Assessing the impacts of emission reduction and meteorology change on India’s air quality during COVID-19 lockdown using the WRF-Chem model

Restrictions on economic activities during the COVID-19 pandemic lockdown period provided a once-in-a-lifetime opportunity for many countries including India to witness the improvement in air quality that can be achieved with a reduction in anthropogenic activities. Many previous studies have reported about the sudden drop in pollution levels across India as an immediate reflex of these economic restrictions or “lockdown”. However, there exist several shortcomings in most of these studies. Firstly, most studies focused only on the lockdown phases and ignored to document the recovery of pollution levels during the unlock phases. Secondly, many studies considered the reduction in emission sources only within India due to lockdown and ignored the reduction in activities in the neighboring counties. Thirdly, many studies could not separate the impact of emission reductions and changing meteorology throughout lockdown phases on the improvement in observed air quality. In the present study, we examine the impacts of changing emissions of air pollutants and meteorology during the entire lockdown and unlock phases of COVID-19 period (February 24 – June 30, 2020) on the air quality over India using the COvid-19 adjustmeNt Factors fOR eMissions (CONFORM) data and Weather Research and Forecasting model coupled to Chemistry (WRF-Chem) model.

We performed systematically designed model simulations to understand and isolate the effects of changing emissions of air pollutants and meteorology on the air quality levels during the COVID-19 period. In one case, we consider business as usual and another one we adjusted anthropogenic emissions using CONFORM data. To quantify and isolate the impact of changing meteorology during the same period, we performed additional simulations for the last five years that is from 2015 to 2019. All simulations are performed for a nested model domain having an inner domain horizontal resolution of 9Km x 9Km and centre around Bilaspur, Chhattisgarh, using 6-hourly ERA-5 reanalysis data. Model simulations are evaluated using available in-situ measurements and remote sensing data from various satellite observations.

Results from our emission-restricted simulations show that immediately before and after the imposition of the lockdown, particulate matter concentration decreased by ~36%, compared to a 20% decrease during the same periods in the last five years. Movement and Industrial restrictions and reductions in Power Production led to a significant decrease in surface distribution of NO_x (NO+NO₂) over different Indian metro cities (Kolkata ~52%, Delhi ~67%). Highly populated and polluted Northern and Western Indian states show distinct spatial variations, with the more critical decrease in CO (~21%), PM_2.5 (~27%), and SO₂ (~17%) pollutant levels. A significant reduction of NO_x to VOCs in VOC-limited urban locations shows a marginal increase in the O₃ distribution over Central and South India. We observed that, although there was an initial improvement in air quality due to strict activity restrictions during the first phase of the lockdown, most Indian megacities still failed to achieve the national standard of air pollutants. More results with greater details will be presented.