PM2.5 diminution and haze events over Delhi during the COVID-19 lockdown period: an interplay between the baseline pollution and meteorology

Surendra K Dhaka,Vinay Kumar,Chetna Chetna,Prabir K Patra,Mizuo Kajino,Yutaka Matsumi,Vivek Panwar,Prakhar Misra,Kazuyo Yamaji,Sachiko Hayashida,Narendra Singh,Masayuki Takigawa,A P Dimri,Tomoki Nakayama

doi:10.1038/s41598-020-70179-8

Abstract

Delhi, a tropical Indian megacity, experiences one of the most severe air pollution in the world, linked with diverse anthropogenic and biomass burning emissions. First phase of COVID-19 lockdown in India, implemented during 25 March to 14 April 2020 resulted in a dramatic near-zeroing of various activities (e.g. traffic, industries, constructions), except the “essential services”. Here, we analysed variations in the fine particulate matter (PM2.5) over the Delhi-National Capital Region. Measurements revealed large reductions (by 40–70%) in PM2.5 during the first week of lockdown (25–31 March 2020) as compared to the pre-lockdown conditions. However, O3 pollution remained high during the lockdown due to non-linear chemistry and dynamics under low aerosol loading. Notably, events of enhanced PM2.5 levels (300–400 µg m−3) were observed during night and early morning hours in the first week of April after air temperatures fell close to the dew-point (~ 15–17 °C). A haze formation mechanism is suggested through uplifting of fine particles, which is reinforced by condensation of moisture following the sunrise. The study highlights a highly complex interplay between the baseline pollution and meteorology leading to counter intuitive enhancements in pollution, besides an overall improvement in air quality during the COVID-19 lockdown in this part of the world.

Highlights

Delhi, a tropical Indian megacity, experiences one of the most severe air pollution in the world, linked with diverse anthropogenic and biomass burning emissions
Unprecedented reductions in anthropogenic activities yielded to very low values of emissions resulting in significantly improved air quality over the Delhi-National Capital Region (NCR) [up to 50% reduction in fine particle
Such studies are not possible in normal conditions, for example even during the very aggressive “odd–even scheme” the air quality over Delhi was only marginally improved[13,14], which besides other factors[15,16], highlighted a remarkable role played by the meteorology and lower atmospheric dynamics

Summary

Introduction

A tropical Indian megacity, experiences one of the most severe air pollution in the world, linked with diverse anthropogenic and biomass burning emissions. Prior and during first week of lockdown, the decreases in PM2.5 concentrations range from ~ 70 to 40% as seen from the observations at 9 stations (Supplementary Fig. S1).

Results

Conclusion