COVID-19 lockdowns highlight a risk of increasing ozone pollution in European urban areas

TLDR

In March 2020, European lockdowns were imposed to curb COVID‑19 spread, profoundly altering atmospheric composition, especially nitrogen dioxide and ozone levels. The study investigates how these lockdowns changed NO₂ and O₃ concentrations across Europe. We analyzed data from 246 monitoring sites in 102 urban areas and 34 countries between February and July 2020, creating counterfactual business‑as‑usual air‑quality series with machine‑learning models to control for weather variability. Lockdowns reduced NO₂ by 34 % and 32 % below expectations in traffic and urban background sites, while O₃ increased 30 % and 21 % higher—an effect equivalent to 7.6 years of average NO₂ decline—and the unchanged total oxidant indicates NO₂ reductions were offset by O₃ increases, suggesting that future NO₂ cuts could raise urban O.

Abstract

Abstract. In March 2020, non-pharmaceutical intervention measures in the form of lockdowns were applied across Europe to urgently reduce the transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus which causes the COVID-19 disease. The aggressive curtailing of the European economy had widespread impacts on the atmospheric composition, particularly for nitrogen dioxide (NO2) and ozone (O3). To investigate these changes, we analyse data from 246 ambient air pollution monitoring sites in 102 urban areas and 34 countries in Europe between February and July 2020. Counterfactual, business-as-usual air quality time series are created using machine-learning models to account for natural weather variability. Across Europe, we estimate that NO2 concentrations were 34 % and 32 % lower than expected for respective traffic and urban background locations, whereas O3 was 30 % and 21 % higher (in the same respective environments) at the point of maximum restriction on mobility. To put the 2020 changes into context, average NO2 trends since 2010 were calculated, and the changes experienced across European urban areas in 2020 was equivalent to 7.6 years of average NO2 reduction (or concentrations which might be anticipated in 2028). Despite NO2 concentrations decreasing by approximately a third, total oxidant (Ox) changed little, suggesting that the reductions in NO2 were substituted by increases in O3. The lockdown period demonstrated that the expected future reductions in NO2 in European urban areas are likely to lead to widespread increases in urban O3 pollution unless additional mitigation measures are introduced.