| 摘要: | 摘要
海洋區域的海鹽氣膠 (SS)測量，需要利用大量衛星觀測反演的氣膠產品。 在本研究中，根據不同波長的氣膠光學厚度 (AOD) 計算 n 階光譜導數，用來分析檢驗海鹽氣膠和其他氣膠類型的光學本質參數，包含粒徑大小分佈和複數折射指數等重要的光學特性。藉由輻射傳送計算模式 (6S)理論的模擬，針對海鹽氣膠以及其他主要類型的氣膠，包括沙塵 (DS)、生質燃燒 (BB) 和人為污染物 (AP) 進行了多頻譜n階導數的特徵分析。 並利用正規化的一階和二階導數所對應的氣膠粒徑大小分佈(Angstrom Exponent，AE)和複數折射指數的本質參數進行氣膠種類的區分，最後應用於中級解析度成像分光輻射度計(MODIS) 以及氣膠觀測網 (AERONET) 的實際觀測資料。研究結果顯示本研究的方法可以有效地辨識海鹽氣膠SS並區分海洋地區的 DS、BB 和 AP，其中SS的粒徑為0.08±0.09，DS的AE為0.11±0.13，BB的粒徑為2.34±0.35，AP的AE為1.18±0.08，較小的 AE 值代表大粒徑氣膠為多數，而較大的 AE 則代表多數為小粒徑的氣膠。氣膠種類的辨識結果可應用於估算各類氣膠的短波輻射強迫及其效率，以評估其對天氣或氣候系統的影響。 而平均輻射強迫的數值則顯示，SS在大氣層頂的輻射強迫最大，為-36 W/m2，在大氣層頂產生最有效的輻射冷卻效果；而AP在底層大氣的絕對強度最大，但在大氣層中則最小，分別為-66.2和32.5 W/m2。 由於大氣氣膠粒子可提供凝結核，所有氣膠種類的氣膠光學厚度與正規化的可降水氣均呈現正相關，其中SS 在低、高、中層的大氣中，對於水氣的凝結是最有效的氣膠，相對於其他3種氣膠產生更多的可降水氣，而 DS 的效率是最低的。此外，SS 在低層大氣中雲量的增加是最有效的氣膠種類，而 AP 在中層和高層大氣中表現出最有效的氣膠種類，其次是 SS 和 DS。
關鍵詞：氣膠光學厚度、MODIS、CERES、氣溶膠類型、海鹽、輻射強迫、可降水氣.
;Abstract
The ground-based measurement of sea salt (SS) aerosol over the ocean requires the massive utilization of satellite-derived aerosol products. In this study, n-order spectral derivatives of multispectral aerosol optical depths (AODs) were examined to characterize SS and other aerosol types in terms of their spectral dependence related to their optical properties such as particle size distributions and complex refractive index (absorption and scattering). Based on theoretical simulations from the second simulation of a satellite signal in the solar spectrum (6S) model, wavelength-dependent spectral derivatives of SS were characterized along with other major types including mineral dust (DS), biomass burning (BB), and anthropogenic pollutant (AP). The approach of partitioning aerosol types with intrinsic values of particle size distribution (Angstrom Exponent, AE) and complex refractive index from normalized first- and second-order derivatives was applied to the datasets from a moderate resolution imaging spectroradiometer (MODIS) as well as by the ground-based aerosol robotic network (AERONET). The results after implementation from multiple sources of data indicated that the proposed approach could be highly effective for identifying and segregating abundant SS from DS, BB, and AP, across an ocean. Consequently, each aerosol’s shortwave direct radiative forcing and its efficiency could be further estimated in order to predict its impact on the climate system. The particle size distribution of SS was 0.08 ± 0.09, together with DS with AE 0.11 ± 0.13, BB with the particle size 2.34 ± 0.35), and AP with AE 1.18 ± 0.08. Smaller magnitudes of AE refer to coarse-mode dominant particles while large AE represents fine-mode dominating aerosols. The averaged radiative forcing (RF) exhibited that the SS had the smallest magnitude at TOA, −36 W/m2, while AP had the lowest magnitude at the BOA but the highest in the atmosphere, with −66.2 and 32.5 W/m2, respectively. SS generated the most efficient radiative cooling effects at TOA of all aerosol types as indicated by its steepest slope in the linear best fit between RF and AOD055 μm. AOD055 μm and normalized precipitable water vapor depicted positive correlation. At low (surface to 680 mb) and middle (680 – 400 mb) atmosphere, SS posed the most efficient aerosol, having the steepest slope of the line of best-fit, and generated more precipitable water vapor than the other 3 aerosol species. DS was the least efficient of all. The highest efficiency of SS in PW was also tailed by the relationship between AOD055 μm and cloud amount. SS exhibited the most effective aerosol species in enhancing cloud amount in low (< 4 km) and high (> 8 km) atmosphere, while AP demonstrated the most efficient in the middle atmosphere (4 – 8 km), followed by SS and DS.
Keywords: Aerosol Optical Depth, MODIS, CERES, aerosol types, sea salt, radiative forcing, precipitable water vapor. |
