Quantifying the structural uncertainty of the aerosol mixing state representation in a modal model

Abstract

Abstract. Aerosol mixing state is an important emergent property that affects aerosol radiative forcing and aerosol-cloud interactions, but it has not been easy to constrain this property globally. This study aims to verify the global distribution of aerosol mixing state represented by modal models. To quantify the aerosol mixing state, we used the aerosol mixing state indices for submicron aerosol based on the mixing of optically absorbing and non-absorbing species (χo), the mixing of primary carbonaceous and non-primary carbonaceous species (χc), and the mixing of hygroscopic and non-hygroscopic species (χh). To achieve a spatiotemporal comparison, we calculated the mixing state indices using output from the Community Earth System Model with the modal MAM4 aerosol module, and compared the results with the mixing state indices from a benchmark machine-learned model trained on high-detail particle-resolved simulations from the particle-resolved stochastic aerosol model PartMC-MOSAIC. The two methods yielded very different spatial patterns of the mixing state indices. In some regions, the yearly-averaged χ value computed by the MAM4 model differed by up to 70 percentage points from the benchmark values. These errors tended to be zonally structured, with the MAM4 model predicting a more internally mixed aerosol at low latitudes, and a more externally mixed aerosol at high latitudes, compared to the benchmark. Our study quantifies potential model bias in simulating mixing state in different regions, and provides insights into potential improvements to model process representation for a more realistic simulation of aerosols.

Evaluating BC Aging Processes in the Community Atmosphere Model Version 6 (CAM6)

“…Recently, Zheng et al. (2021) quantified the uncertainty of the aerosol mixing state based on the mixing state index of primary carbonaceous and nonprimary carbonaceous species (χ c ) and the mixing state index of hygroscopic and non‐hygroscopic species (χ h ) in MAM4. They found MAM4 overestimates the mixing state index χ c and χ h at low latitudes to mid‐latitudes, which also points toward the too rapid transfer from the primary carbonaceous mode to the accumulation mode.…”