Spatially and temporally dense measurements reveal meteorological driver of atmospheric mercury variability

Eric Michael Roy , David A. Gay, Noelle Eckley Selin 

The environmental fate of mercury (Hg) is determined by its atmospheric processing, yet the relative role of surface fluxes, chemistry, and transport on atmospheric loadings remains poorly understood. We use multiyear gaseous elemental Hg (Hg0) concentration measurements at two rural sites and two urban sites in the northeastern US coordinated by the National Atmospheric Deposition Program (NADP) together with models to identify drivers of diel variability. We observe that Hg0 concentrations have similar magnitude and variability between measurement sites within urban or rural regions, with highest concentrations in the urban region. Concentrations within the rural and urban regions tend to converge at midday, reaching a maximum at rural sites and a minimum at urban sites, particularly in summer. Using a boundary layer box model and the GEOS-Chem chemical transport model, we show that this midday convergence is driven by the combined influence of regional surface fluxes and the entrainment of free tropospheric air, rather than in-situ chemical fluxes. Our results demonstrate that diel Hg0 concentration variability can serve as a constraint on free tropospheric loadings of Hg0 and can help identify biases in regional and global atmospheric budgets of Hg.