Elemental Mercury Concentrations and Fluxes in the Tropical Atmosphere and Ocean

Air–sea exchange of elemental mercury (Hg0) is a critical component of the global biogeochemical Hg cycle. To better understand variability in atmospheric and oceanic Hg0, we collected high-resolution measurements across large gradients in seawater temperature, salinity, and productivity in the Pacific Ocean (20°N-15°S). We modeled surface ocean Hg inputs and losses using an ocean general circulation model (MITgcm) and an atmospheric chemical transport model (GEOS-Chem). Observed surface seawater Hg0 was much more variable than atmospheric concentrations. Peak seawater Hg0 concentrations (∼130 fM) observed in the Pacific intertropical convergence zone (ITCZ) were ∼3-fold greater than surrounding areas (∼50 fM). This is similar to observations from the Atlantic Ocean. Peak evasion in the northern Pacific ITCZ was four times higher than surrounding regions and located at the intersection of high wind speeds and elevated seawater Hg0. Modeling results show that high Hg inputs from enhanced precipitation in the ITCZ combined with the shallow ocean mixed layer in this region drive elevated seawater Hg0 concentrations. Modeled seawater Hg0 concentrations reproduce observed peaks in the ITCZ of both the Atlantic and Pacific Oceans but underestimate its magnitude, likely due to insufficient deep convective scavenging of oxidized Hg from the upper troposphere. Our results demonstrate the importance of scavenging of reactive mercury in the upper atmosphere driving variability in seawater Hg0 and net Hg inputs to biologically productive regions of the tropical ocean.单质汞（elemental mercury, Hg⁰）的海气交换是全球汞生物地球化学循环的关键组成部分。为深入厘清大气与海洋中单质汞的时空变异性，我们在太平洋海域（20°N~15°S）布设了高分辨率观测序列，覆盖了海水温度、盐度及初级生产力的大范围梯度区间。本研究采用海洋通用环流模型（MITgcm）与大气化学传输模型（GEOS-Chem），对海洋表层汞的输入与流失过程开展了数值模拟。观测结果显示，表层海水单质汞浓度的变异性远高于大气汞浓度。在太平洋热带辐合带（Intertropical Convergence Zone, ITCZ）中观测到的表层海水单质汞浓度峰值约为130飞摩尔每升（fM），较周边海域（约50 fM）高出约3倍，该特征与大西洋海域的观测结果一致。北太平洋热带辐合带的单质汞海气逸出通量峰值较周边区域高出4倍，且该峰值出现在高风速与高海水单质汞浓度的交汇区域。模拟结果表明，热带辐合带内降水增强带来的汞输入增量，与该海域较浅的海洋混合层共同作用，推高了表层海水单质汞浓度。模拟得到的海水单质汞浓度能够复现大西洋与太平洋热带辐合带的观测浓度峰值，但对峰值幅度存在低估，这一偏差大概率源于对流层上层氧化态汞的深层对流清除作用不足。本研究结果证实，上层大气中活性汞的清除过程，是驱动热带海洋高初级生产力海域海水单质汞变异性及汞净输入通量的核心机制。