| 摘要: | 本研究分析了2009至2018年間中壢、台中與高雄之汞濕沈降資料。三處測站位於台灣西部，周圍有不同程度的人為與工業活動。因此，本研究主要目標為描述各測站的汞濕沈降時間與空間變化分布，以及研討天氣型態和雨水化學成分和汞濕沈降的機制。於此10年期間，台中站(14.2 ng L-1, n = 284)與高雄站(14.2 ng L-1, n = 233)雨水汞體積權重平均濃度比中壢站(11.4 ng L-1, n = 362)雨水汞體積權重平均濃度高25% (p < 0.01)，而三處測站汞濕沈降年沈降量介於22.4至24.6 ug m-2 yr-1，各測站年沈降量並無顯著差異(p > 0.1)。使用Mann-Kendall和Thiel-Sen方法探討雨水汞濃度和汞濕沈降量變化趨勢，中壢站雨水汞濃度變化趨勢為-0.032 ng L-1 month-1或-3.4% yr-1(p < 0.01)、汞濕沈降量變化趨勢-8.1 ng m-2 month-1或-0.43 % yr-1 (p < 0.01)，但台中站和高雄站變化趨勢並不顯著(p > 0.1)。依季節分析，三處測站都在夏季和秋季觀察到較高的雨水汞濃度，夏季汞濕沈降量最高。降雨量和降雨類型為控制汞濕沈降量主要機制，汞濕沈降量和降雨量的相關性(R2 = 0.75–0.82, p < 0.01)高於和雨水汞濃度之相關性(R2 = 0.18–0.32, p < 0.01)。降雨類型也導致雨水汞濃度和汞濕沈降量之空間變化，午後雷雨型降雨增加雨水汞濃度約14至42%，西南季風因帶來更多的對流型降雨，與東北季風相比有更高的汞濕沈降量。主成分分析結果顯示自對流層移除的氣態氧化汞可能為三處測站的雨水汞主要來源，經由個案分析更進一步指出一般午後雷雨事件反映出雨水汞濃度，意味著對流活動移除自由對流層中氣態氧化汞的重要性。;In this study, rainwater mercury (Hg) concentration and wet Hg deposition flux data of Jhongli, Taichung and Kaohsiung in 2009–2018 were analyzed. Located along the western side of Taiwan, these surface sites were surrounded by anthropogenic and industrial activities in various degrees. Therefore, characterization of temporal and spatial variations in wet Hg deposition at these suburban and urban sites was the major research objective of this study. In addition, the associated weather types and co-collected rainwater chemical components were also studied to explore the mechanisms governing wet Hg deposition at these sites. Over the 10-year period, the volume-weighted mean (VWM) Hg concentrations at Taichung (14.2 ng L-1, n = 284) and Kaohsiung (14.2 ng L-1, n = 233) were 25% higher (p < 0.01) than at Jhongli (11.4 ng L-1, n = 362) while there was no significant difference in annual deposition fluxes among 3 sites (22.4–24.6 ug m-2 yr-1, p > 0.1). Mann-Kendall test and Thiel-Sen slope were employed to investigate the trends in rainwater Hg concentration and wet Hg deposition. Significant decreasing trends in Hg concentration (-0.032 ng L-1 month-1 or -3.4% yr-1, p < 0.01) and Hg flux (-8.1 ng m-2 month-1 or -0.43 % yr-1, p < 0.01) were observed at Jhongli, but not at Taichung (p > 0.1) and Kaohsiung (p > 0.1). Seasonally, higher summer and fall concentrations were observed at all sites, with peak Hg deposition flux in summer. Rainfall depth and rainfall types were found to be major factors governing wet Hg deposition. Wet Hg fluxes showed better correlation with rainfall (R2 = 0.75–0.82, p < 0.01) than rainwater Hg concentration (R2 = 0.18–0.32, p < 0.01), demonstrating the importance of rainfall in governing wet Hg fluxes at these sites. Rainfall type also contributed to the spatial variation in rainwater Hg concentration and wet Hg deposition flux among these 3 sites. Afternoon thunderstorms enhanced the rainwater Hg concentration by 14–42%. Southwest monsoon brought more convective rainfall and higher wet Hg deposition flux than the northeast monsoon. Results of principle component analysis (PCA) result indicated gaseous oxidized mercury (GOM) scavenging from the free troposphere could be the major source for rainwater Hg at all sites. A case study further examined the rainwater Hg concentration response from a typical afternoon thunderstorm event and suggested the importance of convective activity in scavenging GOM from higher altitudes. |
