物理和化學作用對NOx日變化影響之理想模擬研究

Abstract

本研究旨在利用海洋、陸地和山脈分布的理想模擬，研究局部排放的日變化所受到的物理和化學作用。在台灣冬季弱綜觀的天氣型態下，由於局部環流和複雜地形之間的相互作用，很難評估局部排放如何影響空氣品質，因此本研究使用與化學模組耦合的三維渦度向量方程雲解析模式 (VVM) 進行理想化模擬，以釐清物理和化學過程對局部排放的影響。化學方面僅使用兩個反應將複雜的化學交互作用簡化為保守的化學系統，實驗中主要針對兩種排放源，分別是廣泛分佈在城市平原地區的交通排放，以及在狹長沿海工業區的高濃度工業污染排放，排放量則根據台灣的歷史空氣汙染排放數據 (臺灣空氣污染物排放量清冊, TEDS) 。在平原地區，交通排放在清晨累積在地表附近，但在邊界層形成後，污染物濃度受到垂直混合被稀釋。另一方面，工業排放的汙染傳送主要為海風控制，在下午隨著海風前緣進入內陸地區，影響內陸地區的空氣品質。在山區，兩種排放源的汙染物皆由海風傳送主導，於傍晚從平原地區被海風傳送到山區。因此，我們可以在平原地區觀察到兩個汙染濃度峰值，而在山區只有一個濃度峰值。再者可以利用停留時間的機率分布來分析不同汙染源在時間上的表現，由此可知工業排放主要停留在五百公尺以下的平原地區，而交通排放則在邊界層頂累積並維持較長的時間。此外，白天氮氧化物之間的轉換以光解反應為主，可以觀察到臭氧的增加，而排放的一氧化氮和二氧化氮比率在影響化學平衡中也扮演重要的角色。在夜間，一氧化氮和臭氧反應生成二氧化氮，二氧化氮是二次氣溶膠的前驅物，經歷夜間反應後可能會導致山區污染。由於不利汙染散布的氣象條件和過多的氮氧化物排放，平原地區的臭氧在第二天晚上耗盡。此研究得知釐清物理過程以了解污染物濃度的局部變化的重要性，持續的汙染排放和化學作用則造成一氧化氮和二氧化氮的比率在兩天之間發生劇烈變化。

This study aims to investigate the diurnal evolution of local emissions based on the physical and chemical views in idealized simulations with ocean, land, and mountain distribution. During the wintertime weak synoptic condition in Taiwan, it is difficult to evaluate how local emissions influence the air quality due to interactions between the local circulation and complex topography. Idealized simulations using the Vector Vorticity equation cloud-resolving Model (VVM) coupled to a chemistry parameterization are performed to study the impacts of physical and chemical processes on local emissions. Two nitrogen reactions are applied to simplify the chemical interactions to a conservative chemical system, and the local emissions are based on historical air pollution data in Taiwan (Taiwan Emission Data System, TEDS). The traffic emissions are broadly distributed over urban plain areas, while high-concentration industrial pollutants are emitted in very limited coastal areas. Over the plain areas, traffic emissions accumulated in the early morning, but the concentration of the pollutants was diluted through vertical mixing after the boundary layer developed. On the other hand, the transport of industrial emissions is controlled by the sea breeze, and highly polluted air is transported with the sea breeze front invading inland regions in the afternoon. In the mountain areas, both emissions are transported by the sea breeze from the plain areas in the evening. Thus, we can observe two concentration peaks over the plain areas, while there is only one concentration peak over the mountain areas. Another aspect of physical effects could be analyzed using the probability distribution of the residence time, which shows that the industrial emissions mostly stay over the plain areas under 500m, while the traffic emissions accumulated at the boundary layer top and persist for an extended period. Besides, the transformation between NOx in the daytime is dominated by photodissociation; an increase in O3 can be observed in the daytime, while the emitted NO/NO2 ratio also plays an important role in affecting chemical equilibrium. During the nighttime, NO and O3 react to produce NO2, which is the precursor of secondary aerosols that would lead to a polluted condition in the mountain areas. Due to unfavorable meteorological conditions and excessive NOx emissions, O3 runs out on day 2 evening over the plain regions. The results highlight the importance of resolving the physical processes to understand the local evolution of pollutant concentration. Excessive emissions and the effect of chemical reactions lead to a drastic change in the NO/NO2 ratio between day 1 and day 2.