Figure 2 shows the observed hourly relative humidity, temperature, PM₁₀ concentration, and noon time wind speed and direction observed by four riverbed monitoring stations, as well as rainfall at general EPA stations near each river (Figure 2a–d). Observations from the Sanyi background station in central Taiwan are also plotted for comparison (Figure 2e). Two annual datasets, starting from September 2010 and September 2012, for all stations are plotted. In these figures, we can roughly divide the data into two periods, namely the winter monsoon and the summer monsoon. From October to April is usually the period of the winter monsoon. It can be seen in Figure 2 that the northeasterly or northerly winds prevailed at most riverbed stations during this period, and the temperature, humidity, and rainfall were relatively low, indicating the dry and windy seasons of the year. This is the period when more dust particles are generated from the riverbeds. On the other hand, during the summer monsoon period, the wind is usually weaker and wind direction is more diversified, and the relative humidity and rainfall are higher, resulting in fewer riverbed events. Among all the rivers, the Beinan is located in the southernmost position and is also on the edge of the prevailing northeasterly wind in winter (Figure 1). Therefore, even in the northeasterly seasons, the local land sea breezes often dominate. The wind direction at noon time (12:00) in Figure 2d shows that the sea breezes from the south are often observed in the riverbed area during the winter monsoon, and the wind direction is different from the northerlies or northeasterlies prevailing at the stations in Central Taiwan. These details can be seen later in the case study.

Comparing the monitoring results of the four rivers, the riverbed event in the Zhuoshui River in Central Taiwan was more serious than events in other rivers (Figure 2). The PM₁₀ concentration in the Zhuoshui River exceeded 400 µg m⁻³ in 16 h per year, of which it exceeded 800 µg m⁻³ in 2.5 h. In Daan/Dajia, the PM₁₀ concentration exceeded 400 µg m⁻³ in 5 h per year, of which one hour exceeded 800 µg m⁻³. For the Beinan River in southeastern Taiwan, PM₁₀ was observed to exceed 400 µg m⁻³ in 5 h per year. Similar to the Daan/Dajia and Zhuoshui Rivers, the Dadu River is also located in Central Taiwan, but the maximum hourly concentrations of PM₁₀ exceeds 400 µg m⁻³ only once per year. In addition, aerosol transported from the Asian continent or local pollution to the riverbeds is limited. At the Sanyi background station in Central Taiwan (Figure 2e), the highest hourly PM₁₀ concentrations in the two years were about 250 µg m⁻³, representing the hourly maximum PM₁₀ value due to long-range transport to the island from abroad. Moreover, the hourly PM₁₀ of the largest city in Central Taiwan (Taichung) is also less than 250 µg m⁻³ in the two years. Thus, the PM₁₀ concentration due to dust generation from these major rivers is extremely high.

Figure 3 summarizes the PM₁₀ and meteorological values from Figure 2. Figure 3a presents the annual and monthly averaged PM₁₀ concentrations of the major rivers. The results show that although the hourly 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 value range is only 21–57 µg m⁻³, indicating that the rivers are located in rural areas. The largest annual averaged PM₁₀ is the Zhuoshui River in Central Taiwan, with a value of 57 µg m⁻³, and the lowest is the Beinan River in southeastern Taiwan, with a value of 21 µg m⁻³. In addition, the maximum monthly average of PM₁₀ was observed in the Zhuoshui River, with a concentration of 89 µg m⁻³, which appeared in December. This maximum monthly average of the Zhuoshui River is more than three times the minimum value observed in July (25 µg m⁻³). Similarly, the monthly average PM₁₀ concentrations during the winter monsoon period (October to April) are all higher than those in the summer monsoon periods (May to September) for other stations, but the ratio of maximum to minimum monthly averages are all around 3. Although the higher PM₁₀ in the riverbeds during the winter monsoon may come from the long-range transport of Asian dust and pollutants, previous studies reported that the monthly or annual average increase in Northern Taiwan is up to about 20 µg m⁻³ [23,26], indicating the impacts of riverbed dust on PM₁₀ concentration.

