Radiocarbon-supported source apportionment of carbonaceous aerosols within the Clean Air Project in India
BORIS DOI
Abstract
Atmospheric aerosols represent a complex mixture of innumerable different chemical species originating from various natural and anthropogenic emission sources and their investigation has become a wide-ranging scientific research topic. In particular the disclosure of their composition and sources are of great interest since air pollution raised awareness due to its adverse effects on climate and human health. Climatic effects triggered by aerosols include changes in the atmospheric radiative balance that lead to increased cooling or warming. Alterations of the hydrological cycle ensuing extreme events such as floods and droughts may also be a consequence of air pollution. Impacts on human health, on the other hand, include impaired brain health as well as a variety of different respiratory and cardiovascular diseases provoked by aerosol-induced inflammation, oxidative stress or even cell death. The tragic consequence is ~4 million premature deaths occurring worldwide every year that are attributable to atmospheric aerosols. Hence, air pollution was classified by the world health organization as an eminent risk factor. This highlights the urgency of efficient air pollution mitigation strategies, however, sound knowledge of source contributions and their associated health impacts is required. Source apportionment of aerosols is therefore key to improve the current situation. However, this task remains challenging due to the huge diversity of compounds, and atmospheric processes and reactions influencing the composition of aerosols. This thesis aims to contribute to a better understanding of source contributions to atmospheric particulate matter (PM) with focus on the situation in India, a country heavily affected by high air pollution levels. The outcome of this work will support projects launched to effectually lower air pollution levels with the overall goal to improve the well-being of people in highly polluted regions. The thesis conglomerates three stand-alone studies concentrating on the improvement and application of source apportionment techniques. Radiocarbon (14C), a powerful source apportionment tool for the discrimination of carbonaceous aerosols originating from fossil and non-fossil sources, serves as a common thread throughout the different sub-projects.
The first study provides the technical description of an improved setup for the 14C analysis of water-soluble organic carbon (WSOC). WSOC represents a prominent fraction of airborne aerosols and its water-solubility is indicative for atmospherically aged species, i.e. compounds that underwent chemical oxidation. This fraction is often challenging to apportion to specific sources as the molecular information has changed since its emission. Hence, 14C measurements of WSOC provide valuable insights into the origin of these species. Due to the low natural abundance of the 14C isotope, its measurement is exceedingly sensitive to carbonaceous contamination and low procedural blanks are a crucial prerequisite for successful analyses. This work presents results from extensive blank assessments and improvements performed on a system for the radiocarbon analysis of WSOC samples that yielded a desirable constant contamination level as low as 0.62 ± 0.12 μg of carbon. The contamination level was achieved by optimizing the conditions of the wet chemical oxidation procedure prior to 14C measurements. Moreover, the development and implementation of a non-coring glass-sintered needle minimized the amount of extraneous carbon-based materials being oxidized together with the sample. The installation of a non-dispersive infrared detector within the setup additionally enables more accurate sample quantification.
A comprehensive source apportionment study is presented in the second part of this work. It was conducted on atmospheric PM samples collected in Delhi during winter 2019. Several
samples were collected per day enabling results with diurnal resolution. Water-soluble organic aerosols (WSOA) were analyzed by means of an extractive electrospray ionization time-of-flight mass spectrometer. This technique provided, due to its non-destructive ionization process, molecular information of the organic PM fraction. Analysis of the resulting mass spectra with positive matrix factorization yielded five discriminative factors representing different sources of WSOA. Three factors were apportioned to primary emissions from biomass combustion contributing to the atmospheric pollution levels predominantly during nighttime. The remaining two factors were attributable to secondary organic aerosols (SOA), i.e. species that were formed due to gas-to-particle conversions from gaseous precursors. Both SOA factors exhibited increased concentrations during daytime. One was more influenced by aged species from biomass combustion while the other experienced influences from biogenic emissions. Supporting 14C measurements facilitated distinct discrimination between SOA from fossil and non-fossil sources as well as the estimation of water-insoluble, traffic-related hydrocarbon-like organic aerosols. The different PM contributors were finally used to explain the measured oxidative potential (OP) of the samples, a metric to estimate their toxicity. Results unveiled primarily emitted organic aerosols from biomass combustion to be the largest contributor to the OP measured in Delhi, while other organic aerosols appeared to be of minor and transition metals of negligible importance.
