台北郊區氣膠成雲凝結核之活化特性探討

The study of cloud condensation nuclei activity for suburban aerosol in Taipei

呂宛蓉(Wan-Jung Lu)

指導教授 ： 洪惠敏

國立臺灣大學/理學院/大氣科學研究所/碩士(2013年)

https://doi.org/10.6342/NTU.2013.00473

摘要

氣膠的物理化學特性，在大氣中扮演著十分重要的角色。它會影響到輻射的收支平衡以及雲凝結核(CCN)的活化特性。本研究在2011/8/12 - 22日利用雲凝結核計數器、掃描式電移動度粒徑分析儀(SMPS)及凝結核計數器，探討台北郊區氣膠與雲凝結核活化特性之間的關聯性。依照不同的氣態污染物之傳入濃度及氣流逆軌跡路線分析，可區分成兩個時期。第一個時期(Period 1)為8月12 - 19日，此期間人為排放氣態污染物濃度較高並具有較長生命期的特徵；第二個時期(Period 2)為8月20 - 22日，有較少的人為污染物。在氣膠粒徑分布的分析，Period 1 亦量測到較明顯的新粒子生成現象，可能由於人為污染物的參與加強了新粒子的生成。由氣膠粒徑分布、凝結核數量濃度(NCN) 及雲凝核數量濃度(NCCN)，能計算出成長至雲滴的最小粒徑(Dss)，並可從中推算吸濕參數( κ )。在 κ 值的分析結果指出台北郊區氣膠 κ ~ 0.18，其中包含Period 1之 κ ~ 0.23，Period 2的 κ ~ 0.14；相較於台北都市 κ ~ 0.05和台中都市 κ ~ 0.07，台北郊區氣膠含有較多的良好吸濕性物質分子比例，較容易成為雲凝結核。
總體而言，當氣膠或其前驅物在傳送過程經過光化學氧化作用，會改變其氣膠的物理化學特性，推測Period 1的氣膠可能是當地自然源的光化反應再加上人為排放污染物的傳入影響，而有較好的氣膠吸濕特性；在有高濃度的氣態污染物(傳送加上當地自然源產生) ，可能促使明顯的新粒子生成現象。而Period 2的氣膠有較低的活化特性，推測氣流路線由台灣東岸傳送至華林並加上當地的的氣態污染物經過較不足夠的光化學反應過程為可能的影響因素。因此，推測都市間(如台灣西部)所排放的污染物若經過足夠的時間進行光化學反應再傳送到郊區，則很有可能會增強新粒子的生成，以及提升氣膠的吸濕能力。

關鍵字

雲凝結核 ； 郊區 ； 氣膠 ； 吸濕特性

並列摘要

The physical chemical properties of aerosols play a significant role affecting radiative energy budgets and the activity of cloud condensation nuclei (CCN). In this study, how the suburban aerosols affecting the cloud formation was investigated using a thermal-gradient diffusion chamber (cloud condensation nuclei counter, CCNc, DMT), a Scanning Mobility Particle Sizer (SMPS, TSI) and a condensation particle counter (CPC, TSI) over Hua-Lin, Northern Taiwan for August 2011. Based on the measured concentration of anthropogenic gaseous pollutants and the back trajectory analysis, this study is divided into two periods；August 12 - 19 as period 1 and August 20-22 as period 2. . In period 1, the air parcels entered Taiwan from the South part and passed through the western Taiwan before reaching Hualin and were associated with significant anthropogenic emission combining with obvious oxidation or photolysis reactions. In period 2, the air parcels entered Taiwan from the East Coast and reached HuaLin shortly and tended to be composed of less anthropogenic emission. In this study, the more significant new particle formation (NPF) events observed in Period 1 is possibly due to the high concentration and diverse of gas species with the addition of anthropogenic emission with the local biogenetics emission. With the measured number concentrations of condensation nuclei (NCN), cloud condensation nuclei (NCCN), and particle number size distribution, the minimum diameter (Dss) required for the activation was estimated and were applied to derive the hygroscopicity parameter ( κ ). The κ values in Hualin are ~ 0.23 in period 1, κ ~ 0.14 in period 2, usually higher than that in Taipei city ( κ ~ 0.05) and Taichung city ( κ ~ 0.07), which showed the higher fraction of hygroscopic species to turn into CCN easily in suburban area ( κ ~ 0.18).
Thus, the results exhibited the anthropogenic emission though aging process such as oxidation and photochemical reaction can change the physical chemical properties of aerosols to enhance new particle formation and increase the hygroscopicity in period 1. The local plants emission with less aging particles was the major factor to decrease the CCN activity in period 2. The particles emitted from the urban and then transported to the suburban area through longer aging processes tend to have higher κ values (higher hygroscopicity).

並列關鍵字

cloud condensation nuclei ； suburban aerosol ； secondary organic aerosol

參考文獻

Albrecht, B.A. (1989). Aerosols, cloud microphysics, and fractional cloudiness. Science (New York, NY), 245(4923), 1227.

Andreae, M. O. (2009). A New Look at Aging Aerosols. Science, 326(5959), 1493-1494.

Andreae, M. O., & Rosenfeld, D. (2008). Aerosol-cloud-precipitation interactions. Part 1. The nature and sources of cloud-active aerosols. Earth-Sci Rev, 89(1-2), 13-41.

Charlson, R. J., Langner, J., Rodhe, H., Leovy, C. B., & Warren, S. G. (1991). Perturbation of the Northern-Hemisphere Radiative Balance by Backscattering from Anthropogenic Sulfate Aerosols. Tellus A, 43(4), 152-163.

Chou, C. C. K., Lee, C. T., Cheng, M. T., Yuan, C. S., Chen, S. J., Wu, Y. L., et al. (2010). Seasonal variation and spatial distribution of carbonaceous aerosols in Taiwan. Atmos Chem Phys, 10(19), 9563-9578.