MOSAIC-Mix: a mixing state resolving sectional model designed to improve the treatments of CCN and optical properties

Description

The need to resolve both aerosol size and mixing state originates from the need for more accurate treatments of cloud condensation nuclei (CCN) activity and optical properties to reduce the uncertainties of aerosol direct and indirect effects in climate simulations. A size and mixing state resolved version of aerosol module MOSAIC, called MOSAIC-Mix, is introduced. In MOSAIC-Mix, aerosol particles are represented in a three-dimensional parameter space of dry size (Ddry), BC mass fraction (wBC), and hygroscopicity (κ). Since it is computational prohibitive to perform high-resolution simulations of the aerosol mixing state using MOSAIC-Mix in 3D regional modeling studies, this study is motivated to systematically develop a low-resolution version of MOSAIC-Mix, which improves CCN concentrations and optical properties while being computational affordable in future 3D regional studies. We performed high-resolution MOSAIC-Mix (with 24 Ddry bins x 35 wBC bins x 30 κ bins) box model simulations as well as low-resolution ones (24 Ddry bins x 1 to 3 wBC bins x 1 to 3 κ bins) for 10 idealized environmental scenarios. For the low-resolution simulations, we optimize the positions of the wBC and κ bin boundaries which give the minimum errors. We found that the errors in CCN concentrations and optical properties with respect to the high-resolution results are reduced when either BC mass fraction or hygroscopicity (or both) is (are) resolved together with size, compared to size-resolved (internal-mixture) simulations. In addition, we showed that among all the bin configurations tested, the 3D bin configuration (24 Ddry bins x 2 wBC bins x 2 κ bins) having wBC and κ bin boundary of 0.3 and 0.1 respectively, had the smallest errors in CCN concentrations (5.38%), optical properties (2.38%), and absorption cross section (3.75%). In the future, low-resolution MOSAIC-Mix will be implemented in the 3D WRF-Chem to evaluate its performance against field campaign observations as well as to test the validity of the internal mixture assumption of aerosol mixing state over local to regional scales.