| 摘要: | 鑒於民眾對於改善能見度的期盼、能見度無逐時觀測以及當前研究缺乏能見度劣化與氣膠物化特性之關係的全面性分析，本研究在台中市東海大學的2017年十月至2019年八月進行高時間解析度的觀測，在移動式測站IMPACT (Integrated Measurements of Pollution and Aerosol Composition & Transformation)放入質量濃度、光學係數、粒徑分布、水溶性無機鹽離子、重金屬元素、微量氣體與氣象狀態等面向的多台儀器進行觀測，使用化學散光模式(IMPROVE, The Interagency Monitoring of Protected Visual Environments)以及事件演變階段的分析方法來探討台中能見度劣化之成因。
在總觀測期間內發現光學機制由散光主導，能見度低值事件(消光係數上升)不僅與「量」有關(粒狀污染物濃度增加)，同時亦受「質」的影響(單位質量的微粒消光效率上升)。在季節變化中，以夏秋兩季消光係數較低；冬春兩季較高。在風花圖的結果發現低風速下容易產生高消光係數、SSA(single scattering Albedo)與MSE(Mass scattering efficiency)。由乾淨情境(Clean)與事件期(Event)分析兩年冬季的化學散光組成，發現能見度劣化期間的硝酸鹽比例較乾淨期間大幅增加(12%增至43%)，此外主要散光成分的硫酸鹽(18%)、硝酸鹽(43%)與有機物(32%)的粒徑有明顯增長，分別在400 – 600 nm、200 – 500 nm與100 – 300 nm的範圍內，造成散光效應促使能見度降低之結果。在事件演變階段(Stage 1與Stage 2)的氣象條件屬於低風速(< 1 m/s)與高溫的情況，在不同相對濕度下會出現對應的硝酸鹽生成機制，白天的低濕度(70%)適合氣相反應；晚間的高濕度(90%)則偏向異相反應，造成反應機制不同，但產物與劣化表現相似的結果。
;In Taiwan, many studies have been carried out over the past to survey the reasons of visibility degradation, but there still lack of the comprehensive analysis in aerosol physical-chemical characteristics. This study established a high time-resolution monitoring system and seted the trailer station – IMPACT at the campus of Tunghai University in Taichung during October 2017 to August 2019. Measurement items include the mass concentration, extinction coefficient(bext), size distribution, water-soluble inorganic ions, heavy metal elements, tracer gas and meteorological data in two years. In this study, to evaluate the reasons of visibility degradations, we used the revised IMPROVE algorithm and the stage variation of events to estimate the chemical components and extinction coefficient (bext) of fine particulate matter (PM2.5).
During the observation period, the extinction mechanism is dominated by the scattering particles. The low visibility events (high bext) are not only caused by high PM2.5 concentrations but also affected by high mass extinction efficiency (MEE). In seasonal variations, the bext is lower in summer and autumn than in winter and spring. Based on the wind-roses analysis, lower wind speed (WS < 1 m/s) would cause higher bext, single scattering Albedo (SSA), and mass scattering efficiency (MSE) during each season. In the IMPROVE chemical scattering of event analysis, the main contributor to bext is Nitrate (43%), followed by OM (32%) and Sulfate (18%). Nitrate would get a higher proportion (12% to 43%) in visibility degradation than clean case. Furthermore, the particle size range would become larger and concentrated, Nitrate is 200 to 500 nm, OM is 100 to 300 nm and Sulfate is 400 to 600 nm. While the increasing particle size make higher scattering effect and lower visibility. In stage 1 and stage 2 of event, wind speed is lower (< 1 m/s) and the temperature is higher than the clean case. The different relative humidity (RH) would correspond to those Nitrate formation-reaction. Low RH (70%) during the daytime is suitable for gas-phase reactions, and high RH (90%) during the nighttime is appropriate for heterogeneous reactions. Surprisingly, daytime and nighttime mechanisms are different, but the final situation cause visibility degradation and the same produce - nitrate. |
