Using a Near-Explicit Model, GECKO-A, to Improve the Mechanistic Understanding of Monoterpene Chemistry as Relevant to Secondary Organic Aerosol (SOA) Formation
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Using a Near-Explicit Model, GECKO-A, to Improve the Mechanistic Understanding of Monoterpene Chemistry as Relevant to Secondary Organic Aerosol (SOA) Formation

  • Author(s): Afreh, Isaac
  • Advisor(s): Barsanti, Kelley C
  • et al.
Abstract

Monoterpenes, which are emitted from biogenic and pyrogenic sources, represent a significant mass fraction of volatile organic compounds (VOCs) emitted to the earth’s atmosphere. Monoterpenes exhibit wide diversity in molecular structure and ranges of atmospheric lifetimes, and can impact climate and air quality through the formation of secondary organic aerosol (SOA). To accurately predict the effects of SOA on climate and air quality, better representation of monoterpene chemistry in chemical mechanisms and improved SOA parameterizations are needed. For air quality modeling, the gas-phase chemistry of and SOA formation from monoterpenes are often represented by one or two model surrogates, despite the complexity in monoterpene chemistry and SOA formation potential. While the simplified approach for representing monoterpenes in models enhances computational efficiency, it results in uncertainties in model predictions.As presented in this thesis, detailed studies of gas-phase chemistry and SOA formation from monoterpenes were performed using a near-explicit chemical mechanism and box model, GECKO-A. This includes the first mechanistic study of SOA formation from camphene, a ubiquitous but understudied monoterpene. The mechanistic study of SOA formation prompted a chamber study of this interesting compound, and comparison between the model predictions and measurement data were performed. Finally, using GECKO-A and published chamber data, gas-phase chemistry and SOA formation from 13 monoterpenes was studied in an effort to suggest a simplified modeling strategy that better represented the complexity of monoterpene chemistry in air quality models. The mechanistic study demonstrated that: (1) in the early stages of oxidation, camphene formed very low volatility products, lower than commonly studied monoterpenes α-pinene and limonene; and (2) the final simulated SOA yield for camphene (46 %) was relatively high, approximately twice that of α-pinene (25 %). The model-measurement comparison for camphene supported that camphene forms significant SOA. The SOA yield trends were similar between the model simulations and the chamber studies when nitrogenic oxides were present. The systematic study of the 13 monoterpenes indicated that monocyclic monoterpenes with internal double bonds generally formed more SOA than bicyclic monoterpenes with the exception of camphene. Cluster analysis (k-means) was explored to optimize the number of monoterpene surrogates.

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