| 英文摘要 |
Due to the huge amount of bare soil existing during drought season, and the monsoon effect, the estuaries of rivers in central Taiwan are susceptible to severe wind erosion. Dust emission decreases the living quality and affects the health of nearby humans. It is important to delineate the potential riverbed areas that are vulnerable to wind erosion. Analysis of sediment yield, geology, particle size, and PM10 shows that the downstream segment of the Choshui River is an area of potential dust emission. Image interpretation derived from SPOT imagery coupled with Digital Elevation Modeling and recurrence analysis of water levels in the study areas are used to delineate the potential areas of dust emission. For an economic sand fixation measure, windbreak facilities and Xanthan Gum can be strongly recommended for use at the areas of potential dust emission.
Among the four rivers (the Taan River, the Tachia River, the Wu River and the Choshui River), the Choshui River has widest distribution of sand diameter. It ranges between 3.5μm and 1400μm and about nineteen percent of the sand is PM10. The Taan, Tachia and Wu Rivers all have similar distribution and range between 150μm and 700μm. These results can explain why, of the four rivers, the Choshui River could cause the greatest amount of river dust. When the wind velocity reaches about 15m/s, much more coarse sand than fine sand was collected at ground level. As the water content in sand reaches 1.22％, the sand begins to be restrained from moving. Only very small amounts of sand could be moved by wind as the water content was more than 4.35％.
Previous (within the past 14 years) aerosol (PM10) episodes caused by aeolian dust from rivers in central Taiwan were investigated in this project. The average number of annual episode days of river dust at Lunbei station was nine days. This station had the highest number of episode days of river dust among the nine stations. The major synoptic weather type for PM10 episodes of river dust was P1 (northeasterly winter monsoon). This type accounts for about 46% of the total synoptic weather types. The synoptic weather type P2 (cold high-pressure system moving eastward to the ocean with a center not exceeding 125oE longitude) was the second major type and accounts for about 21% of the total synoptic weather types. Among the twelve months, November has the highest incidence of river-dust episodes. Chemical compositions for the aerosol samples collected from air monitoring stations were analyzed. These data show that the concentrations of crustal elements in the periods of river-dust episodes (6.69µg/m3) were about 4 times higher than those in the periods of non-river-dust episodes (1.67µg/m3). Aerosols (PM10) caused by aeolian dust from the Choshui River could be transported to the Puzih station. Air quality for most of the air-monitoring stations on the southern side of the Choshui River was obviously affected by river dust during the periods of river-dust episodes. Contrarily, air quality for the air-monitoring stations between the Da An River and the Choshui River was less affected by river dust during the periods of river-dust episodes. Some stations located in the southern part of Taichung City were almost totally unaffected by river dust. Fingerprinting indexes established in this project could well distinguish the differentiating characteristics between river dust and street dust. These fingerprinting indexes were applied to the aerosol samples collected from air- monitoring stations in this study. The fingerprinting characteristics of samples of river-dust episodes and non-river-dust episodes show significant differences. The fingerprinting characteristics also show different levels of river-dust effects for samples collected from seven air-monitoring stations during the periods of river-dust episodes.
This study uses receptor model positive matrix factorization (PMF) to analyze the samples collected near the four rivers in central Taiwan: the Taan River, the Tachia River, the Wu River and the Choshui River. The purpose is to resolve the impact from these four rivers. Two sets of field samples were collected in April and November 2008, respectively. The analysis results showed that due to the northerly winds in November, the particulate matter increased significantly at sites near the Choshui River. The maximum increase between two sets of samples at the Lunbei station was about 1712%.
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