引擎廢氣中氣膠再凝結核化機制之模式模擬

Abstract

Chen (1999) 提出「再凝結核化機制 (Recondensation-induced nucleation mechanism)」假說，用來解釋都市地區的氣膠在數量濃度和質量濃度的日夜變化上呈現出趨勢不一致、時間不同步的現象，此現象無法單純的透過由光化學反應或內燃引擎燃燒所導致的核化過程來闡明。再凝結核化機制的主要概念是，內燃引擎中發生的燃燒過程本身不僅是形成新氣膠的來源，還會將原本空氣中既有的氣膠轉換為粒徑較小、數量較多的狀態並重新回到環境中。整體氣膠質量在這個過程中維持守恆，數量則大幅增加。首先，本研究藉由建立簡單的理想模型，只考慮再凝結核化機制的作用，以初步了解其對整體氣膠性質的影響和重要性。隨後，本研究將再凝結核化機制以詳盡參數法的形式植入Community Multi-Scale Air Quality (CMAQ) 模式的氣膠模組中，以模式模擬的方法分析此機制會如何改變氣膠的各種物理性質，以及評估其對區域空氣品質的可能衝擊。為了滿足模式模擬的需求，本研究亦發展出一套完整的系統性方法，可以對多組不同的排放資料進行客觀分析，最後產生CMAQ模式所需要的排放場。 理想模型的結果指出，數量濃度和表面積濃度這兩項氣膠物理性質對再凝結核化機制的作用非常敏感，但是體積濃度則因為質量守恆而沒有反應。在CMAQ模式模擬方面，結果顯示位於再凝結核化機制活躍的地區，例如都市和大型公路沿線等，氣膠的數量濃度增加可達400%、表面積濃度增加可達20%、至於體積濃度則增加大約2%。體積濃度的增加雖然微弱但是明確，此發現似乎與再凝結核化機制遵守質量守恆的結果相抵觸，但其實是因為氣膠動力過程的複雜回饋所造成。另外，這些由再凝結核化機制所引發的氣膠物理性質改變可能會產生健康風險、能見度下降等負面影響，如何對內燃引擎制定有效的氣膠排放規範將是未來重要的環境議題。

This study intends to verify the hypothesized recondensation-induced nucleation (RIN) mechanism, which was proposed by Chen (1999) to explain several observed inconsistencies between the diurnal variations in the aerosol number and mass concentrations over urban areas. Essentially, the core concept of the RIN mechanism is that the combustion process of an internal combustion engine not only acts as a source of aerosol particles but also converts pre-existing aerosol particles into smaller and more numerous ones while obeying total aerosol mass conservation. This study investigates the RIN mechanism through constructing a simple box model as well as implementing a comprehensive parameterization scheme into the aerosol module of the Community Multi-Scale Air Quality (CMAQ) model for simulations. In the process, a systematic method is developed as a byproduct to prepare the model-ready emission fields from several data sources. The box model results reveal that the aerosol number and surface area are the key aerosol physical properties particularly sensitive to the effects of the RIN mechanism but the aerosol volume is not. To analyze further into the details, a summer case on 2015 July 15th over Taiwan is selected for CMAQ model simulations. The results show that at places where the RIN mechanism is the most active such as major cities and highways, the overall aerosol number concentration can increase up to 400% while the overall aerosol surface area concentration can increase up to 20%. The overall aerosol mass concentration shows weak but clear increases up to 2%, a result seemingly contradictory to the mass conservation principle but in fact due to the feedback from complex aerosol dynamics. The model simulations demonstrate that the changes induced by the RIN mechanism in the aerosol physical properties may lead to potential health risks and visibility degradation. Future particle emission standards for internal combustion engines may need to be re-evaluated.