| 摘要: | 本文於2020年春季(4月22日至5月6日)及秋季(9月17日至9月25日)在中山醫學大學測站，利用半自動儀器觀測短時間(20分鐘)的PM2.5水溶性無機離子，結合鄰近環保署測站監測資料解析數據；此外，為了評估NH3的影響，於秋季採樣期間增加觀測NH3的短時間變化。
在春季前期(4月22日至4月27日)出現海風影響事件，Na+在4/27日以前總共出現了三次不同程度的高值，伴隨持續時間較長的西風，Cl-/Na+在海風期間平均為1.07，氯損失平均為19.14%，ISORROPIA II模式模擬結果，Na2SO4在海風影響期間有明顯的峰值。春季中期(4/27至5/2)，高濃度事件的 NO2、NO及CO等前驅氣體濃度分別上升，推測為交通排放源，且高濃度為經由NO2二次反應形成的NH4NO3主導。春季後期(5月2日至5月6日)，整體低濃度與NO2的二次反應減弱有關。另外從網站氣流場模擬(http://earth.nullschool.net)發現，春季中期開始(4/27)至春季後期(5/6)整個氣流的改變促成了污染物擴散。
秋季(2020年 9月17日至9月25日)的數據發現，在NH4+/SO42->1.5，隨著NH3(g)的濃度上升，NH4+的氣相與顆粒相分配比例ε(NH4+)有明顯的曲線關係，最後到達0.2較穩定的數值；ε(NH4+)分配比率降低推測與環境中的HNO3濃度缺乏有關，0.2為反應達到平衡點。另外，NH3的來源推測是來自植物露水的蒸發再加上鄰近工地的重機具。
秋季前期(2020年 9月17日至9月21日)第一次高濃度事件(ES1)是來自交通活動排放大量的前驅氣體，NO3-由光化反應生成。第二次高濃度事件， SO42-有一段峰值出現，NO3-則較為平緩，與ES1期間恰好相反，由於光化活動並沒有ES1強烈，即使在NH3充足濃度下，PM2.5濃度沒有高過ES1期間。在夜間低溫高濕且風速低的環境條件下，NO3-的峰值是由N2O5水解的異質反應為主導。
秋季後期(9月21日至9月25日)第一次高濃度事件期間，NH3不足且出現明顯的北風，濃度上升的主因應為來自外地傳輸。第二次高濃度事件風速低，顯示受外地傳輸的影響有限，PM2.5濃度的上升是由NO3-主導，但SO42-的高濃度與高占比提供了一個穩定的PM2.5濃度基本數值。從NO2-的數據發現，在相對濕度高於69%的條件下，NO2的液相反應對NO2-生成有重要影響，且大氣中供給鹼度的能力越強，越有利於NO2-的生成，但在相對濕度低於55%的情況下，NO2-主要形成機制應是由NO3-光解形成。
總結來說，本文發現台中都會地區受地形影響，吹南風有利於本地污染擴散，東北季風則利於外地污染傳輸，交通污染源對於PM2.5濃度的上升佔了主導地位。除了污染源和氣象因子外，春季高濃度NO3-產生化學反應機制由NO2二次反應形成的NH4NO3主導，秋季高濃度NO3-則是N2O5水解的異質反應，NH3濃度對污染物的生成有關鍵作用。
;This study used a semi-automatic instrument to observe short-interval (20 minutes) PM2.5 water-soluble inorganic ions at the Chung Shan Medical University station and resolved the results by combining with monitoring data of the neighboring monitoring station of Taiwan Environmental Protection Administration in spring (4/22 to 5/6) and autumn (9/17 to 9/25) of 2020. In addition, short-term variations of NH3 were observed to evaluate the impact of NH3 in autumn.
In the early spring (4/22 to 4/27), sea-breeze events occurred, three high Na+ values with varying degrees accompanied by long-lasting westerly winds before 4/27. The Cl-/Na+ was averaged at 1.07 during the sea-breeze, and the average chlorine loss was 19.14%. Simulations from the ISORROPIA II model showed a pronounced Na2SO4 peak during the sea-breeze event. In the middle spring (4/27 to 5/2), a high-concentration event was observed with increased precursor gas concentrations such as NO2, NO, and CO, which was attributed to traffic emissions dominated by NH4NO3 from secondary reactions of NO2. In the late spring (5/2 to 5/6), the overall low concentration was related to the weakening of secondary reactions of NO2. In addition, the change of the entire airflow from the middle spring (4/27) to the late spring (5/6) enhanced the diffusion of pollutants based on flow field simulation from the website (http://earth.nullschool.net).
The data in autumn (9/17 to 9/25) showed that the concentration ratio of NH4+ gas to particle partition, ε(NH4+), decreased with an increase of NH3 in a curvelike relationship and finally reached at a relatively stable value of 0.2 for NH4+/SO42->1.5. The decrease of ε(NH4+) partition ratio is presumed to be related to the lack of HNO3 concentration in the environment, and 0.2 is the equilibrium point of the reaction. In addition, the sources of NH3 are conjectured to contribute from the evaporation of plant dew plus heavy machinery at the nearby construction site.
In the early autumn (2020 9/17 to 9/21), the first high-concentration event (ES1) was attributed to the emissions of a large amount of precursor gases from traffic activities and photochemical reactions in producing NO3-. During the second high-concentration event, SO42- was peaked in contrast to relatively flat NO3-, which was opposite to ES1. Since the photochemical activities were not as strong as ES1, the PM2.5 concentration was higher than the ES1 period, even under sufficient NH3 concentration. At night, NO3- peaks were presumed to be caused by the heterogeneous reactions of N2O5 hydrolysis when the environment was under low temperature, high humidity, and low wind speed.
During the first high-concentration event in late autumn (9/21 to 9/25), NH3 was insufficient, and with noticeable northerly winds, the rising concentration was from external transport. During the second high-concentration event, the wind speed was low to limit outside transport. NO3- dominated the increase of PM2.5 concentration, but the high concentration and high proportion of SO42- provided a stable PM2.5 base concentration. From the measurements of NO2-, the liquid phase reactions of NO2 played an essential role in NO2- formation when the relative humidity was higher than 69%. And the more potent the ability of alkalinity supply in the atmosphere, the more the formation of NO2-. However, the primary formation mechanism of NO2- should be the photolysis of NO3- when the relative humidity is lower than 55%.
In summary, this study found that the topography of the Taichung metropolis tended to accumulate local pollution for the southerly wind and conducted an external pollution transport under the northeast monsoon. Traffic pollution sources dominated the increase of PM2.5 concentrations. In addition to pollution sources and meteorological factors, NH4NO3 formed from secondary reactions of NO2 dominated the chemical reaction mechanism of high-concentration NO3- in spring. In contrast, the high NO3- concentrations in autumn were attributed to the heterogeneous reactions of N2O5 hydrolysis. The concentration of NH3 plays a crucial role in the formation of pollutants. |
