PNNL

Abstract

Isoprene is an abundant atmospheric volatile organic compound emitted from broadleaf forests. Under low-nitric oxide (NO) concentrations, isoprene is photochemically oxidized by hydroxy radical to form substantial amounts of gas-phase isoprene epoxydiols (IEPOX). In the presence of acidified sulfate aerosols, IEPOX enhances secondary organic aerosol (SOA) formation. Predictions of IEPOX-SOA in regional-scale models are uncertain due to lack of observational data to constrain parameters and simplifying assumptions such as homogeneous well-mixed aerosol particles. Recent chamber experiments have produced key data to constrain models showing phase separated aerosols inhibiting IEPOX-SOA production. We used experimental measurements of IEPOX-SOA tracers, 2-methyltetrols (2-MT) and 2-methyltetrol sulfates (2-MTS), formed at ratios of IEPOX to inorganic sulfate ranging from 1 to 10.5, under humid conditions (~50% relative humidity) to evaluate current regional-scale model implementations, assess underlying assumptions, and provide updates to optimize model performance. Specifically, we evaluated the Community Multiscale Air Quality Model (CMAQ) with analyses of phase state and sensitivities of other key IEPOX-SOA parameters (i.e., organic shell diffusivity, acidity, hygroscopic growth, mass accommodation coefficient, and reaction rates) on model predictions. The base CMAQ parameterization overpredicted chamber concentrations of IEPOX-SOA with an average normalized mean bias (NMBaverage) of 1.63. When phase separation was added the model then underpredicted IEPOX-SOA (NMBaverage = -0.71). Using the phase separated model, CMAQ model performance was optimized (NMBaverage = 0.077) with an updated increased organic diffusivity (Dorg = 2×10-16 m2s-1), and with an increase in kinetic rate constants for IEPOX-SOA tracers (k2-MT = 1×10-3, k2-MTS = 8.83×10-3). Regional-scale models account for hygroscopic growth indirectly through aerosol water mass, resulting in underpredictions of heterogeneous surface area. The optimized model explicitly accounted for hygroscopic growth, which improved heterogeneous surface area predictions. The optimized model parameters improve modeling capabilities of IEPOX-SOA and chemical representations of phase separated particles.