A Simulation Study on PM2.5Sources and Meteorological Characteristics at the Northern Tip of Taiwan in the Early Stage of the Asian Haze Period

The present study utilizes air quality modeling to probe the sources and characteristics of PM2.5(particles less than 2.5 micrometers in aerodynamic diameter) at the northern tip of Taiwan (CAFE station) in the early stage of the Asian haze period. Since CAFE is the first place that is influenced by the Asian haze coming from the north, this study focused on the wind field, PM2.5concentration, and PM2.5composition at CAFE. During the research period (Oct. 16, 2015, to Nov. 15, 2015), four PM2.5episodes occurred at CAFE. This study classified these four episodes into three types, according to their PM2.5sources: the long-range transport (LRT) type, the local pollution (LP) type, and the LRT/LP mix type. For the LRT type, Asian outflows prevailed in a north to northeast wind at the north of Taiwan. The proportion of NO3−in the PM2.5resolvable compositions was very small at CAFE due to evaporation during transport, whereas the relative proportion of sea salt increased due to strong winds. For the LP type, an east wind prevailed and formed a cyclone/lee vortex in northwest Taiwan. Although the background PM2.5concentrations were low (4–20 µg m−3), the cyclone transported local anthropogenic emissions northward and elevated the PM2.5levels at CAFE. For the LRT/LP mix type, an east wind also prevailed, but the background PM2.5concentrations were at an intermediate level (20–30 µg m−3) because the Asian outflows had already transported haze to the West Pacific. The combined LRT and LP increased PM2.5at CAFE. In addition, the proportions of NO3−(nitrate) for the LP and LRT/LP episodes were obviously higher than those on the days before and after. This suggests a considerable contribution on PM2.5from LP.