Modeling impact of volatility-based phase state prediction on particle-phase diffusion limitations to isoprene epoxydiol (IEPOX) uptake on SOA particles in Amazon rainforest

Abstract:

Accurate prediction of secondary organic aerosols (SOA) budget in atmospheric models is important to quantify their impacts on air quality and Earth’s radiative balance. Recent laboratory findings have confirmed that pre-existing highly viscous (glassy) SOA coatings on inorganic seed aerosol create a diffusional barrier hindering SOA formation. Isoprene epoxydiol SOA (IEPOX-SOA) are key components of SOA formed through multiphase chemistry involving gas- and aqueous-aerosol phases. Current atmospheric models represent acid-catalyzed heterogeneous reactive uptake of IEPOX on to SOA particles. But they either lack or assume a simplified characterization of mixing state of aerosols. There is an impending need to predict phase separation between organics and inorganics, as well as phase state of particle-phase diffusion limitations to IEPOX uptake on particles in the atmosphere. Both phase-separation and particle-phase diffusion-limitations are expected to vary dynamically in the atmosphere as a function of SOA composition, temperature, and relative humidity. This work parameterizes dynamic variations in the viscosity of SOA coatings (surrounding an inorganic core) and particle-phase diffusion limitations based on the glass transition temperature (T_g,org) of organic aerosols. We recently implemented a parametrization of T_g,org based on the composition and volatility distributions of SOA species within the Weather Research and Forecasting Model coupled to Chemistry (WRF-Chem). High resolution WRF-Chem simulations (at 10 km grid spacing) are used to predict the dynamic changes in SOA viscosity during the dry season of the Green Ocean Amazon (GoAmazon2014/5) field campaign. Results show that SOA mostly exists as a liquid near the surface but is semi-solid close to the top of the boundary layer and at higher altitudes within the troposphere. We also parameterize the effects of diffusional limitation and organic-inorganic phase separation within organic coatings on IEPOX-SOA formation within the Amazon.