y.M. Shin, I. Shahid, D. Shallcross, S. Tilmes, T. Trickl, T. J. Wallington, T. Wang, H. M. Worden, G. Zeng (2020), Tropospheric Ozone Assessment Report: A critical review of changes in the tropospheric ozone burden and budget from 1850 to 2100. Elem Sci Anth, 8(1), p.53. DOI : 10.1525/elementa.2020.034.

Tropospheric Ozone Assessment Report: Tropospheric ozone from 1877 to 2016, observed levels, trends and uncertainties. Tarasick, D., Galbally, I.E., Cooper, O.R., Schultz, M.G., Ancellet, G., Leblanc, T., Wallington, T.J., Ziemke, J., Liu, X., Steinbacher, M., Staehelin, J., Vigouroux, C., Hannigan, J.W., García, O., Foret, G., Zanis, P., Weatherhead, E., Petropavlovskikh, I., Worden, H., Osman, M., Liu, J., Chang, K.-L., Gaudel, A., Lin, M., Granados-Muñoz, M., Thompson, A.M., Oltmans, S.J., Cuesta, J., Dufour, G., Thouret, V., Hassler, B., Trickl, T. and Neu, J.L., 2019. Tropospheric Ozone Assessment Report: Tropospheric ozone from 1877 to 2016, observed levels, trends and uncertainties. Elem Sci Anth, 7(1), p.39. DOI : 10.1525/elementa.376.

Tropospheric Ozone Assessment Report: Present-day tropospheric ozone distribution and trends relevant to vegetation. Mills G, Pleijel H, Malley CS, Sinha B, Cooper OR, Schultz MG, Neufeld HS, Simpson D, Sharps K,  Feng Z,  Gerosa G,  Harmens H, Kobayashi K,  Saxena P,  Paoletti E,  Sinha V, Xu X,. Elem Sci Anth. 2018;6(1):47. DOI: 10.1525/elementa.302.

Tropospheric Ozone Assessment Report: Present-day distribution and trends of tropospheric ozone relevant to climate and global atmospheric chemistry model evaluation. Gaudel, A., Cooper, O.R., Ancellet, G., Barret, B., Boynard, A., Burrows, J.P., Clerbaux, C., Coheur, P.-F., Cuesta, J., Cuevas, E., Doniki, S., Dufour, G., Ebojie, F., Foret, G., Garcia, O., Granados Muños, M.J., Hannigan, J.W., Hase, F., Huang, G., Hassler, B., Hurtmans, D., Jaffe, D., Jones, N., Kalabokas, P., Kerridge, B., Kulawik, S.S., Latter, B., Leblanc, T., Le Flochmoën, E., Lin, W., Liu, J., Liu, X., Mahieu, E., McClure-Begley, A., Neu, J.L., Osman, M., Palm, M., Petetin, H., Petropavlovskikh, I., Querel, R., Rahpoe, N., Rozanov, A., Schultz, M.G., Schwab, J., Siddans, R., Smale, D., Steinbacher, M., Tanimoto, H., Tarasick, D.W., Thouret, V., Thompson, A.M., Trickl, T., Weatherhead, E., Wespes, C., Worden, H.M., Vigouroux, C., Xu, X., Zeng, G. and Ziemke, J., 2018. Elem Sci Anth, 6(1), p.39. DOI: 10.1525/elementa.291.

Tropospheric ozone assessment report: Global ozone metrics for climate change, human health, and crop/ecosystem research. Lefohn, AS, Malley, CS, Smith, L, Wells, B, Hazucha, M, Simon, H, Naik, V, Mills, G, Schultz, MG, Paoletti, E, De Marco, A, Xu, X, Zhang, L, Wang, T, Neufeld, HS, Musselman, RC, Tarasick, D, Brauer, M, Feng, Z, Tang, H, Kobayashi, K, Sicard, P, Solberg, S and Gerosa, G 2018. Elem Sci Anth, 6: 28. DOI: 10.1525/elementa.279. 

Tropospheric Ozone Assessment Report: Present-day ozone distribution and trends relevant to human health. Fleming, Z.L., Doherty, R.M., von Schneidemesser, E., Malley, C.S., Cooper, O.R., Pinto, J.P., Colette, A., Xu, X., Simpson, D., Schultz, M.G., Lefohn, A.S., Hamad, S., Moolla, R., Solberg, S. and Feng, Z., 2018. Elem Sci Anth, 6(1), p.12. DOI: 10.1525/elementa.273.

