Characterizations of atmospheric mercury concentration and deposition at a tropical mountain background site in East Asia: insight into potential driving mechanisms

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Title:	Characterizations of atmospheric mercury concentration and deposition at a tropical mountain background site in East Asia: insight into potential driving mechanisms
Authors:	阮立錫福;Phu, Nguyen Ly Sy
Contributors:	大氣科學學系
Keywords:	大氣汞;東亞;汞沈降;汞同位素組成;生質燃燒;Atmospheric Mercury;East Asia;Mercury Deposition;Mercury isotopic compositions;Biomass Burning
Date:	2020-01-09
Issue Date:	2020-06-05 16:45:52 (UTC+8)
Publisher:	國立中央大學
Abstract:	全球人為排放清單指出亞洲，特別在東亞和東南亞區域是人為大氣汞排放量最大的區域，此外，近年觀測與模式結果顯示熱帶森林可能是大氣汞主要沈降區。但是，關於上述區域大氣汞濃度和大氣汞沈降的長期研究仍十分有限。本研究首次於熱帶高山森林區，東亞大陸下風處之鹿林山大氣背景監測站(Lulin Atmospheric Background Station, LABS)分析大氣汞物種長期分布與大氣汞乾濕沈降。大氣汞趨勢分析依據氣態元素汞(GEM)夜間(0-8時)觀測濃度，夜間GEM濃度主要受到區域性大氣影響，2006年4月至2016年12月間GEM濃度呈現顯著下降，與近期文獻指出東亞大氣汞排放輸出量降低相符。氣團來源與傳輸途徑之頻率分佈也可能導致GEM分佈變化，此外，觀測到海洋性氣團中大氣汞濃度呈現下降趨勢，顯示北半球大氣汞背景濃度亦呈現下降趨勢。LABS觀測到大氣汞乾濕沈降比為2.8，顯示森林區域大氣汞沈降機制主要為乾沈降。 LABS大氣汞乾濕沈降量均高於溫帶地區沈降量，指出東亞和東南亞熱帶高山森林區可能是大氣汞沈降熱點。降雨量和降雨型態是大氣汞濕沈降季節變化的重要因素，然而，大氣汞乾沈降季節變化則由大氣汞濃度、氣溫和風速控制，GEM為乾沈降主要物種，經葉子吸收，因此植被活動強度在森林生態系統中為控制大氣汞乾沈降的重要機制。利用一年氣態總汞(total gaseous mercury, TGM)同位素監測數據探討驅動LABS的TGM同位素組成變化因素。結果顯示，δ202HgTGM濃度變化與植被活動相關，而Δ199HgTGM的濃度季節變化可能受到氣團來源和傳輸途徑的季節變化影響。因此，這項監測證明了LABS植被活動對總汞分布的作用。以個案研究探討LABS於生質燃燒長程傳輸期間大氣汞與氣膠光學參數之間的關係。發現R2介於0.12至0.7，說明含碳氣膠和汞不僅來自相同來源，而且可能存在於相似的生質燃燒氣膠庫中。此外，顆粒汞與Ångström 吸收指數（AAE370-880）之間的正相關性意味著二價氧化汞可能更容易被有機氣膠吸收。 ;Global anthropogenic emission inventories indicated that Asia is the region with the greatest proportion of anthropogenic mercury (Hg) emissions to the atmosphere, particularly in East and Southeast Asia. In addition, recent observation and modeling studies suggested that tropical forest areas may be hotspots of atmospheric Hg depositions. However, long-term studies on atmospheric Hg concentration and deposition in these regions are still quite limited. This work, for the first time, presents and analyzes the long-term observation of speciated atmospheric Hg, wet and dry deposition of atmospheric Hg at Lulin Atmospheric Background Station (LABS), a high mountain tropical forest site downwind of the East Asian continent. Nighttime (0–8 am) data of gaseous elemental Hg (GEM), which is more representative of regional influence, between April 2006 and December 2016 were used for trend analysis. Significant decreasing trends in GEM concentrations at LABS were observed, in agreement with the reductions in atmospheric Hg export from the East Asia continent that has been suggested by recent studies. Changes in the frequency distribution of air mass origins and transport paths may also contribute to the changes in GEM concentrations at LABS. Besides, the decreasing trends observed with air from the ocean indicated declining background GEM concentrations in Northern Hemisphere. Multi-year of dry and wet Hg depositions at LABS were studied. The dry/wet deposition ratio of 2.8 was observed, indicating that Hg deposition to forest landscape was governed by dry rather than wet deposition. Both dry and wet Hg depositions at LABS were higher than those reported from the temperate region, suggesting that tropical forest mountains in East and Southeast Asia could be hot spots of atmospheric Hg deposition. Rainfall amount and rainfall types were the important factors in determining the seasonal distribution of wet Hg deposition. On the other hand, the seasonal distribution of dry Hg deposition was controlled by atmospheric Hg concentrations, temperature, and wind speed. GEM was the major dry deposition contributor at LABS, suggesting the important role of vegetation activity in forest ecosystems due to the uptake of Hg(0) by foliage. One-year observation data of total gaseous mercury (TGM) isotopic compositions were used to investigate the drivers responsible for the variation of TGM isotopic compositions at LABS. The results indicated that the variability of δ202HgTGM was attributed to vegetation activity, whereas the seasonal variation of ∆199HgTGM was likely driven by the seasonal changes in air mass origins and transport paths. This study, therefore, provides observational evidence for the role of vegetation activity at LABS. A case study was conducted to explore the relationship between atmospheric Hg and aerosol optical parameters during the biomass burning (BB) long-range transport periods at LABS. Positive correlations were found with R2 ranging from 0.12 to 0.7, suggesting carbonaceous aerosols and Hg are not only simultaneously emitted from the same source but also could reside in a similar pool of BB aerosols. Additionally, the positive correlation between particulate bound Hg (PBM) and absorption Ångström exponent (AAE370-880) likely suggests that the Hg(II) may be preferably absorbed by organic aerosols.
Appears in Collections:	[Department of Atmospheric Sciences and Graduate Institute of Atmospheric Physics ] Department of Earth Sciences

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