Nested-grid simulation of mercury over North America

Abstract

We have developed a new high-resolution (1/2° latitude by 2/3° longitude) nested-grid mercury (Hg) simulation over North America employing the GEOS-Chem global chemical transport model. Emissions, chemistry, deposition, and meteorology are self-consistent between the global and nested domains. Compared to the global model (4° latitude by 5° longitude), the nested model shows improved skill at capturing the high spatial and temporal variability of Hg wet deposition over North America observed by the Mercury Deposition Network (MDN) in 2008–2009. The nested simulation resolves features such as land/ocean contrast and higher deposition due to orographic precipitation, and predicts more efficient convective rain scavenging of Hg over the southeast United States. However, the nested model overestimates Hg wet deposition over the Ohio River Valley region (ORV) by 27%. We modify anthropogenic emission speciation profiles in the US EPA National Emission Inventory (NEI) to account for the rapid in-plume reduction of reactive to elemental Hg (IPR simulation). This leads to a decrease in the model bias to +3% over the ORV region. Over the contiguous US, the correlation coefficient (<i>r</i>) between MDN observations and our IPR simulation increases from 0.63 to 0.78. The IPR nested simulation generally reproduces the seasonal cycle in surface concentrations of speciated Hg from the Atmospheric Mercury Network (AMNet) and Canadian Atmospheric Mercury Network (CAMNet). In the IPR simulation, annual mean reactive gaseous and particulate-bound Hg are within 80% and 10% of observations, respectively. In contrast, the simulation with unmodified anthropogenic Hg speciation profiles overestimates these observations by factors of 2 to 4. The nested model shows improved skill at capturing the horizontal variability of Hg observed over California during the ARCTAS aircraft campaign. We find that North American anthropogenic emissions account for 10–22% of Hg wet deposition flux over the US, depending on the anthropogenic emissions speciation profile assumed. The percent contribution can be as high as 60% near large point emission sources in ORV. The contribution for the dry deposition is 13–20%

Gas-particle partitioning of atmospheric Hg(II) and its effect on global mercury deposition

“…Here we follow the emission speciation of Y. Zhang et al (2011), which is consistent with the above studies and greatly reduces the model bias compared to observations of RGM and PBM (see Sect. 4).…”

“…Several model studies also provide support for in-plume reduction of Hg(II) emitted from power plants (Seigneur et al, 2003;Lohman et al, 2006;Seigneur et al, 2006;Vijayaraghavan et al, 2008;Kos et al, 2011;Y. Zhang et al, 2011).…”

“…Y. Zhang et al (2011) use a 86.5:9.9:3.6 (1) and (2), not including partitioning into sea-salt aerosol which is treated separately following Holmes et al (2010).…”

Relative contributions of gaseous oxidized mercury and fine and coarse particle‐bound mercury to mercury wet deposition at nine monitoring sites in North America

“…It is possible that GOM and FPBM were scavenged by dry deposition before the precipitation event occurred because of relatively higher contributions of GOM and PBM to Hg dry deposition at MD08 and OH02 than sites without point sources [Zhang et al, 2012b]. These results share similar findings with modeled Hg deposition using the GEOS-Chem model that suggested domestic anthropogenic sources only contributed 10-22% of the total Hg wet deposition in the U.S. [Zhang et al, 2012c]. Aside from local Hg point sources, other sources and processes may affect the air concentrations of GOM and FPBM, which subsequently affects %GOM and %FPBM.…”

“…Mercury transport models have been evaluated using monitored wet deposition data; however, most models do not include coarse PBM (CPBM) in the deposition budget since particle-bound Hg is typically treated as a fine aerosol [Selin et al, 2007;Bullock et al, 2008;Pongprueksa et al, 2008;Amos et al, 2012;Holloway et al, 2012;Zhang et al, 2012a;Kos et al, 2013]. Although most model results are comparable to mercury wet deposition measurements, the omission of CPBM is another potential explanation for the overpredicted concentrations of GOM and fine PBM (<2.5 μm) in many mercury transport models [Amos et al, 2012;Holloway et al, 2012;Kos et al, 2013;Zhang et al, 2012aZhang et al, , 2012c. Measurements of CPBM indicate that the coarse Hg fraction of total particulate mercury at two sites in New Hampshire can range from 0.35 to 0.6 depending on the season and year sampled [Feddersen et al, 2012].…”