One of the world’s worst cities for air pollution is Delhi, India. Past studies have
shown air pollution in Delhi has a significant burden on the population’s
respiratory and cardiovascular health. It is therefore important to analyse the
sources of these pollutants in order to improve air quality mitigation strategies.
This thesis presents time-resolved source apportionments of the organic
fraction of particulate matter (PM) and of the gaseous volatile organic
compounds (VOCs) that were measured in Delhi. The results of this analysis
are used to identify the sources of these air pollutants and determine their
atmospheric implications.
Online aerosol mass spectrometry (AMS) measurements taken in Old Delhi
during pre-monsoon, monsoon and post-monsoon seasons revealed
concentrations of submicron particulate matter (PM1) reaching ~750 μg m-3
The largest contributing inorganic species to PM1 in the post-monsoon is
nitrate (8%) but this changes to sulphate during the monsoon (24%) and premonsoon (24%). PM1 is dominated by the organics throughout the three
measurement periods (54% - 68%). Source apportionment on the organics
fraction using Positive Matrix Factorisation (PMF) revealed that PM1 traffic
emissions are the largest contributor of primary organic aerosol (POA) in the
pre-monsoon (12%) and monsoon (16%) periods. Two traffic factors were
resolved: a hydrocarbon-like organic aerosol (HOA) and another HOA factor
rich in nitrogen (NHOA). The N within NHOA is previously undocumented
within AMS measurements and is found to be linked to nitrile species. The two
traffic factors are found to have separated due to fuel-type where NHOA is
related to heavy goods vehicles (HGVs) and diesel emissions while HOA is
related to petrol and compressed natural gas emissions. The highest PM1
concentrations are observed in the post-monsoon period during which the
highest form of POA is from a burning-related factor. These factors alone
contribute 35% to the total post-monsoon increase when concentrations
increase by 188%. A high correlation between one burning factor, semi-volatility biomass burning organic aerosol (SVBBOA), and Earth observation
fire counts in surrounding states demonstrates its link to crop residue burning.
Another burning factor, solid-fuel OA (SFOA), is found to have a high
composition of polyaromatic hydrocarbons (PAHs) and novel AMS-measured
marker species for polychlorinated dibenzodioxins (PCDDs) and
polychlorinated dibenzofurans (PCDFs). SFOA is therefore linked to local
open-waste burning. There is also a 522% increase in chloride concentrations
from pre-monsoon to post-monsoon. High correlations with SVBBOA and
SFOA, suggest the post-monsoon increase in chloride is due to crop residue
and open-waste burning. Future air quality mitigation strategies should
concentrate on traffic emissions in order to cause a reduction PM1 across the
whole year. To reduce peak PM1 concentrations during the post-monsoon,
requires targeting of burning-related activities in future air quality policy.
The monitoring and measurements of volatile organic compounds (VOCs) in
Delhi is often overlooked. While the effects of VOCs on people’s heath is still
being debated, the outcome of their interaction with the atmosphere can play
a pivotal role in PM formation, OH reactivity and photochemical ozone creation
potentials (POCP). For the first time, a novel approach to source
apportionment was developed which allows for emission and deposition to be
calculated for positive matrix factorisation (PMF) VOC source factors. Spatial
flux patterns for PMF factors are presented, which give in-depth detail of the
localised fluxes. A total of nine factors were resolved, of which traffic emissions
are the largest (70% of total flux) and are the most significant source of OH
reactivity flux and photochemical ozone creation potential (POCP). The two
traffic factors separate based on the HGV restrictions causing the composition
of one factor (TRAF1) to have a strong connection with diesel emissions and
the other (TRAF2) with petrol and compressed natural gas. The second most
significant source of emissions is an evaporative VOC (EVOC) source (10% of
total flux) which has a high composition of marker species related to asphalt
emissions. These are found to be naphthalene and tetralin-based structures,
along with 1,3-butadiene and styrene which are constituent species of the
adhesives used in asphalt. The peak in emissions of EVOC occurs during
midday and follows surface temperature and solar radiation. Due to their
similar meteorological controls, most of the biogenic VOCs reside within the
EVOC factor. It is estimated that biogenic emissions contribute 25% to total
monoterpene concentrations and 18% to the localised monoterpene fluxes.
However, the traffic factors have the highest composition of monoterpenes
and, as a result, they dominate the monoterpene contributions to
concentrations (60%) and localised fluxes (78%). Two burning-related factors
were separated, one associated with solid-fuel combustion VOCs (SFVOC)
and another associated with pyrotechnical activity VOCs (PVOC). The two
factors are shown to share the largest percentage of the concentration-weighted OH reactivity. This is caused by a high content of furan-based
species. The first observation of a strong urban deposition flux for PMF factors
is also seen for SFVOC and an oxidised VOC (OVOC1) factor. Spatial
deposition flux patterns suggest vegetation could act as a VOC sink in Delhi,
however, the magnitude of which could show it to be small.
Over recent years, multiple PM mitigation strategies have been implemented
in Delhi, but, how effective these changes have been to reduce PM is not clear.
It is however possible to chart the changes in PM by analysing large archives
of PM filter samples that have been collected over multiple years. An
automated offline-AMS method was developed which has enabled high-throughput analysis, allowing PM concentrations to be charted over multiple
years. This novel offline-AMS method uses an organic solvent mix of acetone
and water which can extract high quantities of OA (95.4 ± 8.3 %). PM10 filter
samples collected in Delhi for the years of 2011, 2015 and 2018, were
analysed. PMF analysis was performed on the organic fraction and nine
factors were resolved. These factors can be grouped into four source
categories: cooking, traffic, coal-combustion and burning-related (solid fuel or
open burning) OA. Burning-related OA is the highest contributor when total OA
concentrations are also at their highest, during the winter and post-monsoon.
As a result, burning-related OA likely contributes significantly to acute PM-related health affects in Delhi. Annual averages show a decline in burning related OA concentrations from 2015 to 2018 (47%). This could be linked to
the 2015 ban on open-waste burning; however, compositional analysis of OA
factors suggests municipal waste burning is still present in 2018. Further
mitigation strategies are therefore required to reduce burning-related OA
further. The shutdown of the two coal power stations in Delhi, along with
initiatives to reduce the popularity of coal-use in businesses, residential
homes, and industry, has led to a significant decrease (87%) in coal-combustion OA. Total OA concentrations were also predicted to be reduced
by 17%, therefore, proving these measures brought effective reductions in
PM10. The Bharat stage emissions standards for vehicles likely supressed
PM10 OA concentrations as the annual averages of traffic OA factors do not
reflect the coinciding rapid increase in population and registered vehicles.
However, restrictions on HGVs entering during the day shows possible links to
increases in PM10 concentrations during the winter and post-monsoon months.
This is likely due to the large influx of diesel-engine HGVs entering the city
when the boundary layer is particularly low during the early mornings and
evenings
