| dc.description.abstract | The high PM2.5 air masses from northern China usually deteriorated the air quality in Taiwan via long-range transport (LRT) from the late autumn to early spring every year. The reason is that Taiwan is located in the downwind area of Asian continent under prevailing northeasterly wind. In order to understand the impact of pollutants from LRT on the air quality in Taiwan, this study analyzed long-term air quality and meteorological data in northern Taiwan (including Wanli, Keelung, Yilang, Dongshan, and Tamsui stations) from 2006 to 2015 especially when the prevailing wind is northeasterly. The outcome of analysis referring to the method of Junker et al. (2009) and Chuang et al. (2008a) results in three types of pollution. The present study defined the first type as LRT – Event with the conditions of the daily-average wind speed ? 3.7 m s-1 and the daily-average PM2.5 ? 25 μg m-3. The second type is LRT – Ordinary when the daily-average wind speed ? 3.7 m s-1 and the daily average PM2.5 < 25 μg m-3, and the third type is LRT/LP Mix (Local Pollution) or the Pure LP when the daily-average wind speed < 3.7 m s-1. According to the statistics, LRT – Ordinary type is the most frequent occurring one usually from autumn to next spring, followed by LRT – Event also from autumn to early spring, and the LRT/LP Mix or the Pure LP appearing in every season.
This study applied the Community Multi-scale Air Quality Model (CMAQ) model to simulate a short-term (January to May 2010) scenario with emissions geographically covering the whole East Asia but excluding Taiwan. From such a simulation, this study estimated the variation ratios of LRT PM2.5 concentration in major metropolises to Wanli station, the northern tip of Taiwan. Based on this estimation, the variation ratios of LRT PM2.5 concentration over Wanli in the northern (Hsinchuang station), central (Zongming station), and southern (Xiaogang station) metropolises are 0.83, 0.62, and 0.53, respectively, for the LRT – Event type (n=13). For the LRT – Ordinary type (n=32), the variation ratios of LRT PM2.5 concentrations over Wanli in the three metropolis are 0.72, 0.67, and 1.44, respectively. As to the LRT/LP Mix or the Pure LP type (n=14), the variation ratios of LRT PM2.5 concentration over Wanli are 0.91, 1.10, and 1.72, respectively. Due to complicated mechanisms of atmospheric condition, these three types of pollution have different variation ratios in various regions.
Using 10-year air quality data and the variation ratios of the three types of pollution, the present study made estimates on the PM2.5 concentrations of LRT and LP at different metropolises. The results show that the average contributions of LRT and LP in northern, central, and southern metropolises are at 14 μg m-3 (53%) and 11 μg m-3 (47%), 12 μg m-3 (37%) and 21 μg m-3 (63%), and 18 μg m-3 (36%) and 36 μg m-3 (64%), respectively. The contribution of PM2.5 in northern Taiwan is mainly from LRT. Although the impact of LP is higher in central and southern Taiwan, the present study suggests that the contribution of LRT is not ignorable.
In terms of the contributions of LRT and LP to various stations, the PM2.5 concentrations from LRT in the western Taiwan had gradually decreased since 2013. The annual variations of PM2.5 concentration from LP were not obvious in stations north from Hsinchu, but there was a significantly downward trend in the central and southern Taiwan during the last ten years. Compared to yearly PM2.5 standard (15 μg m-3), the LRT contribution from China to PM2.5 is already near the yearly standard in northern Taiwan. However, it is practical to reduce the PM2.5 contribution from LP sources than expecting reduction of LRT contribution to meet PM2.5 daily standard (35 μg m-3).
| en_US |