ISORROPIA II

Overview

The ISORROPIA II package performs aerosol thermodynamical equilibrium. It partitions nitrate (HNO3 and NIT) and ammonia (NH3 and NH4) between the gas and aerosol phases. Inputs to the partitioning routine include temperature and RH. ISORROPIA II has significant benefits over previous implementations of ISORROPIA, especially for partitioning of nitrate at low RH.

Authors and collaborators:

• Thanos Nenes (Georgia Tech) -- Principal Investigator
• Havala O. T. Pye (Caltech)

From Pye et al [2009]:

ISORROPIA II [Fountoukis and Nenes, 2007] is implemented in GEOS-Chem to compute gas-aerosol equilibrium partitioning of nitric acid and ammonia. Particles in this study are not size-resolved; however, they can be generally assumed to represent PM2.5 since formation of sulfate-nitrate-ammonium on coarse mode sea salt and dust is excluded. Submicrometer-sized particles are likely to reach gas-aerosol equilibrium on time-scales less than the 1 hour computational time step used here [Meng and Seinfeld, 1996].

Sodium and chloride from accumulation mode sea salt are considered in the gas-aerosol equilibrium along with sulfate, nitrate, and ammonium. Calcium, magnesium, and potassium concentrations are not considered in the present study because of the issues with dust emissions previously mentioned. All inorganic aerosols are assumed to exist on the upper, metastable branch of the hygroscopic hysterisis curve. Although this assumption may not hold at higher altitudes in the free troposphere [Wang et al., 2008], since the focus of this study is mainly on surface-level concentrations, where humidities reach high values on a daily basis, the metastable assumption is acceptable.

--Bob Y. 15:48, 22 February 2010 (EST)

Implementation notes

As of Jan 27, 2010, ISORROPIA II is currently being implemented into GEOS-Chem v8-02-05 (version in testing).

Code structure

The main-level Code directory has now been divided into several subdirectories:

GeosCore/    GEOS-Chem "core" routines
GeosTomas/   Parallel copies of GEOS-Chem routines that reference TOMAS
GeosUtil/    "Utility" modules (e.g. error_mod.f, file_mod.f, time_mod.f, etc.
Headers/     Header files (define.h, CMN_SIZE, CMN_DIAG, etc.)
ISOROPIA/    Directory where ISORROPIA II code resides   
KPP/         KPP solver directory structure
bin/         Directory where executables are placed
doc/         Directory where documentation is created
help/        Directory for GEOS-Chem Help Screen
lib/         Directory where library files are placed
mod/         Directory where module files are placed
obsolete/    Directory where obsolete versions of code are archived

ISORROPIA II consists of the following files:

Files in ISOROPIA/ subdirectory:
---------------------------------
Makefile            Makefile for ISORROPIA II code
isoropiaIIcode.f    Source code file with ISORROPIA II routines
isrpia.inc          ISOROPIA II header file with common blocks

Files in GeosCore/ subdirectory:
--------------------------------
isoropiaII_mod.f    "Interface" code between GEOS-Chem and ISORROPIA II

Additional Documentation

[havala@caltech.edu Havala Pye]wrote:

1. Documentation (including a user manual) for ISORROPIAII can be found on the ISORROPIA website.
2. isoropiaIIcode.f is essentially ISOFWD.FOR and ISOCOM.FOR of the ISORROPIA II box model pasted together.
3. For more information on ISORROPIA II, see
   • Fountoukis, C. and Nenes, A.: ISORROPIA II: a computationally efficient thermodynamic equilibrium model for K+–Ca2+–Mg2+–NH4+–Na+–SO42−–NO3−–Cl−–H2O aerosols, Atmos. Chem. Phys., 7, 4639-4659, 2007. pdf
4. The implementation by Pye et al. 2009 JGR did not include Ca, K, or Mg since dust emissions were not used.

--Bob Y. 10:28, 29 January 2010 (EST)

Validation

See Pye et al [2009]

References

1. Fountoukis, C., and A. Nenes (2007), ISORROPIA II: A computationally efficient thermodynamic equilibrium model for K+-Ca²⁺-Mg²⁺-NH₄⁺-Na⁺-SO₄^2-NO₃^--Cl-H₂O aerosols, Atmos. Chem. Phys., 7(17), 46394659.
2. Meng, Z. Y., and J. H. Seinfeld, Time scales to achieve atmospheric gas-aerosol equilibrium for volatile species, Atmos. Environ., 30(16), 28892900, doi:10.1016/1352-2310(95)00493-9., 1996.
3. Pye, H. O. T., H. Liao, S. Wu, L. J. Mickley, D. J.Jacob, D. K. Henze, and J. H. Seinfeld, Effect of changes in climate and emissions on future sulfate-nitrate-ammonium aerosol levels in the United States, J. Geophys. Res., 114, D01205, 2009. PDF
4. Wang, J., A. A. Hoffman, R. J. Park, D. Jacob, and S. T. Martin, Global distribution of solid and aqueous sulfate aerosols: Effect of the hysteresis of particle phase transitions, J. Geophys. Res., 113, D11206, doi:10.1029/2007JD009367, 2008. PDF

--Bob Y. 16:00, 22 February 2010 (EST)

Previous issues now resolved

None at this time.

Outstanding issues

Aerosol pH

This issue is somewhat resolved.

Havala Pye wrote:

At the GEOS-Chem User's meeting (2009) Becky Alexander and her student Eric noted that ISORROPIA II sometimes returns a negative H+ concentration. In my tests, the negative values occurred in N. India and Western Russia area during limited times of the year. A quick fix has been implemented. A condition I tracked down was returning a negative [H+] when solving the HSO4 = H + SO4 equilibrium (CALCHS4 in isorropiaIIcode.f) when the actual answer should have been ~1e-27 mol/m3. (I plugged the same input values into a spreadsheet and it returned the same negative answer so it seems like a numerical precision issue.) For my case study, it came down to subtracting two numbers of very similar value (first 8 digits the same) which resulted in a negative. I put in a fix to reset H+ to 1d-30 in CALCHS4 if it goes negative.

--Bob Y. 10:29, 29 January 2010 (EST)