Model Description

ISORROPIA (II) calculates the composition and phase state of an ammonia-sulfate-nitrate-chloride-sodium-(calcium-potassium-magnesium)-water inorganic aerosol in thermodynamic equilibrium with gas phase precursors.


ISORROPIA was originally developed at the Division of Marine and Atmospheric Chemistry of the Rosenstiel School of Marine and Atmospheric Science, University of Miami, and ISORROPIA II at the Schools of Earth & Atmospheric Sciences and Chemical & Biomolecular Engineering at the Georgia Institute of Technology.

The objective was to develop a computationally efficienct and rigorous aerosol thermodynamics module for use in regional and global aerosol models. The complete theory of ISORROPIA, together with a detailed description of the equations solved, the activity coefficient calculation methods and the computational algorithms used can be found in Nenes et al., 1998a,b and Fountoukis and Nenes, 2007. The performance and advantages of ISORROPIA over the usage of other thermodynamic equilibrium codes has been assessed in numerous studies (e.g., Nenes et al. 1998b; Ansari and Pandis, 1999ab; Yu et al., 2005). It has been evaluated wth numerous in-situ data sets (e.g., Yu et al., 2005; Nowak et al., 2006; Fountoukis et al., 2009).

Model Description

ISORROPIA-II can solve for two classes of problems:

1. Forward (or “closed system”)

Known quantities are temperature, relative humidity and the total (i.e. gas+aerosol) concentrations of NH3, H2SO4, Na, (Ca, K, Mg), HCl and HNO3.
These computations are needed when the total concentrations of precursors are solved for in a model.
2. Reverse (or “open system”)

Known quantities are temperature, relative humidity and the aerosol phase concentrations of NH3, H2SO4, Na, (Ca, K, Mg), HCl and HNO3.
These computations are needed in detailed models of aerosol dynamics, such as MADM (Pilinis et al., 2000).

In both types of problems, the aerosol can be either in a thermodynamically stable state (where salts precipitate once the aqueous phase becomes saturated) or in a metastable state (where the aerosol is composed only of a supersaturated aqueous phase).



The source code of ISORROPIA is written primarily in FORTRAN 77. The only extensions to the ANSI standard implemented were the usage of INCLUDE statements, trailing comments, and variable names with length larger than 6 characters (but not larger than 9). These extensions are supported by the majority of FORTRAN 77 compilers, and are also included in the Fortran 90 standard, so they should not pose any portability problems.


The model has extensively been tested on many platforms (DEC Alpha, HP and IBM RISC workstations under UNIX, Intel-based PC’s under MS-DOS and Windows 95) and compilers (both FORTRAN 77 and Fortran 90). The results during these tests were found to be independent of the system used.


Original (v.1.0)

Since its original release in 1997 (version 1.0), the code has been widely used in air quality and global aerosol modeling studies. As a result, it is regularly updated and expanded. It’s most recent extension, known as ISORROPIA II, incorporate a larger number of aerosol species (Ca, Mg, K salts) and was developed with support from NOAA. ISORROPIA-II is designed to be a superset of ISORROPIA, and uses the exact same routines as ISORROPIA to compute the equilibrium composition when crustal species are not present (i.e., when concentration of Ca, K, Mg are zero). This means that replacing ISORROPIA with ISORROPIA-II should produce identical results, except when crustal species are present.


ISORROPIA has been phased out and replaced by ISORROPIA-II. Updates to the source code of ISORROPIA 1.7 were discontinued June 1, 2010 (although the source code and manual for 1.7 are available for download), but support will be provided for ISORROPIA-II.

Users of ISORROPIA are encouraged to download ISORROPIA-II, which can be found on Code Repository, at their earliest convenience. Those that have chosen not to update the source code to ISORROPIA-II will still continue to recieve support (as routines of 1.7 are incorporated in ISORROPIA-II) but will have to manually maintain their copy.

Vectorized Form

A code (called HETV) which is based on the algorithms of ISORROPIA for sulfate, nitrate and ammonium aerosol systems has been developed. This code is optimized for running on vectorized architectures.


The development of the first release of the code (version 1.0) was done with support from the Environmental Protection Agency under grant R-824793010, from the National Science Foundation under grant ATM-9625718, and from the ONR studentship by grant N000149510807.

Versions 1.1 to 1.6 were developed with the contributions of many people. In particular we would like to acknowledge the contributions (in alphabetical order): Asif Ansari, Veronique Bouchet, Prakash Bhave, Bill Hutzel, Kevin Kapaldo, Bonyoung Koo, Sonia Kreidenweis, Paul Makar, Federico San Martini, Chris Nolte, Betty Pun, Armistad Russell, Uma Shankar, Jason West, Douglas Waldron, Ashraf Zakey, Yang Zhang. The feedback and bug reports from many other individuals using the code is greatly appreciated.

Versions 1.7 and 2.0 was developed with support from the National Oceanic and Atmospheric Administration (NOAA) under contract NMRAC000-5-04017. We would also like to acknowledge the contributions of Prakash Bhave and Chris Nolte.

Version 2.1 is maintained by the authors without formal support from funding agencies.