Atmospheric aging of biomass burning emissions : formation of secondary aerosols and changes in toxicity
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Date
2026-09-24
Authors
Γεωργοπούλου, Μαρία
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Abstract
Biomass burning (BB) is a major global source of particulate matter (PM) and volatile organic compounds (VOCs). After emission, BB pollutants undergo atmospheric aging driven by sunlight and oxidants such as hydroxyl (OH) and nitrate (NO3) radicals. These processes alter aerosol mass, chemical composition, oxidation state, optical properties, and oxidative potential (OP), thereby modifying their effects on air quality, human health, and climate. However, the mechanisms governing secondary organic aerosol (SOA) formation and toxicity across realistic day–night cycles remain incompletely understood. This doctoral thesis investigates how these processes control SOA formation, chemical evolution, and water-soluble oxidative potential (WS-OP), an acellular indicator of the ability of aerosol components to induce oxidative stress. The work combines atmospheric simulation-chamber experiments, high-resolution gas- and particle-phase measurements, and size-resolved ambient observations. The aerosol WS-OP was quantified using a semi-automated dithiothreitol (DTT) assay.
A novel experimental platform was developed by coupling, for the first time, the FORTH mobile atmospheric simulation chamber with the CESAM reactor at the LISA laboratory (Laboratoire Interuniversitaire des Systèmes Atmosphériques, CNRS) in Créteil, France. The system enabled the continuous production of stable and atmospherically representative fresh and aged aerosols from pellet combustion for up to 72 h under controlled conditions, supporting realistic simulations of urban pollution and long-term exposure studies in preclinical models. Photochemical aging increased the aerosol oxygen-to-carbon (Ο:C) ratio by approximately 35% and enhanced oxidative potential by up to a factor of two, demonstrating that atmospheric processing can increase the intrinsic oxidative activity of BB aerosol.
Subsequently, experiments with olive-wood and olive–pine emissions showed that oxidation history and relative humidity determine the trajectory of aerosol mass, composition, and oxidative activity. Under initially dry conditions (RH 12–24%), organic aerosol mass increased by 35–90% and O:C by 56% on average. Despite sequence-dependent differences during intermediate stages, the final day–night and night–day mass spectra converged (spectral angle θ=3°). In addition, atmospheric aging increased the WS-OP by approximately 60%, with the final mean values of both experimental sequences converging to 68 ± 18 pmol min⁻¹ μg⁻¹.
In the next phase of the work, the SOA formation during the nighttime and daytime oxidation of some of the major volatile organic compounds emitted during biomass buring: phenol, m-cresol, and catechol was investigated. During nighttime oxidation, more than 95% of the products of phenol- and m-cresol-derived remained in the gas phase, forming reservoirs of oxygenated and nitrogen-containing secondary vapours. Subsequent OH exposure of these products increased m-cresol SOA 9.4-fold even though approximately 96% of the precursor had already reacted at night, demonstrating that nighttime products can prime aerosol formation after sunrise. Catechol followed a different pathway: 24% of the nighttime products were already in the particle phase, and subsequent OH exposure mainly increased aerosol oxidation state rather than mass.
Finally, the water-soluble oxidative potential of fine (PM2.5) and ultrafine (PM0.1) ambient particles was compared, for the first time in Greece, between a regional-background site in Poros and an urban site (National Observatory of Athens; NOA) in Thissio, Athens. At Poros, the mass-normalized DTT activity (DTTm) of PM0.1 averaged 50.9 ± 20.8 pmol min-1 μg-1 and was approximately four times that of PM2.5. This OP was associated mainly with organic aerosol and nitrate, whereas PM2.5 activity was more closely related to chloride, sulfate, and more-oxidized oxygenated organic aerosol (MO-OOA), indicating marine, shipping, and transported aged influences. In Athens the size dependence of the WS-OP had the opposite behaviour. The PM2.5 activity was on average 33.3 ± 6.8 pmol min-1 μg-1, twice that of PM0.1. Covariance between biomass-burning organic aerosol (ΒΒΟΑ) factor and potassium (K+) confirmed fresh biomass-burning influence on PM0.1 composition. The reversal between the two environments demonstrates that intrinsic particle oxidative activity is controlled by source mixture, chemical composition, and atmospheric processing rather than by particle mass or size alone.
Overall, the thesis links oxidation history and molecular product formation with aerosol phase transfer, size distribution, and potential toxicity. The results demonstrate that PM mass and bulk oxidation state are insufficient descriptors of health-relevant aerosol properties and support the inclusion of precursor-specific chemistry, sequential day–night processing, and size-resolved composition in assessments of biomass-burning pollution and its potential health effects.
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Keywords
Biomass burning, Atmospheric aging, Secondary organic aerosol, Daytime and nighttime oxidation, Oxidative potential, Particulate matter, Volatile organic compounds, Aerosol toxicity, Coupled atmospheric simulation chamber system, Production of realistic aerosols