Model simulation of dust emissions from river basins in Taiwan

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Dust emissions from the river basins of Taiwan often lead to high aerosol concentrations nearby the rivers. The observed PM10 concentrations can be as high as 400-500 μg m-3, about three times more than the value of the air quality standard. This study incorporates the dust emissions from eight major riverbeds of Taiwan into the Asian dust model, i.e., TAQM-DUST, in order to understand the impacts of riverbed emission on the aerosol concentration of Taiwan. The eight major river basins of Taiwan include Da-an, Da-jia, Ta-tu（Wu）, Juo-Shui（LTK）, Gao-ping, Lan-Yang, Bei-nan and Tseng-wen rivers. The critical values of wind speed and relative humidity required by the model for dust emission are analyzed from the data obtained from the nearby stations of the river basins. These critical values include monthly minimum wind speeds and maximum relative humidities. The dust emission of the eight major riverbeds of Taiwan is parameterized with the same equation as the Asian dust emissions in the model, except that the riverbed emission weighting factor is modified according to the local observations. Model result of two dust emission cases show that the dust emissions from the eight major river basins of Taiwan will enhance the PM10 concentrations nearby the riverbeds and the downstream areas in the western coast of Taiwan. The maximum PM10 enhancement depends on the wind speeds of the northeasterlies. When the northeasterly is strong, the maximum PM10 enhancement can reach 200-300 μg m-3 and the concentration covers a wide range of areas from south of Taichung to whole southern Taiwan. About the emission height, the riverbed emission can reach 10 μg m-3 of maximum PM10 enhancement at about 1-2 km above the ground over central Taiwan. Over southern Taiwan, the maximum PM10 enhancement are concentrated below 1 km above the ground and can last for 1-2 days. When average over the whole island, dust emissions from the eight major riverbeds can contribute up to 10-30% of the aerosol concentrations in Taiwan, depends on the cases and the strength of the wind speeds.