Multi-Pollutant Model - 30 September 2008 Version 4.7 of the CMAQ provides a version of the model that can simulate the atmospheric fate of mercury compounds and other Hazardous Air Pollutants (HAPs). Roselle et al. 2007 presented a prototype of this model and called its mechanism CB05TXHG_AE4_AQ. The mechanism included the same mercury compounds and HAPs in the CB05 based mechanisms for mercury compounds and HAPs in CMAQ version 4.6. Version 4.7 upgrades this prototype based on the new science options available. The upgrade had to modify new options for the photochemical mechanism, aerosol physics, cloud chemistry and vertical diffusion. The result produced a new mechanism called CB05TXHG_AE5_AQ or the Multiple Pollutant mechanism and specific option settings needed to build and run this version of the CMAQ model. The following paragraphs describe the Multiple Pollutant model and the changes to standard build setting in CMAQ version 4.7 In the Multi-Pollutant model, phase, i.e., gas or aerosol, determines the chemical and physical processes that they undergo. Each HAPs is transported and deposited. Wet deposition is determined by the Henry's Law Constant or scavenging rate of the aerosol mode. Aerosol mode also determines dry deposition velocity for aerosol phase pollutants. For the gas phase HAPS, dry deposition has a nonzero velocity if the EPI Suite program (USEPA, 2005) and the SPECTRUM Laboratory database (http://www.speclab.com/price.htm) indicate dry deposition as a fate determining process. Check the NR_DEPV.EXT and GC_DEPV.EXT files for which gas phase HAPs undergo dry deposition. In version 4.7, five HAPs in the NR species have explicitly calculated velocities. Their values are calculated within simulations if the model is compiled the acm2_inline_txhg option and if the environment variable, CTM_ILDEPV, is set to T (true) or Y (yes). MCIP version 3.4 also provides the velocities in the METCRO2D files when the MCIP is run calculates maximum number of deposition velocities. The CB05TXHG_AE5_AQ mechanism adapts the CB05CL_AE5_AQ mechanism by adding the reactions for mercury compounds and reactive tracers in the CB05HG_AE4_AQ and CB05CLTX_AE4_AQ mechanisms in CMAQ versions 4.6. The new mechanism also adds a reaction between elemental mercury and monatomic chlorine that produces HGIIGAS (Donohoue et al. 2005). The nature of reaction changes the definition of the model species from divalent gaseous mercury because the new reaction should produce monovalent gaseous mercury. To uphold the point, the definition of HGIIGAS becomes reactive gaseous mercury representing sum of the two valence states. The monovalent state is assumed quickly converted into the divalent state but the model physics does not represent the process. Two methods compute the chemical transformation of gas phase HAPs (Table 1). The first is done within the standard numerical solver for ozone and radical chemistry such as the Euler Backward Iterative solver (Hertel et al., 1993). The chemical reactions are listed in the mech.def file. The method may affect the solution for ozone and radical concentrations if the pollutant has high enough concentrations. The second method estimates loss from chemical reactions based on the solution from ozone and radical chemistry and does not alter the ozone and radical concentrations. Luecken et al. (2006) describes both methods. The first method mentioned above treats two types of model species. Type one destroys and produces model species influencing ozone and radical concentrations. Formaldehyde, acetaldehyde, benzene and elemental mercury belong to type one. Type two does not alter ozone and radical concentrations and serves as tracers of emitted pollutants. Tracers for formaldehyde, acetaldehyde, and acrolein emissions allow determining photochemical production of the given pollutant. The type two method also applies to emissions tracers for toluene, alpha-pinene, beta-pinene and three xylene isomers (Table 2). Adapting the new aerosol module resulted from adding aerosol species representing mercury and other toxic metals (Table 3). Based on the emissions of these pollutants, concentrations of the added species should be minor regarding the bulk composition of aerosols. Their concentrations then are not used to determine the rates of aerosol microphysics and deposition but they do coagulate and mode merge. The species representing particulate mercury differ from the other metallic aerosols species because photochemistry produces mass for particulate mercury. The gas species, HGIIAER, represents the produced mass. The mass goes directly into the accumulation