Elemental Mercury Concentrations and Fluxes in the Tropical Atmosphere and Ocean

Air-sea exchange of elemental mercury (Hg0) in the open ocean is an important 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 temperature, salinity, and productivity in the Pacific Ocean (20°N-15°S). Surface seawater Hg0 was much more variable than atmospheric concentrations. Peak seawater Hg0 (~130 fM) observed in the inter-tropical convergence zone (ITCZ) were ~3-fold greater than surrounding areas (~50 fM), and were comparable to latitudinal gradients found in the Atlantic Ocean. Peak evasion in the northern ITCZ was four times higher than surrounding oceanographic regimes and located where high wind speed and elevated seawater Hg0 coincided. A modeling analysis using the MITgcm-Hg (3D ocean circulation model) and atmospheric inputs from the GEOS-Chem global Hg model (3D biogeochemical model) suggests that higher Hg inputs from enhanced precipitation in the ITCZ combined with the shallow ocean mixed layer in this region can explain observations. Modeled seawater Hg0 reproduced the observed seawater Hg0 peaks in the ITCZ of the Atlantic and Pacific Oceans but underestimated the magnitude, likely due to insufficient deep convective scavenging of oxidized Hg from the upper troposphere in the model.