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.


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 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.