台灣中部能見度與細懸浮微粒物化特性之長期趨勢分析

Abstract

大氣氣膠，特別是粒徑小於2.5微米的細懸浮微粒 (PM2.5) 和小於1微米的微粒 (PM1.0)，在空氣污染、氣候變遷及人類健康影響等方面備受關注，並對能見度的降低具有顯著影響。本研究聚焦於臺中地區，探討PM2.5與PM1.0的長期變化趨勢，分析其化學組成、粒徑分佈及光學特性對能見度的影響。研究採用高時間解析度的觀測數據，分析消光係數、散射係數及吸收係數的趨勢，並結合修正的IMPROVE公式與Theil-Sen方法進行統計分析。 研究結果顯示，五年半的觀測期間，消光係數自 106 1/Mm 下降至 66.7 1/Mm（-37.1%），能見度提升約5.6公里。PM2.5濃度每年顯著下降1.74 μg/m³，其中硫酸銨 (Ammonium Sulfate, AS) 每年減少1.01 μg/m³，對消光係數的貢獻每年下降4.08 Mm⁻¹，可能與船舶燃料減排有關。硝酸銨 (Ammonium Nitrate, AN)、黑碳 (Elemental Carbon, EC) 和有機物質(Organic Matter, OM) 所造成的消光係數亦分別每年減少2.56、1.92和1.73 Mm-1，但OM的相對貢獻比例每年上升3.39%，可能成為未來減排的重點。PM2.5的減少主要集中於1.0–2.5 μm範圍，而PM1.0未呈現顯著下降，顯示需進一步探討其形成機制及排放來源，以促進能見度的進一步改善。 此外，能見度良好的乾淨期每年增加3.64%，而能見度劣化事件的發生頻率每年下降1.42%。在劣化事件期間，PM2.5濃度每年下降3.67 μg/m³，AS、AN、EC和OM對消光係數的貢獻分別每年下降7.19、6.34、3.29和2.08 Mm-1，改善幅度高於常態時期。然而，PM1.0在劣化事件期間依然未顯示下降趨勢。粒徑分佈分析顯示，積聚模態 (100–1000 nm) 內的表面積與體積濃度增加，而1.0–2.5 μm範圍則呈現下降，表明化學物種濃度的減少在此範圍內更為顯著。此外，秋冬季粗顆粒比例較高，顯示該季節易受粗顆粒影響，進一步導致能見度劣化事件的發生。

Atmospheric aerosols, especially particulate matter with diameters less than 2.5 μm (PM2.5) and 1 μm (PM1.0), are a critical focus of research due to their significant impact on air pollution, climate change, and human health, as well as their significant contribution to visibility degradation. This study investigates the long-term variations of PM2.5 and PM1.0 in the Taichung region, analyzing their chemical composition, particle size distribution, and optical properties in relation to visibility. High temporal resolution observations were used to investigate trends in extinction, scattering, and absorption coefficients, combined with statistical analysis using a revised IMPROVE equation and the Theil-Sen method. The results show that the extinction coefficient decreased from 106 ± 62.2 1/Mm to 66.7 ± 50.8 1/Mm (-37.1%) over a period of five-and-a-half years, corresponding to an approximate visibility improvement of 5.6 km. PM2.5 concentrations decreased significantly by 1.74 μg/m³ per year, with ammonium sulfate (AS) showing the largest decrease of 1.01 μg/m³ per year, contributing to an annual decrease in the extinction coefficient of 4.08 Mm-1. This trend is likely to be related to reductions in marine fuel emissions. Ammonium nitrate (AN), elemental carbon (EC), and organic matter (OM) also contributed to annual decreases in the extinction coefficient by 2.56, 1.92, and 1.73 Mm⁻¹, respectively. However, the relative contribution of OM to the extinction coefficient increased by 3.39% per year, highlighting its potential as a key focus for future emission reduction strategies. The reduction in PM2.5 was mainly concentrated in the 1.0-2.5 μm range, with no significant reduction observed for PM1.0, indicating the need for further investigation of its formation processes and emission sources to achieve further visibility improvements. In addition, the frequency of clean periods with good visibility increased by 3.64% per year, while the frequency of visibility degradation events decreased by 1.42% per year. During these degradation events, PM2.5 concentrations decreased significantly by 3.67 μg/m³ per year, with AS, AN, EC, and OM contributing to the annual reduction in extinction coefficient by 7.19, 6.34, 3.29, and 2.08 Mm-1, respectively, representing greater improvement rates than during normal periods. However, PM1.0 showed no decreasing trend during the degradation events. Particle size distribution analysis showed increased surface area and volume concentrations within the accumulation mode (100-1000 nm), while concentrations in the 1.0-2.5 μm range decreased, suggesting a more pronounced reduction in chemical species concentrations within this size range. In addition, the higher proportion of coarse particles (>1 μm) during autumn and winter suggests a seasonal susceptibility to coarse particle influence contributing to visibility degradation events.