Figure 3b counts the total number of days in which the daily averaged PM₁₀ of all riverbeds exceeded the air quality standard of 125 µg m⁻³ in each month. The results showed that the daily average PM₁₀ exceedance time of all rivers was within 3 days per month, of which the maximum 3 days was observed in the Dadu River in April. In addition, for the entire year, the maximum number of 10 days was observed in the Zhuoshui River. This is because in the Zhuoshui River, the daily PM₁₀ exceeds the standard for nearly half a day to 2.5 days per month in the winter. On the other hand, the Daan/Dajia Rivers exceed the standard value for half a day in September and October, while the Beinan River only exceeds the standard for half a day in October. The low daily average PM₁₀ exceedance time of all rivers in Figure 3b indicates that the high riverbed concentrations occur sporadically within a few hours, and the background concentration of the rivers is relatively low during the rest of the day, and thus the daily average PM₁₀ is not high.

Figure 3c shows the average concentration of PM₁₀ for 24 h. Similar to the monthly average PM₁₀ in Figure 3a, the hourly average PM₁₀ of the riverbed is not high, with the highest values being 65 µg m⁻³, and appears after noon time (14:00) in the Zhuoshui River. The Daan/Dajia, and Beinan Rivers also have similar high concentrations during the day and low concentrations at night, while the hourly variation of the Dadu River is different from that of other rivers. The high concentration appeared in the morning, and the maximum value was as high as 60 µg m⁻³ at 7 a.m. In addition to the morning hours, the other hourly average values of the Dadu River are almost all slightly lower than in the Zhuoshui River, which is the river with the second highest concentration.

In order to inspect the occasional high concentrations of PM₁₀ in the riverbed, this study counts the hours of high PM₁₀ concentration. Since the EPA does not have an hourly air quality standard for PM₁₀, we have used the 125 µg m⁻³ from the PM₁₀ daily air quality standard as the criteria for selecting hourly high-concentration cases. Figure 3d shows the total number of hours exceeding 125 µg m⁻³ per month for all the rivers in an annual average. The results further highlight that the high concentration of PM₁₀ varies by month and river. During the winter monsoon from October to April of the following year, the maximum number of exceedance hours per month is more than 80 h, while during the summer monsoon period, the value is almost zero in all rivers. Comparing different months, Figure 3d shows that the distribution of PM₁₀ exceedance time is similar to the monthly average distribution in Figure 3a, showing a bimodal type, among which December, February, and April are the months with more exceedance hours for the Daan/Dajia, Dadu, and Zhuoshui Rivers in Central Taiwan. The high number of hours exceeding the criteria in these several months also resulted in high averaged concentrations in these rivers (Figure 3a). In Figure 3d, January is the month when the PM₁₀ exceedance time is the lowest in the winter monsoon, which also results in the lowest monthly average concentration in the season. In addition to these few months, the number of hourly PM₁₀ values exceeding the criteria in the Zhuoshui River was also high in October, and in the Beinan River in southeastern Taiwan was also high in September and October.

When comparing different rivers, Figure 3d shows that the maximum time exceeding the criteria per month was 86 h, which occurred in the Zhuoshui River in December, and the total exceedance is 465 h for the same river in a year. Converting 465 or 86 h into days, there are almost 19 days a year, or 4 days a month, of exceedance time. Interestingly, 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]. However, if the 465 or 86 h of over-criteria time are evenly distributed to 365 in a year, or 30 days in a month, the riverbed has from 1 h to nearly 3 h a day when PM₁₀ concentration has reached a value of 125 µg m⁻³. For the Daan/Dajia, and Dadu Rivers, the total number of hours exceeding 125 µg m⁻³ per month corresponds to about 2 h and 1 h per day in the severe months, respectively. Therefore, although the daily PM₁₀ exceedance time is low, high dust concentrations are often observed over these rivers, especially in severe months.

Figure 3e counts the total number of hours when PM₁₀ exceeds 125 µg m⁻³ in each 24 h. It can be seen from the figure that the PM₁₀ exceedance hour is unevenly distributed within 24 h. Most riverbeds show high exceedance hours during the daytime. Among all the rivers, the maximum exceedance occurs in the Zhuoshui River, and the over-criteria time from 8:00 to 16:00 during the day is about 25 to 35 h per year, indicating that the riverbed emissions may be higher during these periods. The Daan/Dajia, and Beinan Rivers have similar high PM₁₀ concentrations at noon time, but the frequency is less. Different from other rivers, the Dadu River’s maximum exceedance occurred in the morning, and the value (20 h) is only slightly lower than that of the Zhuoshui River at the same time, similar to the result in Figure 3c. In addition to the daytime exceedance, Figure 3e shows that most rivers in Central Taiwan have also observed high PM₁₀ events at night, implying that the PM₁₀ in these rivers may have other sources.

