| Summary: | 碩士 === 國立臺灣大學 === 大氣科學研究所 === 94 === Mineral dust is an important source of atmospheric ice nuclei (AIN) (Isono et al., 1959) and plays an important role on atmospheric radiation (Twomey, 1974) and the balance of ecosystem (Jickells et al., 2005). For Taiwan, most of the desert dust came from Northern China and Mongolia during spring. However, judging from the seasonal variation of aerosol chemical compositions observed here, a great portion of mineral dust aerosols in Taiwan should be locally produced.
In this study, we build up a dust deflation module appropriate for use in the Taiwan area to provide explicit information on local dust emission. Simulations were performed for the period of 2000/09-2001/08 using the MM5 and TAQM/kosa model.
In the regional-scale version of TAQM/kosa, the three predictors used for determining the dust mobilization condition are the friction velocity (or surface wind speed), the relative humidity and dominant weather system (Wang et al., 2000). Through sensitivity tests, we designed new emission thresholds appropriate for the Taiwan area. According to the new analysis, we also try to build up modified thresholds for dust emission that can be generally used for various regions. We found that soil moisture can be used to replace the “relative humidity” criteria, provided that the meteorology model can produce reasonable soil moistures.
Comparison between locally produced and long-range transported dust concentrations shows that annually local dust contributes to about 71% of the airborne mineral dust in Taiwan. Local dust concentration is the highest in winter due to strong winds associated with the northeast Winter Monsoon in East Asia. But percentage-wise, autumn has the highest local dust proportion. Spatially, local dust is produced mainly in western Taiwan, with average dust mass concentration of about 6.6 μg/m3. The number concentration of dust varied from 105 to 10 per liter and the number concentration of potential ice nuclei is about 100 per liter. Surface radiative forcing caused by local dust is -1.4 W/m2, whereas air heating rate may reach about 0.034 K/day with max. occur mainly in the boundary layer.
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