| 摘要: | 近幾年政府致力於改善台中地區的空氣品質，而能見度為民眾主要關注空污議題之一。本研究中以懸浮微粒質量濃度與能見度之間的關係為出發點，探討不同時間尺度及不同粒徑之懸浮微粒的質量濃度與成份特性對能見度之影響。研究工作於2017年10月至2019年08月期間在台中東海大學設立移動式監測站，透過 TEOM 與BAM 之自動監測儀器分別進行PM1與PM2.5質量濃度之即時量測，且於採樣過程進行局部加熱或調理。含碳成份之量測則使用七波段碳成份吸光儀與氣膠有機碳元素碳監測儀，水溶性無機離子成份之量測使用在線式氣體與氣膠成份監測儀。
研究結果顯示，台中東海地區整個觀測期間加熱過之 PM1 與 PM2.5 平均值質量濃度分別為 13.4 µg/m3及 24.2 µg/m3 ，環境之 PM1 及 PM2.5 平均值量濃度分別為 18.9 µg/m3 及 28.1 µg/m3，整個觀測期間 PM2.5 質量濃度在 2018 年秋冬季節期間有下降之趨勢，然而 PM1 質量濃度變化幅度相較於 PM2.5 並不顯著。環境情況下與加熱或調理過之 PM 質量濃度有相當之差異，且加熱過後之 PM2.5 與微粒散光係數有較好的相關性。重建之PM2.5質量濃度約占實際質量濃度80%至90%，且重建和量測的PM2.5質量濃度之間發現良好的相關性，說明硫酸鹽、硝酸鹽、元素碳及有機物質之化學成分成份幾乎可以代表量測所得的PM2.5質量濃度。環境與加熱後之 PM1/PM2.5 約為至 62%與 48%， PM1/PM2.5 在加熱後平均比值下降表示 PM1 中含有較多的揮發性物質。
水溶性無機離子(NO3-, SO42-, NH4+)與含碳物質(OC, EC)為PM2.5之主要貢獻成份，季節變化對於成份之影響方面，PM2.5之OC、NO3-及NH4+冬季和春季期間濃度最高，夏季濃度最低，與PM2.5質量濃度有類似之趨勢。在2018年秋季與冬季期間可能受到汽油輕型車輛所產生之污染影響，在夏季期間相較於交通污染來源，可能有較多二次有機物生成，且可能多來自於南風或西北風之污染來源，南風的污染來源多來自於工業區，工業區之污染物可能含有揮發性有機物質，其可作為形成SOC之前驅物。能見度劣化期間，水溶性無機離子約占PM2.5 四到五成，含碳物質約占PM2.5 一到兩成，而且當消光係數高於200 Mm-1時，硝酸鹽之占比有顯著增加，硝酸鹽之生成又會受到臭氧及相對濕度影響，夜晚期間在高相對濕度下，氮氧轉化率 (NOR) 有明顯增加，白天期間當臭氧增加時， NOR 也有顯著增加。
根據以上結果表示，從質量及成份面相來看都有減量，但能見度仍無顯著之改善原因可能為在化學成份及質量濃度上同時影響能見度，且粒徑的變化可能會抵消減量之效益，因此需考慮化學物質在粒徑上的分布。總觀測期間之減量主要可能多為硫酸鹽，然而硝酸鹽對於能見度之影響更為顯著，因此減量及改善策略可能要從硫酸鹽之減量漸進到硝酸鹽之減量控制。
;To investigate the effect of PM mass concentration and composition characteristics on visibility at different time scales and particle sizes. PM mass concentration, chemical composition, and meteorological conditions were measured in Tunghai University from October 2017 to August 2019. Tapered element oscillating microbalance (TEOM) and β-attenuation monitor (BAM) were used to measure the after heating and ambient cases of particulate matter in real-time. The carbonaceous species in PM2.5 were measured using a aethalometer (AE33) and the semi-continuous OC-EC field analyzer. Water-soluble inorganic ion components were measured with an online gas and aerosol component monitor.
The results revealed that the average mass concentrations of PM1 and PM2.5 in after heating cases are 13.4 µg/m3 and 24.2 µg/m3, respectively. The average mass concentrations of ambient PM1 and PM2.5 are 18.9 µg/m3 and 28.1 µg/m3, respectively. The mass concentration of PM2.5 has significantly decreased. However, PM1 has a slower decreasing tendency than PM2.5 during the observation period. A strong correlation (R2=0.88) is found between the measured and reconstructed PM2.5 mass concentration with a slope of 0.89, indicating that the total mass of major chemical components in PM2.5 could closely represent the measured PM2.5. Water-soluble inorganic ions (NO3-, SO42-, NH4+) and carbonaceous species (OC, EC) are the main components of PM2.5. The mass concentrations of OC, NO3-, and NH4+ in PM2.5 are higher in winter and spring, and lower in summer. Compared with the sources of traffic pollution during the summer, there may be more secondary organic matter formation, and more likely from the southerly or northwesterly winds. The sources of southerly wind probably come from the industrial zone. Pollutants in industrial zone may contain volatile organic substances, which can be used as precursors to form SOC. When visibility is degrading, the water-soluble inorganic ions account for about 40 to 50% of PM2.5 and carbonaceous species account for about 10 to 20% of PM2.5. Nitrate may increase significantly in low visibility, and ozone and relative humidity may affect the formation of nitrate.
According to the above results, simultaneous changes in chemical composition and mass concentration affect visibility, but the changes in particle size may offset the benefit of reduction. Thus, it is still necessary to consider the chemically resolved aerosol size distributions. In terms of reduction and improvement strategies, it can not only focus on the reduction of sulfate, but still need to strengthen the reduction of nitrate. |
