| 摘要: | 本研究於2002年9月~2003年4月在台北縣石門鄉進行氣膠光學特性的量測，本研究以散光儀(TSI model 3563)量測氣膠的散光係數以及以雷射氣膠數目粒徑分佈監測儀(PMS model PCASP-X)量測大氣中氣膠的粒徑分佈，並採用同時間進行的質量濃度及化學組成量測資料（梁，2003），以瞭解氣膠光學特性和粒徑分布、化學組成間的關係。考量氣流軌跡來源的影響，本研究配合後推氣流軌跡線、風向分類出不同污染來源時的氣膠光學特性，以期能瞭解長程傳輸對氣膠光學特性的影響。最後，由於氣膠會影響地球輻射平衡，造成氣候改變，因此本研究也對石門地區的輻射作用力進行一探討。 研究顯示影響散光係數的主要粒徑為0.4~1?m的氣膠微粒，當這些粒徑氣膠的數目濃度增加時，將會使得散光係數增加。背散光率(backscatter fraction)的變化和總散光係數的變化成一相反的趨勢，也就是總散光係數增加時，背散光率則是降低的，這原因為當影響總散光係數0.4~1?m的微粒增加時，其對前散光的影響效應較背散光來的大，也就是說前散光係數較背散光係數增加的多。另外，背散光率會隨著粒徑的減小而增加。 石門地區地面風向主要可分成來自陸地和海面，從體積濃度來看 風來自海面（東北風）和風來自陸地（西南風）時其氣膠總體積濃度相當，但風來自海面時氣膠的粒徑分布以粗粒徑為主;相反地風來自陸地時則轉為以細粒徑為主，因此風向來自陸地時的散光係數值(124±65Mm-1)較高於風向來自海面時的散光係數值(91±64Mm-1)。氣膠會隨污染來源的不而有所不同，根據本研究分類出八種氣流軌跡線後發現，當氣流來自大陸沿岸時，在粒徑分布上屬於細粒徑為主，且含有較多的硫酸鹽，因此散光係數較高，而來自海洋傳輸時，在粒徑分布上屬於粗粒徑為主，主要以海鹽為主，散光係數都不高。 石門當地的臨界單一氣膠反照率為0.86，當黃沙來臨時，氣膠的反照率會大於此值，有較大的輻射冷卻效應。在計算輻射作用力時，根據本研究所算的光學厚度與AERONET實測值相比之後發現低了很多，這顯示了混合層高度並非氣膠層厚度，假設在台北市區AERONET實測值可代表石門地區氣膠的光學厚度，則可推算得石門地區氣膠層的平均厚度為5034公尺，因此以大氣混合層厚度代表氣膠層厚度會對本研究氣膠輻射作用力有低估的情形。 Measurements of optical properties of ambient aerosol particles were carried out from September 2002 to April 2003 at Shi-Men, Taipei. In order to understand the influence of aerosol size distribution and chemical composition on aerosol optical properties. Light scattering coefficient and aerosol particle number size distribution were made by Nephelometer and PMS, filter samples of aerosols were collected and analyzed for mass and chemical species simultaneously. In order to understand the effect of long-term transport to aerosol optical properties, this study use backward air trajectory and wind direction to categorize the aerosol optical properties into different pollutant. Because of aerosol particles will influence the Earth’s radiative balance and cause climate change, computing the radiative forcing at Shi-Men finally. The results show that aerosols size between 0.4 and 1?m contributed the light scattering coefficient mainly. Light scattering coefficients will increase following 0.4 to 1?m particle number increasing. Backscatter fraction and total light scattering coefficient have an inverse relationship, in other way, light scattering coefficient increasing as backscatter fraction decreasing. The reason of this is particle number at this active size is sensitive to forward scattering than backscattering, this implies forward scattering increasing more than backscattering. Moreover, backscatter fraction increasing following the decreasing in aerosol size. The predominant wind direction can be divided into two types, one is the airflow from the sea, and the other is from the land. The airflow from the sea and from the land have the quite same aerosol volume, when the predominant airflow from the sea the aerosol consisted of coarse mode ; on the contrary, as the wind shifted from the sea to the land, the aerosol size distribution shifted to the fine mode; therefore, the wind from the land have highly light scattering coefficient, the mean value and standard deviation are 124±45Mm-1,91±64-1 Mm-1,respectively. With the different sources of air mass, aerosols have different properties, our research categorize the backward air trajectory into 8 types. When Air mass from the coastal of Mainland China, the aerosol consists of fine mode and the predominant chemical species is sulfate, so it can cause the high light scattering coefficient; when air mass from the ocean , their size distribution is characterized by coarse mode, sea salt is the predominant species, having lower light scattering coefficient. A critical value of aerosol albedo at Shi-Men is 0.86, aerosol single scattering albedo will greater than this value during the Yellow Dust period, having negative radiative forcing(cooling). |
