Long-term trends in urban ozone in Europe and the USA

High emissions of NO_x 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 NO_x 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, long-term trends in urban ozone concentrations over the past 20 years across over 400 sites in Europe and the USA will be explored. 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. Using ground-based data from air quality monitoring networks, seasonal trends in ozone and NO₂ concentrations are explored alongside trends in peak ozone and MDA8.  Global 5-year trends in ozone and NO₂ will also be examined, extending the analysis to include urban centres in China and South America. The period between 2018-2022 will be investigated, allowing us to take a closer look into the impact of COVID-19 in cities across the world. This study aims to assess trends in urban ozone and NO₂ across Europe and the USA, and to explore how lockdowns and restrictions during the COVID-19 pandemic might reveal insights into a lower NO_x emission future.