In addition to monthly and hourly variations in high riverbed concentrations, the wind directions in all rivers that cause hourly PM₁₀ to exceed 125 µg m⁻³ are also summarized. In order to simplify the wind problem, all wind directions are only divided into four directions, 0 to 90 degrees as northeasterlies (NE), 90 to 180 degrees as southeasterlies (SE), 180 to 270 degrees as southwesterlies (SW), and 270 to 360 degrees as northwesterlies (NW). Figure 3f shows the total number of hours per year for different wind directions leading to high riverbed concentrations. In Figure 3f, the Zhuoshui River has the longest time exceeding the standard per year (465 h), and the Beinan River has the shortest time exceeding the standard per year (35 h). Regardless of the wide range of exceedance time, Figure 3f shows that the main wind directions are northeasterly and northwesterly winds, which account for at least 70% of all riverbed high-concentration events, observed mainly in the northeasterly monsoon seasons. In addition, southwesterly and southeasterly winds are also important, accounting for at least about 15% of the high-concentration events for all the rivers.

Furthermore, we also checked other meteorological conditions that caused the hourly PM₁₀ value to exceed 125 µg m⁻³, including wind speed, temperature, and humidity. The results showed that all rivers have a wide range of wind speed during high concentrations of hourly PM₁₀. The maximum wind speed was as high as 11 m s⁻¹, but the minimum wind speed was only 0.2 m s⁻¹. The temperature range was likely to be from 11 to 34 °C, and the humidity range was from 33% to 94% at the high hourly concentration of PM₁₀. The wide ranges of the meteorological conditions at high PM₁₀ concentration may be caused by the meteorological conditions of various weather types and will be explored in the following sections.

Figure 4 shows the weather maps of the first events of the typhoon and non-typhoon types, in which the latter is further classified into ahead of front, ahead of anticyclone, and behind anticyclone. In Figure 4a, the first typhoon type was observed on 27 October 2010 (also see Table 1). In this event, Typhoon Chaba approached Taiwan from near the equator, and later turned northeast to the Japanese coast without passing through Taiwan. On the other hand, a strong anticyclonic circulation occupied China on the same day, and there was an intensive pressure gradient in the coastal areas of China. Due to this strong pressure gradient, strong northeasterly winds prevailed in Taiwan. The approach of the cyclonic circulation of the typhoon accelerated the strong northeasterlies, resulting in the rising of the riverbed sands and dust particles, and has led to the daily PM₁₀ exceedance over the Daan/Dajia and Zhuoshui Rivers (Table 1). Although typhoons are warm and humid cyclones, and often produce rainfall, Typhoon Chaba was far away from Taiwan, so less than 1 mm of precipitation was observed in southeastern Taiwan, and no precipitation was observed in the riverbed area of Central Taiwan.

For the non-typhoon type, Figure 4b–d show the weather map of the first consecutive event from 20 November to 30 November 2010. The event includes weather conditions such as Taiwan’s location ahead of the surface front (Figure 4b), ahead of the anticyclonic system (Figure 4c), and behind the anticyclonic system (Figure 4d). This event caused the daily averaged PM₁₀ values of the Zhuoshui River to exceed the standard for two days (22/11/2010 and 28/11/2010 in Table 1). Figure 4b shows the ahead of the front type of weather on 21 November 2010. In this type, a surface front was located on the southeast coast of China, which was about to pass through Taiwan.

After the passage of the surface front, Taiwan was under northeasterly winds ahead of the anticyclonic system. Figure 4c shows the weather map of this type on 25 November 2010. The northeasterly wind lasted for several days until the anticyclonic center moved off the coastal areas of China. Figure 4d shows that after the anticyclonic center passed through areas to the north of Taiwan on 28 November 2010, most areas of Taiwan were dominated by weak easterly winds.

Figure 5 and Figure 6 plot PM₁₀ concentration and meteorological data of all riverbeds for the typhoon and non-typhoon types discussed in Figure 4. Figure 5 shows the typhoon type data from 24 October to 2 November 2010. In this type, Typhoon Chaba approached Taiwan from 26 to 27 October. Before the arrival of the typhoon on 24 October, land breezes (easterlies on the western coast) were observed in most riverbeds from the evening to early morning, and then sea breezes (westerlies on the western coast) during the day (Figure 5). The land–sea breeze is in the reverse wind direction in the Beinan River over the southeastern coast of Taiwan.

