| 摘要: | 大氣氣膠的存在能改變太陽輻射收支，為潛在造成氣候變遷原因之一。IPCC(2007)指出人為造成之氣膠直接輻射驅動力(radiative forcing)約 -0.5 W m-2，但具有相當大的不確定性，Liou et al. (2007)建議目前大氣輻射傳送模擬主要問題在於：氣膠之輻射效應、複雜地形輻射效應、海表面複雜之反照。本篇研究目的為探討鹿林山大氣背景站大氣輻射特徵及估算氣膠直接輻射驅動力。吾人嘗試整合鹿林山背景站Kipp&Zonen全天空輻射通量計、AERONET太陽光度計(Cimel’s sunphotometer, CIMELs) 與一維大氣輻射傳送模式(libRadtran)，藉由個案分析建立觀測與模擬之間的相關性，並探討儀器與模式之誤差及不確定性，統整分析2010-2011年鹿林山背景站大氣輻射與氣膠光學之特徵，最後以三種不同方法估算氣膠輻射驅動力。
由個案分析結果顯示，模式於晴空狀態下大氣輻射通量模擬具有良好的表現。經由濾雲可得到晴空下地面輻射通量，將此輻射通量減去模式模擬無氣膠時的地面輻射通量，可求得瞬間輻射驅動力，經由三種不同方法估算，結果顯示2010-2011年平均地面短波氣膠直接輻射驅動力(ARF)為：-14.6?-6.5 Wm-2 (Global)、11.0?14.1 Wm-2 (Diffuse)、-23.2?-17.1 Wm-2 (Direct)，鹿林山於春季(3-5月)有較高之背景污染物，使地面輻射通量下降，造成氣膠輻射驅動力產生極值。而每單位AOD可造成地面短波輻射的改變(ARFE)為： -237.8?-94.2 Wm-2τ-1 (Global)、167.8?191.8 Wm-2τ-1 (Diffuse)、-350.2?-245.9 Wm-2τ-1 (Direct)。另外亦發現，6至9月期間氣膠具有較大的吸光性，使得相同AOD下，輻射通量減少較多。未來研究除嘗試以更精確之氣膠光學參數(如由in-situ觀測取得)改進模擬結果，此外若以本研究估算之氣膠輻射驅動力作為基礎，則可直接由觀測之地面輻射通量回推氣膠輻射驅動力，並反演AOD，可做為輻射通量計觀測的延伸應用。
Aerosols can alter solar radiation in the earth’s atmosphere and have implications for future climate change. The Intergovernmental Panel on Climate Change (IPCC, 2007) has indicated that anthropogenic direct aerosol radiative forcing (ARF), which is estimated to be -0.5 Wm-2, has a large uncertainty. Liou et al. (2007) suggested the current challenges on the atmospheric radiative transfer model to be aerosol-radiative effect, surface complexity terrain-radiative effect, and wind-driven air-sea interface-radiative effect. We investigated atmospheric radiation and direct aerosol-radiative forcing at Lulin Atmospheric Background Station (LABS), Taiwan. We integrated measurements from the Kipp & Zonen solar instrument systems, the Aerosol Robotic Network (AERONET) Cimel sunphotometer (CIMELs), and a radiative transfer model (libRadtran) to estimate ARF. The good agreement between observation and simulation for clear-sky implies the model can represent solar radiation at the surface for the mountain site. We also discussed model and instrument uncertainties and analyzed data from 2010 to 2011 to clarify the characteristics of atmospheric radiation and aerosol optical properties at LABS. We applied three different methods (i.e., direct calculation, linearly interpolated, and model calculation) to estimate direct aerosol radiative forcing.
The results show that the mean downward shortwave ARFs at the surface are -14.6?-6.5 Wm-2 (Global), 11.0?14.1 Wm-2 (Diffuse), and -23.2?-17.1 Wm-2 (Direct), respectively. We observed the seasonal maximum values of ARFs in spring because of the higher aerosol optical depth (AOD). We estimated the aerosol radiative forcing efficiency (AREF
ARF/AOD) to be -237.8?-94.2, 167.8?191.8, and -350.2?-245.9 Wm-2τ-1 for Global, Diffuse, and Direct, respectively. We attributed the higher AREF values during June and September to aerosol with larger light absorption during this season.
In the future, we will attempt to improve our simulation results by using inputs of aerosol optical properties that are more accurate (e.g., obtained from in-situ). The empirical relationship between ARF and AOD from this study could also be used to estimate AOD, when surface radiation flux measurements are available. The AOD retrieved from the global distribution of radiometer measurements could benefit aerosol and radiation communities. |
