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|Authors: ||林正直;Lin, Cheng-Chih|
|Issue Date: ||2017-10-27 12:28:16 (UTC+8)|
;By using the pollutants concentrations observed by the Taiwan Air Quality Monitoring Network (TAQMN) from Taiwan Environmental Protection Administration (EPA) and both NCEP and ECWMF reanalysis data, this research effort especially investigated the influence of long-range transport of the Southeast Asian biomass burning aerosols on the downwind surface air quality, particularly in Taiwan, under different weather and climate regimes.
Climatologically, the East Asian high merges with the westward expansion of the northwestern Pacific subtropical high in March. The local East-West cell/circulation is enhanced by a well-organized convergent center that forms over Indochina at 925 h Pa in March. This allows the emitted air pollutants from the biomass burning to be uplifted to the free troposphere and immediately transported downwind to the east. Using the data collected from the 2010 7-SEAS/Dongsha Experiment, six cases of long-range transport events involving air pollutants during March-April were identified at the Hengchun air quality monitoring station in southern Taiwan. These events were related to active burning phases over Indochina Peninsula. The air pollutants produced by these events were transported to Taiwan after a 2e3 day journey. A composite analysis for these identified six cases showed that the boundary layer of the southwesterly flow confluence coupled with a well-organized convergent center located over a thermal low under clear skies over the Indochina Peninsula may induce a distinct ascending motion to form the upward branch of the transient local East-West cell/circulation during the burning phase. This inferred upward motion together with the strong thermal buoyancy created by the active biomass burnings could carry the air pollutants to the lower free troposphere where they would effectively be conveyed downwind along the westerly flow above 850 h Pa level. The air pollutants were brought down to the surface by downward branch of the transient local East-West cell/circulation, which was induced by the subsidence of a cold surge anticyclone. Using continuous point sources, the six identified cases were simulated with the Hybrid Single-Particle Lagrangian-Integrated Trajectory (HYSPLIT) particle dispersion forward model to confirm our supposition regarding the mechanism for the long-range transport of Southeast Asian biomass burning pollutants that has a significant impact on the surface air quality of the downstream areas, particularly in Taiwan.
The transport of aerosols highly relies upon the air flow to convey, however, the air flow pattern is completely dominated by the large-scale circulation condition. One of the missions for the 2013 7-SEAS/BASELInE (Seven SouthEast Asian Studies/Biomass-burning Aerosols & Stratocumulus Environment: Lifecycles and Interactions Experiment) campaign was to capture/confirm the downwind effect on the surface air quality due to the long-range transport of Southeast Asia biomass burning (SEA BB) pollutants that was first discovered during 2010 Dongsha experiment. Fortunately, this phenomena was directly observed by Lidar system at Hengchun in southern Taiwan. The most significant findings in this study is the upper level ridge-trough short wave within 20°-35°N was very active during 2013 spring. Through the three dimensional structure analysis, it was also found that the sinking motion behind the upper level active short wave trough is the major mechanism to help enhancing the subsidence of cold surge leading edge. In turn, the enhanced subsidence of cold surge leading edge might bring down the long-range transport of the SEA BB pollutants to the surface. Furthermore, the HYSPLIT backward air trajectories model was performed to help identifying the SEA BB pollutants in the mid-troposphere while the fine resolution WRF model simulation was employed to demonstrate the detailed time evolution of the brought-down aerosols process in coping with the evidence from particle depolarization Lidar observation. For the future study, we will further investigate the inter-annual variation between the atmospheric circulation and long range transport of aerosol over Southeast Asia in recent decade during springtime.
|Appears in Collections:||[大氣物理研究所 ] 博碩士論文|
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