Simulating IEPOX multiphase chemistry chamber experiments using MOSAIC box model

Isoprene epoxydiols (IEPOX) is a major precursor of isoprene-derived secondary organic aerosols (SOA) in the atmosphere. Recent advances in understanding of the IEPOX-SOA formation pathways based on laboratory experiments stimulate growing interest in analyzing limiting factors in the IEPOX-SOA formation. To date, computational approaches to model IEPOX-SOA formation are very limited. We will present a modeling study of the multiphase chemistry of IEPOX that utilizes the Model for Simulating Aerosol Interactions and Chemistry (MOSAIC). The model was extended to include process representations of gas-phase IEPOX uptake onto acidified sulfate particles and its subsequent reactions with liquid water and sulfate to form aqueous IEPOX-SOA. The model validation was performed against chamber experiments conducted under various conditions, including different aerosol acidity and relative humidity. In addition to particle size distribution monitoring by a scanning mobility particle sizer, the experiments applied single-particle measurements through SPLAT II and miniSPLAT to characterize in-situ and in real-time particle density, shape, and compositions. Three plausible mechanisms for IEPOX reactive uptake (ɣ_IEPOX) and the formation of two key IEPOX-SOA components, namely 2-methyltetrols and organosulfates, were tested. In addition, the model sensitivity to particle-phase reaction rate constant (k_particle) were assessed. Our preliminary results showed that the model reproduces changes in particle size distribution at steady state, although the simulated particle growth rate is significantly faster (half-life of the rise: ~10 seconds) compared to observed (~15 minutes). The model captures the measured variation of methyltetrols with humidity qualitatively, i.e., higher humidity decreases and increases simulated tetrol production, which is consistent with measurements. However, the model tends to underestimate organosulfate and overestimate methyltetrols, for both ammonium bisulfate and ammonium sulfate seeds at dry and wet conditions, and across different k_particle. For atmospheric relevance, we further investigate the suppression of ɣ_IEPOX by α-pinene SOA coatings. Future research will investigate a better representation of aqueous IEPOX-SOA formation to be applied in 3D models.