既存粒子對光化反應中新粒子生成影響之探討

懸浮微粒於大氣中扮演重要角色，除了改變大氣輻射收支，對於雲的生成及降雨亦有影響，同時也會造成人類健康上的危害。懸浮微粒的來源很廣，分為人為來源及自然來源，前者如工業排放，後者如海水飛沫、地表揚塵等。近期實驗研究及觀測發現，光化反應為粒子生成的重要過程；而粒子的形成與粒徑變化主要經由核化、凝結、碰撞合併等過程所影響，於大氣環境中存在了許多懸浮微粒，粒子間的碰撞同時也影響了粒徑分布，氣態物質在凝結過程亦會凝結至既有的粒子上，當粒子在形成的過程中有既存物質參與反應時，所得到的粒徑分布亦有所不同。本研究目標在探討，當環境中有粒子存在時，對於光化反應生成粒子之影響，利用實驗以及模擬計算，推測可能的反應機制，以及既有粒子所扮演之角色。實驗選用黑碳及硫酸銨兩種粒子，分別做為不可溶與可溶粒子來進行探討，並藉由SMPS系統量測所產生之粒徑分布；由結果可得知，既存粒子會影響新粒子的生成，當環境中有物質存在時，會使得新生粒子數量減少且粒徑較小，但環境中粒子的數量及狀態之差異亦會有影響，藉由改變粒子之條件進行探討，並與模擬計算結果進行比較得知，新生粒子應是藉由氣態物質經核化而生成，推測既存粒子可能影響有二，一為凝結消耗作用，另一為碰撞消耗作用，由於凝結需要液態水，當粒子潮解或帶水時較為顯著，因此硫酸銨粒子的影響較黑碳大；而碰撞效應於本實驗設計條件中，由於時間短所以效果不明顯，根據模擬計算結果，粒子數量多且新生粒子粒徑較小時，碰撞蒐集效率較高；此外發現大粒子的數量增加，應是經由較小粒徑硫酸銨粒子，與新生粒子碰撞，或經氣態物質凝結成長所得。藉由改變環境相對濕度進行實驗，得知新生粒子之數量及大小與濕度之相關性高，但無法只藉由過氧化氫與水核化產生，應有其他機制參與其中。

關鍵字

懸浮微粒；光化反應；碰撞合併；凝結成長

並列摘要

Aerosol particles play an important role in precipitation by acting as ice and cloud condensation nuclei, which also affect atmospheric radiation, and make impact on the ecosystem. There are many sources of aerosol formation. It can either be produced by human activities, or formed through nature processes. The particle size distribution depends on the nucleation, condensation and coagulation processes. With the presence of existing particles, the nucleation process might be alternated due to the condensation of gas phase chemical species which can contribute to the nucleation process. Besides, the collision between the existing particles with the nucleated particles can also affect the overall aerosol size distribution. In this study, the new particle formation via moisture air exposed to ultraviolet radiation is quantified and the impact of the existing particles on such particle nucleation process is investigated. Two types of existing particles are studies; ammonium sulfate and commercial black carbon representing as soluble and insoluble particles, respectively. The observed particles generated from the moisture air exposed to ultraviolet radiation are mainly in the nuclei mode with mode size of 7 nm and the total number concentration is dependent on the relative humidity of the air flow. The total number concentration of nucleated particles is more significantly decreased with the presence of ammonium sulfate particles than that of black carbon particles. With further model study, the existing particles can affect the new particles formation mainly by the condensation process than the coagulation process (less than 1%) for this study. As to the mechanism for the new particle formation under ultraviolet radiation, the simulated concentration of hydrogen peroxide for the given experimental condition is too low to formed particles by binary nucleation with water, which suggests there are other mechanisms or species such as other reactive trace gases involved in the process.

並列關鍵字

aerosol ； photolysis reaction ； coagulation ； condensation growth

參考文獻

蔡宜君, 2009. 氣膠總體參數法之研發與應用, 臺灣大學.

Aloisio, S., J. S. Francisco, and R. R. Friedl (2000), Experimental evidence for the existence of the HO2-H2O complex, J Phys Chem A, 104(28), 6597-6601.

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既存粒子對光化反應中新粒子生成影響之探討

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