以光達觀測分析台北之邊界層與氣膠特性

Abstract

氣膠粒子是重要空氣污染物之一，對於人體健康有極大的影響，也會經由散 射或是吸收輻射的過程影響氣候，對於雲的形成過程也扮演了凝結核或是冰核的 角色。而這些不同的效應會因為氣膠粒子本身的成分組成、混合狀態及空間分佈 而有極大的不同，但是這個特徵與機制尚未被完全了解。 由於光達(LIDAR, Light Detection and Ranging system)的觀測具有高空間時間 解析度特性，為研究氣膠及邊界層的有效工具，因此本研究使用RCEC/NTU-AS 拉曼光達來分析氣膠粒子的成分組成及垂直分佈，並推演出的台北邊界層高度。 本研究分析所使用的參數有：(1)背向散射率，可反映出氣膠的濃度；(2)消偏振率， 主要反映出氣膠粒子的形狀，較低的消偏振率反映出粒子為球性，而不規則形的 粒子則會有較高的消偏振率；(3) Ångstrom exponent(α)，此參數可得知粒子的粒徑 大小，α 值越小代表粒子越大，α 值越大則代表較小的粒子。本研究首先分析了台 北地區這些參數的季節分佈。 本研究並針對2007 年5 月7 日、1 月28 日及2008 年3 月4 日進行分析，此 三個案其間台灣附近都有高壓存在，2007 年5 月7 日顯示了氣膠粒子吸濕過程在 特性上的改變，而2007 年1 月28 日及2008 年3 月4 日為大陸沙塵個案，由光達 資料得知當時亦存在其他種類之氣膠。藉由消偏振率及Ångstrom exponent 的分 析，我們發現了幾個重要的現象例如潮解、成雲作用、氣膠核化以及邊界層擾動 造成氣膠的混合現象。利用這樣的結果可以得到一個消偏振率及Ångstrom exponent 變化的概念圖，對於日後光達訊號的相關分析將有所助益。

Aerosol particles are a major air pollutant that affects human health, play important roles in climate by scattering or absorbing solar radiation, and can modify cloud properties by acting as cloud condensation nuclei and ice forming nuclei. The significance of these effects varies with their composition and mixing state as well as the spatial distribution which unfortunately are not well understood. With the advantage of high temporal and spatial resolutions, LIDAR (Light Detection and Ranging system) is a powerful tool for aerosol and boundary layer analysis. This study applies the RCEC/NTU-AS Raman lidar to analyze the composition and vertical distribution of aerosol particles, and deduce from them the boundary layer characteristics at Taipei. The parameters analyzed include (1) the backscattering signals which are representative of aerosol concentration; (2) the depolarization ratio which reflects the irregular shape of aerosols as non-spherical aerosols give higher depolarization ratio than spherical ones; and (3) the backscatter-related Ångstrom exponent, α, which indicates the size of aerosol particles as it approximately decreases with increasing particle size. Seasonal variation of these parameters over Taipei during 2006-2007 are analyzed in this study. In addition, we focus on the cases on 5 May 2007, 28 Jan 2007 and 4 Mar 2008, when Taipei was situated at the outskirt of a high pressure system. The 5 May 2007 case shows the swelling process of aerosol, and the cases of 28 Jan 2007 and 4 Mar 2008 were Asian dust incursion events. Besides mineral dust, the lidar data also revealed other types of aerosols in these two cases. By combining the analyses of backscattering intensity, Angstrom exponent and depolarization ratio, we identified several important phenomena such as deliquescence-dehydration, swelling, cloud drop activation, aerosol nucleation and boundary layer turbulence mixing, as well as aerosol mixing states. The findings are summarized and reduced to a conceptual diagram which could be a useful tool for facilitating further analysis of lidar signals. Keywords: aerosol, lidar, depolariztion, Ångstrom exponent, deliquescence, swelling