  Diwali is among the most important Indian festivals, and elaborate firework
displays mark the evening's festivities. This study assesses the impact of
Diwali on the concentration, composition, and sources of ambient PM2.5. We
observed the total PM2.5 concentrations to rise to 16 times the pre-firework
levels, while each of the elemental, organic, and black carbon fractions of
ambient PM2.5 increased by a factor of 46.1, 3.7, and 5.6, respectively. The
concentration of species like K, Al, Sr, Ba, S, and Bi displayed distinct peaks
during the firework event and were identified as tracers. The average
concentrations of potential carcinogens, like As, exceeded US EPA screening
levels for industrial air by a factor of ~9.6, while peak levels reached up to
16.1 times the screening levels. The source apportionment study, undertaken
using positive matrix factorization, revealed the fireworks to account for 95%
of the total elemental PM2.5 during Diwali. The resolved primary organic
emissions, too, were enhanced by a factor of 8 during Diwali. Delhi has
encountered serious haze events following Diwali in recent years; this study
highlights that biomass burning emissions rather than the fireworks drive the
poor air quality in the days following Diwali.
