臺中都會區PM2.5化學組成、形成機制及其氧化潛勢之探討

Abstract

近年來空氣污染及其對環境和民眾健康之影響日益受矚，尤其是都會區之細懸浮微粒(PM2.5)及其二次氣膠。然而，由於缺乏此領域之系統性研究，目前對這些問題及其與大氣環境複雜之關係的瞭解仍然有限，尤其是在臺灣。本研究於臺中都會區進行為期一年之採樣，藉以研究PM2.5及其前驅物之季節性和日夜變化，並著重於探討其化學組成、形成機制、氧化潛勢(OP)及潛在來源，同時搭配多項統計模型與軟體如ISORROPIA-Ⅱ、主成分分析、雙變量條件機率函數以及結合k-平均演算法之逆軌跡模式，綜合性地闡釋二次污染物的形成機制，解析OP與化學成份之間的關係，並探討各物種之潛在來源。 本研究發現冬季PM2.5質量濃度顯著較高(27.7 ± 9.7 μg/m3)，其次為秋季(22.5 ± 8.3 μg/m3)、春季(19.2 ± 6.4 μg/m3)和夏季(11.0 ± 3.1 μg/m3)。二次無機氣膠主要之形成機制為日間勻相形成的硫酸鹽以及夜間非勻相形成的硝酸鹽，此外，本研究亦凸顯低溫情形下氮氧化物排放管制之重要性；而氣膠酸鹼度在二次無機氣膠(尤其是硝酸鹽)形成及PM2.5濃度扮演關鍵角色，硫酸鹽則會對氣膠pH值造成顯著影響。二次有機氣膠形成則可能由本地工業和交通排放的甲苯和二甲苯異構物經光化反應後產生，而與二次有機氣膠相比，來自交通和燃燒源的一次有機氣膠與PM2.5之相關性較高。在主成分分析中，OPv（體積量化之OP）與二次無機氣膠的形成以及當地的燃燒、工業和交通相關排放（包括氣體、含碳物質和金屬）指標物有關；OPm（質量量化之OP）亦與含碳物質、金屬有關，但不包含二次無機氣膠，因此瞭解二次無機氣膠的形成有助於降低OPv與PM2.5濃度，而含碳物質及金屬的減量則能更有效降低氣膠毒性。最後，春季、秋季和冬季經常發生長程傳輸的現象，並促進了二次有機氣膠之形成，然而長程傳輸並非事件日的成因，其可能扮演著使污染物擴散的角色；反之，短程的區域傳輸則可解讀為當地污染物之累積。本研究能為相關決策單位提供有效減量策略以減緩空氣污染並促進公眾健康。

In recent years, there has been a growing concern about air pollution and its impact on the environment and public health, especially for fine particulate matter (PM2.5) and its associated secondary aerosols in urban areas. However, there is still a limited understanding of these issues and their intricate relationship with the atmospheric environment due to the lack of systematic studies, especially in Taiwan. This study was conducted in Taichung urban area and involved year-long seasonal and diurnal sampling of PM2.5 and its gaseous precursors, focusing on the chemical composition, formation mechanism, oxidative potential (OP), and potential sources. The primary aim of the study was to identify the factors influencing PM2.5 concentrations and reduce the aerosol toxicity. Several statistical models and software were utilized to provide better insights, including ISORROPIA-Ⅱ, principal component analysis (PCA), conditional bivariate probability function (CBPF), and backward trajectories coupled with k-means clustering. This comprehensive investigation sheds light on the formation of secondary pollutants, explores the relationship between OP and chemical composition, and identifies potential sources for various species. This study observed significantly higher PM2.5 mass concentrations in winter (27.7 ± 9.7 μg/m3), followed by autumn (22.5 ± 8.3 μg/m3), spring (19.2 ± 6.4 μg/m3), and summer (11.0 ± 3.1 μg/m3). The dominant mechanism of SIA (secondary inorganic aerosols) was the nighttime heterogeneous formation of NO3- and the daytime homogeneous formation of SO42-. Additionally, the importance of controlling NOx emissions under low temperature conditions was highlighted. The aerosol acidity calculated from ion balance plays a crucial role in the formation of SIA (especially for NO3-) and PM2.5 concentrations, while SO42- may exert significant impact on aerosol pH. The formation of SOA (secondary organic aerosols) could result from toluene and xylene isomers emitted from local industrial and traffic sources through the photochemical reactions, while POA (primary organic aerosols) from traffic and combustion sources exhibited stronger correlations with PM2.5 compared to SOA. In the PCA model, OPv (volume-normalized OP) was associated with the formation of SIA as well as the indicators of combustion, industrial, and traffic-related emissions (including gases, carbonaceous matter, and metals) from the local region. OPm (mass-normalized OP) was also related to carbonaceous matter and metals, but excluding the SIA. Consequently, the understanding of SIA formation could be beneficial in reducing both OPv and PM2.5 concentrations, while the reduction of carbonaceous matter and metals is more helpful to lower the aerosol toxicity. Finally, long-range transport frequently occurred in spring, autumn, and winter and was found to enhance the formation of SOA. However, long-range transport mainly served as the dispersion of pollutants, while regional transport (air masses transported for short distances) accumulated local emissions. This work offers implications and effective strategies for policy-makers to mitigate air pollution and improve public health.