Abstract
Extremely high concentrations of dust particles are occasionally generated from the riverbeds of Taiwan, affecting the visibility and traffic safety of the local and nearby areas. The condition is most severe during the winter monsoon when surface wind is strong. This study analyzes the concentration of particulate matter of 10 µm or less (PM10), wind direction, wind speed, temperature, and humidity of riverbed stations adjacent to the Daan, Dajia, Dadu, Zhuoshui, and Beinan Rivers in Taiwan for a period of two years. The weather conditions that cause the high concentration of PM10 are classified into typhoon and non-typhoon types, and the latter type is further classified into three stages: ahead of front, ahead of anticyclone, and behind anticyclone. The associated meteorological influences of these weather types on high-concentration events in the riverbed are explored. The monitoring data show that the hourly PM10 concentration of the four riverbed stations exceeded 125 µg m−3 for 35–465 h per year, and the maximum PM10 in the Daan (and Dajia), and Zhuoshui Rivers was more than 800 µg m−3. Weather analysis showed that the extreme PM10 concentration on the riverbed was caused by weather types: typhoon and ahead of anticyclone, in which the peak hourly concentration reached average values of more than 600 and 400 µg m−3, respectively. The high PM10 caused by the typhoon type mainly occurred in October, with an average wind speed of 6 m s−1, high temperature of 25 °C, and mostly northeasterly winds. The ahead of anticyclone type mainly occurred in December, with an average wind speed of 5 m s−1, and northeasterly and northwesterly winds. Both weather types of riverbed events were observed during the daytime, especially at noon time, when strong wind speed, high temperature, and low relative humidity is favorable for riverbed dust generation. On the other hand, the main months of the high PM10 concentrations of the ahead of front and behind anticyclone stages are February and April. The peak PM10 concentrations of these two types of riverbed events are both about 300 µg m−3, but sporadic riverbed dust in these weather stages is mixed with Asian dust or pollution transported to the rivers through weak northwesterly and northeasterly winds. The high concentrations of these two types of riverbed events can occur at any time; but for the Dadu River, the high concentrations are often observed in the morning, when land breezes from the southeast bring local pollutants to the river.
Highlights
Dust particles are important suspended particles in the atmosphere
The results show that the hourly particulate matter of 10 μm or less (PM10) concentration in the riverbed is as high as 800 μg m−3, the annual average PM10 concentration in all major rivers is not high
The number of days when daily PM10 exceeded the standard in the Zhuoshui River was only 10 days per year, and in December it was only 1.5 days (Figure 3b). These results indicate again that high concentrations of PM10 are mostly sporadic for a few hours of the day, rather than continuously occurring, so that the daily average value exceeding the standard is rare, which is different from the high concentration of the Asian dust event observed for several consecutive days [36]
Summary
Dust particles are important suspended particles in the atmosphere. In addition to affecting air quality and human health, they have an impact on the climate [1,2,3,4]. The rainfall due to frontal passage often affects the northern areas of the island, but not the central or southern areas, where these major rivers are located During this period, the river waters recede and the exposed area of the dry riverbed increases. Compared with Asian dust or pollutant events, the impact area of suspended particles in riverbed events is smaller but can cause extremely high concentrations in local and nearby areas. Lin et al [27] analyzed the concentration and composition of suspended particles in the lower reaches of the Zhuoshui River They found that during the northeast monsoon, the concentration of the suspended dust particles in the riverbed would double than during other period, which would lead to an increase in the proportion of crustal elements and a decrease in pollutant elements. The meteorological conditions in different weather types and the monthly and hourly average meteorology were summarized to explain the causes of high PM10 concentration in the riverbeds
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