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Ozone pollution and the TOAR project

What is tropospheric ozone?


Tropospheric ozone is a greenhouse gas and pollutant detrimental to human health and crop and ecosystem productivity. Since 1990 a large portion of the anthropogenic emissions that react in the atmosphere to produce ozone have shifted from North America and Europe to Asia. This rapid shift, coupled with limited ozone monitoring in developing nations, has left scientists unable to answer the most basic questions:

  • Which regions of the world have the greatest human and plant exposure to ozone pollution?

  • Is ozone continuing to decline in nations with strong emission controls?

  • To what extent is ozone increasing in the developing world?

  • How can the atmospheric sciences community facilitate access to the ozone metrics necessary for quantifying ozone’s impact on human health and crop/ecosystem productivity?

TOAR is designed to answer these questions through the development of an assessment report based on expert opinion and analysis, and the generation of a range of ozone metrics at hundreds of sites around the world.

The IGAC- funded Tropospheric Ozone Assessment Report (TOAR) project is an international collaboration of scientific experts who have written up research papers on all aspects of tropospheric ozone. The aim of TOAR is to provide the research community with an up-to-date scientific assessment of tropospheric ozone’s global distribution and trends from the surface to the tropopause.

The TOAR report has been split into chapters which have each become papers in the journal, Elementa. The papers can be downloaded here. The subjects of the papers are:

  • Tropospheric ozone sources, sinks and budgets

  • Ozone measurement techniques and instrumentation

  • The metrics for measuring tropospheric ozone

  • The effect of tropospheric ozone on human health

  • The effect of tropospheric ozone on vegetation and crops

  • Tropospheric ozone and effects on climate change

  • Global-scale model performance for global and regional ozone

  • The TOAR database

The effects of ozone on human health


I was one of two lead authors on the chapter on the effects of ozone on human health. The paper was published in February 2018 and can be viewed online and downloaded here.

Fleming ZL, Doherty RM, von Schneidemesser E, Malley CS, Cooper OR, Pinto JP,

Colette A, Xiaobin X, Simpson D, Schultz MG, Lefohn AS, Hamad S, Moolla R, Solberg S, Feng Z: Tropospheric Ozone Assessment Report: Present-day ozone distribution and trends relevant to human health. Elem Sci Anth. 2018;6(1):12. DOI:

We quantified the present-day global and regional distributions (2010–2014) and trends (2000–2014) for five ozone metrics relevant for short-term and long-term human exposure. These metrics were explored at ozone monitoring sites worldwide, which were classified as urban or non-urban based on population and nighttime lights data.

Ozone is present at levels that are harmful to human health in many parts of the world and there are vast areas where there are no measurements. The metrics defining peak ozone levels have tended to decrease over recent years in north America and Europe, but increase in Asia.


Present day (2010-2014) number of days of 8 hourly ozone levels over 70 ppb at urban stations


Figures for trends of the annual 8 hour daily maximum (4MDA8) ozone at the stations between 2000 and 2014 (blue is a decrease and red an increase, light blue and orange are less statistically significant trends and the non-significant trends are shown in green.

Summaries of the findings can be read in these Geographical and EOS news articles


Ozone trends in Santiago de Chile


With high summer temperatures intense sunlight throughout the year at this city at 33 degrees S and at 800 m of elevation, ozone levels can reach harmful levels, especilally in the east of the city as the predominant winds bring the mix of VOCs and NOx towards the richer areas of town, in the foothills of the Andes. While they the population believes they are out of and above the smog of the centre of the city, they are exposed to much higher ozone in the summer. Read more about it in this paper (Seguel, R.J., Gallardo, L, Fleming, Z.L., Landeros, S.: Two decades of ozone standard exceedances in Santiago de Chile, Air Quality, Atmosphere & Health, Air Qual Atmos Health, 2020) by my colleague Rodrigo Seguel.

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