Tropospheric Ozone Assessment Report: Assessment of global-scale model performance for global and regional ozone distributions, variability, and trends. Young, P.J., Naik, V., Fiore, A.M., Gaudel, A., Guo, J., Lin, M.Y., Neu, J.L., Parrish, D.D., Rieder, H.E., Schnell, J.L., Tilmes, S., Wild, O., Zhang, L., Ziemke, J.R., Brandt, J., Delcloo, A., Doherty, R.M., Geels, C., Hegglin, M.I., Hu, L., Im, U., Kumar, R., Luhar, A., Murray, L., Plummer, D., Rodriguez, J., Saiz-Lopez, A., Schultz, M.G., Woodhouse, M.T. and Zeng, G., 2018. Elem Sci Anth, 6(1), p.10. DOI: 10.1525/elementa.265.

Tropospheric Ozone Assessment Report: Database and Metrics Data of Global Surface Ozone Observations. Schultz, M.G., Schröder, S., Lyapina, O., Cooper, O., Galbally, I., Petropavlovskikh, I., von Schneidemesser, E., Tanimoto, H., Elshorbany, Y., Naja, M., Seguel, R., Dauert, U., Eckhardt, P., Feigenspahn, S., Fiebig, M., Hjellbrekke, A.-G., Hong, Y.-D., Christian Kjeld, P., Koide, H., Lear, G., Tarasick, D., Ueno, M., Wallasch, M., Baumgardner, D., Chuang, M.-T., Gillett, R., Lee, M., Molloy, S., Moolla, R., Wang, T., Sharps, K., Adame, J.A., Ancellet, G., Apadula, F., Artaxo, P., Barlasina, M., Bogucka, M., Bonasoni, P., Chang, L., Colomb, A., Cuevas, E., Cupeiro, M., Degorska, A., Ding, A., Fröhlich, M., Frolova, M., Gadhavi, H., Gheusi, F., Gilge, S., Gonzalez, M.Y., Gros, V., Hamad, S.H., Helmig, D., Henriques, D., Hermansen, O., Holla, R., Huber, J., Im, U., Jaffe, D.A., Komala, N., Kubistin, D., Lam, K.-S., Laurila, T., Lee, H., Levy, I., Mazzoleni, C., Mazzoleni, L., McClure-Begley, A., Mohamad, M., Murovic, M., Navarro-Comas, M., Nicodim, F., Parrish, D., Read, K.A., Reid, N., Ries, L., Saxena, P., Schwab, J.J., Scorgie, Y., Senik, I., Simmonds, P., Sinha, V., Skorokhod, A., Spain, G., Spangl, W., Spoor, R., Springston, S.R., Steer, K., Steinbacher, M., Suharguniyawan, E., Torre, P., Trickl, T., Weili, L., Weller, R., Xu, X., Xue, L. and Zhiqiang, M., 2017.  Elem Sci Anth, 5, p.58. DOI: 10.1525/elementa.244.

Regional trend analysis of surface ozone observations from monitoring networks in eastern North America, Europe and East Asia. Chang K-L, Petropavlovskikh I, Cooper OR, Schultz MG, Wang T. Elem Sci Anth. 2017;5:50. DOI: 10.1525/elementa.243.

Long-term changes of regional ozone in China: implications for human health and ecosystem impacts. Xu, X., Lin, W., Xu, W., Jin, J., Wang, Y., Zhang, G., Zhang, X., Ma, Z., Dong, Y., Ma, Q., Yu, D., Li, Z., Wang, D. and Zhao, H., 2020. Elem Sci Anth, 8(1), p.13. DOI:

Past Events

TOAR Workshop 1.04
IGAC Sponsored
Jülich, Germany
IGACnews Workshop Summary
Workshop Summary (Full Version)665.2 KB, TOAR List of Metrics (Updated 27 June 2016)341.64 KB
TOAR Workshop 1.03
IGAC Sponsored
Beijing, China
IGACnews Workshop Summary
Workshop Summary (Full Version)1.59 MB
TOAR Workshop 1.02
IGAC Sponsored
Madrid, Spain
IGACnews Workshop Summary
Workshop Summary (Full Version)257.28 KB
TOAR Workshop 1.01
IGAC Sponsored
Boulder, CO, USA
IGACnews Workshop Summary
Workshop Summary (Full Version)302.47 KB