| 摘要: | 本研究旨在探討台灣北部冬季層狀雲之化學及微物理特性，於1996 - 2007年冬季在陽明山國家公園竹子山區( 25°11' N，121° 31' E，1,103 m )進行雲化學及微物理觀測，探討不同雲系及不同氣流來源之雲水特性，並討論年際變化。 實驗期間共觀測到101個雲事件，雲化學分析結果發現海鹽離子( Na+、Cl- )佔總離子濃度50 %以上，具有海洋性層雲之特性，硫酸根離子( SO42- ) 濃度次之，約佔17 %；平均 pH 值為 3.97，95 % 之雲水樣本 pH 值小於酸雨標準5.0。以nss-SO42-/NO3-作為長程傳送指標，平均比值為2.46，顯示陽明山雲霧水以長程傳送為主要途徑。依據生成特性及綜觀天氣條件可將雲事件分為東北季風雲系( Monsoon )及鋒面雲系( Front )兩類，結果顯示東北季風雲系之總離子濃度較鋒面雲系高出約2倍。進一步計算單位體積雲水中所含的非海鹽性硫酸根離子( nss-SO42- )負載量，東北季風雲系亦高於鋒面雲系1.7倍，分別為6.73??g m-3 及4.07 ?g m-3。 雲微物理特性部份，雲滴粒徑平均為12.9 ± 4.0 ?m，雲滴數為227 ± 276 cm-3，液態水含量則為0.23 ± 0.31 g m-3，雲滴粒徑越大( > 15 ?m ) 液態水含量成長越快且數值較高。離子濃度與雲滴粒徑及雲滴數成指數關係，相關性 r 分別為 -0.34 及 0.35，且雲滴數越多雲滴粒徑越小。東北季風雲系之雲滴粒徑較小、雲滴數較多、液態水含量較小，分別為12.2 ± 3.0 ?m、230 ± 316 cm-3、0.17 ± 0.15 g m-3；鋒面雲系反之，分別為13.2 ± 4.7 ?m、203 ± 200 cm-3、0.23 ± 0.43 g m-3。 氣流來源分析顯示在觀測期間94 %之氣流源自中國，此時雲水中離子濃度及負載量皆有最大值，總離子濃度為受到台灣內陸之區域性影響的2.6倍，nss-SO42-負載量亦為3倍。其中沙塵事件之總離子濃度為非沙塵時期之2倍，塵土離子 Ca2+ 佔總離子濃度之比例由3 %增加至7 %，濃度更增加5倍之多，使得 pH 值最高曾達7.08，顯示在此氣流來源下，北台灣冬季雲水受中國排放之污染物及沙塵影響甚鉅。 年際變化部份，陽明山雲霧水之nss-SO42-負載量自1996年至2007年間有上升趨勢，代表雲水酸度之 pH 值則逐年下降。1996年之雲水樣本約有86 %之 pH 值小於酸雨標準之5.0，至2007年已增至99 %，顯示雲水酸化的問題日益顯著。 The purpose of this study is to characterize the chemistry and microphysics of the wintertime stratiform clouds in northern Taiwan. The experiment of cloud water collection was established at Mt. Bamboo (25°11' N，121° 31' E，1103 m MSL) in the winters of 1996 - 2007. Cloud events were categorized to study the relationship between cloud varions types. Backward trajectory analysis was used to categorize the source of cloud-forming air mass that arrived at the site. We also summarized the 12-year cloud measurements, reported the trend and explained the significance. In total, 101 cloud events were obtained. Principal ions in cloud water were found to be Cl-, Na+ and SO42- , each contributing 27 %, 24 %, and 17 % to total ions, respectively. On average, pH in cloud water was 4.0. Ninety-five percent of cloud water samples had the pH value less than 5.0. Average ratio of nss-SO42- / NO3- in the cloud water was 2.46, indicating the long-range transport of pollutants. Cloud events can be attributed to northeast monsoon flow (Monsoon) and frontal passage (Front), the average total ion concentration of monsoon clouds were more than twice that of frontal clouds. Consequently, their cloud water loadings of nss-SO42- were 6.73 ?g m-3 and 4.07 ?g m-3, respectively. On average, cloud droplet diameter (Dv), cloud droplet number (N) and liquid water content (LWC) were 12.9 ± 4.0 ?m, 227 ± 276 cm-3 and 0.23 ± 0.31 g m3, respectively. The larger cloud droplet diameter ( > 15 ?m ) is, the higher then liquid water content is. The total ion concentration was an exponential function of droplet diameter and cloud number, with a correlation of r was -0.34 and 0.35, respectively. Backward trajectory analysis indicated that about 94 % of air mass came from the industrial region in China. Highest total ion concentration and cloud loading of nss-SO42- were associated with the air mass originated from China. For dust events, the average total ion concentration was more than five times that of non-dust events. The contribution of Ca2+ to total ions in cloud water was 3 % and 6 % for non-dust and dust events, respectively. During the winters of 1996 through 2007, significant trends of increasing nss-SO42- loading and decreasing pH can be found. In the year of 1996, 86 % of pH values in cloud water were less than 5.0, the year of 2007 that 99 % of cloud water samples had the pH less than 5.0,except for possibly due to increasing SO2 emission in China recently. |
