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Springtime is the biomass burning (BB) season in Indochina. BB emissions are transported to East Asia and Taiwan at elevations of between 2000 and 4000 m and then transported downward to the near-surface layer when weather conditions are favorable. In this study, a numerical model, the Weather Research Forecast with Chemistry (WRF–Chem) model with tracer simulation was employed to explore the downward transport mechanisms and evaluate their potential effects on the planetary boundary layer (PBL) and near-surface layer during the springtime (Feb.–April) in 2018. Ambient backscatter was measured through LiDAR, aerosol optical depth was measured by MODIS and Himawari-8 satellites and by ground observations from the stations of AERONET. The data were applied to identify the transport mechanisms and evaluate the performance of the model. We summarized the downward transport mechanisms surrounding Taiwan, including the thermal effects of mountain downslope circulations (MDC), frontal secondary circulation (FSC) downward transport, and the mixing of MDC and FSC (MMF). Over Taiwan, the contributions of BB emissions for the MDC to the near-surface layer (<500 m), PBL (<1500 m), and free atmosphere (1500–6000 m) were 0.4%, 4.7%, and >95%, respectively, during springtime in 2018. For the FSC (MMF) mechanisms, the BB contributions to the near-surface layer, PBL, and free atmosphere were 2.5% (3.3%), 15.7% (17.1%), and 84.3% (82.9%), respectively. MMF occurred the most frequently among these three mechanisms and had the largest effect on the near-surface layer and PBL. The downward contributions of BB in different regions, including Taiwan, southern China, and the East China Sea (ECS), were also evaluated. Southern China had the highest value (11.0% [2.1%]) in the PBL (near-surface layer), followed by Taiwan and the ECS (8.2% [1.2%] and 4.5% [0.4%]). Overall, this research fills a gap in our understanding of the mechanisms by which BB emissions are transported to the surface and how they affect air quality in East Asia in general and Taiwan in particular. • We analyzed the BB emissions transport from Indochina to East Asia in general. • The downward transport mechanisms of aloft BB emissions have been explored. • The Impacts of BB emission on the PBL over the study regions were discussed. [ABSTRACT FROM AUTHOR]

Abstract: Carbon monoxide (CO) mixing ratios were observed from 30 January to 7 April 2008 at Mt. Lulin (23.51°N, 120.92°E, 2862masl) in central Taiwan to investigate characteristics of CO during biomass burning periods. During the sampling campaign, the average mixing ratio of CO was 234±63ppb with higher levels observed in March. The elevated CO in March can, on the basis of backward trajectories and satellite fire spots analyses, possibly be attributed to biomass burning activities in the Asian continent. Significant diurnal variations of CO mixing ratios were observed at the remote site. The higher CO levels in the afternoon were influenced by the transport of boundary layer pollution to the site during daytime upslope flow. Backward trajectory analysis showed that air masses mainly originated from India (ID), the Indochina Peninsula (IP) and South Coastal China (SC), which together accounted for 85% of the total trajectories. Higher mixing ratios of CO were found in the ID, IP, and SC categories, indicating significant impacts of anthropogenic emissions on the Pacific region. Furthermore, the air parcels were divided into two categories, those that passed over the fire regions and those that did not. The result showed that the average difference of CO levels between the two categories was approximately 79ppb, suggesting that Asian biomass burning plays an important role in CO levels at this remote site during the springtime. [Copyright &y& Elsevier]

Abstract: To quantify the possible sources of the high ambient ozone concentration in the low troposphere over Taiwan, ozone sounding data from a two-year intensive field measurement program conducted in April and early May of 2004 and 2005 in northern Taiwan has been examined. We found that the vertical ozone distributions and occurrence of enhanced ozone in the lower troposphere (below 6 km) mainly resulted from (1)Type NE: the long-range transport of ozone controlled by the prevailing northeasterly winds below 2 km, (2)Type LO: the local photochemical ozone production process, and (3)Type SW: the strong southwest/westerly winds aloft (2–6 km). In the boundary layer (BL), where Asian continental outflow prevails, the average profile for type NE is characterized by a peak ozone concentration of nearly 65 ppb at about 1500 m altitude. For type LO, high ozone concentration with an average ozone concentration greater than 80 ppb was also found in the BL in the case of stagnant atmospheric and sunny weather conditions dominated. For type SW, significant ozone enhancement with average ozone concentration of 70–85 ppb was found at around 4 km altitude. It is about 10 ppb greater than that of the types NE and LO at the same troposphere layer owing to the contribution of the biomass burning over Indochina. Due to Taiwan''s unique geographic location, the complex interaction of these ozone features in the BL and aloft, especially features associated with northeasterly and south/southwesterly winds, have resulted in complex characteristics of ozone distributions in the lower troposphere over northern Taiwan. [Copyright &y& Elsevier]

