  A flexible treatment for gas- and aerosol-phase chemical processes has been
developed for models of diverse scale, from box models up to global models. At
the core of this novel framework is an "abstracted aerosol representation" that
allows a given chemical mechanism to be solved in atmospheric models with
different aerosol representations (e.g., sectional, modal, or
particle-resolved). This is accomplished by treating aerosols as a collection
of condensed phases that are implemented according to the aerosol
representation of the host model. The framework also allows multiple chemical
processes (e.g., gas- and aerosol-phase chemical reactions, emissions,
deposition, photolysis, and mass-transfer) to be solved simultaneously as a
single system. The flexibility of the model is achieved by (1) using an
object-oriented design that facilitates extensibility to new types of chemical
processes and to new ways of representing aerosol systems; (2) runtime model
configuration using JSON input files that permits making changes to any part of
the chemical mechanism without recompiling the model; this widely used,
human-readable format allows entire gas- and aerosol-phase chemical mechanisms
to be described with as much complexity as necessary; and (3) automated
comprehensive testing that ensures stability of the code as new functionality
is introduced. Together, these design choices enable users to build a
customized multiphase mechanism, without having to handle pre-processors,
solvers or compilers. This new treatment compiles as a stand-alone library and
has been deployed in the particle-resolved PartMC model and in the MONARCH
chemical weather prediction system for use at regional and global scales.
Results from the initial deployment will be discussed, along with future
extension to more complex gas-aerosol systems, and the integration of GPU-based
solvers.
