Sources and concentrations of atmospheric organic pollutants in gaseous and particulate phase
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Date
2026-07-24
Authors
Ματραλή, Αγγελική
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Abstract
Air pollution is one of the leading factors driving premature mortality currently on the globe, with an estimated contribution in the order of 10 million deaths annually attributed to inhalation of airborne fine particles. A major fraction of submicron aerosol is organic (OA) and can be either primary (directly emitted in the particle phase) or secondary (formed from gas to particle conversion processes). The secondary OA (SOA) has been found to be more hazardous and harder to regulate, with chemical transport models (CTMs) often underestimating its concentration. A central reason for this discrepancy is the incomplete understanding of the multigenerational oxidation pathways that lead to SOA formation.
This thesis investigates the formation, chemical evolution, and atmospheric relevance of secondary organic aerosols from both anthropogenic and biogenic precursors. It combines controlled smog chamber and oxidation flow reactor experiments with field measurements in urban and remote environments. Emphasis is placed on the role of intermediate volatility organic compounds, the interaction between anthropogenic and biogenic emissions, and the ability of measured gas phase precursors to explain observed SOA formation.
The first part of this work investigates the mass balance gap between observed and predicted SOA produced during the oxidation of road transport emissions. Parallel analysis of the levels of organic vapors and their photooxidation was conducted in two environments heavily influenced by vehicular emissions. The precursors collected on sorption tubes were analyzed offline with thermal desorption gas chromatography mass spectrometry (TD-GC-MS). The potential SOA that could be formed in these environments was quantified using an oxidation flow reactor. For both environments the estimated SOA from the measured precursors could account for practically all the observed SOA. However, the importance of intermediate-volatility organic compounds (IVOCs) varied strongly between the two environments. In the Fréjus tunnel, where diesel-fueled vehicles had a stronger influence, IVOCs dominated the predicted SOA formation, contributing approximately 70%. In contrast, in the gasoline-influenced environment of an underground parking, IVOCs accounted for only about one quarter of the total SOA. These results highlight the importance of intermediate volatility precursors in the diesel-dominated vehicular European fleet.
The second part of the thesis focuses on biogenic SOA formation from sesquiterpenes. Although sesquiterpene emissions are substantially lower than monoterpene emissions, their ozonolysis can produce semi-volatile and intermediate-volatility products that continue to react in the atmosphere. This work shows that this sesquiterpene SOA source has been seriously underestimated, with the aging of ozonolysis products reacting with the OH radical producing SOA at mass yields exceeding 100%. This later generation SOA is quite oxidized (O:C above 0.5) and its composition is consistent with field observations. The corresponding multigenerational chemistry was parameterized and implemented in a chemical transport model. The simulations suggest that, during summer in European forested regions, inclusion of this chemistry can increase sesquiterpene-derived SOA concentrations by up to 1.5 μg m-3, corresponding to a factor of 3–5 enhancement compared with previous estimates.
The third part of the thesis examines organic vapors and particulate organics in an urban environment. A month-long measurement campaign was conducted at an urban background station in Athens during January and February 2025. Individual organic compounds in both the gas phase and the submicron particulate phase were measured using a PTR-ToF-MS coupled with a CHARON inlet. Source apportionment of the VOCs indicated contributions from various primary and oxidized factors. The concentrations of major compounds did not appear to have changed significantly over the last decade, despite the factor of 2.6 reduction in estimated anthropogenic emissions assumed in the most recent European emissions inventory for this area. Comparison with a collocated PTR-Quad-MS and low-cost total VOC electrochemical sensors was conducted assessing the sensitivity of these instruments to specific VOCs. In the particle phase, OA components showed significant fragmentation in the PTR-ToF-MS, and the 215 identified species or fragments represented approximately 10% of the total OA. Biomass-burning tracers, including dehydroabietic acid, abietic acid, nitrophenol, and eugenol, were detected, while oleic, palmitic, and stearic acids were quantified as cooking-related OA tracers, corresponding to approximately 0.5–1% of the cooking OA factor.
The final part of the thesis investigates summertime aerosol composition and sources at a remote forested site in the Eastern Mediterranean. The SPRUCE-22 field campaign was conducted in July 2022 at a remote mountainous site in Greece. Particle chemical composition, size distributions, and concentrations of organic and inorganic gases were measured at high time resolution, complemented by offline analysis of collected samples. Despite the remote location, average PM₁ concentrations were unexpectedly high, reaching 13 μg m-3 dominated by organics, in a previously sulfate rich area pertaining to reductions in SO₂ emissions from coal-burning power plants in Greece. The observed concentrations were uniformly distributed across Greece with PM2.5 concentrations at the site occasionally higher than, those observed in Athens, in the absence of influences from local sources. This background aerosol was highly oxidized with an average O:C ratio of 0.83. Mean isoprene and monoterpene mixing ratios were 0.6 ppb and 0.5 ppb, respectively. Source apportionment identified highly and moderately oxidized OA as well as biogenic SOA. Radiocarbon analysis was used in conjunction with the source apportionment factors to further elucidate the origin of the observed PM. The biogenic SOA was also relatively aged, with an O:C ratio of 0.63, and was primarily transported to the site. Nearby new particle formation events are related to the concentrations of precursor species and origin of the air masses. The oxidative potential of the particulate matter per unit mass was high, emphasizing the potential health relevance of regionally transported aerosol even at remote locations.
The results of this work highlight the importance of specific gaseous precursors in explaining organic aerosol by accounting for the in situ observed SOA of traffic gaseous precursors to highlighting the importance of multigeneration reactions of biogenic IVOCs like β-caryophyllene. Field measurements indicate the dominance of long-range transport on a regional scale during a photochemically active period. Conversely, the urban wintertime pollution of the Greek capital is assessed (with respect to organic vapors) and related to equivalent measurements of more than a decade ago. New insights are gained with respect to SOA precursors, while on-line measurements of major PM constituents on the molecular level are first conducted in Athens using high resolution mass spectrometry.
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Keywords
Secondary organic aerosol, Atmospheric chemistry, Intermediate volatility organic compounds