A revised mineral dust emission scheme in GEOS-Chem: improvements in dust simulations over China

Mineral dust plays a significant role in climate change and air quality, but large uncertainties remain in terms of dust emission prediction. In this study, we improved the treatments of dust emission process in a Global 3-D Chemical Transport model (GEOS-Chem) v12.6.0, by incorporating the geographical variation of aerodynamic roughness length (Z₀), smooth roughness length (Z_0s), soil texture, introducing Owen effect and Lu and Shao (1999) formulation of sandblasting efficiency α. To investigate the impact of the modifications incorporated in the model, several sensitivity simulations were performed for a severe dust storm during March 27, 2015 to April 2, 2015 over northern China. Results show that simulated threshold friction velocity is very sensitive to the updated Z₀ and Z_0s field, with the relative difference ranging from 10% to 60% compared to the original model with uniform value. An inclusion of Owen effect leads to an increase in surface friction velocity, which mainly occurs in the arid and semi-arid regions of northwest China. The substitution of fixed value of α assumed in original scheme with one varying with friction velocity and soil texture based on observations reduces α by 50% on average, especially over regions with sand texture. Comparisons of sensitivity simulations and measurements show that the revised scheme with the implement of updates provides more realistic threshold friction velocities and PM₁₀ mass concentrations. The performance of the improved model has been evaluated against surface PM₁₀ observations as well as MODIS aerosol optical depth (AOD) values, showing that the spatial and temporal variation of mineral dust are better captured by the revised scheme. Due to the inclusion of the improvement, average PM₁₀ concentrations at observational sites are more comparable to the observations, and the average mean bias (MB) and normalized mean bias (NMB) values are reduced from -196.29μg m^-3 and -52.79% to -47.72μg m^-3 and -22.46% respectively. Our study suggests that the erodibility factor, sandblasting efficiency and soil-related properties which are simply assumed in the empirical scheme may lack physical mechanism and spatial-temporal representative. Further study and measurements should be conducted to obtain more realistic and detailed map of these parameters in order to improve dust representation in the model.