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Determining Biogenic and Anthropogenic Contributions to Secondary Organic Aerosol

Mehra, Archit

[Thesis]. Manchester, UK: The University of Manchester; 2021.

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Abstract

Aerosols are ubiquitous in the troposphere with impacts upon climate, air quality and human health. Secondary organic aerosol (SOA) is a major component of aerosol, formed from oxidation of anthropogenic and biogenic volatile organic compounds (VOC). Emissions and speciation of VOCs are dependent upon local ecosystems and human activity, thus unique blends of VOCs are observed at local scales. VOCs are diverse in their composition and reactivity, yet our knowledge of the mechanisms by which their degradation leads to SOA formation is limited. It remains unclear if the limited species studied are representative of the emissions which are most important for SOA on local and regional scales. This thesis addresses the uncertainties surrounding SOA in two ways using iodide-anion chemical ionisation mass spectrometry (CIMS): firstly, laboratory experiments were carried out to characterise the SOA from previously understudied anthropogenic and biogenic VOC. Secondly, understanding derived from single component experiments was applied to SOA from sage plant species native to coastal southern California, and ambient measurements of SOA carried out as part of the Air Pollution and Human Health (APHH) project in Beijing during the summer of 2017. Study of SOA from emissions of California Sage plants, for the first time, identifies a contribution from highly oxygenated organic molecules (HOM) and oligomers, which are attributed the dominant emissions from oxygenated monoterpenes. Products from oxidation of aromatic VOCs, though similar, show variations in the relative proportion of ring-retaining, ring-scission and HOM products. Ring-retaining products, specifically HOM are more abundant for the more substituted aromatic isomers while while ring-scission products, many more oxidised than previously reported, are more important for those less substituted. Both in the case of biogenic and anthropogenic SOA, the majority of ion signal is not unique to a given precursor or system. Positive matrix factorisation (PMF) was applied to ambient SOA measurements from Beijing, identifying factors with distinct temporal variability including those related to aromatic oxidation, including HOM formation and those related to monoterpene and sesquiterpene oxidation. Several factors were dominated by ions potentially formed from both anthropogenic and biogenic sources, highlighting the challenge of distinguishing unique contributions to SOA using online mass spectrometry.