Followed by the local land–sea breeze on early 25 October, the northeasterlies ahead of the surface anticyclonic system started to affect the coastal areas of East Asia. From 26 to 27 October, the northeasterlies reached their maximum surface wind speed due to the approach of the typhoon. The maximum surface wind speed observed at the Daan/Dajia Rivers was close to 10 m s⁻¹, while at other sites in Central Taiwan it remained at 7 m s⁻¹, and at the Beinan River in southeastern Taiwan it decreased to 5 m s⁻¹. Under the extremely high wind speeds, all riverbed stations observed an increase in PM₁₀ concentration (Figure 5). On 26 and 27 October, the hourly maximum of PM₁₀ was around 350 and 900 µg m⁻³ at the Daan/Dajia Rivers, respectively, while at the Zhuoshui River, it exceeded 800 µg m⁻³ on both days (also see Figure 2). The PM₁₀ concentration of the Dadu and Beinan Rivers is lower, but the maximum hourly value is also about 200 µg m⁻³. In addition, the hourly PM₁₀ concentrations at the Sanyi background station in Central Taiwan during the entire event was less than 70 µg m⁻³ (Figure 5a), indicating the impacts of long-range transport of aerosol from the Asian continent were small. Also, note that although the prevailing wind was strong throughout the day, the riverbed dust concentration was only enhanced during the day and reached its peak at noon time, when the temperature was warmer and relative humidity of all the rivers decreased to less than 60%, indicating these conditions are conducive to riverbed dust generation.

Among all the rivers, the Zhuoshui River has more obvious riverbed dust enhancement than other rivers (Figure 5c). Except for the extremely high PM₁₀ concentration on 26 and 27 October, PM₁₀ concentration exceeding 200 µg m⁻³ continued for several days in the river until 31 October. The maximum hourly wind speed observed on the Zhuoshui River during the event (7 m s⁻¹) was lower than the wind speed observed on the Daan/Dajia Rivers (10 m s⁻¹), and was roughly the same as the wind speed observed on the Dadu River. In addition, the temperature (23–24 °C) and relative humidity (49–58%) at the peak PM₁₀ concentration from 27 to 31 October are similar to those of the Dadu River (21–25 °C, 46–60%). However, the maximum concentration of PM₁₀ in the Zhuoshui River was more than 800 µg m⁻³ and PM₁₀ enhancement lasted for a few days, while the concentration in the Dadu River reached 200 µg m⁻³ only in the first two days. This suggests that in addition to meteorological conditions, different riverbed characteristics are critical to riverbed dust generation.

After the typhoon, the PM₁₀ concentrations of most rivers decreased, but the opposite occurred in the Beinan River (Figure 5d). From 30 October to 2 November 2010, high PM₁₀ concentrations were observed in the Beinan River at noon time, and the maximum concentration exceeded 500 µg m⁻³. The wind speed on these days only reached around 4 m s⁻¹, and was less than the peak wind speed (close to 5 m s⁻¹) during the typhoon period. However, the Beinan River is located on the southeastern coast of Taiwan, directly facing the warm and humid air brought by the typhoon’s peripheral circulation, which suppressed the uplifting of dust particles from riverbeds. After the typhoon left, the northeast monsoon followed and the relative humidity decreased to a minimum below 40%. Thus, the riverbed dust particles were uplifted under windy and dry conditions, showing the importance of humidity as a factor. In addition, Figure 5d shows that the PM₁₀ concentration in the town near the riverbed increased during this period. Similarly, the PM₁₀ concentrations in towns near other riverbeds also increased during the typhoon period (Figure 5). Since the concentration of PM_2.5 in towns and PM₁₀ in the background Sanyi station were both low during the event, indicating low local pollution or foreign dust concentrations. The high PM₁₀ concentration in towns suggests that the coarse particles from riverbeds have affected those nearby towns.

