10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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The Predicted Impact of Organic Coatings on Isoprene-Derived Secondary Organic Aerosol Formation
WILLIAM VIZUETE, Mutian Ma, Yue Zhang, Sri Hapsari Budisulistiorini, Havala Pye, Jason Surratt, Yuzhi Chen, Ryan Schmedding, Sarah Farrell, University of North Carolina at Chapel Hill
Abstract Number: 492 Working Group: Aerosol Chemistry
Abstract Fine particulate matter (PM2.5) is known to have an adverse impact on public health and is an important climate forcer. Secondary organic aerosol (SOA) contributes to PM2.5 and the multiphase reactions between gas- and particle-phase constituents is an important source. Aerosol-phase state is thought to influence the reactive uptake of gas-phase precursors to aerosol particles by altering diffusion rates within particles. Current air quality models (AQM), however, do not include this feature. This work examines the predicted impact of organic coatings on the formation of SOA from the acid-catalyzed multiphase reactions of isoprene epoxydiols (IEPOX). New experimental studies have provided a systematic quantification of the reactive uptake coefficient (γ) of trans-β-isoprene epoxydiol (trans-β-IEPOX), the predominant IEPOX isomer, onto acidic sulfate particles with varying organic coatings. In some experiments the uptake coefficient was reduced by half of the original value when coatings were present. This new experimental data, and published kinetic rates, now allow for examination of constrains on critical parameters needed to simulate coating effects on reactive uptake. This work developed a 0-dimensional box model that incorporated the latest kinetic data that accounts for coating influences. Meteorological inputs and gas phase concentration data were provided from field measurements at the Look Rock, Tennessee (LRK) ground site during the 2013 Southern Oxidant and Aerosol Study (SOAS). The LRK site also provided particle-phase chemical data of IEPOX-derived organosulfates and 2-methyltetrols that was used for model performance evaluations.
Including a coating influence in the reactive uptake algorithm reduced predicted IEPOX-derived SOA on average by 31%. One critical parameter in the model algorithm was the inorganic Henry’s Law coefficient (H), with a range of reported values from 2.7 6 M/atm to 4.0 8 M/atm. Sensitivity simulations from this work found that H had the greatest impact on predictions when a 10-fold increase led to 8-fold increase in SOA concentrations. The organic diffusion coefficient (Dorg) and organic Henry’s Law coefficient (Horg) were also found to be a critical parameter. For these simulations, the reported 0.3 M/atm was used for Horg. In the literature Dorg has been derived as a function of relative humidity (RH) either from experimental measurements of IEPOX uptake on -pinene coated sulfate aerosols, or estimates from viscosity data of -pinene SOA. In these model runs when the Dorg is , IEPOX uptake was completely inhibited and when Dorg reached organic coatings had no effect. Using the LRK filter data, and published range of parameter values, a simulation was completed that produced the best model performance where the empirically derived Dorg from -pinene coated aerosols was used with a H of 8.0 7 M/atm and Horg of 0.3 M/atm. This simulation increased SOA concentrations improving the normalized mean bias to -24.7%.
These results suggest that the organic coating layer could have significant impact on the IEPOX-derived SOA formation and should be considered in AQMs. This work relied on organic diffusion coefficients derived from our previous -pinene SOA experiments, and thus, only represents one possible type of organic coatings in the atmosphere. Other organic coatings, such as those derived from anthropogenic and other biogenic sources (including isoprene itself) could have a different impact on the acid-catalyzed reactive uptake of IEPOX. Furthermore, part of the Dorg used in this work were derived with flow tube experiment data occurring only under RH conditions between 15-50% while the Dorg for higher RH are based on viscosity data and experimental extrapolation. Further experimental studies are needed to constrain these parameters in >50% RH environments.