CMAQv5.0 Bidirectional Exchange of Hg

Bidirectional Hg exchange release notes September 27th 2011

Jesse Bash

Emissions of elemental Mercury Hg⁰ from natural processes are estimated to be as large as or larger than anthropogenic emissions and thus represent a critical process which must be accurately described in the modeling of the transport and fate of mercury. Recent ecosystem‐scale measurements indicate that a fraction of recently deposited mercury is recycled back into the atmosphere on a timescale from days to months (Graydon et al. 2006), and that an atmospheric compensation point, i.e. an ambient concentration below which there is a net evasive flux and above which there is a net deposition flux, exists at background ambient concentrations (Bash et al. 2009). Modeled Hg⁰ emissions from natural sources are typically uncoupled from dry deposition estimates and unconstrained by air‐biosphere gradient processes. A module has been developed for the Community Multiscale Air Quality (CMAQ) model to parameterize concentration dependent processes of bidirectional mercury exchange. The Hg⁰ air‐surface exchange was modeled as a function of a dynamic compensation point. The compensation point is modeled as a function of sources and sinks of Hg⁰ in vegetation and soil media using partitioning coefficients.

Emissions

CMAQ v5.0 with bidirectional Hg⁰ exchange estimates emissions in-line, thus Hg⁰ emissions from direct natural, except volcanic and forest fire emissions, and recycled/re-emitted sources should be removed from the emissions inventory to avoid double counting of this source.

Run time configuration

Hg⁰ bidirectional flux is implemented by setting the CTM_HGBIDI environment variable to Y and by including Hg as a deposited species in the name list for gas phase species.

Set ASXfile to the output path for the additional CMAQ output file with the soil, vegetation, and water concentrations of Hg species.

1. INIT_MEDC_1 - Set this to point to the previous days ASXfile to initialize the soil, vegetation, and surface water Hg species concentrations.

Effects of Changes:

Hg⁰ dry deposition is reduced when using the Hg⁰ bidirectional flux option.

Example of Run Script Modifications:

...
# Algorithm for bi-directional flux for gaseous mercury species [ N|F ]
# Default value is false or no
setenv CTM_HGBIDI Y
...
# output files
...
set ASXfile = $EXEC"_"$EPI.MEDIA.${APPL}
...
set ICFILE = $S_CGRID
set MD_ICFILE = $OUTDIR/$ASXfile

References

Bash, J.O., 2010 Description and initial simulations of a dynamic bidirectional air-surface exchange model for mercury in Community Multiscale Air Quality (CMAQ) model, Journal of Geophysical Research, 115, D06305

Bash, J.O., D.R. Miller, 2009, Growing season total gaseous mercury (TGM) flux measurements over an Acer rubrum L. stand, Atmos. Envrion. 43, 5953-5961

Graydon, J. A., V. L. St. Louis, S. E. Lindberg, H. Hintelmann, and D. P. Krabbenhoft, 2006, Investigation of mercury exchange between forest canopy vegetation and the atmosphere using a new dynamic chamber, Environ. Sci. Technol., 40(15), 4680– 4688.