含碳物質對成雲之衝擊：焚雲效應與覆膜效應

本研究結合理論發展，並利用一雲氣塊模式來探討當雲凝結核內包含煤灰粒子和可形成薄膜的有機化合物對雲滴活化過程的影響。煤灰粒子的加熱會提高粒子的臨界飽和比，具有延遲氣懸膠活化成雲滴的特性，這個個體效應在粒子大且含有較高的煤灰質量時具有較顯著的作用。再者，氣懸粒子的整體加熱具有兩個相反的效應，其一為粒子加熱效應會提高粒子的柯勒曲線，減緩所有粒子的凝結過程，導致氣塊飽和比升高，使得雲滴數量濃度增加；其二為加熱影響氣相環境溫度，使得環境飽和蒸氣壓升高而氣塊飽和比降低，進而造成雲滴數量濃度減少。在本研究中，考慮四種加熱情況的模擬，分別為(1)煤灰粒子不加熱，僅以不可溶解的物質存在於雲凝結核中；(2)煤灰加熱氣懸粒子，但假設僅影響粒子溫度，熱量不傳至空氣中；(3)煤灰加熱僅作用於氣相環境，影響氣相溫度，但氣懸粒子溫度不受影響；(4)煤灰粒子加熱同時影響粒子與氣相溫度。結果顯示雲滴數量濃度變化是正是負，取決於煤灰加熱粒子和環境兩種相反作用的競爭效應，其相對大小受控於氣懸粒子內煤灰質量多寡以及煤灰粒子與雲凝結核的混合狀態。可形成薄膜的有機化合物(Film Forming Compound, FFC)覆蓋在粒子表面會導致水的凝結係數下降，阻礙粒子的質量成長。過去對這方面的研究，凝結係數和覆蓋率的關係多以Heaviside function描述，即FFC覆蓋率必須超過某個臨界值，凝結係數才會遽降為一定值。然而根據以往的實驗數據，此關係應為連續。本研究發展一個新的方法來表現此連續關係，理論分析相當吻合實驗結果。FFC具有遲緩粒子質量成長的效應，會導致雲內飽和比提高，使得較多的粒子有機會被活化成雲滴，進而增加雲滴數量濃度。然而，FFC也會巨幅加長成長特性時間，使得某些粒子來不及被活化，造成雲滴數量濃度減少。因此，FFC對雲滴活化過程的影響可能不如前人所認為是單向的。模擬結果顯示此效應的強弱與FFC總量、FFC分配、凝結係數被改變的程度、初始氣膠粒徑譜以及上升速度等有關。

關鍵字

活化；含碳物質

並列摘要

This study applies theoretical analyses and numerical simulations to show how the cloud drop activation process is influenced by the presence of Black Carbon (BC) and Film-Forming Compounds (FFC). BC heating has the effect of raising critical saturation ratio of the droplet and may delay the activation of the aerosol into a cloud drop. The effect is stronger in larger Cloud Condensation Nuclei (CCN) that contain higher amount of BC. Total heating effect that contributes by the sum of whole aerosol population has two opposite effects. One is that BC heating will raise the köhler curves of droplets and then retard their growth. This will cause the parcel supersaturation to raise thus results in an increase of cloud drop concentration. However, BC heating on air parcel would cause a decrease in parcel supersaturation, thus causing the cloud drop number concentration to decrease. We investigate these mechanisms by considering four heating scenarios : (1) The BC has no heating effect, just sever as an insoluble core within CCN; (2) The BC heats droplets and raises the temperature of droplets but not the air; (3) The BC heats the air only, not the droplets; (4) The BC heating exerts on both droplets and air parcel. Simulation results show that BC heating effect can positively or negatively influence the cloud drop number concentration, depending on the relative strength of the two opposite heating effects, which is determined by the mixing state of the BC in CCN as well as the total BC mass. The FFC effect causes a significant decrease in water accommodation coefficient (

並列關鍵字

carbonaceous compounds ； activation

參考文獻

Ackerman, A., and O. Toon, 1996: Unrealistic dessication of marine stratocumulus clouds by enhanced solar absorption. Nature, 380, 512– 515.

Archer, R. J., and V. K. La Mer, 1955: The rate of evaporation of water through fatty acid. J. Phys. Chem., 59, 200-208.

Barnes, G. T., and V. K. La Mer, 1962: The evaporation resistances of monolayers of long-chain acids and alcohols and their mixtures. Retardation of Evaporation by Monolayers, V. K. La Mer, Ed., Academic Press, 9-33.

Barnes, G. T., 1986: The effects of monolayers on the evaporation of liquids, Adv. Colloid Interface Sci., 25, 89-200.

Bigg, E. K., 1986: Discrepancy between observation and prediction of concentrations of cloud condensation nuclei, Atmos. Res., 20, 82-86.

國際替代計量

含碳物質對成雲之衝擊：焚雲效應與覆膜效應

全文下載

主題瀏覽