Quantifying the Importance of Aerosol Mixing State for Aerosol Optical Properties on the Regional Scale

Aerosol mixing state, defined here as the distribution of chemical species across the particle population, impacts aerosol optical properties. However, it is difficult to quantify this impact because chemical transport models represent the aerosol mixing state highly simplified. To address this, we coupled the Weather Research and Forecast (WRF) model and the particle-resolving aerosol physics and chemistry model PartMC-MOSAIC. The new model explicitly resolves and tracks the size and composition of individual particles as they undergo transformations by coagulation and condensation in the atmosphere, and simulates stochastic particle transport between grid cells using velocity and turbulent mixing fields provided by the WRF model.

We simulated a scenario representing Northern California for a two-day period. The aerosol mixing state in the region varies spatially as well as temporally due to the spatial distribution of different emission sources, the transport of aerosols and trace gases and the aerosol aging resulting from coagulation and condensation. To quantify the impact of aerosol mixing state on optical properties, a composition-averaging technique was used, which simplifies the highly detailed per-particle composition to a level of detail that lower-detail sectional models are able to represent. The optical properties were calculated using a core-shell Mie code for both the reference simulation, which tracks the full mixing state, and the composition-averaged simulation, which assumes an internal mixture. We then compare absorption and scattering as well as single scattering albedo of the particle-resolved simulation to the composition-averaged simulation.

We present the spatial maps of error in simulated optical properties between the particle-resolved and composition-averaged calculations. We discuss how the differences between the two representations vary as a function of mixing state parameter chi, a metric where values range from 0 – reflecting a fully externally mixed particle population – to 1, reflecting a fully internally mixed particle population.