10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Effects of Temperature on Nucleated Particles from α-Pinene Ozonolysis Measured by a FIGAERO-Chemical Ionization Mass Spectrometer
QING YE, Mingyi Wang, Victoria Hofbauer, Dexian Chen, Jasper Kirkby, Neil Donahue, CLOUD Collaboration, Carnegie Mellon University
Abstract Number: 1738 Working Group: Aerosol Chemistry
Abstract Oxidation products from α-pinene ozonolysis form particles that will scatter sunlight and nucleate clouds. These products, formed via various reaction pathways under different environmental conditions, have a wide range of volatilities and structures that will determine both their gas-particle phase partitioning and their tendency to participate in condensed phase reactions. It has been shown that highly oxygenated multifunctional molecules (HOMs) from α-pinene ozonolysis contribute to nucleation and particle initial growth. Thus understanding the formation of HOMs is crucial to constrain particle loading in the pre-industrial atmosphere. Recent research shows that HOMs are formed via autooxidation process where highly oxygenated RO2 radicals are formed through multiple cycles of intramolecular hydrogen abstraction and O2 addition to the initial RO2 radicals from α-pinene ozonolysis. The autooxidation process, which is a unimolecular reaction, is strongly temperature dependent, and low temperature slows down the autooxidation rate. Therefore, nucleation and initial particle growth is expected to reduce. On the other hand, low temperature favors condensation of materials with higher volatilities (semi-volatiles become low-volatiles and low-volatiles become extremely low-volatiles at low temperature), which are presumably less polar and oxygenated. Thus, composition and properties of the nucleated particles at different temperatures could be very different.
Chemical ionization mass spectrometry coupled with a filter inlet for gas and aerosols (FIGAERO-LToF-CIMS) can measure the chemical composition of gas and condensed phase products from α-pinene ozonolysis with high time resolution and high mass resolution. We deployed a FIGAERO-LToF-CIMS with iodide as the reagent ion in the experiments in the Cosmics Leaving Outdoor Droplets (CLOUD) chamber at CERN. In the experiments, particles were formed through pure organic nucleation under 3 different temperatures, 25oC, 5oC and -50oC with 600 ppt α-pinene and 40 ppb ozone without the addition of NOx and under dark conditions. When gas phase products were analyzed by the CIMS, particle phase products were being collected on the Teflon filter for 30 mins. During the particle measurement mode, the filter was heated up to 170oC from room temperature to evaporate materials collected on the filter and the materials were then analyzed by the mass spectrometer.
Our preliminary results show that we are able to clearly measure the monomers (C8−10 compounds) and dimers (C16−20 compounds) in the particle phase under all temperatures in our experiments. At 5oC, both the monomers and dimers mass range contain products with higher oxygen number than at -50oC, and the overall O/C is higher at 5oC than at -50oC. At 25oC, oxidized products in the particles contain molecules with 6 oxygens or more while at -50oC, molecules with 3 oxygens start to appear in the particles phase. In addition, we observed more fragmentation (increased fraction of C8−9 compounds in monomers and increased fraction of C16−19 compounds in dimers) at higher temperature than at low temperature. Our work will provide better understanding on particle formation and properties of the nucleated particles from α-pinene ozonolysis at different atmospheric temperature conditions.