衛星搭載ライダーCALIOPによって観測された東アジア人口稠密地域上空におけるダスト偏光解消度の低下

概要

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The microphysical properties of dust over Eastern Asia are examined by using optical properties from the data collected by the space-borne lidar CALIOP onboard the satellite CALIPSO. The dust was frequently observed over northern China and northern India. The particle depolarization ratio (PDR) and the color ratio (CR) measured by the lidar respectively decreased and slightly increased from west to east above China. It is shown that the PDR decrease is not caused only by the change in shape distribution of the dust particles due to the shape-dependent falling velocity or the external mixturing of the dust with the secondary sulfate aerosols in accumulation mode, although it was suggested that the external mixturing of dust and sea salt particles could be the cause of the observed changes in PDR and CR. If soluble materials such as sulfate or sea salt are internally mixed with dust deliquesce in the atmosphere, the shape of the dust particles is more spherical, and their size is much larger. Therefore, this type of internal mixture should lead to the observed decrease in PDR and slight increase in CR. Moreover, since recent studies show that calcium-containing minerals react to form nitrate salts in polluted urban areas and these salts deliquesce under very dry conditions, this type of dust might also reduce PDR.