Impact of lockdown during the COVID-19 outbreak on multi-scale air quality

Abstract

One of the multi-facet impacts of lockdowns during the unprecedented COVID-19 pandemic was restricted economic and transport activities. This has resulted in the reduction of air pollution concentrations observed globally. This study is aimed at examining the concentration changes in air pollutants (i.e., carbon monoxide (CO), nitrogen dioxide (NO₂), sulfur dioxide (SO₂), ozone (O₃), and particulate matters (PM_2.5 and PM₁₀) during the period March-April 2020. Data from both satellite observations (for NO₂) and ground-based measurements (for all other pollutants) were utilized to analyze the changes when compared against the same months between 2015 and 2019. Globally, space borne NO₂ column observations observed by satellite (OMI on Aura) were reduced by approximately 9.19% and 9.57%, in March and April 2020, respectively because of public health measures enforced to contain the coronavirus disease outbreak (COVID-19). On a regional scale and after accounting for the effects of meteorological variability, most monitoring sites in Europe, USA, China, and India showed declines in CO, NO₂, SO₂, PM_2.5, and PM₁₀ concentrations during the period of analysis. An increase in O₃ concentrations occurred during the same period. Meanwhile, four major cities case studies i.e. in New York City (USA), Milan (Italy), Wuhan (China), and New Delhi (India) have also shown a similar reduction trends as observed on the regional scale, and an increase in ozone concentration. This study highlights that the reductions in air pollutant concentrations have overall improved global air quality likely driven in part by economic slowdowns resulting from the global pandemic.

Keywords: Air pollution; COVID-19; Global analysis; Ground-based measurements; Satellite measurements.

Fig. 1

Global tropospheric NO₂ column loadings with 30% cloud screened, presented in μmol m⁻² during March and April 2020. Top panel figures show NO₂ observed in (A) March 2020, and (B) April 2020. Figures on the bottom panel illustrate the difference of NO₂ loadings in (C) March 2020, and (D) April 2020 when compared against the average loadings for the respective month in the period 2015–2019.

Fig. 2

Percent changes of gaseous pollutants in March 2020 (left figures) and April 2020 (right figures) measured over ground-based monitoring stations in Europe. The pollutants are (A, B) carbon monoxide (CO); (C, D) nitrogen dioxide (NO₂); (E, F) ozone (O₃); and (G, H) sulfur dioxide (SO₂). The changes are based on the concentration difference between the period 2015–2019 and 2020 for respective month.

Fig. 3

Percent changes of particulate pollutants in March 2020 (left figures) and April 2020 (right figures) measured over ground-based monitoring stations in Europe. The particulate pollutants are (A, B) PM₁₀, and (C, D) PM_2.5. The changes are based on the concentration difference between the period 2015–2019 and 2020 for respective month.

Fig. 4

Percent change of O₃ in March and April 2020, observed in ground-based monitoring stations across the contiguous USA as a departure from the average concentrations in the base period.

Fig. 5

2020 monthly average concentrations of PM_2.5, PM₁₀, NO₂, O₃, CO and SO₂ in New York City, USA (A), Milan, Italy (B), Wuhan, China (C) and New Delhi, India (D) as a fraction of the average concentration in the base period (the vertical bars present ± 1 standard deviation).

Fig. 6

Scatterplot depicting relationships between NO₂ and O₃ measured in April at (A, B) New Delhi, India, and (C, D), Milan, Italy. Figures on the left panel show the relationships during the period 2015–2019 while figures on the right panel are for 2020. (NO₂ concentrations are averaged values from 11.00 a.m. to 6.00 p.m. i.e. during the time of photochemical activity; and O₃ concentrations are maximum 8-h value during the same time window – reflecting maximum photochemical activity).