HYSPLIT4 Predicted and Measured PM10 Air Concentrations

 Roland R. Draxler

Prepared for the CSIRO Wind-blown Dust Workshop; the emission, transport and impacts of wind-blown dust from soil erosion: modelling and observation, November 8-10, 2004, Aspendale, VIC, Australia


Introduction -  A model for the emission of PM10 dust had been constructed (Draxler et al., 2001) using the concept of a surface roughness dependent threshold friction velocity. Surface roughness is correlated with soil properties. A dust emission rate is computed from each model grid cell when the local wind velocity exceeds the threshold velocity for the soil characteristics of that emission cell. The dominant mechanism for the PM10 emission is "sand-blasting".The emitted material is dispersed and transported using HYSPLIT, a Lagrangian transport and dispersion model (Draxler and Hess, 1998). The model was initially tested over Kuwait, Iraq and Saudi Arabia (Draxler et al., 2001), where it predicted about the right number of dust events (18%). The model results also agreed quantitatively with measurements at four locations in Saudi Arabia and one in Kuwait for one major dust event (>1000 Ķg/m3 ). However, for several smaller scale dust events (200-1000 Ķg/m3) the model substantially over-predicted the air concentrations. Part of the over-prediction was attributed to the modelís sensitivity to the threshold friction velocity and the surface soil texture coefficient (the soil emission factor), and the difficulty in accurately representing these parameters in the model. In this application, the model is applied over other domains where detailed digital soil characteristics are not available. The emission module was modified (Draxler, 2002) to use HYSPLITís one-degree land-use file by assuming that a "desert" land-use grid cell corresponds to the Kuwait "active sand sheet" soil type category. To compensate for the greater number of potential dust emission cells the original PM10 flux equation was replaced by a relationship not dependent upon soil type and with a substantially lower emission flux. This modified model was tested for two Chinese dust storms and one from Mexico, Australia, and North Africa. Model results are compared to the TOMS Aerosol Index and where available, measured PM10 air concentrations.  


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