The second selected weather type is without typhoon, which occurred from 20 November to 30 November 2010, and Figure 6 shows the observed data of this type. In this type, three meteorological stages were observed, including ahead of a surface front, ahead of an anticyclonic system, and behind an anticyclonic system. At the beginning of this type on 21 November, a surface front was approaching Taiwan (Figure 4b). At this time, the wind speed was low and the atmosphere was stable. In addition, due to the approach of the cyclonic circulation of the low-pressure system, southwesterly winds were easily seen in some areas of Western Taiwan [36], causing the pollutants to accumulate over the marginal sea and northwestern Taiwan. In the riverbed area of Central Taiwan, the sea breeze from the northwest prevailed at noon time, bringing pollutants into the riverbed areas (Figure 6a–c). Under the northwesterly wind, high PM₁₀ concentration was observed over the Daan/Dajia Rivers, with the maximum value exceeding 400 µg m⁻³ (Figure 6a). Since concentrations of PM₁₀ in the Sanyi background station and the PM_2.5 in riverside towns were both about 200 µg m⁻³ at the same time, it suggests that other than the riverbed dust raised by the wind speed, the pollutants carried by the westerly wind affected the riverbeds of the Daan/Dajia Rivers and surrounding areas. The PM₁₀ concentration of other rivers in Central Taiwan also increased, but the time was slightly delayed from north to south (Figure 6b,c).

After the frontal passage on early 22 November, the northeast monsoon prevailed ahead of the anticyclonic system. On 25 November, another anticyclonic system with a strong pressure gradient affected Taiwan and once again triggered strong northeasterly winds. Figure 6 shows that the Daan/Dajia Rivers were affected by wind speeds close to 6 m s⁻¹ and 7 m s⁻¹ at noon time on 22 and 25 November, respectively. On the same days, the Dadu and Zhuoshui Rivers exceeded 4 m s⁻¹ and 5 m s⁻¹, respectively, while the Beinan River was about 3 m s⁻¹ on both days. The wind direction was northeasterly in the Daan/Dajia Rivers, the same as in the typhoon type, but turned into northerly or northwesterly in the Dadu and Zhuoshui Rivers due to the eastward movement of the anticyclone and its interaction with the terrain. The northwesterly wind was seen in the Beinan River, but unlike the rivers in Central Taiwan, the local sea breeze from the south also prevailed during the daytime (Figure 6d). However, although the humidity of all rivers is approximately the same, only a significant increase in PM₁₀ was observed on the Zhuoshui and Beinan Rivers. At noon time on 22 and 25 November, the hourly PM₁₀ peak value of the Zhuoshui River reached about 400 µg m⁻³ (Figure 6c). During these stages, the PM_2.5 and PM₁₀ concentrations were not high in the riverside town and background Sanyi station, suggesting that the high value of suspended particles was mainly from riverbed dust. Similarly, on 25 November, PM₁₀ concentration close to 200 µg m⁻³ was also observed on the Beinan River when the northeasterly wind prevailed (Figure 6d). Interestingly, the PM₁₀ concentration in the riverside town reached 350 µg m⁻³, much higher than observed over the riverbed. Although the PM₁₀ concentration in the Sanyi background station in Central Taiwan is low (less than 42 µg m⁻³), according to the EPA background station over the northeastern tip of Taiwan (Wanli Station), the maximum hourly PM₁₀ concentration observed on that day was close to 140 µg m⁻³. Therefore, in addition to the dust generated from the riverbed, the PM₁₀ in the riverside town of Beinan River may be partially affected by the long-range transport of coarse particles (such as Asian dust) due to its location over the southeastern coast and directly facing the northeasterly wind.

Late on 26 November, the center of the anticyclonic system left the coastal areas of China and passed over the marginal sea to the north of Taiwan (Figure 4d). Meteorological conditions had shifted to the stage of the behind anticyclonic system. The prevailing wind changed from northeasterly to easterly winds over northern Taiwan, and the wind speed also weakened (Figure 4d). At this stage, most of the riverbed areas were heavy polluted and the PM_2.5 concentrations were high (Figure 6). At the same time, the PM₁₀ at the Sanyi background station also reached more than 100 µg m⁻³, suggesting a high concentration of particles imported from abroad. In addition to the long-range transport, there were also a few cases of high dust concentration in riverbeds during the daytime. For example, at noon on 28 November, an hourly PM₁₀ of 400 µg m⁻³ was observed over the Zhuoshui River. At the same time, the relative humidity was extremely low, reaching a minimum of around 40%, and was favorable for riverbed emission under a maximum wind speed of 3.5 m s⁻¹. At night, high concentrations of PM₁₀ were also observed, especially in the Dadu River (Figure 6b). The land breeze from the east suggests that the high concentration of the river is related to the pollution of the main city in Central Taiwan (Taichung), which is located 10 km to the east of the river.

