| 摘要: | 每年的三到五月為大陸沙塵暴發生頻率最高的月份，並且帶來大量的揚沙，造成空氣品質的下降、能見度的衰減、氣膠化學組成的改變。為了獲得當沙塵暴發生時氣膠物理特性、氣膠粒徑分布以及化學性質，本研究於民國90年3月至5月在台北縣石門鄉進行為期45天連續24小時採樣，量測當地氣膠的光學性質、氣膠粒徑分布以及沙塵暴所帶來的氣膠化學組成，並配合當地氣象條件的相互分析，以瞭解湍上風區氣膠的光學特性以及粒徑分布，並解析沙塵暴帶來的影響。 研究顯示石門地區氣膠的散光性質（σsp）、粒徑分布與當地的風向有很大的關係。在盛行從海面而來氣流（以下稱之為海風）的期間，氣膠的體積濃度主要來自於超微米（super-micrometer）的微粒，平均的σsp為0.0826km-1；而當風向轉移成從陸地而來的氣流（以下稱之為陸風）時，氣膠的總體積濃度較海風小，主要為次微米（sub-micrometer）的氣膠，平均的σsp為0.101 km-1。此外，氣膠散光係數、數目濃度、體積濃度的平均值也會隨著假日與平日有顯著的變化。 於石門採樣期間共採集到三次沙塵暴（由環保署公告）氣膠散光係數以及粒徑分布資料。三月沙塵暴的平均σsp為0.3438 km-1、四月沙塵暴的σsp為0.1283 km-1、五月沙塵暴的σsp為0.1681 km-1，雖然四月份的沙塵暴較為猛烈，但是所造成的散光效應並不是特別顯著，這是由於超微米氣膠所佔比例較多的原因所導致。 由於氣膠化學組成的多變性，氣膠對太陽輻射同時具有散射（冷卻）與吸收（增溫）的能力。氣膠對於太陽輻射的冷卻效應主要與氣膠的反射輻射回大氣層有關，而氣膠的反射又與氣膠的化學組成有著連帶的關係。由於氣膠不同的化學組成之生存期並不相同，所以氣膠的反射率會隨著時間以及空間的不同而有所變化。氣膠反射率與氣膠的光學性質有關，定義為氣膠的散射效應對於消光效應的比值。由高雄小港（1999）、台北氣象局（2000）與石門（2001）於採樣時期分析氣膠化學組成發現，高雄小港的氣膠平均反射率為0.87，台北氣象局的氣膠平均反射率為0.816，石門的氣膠平均反射率為0.915。由研究結果顯示，氣膠反射率大於0.85時，氣膠對於太陽輻射有冷卻的效應，所以高雄以及石門地區大氣有降溫的可能性。 Dust Storm from Mainland China brings a great amount of aerosols to degrade air quality, reduce visibility, and modify aerosol chemical properties. From time to time, the most frequent occurrence of dust storm is from March to May each year. To investigate aerosol optical property, size distribution, and chemical properties at normal days and dust storm event, this study collected atmospheric aerosols continuously at Shi-Men site in Taipei County for 45 days starting from March to May in 2001. The results show the light scattering coefficient (ssp) and size distribution of aerosols are related to local wind direction. For predominant airflow from the sea, aerosol volume is mainly contributed from super-micrometer particles with an average ssp at 0.083km-1. As the wind shifted from the sea to from the land, aerosol volume is turned to the control of sub-micron particles having an average ssp at 0.101km-1. In addition, aerosol ssp, number concentration, and volume concentration are distinguished between holidays and non-holidays. During the intensive study period, Taiwan Environmental Protection Administration announced three dust storm events. The average ssp during the dust storm event was 0.344km-1 in March, 0.128km-1 in April, and 0.168km-1 in May. Although the dust storm in April was the strongest, the aerosol ssp was not correspondingly high which was due to the majority existence of low scattering efficiency super-micron particles. Owing to its various chemical compositions, atmospheric aerosol is capable of both scattering (cooling) and absorbing (heating) solar radiation. The cooling effect is related to the backscattering of radiation to the upper atmosphere. A critical value of aerosol albedo (defined as the ratio of aerosol light-scattering coefficient to extinction coefficient) to have negative radiative forcing (and thus cooling) is found at 0.85. For the historical data and date from this study, the average albedo was calculated at 0.87 in Kaohsiung City, 0.82 in Taipei, and 0.92 at Shi-Men, respectively. Consequently, a radiative cooling by the atmospheric aerosol in Kaohsiung City and Shi-Men is evidentable. |
