Abstract
Abstract. Since early 2020, the COVID-19 pandemic has led to lockdowns at national scales. These lockdowns resulted in large cuts of atmospheric pollutant emissions, notably related to the vehicular traffic source, especially during spring 2020. As a result, air quality changed in manners that are still currently under investigation. The robust quantitative assessment of the impact of lockdown measures on ambient concentrations is however hindered by weather variability. In order to circumvent this difficulty, an innovative methodology has been developed. The Analog Application for Air Quality (A3Q) method is based on the comparison of each day of lockdown to a group of analog days having similar meteorological conditions. The A3Q method has been successfully evaluated and applied to a comprehensive in situ dataset of primary and secondary pollutants obtained at the SIRTA observatory, a suburban background site of the megacity of Paris (France). The overall slight decrease of submicron particulate matter (PM1) concentrations (−14 %) compared to business-as-usual conditions conceals contrasting behaviors. Primary traffic tracers (NOx and traffic-related carbonaceous aerosols) dropped by 42 %–66 % during the lockdown period. Further, the A3Q method enabled us to characterize changes triggered by NOx decreases. Particulate nitrate and secondary organic aerosols (SOAs), two of the main springtime aerosol components in northwestern Europe, decreased by −45 % and −25 %, respectively. A NOx relationship emphasizes the interest of NOx mitigation policies at the regional (i.e., city) scale, although long-range pollution advection sporadically overcompensated for regional decreases. Variations of the oxidation state of SOA suggest discrepancies in SOA formation processes. At the same time, the expected ozone increase (+20 %) underlines the negative feedback of NO titration. These results provide a quasi-comprehensive observation-based insight for mitigation policies regarding air quality in future low-carbon urban areas.
Highlights
With the worldwide spread of the SARS-CoV-2 coronavirus, the COVID-19 outbreak has been responsible for millions of premature deaths
Species usually considered as markers for primary traffic emissions (NOx, BCff and hydrocarbon-like organic aerosols (HOAs)) exhibit a median decrease of concentrations by 42 %–62 % (Table 5) at SIRTA
For nitrogen oxides (NOx), this is very consistent with previous results in the Paris region using machine-learning approaches (−42 %, Grange et al, 2021)
Summary
With the worldwide spread of the SARS-CoV-2 coronavirus, the COVID-19 outbreak has been responsible for millions of premature deaths. The corresponding stay-at-home orders resulted in a sudden halt of economic activities and, as a consequence, in an unprecedented drop of emission of pollution sources. From this perspective, and despite tragic death records, these lockdowns are unique opportunities to characterize an extreme end of mitigation policy scenarios and future low-carbon megacities from direct observations. Without climatologically representative values, specific care must be taken when comparing concentrations observed during and outside the lockdown period The robustness of this assessment depends on the way meteorology is handled and on what reference period is chosen to compare with the lockdown period. The 12 million inhabitants of the region, representing around 20 % of the total French population, were placed under lockdown from 17 March 2020 to 10 May 2020, further designated as LP2020
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