Table 2 summarizes the times, peak PM₁₀ concentrations, and the corresponding meteorological conditions at the peak concentration for each weather type in Table 1. There are 6–11 cases for each type. One case in Table 2 represents one day listed in Table 1. In Table 2, September and October are the months in which the typhoon type cases occurred (also see Table 1). Because there are more typhoon cases observed in October in the Zhuoshui River, they are listed as the main month and river in Table 2. The result in Table 2 explains that in Figure 3d, the higher PM₁₀ exceedance hours observed in October, especially in the Zhuoshui River, resulted from typhoons. In the non-typhoon type in Table 2, the riverbed events in ahead of front, ahead of anticyclone, and behind anticyclone stages were mainly observed in April, December, and both February and April, respectively. Riverbed events in these weather stages were mainly observed in the Zhuoshui and Dadu Rivers.

In addition, Table 2 shows the time of peak PM₁₀ concentration for each type. We roughly classified the time of day as morning (06:00–09:00 local time), noon time (10:00–15:00 local time), afternoon (16:00–17:00), and night (18:00–05:00). Although more peak concentrations of riverbed events were observed at noon time from 11:00 to 15:00, peak concentrations were also observed in the morning in the typhoon type. For example, on the Daan/Dajia and Zhuoshui Rivers, a typhoon type on 28 September 2012 was observed at 8:00–9:00 a.m. The occurrence time of the typhoon type is similar to that in the ahead of anticyclone stage, except that the peak time of the ahead of anticyclone stage appears to be more extended, and was observed from 09:00 to 15:00 during the day. These two types also explain that the high concentrations of PM₁₀ in Figure 3e appeared at noon time and a few in the morning. Different from these two types, in the ahead of front stage, peak PM₁₀ concentration appeared throughout the day, while in the behind anticyclone stage, peak PM₁₀ concentration was only observed in the morning and at night. Thus, the peak PM₁₀ concentrations in these two stages resulted from different mechanisms.

Table 2 also lists the average concentration of the peak PM₁₀ in the riverbed, peak PM_2.5 of the riverside town, and peak PM₁₀ concentration of the day at the Sanyi background station for each weather type. Among them, the peak PM₁₀ concentration in the riverbed caused by the typhoon types exceeded 600 µg m⁻³ on average, which is the highest concentration of all types. The ahead of anticyclone stage of the non-typhoon type exceeding 400 µg m⁻³ is the second highest concentration. In addition, the PM_2.5 concentration of these two types is low, with an average of 37 to 64 µg m⁻³, respectively, indicating that the concentration of pollutants from either local areas or imported is low. Furthermore, during these two types of events, the highest concentration of PM₁₀ at the Sanyi background station in Central Taiwan is about 60 µg m⁻³, implying that the contribution from Asian dust was low. Therefore, the high riverbed PM₁₀ concentrations of these two types is mainly caused by the riverbed dust generation. On the contrary, the PM₁₀ in the ahead of front and behind anticyclone stages in the non-typhoon types are both around 300 µg m⁻³ on average, but their PM_2.5 concentration is higher, both being about 130 µg m⁻³, indicating that part of the riverbed aerosol may come from pollution rather than riverbed emission. Also, the background PM₁₀ concentration at Sanyi is close to 150 µg m⁻³ on average, indicating that the pollution from the Asian continent contributes a lot during these two weather stages.

