| dc.description.abstract | Yellow sand (YS) phenomenon is very active in springtime of East Asia. For the right atmospheric condition, the dust storm from northwestern region of China, the desert area, will transport dusts to Taiwan. During the period effected by the dust storm, the aerosol concentration in Taiwan increases significantly. Not only the air quality, but also the human health is threatened. Therefore, it is important to understand the effects of YS to our atmosphere. In this study, the concentrations of PM10 and PM2.5 in metropolitan Taipei from March to May in 2002 were monitored. In addition to automatic instruments installed in EPA aerosol supersite, manual sampling equipments are employed to get more detailed information of both chemical and physical characteristics of aerosols.
This year, the YS invaded Taiwan eight times. From the real-time monitoring concentrations of PM10, the worst two batches were the second (March 6-9) and the third (March 18- 20) ones. The maximum hourly PM10 concentration was higher than 160 mg/m3. Among the eight batches of YS, the average of PM2.5 and PM10 was 28.7 and 66.3 mg/m3, respectively. In contrast to 37.4 and 55.4 mg/m3, the average of PM2.5 and PM10 during non-yellow-sand (NYS) periods, the concentration of the coarse particles (PM10-2.5) increased a great amount during the YS periods. It is found that on the arrival of a YS batch, the wind direction shifted from northeast to from north and the wind speed was lifted. Meanwhile, the PM10-2.5 level was increased during this time period, which is different from the level in the NYS periods. This demonstrates that one may use the change of WD and WS to infer the arrival of a YS event. Certainly, this inference is better verified by the change of the trend of PM10-2.5 level.
From the chemical compositions of the aerosols, it could be found that the average mass fractions of sulfate, nitrate and carbon are lower in YS periods than that in NYS periods. It demonstrates that the dust storm brings little PM2.5, most PM2.5 of is contributed from local activities. In addition, the PM10 volume size distributions show that fine fraction is predominant in NYS periods, while coarse mode is more significant in NY periods.
During NY periods, PM2.5 and PM10 were also collected manually, the results showed a consistency in variations of mass and carbonaceous content with automatic continuous measurements. This demonstrates the reliability of the manual collection as well as the automatic continuous measurement. From the HYSPLIT model (Draxel, 1999), one can categorize the 72-hour backward air trajectory into 6 types. For the periods associated with the YS backward air trajectory, the PM10-2.5 level is higher than PM2.5 level. The trend is reversed for the NYS periods. For aerosol carbonaceous contents, the variation between YS and NYS is very little. As to the elemental contents, the air trajectory associated with source regions carry predominant Si, Ca, Al, and Fe in the coarse particles; the enhancement factor calculation shows these elements were contributed from crustal materials. For water-soluble ions, the air trajectories from source regions transport predominant Ca2+ and Mg2+. From the application of enhancement factor to the water-soluble ions, Cl－and Mg2＋were originated from the sea, while Ca2+, SO42-, and K＋ were from non-sea-salt sources. | en_US |