Air Quality Before and After COVID-19 Lockdown Phases Around New Delhi, India.

Abstract

Background: The COVID-19 pandemic has had a deep global impact, not only from a social and economic perspective, but also with regard to human health and the environment. To restrict transmission of the virus, the Indian government enforced a complete nationwide lockdown except for essential services and supplies in phases from 25 March to 31 May 2020. Ambient air quality in and around New Delhi, one of the most polluted cities of world, was also impacted during this period.

Objective: The aim of the present study was to assess and understand the impact of four different lockdown phases (LD1, LD2, LD3 and LD4) on five air pollutants (particulate matter (PM) PM_2.5, PM₁₀, nitrogen oxide (NO_x), sulfur dioxide (SO₂) and ozone (O₃)) compared to before lockdown (BLD) at 13 air monitoring stations in and around New Delhi.

Methods: Secondary data on five criteria pollutants for 13 monitoring stations in and around New Delhi for the period 1 March to 31 May 2020 was accessed from the Central Pollution Control Bard, New Delhi. Data were statistically analyzed across lockdown phases, meteorological variables, and prevailing air sources around the monitoring stations.

Results: Pollutant concentrations decreased during LD1 compared to BLD except for O₃ at all stations. PM_2.5 and PM₁₀ remained either close to or higher than the National Ambient Air Quality Standards (NAAQS) due to prevailing high-speed winds. During lockdown phases, NO₂ decreased, whereas O₃ consistently increased at all stations. This was a paradoxical situation as O₃ is formed via photochemical reactions among NO_x and volatile organic compounds. Principal component analysis (PCA) extracted two principal components (PC1 and PC2) which explained up to 80% of cumulative variance in data. PM_2.5, PM₁₀ and NO₂ were associated with PC1, whereas PC2 had loadings of either O₃ only or O₃ and SO₂ depending upon monitoring station.

Conclusions: The present study found that air pollutants decreased during lockdown phases, but these decreases were specific to the site(s) and pollutant(s). The decrease in pollutant concentrations during lockdown could not be attributed completely to lockdown conditions as the planetary boundary layer increased two-fold during lockdown compared to the BLD phase. Such restrictions could be applied in the future to control air pollution but should be approached with caution.

Competing interests: The authors declare no competing financial interests.