Unusual PM episode caused by yellow sand over Taiwan on April 25, 2009

This study analyzes the unusual particulate matter episode caused by yellow sand (YS), which affected Taiwan on April 25, 2009. During this YS event, the PM10 hourly concentrations higher than 1000 μg m-3, about 20 times higher than local normal concentration, were observed at several sites over northern Taiwan. Moreover, of Taiwan's 76 air quality monitoring stations, 69 stations recorded daily average concentrations of PM10 which exceeded Taiwan's air quality standard of 125 μg m-3. Comparing the time periods when such high PM10 concentrations occurred in different areas in Taiwan, the transport phenomena of PM10 could be shown. The maximum concentrations of the central and southern PM10 occurred about 6 hours and 15 hours later, respectively, than those of the northern region. The maximum hourly values of PM2.5 and PM10 particles at 175 and 814 μg m-3, respectively, were recorded at the Taipei aerosol supersite (TAS) located in northern Taiwan. The ratios of PM10 to PM2.5 decreased from 0.6 to 0.2 during YS period as shown in Fig.1. This YS event not only greatly increased the PM2.5 and PM10 concentrations, but enhanced the sulfate fraction of PM2.5. The PM2.5 sulfate 22 μg m-3 at TAS site during YS period was even higher than that derived from local accumulation. Furthermore, the particles’ number size spectra at TAS site showed a decrease in the amount of particles with a diameter of less than 200 nm, indicating that the rise of the mass concentration of PM2.5 might increase along with the increase of PM2.5 particles with diameters larger than 200 nm. Several studies have indicated that the presence of yellow sand may be associated with increased health risks. The results of this study can provide the basis for further investigation into its effects on human health. Figure 1. Hourly PM2.5, PM10 and PM2.5 to PM10 ratios at TAS during yellow sand period.