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CMAQ Aerosol Module
Research Programs
CMAQ has been used extensively in recent years to simulate ambient fine particle concentrations for both regulatory and research-grade applications. To keep pace with advancements in atmospheric aerosol research and to respond to various needs of the growing CMAQ user community, the aerosol module must be updated frequently. The aerosol module incorporated into the first public release of CMAQ in June 1998 is described in detail by Binkowski and Roselle (2003). In June 2002, a new aerosol module, aero3, was released with improved treatments of secondary organic aerosol (SOA) formation, sulfate nucleation, inorganic equilibrium partitioning, and heterogeneous nitrate formation (Schere, 2002). In September 2003, Version 4.3 of CMAQ was released with updates to the SOA and heterogeneous nitrate treatments (Pleim et al. 2003). Recently, the aerosol module was enhanced for CMAQ Version 4.4, released to the public in Fall 2004.
The CMAQ aerosol module in version 4.4 has been revised to improve computational efficiency, numerical stability, and the in-line code documentation. Testing of the 2003 CMAQ release revealed that 47% of the aerosol module computational burden is spent on calculating coagulation coefficients and 13% on partitioning secondary organic aerosol (SOA) material between the gas and aerosol phases. A new subroutine was developed to calculate coagulation coefficients in a more efficient manner than the approach used previously. In the new subroutine, coagulation coefficients for particle number and aerosol third moment are calculated from analytical expressions reported by Binkowski and Shankar (1995), and the second moment coagulation coefficients are calculated using a combination of analytical expressions and correction factors similar to the approach outlined by Whitby et al. (1991). The coagulation coefficients obtained using the new subroutine are within one percent of the previous model version, but the computational efficiency of the new routine is sixty times faster. The SOA partitioning calculation involves the iterative solution of a system of quadratic equations. In the revised aerosol module, the initial guess for this iterative solution has been revised to incorporate information on SOA concentrations from the previous time step. This reduces the number of iterations required for convergence, and hence, the computational burden, by approximately sixty percent. The net effect of both improvements is a factor of two increase in computational efficiency of the CMAQ aerosol module.
Model Development
During developmental testing of previous CMAQ versions, small numerical perturbations caused by the use of different FORTRAN compilers were found to produce large (~10 ug m-3), transient impacts on aerosol-phase nitrate concentrations over the arid southwestern U.S. These effects have been attributed to numerical instabilities in the ISORROPIA thermodynamics module. In collaboration with the developer of ISORROPIA at Georgia Institute of Technology, two problems were identified and corrected in the thermodynamics module. As a result, the large numerical instabilities observed in previous CMAQ versions are now removed. Recently, a second family of numerical instabilities has been identified in the ISORROPIA module which results in small (~1 ug m-3), transient impacts on aerosol-phase nitrate concentrations in low-humidity environments. A solution to this numerical error is under investigation, with an anticipated release in FY-2005. A significant effort has been made to strengthen the in-line documentation in the CMAQ aerosol code. Every major scientific formula in the aerosol chemistry and dynamics routines has been annotated with comments that permit users to trace the formula back to an equation in a journal article or published report where it is described thoroughly. These in-line documentation enhancements should aid the CMAQ users seeking to modify scientific algorithms within the model. The above revisions are described in further detail by Bhave et al. (2004) and are incorporated into the 2004 CMAQ public release.
In addition to the aerosol module revisions for the 2004 public release, a set of diagnostic tools are under development that will enable users to probe the sources of modeled aerosol concentrations. With one tool, the ambient sulfate concentrations formed via different pathways can be determined quantitatively. For example, Figure 2-2 illustrates model calculations of the fractions of ground-level aerosol sulfate produced by aqueous-phase oxidation and gas-phase oxidation, when averaged over the month of January 2001. With a second diagnostic tool, one may calculate the contributions from individual emission source categories and/or geographic regions to the ambient primary carbonaceous aerosol burden. An example of the results obtained using this tool are shown in Figure 2-3, where the modeled concentrations of primary carbon originating from biomass combustion and food cooking are calculated and averaged over the June 15 - August 31, 1999 time period. Both of these diagnostic tools are anticipated for release in FY-2005.
REFERENCES
Bhave, P.V., S.J. Roselle, F.S. Binkowski, C.G. Nolte, S. Yu, G.L. Gipson, and K.L. Schere. CMAQ aerosol module development: recent enhancements and future plans. In The 2004 Models-3 Users Workshop, Chapel Hill, North Carolina, October 18–20, 2004. Community Modeling and Analysis System (CMAS) Center, Chapel Hill, NC (2004).
Binkowski, F.S., Roselle, S.J., 2003, Models-3 Community Multiscale Air Quality (CMAQ) model aerosol component 1. Model description. Journal of Geophysical Research, 108(D6), 4183, doi:10.1029/2001JD001409.
Binkowski, F.S. and U. Shankar. The Regional Particulate Matter Model 1. Model description and preliminary results. Journal of Geophysical Research-Atmospheres 100(D12):26191–26209 (1995).
Pleim, J., Gipson, G., Roselle, S., Young, J., 2003, New features of the 2003 release of the CMAQ model. 2nd Annual CMAS Models-3 Conference, Research Triangle Park, NC.
Schere, K.L., 2002, The Models-3/CMAQ Model: 2002 Release - New Features. 1st Annual CMAS Models-3 Conference, Research Triangle Park, NC.
Whitby, E.R., P.H. McMurry, U. Shankar, and F.S. Binkowski. Modal aerosol dynamics modeling. EPA/600/3-91/020, Atmospheric Research and Exposure Assessment Laboratory, U.S. Environmental Protection Agency, Research Triangle Park, NC, (1991).