10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Developing Model Surrogates for Monoterpenes to Improve Predictions of Secondary Organic Aerosol
ISAAC AFREH, Bernard Aumont, Marie Camredon, Richard Valorso, Kelley Barsanti, University of California, Riverside
Abstract Number: 1560 Working Group: Aerosol Modeling
Abstract Global emissions of biogenic volatile organic compounds (BVOCs) are on the order of 1000 Tg carbon yr-1. Monoterpenes (C10H16) account for approximately one-fifth of the total estimated BVOC emissions. Monoterpenes also represent a significant mass fraction of VOCs emitted during biomass burning, particularly from coniferous fuels. These atmospherically relevant monoterpenes exhibit large diversity in molecular structure. For example, in pine and spruce biomass burning samples, approximately thirty isomers were identified. Once emitted to the atmosphere, monoterpenes can impact climate and air quality through the production of secondary organic aerosol (SOA). The extent of SOA formation is dependent on the mass of the emitted compound, its atmospheric reactivity, and the volatility of oxidation and reaction products. To represent SOA formation in chemical transport models a gas-phase chemical mechanism is used to represent the reactivity of VOCs and subsequent oxidation product formation. To enhance computational efficiency, most gas-phase mechanisms adopt simplification strategies whereby individual VOCs are lumped into surrogate species, largely based on the reactivity of these compounds with hydroxyl radical. While such mechanisms are routinely used for modeling SOA, the lumping approaches have not been optimized to best represent the reactivity and properties of VOCs that serve as precursors to SOA. Monoterpenes are often represented by one model surrogate in gas-phase chemical mechanisms and SOA parameterizations used in chemical transport modeling, even though monoterpenes are known to have diverse chemical structures, different reaction rates with atmospheric oxidants (by orders of magnitude), and varied propensity for SOA formation.
In this work, optimization of the number and identities of monoterpene model surrogates was pursued by evaluating factors that influence SOA production including the reaction rates and products of monoterpenes with atmospheric oxidants, and the products and extents of SOA formation. GECKO-A, an explicit chemical mechanism generator and SOA model, was used to simulate SOA formation from ten individual monoterpenes under a range of NOx conditions, precursor concentrations, and particle loadings. The relative reactivity of the monoterpenes, the gas- and particle-phase products formed, and the rates and yields of SOA production were considered in optimizing the surrogates. Results to be presented include the SOA yields for each monoterpene under the different simulation conditions, the distribution of gas- and particle-phase species in the carbon oxidation state – volatility space, and the mass-based contributions of compounds to SOA as a function of number of carbons and functional groups. Further, the criteria for defining the optimum number of surrogates for monoterpenes will be presented. The GECKO-A based SOA formation analyses developed in this work form the basis for future work on optimization of model surrogates to better represent other classes of SOA precursors in gas-phase chemical mechanisms and SOA parameterizations used in chemical transport modeling. A broader consideration of the chemistry relevant for SOA formation and representation of that chemistry in chemical mechanisms will ultimately lead to more accurate predictions of the air quality and climate impacts of SOA.