Increasing free tropospheric ozone (O3), combined with the high elevation and often deep boundary layers at western U.S. surface stations, poses challenges in attaining the more stringent 70 ppb O3 National Ambient Air Quality Standard. As such, use of observational data to identify sources and mechanisms that contribute to surface O3 is increasingly important. This work analyzes surface and vertical O3 observations over California and Nevada from 1995 to 2015. Over this period, the number of high O3 events (95th percentile) at the U.S. Environmental Protection Agency Clean Air Status and Trends Network (CASTNET) sites has decreased during summer, as a result of decreasing U.S. emissions. In contrast, an increase in springtime 5th percentile O3 indicates a general increase of baseline O3. During 2012 there was a peak in exceedances and in the average spring-summer O3 mixing ratios at CASTNET sites. Goddard Earth Observing System-Chem results show that the surface O3 attributable to transport from the upper troposphere and stratosphere was increased in 2013 compared to 2012, highlighting the importance of measurements aloft. Vertical O3 measurements from aircraft, ozonesondes, and lidar show distinct seasonal trends, with a high percentage of elevated O3 laminae (O3 > 70 ppb, 3–8 km) during spring and summer. Analysis of the timing of high O3 surface events and correlation between surface and vertical O3 data is used to discuss varying sources of western U.S. surface O3.
Potentially harmful levels of the air pollutant ozone are present in many regions around the world, a widespread study has shown.
Levels of ozone in the troposphere – the lower region of the atmosphere – have decreased in much of Europe and the US over the past 15 years. However, they are increasing in parts of East Asia in line with economic development.
Relatively high levels are found in both in urban and rural areas of the globe, with identifiable hot-spots in regions with major emissions, according to the results.
The study uses data from more than 4,800 monitoring stations across the globe, and is the biggest assessment to date on ground-level ozone, which can be harmful to breathe.
The highest daily ozone levels were measured in five different ways, and statistical analysis was used to calculate peak levels and to interpret trends and changes in the data at urban and rural locations.
The study is a component of the Tropospheric Ozone Assessment Report (TOAR), an international effort to improve scientific understanding of ozone’s global distribution and trends.
The international team of researchers behind the study, who volunteered their time and expertise to take part, have released their data, including statistics and graphics available, for wider use.
The ability to quantify the changes in high and peak ozone levels over the past 15 years and more for urban regions worldwide is an exciting development. We hope this will be useful to air quality managers, to inform and evaluate strategies to protect human health from the adverse effects of ozone.Professor Ruth DohertySchool of Geosciences, University of Edinburgh
Despite some improvements in air pollution 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.Dr Zoë FlemingNational Centre for Atmospheric Science (NCAS), University of Leicester
The research, led by the Universities of Leicester and Edinburgh in collaboration with 12 other institutions worldwide, was published in the journal Elementa: Science of the Anthropocene.
The TOAR project is funded by the International Global Atmospheric Chemistry project (IGAC), the World Meteorological Organization (WMO), the National Oceanic and Atmospheric Administration (NOAA) and Forschungszentrum Jülich with the support of a large international team.
Tropospheric Ozone Assessment Report
To read the paper in Elementa: Science of the Anthropocene, click here