A Synthesis of Global Observations on the Air-Surface Exchange of Elemental Mercury Vapor

English Abstract:

Both the atmosphere and earth surface are transient reservoirs of mercury (Hg) in the environment. Reliable quantification of the air-surface exchange of elemental mercury vapor (Hg⁰) is crucial for understanding the global biogeochemical cycling of Hg. There have been extensive measurement and modeling efforts made to estimate the exchange fluxes between the atmosphere and various surfaces (e.g., soil, canopy, water, snow, etc.). However, large uncertainty remains in these estimates due to the complexity of Hg⁰ bi-directional exchange, the limitations of flux quantification techniques and the challenges in model parameterization. Due to the semi-volatile nature of Hg⁰ and poorly understood redox transformation of Hg in different environmental media, the flux of deposition and re-volatilization of previously deposited Hg exhibits distinct characteristics subject to the influence of environmental variables such as natural surface types, temperature, light, moisture, atmospheric turbulence, presence of reactants that facilitate Hg redox chemistry, and relative levels of Hg in the media where Hg⁰ exchange occurs. In this study, we provide an integrated synthesis on the current state of scientific understanding in the air-surface exchange of Hg⁰. Specifically, the development of advancement of flux quantification methods, existing field observational data of Hg⁰ flux, environmental factors driving the exchange process, and modeling efforts to scale up the measured flux for global assessment are presented. The existing knowledge gap and the associated research need to further understand the air-surface exchange of Hg⁰ are discussed.