Detailed chemical characterization of submicrometer organic particles in the Amazon rainforest
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Abstract
Organic aerosols (OA) are a major component of the atmosphere and strongly influence climate, cloud formation, radiative forcing, air quality, and human health. Despite substantial advances in analytical chemistry, fundamental knowledge gaps remain regarding low- and semi-volatile compounds, highly oxidized species, and reactive organic molecules. This dissertation aims to improve the molecular-level understanding of organic aerosols by applying high-resolution mass spectrometry (HR-MS) to investigate their chemical composition, formation pathways, and environmental and anthropogenic controls within an integrated atmospheric and ecosystem framework.
The results demonstrate that the extreme chemical complexity of organic aerosols requires the combined use of high-resolution mass spectrometry and chromatographic separation to resolve isomeric structures, functional groups, and oxidation states. Non-targeted HR-MS approaches detect thousands of molecular formulas, while targeted analyses provide quantitative insights into specific precursor and marker compounds. Observations from the central Amazon rainforest show that biogenic secondary organic aerosols derived from isoprene, monoterpene, and sesquiterpene oxidation dominate the background composition, with pronounced seasonal differences between wet and dry periods. Molecular composition varies systematically with particle size and sampling height, reflecting distinct formation pathways, aging processes, and source contributions in ultrafine and fine particles. In particular, ultrafine particles are closely linked to specific biogenic oxidation products and play a key role in new particle formation and early growth.
Meteorological processes, especially rainfall events, strongly modify the chemical environment within the forest canopy and can trigger local new particle formation, followed by rapid growth to climatically relevant particle sizes. In addition, the availability of biogenic precursor compounds is shown to depend not only on atmospheric chemistry but also on soil biogeochemical processes. The molecular characterization of dissolved organic matter in Amazonian soils reveals pronounced phosphorus limitation and potential nitrogen and sulfur co-limitations, highlighting the role of nutrient cycling in controlling the supply of biogenic emissions that ultimately contribute to secondary organic aerosol formation.
Anthropogenic influences are addressed by investigating organosulfates, which constitute an important fraction of organic aerosol mass but are difficult to quantify due to their molecular diversity. By combining solid-phase extraction, HR-Orbitrap-MS, and charged aerosol detection, this work demonstrates that a substantial fraction of organosulfates occurs in chromatographically unresolved form and has therefore been largely underestimated in previous studies. These findings emphasize the importance of sulfur emissions for organic aerosol formation and highlight the potential of emission control strategies to improve air quality.
Overall, this dissertation provides an integrated, molecularly resolved perspective on organic aerosols that links chemical composition with particle properties, meteorological drivers, ecosystem controls, and anthropogenic emissions. The results highlight the central role of high-resolution mass spectrometry in unraveling the complexity of organic aerosols and contribute to a more mechanistic understanding of their formation, transformation, and impacts on climate and air quality.