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Graduate Student, California Institute of Technology


Deciphering the chemistry of organic aerosol and its impacts on climate, air quality, and health.

Atmospheric aerosol is a highly variable and complex mixture, largely composed of organic compounds derived from the oxidation of anthropogenic and biogenic emissions. Aerosol particles are ubiquitous in the atmosphere, exerting large but uncertain effects on Earth’s radiative budget directly, by scattering and absorbing radiation, and indirectly, by altering cloud albedo and lifetime as cloud condensation nuclei. Exposure to ambient aerosol has also been shown to cause damaging effects to the respiratory and cardiovascular systems. Owing to the chemical complexity of atmospheric organic aerosol (OA), which generally consists of thousands or more compounds of diverse chemical classes, molecular-level analysis represents one of the most formidable challenges in atmospheric chemistry, resulting in a limited understanding of the fundamental chemical processes that govern the formation, properties, and fate of OA in the atmosphere. Through a combination of advanced mass spectrometric, chromatographic, and synthetic techniques, my research seeks to decipher the molecular composition, origins, and formation mechanisms of OA that are essential to unraveling its impacts on climate, air quality, and health.


A large source of low-volatility secondary organic aerosol.

Abstract: Forests emit large quantities of volatile organic compounds (VOCs) to the atmosphere. Their condensable oxidation products can form secondary organic aerosol, a significant and ubiquitous component of atmospheric aerosol, which is known to affect the Earth's radiation balance by scattering solar radiation and by acting as cloud condensation nuclei. The quantitative assessment of such climate effects remains hampered by a number of factors, including an incomplete understanding of how biogenic VOCs contribute to the formation of atmospheric secondary organic aerosol. The growth of newly formed particles from sizes of less than three nanometres up to the sizes of cloud condensation nuclei (about one hundred nanometres) in many continental ecosystems requires abundant, essentially non-volatile organic vapours, but the sources and compositions of such vapours remain unknown. Here we investigate the oxidation of VOCs, in particular the terpene α-pinene, under atmospherically relevant conditions in chamber experiments. We find that a direct pathway leads from several biogenic VOCs, such as monoterpenes, to the formation of large amounts of extremely low-volatility vapours. These vapours form at significant mass yield in the gas phase and condense irreversibly onto aerosol surfaces to produce secondary organic aerosol, helping to explain the discrepancy between the observed atmospheric burden of secondary organic aerosol and that reported by many model studies. We further demonstrate how these low-volatility vapours can enhance, or even dominate, the formation and growth of aerosol particles over forested regions, providing a missing link between biogenic VOCs and their conversion to aerosol particles. Our findings could help to improve assessments of biosphere-aerosol-climate feedback mechanisms, and the air quality and climate effects of biogenic emissions generally.

Pub.: 28 Feb '14, Pinned: 31 Aug '17

Formation and evolution of molecular products in α-pinene secondary organic aerosol

Abstract: Much of our understanding of atmospheric secondary organic aerosol (SOA) formation from volatile organic compounds derives from laboratory chamber measurements, including mass yield and elemental composition. These measurements alone are insufficient to identify the chemical mechanisms of SOA production. We present here a comprehensive dataset on the molecular identity, abundance, and kinetics of α-pinene SOA, a canonical system that has received much attention owing to its importance as an organic aerosol source in the pristine atmosphere. Identified organic species account for ∼58–72% of the α-pinene SOA mass, and are characterized as semivolatile/low-volatility monomers and extremely low volatility dimers, which exhibit comparable oxidation states yet different functionalities. Features of the α-pinene SOA formation process are revealed for the first time, to our knowledge, from the dynamics of individual particle-phase components. Although monomeric products dominate the overall aerosol mass, rapid production of dimers plays a key role in initiating particle growth. Continuous production of monomers is observed after the parent α-pinene is consumed, which cannot be explained solely by gas-phase photochemical production. Additionally, distinct responses of monomers and dimers to α-pinene oxidation by ozone vs. hydroxyl radicals, temperature, and relative humidity are observed. Gas-phase radical combination reactions together with condensed phase rearrangement of labile molecules potentially explain the newly characterized SOA features, thereby opening up further avenues for understanding formation and evolution mechanisms of α-pinene SOA.

Pub.: 02 Nov '15, Pinned: 31 Aug '17