臺北都會區褐碳來源解析與特定來源光學特性之探討

Abstract

大氣含碳氣膠因其吸光性質對全球氣候變化具有明顯影響，其中黑碳與褐碳被視為大氣輻射強化重要的貢獻物質。近年來，大量的研究揭示黑碳的光學特性，以及對於大氣輻射平衡的顯著影響。相較之下，因褐碳複雜的生成機制、來源多樣化且相關的研究相對較少，使褐碳的物化與光化性質及其來源仍充滿不確定性。 目前估算褐碳的吸光能力主要包括兩種方法: 來源歸因法和溶劑萃取法。但這些方法通常依賴於一些特定的假設，例如歸因分析法假設黑碳之波長指數為定值，而溶劑萃取法假設不能被溶劑萃取之有機碳幾乎不吸收光等。然而，氣膠波長指數與不能被溶劑萃取之有機碳的吸光能力都會隨氣膠的化學組份、排放特徵等因素改變。因此，這些假設可能導致褐碳的吸光能力估算出現誤差。 為了降低這些方法在探討褐碳上的不確定性，本研究於國立臺灣大學環境工程學研究所進行為期十個月之採樣，分析台北地區氣膠光吸收的來源貢獻，以及使用多元線性回歸探討含碳氣膠的特定來源光學特性。最後，利用多元線性回歸的分析結果結合最小平方法導出修正因子，以提高來源歸因法估算褐碳光吸收的準確性。 分析結果顯示，在370 nm波長下，超過70%的主要氣膠光吸收來自當地排放來源。多線性回歸結果顯示化石燃料燃燒是氣膠光吸收的主要來源，其中元素碳（EC）、水溶性有機碳（WSOC）和非水溶性有機碳（WIOC）在370 nm波長下的來源特定質量吸收截面分別為9.17 m2 g-1、5.66 m2 g-1以及3.25 m2 g-1。此外，修正因子的結果顯示利用來源歸因法估算的褐碳光吸收可能被低估最多達9%，揭示了在探討氣膠的光學特性時可能出現顯著偏差。

Atmospheric carbonaceous aerosols significantly influence global climate change due to their light-absorbing properties. Among these aerosols, black carbon (BC) and brown carbon (BrC) are prominent contributors to atmospheric radiative forcing. While extensive researches have revealed the optical properties of BC and its substantial impacts on atmospheric radiative balance, uncertainties remain regarding BrC. These uncertainties largely stem from its complex formation mechanisms, diverse sources profiles and relatively limited research, resulting in an inadequately understanding of BrC’s physical, chemical, and optical properties. Two popular methods for examining BrC’s optical properties are the attribution approaches and the solvent-extraction method. These methods often rely on specific assumptions that can introduce bias. Specifically, the attribution approaches typically assumes a constant absorption Ångström exponent value of BC (AAEBC), while the solvent extraction method often assumes negligible light absorption by unextracted fractions of organic carbon (OC). Nonetheless, both the AAE value and the light absorption capacity of the unextracted fractions of OC can vary with factors such as aerosol chemical composition of the aerosols and their emission characteristics. This variability can lead to inaccuracies in estimating the light absorption of BrC. To tackle these challenges, this study conducted a 10-month sampling campaign at the Graduate Institute of Environmental Engineering, National Taiwan University, focusing on the source contributions to aerosol light absorption in the Taipei area. A multi-linear regression (MLR) analysis was subsequently employed to elucidate the source-specific optical properties of carbonaceous aerosol. Finally, the MLR analysis was used to derive correction factors, thereby enhancing the accuracy of the BrC light absorption estimates obtained via the attribution approach. The results indicate that local emission sources contributed over 70% to aerosol light absorption at 370 nm. In addition, MLR analysis suggest that fossil fuel combustion is the dominant emission source for aerosol light absorption, yielding source-specific mass absorption cross-sections at 370 nm of 9.17 m2 g-1 for elemental carbon, 5.66 m2 g-1 for water-soluble and 3.25 m2 g-1 for water-insoluble organic carbon. The derived correction factor further revealed that the attribution approach could underestimate BrC light absorption by up to 9%, highlighting the potential for significant bias in investigations of aerosol optical properties.