  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.
