| 摘要: | 大氣氣膠對氣候變遷和全球暖化具有重要影響，水溶性無機離子(Water-Soluble Inorganic Ions, WSIIs)是氣膠重要化學成分，本文於2023年11月至2024年1月臺中市區及2024年2月至3月鹿林山區進行PM2.5 WSIIs實時濃度監測，以探討PM2.5質量濃度增高期間，WSIIs濃度變化趨勢及其來源，並分析pH值與氣膠含水量(Aerosol Water Content, AWC)在二次無機離子生成過程中的影響。
臺中市區採樣期間發生三次PM2.5高濃度事件，第一、二次高濃度事件NO3-濃度的增高凸顯，主要受到移動污染源排放和低風速與高相對濕度影響；第三次高濃度事件SO42-濃度明顯增高，推論主要受到中國長程污染傳輸及跨年活動影響。鹿林山區主要受到生質燃燒(Biomass Burning, BB)煙團及人為排放源影響，WSIIs以SO42-為主，顯示兩地高濃度事件WSIIs的成分與來源存在顯著差異。以熱力學平衡模式模擬臺中市區採樣期間氣膠呈現酸性，pH值平均為3.3 ± 0.7，AWC平均為5.1 ± 4.4 μg m-3，夜間在高相對濕度下，有利於液相反應生成二次氣膠，pH值與AWC高於日間。本文分析WSIIs的可能來源與反應途徑，臺中市區日間CO和NOx的提高可推演出移動污染源的貢獻，透過光化學反應將NO2轉換為氣相HNO3，然後在NH3充足的環境下生成NH4NO3。夜間高相對濕度和低風速，促使白天生成的N2O5通過液相反應形成NO3-。SO42-濃度的變化與區域性傳輸影響及SO2光化學反應生成有關。NO2-的生成主要經由夜間高相對濕度的液相反應，且與夜間NO3-濃度變化趨勢相似。相較之下，鹿林山區春季WSIIs的主要來源為BB煙團長程傳輸，採樣期間SO42-、NO3-、NH4+和K+濃度顯著增加，日間可以觀察到PM2.5和CO濃度上升，O3濃度下降的山谷風特徵，但受BB事件掩蓋，山谷風貢獻不明顯。臺中市區與鹿林山區的主要WSIIs結合型態存在差異，臺中市區NH3充足，WSIIs主要以(NH4)2SO4和NH4NO3的形式存在；鹿林山區NH3則較為不足，可能形成NH4SO4、(NH4)3H(SO4)2、NH4NO3及NO3- (沒有與NH4+結合的NO3-)氣膠。
總結來說，臺中市區高濃度事件主要受到本地污染物累積影響，污染來源以移動污染源為主，其次是受到中國境外污染長程傳輸影響；鹿林山區則以BB煙團及人為排放源長程傳輸影響為主。本文解析臺中市區和鹿林山區WSIIs高濃度主要來源和生成途徑，提供管制WSIIs高濃度發生的科學依據。
;Atmospheric aerosols have significant impacts on climate change and global warming. Water-soluble inorganic ions (WSIIs) are major chemical components of aerosols. This study conducted real-time monitoring of PM2.5 WSIIs concentrations in urban Taichung from November 2023 to January 2024 and at the Lulin background site from February to March 2024. The aim was to investigate the variation trends and sources of WSIIs during periods of elevated PM2.5 mass concentrations, and to analyze the influence of pH and aerosol water content (AWC) on the formation of secondary inorganic aerosols.
During the sampling period in Taichung, three high PM2.5 events occurred. The first and second events were characterized by a significant increase in NO3- concentrations, mainly influenced by mobile emission sources, low wind speed, and high relative humidity. The third event showed a clear increase in SO42- concentrations, which was attributed to long-range transport from China and local New Year festivities. In the Lulin background site, WSIIs were mainly influenced by biomass burning (BB) plumes and anthropogenic emissions, with SO42- being the dominant component, indicating significant differences in WSIIs composition and sources between the two sites. Thermodynamic equilibrium model simulations suggested that aerosols in Taichung were acidic during the sampling period, with an average pH of 3.3 ± 0.7 and AWC of 5.1 ± 4.4 μg m-3. Higher pH and AWC values were observed at night under high relative humidity, favoring aqueous-phase reactions and the formation of secondary aerosols. This study analyzed the possible sources and reaction pathways of WSIIs. During daytime in Taichung, increased concentrations of CO and NOₓ suggested contributions from mobile sources. Photochemical reactions converted NO2 to gaseous HNO3, which then reacted with NH3 to form NH4NO3 under ammonia-rich conditions. At night, high relative humidity and low wind speeds promoted aqueous-phase reactions of N2O5 formed during the day, leading to NO3-formation. Variations in SO42- concentrations were associated with regional transport and SO2 photochemical oxidation. NO2- formation was mainly attributed to aqueous-phase reactions at night under high humidity, showing a trend similar to nighttime NO3-. In contrast, springtime WSIIs at the Lulin site were primarily influenced by long-range transport of BB plumes. Significant increases in SO42-, NO3-, NH4+, and K+ were observed during the sampling period. Diurnal variations in PM2.5 and CO concentrations, along with decreased O3 levels, suggested mountain-valley wind characteristics, though the contribution of valley winds was not significant due to BB events. The major WSIIs compound forms differed between Taichung and Lulin. In Taichung, sufficient NH₃ led to the formation of (NH4)2SO4 and NH4NO3. In contrast, Lulin had limited NH₃, potentially resulting in the formation of NH4SO4, (NH4)3H(SO4)2, NH4NO3, and unneutralized NO3- aerosols.
In summary, high-concentration events in Taichung were primarily driven by local pollutant accumulation, mainly from mobile sources, with additional influence from long-range transport of pollutants from China. In Lulin, high WSIIs concentrations were mainly influenced by long-range transport of BB plumes and anthropogenic emissions. This study analyzed the major sources and formation pathways of high WSIIs concentrations in both urban and background areas, providing scientific evidence for the control of WSIIs pollution events. |
