Accounting for large dust particles in the GEOS-5 model: consequences for surface PM10 concentration and optical depth

The size of dust particles is an important property that determines how far dust plume can travel and how dust particles interact with solar and terrestrial radiation. Currently, the GEOS-5 model accounts for dust particles smaller than 20 micron in diameter with simple assumptions of size fractions in the erodible soil; and it overestimates the mass of relatively fine (<6 micron) dust particles but underestimates that of coarse particles. In this study, we extend the GEOS-5 dust particle size to 100 micron by adding two more size bins and constrain the dust particle size fractions in the soil by using observed particle size distribution in the fresh dust plumes in the air over the Sahara desert during the Fennec campaign. We found that this new representation of dust particle sizes in the GEOS-5 model yields a substantial reduction of high bias in simulated surface PM10 concentrations against the AMMA measurements compare to the previous model results, however the simulated aerosol optical depth (AOD) is significantly lower than the MODIS AOD. We further use the AERONET AOD measurements and CALIPSO lidar profiles and conduct several sensitivity studies to reconcile the differences, including identifying the potential bias in the dust mass extinction efficiency, vertical profile, and size distributions that could yield a consistency in model simulated column AOD and surface PM10 concentrations that agree better with observations.