The mountainous terrain cutting through the center of Taiwan to the northeast disrupts the flow of low-altitude incoming air masses, essentially shielding eastern Taiwan from much of the brunt of the incoming wintertime plumes. It is an island nation sitting just southeast of the main continental land mass of Asia and on the southern boundary of the East China Sea thus during northeast monsoon season, it can be the recipient of even relatively small Asian outflow plumes. Taiwan is often a receptor for transboundary pollution coming from East Asia ( Chuang et al., 2008 S.H. Dust-related plumes are more common in late winter and springtime ( Hung et al., 2019), while other transboundary plumes in East Asia are populated by anthropogenic sulfate particles and may arrive throughout the northeast monsoon season ( Hung et al., 2019). Some of these transboundary events in East Asia are dust-related, originate from one of two deserts in north-northwest China and are physically dominated by larger particles that completely occlude the visual range ( Wang et al., 2010).
On the other hand, the incoming plume may completely dominate the total PM 2.5 concentration ( Chuang et al., 2008 Kaneyasu et al., 2014). PM 2.5 transboundary events may be particularly intense if local pollution in the receptor area significantly mixes in with the transported plume ( Hung et al., 2019). Furthermore, PM 2.5 is a small-diameter subset of total particles, and thus is the most buoyant and can be easily carried on the wind. aerosols less than or equal to 2.5 μm in diameter) are a common component of these air masses as they are a conglomeration of hundreds if not thousands of different compounds, of which the less reactive and less volatile components are capable of long transport ( Ueda et al., 2016 Shimada et al., 2018). From late autumn to early spring, northeast monsoon weather patterns are routine over East Asia and deliver air masses across the Asian continent ( Luo et al., 2018), and as outflow from the continent across the East China Sea and to Korea ( Lee et al., 2015 Kim et al., 2018), Japan ( Matsuda et al., 2010 Kaneyasu et al., 2014 Lee et al., 2019) and Taiwan ( Chuang et al., 2008 S.H. suspended liquid or solid particle in air) pollution impacts areas all around the world ( Querol et al., 2004 Itahashi et al., 2017 Anil et al., 2019 Shairsingh et al., 2019 Grenier et al., 2020), and East Asia is a particularly unique arena for these events. Employing the 100% emission model scenario and scaling up to the average episode hours each winter, the PM 2.5 surplus delivered via plumes on the northeast monsoon is equivalent to a 0.5 μg m −3 surplus for the whole year. Due to the length of the episode and the significant reduction in emissions, Taiwan avoided a PM 2.5 surplus of 19.2 μg m −3 on average during the episode, equivalent to a 0.5 μg m −3 reduction for the whole 3-month winter season.
CMAQ v5.2.1 modeling of the LRT event with 100% emission and reduced emission scenarios, revealed emissions in China were approximately 50% less than normal periods. The whole episode persisted for about 60 h, longer than most LRT episodes from China to Taiwan. The attenuated transport event arrived to northern Taiwan with a PM 2.5 concentration <45 μg m −3 and most often <35 μg m −3, which is 2–3 times lower than LRT episodes of similar back-trajectory and synoptic patterns. We found NO 2 column concentrations were reduced by 24% during the CNY Week 3 this year compared to previous years. Thus, movement of the Chinese populace from city to city was already greatly reduced by the time of the LRT episode, although the reductions in industrial output are less clear. In 2020, the initial COVID-19 lockdowns in China overlapped with Week 3 of the Chinese New Year (CNY) holiday, and an Asian outflow event. Long-range transport (LRT) of air pollutants from East Asia during the northeast monsoon season impacts several downwind locations.
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