Amplified ozone pollution in cities during the COVID-19 lockdown

Abstract

The effect of lockdown due to coronavirus disease (COVID-19) pandemic on air pollution in four Southern European cities (Nice, Rome, Valencia and Turin) and Wuhan (China) was quantified, with a focus on ozone (O₃). Compared to the same period in 2017-2019, the daily O₃ mean concentrations increased at urban stations by 24% in Nice, 14% in Rome, 27% in Turin, 2.4% in Valencia and 36% in Wuhan during the lockdown in 2020. This increase in O₃ concentrations is mainly explained by an unprecedented reduction in NO_x emissions leading to a lower O₃ titration by NO. Strong reductions in NO₂ mean concentrations were observed in all European cities, ~53% at urban stations, comparable to Wuhan (57%), and ~65% at traffic stations. NO declined even further, ~63% at urban stations and ~78% at traffic stations in Europe. Reductions in PM_2.5 and PM₁₀ at urban stations were overall much smaller both in magnitude and relative change in Europe (~8%) than in Wuhan (~42%). The PM reductions due to limiting transportation and fuel combustion in institutional and commercial buildings were partly offset by increases of PM emissions from the activities at home in some of the cities. The NO_x concentrations during the lockdown were on average 49% lower than those at weekends of the previous years in all cities. The lockdown effect on O₃ production was ~10% higher than the weekend effect in Southern Europe and 38% higher in Wuhan, while for PM the lockdown had the same effect as weekends in Southern Europe (~6% of difference). This study highlights the challenge of reducing the formation of secondary pollutants such as O₃ even with strict measures to control primary pollutant emissions. These results are relevant for designing abatement policies of urban pollution.

Keywords: Air quality; Coronavirus; Lockdown; Ozone; Risk assessment.

Graphical abstract

Fig. 1

Location of the air quality monitoring stations in China (Wuhan), France (Nice), Italy (Turin and Rome) and Spain (Valencia).

Fig. 2

Daily ozone (O₃), nitrogen dioxide (NO₂) and particulate matter with an aerodynamic diameter lower than 2.5 µm (PM_2.5) mean concentrations (μg.m⁻³) by joining all stations in Nice, Rome, Turin, Valencia and Wuhan from 1st January (Day of the Year, DOY = 1) to DOY = 150, averaged over the 3 previous years (2017–2019), and from DOY = 1 to the end date of the analyzed period in Europe (DOY = 109) and the lockdown in Wuhan (DOY = 98) in 2020. Vertical line: start date of the lockdown in 2020. Smoothing line: locally weighted smoother (LOESS).

Fig. 3

Mean bias (±standard error, in %) at city-scale of 24-hour mean concentrations (PM_2.5, PM₁₀, NO, NO₂ and O₃) at all stations and traffic stations in Nice, Rome, Turin, Valencia and Wuhan between the lockdown period in 2020 and the same time period averaged over the 3 previous years (2017–2019).

Fig. 4

Mean concentrations (±standard error, in μg.m⁻³) by joining all stations at city-scale of 24-hour mean concentrations (PM_2.5, PM₁₀, NO, NO₂ and O₃) in Nice, Rome, Turin, Valencia and Wuhan between the lockdown period in 2020 and the weekday and weekend of the equivalent time period averaged over the 3 previous years (2017–2019). Different letters represent significant differences between groups per city (Kruskal-Wallis test, p-value <0.05 significant).