Ambient aerosol variation in India during the COVID-19 lockdown : Simulations from high resolution chemical transport model (CHIMERE)

On March 10^th 2020, COVID-19 was categorised as a global pandemic by the World Health Organisation (WHO), concerned both by the alarming levels of spread and severity across 114 countries. On account of this, India announced the first nationwide lockdown on 25^th March 2020. The present study examines the impact of the lockdown period on the atmospheric levels of overall aerosols (PM_2.5) and on individual aerosol species (Black Carbon (BC), Organic Matter (OM), Sulphate (SO₄^2-), Nitrate (NO₃^-) and Ammonium (NH₄⁺) over the Indian subcontinent using a chemical transport model, CHIMERE. In this study, CHIMERE is forced externally by Weather Research and Forecasting (WRF) model as a meteorological driver in offline mode. The model was run for usual and pandemic lockdown scenarios to estimate the reductions in aerosol species concentration during the lockdown period. The cessation of industrial and transportation activity caused a significant drop in PM_2.5 concentration of 20–30% over India, and notably 48% over Delhi. In-land regions saw a sharp decline in PM_2.5 concentration (39–48% at Delhi, Bengaluru, and Raipur) compared to coastal locations (11–24% decrease at Kolkata, Mumbai, Chennai, Ahmedabad, and Bhubaneswar), which is explained by the marine influence at those coastal locations. The decrease in measured PM_2.5 concentration is very well reproduced by the model for the agricultural state of Telangana, where agricultural residue burning is prominent as a major source of anthropogenic emissions. The elimination of traffic and industrial emissions during the lockdown period resulted in a significant decrease of aerosol species concentration (BC (10-40%), OM (3-10%), SO₄^2- (30-80%), NO₃^- (70-90%) and NH₄⁺ (60-80%)) over the subcontinent. The study also aims to understand the effect of lockdown on the aerosol optical property and found a reduction of 10-40% in the aerosol optical depth (AOD) over the Indian region. The reductions in the simulated mass concentration of PM_2.5 are in eminent agreement (bias ≤ 40%) with the available measurements, rendering the model to be effective for simulating low emission scenarios over India.