Golam Sarwar, Sergey Napelenok, Havala Pye, Prakash Bhave, Shawn Roselle, Jeff Young, Kathleen Fahey
Replaced ISORROPIAv1.7 with ISORROPIAv2.1 (Fountoukis and Nenes, 2007). In addition to sulfate, nitrate, chloride, ammonium, and sodium, ISORROPIAv2.1 treats the thermodynamics of ions commonly found in crustal material: Ca2+, K+, and Mg2+. While it solves for the same number of gas-phase components, the number of liquid-phase components that it solves for has increased from 12 to 15. It also incorporates optimizations in activity coefficient calculations to minimize the impact on model runtime. The AERO6 module in CMAQv5.0 speciates PMother into several aerosol species including Mg2+, K+, Ca2+ (please see AERO6 notes for details). The ISORROPIAv2.1 takes advantage of the newly added aerosol species in CMAQ and their thermodynamics for calculating the partitioning of species into gas and aerosol phases.
Along with ISORROPIAv2.1, the sea-salt emission speciation in CMAQ has been updated. In CMAQv4.7.1, sea-salt emissions were speciated into Na+, Cl-, and SO42-. The Na+ species was actually a sum of all sea-salt cations. In AERO6, sea-salt emissions in the accumulation mode are speciated into Na+, Cl-, SO42-, Ca2+, K+, and Mg2+ following Millero, 1996. However, all cations in coarse mode sea-salt (i.e., Na+, Ca2+, K+, and Mg2+) are lumped into a species called ASEACAT. The molecular weight of ASEACAT is calculated as the weighted average of sea-salt cations.
Similar to the previous versions of the CMAQ model, AERO6 uses ISORROPIA in the “reverse mode” to calculate the condensation/evaporation of volatile inorganic gases to/from the gas-phase concentrations from known coarse particle surfaces. However, in addition to the coarse-mode aerosol concentrations of Na+, SO42-, NH4+, NO3-, and Cl-, it now uses the coarse mode aerosol concentrations of Ca2+, K+, and Mg2+ to compute the inorganic flux rates. For thermodynamic calculations in ISORROPIAv2.1, coarse-mode Na+, Ca2+, K+, and Mg2+ concentrations are calculated from ASEACAT, windblown dust, and anthropogenic coarse material using prescribed speciation factors. Speciation of anthropogenic coarse material into anions is accounted for in the emissions module.
Similar to the previous versions of the CMAQ model, AERO6 uses ISORROPIA in the “forward mode” to calculate instantaneous thermodynamic equilibrium between the gas phase and fine particle modes. However, in addition to the aerosol concentrations of Na+, SO42-, NH4+, NO3-, Cl-, it now also uses the aerosol concentrations of Ca2+, K+, and Mg2+ to determine these equilibria. For thermodynamic calculations in ISORROPIAv2.1, fine-mode Na+, Ca2+, K+, and Mg2+ concentrations, available in AERO6, are used. Updates to the sea-salt emissions speciation (see above) are accounted for when calculating fine-mode Na+, Ca2+, K+, and Mg2+ concentrations.
In previous versions of the CMAQ model, aerosol liquid water content is calculated using the subroutine CALC_H2O that employs algorithms from ISORROPIAv1.7. In AERO6, CALC_H2O and associated subroutines (GETSC and GETM0I) have been updated based on algorithms in ISORROPIAv2.1.
Impact of the changes
In winter, they increase coarse nitrate in areas with eleavated crustal materials and decrease fine nitrate. Their impact on fine sulfate and coarse sulfate is small. They decrease ammonium. Impact on ozone is small.
Model sensitivity runs were conducted using ISORROPIAv1.7 and v2.1 by perturbing EC emissions in one grid-cell in the modeling domain. Predictions with the perturbed EC emissions were compared to the predictions without the perturbed emissions. For the model with ISORROPIAv1.7, predicted nitric acid, aerosol nitrate, ammonia, and ammonium with perturbed EC emissions differed from those without the perturbed emissions. However, for the model with ISORROPIAv2.1, differences in the predictions of these compounds between the perturbed and non-perturbed EC emissions were significantly smaller.
Fountoukis C. and Nenes, A., 2007. ISORROPIA II: a computationally efficient thermodynamic equilibrium model for K+-Ca2+-Mg2+-NH4+-Na+-SO42- -NO3- -Cl--H2O aerosols. Atmospheric Chemistry and Physics, 7, 4639-4659.
Millero, F. J., 1996. Chemical Oceanography, second ed. CRC Press, Boca Raton, FL.
isocom.f, isofwd.f, isorev.f, isrpia.inc, aero_subs.f, AERO_DATA.F, AERO_EMIS.F