10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Volatility Distribution of Primary Organic Aerosol from Food-Cooking Emissions and Its Evolution upon Oxidation
MANPREET TAKHAR, Arthur W. H. Chan, University of Toronto
Abstract Number: 1311 Working Group: Aerosol Chemistry
Abstract Food cooking emissions represent one of the most important sources of primary organic aerosol (POA) in urban areas and have been shown to contribute 10-34% of total organic aerosol. However, the evolution of cooking emissions in the atmosphere is not fully understood. The gas-particle partitioning of cooking-related organic compounds is largely determined by their vapor pressures and volatilities, which can also have a significant impact on aerosol chemistry. In this work we study the volatility distribution of primary and secondary organic aerosol (P/SOA) from cooking emissions. We use cooking oils (canola oil, olive oil, peanut oil) as a surrogate to investigate the chemical composition and volatility using thermal desorption-gas chromatography mass spectrometry (TD-GC/MS), and a thermodenuder. In particular, we focus on the composition of intermediate volatile organic compounds (IVOCs) emitted from food-cooking emissions. Heated cooking oil particles were oxidised in a quartz flow tube reactor in presence of ozone or hydroxyl radicals to simulate atmospheric aging. Tenax tube samples and quartz filter samples were collected for analysis of IVOCs and particle-phase organics. We hypothesize that the overall volatility of oxidised cooking aerosol decreases upon oxidation due to addition of functional groups.
The unoxidized cooking emissions were found to be mainly comprised of saturated fatty acids ranging from C12-C20, along with palmitoleic acid, oleic acid, linoleic acid and sterols. The oxidised cooking emissions were comprised of dicarboxylic acids, hydroxy fatty acids, and short chain fatty acids. The organic compounds are therefore becoming more functionalized upon oxidation as hypothesized. The vapour pressures of organic compounds identified in cooking oil samples were estimated based on GC retention times and compared to the temperature dependent volatility distribution measured using the thermodenuder. Upon oxidation, the thermodenuder showed a decrease in the overall volatility as indicated by increased mass fraction remaining at high temperatures. However, the organic compounds identified using GC/MS are smaller and have higher vapor pressures. The inconsistency between the thermodenuder and GC/MS results could be ascribed to a few reasons. The first is the fragmentation of higher molecular weight compounds in the TD-GC/MS due to high desorption temperatures, thus leading to higher apparent volatility. Another possible explanation is the change in the morphology of the particles upon oxidation. Upon oxidation the particles may become sufficiently viscous to hinder evaporation or growth. Thus, while the compounds become more volatile, they also become more difficult to evaporate upon heating. The mass transfer limitations within the particle were investigated using thermodenuder mass transfer models to evaluate the significance. Overall, our results highlight the need to understand both the chemical composition and the particle-phase state to understand the evolution of cooking emissions in the atmosphere.