The influence of the surface wind speed and wind direction on riverbed dust generation is also shown in Table 2. At the peak PM₁₀ concentrations of the typhoon type, the surface wind speeds reach an average value of 6.5 m s⁻¹, while more than 5 m s⁻¹ in the ahead of anticyclone stage, and thus high wind speed is critical for riverbed dust generation. In the typhoon type, the wind directions are mostly northeasterly winds, but in the ahead of anticyclone type, only half of the wind directions are northeasterly winds, and the other half are northwesterly winds. Similarly, Tsai et al. [36] also reported that after a frontal passage over Taiwan, the wind direction changes from northwesterly to northeasterly in Northern Taiwan. In contrast to these two types, the averaged wind speeds in the stages of ahead of front and behind anticyclone are lower, with only 1 m s⁻¹ of average wind speed. In addition, the wind directions of these two types are more diversified, and northeasterlies, northwesterlies, and southeasterlies were all observed in both types at the peak PM₁₀ concentrations. Among them, northwesterly is the main wind direction in the ahead of front stage of the non-typhoon type. As discussed in the case study, most of it is related to the sea breeze during the day and the cyclonic circulation, and the wind direction may carry pollutants to the riverbeds. The southeasterlies in the two stages are mostly observed in the Dadu River. The main city in Central Taiwan is located to the east of the river, and the land breeze can carry pollutants to affect the concentration of the river. Northeasterly is the main wind direction for the behind anticyclone stage. The high concentrations of PM_2.5 at the riverside town and PM₁₀ at the background station show that the riverbed is affected by local pollution and foreign aerosol at this time. This is because during the winter monsoon, northeasterly winds can bring pollution from Asia, resulting in high background pollution in Taiwan. Liu et al. [23] and Hung et al. [26] also showed that due to the long-range transport of dust or pollutant from the Asian continent, the averaged background concentration of PM₁₀ in Northern Taiwan can be as high as 100 µg m⁻³ during the event, and April is the month with the highest background concentration. Our results are consistent with those studies.

The average temperature and relative humidity in Table 2 also give an explanation for the high PM₁₀ concentration in the riverbeds. The typhoon type has the highest average temperature (25 °C), and the weather is usually warm and humid. However, this type of low relative humidity also shows that the moist air has not reached Central Taiwan, where most of the rivers are located, so it is conducive to dust emissions. For the ahead of anticyclone type, the temperature is lower (20 °C), but the dry air (mean humidity of 60%) is also favorable for riverbed emissions. In contrast, the behind anticyclone and ahead of front stages in the non-typhoon types both have high humidity close to 80%. Particularly for the behind anticyclone stage, the cold and humid air is not favorable for dust emission, and thus the riverbed events were significantly affected by local pollutant or foreign aerosol.

Table 2 shows that the riverbed events were mainly observed during different weather stages in December, February, and April, respectively, and also often observed at noon time. To further explain the high riverbed PM₁₀ concentrations observed in these months rather than in the same weather stages of other months and times, the monthly and hourly mean wind speed, temperature, relative humidity, and precipitations of all the rivers are plotted and the general meteorological conditions are discussed (Figure 7). Figure 7a shows that during the winter monsoon from October to April, higher monthly average wind speeds were observed in all the rivers than in the summer monsoon. In addition, the peak monthly wind speeds were observed from December to February. Thus, during the ahead of anticyclone stage of these months, the strong wind speeds can result in higher riverbed emission compared to other months. In contrast, April is at the end of winter monsoon. The wind speed decreases (Figure 7a) and thus the weak wind speed behind the anticyclone can easily result in the accumulation of local pollution compared to other months, and enhance the riverbed concentration. In addition, a previous study [23] also found that in December and April, the amount of Asian dust transported to Northern Taiwan was higher than in other months. Under high background concentration, more riverbed events can also be observed. However, January is the month with the lowest riverbed concentration (Figure 3), despite the maximum wind speed of the months. It is possible that due to the low temperature in January (Figure 7b), Asian deserts are easily covered by snow, resulting in the reduction of Asian dust emissions and transport. The low foreign dust concentration in this month also resulted in the lower PM₁₀ concentration in the riverbed areas. The similar low PM₁₀ concentration is also seen in a previous study of Northern Taiwan [23].

In addition, Figure 7c shows that the monthly average relative humidity is around 70% for all the rivers, and Figure 7d shows that rainfall in all the rivers during the winter monsoon is extremely low, different from that during the summer monsoon (May to September). The dry condition is favorable for riverbed dust generation during those months.

The average meteorological values in a day (Figure 7e–h) show that higher wind speeds often occur around noon for all the rivers, and the maximum wind speed is between 13:00–14:00. The highest temperature was also observed at noon, while the hourly average relative humidity of all rivers reaches a minimum at noon time. The strong wind speed, high temperature, and low humidity at noon time are conducive to dust emission from the riverbed, as shown in Table 2. Furthermore, the hourly average rainfall in Figure 7h shows that the 24-h rainfall of all rivers remains mostly around 50 mm to 100 mm. Among them, the minimum mean precipitation was observed over the Zhuoshui River most of the time, which may be one of the reasons for the high riverbed PM₁₀ concentration shown in Figure 3.