Influence of the COVID-19 Lockdown on Delhi’s Air Quality

To reduce the spread of COVID-19, a nationwide lockdown was implemented in India from Mar 24-May 31, 2020. Lockdowns were implemented in four phases – Phase 1 (Mar-25-Apr-14), 2 (Apr-15-May-03), 3 (May 04-17) and 4 (May 18-31), with restrictions easing with time. During phases 1 and 2, activities such as transportation and construction were severely restricted, and several factories and businesses were shut down. Power plants in several parts in India operated at a lower capacity or were shut down owing to lower demand. These changes offered insights on how future emission reductions might affect air quality in Delhi. (Patel et al., 2021). 

Here, we assess impacts of the COVID-19 lockdown on ambient air quality in Delhi, building on over three years of real-time measurements of black carbon (BC) and non-refractory sub-micrometer aerosol (NR-PM1) composition from the Delhi Aerosol Supersite and public data from the regulatory monitoring network. We performed source apportionment of organic aerosol (OA) using positive matrix factorization and robust statistical analyses to differentiate lockdown-related impacts from baseline seasonal and inter-annual variability. We used a combination of statistical tools to demonstrate the influence of the COVID-19 lockdown on Delhi’s air quality. In core analyses, we used the method of “robust differences” (Bekbulat et al., 2021) to compare the concentrations in 2020 to recent trends (2017-2019 average). As a supplemental and complementary analysis technique we computed the percentage difference between concentrations in each phase of the lockdown and the baseline pre-lockdown period (01-24 March). We compared this seasonal evolution in air pollutants for 2020 with that for prior years. Further, we conducted statistical tests to determine significance of the difference in concentrations relative to historical concentrations and concentrations before the lockdown. Overall, results provide detailed insights into the influence of COVID-19 lockdowns and related restrictions on the air quality in Delhi, India.

References: 

1. Patel, K., Campmier, M., J., Bhandari, S., Baig, N., Gani, S., Habib, G., Apte, J., S., Hildebrandt Ruiz, L.: Persistance of primary and secondary pollutants in Delhi: Concentrations and composition from 2017 through the COVID pandemic, ES&T Letters, https://doi.org/10.1021/acs.estlett.1c00211, 2021
2. Bekbulat, B.; Apte, J. S.; Millet, D. B.; Robinson, A. L.; Wells, K. C.; Presto, A. A.; Marshall, J.    D. Changes in Criteria Air Pollution Levels in the US before, during, and after Covid-19 Stay-at-Home Orders: Evidence from Regulatory Monitors, Science of the Total Environment, 769, 144693, https://doi.org/10.1016/j.scitotenv.2020.144693, 2021