mode and does not divided between the fine modes unlike the CB05HG_AE4_AQ mechanism in CMAQ version 4.6. The change assumes that the surface area of the accumulation mode dominates condensation onto aerosol modes. Adapting the cloud module added the in-cloud scavenging for metallic aerosol species and the cloud chemistry for mercury compounds. For the metallic species, the method follows the same approach as elemental carbon in the fine modes and unidentified material in the coarse mode. For mercury, in-cloud chemistry follows the method outlined in Bullock and Brehme (2002) and the mercury release notes for CMAQ version 4.6. Although the reactions involving mercury do not directly change other aqueous species, mercury chemistry can alter particulate sulfate predictions for two reasons. First, solving the modified chemistry changes the minimum time step used in the numerical solution if the mercury reactions have the fastest rate of change. Second, the mercury chemistry requires using the gas phase HO2, HOCl and Cl2. The three gas species affect pH and ion balance in cloud droplets based on Lin et al. (1998). Side effects increase wet deposition of each compound and possibly produce gaseous HOCL from clouds with low or no participation. See release notes on Hazardous Air Pollutants about the cloud chemistry for trivalent and hexavalent compounds. To use vertical diffusion that calculates dry deposition velocities, biogenic emissions and plume rise, the two changes revised the new vdiff option. First, reading aerosol emissions adds routines for particulate mercury and other metals. Second, calculation of emissions includes a constant source of molecular chlorine over open oceans. The source is set off by default. The run script turns the chlorine source on by setting the environment variable, CTM_CL2_SEAEMIS, to true. This source mimics production of molecular chlorine observed in the marine boundary layer that may come from the heterogeneous chemistry of sea salt aerosols (Spicer et al. 1998). Knipping and Dadbub (2002 and 2003) have proposed a reaction mechanism for the production. We do not attempt to use the mechanism because its reaction efficiencies are not well defined and because the CB05CL_AE5_AQ mechanism does not include all the nitro- chlorine compounds needed. As mention above, building CMAQ with the Multi-Pollutant mechanism requires different build settings than the standard version of CMAQ. Table 4 shows the build settings needed to construct CCTM using this mechanism with its EBI solver. Settings not specified in Table 4 remain the same as the standard version. NOTE that the smvgear and ros3 options for the chem module also work for this mechanism. NOTE: You must use the I/O API version 3.1beta or newer to support the larger number of variables required by the Multipollutant version of CMAQ. To run the CMAQ with the Multi-Pollutant mechanism, the user needs emissions files containing rates listed in the GC_EMIS.EXT, NR_EMIS.EXT and AE_EMIS.EXT files. The files contain emissions that are not identical to the original CB05CL mechanisms. A user must complete SMOKE processing with correct ancillary files such as GSREF and GSPRO and the merged NEI/Toxics database. To obtain these items contact the CMAS Help desk at www.cmascenter.org. References Bullock, O. R. and K. A. Brehme, 2002. Atmospheric mercury simulation using the CMAQ model: formulation description and analysis of wet deposition results. Atmospheric Environment, 36, 2135-2146.. Donohoue, D.L., Bauer, D. and Hynes, A.J., 2005. Temperature and Pressure Dependent Rate Coefficients for the Reaction of Hg with Cl and the Reaction of Cl with Cl: A Pulsed Laser Photolysis-Pulsed Laser Induced Fluorescence Study Journal of Physical Chemistry A, 109, 7732-7741. Hertel, O., R. Berkowicz and J. C. Hov, 1993. Test of two numerical schemes for use in atmospheric transport-chemistry models. Atmospheric Environment, 27, 2591-2611. Knipping, E. M.and Dabdub, D. J., 2003. Impact of Chlorine Emissions from Sea-Salt Aerosol on Coastal Urban Ozone. Environmental Science and Technology 2003, 37, 275-284. Knipping, E. M.and Dabdub, D. J., 2002. Modeling Cl2 formation from aqueous NaCl particles: Evidence for interfacial reactions and importance of Cl2 decomposition in alkaline solution. Journal of Geophysical Research, 107 (D18), 4360-4390. Lin, C.