Variation in chemical composition and sources of PM2.5 during the COVID-19 lockdown in Delhi

TL;DR

This study analyzes how the COVID-19 lockdown in Delhi drastically reduced certain sources of PM2.5, revealing the impact of human activities on air pollution and highlighting changes in chemical composition and sources during different lockdown phases.

Contribution

It provides the first high-resolution chemical speciation and source apportionment of PM2.5 during lockdown, showing specific reductions in vehicular and domestic emissions and their effects on air quality.

Findings

Vehicular emissions decreased by 96% during lockdown.

Organic and elemental carbon sources were significantly reduced.

Unexpected rise in ozone levels correlated with decreased NOx during lockdown.

Abstract

The Government of India (GOI) announced a nationwide lockdown starting 25th March 2020 to contain the spread of COVID-19, leading to an unprecedented decline in anthropogenic activities and in turn improvements in ambient air quality. This is the first study to focus on highly time-resolved chemical speciation and source apportionment of PM$_{2.5}$ to assess the impact of the lockdown and subsequent relaxations on the sources of ambient PM$_{2.5}$ in Delhi, India. The elemental, organic, and black carbon fractions of PM$_{2.5}$ were measured at the IIT Delhi campus from February 2020 to May 2020. We report source apportionment results using positive matrix factorization (PMF) of organic and elemental fractions of PM$_{2.5}$ during the different phases of the lockdown. The resolved sources such as vehicular emissions, domestic coal combustion, and semi-volatile oxygenated organic aerosol (SVOOA) were found to decrease by 96%, 95%, and 86%, respectively, during lockdown phase-1 as compared to pre-lockdown. An unforeseen rise in O$_3$ concentrations with declining NO$_x$ levels was observed, similar to other parts of the globe, leading to the low-volatility oxygenated organic aerosols (LVOOA) increasing to almost double the pre-lockdown concentrations during the last phase of the lockdown. The effect of the lockdown was found to be less pronounced on other resolved sources like secondary chloride, power plants, dust-related, hydrocarbon-like organic aerosols (HOA), and biomass burning related emissions, which were also swayed by the changing meteorological conditions during the four lockdown phases. The results presented in this study provide a basis for future emission control strategies, quantifying the extent to which constraining certain anthropogenic activities can ameliorate the ambient air.