Modeling Coastal Urban Atmospheric Mercury Emissions and Regional Transport in Boston, MA

Mercury can have serious health and ecological consequences, but the chemistry, transport and deposition of gaseous elemental mercury (Hg(0)) in the atmosphere are still not well understood. Estimates of anthropogenic emissions also remain uncertain. To better constrain urban- and regional-scale chemistry and emissions, we measured concentrations of Hg(0) at an urban site in Boston, MA from Aug 2017 to Sept 2018. Average concentrations of Hg(0) at this site were 1.38 +/- 0.21 ng/m3. These measurements were evaluated with comparison to high resolution meteorological data from NOAA’s HRRR model. Periods of stagnant conditions and low atmospheric mixing, taken to be times at which wind speed and boundary layer height were at least a standard deviation lower than average, correlated with a buildup of Hg(0) to concentrations of 1.64 +/- 0.26 ng/m2 and peaks in the recorded data. Further, we used NOAA’s HYSPLIT model to calculate back trajectories for air masses arriving at the emission site and deduce the parcel origin. Ocean re-emissions contributed to higher Hg(0) levels in Boston (1.51 +/- 0.22 ng/m3), whereas air parcels originating from the Midwestern US largely contained only background-level concentrations (1.33 +/- 0.17 ng/m3), when controlling for the effect of stagnant conditions. Results from a box model analysis show the sensitivity of local Hg(0) concentrations to varying assumptions of mixing, background flux, meteorology, and emissions. We use the results to suggest how further measurements can be used to identify major sources and understand how effective local policy could be in reducing mercury.