Finally, a radiocarbon-based source apportionment study was performed to compare the composition of carbonaceous aerosols in four highly polluted Indian cities during the transition from cold season to warm season. Trends observed for the three cities located in the Indo-Gangetic Plain (Delhi, Kanpur, Lucknow) were comparable. However, results for Pune, which is located in the Deccan Plateau in Western India, deviated significantly from the other cities. Samples collected in Delhi, Kanpur, and Lucknow were dominated by non-fossil influences, while carbonaceous aerosols in Delhi experienced slightly higher contributions from fossil sources than the other two cities in this region. In the Indo-Gangetic Plain, the largest fraction was apportioned to non-fossil organic carbon, but the relative influence of fossil emissions increased towards the warm season. Simultaneously, the atmospheric concentrations from carbonaceous aerosols decreased which is most likely attributable to less emissions from biomass combustion for residential heating. Air pollution in Pune, on the other hand, was predominantly assigned to fossil-fuel combustion without any obvious differences between cold and warm season. Moreover, the atmospheric concentrations of carbonaceous aerosols were significantly lower in Pune than for the other three cities. Hence, the higher relative importance of fossil species in Pune is more likely to be ascribed to lower levels of emissions from non-fossil sources rather than higher emissions from fossil ones. The findings of this study highlight that influences on the atmospheric aerosol composition may strongly vary for different Indian regions.
The first study provides the technical description of an improved setup for the 14C analysis of water-soluble organic carbon (WSOC). WSOC represents a prominent fraction of airborne aerosols and its water-solubility is indicative for atmospherically aged species, i.e. compounds that underwent chemical oxidation. This fraction is often challenging to apportion to specific sources as the molecular information has changed since its emission. Hence, 14C measurements of WSOC provide valuable insights into the origin of these species. Due to the low natural abundance of the 14C isotope, its measurement is exceedingly sensitive to carbonaceous contamination and low procedural blanks are a crucial prerequisite for successful analyses. This work presents results from extensive blank assessments and improvements performed on a system for the radiocarbon analysis of WSOC samples that yielded a desirable constant contamination level as low as 0.62 ± 0.12 μg of carbon. The contamination level was achieved by optimizing the conditions of the wet chemical oxidation procedure prior to 14C measurements. Moreover, the development and implementation of a non-coring glass-sintered needle minimized the amount of extraneous carbon-based materials being oxidized together with the sample. The installation of a non-dispersive infrared detector within the setup additionally enables more accurate sample quantification.
A comprehensive source apportionment study is presented in the second part of this work. It was conducted on atmospheric PM samples collected in Delhi during winter 2019. Several
samples were collected per day enabling results with diurnal resolution. Water-soluble organic aerosols (WSOA) were analyzed by means of an extractive electrospray ionization time-of-flight mass spectrometer. This technique provided, due to its non-destructive ionization process, molecular information of the organic PM fraction. Analysis of the resulting mass spectra with positive matrix factorization yielded five discriminative factors representing different sources of WSOA. Three factors were apportioned to primary emissions from biomass combustion contributing to the atmospheric pollution levels predominantly during nighttime. The remaining two factors were attributable to secondary organic aerosols (SOA), i.e. species that were formed due to gas-to-particle conversions from gaseous precursors. Both SOA factors exhibited increased concentrations during daytime. One was more influenced by aged species from biomass combustion while the other experienced influences from biogenic emissions. Supporting 14C measurements facilitated distinct discrimination between SOA from fossil and non-fossil sources as well as the estimation of water-insoluble, traffic-related hydrocarbon-like organic aerosols. The different PM contributors were finally used to explain the measured oxidative potential (OP) of the samples, a metric to estimate their toxicity. Results unveiled primarily emitted organic aerosols from biomass combustion to be the largest contributor to the OP measured in Delhi, while other organic aerosols appeared to be of minor and transition metals of negligible importance.
Finally, a radiocarbon-based source apportionment study was performed to compare the composition of carbonaceous aerosols in four highly polluted Indian cities during the transition from cold season to warm season. Trends observed for the three cities located in the Indo-Gangetic Plain (Delhi, Kanpur, Lucknow) were comparable. However, results for Pune, which is located in the Deccan Plateau in Western India, deviated significantly from the other cities. Samples collected in Delhi, Kanpur, and Lucknow were dominated by non-fossil influences, while carbonaceous aerosols in Delhi experienced slightly higher contributions from fossil sources than the other two cities in this region. In the Indo-Gangetic Plain, the largest fraction was apportioned to non-fossil organic carbon, but the relative influence of fossil emissions increased towards the warm season. Simultaneously, the atmospheric concentrations from carbonaceous aerosols decreased which is most likely attributable to less emissions from biomass combustion for residential heating. Air pollution in Pune, on the other hand, was predominantly assigned to fossil-fuel combustion without any obvious differences between cold and warm season. Moreover, the atmospheric concentrations of carbonaceous aerosols were significantly lower in Pune than for the other three cities. Hence, the higher relative importance of fossil species in Pune is more likely to be ascribed to lower levels of emissions from non-fossil sources rather than higher emissions from fossil ones. The findings of this study highlight that influences on the atmospheric aerosol composition may strongly vary for different Indian regions.
Date of Publication
2024
Year of graduation
2024
Theses Type
dissertation
Subject(s)
Language(s)
en
Author(s)
Faculty/Graduate School
Access(Rights)
open.access
Primary OA Publication
true