AEROSOL_NOTES - 15 Mar 2006


1.  ISORROPIA.  Recomputed tabulated Kusik-Meissner binary activity 
    coefficients:

    The ISORROPIA aerosol equilibrium model can be run either in "online" 
    mode, in which activity coefficients are computed each time it is 
    called, or in "tabulated" mode, which instead uses lookup tables for 
    the activity coefficients of each of 13 salts. The tabulated mode is 
    used in the default configuration of CMAQ. There are six sets of 
    tables, computed for ambient temperatures of 198, 223, 248, 273, 298, 
    and 323 K.  Each table is indexed by ionic strength.  
  
    There were some inconsistencies between the online computation method 
    and the tabulated activity coefficients.  The tabulated coefficients 
    have been recalculated by calling ISORROPIA in online mode for each of 
    the above six temperatures and for the midpoint of each discretization 
    interval of the ionic strength.
  
    In addition, 10**GAMA has been replaced by EXP(LN10*GAMA) in subroutine 
    CALCACT for computational efficiency.

    The aerosol nitrate predictions are frequently lower as a result of 
    these changes, especially during winter, and are more numerically 
    stable. 

2.  Aerosol processing in clouds:

    A problem was identified in processing the aerosol size distribution 
    through the aqueous chemistry module.  In the past, aerosol species EC 
    and A25 were lumped together into aqchem species "PRI" and species 
    AORGA, AORGB, and AORGPA were lumped into aqchem species "ORG".  The 
    species lumping caused a problem because: (1) the molecular weights are 
    different for each species, affecting the 3rd moment calculation within 
    AQCHEM; and (2) secondary organics were included in the 3rd moment 
    calculation, which is later used to update the 2nd moment...but in the 
    current CMAQ, the 2nd moment is transported without the secondary 
    organic aerosols (see subroutine GETPAR in the aerosol module); only 
    primary organics contribute to the 2nd moment during transport.  To 
    correct this problem, modeled aerosol species were added to AQCHEM for 
    elemental carbon, primary organic, biogenic secondary organic, and 
    anthropogenic secondary organic.  In addition, the aerosol "3rd moment" 
    calculation was updated to include EC and primary organics (not 
    secondary).
  
    All of the chemical mechanisms have been modified to include the 
    mapping of aerosols to the new aqueous species (EC, ORGA, ORGB, ORGP). 
    Changes are isolated to include file AE_A2AQ.EXT.
  
    In our testing, the standard deviation of the aerosol size distribution 
    is reduced somewhat and less often approaches the maximum value of 
    2.5.  In response to the change in standard deviation, the geometric 
    mean diameter of the aerosol size distribution is increased.  Because 
    of the changes in the aerosol distribution, generally the particle dry 
    deposition was reduced and sulfate air concentrations were slightly 
    increased.