Air-sea exchange of gaseous elemental mercury (Hg⁰) is not well constrained, even though it is a major component of the global Hg cycle. Lack of Hg⁰ flux measurements to validate parameterizations of the Hg⁰ transfer velocity contributes to this uncertainty. We measured the Hg⁰ flux on the Baltic Sea coast using micrometeorological methods (gradient-based and relaxed eddy accumulation [REA]) and also simulated the flux with a gas exchange model. The coastal waters were typically supersaturated with Hg⁰ (mean ± 1σ = 13.5 ± 3.5 ng m⁻³; ca. 10% of total Hg) compared to the atmosphere (1.3 ± 0.2 ng m⁻³). The Hg⁰ flux calculated using the gas exchange model ranged from 0.1–1.3 ng m⁻² h⁻¹ (10th and 90th percentile) over the course of the campaign (May 10–June 20, 2017) and showed a distinct diel fluctuation. The mean coastal Hg⁰ fluxes determined with the two gradient-based approaches and REA were 0.3, 0.5, and 0.6 ng m⁻² h⁻¹, respectively. In contrast, the mean open sea Hg⁰ flux measured with REA was larger (6.3 ng m⁻² h⁻¹). The open sea Hg⁰ flux indicated a stronger wind speed dependence for the Hg⁰ transfer velocity compared to commonly used parameterizations. Although based on a limited data set, we suggest that the wind speed dependence of the Hg⁰ transfer velocity is more consistent with gases that have less water solubility than CO₂ (e.g., O₂). These pioneering flux measurements using micrometeorological techniques show that more such measurements would improve our understanding of air-sea Hg exchange.

中文翻译：

---

气态元素汞气海交换的重要观察

气态元素汞 (Hg ⁰ )的海气交换并没有受到很好的限制，尽管它是全球汞循环的一个主要组成部分。缺乏 Hg ⁰通量测量来验证 Hg ⁰传输速度的参数化导致了这种不确定性。我们使用微气象学方法（基于梯度和松弛涡流积累 [REA]）测量了波罗的海沿岸的 Hg ⁰通量，并使用气体交换模型模拟了通量。与大气 (1.3 ± 0.2 ng m ^-3 )相比，沿海水域通常被 Hg ⁰过饱和（平均值 ± 1 σ  = 13.5 ± 3.5 ng m ^-3；约占总 Hg 的 10% ）。汞⁰在整个活动过程中（2017 年 5 月 10 日至 6 月 20 日），使用气体交换模型计算的通量范围为 0.1-1.3 ng m ^-2 h ^-1（第 10 个和第 90 个百分位数），并显示出明显的日间波动。用两种基于梯度的方法和 REA 确定的平均沿海 Hg ⁰通量分别为 0.3、0.5 和 0.6 ng m ^-2 h ^-1。相比之下，用REA 测量的平均公海Hg ⁰通量更大（6.3 ng m ^-2 h ^-1）。外海汞柱⁰通量指示用于汞更强的风速依赖性⁰传输速度与常用参数化相比。尽管基于有限的数据集，我们建议 Hg ⁰传输速度的风速依赖性与水溶性低于 CO _{2 的}气体（例如 O ₂）更一致。这些使用微气象技术的开创性通量测量表明，更多此类测量将提高我们对海气汞交换的理解。