Highly time-resolved chemical speciation and source apportionment of organic aerosol components in Delhi, India, using extractive electrospray ionization mass spectrometry

In recent years, the Indian capital city of Delhi has been impacted by very high levels of air pollution,
especially during winters. Comprehensive knowledge of the composition and sources of the organic aerosol
(OA), which constitutes a substantial fraction of total particulate mass (PM) in Delhi, is central to formulating
effective public health policies. Previous source apportionment studies in Delhi identified key sources of primary
OA (POA) and showed that secondary OA (SOA) played a major role but were unable to resolve specific SOA
sources. We address the latter through the first field deployment of an extractive electrospray ionization timeof-
flight mass spectrometer (EESI-TOF) in Delhi, together with a high-resolution aerosol mass spectrometer
(AMS). Measurements were conducted during the winter of 2018/19, and positive matrix factorization (PMF)
was used separately on AMS and EESI-TOF datasets to apportion the sources of OA. AMS PMF analysis yielded
three primary and two secondary factors which were attributed to hydrocarbon-like OA (HOA), biomass burning
OA (BBOA-1 and BBOA-2), more oxidized oxygenated OA (MO-OOA), and less oxidized oxygenated OA (LOOOA).
On average, 40%of the total OA mass was apportioned to the secondary factors. The SOA contribution to
total OA mass varied greatly between the daytime (76.8 %, 10:00–16:00 local time (LT)) and nighttime (31.0 %,
21:00–04:00 LT). The higher chemical resolution of EESI-TOF data allowed identification of individual SOA
sources. The EESI-TOF PMF analysis in total yielded six factors, two of which were primary factors (primary
biomass burning and cooking-related OA). The remaining four factors were predominantly of secondary origin:
aromatic SOA, biogenic SOA, aged biomass burning SOA, and mixed urban SOA. Due to the uncertainties in
the EESI-TOF ion sensitivities, mass concentrations of EESI-TOF SOA-dominated factors were related to the
total AMS SOA (i.e. MO-OOA C LO-OOA) by multiple linear regression (MLR). Aromatic SOA was the major
SOA component during the daytime, with a 55.2% contribution to total SOA mass (42.4% contribution to total
OA). Its contribution to total SOA, however, decreased to 25.4% (7.9% of total OA) during the nighttime. This
factor was attributed to the oxidation of light aromatic compounds emitted mostly from traffic. Biogenic SOA
accounted for 18.4% of total SOA mass (14.2% of total OA) during the daytime and 36.1% of total SOA mass
(11.2% of total OA) during the nighttime. Aged biomass burning and mixed urban SOA accounted for 15.2%
and 11.0% of total SOA mass (11.7% and 8.5% of total OA mass), respectively, during the daytime and 15.4%
and 22.9% of total SOA mass (4.8% and 7.1% of total OA mass), respectively, during the nighttime. A simple
dilution–partitioning model was applied on all EESI-TOF factors to estimate the fraction of observed daytime
concentrations resulting from local photochemical production (SOA) or emissions (POA). Aromatic SOA, aged
biomass burning, and mixed urban SOA were all found to be dominated by local photochemical production,
likely from the oxidation of locally emitted volatile organic compounds (VOCs). In contrast, biogenic SOA was
related to the oxidation of diffuse regional emissions of isoprene and monoterpenes. The findings of this study
show that in Delhi, the nighttime high concentrations are caused by POA emissions led by traffic and biomass
burning and the daytime OA is dominated by SOA, with aromatic SOA accounting for the largest fraction.
Because aromatic SOA is possibly more toxic than biogenic SOA and primary OA, its dominance during the
daytime suggests an increased OA toxicity and health-related consequences for the general public.

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