再凝結核化機制的實驗與數值模擬

Experimental and Numerical Study on the Mechanism of Nucleation by Re-condensation

郭力維(Li-Wei Kuo)

指導教授 ： 陳正平

國立臺灣大學/理學院/大氣科學研究所/碩士(2009年)

https://doi.org/10.6342/NTU.2009.02582

摘要

此研究根據Chen (1999)提出的「再凝結核化機制」假說，進一步探討引擎燃燒過程產生將空氣中既有氣膠粒子轉換為更多較小新粒子的過程。此過程中，氣懸粒子的數量濃度增加，但在質量濃度上並沒有增加。本研究嘗試同時以實驗與數值模式相互檢驗，測出引擎燃燒過程對氣懸粒子數量濃度和粒徑譜的影響，求得定量的關係。
實驗的部分延續並改進黃 (2005)利用機車引擎燃燒實測，分析進氣氣體之粒子濃度(Ni)與燃燒所產生之粒子濃度(No)間的關係。實驗結果顯示，隨著Ni增加，No亦增加，但部分實驗顯示當Ni達約2*10^4 #/cm^3時，No增加的趨勢減緩甚至下降，顯示其過程並非是簡單的蒸發、凝結核化而已，推測可能牽涉到粒子是否完全蒸發以及核化所需相對濕度與相對酸度之供給量問題，這部分進一步以數值模擬來探討相關機制。
數值模擬結果顯示，雖然Ni愈高，蒸發產生愈大量的酸氣及可凝結氣體，進而有愈高的核化速率，但當Ni太高時，反而會提早在比較高溫且相對濕度較低的情況下就開始核化，反而會降低了核化速率。此外，當Ni太多時，雖然會提供較多的酸氣，但殘留下未蒸發的氣懸粒子也會增多，而酸氣及水氣會吸附在殘留的粒子上，降低核化。殘留的粒子太多時，也會加快粒子間碰撞的速率而降低數量濃度，所以No隨著Ni的增加會有一個高峰值。目前由實驗及數值模擬的結果顯示，兩者間有著相似的趨勢。

關鍵字

氣懸粒子 ； 數量濃度

並列摘要

This study intend to verify the mechanism of “nucleation by re-condensation” proposed by Chen (1999), which hypothesize that engine combustion tend to convert existing ambient aerosol particles into more numerous and smaller particles without affecting the total aerosol mass. Both experimental and numerical studies are conducted in this research to prove this hypothesis.
The laboratory experiment follows the procedure of Huang(2005) but with improvements that allow more quantitative analysis of the processes. Huang(2005) proved the aerosol number concentration is increased after a motor engine combustion. The results show that the output number concentration (No) in the exhaust air generally increases with increasing input number concentration (Ni) in the intake air, except in a few occasions, when Ni is roughly greater than 2*104 #/cm3, No becomes less. This implies that the nucleation by re-condensation process involves not simply evaporation and nucleation but also the amount of residual particles and timing of nucleation, which depends on the supply of relative acidity and relative humidity Numerical simulation of aerosol evolution is designed to analyze the details of these mechanisms.
Simulation results show that, when Ni increases, there is more condensable acid vapor available, this generally leading to higher nucleation rates. However, as more acid vapor is available, the nucleation would start early at which time the air temperature is higher thus the relative humidity is lower. At some point the lower relative humidity produces a lower nucleation rate. Therefore, when the Ni is too high, the nucleation rate actually decreases. In addition, as Ni increases, the number of residual particles may also increase The residual particles would absorb the acid vapor thus decrease the nucleation rate, as well as collect the newly formed particles, both cause the total number concentration to decrease. So, the relation between Ni and the maximum No is not linear, and there exist an optimal Ni that produces the highest No. Some of the observation results also reveal such features.

並列關鍵字

aerosol ； number concentration

參考文獻

黃公度， 2005: 引擎排放對氣膠粒徑譜之影響， 台灣大學大氣科學系碩士論文

Burtscher, H., 1992: Tutorial/Review. Measurement and characteristics of combustion aerosols with special consideration of photoelectric charging and charging by flame ions, J. Aerosol Sci., 23, 549-595

Chen, J.-P., 1994: Theory of Deliquescence and Modified Köhler Curves, J. Atmos. Sci., 51, 3505-3516.

Chen, J.-P., 1999: Particle nucleation by recondensation in combustion exhausts, Geophys. Res. Lett., 26, 2403-2406.

Chen, J.-P. and Lamb, D., 1994: Simulation of cloud microphysical and chemical process using a multicomponent framework. Part I: Description of the microphysical model, J. Atoms. Sci., 51, 2613-2630.