Abstract. In recent years, the Indian capital city of Delhi has
been impacted by very high levels of air pollution, especially during
winter. 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 time-of-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 (LO-OOA). 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 + 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.
