台灣中部地區微粒粒徑分布、化學成分及吸濕性對氣膠消光係數的影響

Abstract

大氣氣膠藉由散射與吸收改變傳遞至人眼的太陽光，進而決定民眾對能見度觀感。為了研究能見度劣化的原因，本研究結合Mie theory與IMPROVE方法，搭配即時且長期的氣膠物化特性採樣，試圖解析化學成分與粒徑分布對消光係數之影響，同時探討微粒吸濕性以及氣象條件對能見度的負面影響。 Revised IMPROVE分析顯示化學物質對消光係數的貢獻按以下順序排列：有機物（OM）（37.2%）、硝酸銨（AN）（20.5%）、硫酸銨（AS）（18.7%）、元素碳（EC）（12.8%）、土壤（7.9%）和海鹽（2.9%）。在相對濕度的影響下，事件期間二次無機氣膠的貢獻可高達71%。為了更準確地估計當地污染對消光係數的影響，本研究基於EC示踪法和多元線性回歸開發適用於本地的IMPROVE演算法，結果顯示 POA 具有顯著的散光能力（MSE=6.22 m2/g），而SOA表現出中度散光能力（MSE=3.78 m2/g）。此外，POA和SOA都表現弱吸光能力，其MAE值分別為0.17 m2/g 和0.30 m2/g。根據這些信息，我們還推斷了POA和SOA的成分和粒徑範圍。通過Revised與Localized IMPROVE演算法，我們發現二次硝酸鹽（AN）的貢獻在從乾淨期到事件期間增加了約20%，原生性有機氣膠（POA）的貢獻則增加了約13%，表明POA與AN皆為能見度劣化的元凶。 基於米氏理論、粒徑分佈和化學成分數據，計算不同粒徑對消光係數的貢獻。對消光分佈的分析結果顯示，隨著能見度的降低，消光分佈更加集中在500 nm附近，這意味著直徑接近可見光波長的顆粒物對消光係數有顯著的影響。此外，本研究發現100 nm和400-700 nm 處微粒數目的增長，並在最後探討這些粒徑範圍內生成的微粒的化學成分。

Atmospheric aerosols alter the solar radiation reaching the human eye through scattering and absorption, thereby influencing perceived visibility. To investigate the causes of visibility degradation in the Taichung area, this study employed Mie theory and the IMPROVE method in conjunction with real-time and long-term aerosol physical and chemical characterization. This study aimed to elucidate the impact of chemical composition and particle size distribution on the extinction coefficient and examined the negative effects of particle hygroscopicity and meteorological conditions on visibility. The Revised IMPROVE analysis revealed that the chemical species contributing to light extinction ranked in the following order: organic matter (OM) (37.2%), ammonium nitrate (AN) (20.5%), ammonium sulfate (AS) (18.7%), elemental carbon (EC) (12.8%), soil (7.9%), and sea salt (2.9%). The heightened contributions (71%) of secondary inorganic aerosols were observed during the event period under the influence of relative humidity. To achieve a more accurate estimation of the organic matter on extinction coefficients, a Localized IMPROVE algorithm was established based on the EC-tracer method and multi-linear regression. The coefficients unveiled that POA possesses a substantial light scattering ability (6.22 m2/g), whereas SOA exhibits a moderate light scattering ability (3.78 m2/g). Additionally, both POA and SOA demonstrated limited light absorption capabilities, with values of 0.17 m2/g and 0.30 m2/g, respectively. Based on this information, we also inferred the composition and particle size range of POA and SOA. The application of both Revised and Localized IMPROVE algorithms revealed substantial increases of approximately 20% in secondary nitrate and 13% in primary organic aerosol (POA) contributions to light extinction from the clean to the event periods, indicating both POA and AN are the culprits of visibility degradation. The size-dependence extinction coefficient was calculated in this study based on the Mie theory, particle size distribution, and chemical composition data. Analysis of the extinction distribution exhibited that the extinction distribution becomes more concentrated around 500 nm as the visibility impairment, which means particles with diameters close to the visible light wavelength have a pronounced impact on light extinction. Furthermore, we found a number enhancement around 100 nm and 400-700 nm as events degrade, and the study concludes with a discussion of the chemical compositions of the particles generated within these size ranges.