東亞氣膠金屬對海洋的貢獻：來源、轉變以及沉降通量

Abstract

生物可利用的溶解氣膠金屬沉降會影響海洋浮游生物的生長與組成，以及海洋中物質的循環。由於氣膠沉降前後經過了複雜的過程，因此要精準估算溶解氣膠金屬的通量是極具挑戰的！本研究我們於東海採集了5種不同粒徑的氣膠和亞北極太平洋採集了表水的懸浮顆粒，並利用元素間比值和鐵同位素來量化天然性和人為性氣膠對於西北太平洋的貢獻，與了解大氣傳播過程對氣膠的影響。

為了調查大氣傳輸對氣膠溶解度和通量的影響，我們沿用了國際常用的氣膠溶出方法來比較氣膠和中國沙漠的沙子中可溶出金屬的差異，其中包含了超純水(Ultrapure water)、醋酸銨緩衝溶液(acetate buffer)和柏格(Berger)等溶出方法。結果顯示東海氣膠的金屬經過大氣傳輸後其溶解度都有顯著的提升。整體而言，氣膠金屬的沉降通量隨著粒徑越大而越高，意旨氣膠中的粗顆粒是氣膠金屬對於海洋的主要貢獻者。從鎘、鋅和鉛的高富集因子得知，就算是粗粒徑的氣膠某些金屬(鎘、鋅、鉛)的主要來源還是有可能來自人為氣膠。此外，我們還發現岩石性元素(如:鋁、鐵和鈦)於不同溶出方法所得到的濃度或溶解度都有較大的差異，因此，若沒有事先研究金屬濃度於不同粒徑氣膠上的分布來得到較精準的平均氣膠沉降速率，岩石性的金屬通量會導致嚴重高估的現象。至於金屬濃度隨粒徑的分布或大氣傳輸的影響可能有地區性的差異，為了更精準估算全球氣膠金屬的通量，在其他大洋區域的類似研究是有必要至少調查一次的！

我們還特別使用鐵同位素組成來精準調查氣膠溶解鐵來源對鄰近海洋及亞北極太平洋表水層水的貢獻，其結果觀察到岩石性氣膠和人為性氣膠的鐵同位素值分別為0.2和-4.5‰。假設人為性氣膠鐵同位素的端點為-4.5‰，那麼其對東海的超純水及醋酸銨緩衝溶液溶出的鐵通量的貢獻分別為30%和8.7%，至於亞北極太平洋收集到的懸浮顆粒的貢獻也是低於10%。簡言而之，雖然東亞陸地傳輸大量高溶解的人為氣膠鐵至西北太平洋，但岩石性氣膠仍然是東海、西北太平洋和亞北極太平洋溶解性氣膠鐵的主要來源。

Aerosol dissolvable metals are considered to be readily bioaccessible so that their input would influence the growth and composition of marine phytoplankton and thus material cycling globally. However, it is highly challenging to measure or estimate reliable deposition fluxes of aerosol dissolvable metals in the ocean due to the impacts of complicated processes involved in pre- and post-deposition of aerosols. In this study, we applied the elemental and isotopic composition of size-fractionated aerosol metals collected in the East China Sea (ECS) and the suspended particulate matter (SPM) in the surface water of the subarctic North Pacific Ocean (SNPO) to quantify the contribution of East Asian anthropogenic and lithogenic aerosols in the oceans and also to understand their transformation processes during the transport processes.

To investigate the impacts of the aeolian transport processes on the solubilities and fluxes of aerosol metals in the ocean, we first collected lithogenic dust from major Chinese deserts and size-fractionated aerosols from the ECS to determine the variations of their dissolvable metals by using three operationally defined leaching protocols (pure water, buffer, and Berger leaches). Referred by the solubility differences between the desert dusts and the ECS largest size aerosols, atmospheric transport processes tremendously enhance all of the three solubilities for most elements in the ECS aerosols. Overall, the metal fluxes of each aerosol treatment increased with increasing particle sizes, indicating coarse aerosol is the primary metal supplier into the surface ocean. Due to the high metal enrichment factor, Cd, Zn, and Pb elements in coarse aerosols could come from anthropogenic aerosol aggregation. In addition, we found highly divergent variations for lithogenic type elements with all aerosol sizes among the three leaching treatments. Without knowing aerosol size-fractionated information, the deposition fluxes of lithogenic type elements would be significantly overestimated. The mass distribution or impacts of the transport processes could be region specific so that similar field studies are essential in other regions to obtain reliable aerosol dissolvable metal data for global modeling.

Specifically, we have applied Fe isotopic composition to investigate the contribution of East Asian aerosol Fe to the surface water of the adjacent oceanic regions and the SNPO. We observed that the isotopic value of lithogenic and anthropogenic aerosols ranged from 0.2 to -4.5 ‰. Assuming the value of anthropogenic aerosol Fe to be -4.5 ‰, the contribution of anthropogenic aerosols were 30% and 8.7% of the fluxes of dissolved Fe and buffer Fe in the ECS, respectively; the contribution to the SPM collected in the SNPO and WPS were both less than 10%. In brief, lithogenic aerosol is still the major sources of aeolian Fe for dissolvable aerosol Fe in the ECS, the WPS, and the SNPO.