南海北部海域大氣汞的時序變化

Abstract

本研究主要探測南海北部大氣汞的分佈及循環。利用南海序列研究計畫（SEATS）的航次於南海航行的時間進行大氣汞的量測，其項目有氣態元素汞（GEM）及懸浮微粒汞（PM）。自2005年3月至2007年1月共進行7個不同季節的航次現場量測及採樣；另外加上先前2003年5月至2005年3月共9個航次的大氣元素汞資料（未發表），一併進行討論。因此，大氣之元素汞及懸浮微粒汞分別有15及7個航次資料。大氣元素汞的採樣及分析是使用自製的採樣分析一體式汞齊濃縮及冷蒸氣原子螢光光譜法；懸浮微粒汞的部份則是以高流量採樣器採集顆粒標本於濾紙，再使用密閉式微波消化法處理，配合微量汞偵測儀及原子螢光法分析。 在大氣元素汞方面，春季為3.7±1.1 ng/m3（n=3），夏季為3.4±1.3 ng/m3（n=4），秋季為4.2±1.5 ng/m3（n=4）及冬季為4.8±2.1 ng/m3（n=4）。季節趨勢大致上看來，冬季最高，依次為秋、春及夏季。日變化方面，夏季具有明顯的白天高、夜晚低的變化，但各航次的日夜變化不甚明顯，然而在2003年8月的OR1_690航次及2006年10月的OR1_812航次，在高頻量測中具有白天高、夜晚低的趨勢。 在懸浮微粒汞方面，春季為17.4±5.9 pg/m3（n=1）夏季為17.6±7.1 pg/m3（n=2）秋季為18.6±14.0 pg/m3（n=1）冬季為18.0±6.3 pg/m3（n=2）。在季節變化方面，以秋、冬二季較高。各航次之間，顆粒汞與大氣元素汞有類似的濃度變化趨勢，推論兩者有相同的來源。 測得之數據配合大氣遙測的溫度及風場資料（NOAA-NCEP），再利用氣團軌跡回推圖來推測氣團的傳輸路徑及大氣汞可能來源。夏季的風向大致為南風，秋冬春季大致為東北風。由氣流軌跡回推圖來看，在秋冬春季，氣團源自蒙古、中國東北地區、日本、韓國及呂宋島北方；夏季氣流則從印度洋經中南半島。由此可佐證秋、冬及春季，大氣汞可能來自北方陸地，而夏季則來自海洋，說明邊緣海大氣汞的濃度受到周圍陸源物質及人為污染源的影響。

The distribution of gaseous elemental mercury (GEM) and particle-associated mercury (PM) were determined in the surface atmosphere of the northern South China Sea during 7 cruises of the SEATS (SouthEast Asian Time-series Study) between March 2005 and January 2007. In addition, there were 9 cruises for GEM investigation (Tseng et al., unpublished data) between May 2003 and March 2005. Therefore, the atmospheric Hg dataset were presented in this study a total of 15 and 7 cruises for GEM and PM, respectively. The sampling and analysis of the GEM were performed on board ship by using a custom-made on-line mercury analyzer based on gold (Au) amalgamation techniques together with cold vapor atomic fluorescence detection (CVAFS), while the analysis of the PM was performed in the laboratory by the closed microwave digestion, followed by the Au-CVAFS detection by using a customized on-line Hg analyzer. Distinct inter-annual and seasonal patterns were observed for the GEM, which concentrations were higher in winter and lower in summer. The seasonal GEM averaged 3.7±1.1 (n=3), 3.4±1.3 (n=4), 4.2±1.5 (n=4) and 4.8±2.1 (n=4) ng/m3 in spring, summer, autumn, and winter, respectively. A significant diurnal variation in GEM was occurred in summer with a peak at noontime and a low at nighttime. The PM averaged 17.4±5.9, 17.6±7.1, 18.6±14.0 and 18.0±6.3 in spring (n=1), summer (n=2), autumn (n=1) and winter (n=2). No obviously seasonal variation in PM was observed, but its concentration was slightly higher in autumn and winter. However, both of the GEM and PM had the likely similar distribution trend in annual variability. According to source tracking from the NOAA-HYSPLIT back trajectory and the NCEP wind vector, we found air masses in autumn, winter, and spring were from Eurasia continents, including Mongolia, China, Japan, and Korea, which exported the terrestrial and industrially anthropogenic materials to the SCS. On the other hand, air masses in summer were always from the Indochina Peninsula and Indian Ocean, which brought with the clean marine materials. The results demonstrated that the East Asian monsoons with the prevailing northeast wind in winter and southwest in summer control the sources of the GEM and PM associated with fine particles and further the long-rang Hg transportation would affect the Hg atmospheric distribution and cycling in the South China Sea.