-J. and Pehkonen, S.O., 1998. Oxidation of elemental mercury by aqueous chlorine: implications for tropospheric mercury chemistry. Journal of Geophysical Research, 103 (D21), 28,093-28,102. Luecken, D. J., W. T. Hutzell and G. L. Gipson 2006. Development and analysis of air quality modeling simulations for hazardous air pollutants. Atmospheric Environment, 40, 5087-5096. Sarwar, G., D. Luecken, G. Yarwood, G. Whitten, B. Carter, 2008. Impact of an updated Carbon Bond mechanism on air quality using the Community Multiscale Air Quality modeling system: preliminary assessment. Journal of Applied Meteorology and Climatology, 47, 3-14. Spicer, C. W., Chapman, E. G., Finlayson-Pitts, B. J., Plastridge, R. A., Hubbe, J. M., Fast, J. D. and Berkowitz, C. M., 1998. Unexpectedly high concentrations of molecular chlorine in coastal air. Nature, 394, 353-356. US Environmental Protection Agency, cited 2005. Estimations Programs Interface for Windows (EIPWIN), version 3.12. Available online at http://www.epa.gov/opptintr/exposure/pubs/episuitedl.htm Tanaka P.L., D.T. Allen, E.C. McDonald-Buller, S. Chang, Y. Kimura, G. Yarwood and J.D. Neece, 2003. Development of a chlorine mechanism for use in the carbon bond IV chemistry model. Journal of Geophysical Research, 108, 4145. Yarwood, G., S. Rao, M. Yocke, and G.Z. Whitten, 2005. Updates to the Carbon Bond Mechanism: CB05. Report to the U.S. Environmental Protection Agency, RT-04-00675. Available online at http://www.camx.com/publ/pdfs/CB05_Final_Report_120805.pdf. Table 1 Gas Phase HAP Species in CB05TXHG_AE5_AQ ===================================================================== Species Name Compound CAS# In mech.def ===================================================================== FORM total FORMALDEHYDE 50-00-0 Yes ALD2 total ACETALDEHYDE 75-07-0 Yes BENZENE BENZENE 71-43-2 Yes ACROLEIN total ACROLEIN 107-02-8 Yes BUTADIENE13 1,3-BUTADIENE 106-99-0 Yes HG Elemental Mercury NA Yes HGIIGAS Reactive Gaseous Mercury NA Yes HGIIAER Particulate Mercury Precursor NA Yes FORM_PRIMARY FORMALDEHYDE emissions 50-00-0 Yes ALD2_PRIMARY ACETALDEHYDE emissions 75-07-0 Yes ACROLEIN_PRIMARY ACROLEIN emissions 107-02-8 Yes ACRYLONITRILE ACRYLONITRILE 107-13-1 No CARBONTET CARBON TETRACHLORIDE 56-23-5 No PROPDICHLORIDE PROPYLENE DICHLORIDE 78-87-5 No DICHLOROPROPENE 1,3-DICHLOROPROPENE 542-75-6 No CL4_ETHANE1122 1,1,2,2TETRACHLOROETHANE 79-34-5 No CHCL3 CHLOROFORM 67-66-3 No BR2_C2_12 1,2DIBROMOETHANE 106-93-4 No CL2_C2_12 1,2DICHLOROETHANE 107-06-2 No ETOX ETHYLENE OXIDE 75-21-8 No CL2_ME METHYLENE CHLORIDE 75-09-2 No CL4_ETHE PERCHLOROETHYLENE 127-18-4 No CL3_ETHE TRICHLOROETHYLENE 79-01-6 No CL_ETHE VINYL CHLORIDE 7501-4 No NAPHTHALENE NAPHTHALENE 91-20-3 No QUINOLINE QUINOLINE 91-22-5 No HYDRAZINE Hydrazine 302-01-2 No TOL_DIIS 2,4-Toluene Diisocyanate 584-84-9 No HEXAMETHY_DIIS Hexamethylene 1,6-Diisocyanate 822-06-0 No MAL_ANHYDRIDE Maleic Anhydride 108-31-6 No TRIETHYLAMINE Triethylamine 121-44-8 No DICHLOROBENZENE 1,4-Dichlorobenzene 106-46-7 No ===================================================================== Table 2. Additional Gas Phase Species in CB05TXHG_AE5_AQ ===================================================================== Species Name Compound CAS# In mech.def ===================================================================== TOLU Toluene Emissions 108-88-3 YES MXYL M-Xylene Emissions 108-38-3 YES OXYL O-Xylene Emissions 95-47-6 YES PXYL P-Xylene Emissions 106-42-3 YES APIN Alpha-Pinene Emissions 80-56-8 YES BPIN Beta-Pinene Emissions 127-91-3 YES ===================================================================== Table 3. Aerosol Phase HAP species in CB05TXHG_AE5_AQ (Note that species exist in each aerosol mode) ===================================================================== String in Aerosol Species Represents PHG Mercury Compounds BE Beryllium Compounds NI Nickel Compounds CR_III Chromium (III) Compounds CR_VI Chromium (VI) Compounds PB Lead Compounds MN Manganese Compounds CD Cadmium Compounds DIESEL Diesel Emissions ===================================================================== Table 4. Option setting needed in CCTM build scrip if using EBI solver. NOTE that unspecific options remain same as CCTM with aerosols. ===================================================================== #Select a HAP mechanism set Mechanism = cb05txhg_ae5_aq #VDIFF has two options set ModVdiff = ( module acm2_txhg $Revision; ) # OR the standard CCTM setting set ModVdiff = ( module acm2_inline_txhg $Revision; ) # Select correct EBI solver # NOTE THAT ros3 and smvgear options also work set ModChem = ( module ebi_cb05txhg_ae5 $Revision; ) #AERO option required set ModAero = ( module aero5_txhg $Revision; ) #cloud processing and aqueous chemistry setting set ModCloud = ( module cloud_acm2_ae5_txhg $Revision; ) =====================================================================