台中地區都市及郊區 PM2.5硝酸之同位素特徵及來源解析

Abstract

硝酸鹽是二次氣膠的重要成分，在污染事件中常見硝酸鹽在 PM₂.₅中的貢獻增加，因此了解氣膠硝酸鹽的來源對改善空氣品質至關重要。硝酸鹽的同位素組成可用於識別硝酸鹽二次氣膠的前驅物 NOx的氧化途徑及其來源，有助於理解硝酸鹽的形成及各來源貢獻。本研究於 2020 年 8 月和 10 月在台中市都市測站(ZM)和郊區測站(TH）收集 PM₂.₅樣本，且在 TH 站收集日夜之樣本，以研究都市及郊區 NOx污染來源及氣膠硝酸鹽生成的日夜差異。透過分析 PM₂.₅硝酸成分的同位素組成，利用 δ¹⁸O-NO₃-值評估 NOx氧化途徑對硝酸鹽生成的影響，並利用δ¹⁵N-NO₃-值配合混合同位素模型 MixSIAR，在考慮同位素分餾效應調整後進行模擬 NOx來源相對貢獻之模擬。 氧化途徑模擬結果顯示，在台中地區兩測站夏秋兩季皆是 OH 途徑主導了NOx的氧化，秋季的 N₂O₅途徑相對貢獻在 TH 站增加約 4％，在 ZM 站增加約3%。位處亞熱帶地區的台中市雖然夏秋以 OH 氧化為主要途徑，但 N₂O₅水解途徑貢獻的變化對於二次硝酸鹽的生成有顯著影響。TH 站夏季的氧化途徑並無明顯日夜差異，但秋季觀察到 OH 途徑的日低夜高現象與大多數研究的趨勢不同。可能由於地形效應的影響，位於台中盆地內的地區容易累積污染物，白天在上方殘留層（RL）形成的硝酸鹽，隨後於夜間邊界層(PBL)高度下降後降落至地表附近，進而影響夜間至清晨的空氣品質。此外，有新的研究顯示夜間的高相對濕度環境可能導致微小液滴的表面自發生成 OH 自由基，因而在夜間增加了 OH 途徑的貢獻。此外，配合穩定同位素混合模型進行 NOx污染源的相對貢獻模擬，模擬結果顯示在 TH 站，NOx來源的相對貢獻為燃煤>生質燃燒>車輛排放>微生物燃燒。在 ZM 站，相對貢獻為生質燃燒>燃煤>交通排放>微生物燃燒。

Nitrate is an important component of secondary aerosols, and its contribution to PM₂.₅ increases during pollution events. Therefore, understanding the sources of aerosol nitrates is crucial for improving air quality. The isotopic composition of nitrates can be used to identify the oxidation pathways of precursor NOx and help to understand nitrate formation. Additionally, δ¹⁵N-NO₃-and stable isotopic mixing models can provide insights into the contribution of pollutant NOx. In this study, the aerosol samples were collected in August and October 2020 in two sites, the urban site (TH) and the suburban site (ZM), in Taichung City. Besides, daytime and nighttime samples were collected in the TH site to study the effect of nitrogen oxidation pathways by analyzing the isotopic composition of NO₃-. δ¹⁸O-NO₃- values were applied to evaluate the relative importance of aerosol nitrate gas-to-particle formation pathways. Furthermore, δ¹⁵N-NO₃-values were used to access the relative contribution of sources adjusted by the isotopic fractionation effect. δ15N-NO3- were simulated utilizing the isotope mixing model, MixSIAR, an integrated framework of various Bayesian isotope mixing models. The simulation result indicated that the OH pathway led the atmospheric nitrate oxidation process in Taichung in both seasons. However, the variations in the contributions of the N₂O₅ hydrolysis pathway played a significant role in the formation of secondary nitrates and PM₂.₅. At the TH site, no obvious difference between daytime and nighttime in summer was observed. However, N₂O₅ pathway relative contribution increased by 4% in autumn compared with summertime. This could be attributed to the unstable weather conditions during the autumn season. Additionally, the Taichung Basin is prone to the accumulation of pollutants due to the influence of terrain effects. During the daytime, nitrate formed in the residual layer (RL) easily descends to the vicinity of the ground surface after the decrease of the planetary boundary layer (PBL) height during nighttime, affecting the air quality from nighttime to early morning. Furthermore, the high humidity environment during nighttime may contribute to the spontaneous generation of OH radicals on the surface of submicron liquid droplets, leading to the contribution of the OH pathway during nighttime. The MixSIAR simulation result shows the relative contribution of various NOx sources. At the TH site, the relative contributions of NOx sources were coal combustion > biomass burning > vehicle emission > biogenic soil emission. At the ZM site, the contributions were biomass burning > coal combustion > traffic emissions > microbial combustion.