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Experimental and Numerical Study on the Mechanism of Nucleation by Re-condensation
|Publication Year :||2009|
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。
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.
|Appears in Collections:||大氣科學系|
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