Transport characteristics of Chinese haze over Northern Taiwan in winter, 2005-2014

Haze over east China has been extensively studied, but its impacts on downwind areas remain unclear. In this study, we applied long-term (9 yr) air quality, meteorological, and ground-based remote sensing data for investigating the transported haze events over Northern Taiwan in winter. Thirty-six haze events were identified using a statistical method and information pertaining to wind direction and episode duration. In contrast to haze events over China, the transported haze exhibits low relative humidity (approximately 70%) and high wind speed (approximately 5 m s^-1) and is associated with a migrating high-pressure system. The mass concentration of fine particulate matter (PM_2.5) for the haze events was 57.1 ± 13.6 μg m^-3, nearly four times higher than that of the background (13.7 ± 7.4 μg m^-3). Such high PM levels persisted for 120 h in winter. Back trajectory analysis suggests that the haze particles were transported from the east coast of China, particularly from the Yangtze River Delta (YRD) area with a traveling time of nearly 28 h. Lidar observations clarify that the Chinese haze was transported to Northern Taiwan and accompanied by low clouds and a temperature inversion layer, which confined the particles to a height of less than 1 km. Moreover, a relatively high PM_2.5 concentration (65.0 μg m^-3) was observed when air mass stagnated over YRD before advancing to Taiwan. This scenario, most frequently seen in the winter of 2013, resulted in the highest historical PM_2.5 concentration (approximately 77.2 μg m^-3). We propose that the heavy haze year is primarily attributable to synoptic weather variability and less to climate.