Long-term Ozone Trends in Different Urban Developments

High emissions of NOx and anthropogenic VOCs from urban areas are a major source of tropospheric ozone production. Tropospheric ozone is a secondary air pollutant that is harmful to human health as well as crop and ecosystem productivity, and an important greenhouse gas. It is formed from the chemical processing of NOx and VOCs in a non-linear cycle, making ozone reduction strategies challenging. Urban centres across the world are developing at different rates and emitting different combinations and concentrations of chemical species, resulting in location specific drivers of urban ozone concentrations. Alongside this, different countries and cities have implemented a wide range of location specific air quality and climate change measures to reduce air pollution and greenhouse gas emissions. The diversity of these policies over the past few decades has further led to different outcomes for secondary pollutant formation across the globe.

As part of TOAR-II, the Urban Ozone Working Group explores the long-term trends in urban ozone concentrations over the past 20 years on a global scale. Using ground based data from global Air Quality networks, trends in ozone concentrations of > 12 cities across different continents are presented, and temporal changes in ozone trends are identified. This study utilises the TOAR database, accessed via the TOAR Data Portal, which collects hourly data of long-term surface air quality measurements from over 10,000 stations globally. Where sufficient long-term NOx and VOC data are available, ozone concentration trends are compared to trends in its precursor species. 

Trends in ozone concentrations are then related to the state of development of the city using factors such as GDP and the UN City Development Index, and an assessment of how cities in different phases of development are contributing to tropospheric ozone across the world is discussed. Trends in ozone will also be compared to country and city specific changes in air quality and net zero policy interventions.