Abstract
Dust storms and long-range transport of pollutants are major environmental concerns of Taiwan during the winter monsoon season when northeasterly winds prevail following passages of cold fronts. To quantify the impact on air quality, we develop an objective method to classify and study the long-range transport processes by examining the frontal passages in two representative years. We have found that there is about one frontal passage per week in winter and spring, consistent with the climatological average. The long-range transport events are classified into three types according to their degrees of impact on levels of dusts and air pollutants in Taiwan, namely dust storms (DS), long-range transport with pollutants (FP), and long-range transport of background air masses (BG). DS cases occurred 4.7% of the time over 14 months and had a large average PM10 concentration of 127.6 μg m^(-3) at Wan-Li station. FP cases occurred 1.9% of the time and the mean concentration of PM10 during the FP periods was about 85 μg m^(-3). BG cases happened 18.6% of the time and the mean concentration of PM10 was 32.8 μg m^(-3). Dust storms and air pollutants tend to be transported in different air parcels as evidenced by a lack of correlation between dust aerosols and air pollutants. The frequency of local pollution (LP) cases was 71.7% in winter and spring. The average PM10 concentration of LP cases at the Wan-Li station was 47.4 μg m^(-3). However, about one to two-thirds of the PM10 during LP cases can be attributed to the long-range transport. When this contribution is taken into account, we estimate that the contribution of long-range transport to PM10 abundance in northern Taiwan during winter and spring to be in the range of 50% to 75%.
Highlights
Dust storms are common during late winter and spring in East Asia
We show below that Asian continental air behind a front can be further classified into three categories: (1) the dust storm case that consists primarily of Asian mineral dust but not necessarily air pollutants as mentioned earlier; (2) the frontal pollution case that contains a significant amount of air pollutants from the Asian continent but little dust; and (3) the background air case that contains relatively clean background air from the East China Sea north of Taiwan
Based on measurements of PM10 and other air pollutants at Taiwan Environment Protection Administration (TEPA) air quality ground stations, we show below that the long-range transport associated with frontal passages can be further classified into three types, namely dust storm (DS), long-range transport with pollutants (FP), and long-range transport of background air masses (BG)
Summary
Dust storms are common during late winter and spring in East Asia In a subsiding high-pressure system atmospheric stability tends to be high and prevents effective vertical mixing These processes are very important to long-range transport of dust and air pollutants and are a major focus of this study. Surface temperatures drop rapidly, and relatively strong northeasterly winds prevail which can transport dust and air pollutants from the Asian continent to Taiwan. As we will see later that most of the frontal passages over Taiwan have background/moderate levels of air pollutants This is probably a result of a well-mixed boundary layer over the East China Sea upwind of Taiwan, which is known for highly efficient turbulence mixing because of the cold winter monsoon winds over the relatively warm Kuroshio Current. We evaluate the contribution of long-range transport to distributions of PM10 in northern Taiwan
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