The impact of the urban heat island (UHI) effect on environmental and regional climate has been receiving wide attention in recent decades. Taiwan, especially Taipei (located in northern Taiwan), is experiencing a significant UHI effect due to its high population density and the uniqueness of the geographic structure. In order to evaluate the impacts of urbanization and UHI effect over northern Taiwan, a next generation mesoscale model, Weather Research and Forecasting (WRF) model coupled with the Noah land surface model and Urban Canopy model (UCM), was used to study this issue. By using the WRF–Noah–UCM model, it has significantly improved our simulation results for the prediction of the UHI effect, boundary layer development, and land sea breeze. Observations of weather stations and Lidar showed that the near surface air temperature was nearly 34–35°C and the boundary layer height was nearly 1500m around noon in Taipei on 17 June 2006. Around midnight, the air temperature ranged from 26 to 28°C. Our model can predict well for boundary layer development during the daytime and the UHI effect in northern Taiwan. Sensitivity tests indicate that the anthropogenic heat (AH) plays an important role for the boundary layer development and UHI intensity in the Taipei area, especially during nighttime and early morning. When we increase AH by 100Wm−2 in the model, the average surface temperature could increase nearly 0.3°C in Taipei. Furthermore, we found the UHI effect also has a significant impact on land sea circulation. It could enhance the sea breeze in the daytime and weaken the land breeze during the nighttime and hence had a significant impact on the air pollution diffusion in northern Taiwan. [Copyright &y& Elsevier]

Taichung City has become the second largest city in Taiwan, where PM 2.5 and O 3 pollution events occur frequently. The causes of these events urgently need a comprehensive and in-depth study. Therefore, a series of well-planned observation experiments were conducted in the city and its surrounding areas. The first experiment supported the current simulation study of the major causes of air quality deterioration in Central-Western Taiwan. First, we utilized the Weather Research and Forecasting (WRF)/Community Multiscale Air Quality (CMAQ) modeling system to simulate the intensive observation period (IOP) from 31 October 2021 to 22 November 2021. The performance of WRF-CMAQ is well justified by comparison with the observations. Next, we selected the PM 2.5 (particles with aerodynamic diameters equal to or <2.5 μm) pollution event occurring on 3–6 November 2021 for detailed analysis. The results reveal that the meteorological conditions of this event were the prevailing easterly/southeasterly winds blocked by the Central Mountain Range (CMR). Central Taiwan is located on the leeward side of the CMR and embedded in a calm wind area, which is especially favorable for the accumulation of PM 2.5 during the nighttime. At noon, the PM 2.5 concentration increased from the surface to the planetary boundary layer height (PBLH), even though the PBLH was overestimated. In the afternoon, the clean sea breeze would dilute the PM 2.5 in the Taichung Basin and the urban PM 2.5 plume would be transported to the mountains. The current simulation suggests that the aloft return PM 2.5 layer would subside along with the sinking PBLH and deteriorate the PM 2.5 concentration at night. At the downtown site of the IOP (UAPRS), from the trend of radicals and nitrogen oxides, ozone (O 3) is closely involved in the formation of nitrate (NO 3 −) during both daytime and nighttime. If Taichung City is Volatile Organic Compounds (VOC)-limited according to our previous study, then reducing VOC emissions could decrease O 3 and thus the NO 3 − concentration in Taichung City. [Display omitted] • The modeling of an intensive observation campaign is comprehensively evaluated. • The modeling performed well and reproduced meteorological and chemical fields. • Aloft return PM 2.5 layer subsidence could enhance pollution at night. • NO 3 − is a major species during nighttime PM 2.5 pollution events. • Reducing VOCemissions could decrease NO 3 − in the Taichung Basin. [ABSTRACT FROM AUTHOR]