Warm season average present-day daily maximum ozone levels (AVGMDA8) at urban (left) and non-urban sites (right). Source: University of Leicester. Click to enlarge.

The data come from certified monitoring stations with rigorous calibration procedures. Statistical analysis was carried out on the data, in order to calculate peak and high ozone levels and to interpret trends and changes in the data. The study uses five different recognized methods for measuring the daily or seasonally highest ozone levels. These metrics show similar patterns and highlight the different assessment methods. TOAR has created a large publicly available database which also includes a complete set of statistics and graphical downloads.

In some parts of Asia, Africa and South America, the researchers identified data gaps which prevented them from characterizing ozone pollution exposure patterns. However, overall, Southern Hemisphere sites tend to have lower ozone pollution levels, and fewer days above 70 ppb.

Previously, analyses of ozone trends at individual or smaller groups of sites often left researchers unable to draw robust conclusions about regional trends in areas such as Europe and North America. The large number of sites now included in this more comprehensive dataset have allowed for more robust conclusions that reveal that a decrease predominates in these regions.

Trends in daily maximum ozone levels (known as 4MDA8) at urban and non-urban sites. The steepness of the arrows up or down illustrates the size of the trend, with blue being a decrease and red, an increase. Source: University of Leicester. Click to enlarge.

The international scientists who compiled and analyzed the global ozone pollution database hope it will give scientists and public health managers better insight on trends and patterns of human health exposure around the world.

TOAR is not just a report. We created the largest database of surface ozone from hourly observations at more than 4,800 monitoring sites worldwide, and we’re making these data freely available to anyone who wants to investigate the impact of ozone on human health, vegetation, and climate.

—Dr. Owen Cooper

Cooper, from the Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, chairs the TOAR Steering Committee and has been working to develop the assessment since 2014.

Tropospheric (ground-level) ozone is a greenhouse gas and air pollutant that, at high levels, is detrimental to human health and crop and ecosystem productivity. Ground-level ozone is a secondary pollutant, meaning that it is not emitted directly, but forms when sunlight triggers reactions between natural and human-caused chemical emissions, known as ozone precursor gases. Emissions from vehicles, power plants, industrial operations, and other human activities are a primary cause of surface ozone, which is one of six main pollutants regulated in the US by the Clean Air Act.

Scientists have understood that the precursor gases that form ozone have been declining in North America and Europe since the 1990s, but levels have been increasing in Asia. However, limited and scattered ozone datasets left scientists unable to answer basic questions about the distribution and trends in ozone pollution in many parts of the world: In which regions of the world do people face the greatest ozone exposure? To what extent is ozone increasing in developing countries? Have air quality regulations reduced ozone levels in developed nations?

To address those and other questions, the TOAR research team produced the first global-scale scientific assessment of tropospheric ozone, based on all available surface observations and the peer-reviewed literature.

Despite some reductions in air pollutant emissions in Europe and North America, human health impacts from ozone are still a cause for concern across the world and are rising in parts of East Asia, with the potential for serious health effects on their populations.

—Zoё Fleming, National Centre for Atmospheric Science at University of Leicester

Fleming and Ruth Doherty of the University of Edinburgh were co-lead authors on the study, with co-authors from 12 international institutions. The other institutions involved were the Institute for Advanced Sustainability Studies (IASS) and Forschungszentrum Jülich in Germany; the Universities of Colorado, North Carolina-Chapel Hill, and Maryland as well as A.S.L. and Associates in the US; the Stockholm Environment institute in the UK; INERIS in France; the Chinese Academy of Meteorological Sciences and Chinese Academy of Science in China; NILU (Norwegian Institute for Air Research) and the Norwegian Meteorological Institute in Norway; Chalmers University in Sweden; and the University of Witwatersrand in South Africa.

Future TOAR studies will provide a global assessment of the ozone levels experienced by vegetation, observed ozone levels that affect climate and a historical analysis that explores how ozone levels have changed around the world since the early 20

The TOAR database of surface ozone metrics is now publicly available and can be used by scientists and policymakers around the world to quantify the impacts of ozone on human health and vegetation.

TOAR is a project of the International Global Atmospheric Chemistry project, with support from NOAA, Forschungszentrum Jülich and the World Meteorological Organization.

Resources

• Zoë L. Fleming, Ruth M. Doherty, Erika von Schneidemesser, Christopher S. Malley, Owen R. Cooper, Joseph P. Pinto, Augustin Colette, Xiaobin Xu, David Simpson, Martin G. Schultz, Allen S. Lefohn, Samera Hamad, Raeesa Moolla, Sverre Solberg, Zhaozhong Feng (2018) “Tropospheric Ozone Assessment Report: Present-day ozone distribution and trends relevant to human health” Elementa: Science of the Anthropocene 6(1):12 doi: 10.1525/elementa.273

• TOAR Surface Observation Database