Novel Techniques for the Measurement of Atmospheric Organic Aerosol Physiochemical Properties

Abstract

Atmospheric aerosols significantly impact both human health, as they increase the frequency of cardiovascular and respiratory diseases, and climate change, as they affect the Earth’s energy balance. Organic aerosol (OA) comprises a substantial fraction of atmospheric aerosol, but there is limited knowledge about its sources, chemical evolution, and physical properties, leading to considerable uncertainty in atmospheric chemical transport model predictions. The magnitude of the impact of OA on human health and climate change is strongly dependent on an improved understanding of the evolution of organic compounds in the atmosphere. Hygroscopicity, oxidation level, and volatility are three of the most crucial properties of OA because they control its atmospheric fate and climate relevance. Hygroscopicity is a measure of a compound’s ability to interact with and absorb water. The oxidation level is a useful surrogate for changes in chemical composition since the atmosphere is an oxidizing environment. Volatility determines, to a large extent, the partitioning of compounds between the particulate and vapor-phases. This work develops and tests novel techniques that can be used to measure these critical properties of OA and provide a foundation to update these properties in atmospheric models.The first part of this work describes the development and testing of a technique that quantitatively relates the hygroscopicity and oxidation level of OA components to their volatility. The technique utilizes a thermodenuder (TD) along with aerosol mass spectrometry and size-resolved cloud condensation nuclei measurements to separate the OA by volatility and characterize its hygroscopicity and oxidation level. The technique was tested with secondary OA (SOA) from the ozonolysis of α-pinene and the results indicated that the least volatile components in this SOA system were the least hygroscopic and least oxidized. The second part of this work improves the characterization of the volatility distribution of OA by combining TD and isothermal dilution measurements. The technique was tested with SOA from the ozonolysis of α-pinene and cyclohexene. The results from this work demonstrated the challenges that arise in estimating the volatility distributions of OA using thermal evaporation techniques and the benefits of combining techniques to measure properties of OA.The third part of this work combines the techniques from the first and second parts of this thesis and applies them to SOA from the ozonolysis of α-pinene, limonene, and cyclohexene. The results suggested that some OA systems have a more complex relationship between these properties than originally thought. Use of the techniques developed in this thesis to different OA, both laboratory and ambient, can supply needed parameters that can be incorporated in atmospheric models.